UNIVERSITÀ DEGLI STUDI DI SASSARI
SCUOLA DI DOTTORATO DI RICERCA
Scienze e Biotecnologie
dei Sistemi Agrari e Forestali
e delle Produzioni Alimentari
Indirizzo in Scienze e Tecnologie Zootecniche
Ciclo XXV
ENERGY CONSUMPTION ANALYSIS OF SHEEP MILK
COOLING SYSTEMS
Direttore della Scuola
Prof.ssa Alba Pusino
Referente di Indirizzo
Prof. Nicolò P.P. Macciotta
Docente Guida
Prof. Antonio Pazzona
Correlatore
Prof.ssa Lelia Murgia
Dottorando
Dott. Marco Cossu
Anno accademico 2011-2012
ABSTRACT
In this study the energy consumption of milk cooling systems for sheep milk was
quantified, depending on number of milkings and performance class. The aim was to
produce updated data about the incidence of energy consumption for milk cooling,
useful for energy auditing research in livestock. The cost in electricity bills and the
weight on the current price of sheep milk were calculated. The experimental work was
carried out on 22 milk cooling systems in Sardinia, equipped with open-type tank and
direct expansion system. A performance test was performed to determine the cooling
time, monitoring the milk cooling energy consumption simultaneously. Factors affecting
energy consumption were identified, such as the number of milkings, power/volume
ratio and performance class. The average energy consumption was 1.795 kWh/100 l for
two milkings and 2.427 kWh/100 l for four milkings tanks. The energy consumption for
storage of cooled milk was estimated averagely 0.120 kWh/100 l. Malfunctioning
systems in the sample consumed averagely 26% more than those with regular cooling
time. The electricity cost for cooling accounts for 0.63% on the current price of sheep
milk. The study highlights the need for regular maintenance in old tanks and a
modernisation of milk cooling systems in sheep breeding farms in Sardinia, influenced
by the introduction of a milk quality payment scheme, taking into account the
importance of a correct and fast cooling process.
Marco Cossu – Energy consumption analysis of sheep milk cooling systems
Tesi di Dottorato in Scienze e Biotecnologie dei Sistemi Agrari e Forestali e delle produzioni alimentari
Indirizzo di Scienze e Tecnologie Zootecniche – XXV Ciclo - Università degli Studi di Sassari
2
RIASSUNTO
In questo studio è stato quantificato il consumo di energia elettrica degli impianti per la
refrigerazione del latte ovino, in funzione del numero di mungiture e della classe di
prestazione degli impianti. L’obiettivo è stato quello di produrre dati aggiornati circa
l’incidenza del consumo elettrico per la refrigerazione del latte, nell’ambito delle
ricerche di energy auditing nel settore zootecnico. Si è inoltre appurato il relativo costo
in bolletta elettrica ed il peso percentuale sul prezzo attuale del latte ovino, nell’ottica di
un suo prossimo regime di pagamento a qualità. Il lavoro sperimentale è stato eseguito
su 22 impianti in Sardegna, dotati di serbatoio di tipo aperto ad espansione diretta. Su
di essi sono stati effettuati controlli prestazionali tesi ad appurare il tempo di
refrigerazione, ed un monitoraggio dei consumi elettrici, tramite strumentazione
specificamente assemblata. Sono stati individuati i fattori che incidono sui consumi
elettrici, come il numero di mungiture dell’impianto, il rapporto potenza/volume e la
classe di prestazione. I consumi elettrici medi dei tank in classe II, maggiormente diffusi
sul territorio, si sono attestati su 1,795 kW/100 l per gli impianti a due mungiture e di
2,427 kWh/100 l per quelli a quattro mungiture. I consumi per la conservazione del
latte refrigerato alla temperatura di 4-5°C sono stati stimati mediamente in 0,120
kWh/100 l. Sono stati inoltre messi in evidenza i consumi degli impianti malfunzionanti
presenti nel campione, maggiori mediamente del 26% rispetto a quelli con tempi di
refrigerazione regolari. Il costo dell’energia elettrica per la refrigerazione incide per lo
0,63% sul prezzo attuale del latte ovino. Lo studio mette in evidenza la necessità di una
manutenzione regolare in impianti con età elevata e un rinnovamento impiantistico
nelle aziende zootecniche ovine della Sardegna, condizionato dall’introduzione di un
regime di pagamento basato sulla qualità del latte, che tenga in attenta considerazione
l’importanza di un corretto e rapido processo di refrigerazione.
Marco Cossu – Energy consumption analysis of sheep milk cooling systems
Tesi di Dottorato in Scienze e Biotecnologie dei Sistemi Agrari e Forestali e delle produzioni alimentari
Indirizzo di Scienze e Tecnologie Zootecniche – XXV Ciclo - Università degli Studi di Sassari
3
TABLE OF CONTENTS
I.
INTRODUCTION .................................................................................................................. 6
1.
ENERGY DEMAND IN THE MODERN DAIRY FARM .................................................. 8
2.
ENERGY DEMAND FOR MILK COOLING ....................................................................11
3.
THE MILK COOLING SYSTEM.......................................................................................13
3.1 Compressor...................................................................................................................16
3.2 Condenser......................................................................................................................17
3.3 Lamination device .....................................................................................................17
3.4 Evaporator....................................................................................................................18
3.5 Control devices and other system components .............................................18
3.6 Refrigeration systems ..............................................................................................19
3.6.1
Direct expansion ........................................................................19
3.6.1
Indirect expansion......................................................................20
3.7 Milk cooling systems typologies...........................................................................21
3.8 Standard regulation for milk cooling systems ..............................................22
3.8.1
Milk cooling system manufacturing ..........................................22
3.8.2
Equipment for regulation and control .......................................23
3.8.3
Performance...............................................................................24
4.
DIMENSIONING A MILK COOLING SYSTEM AND MANAGEMENT COSTS .........27
5.
ENERGY SAVING IN MILK COOLING...........................................................................30
5.1 Pre-cooling....................................................................................................................30
5.2 Instant cooling ............................................................................................................32
5.3 Heat recovery...............................................................................................................33
6.
STANDARD REGULATION FOR PERFORMANCE TESTS ........................................35
6.1 Complete test ...............................................................................................................35
6.1.1
Ambient temperature..................................................................36
6.1.2
Milk rate.....................................................................................36
6.1.3
Initial milk temperature .............................................................37
6.1.4
Age of the milk cooling system...................................................38
6.2 Simplified test..............................................................................................................40
Marco Cossu – Energy consumption analysis of sheep milk cooling systems
Tesi di Dottorato in Scienze e Biotecnologie dei Sistemi Agrari e Forestali e delle produzioni alimentari
Indirizzo di Scienze e Tecnologie Zootecniche – XXV Ciclo - Università degli Studi di Sassari
4
II.
AIM OF THE STUDY..........................................................................................................42
III. MATERIALS AND METHODS .........................................................................................44
1.
MATERIALS FOR PERFORMANCE TESTS..................................................................45
2.
MATERIALS FOR MONITORING ELECTRICITY CONSUMPTION .........................46
3.
SPREADSHEET FOR CALCULATING THE STANDARD COOLING TIME ..............52
3.1 Spreadsheet calculation method.........................................................................55
IV.
RESULTS…...........................................................................................................................57
1.
SURVEY ON SHEEP MILK COOLING EQUIPMENTS.................................................57
2.
MILK COOLING SYSTEMS SAMPLE.............................................................................64
3.
PERFORMANCE TESTS ..................................................................................................66
4.
ELECTRICITY CONSUMPTION MONITORING ..........................................................73
5.
ENERGY UTILIZATION INDEX .....................................................................................78
5.1 EUI calculation for performance classes .........................................................80
5.2 Error check for monitoring and calculation ..................................................81
6.
ELECTRICITY CONSUMPTION FOR COOLED MILK STORAGE.............................83
7.
MALFUNCTIONING SYSTEMS ......................................................................................85
8.
ANNUAL ENERGY COST FOR SHEEP MILK COOLING.............................................87
V.
DISCUSSION ........................................................................................................................90
VI.
CONCLUSIONS................................................................................................................. 100
VII. REFERENCES ................................................................................................................... 102
VIII. ACKNOWLEDGMENTS.................................................................................................. 108
Marco Cossu – Energy consumption analysis of sheep milk cooling systems
Tesi di Dottorato in Scienze e Biotecnologie dei Sistemi Agrari e Forestali e delle produzioni alimentari
Indirizzo di Scienze e Tecnologie Zootecniche – XXV Ciclo - Università degli Studi di Sassari
5
I.
INTRODUCTION
In the modern agriculture, like in other economic sectors, energy consumption and its
supply sources are gaining a strong importance. The technological progress allowed
replacing a large amount of labor force with an engine force that led to an increasing
use of different energy sources. Even dairy farming is walking through a process of
intense productive a technological renovation. The productivity of the farms shows that
there has been an increasing concentration of farms in large enterprises. The
availability of facilities and equipment increased, with a reduction of the workforce and
its gradual replacement by systems with a steadily higher energy demand.
Consequently, even if the energy cost still represents a high portion of total production
costs, the amount of energy consumed will assume a stronger role, together with the
gradual and increasing technology transfer in livestock production sector.
The need for saving or self-produce energy is due to several European regulations that
have interpreted the so-called "20-20-20" of the Kyoto Protocol, whose measures were
revised and strengthened in the recent United Nations Conference Rio +20. The aim is
to reduce the carbon dioxide (CO2) emissions causing global warming, rationalising
energy consumption and increasing power production ratio from renewable energy
sources by 20% within 2012, compared to emissions in 1990. Furthermore the Kyoto
Protocol introduced three mechanisms regulating CO2 emissions, through the
establishment of the so called “emission credits”:
 Clean Development Mechanism (CDM) allows industrialized countries to
implement projects developing environmental benefits in terms of reducing
greenhouse gas emissions and economic and social development, generating
emission credits (CERs) for the countries that promote interventions;
 Joint Implementation (JI) allows industrialized countries to implement projects
to reduce greenhouse gas emissions in another country and use credits, together
with the host country;
 Emissions Trading (ET) allows the exchange of emission credits between
industrialized countries, a country that has achieved a reduction of their
greenhouse gas emissions than its target can thus yield (using ET) such “credits"
Marco Cossu – Energy consumption analysis of sheep milk cooling systems
Tesi di Dottorato in Scienze e Biotecnologie dei Sistemi Agrari e Forestali e delle produzioni alimentari
Indirizzo di Scienze e Tecnologie Zootecniche – XXV Ciclo - Università degli Studi di Sassari
6
to a country that, on the contrary, has not been able to meet its commitments to
reduce greenhouse gas emissions (United Nations, 1998).
Europe was the continent that developed the world’s first Emission Trading System
(ETS) in order to achieve the targets of the Kyoto Protocol. In fact the European Union
(EU) was aware of the negative effects of economic and demographic growth to
environmental resources, causing damage to the biosphere. Furthermore an internal
and external policy was strategic to secure energy sources and increase energy
efficiency, through development of alternative supplies and renewable energy systems,
since about 50% of EU energy requirements currently derive from imported products
(EU Parliamentary Meeting, 2006). The EU interpreted the Kyoto Protocol and met its
needs through the Green Paper in 2006.
The Green Paper is considered the most important regulation in developing EU energy
policy. The aims of the document are to contrast the climate change by promoting
renewable energy sources with low CO2 emissions; diffusion of technologies for energy
efficiency; coordinating and securing the EU supply sources by diversifying the energy
mix. The paper states that the economic sustainability of energy produced inside the EU
borders could be achieved only with an open energy market, based on the competition
between energy supply companies. In fact the main objective of the EU internal energy
market is to promote the competitiveness of EU industry, which requires a stable and
predictable regulatory framework in the long-term period (Commission of the
European Communities, 2006).
The EU also supports a common European CO2 tax and a modernised common
agricultural policy that gradually transfers public subsidies to biofuels and biomass
energy production, or to technologies regarding the Carbon Capture and Sequestration
(CCS). However the main EU economies are still characterised by a high degree of
energy import: United Kingdom imports only 10% of the energy consumed, but Spain
and Italy import respectively 80 and 85%. Due to differences in the size of economies in
the EU, substantial variation exists in total energy consumption: Germany, the largest
energy consumer, uses 13 times more energy than Hungary, the smallest consumer.
Germany is also the largest energy importer in absolute terms, though its degree of
imports in energy consumption is not as high as for Italy or Spain (Leimbach and
Müller, 2008).
Marco Cossu – Energy consumption analysis of sheep milk cooling systems
Tesi di Dottorato in Scienze e Biotecnologie dei Sistemi Agrari e Forestali e delle produzioni alimentari
Indirizzo di Scienze e Tecnologie Zootecniche – XXV Ciclo - Università degli Studi di Sassari
7
1.
ENERGY DEMAND IN THE MODERN DAIRY FARM
The estimation of the energy needs for the national livestock sector is quite complex.
The Italian production system is heterogeneous in terms of structure and basic farming.
Moreover, energy demanding technologies applied to the production process, change
basically depending on the structural characteristics and the productivity of the
livestock. These characteristics, which are the most important variables explaining the
different energy needs of dairy farms, are only partially known (Rossi et al., 2009).
The multifunctionality of the agricultural sector is characterized by a systematic
approach available to contributions from different disciplines and alternative
technologies, which are concurrent with each other though. The production of
renewable energy developed a growing interest in the agricultural sector, especially the
biomass cultivation and the installation of photovoltaic systems, which has become a
priority for many farming activities. The model of distributed generation, the energy
efficiency of the process during crop production, the modernisation of facilities and the
establishment of a functional framework to promote technological innovation and
environmental sustainability, are targets which received a steady growing interest
within the agricultural sector. The development of energy efficiency regulations in the
agricultural field requires the availability of new skills, such as plant biology experts, to
optimize the production process for energy saving and environmental monitoring of the
agro-food chain (ENEA, 2010). The interventions regarding this sector in order to
improve energy efficiency estimate a potential energy saving of 1.4 Mtoe in 2016 and
5.5 Mtoe within 2020. In fact the agricultural sector is suitable for application of
technologies to produce energy in Italy, as it accounts for only 3% on the national
demand for primary energy, with 3.3 Mtoe of primary energy consumed (ENEA, 2011)
and 2% for domestic energy consumption (Terna, 2011). Agriculture requires low
power installations and is characterized by a high availability of space for placing
energy production systems. Therefore it is possible to predict a future scenario in which
agriculture, with the diffused generation, becomes largely energy self-sufficient. The
benefits will be both environmental and economic.
Energy consumption for the agricultural sector in Sardinia in 2010 amounted to 197.5
GWh, equal to 11.7% of national energy demand for agriculture (Table 1). The regions
Marco Cossu – Energy consumption analysis of sheep milk cooling systems
Tesi di Dottorato in Scienze e Biotecnologie dei Sistemi Agrari e Forestali e delle produzioni alimentari
Indirizzo di Scienze e Tecnologie Zootecniche – XXV Ciclo - Università degli Studi di Sassari
8
where energy consumption is concentrated are Lombardy and Emilia Romagna, where
they remained basically stable over the last two years, and currently account for 1.8%
on the total national energy consumption.
Table 1. National and regional energy consumption for the economic-productive sectors in 2009
and 2010. Data in GWh (Terna, 2011).
Regions
Agriculture
Industry
Service
Domestic
TOTAL
2009
2010
2009
2010
2009
2010
2009
2010
2009
2010
309.3
309.0
12,451.1
13,153.4
6,806.2
6,901.0
4,993.7
5,070.2
24,560.3
25,433.6
3.5
4.4
354.7
429.5
301.8
334.9
162.0
184.0
822.0
952.8
Lombardy
849.4
840.5
31,437.8
34,279.2
18,461.4
18,717.1
11,800.4
12,044.8
62,549.1
65,881.7
Trentino Alto Adige
241.1
232.0
2,333.0
2,489.9
2,452.7
2,594.6
1,228.4
1,261.3
6,255.2
6,577.8
Veneto
617.4
618.8
14,971.0
15,447.4
7,949.2
8,059.1
5,558.7
5,621.9
29,096.3
29,747.2
Friuli Venezia Giulia
125.4
123.3
5,143.2
5,841.9
2,339.8
2,329.3
1,395.9
1,426.1
9,004.2
9,720.5
Liguria
34.2
34.0
1,535.9
1,634.2
2,963.2
2,953.4
1,907.1
1,930.4
6,440.4
6,552.1
Emilia Romagna
933.0
924.5
11,400.5
12,163.6
8,476.1
8,939.1
5,275.5
5,283.7
26,085.2
27,310.9
Northern Italy
3,113.4
3,086.5
79,627.2
85,439.0
49,750.3
50,828.8
32,321.7
32,822.3
Tuscany
283.6
287.1
8,661.3
8,955.1
6,579.5
6,619.1
4,369.5
4,402.0
Umbria
101.3
104.0
2,994.4
3,178.8
1,291.7
1,311.8
977.6
980.4
5,364.9
5,575.0
Marche
127.1
124.8
3,273.4
3,231.7
2,367.8
2,387.5
1,643.0
1,643.7
7,411.4
7,387.6
Lazio
330.8
328.2
4,737.8
4,829.7
10,930.9
10,983.7
7,118.6
7,112.3
23,118.1
23,253.8
Central Italy
842.8
844.0
19,667.0
20,195.3
21,169.8
21,302.1
14,108.7
14,138.3
55,788.3
56,479.7
Abruzzo
82.5
83.6
2,953.1
2,988.4
1,946.4
1,949.6
1,269.7
1,323.2
6,251.7
6,344.7
Molise
29.6
30.8
723.4
698.6
380.1
379.9
300.4
302.5
1,433.5
1,411.7
Campania
267.7
271.3
4,830.9
5,001.7
6,210.4
6,289.7
5,829.0
5,891.3
17,138.0
17,454.0
Puglia
514.8
510.8
7,192.5
8,230.6
4,459.6
4,515.5
4,260.6
4,265.3
16,427.5
17,522.2
Basilicata
67.0
63.1
1,491.7
1,499.9
616.5
598.3
522.6
525.4
2,697.9
2,686.6
Calabria
122.2
117.9
956.0
959.6
2,324.6
2,327.3
2,147.5
2,143.5
5,550.3
5,548.3
Sicily
406.7
404.9
6,724.9
7,157.5
5,564.9
5,676.2
5,874.9
5,848.2
18,571.4
19,086.9
Piedmont
Valle d'Aosta
Sardinia
164,812.6 172,176.6
19,893.9
20,263.2
203.0
197.5
6,339.3
6,268.7
2,412.3
2,417.1
2,289.2
2,290.5
11,243.9
11,173.8
Southern Italy
1,693.7
1,679.8
31,211.8
32,805.0
23,914.8
24,153.6
22,494.0
22,589.8
79,314.2
81,228.3
ITALY
5,649.9
5,610.3
130,505.9 138,439.3
94,834.9
96,284.5
68,924.4
69,550.5
299,915.2 309,884.5
Dairy sheep farming is still one the leading sectors in the Sardinian economy. In 2011
Sardinia held 3,008,467 sheep, 52% of national ewes population (increased by 7%
compared to 2000) and 251,375 cattle. Sardinia is also the Italian region with the
highest number of goats, with 237,320 animals surveyed in 2011, with an increase by
13% compared to 2000. Sardinia was the largest national producer of sheep (2,832,349
q) and goat (114,052 q) milk, contributing to national production respectively for 65
and 45% (Table 2).
Marco Cossu – Energy consumption analysis of sheep milk cooling systems
Tesi di Dottorato in Scienze e Biotecnologie dei Sistemi Agrari e Forestali e delle produzioni alimentari
Indirizzo di Scienze e Tecnologie Zootecniche – XXV Ciclo - Università degli Studi di Sassari
9
Table 2. Milk delivered to milk processing facilities (q). Data per Italian region (ISTAT 2010).
Regions
Piedmont
Valle d'Aosta
Lombardy
Liguria
Trentino-Alto Adige
Cow milk
Sheep milk
Goat milk
Buffalo milk
TOTAL
8,283,282
17,275
23,977
7,978
8,332,512
324,411
-
3,687
-
328,098
40,985,825
2,910
42,295
20,423
41,051,453
233,446
-
1,185
-
234,631
5,145,290
-
9,700
-
5,154,990
Bolzano
3,648,418
-
4,889
-
3,653,307
Trento
1,496,872
-
4,811
-
1,501,683
Veneto
10,038,990
2,772
20,589
5,850
10,068,201
Friuli-Venezia Giulia
1,905,028
-
91
10,572
1,915,691
Emilia-Romagna
21,752,820
12,046
-
-
21,764,866
Tuscany
727,186
687,862
1,151
2,772
1,418,971
Umbria
631,729
29,613
114
-
661,456
Marche
564,737
34,112
-
2,620
601,469
4,521,244
416,504
16,097
266,950
5,220,795
Abruzzo
318,593
34,278
348
-
353,219
Molise
707,421
-
-
333
707,754
Campania
2,416,010
19,826
554
1,420,178
3,856,568
Puglia
2,424,038
Lazio
2,369,246
36,336
8,239
10,217
Basilicata
241,510
1,258
3,333
765
246,866
Calabria
601,107
23,886
250
46
625,289
1,780,745
Sicily
1,580,213
171,195
3,686
25,651
Sardinia
2,383,726
2,832,349
114,052
220
5,330,347
105,731,814
4,322,222
249,348
1,774,575
112,077,95
ITALY
Nowadays the economic budget of dairy farms is burdened by high power and heat
demand caused, in most cases, by the excessive size of plants and machineries.
Considering the current global economic situation, where the cost of prime materials
and production factors are not followed by an adequate increase of the agricultural
product price, the success of agro-livestock depends mainly on the ability of the farmer
to reduce production costs including energy, through the introduction of new
technologies and good farming practices, management systems and machineries (Rossi
et al., 2009). Rationalizing energy consumption through conservation and self-energy
production allows both to reduce or eliminate farming costs that reduce profit margins,
and to diversify income for the farmer, improving the market competitiveness of the
company.
In order to identify strategies for higher energy efficiency, it is possible to carry out
energy auditing studies, which can systematically evaluate the efficiency of the
organization of the power management system, monitoring both heat and electrical
energy consumption of all users. The energy auditing study identifies the critical points
Marco Cossu – Energy consumption analysis of sheep milk cooling systems
Tesi di Dottorato in Scienze e Biotecnologie dei Sistemi Agrari e Forestali e delle produzioni alimentari
Indirizzo di Scienze e Tecnologie Zootecniche – XXV Ciclo - Università degli Studi di Sassari
10
to reduce energy consumption and make the company more energy efficient. Through
these studies, possible sources of leakage or waste can be found, in order to provide
decision support for optimising consumption, considering also the national incentive
mechanisms for the implementation of technologies to self-produce and save energy.
The energy consumption data allows the estimation of the annual energy requirement
of the company, providing suggestions on the size of energy production systems, such
as photovoltaic, solar thermal, wind or biogas. These technologies should be chosen
depending on the size of the company, facilities and infrastructures available, final
destination of the generated energy, environmental issues etc.. Installations for
renewable energy production are widely promoted by the Italian national government.
After planning the energy production plant, a technical-economic analysis of the
investment can be performed in order to quantify the convenience with the main
economic indicators (Net present value, internal rate of return, payback time etc.).
2.
ENERGY DEMAND FOR MILK COOLING
Many studies regarding energy auditing in the farms can be found in literature,
especially concerning cattle farms. Some studies are focused on energy consumption
and energy efficiency of the breeding system, both in terms of direct energy (fuels,
lubricants, electricity, gas etc.) and indirect energy consumption, that is the energy
required to provide the production factors used in the farm (Marijke et al., 2006). Other
papers consider only the direct consumption of electricity and thermal energy for the
operations of buildings of the farm, identifying the users with the greatest impact on
energy consumption. In a dairy farm the largest impact seems to be the milk cooling
(43% of total consumption of energy in the milking parlor), the heat for the washing
water (27%) and the vacuum pump (15%), while other utilities seem to be secondary
(Institut de l'Elevage, 2009). On the other hand if the milking operation takes place via
robotic systems, the most demanding electrical loads are the vacuum pump of the
milking plant, because of the higher number of operating hours per day, followed by the
compressor of the cooling tank, the electric boiler for hot water production and the
automatic cleaning system of the milking plant (Rasmussen and Pedersen, 2004). Along
Marco Cossu – Energy consumption analysis of sheep milk cooling systems
Tesi di Dottorato in Scienze e Biotecnologie dei Sistemi Agrari e Forestali e delle produzioni alimentari
Indirizzo di Scienze e Tecnologie Zootecniche – XXV Ciclo - Università degli Studi di Sassari
11
the same trend there are studies establishing that the consumption of electricity due to
the milk tank, water heating and vacuum pump, is equal to 36-55% of the electricity
used in the whole company (Peebles et al., 1994). Therefore milk cooling is one of the
main energy users in a cattle farm. The electrical energy consumption for this operation
can be estimated 10-18% of the whole energy consumed; the amount ranges from 1.76
to 2.42 kWh/100 kg of milk (Peebles et al., 1994). In other studies the cooling of cow
milk on annual basis is the most energy-consuming operation, with 96.7 kWh/cow,
corresponding to 1.1 kWh/100 l milk refrigerated at 4°C, equal to approximately 2124% of the whole electricity consumption of the farm, depending on consistency of
livestock (Murgia et al., 2008).
In order to estimate the energy demand for milk cooling, evaluations using objective
parameters should be performed, such as energy efficiency indicators, which allow
identifying critical operations, suggesting the adoption of energy saving measures.
These energy efficiency indicators can be expressed in term of "Energy Cost" (EP,
Energy Price) of agricultural products: it expresses the amount of energy (MJ) required
to produce a unit of product, like 1 l of milk (Refsgaard et al., 1998, Meul et al., 2007). In
other circumstances, when the indirect energy required to produce a unit of product
can not be defined or estimated, the Energy Utilization Index (EUI) can be used: it
defines the energy consumed for each animal breaded (kWh/head) or for each unit of
finished product (kWh/l of milk or kWh/100 l), expressed on daily and annual basis
(Edens et al., 2003; Ludington and Johnson, 2003). In the United States, the cooling
tanks for cow's milk, usually characterized by high rated volume (several thousand
litres), in good shape and with no energy saving devices (ECM, Milk Cooling Energy
Conservation Measures), show a EUI ranging averagely between 0.8 and 1.2 kWh/100 l
of cooled milk. If a device based on plate heat exchangers is installed for pre-cooling
milk, the EUI decreased to 0.6-0.9 kWh/100 l. If in addition to the pre-cooler a variable
frequency drive (VFD) is added, the rotation speed of the pump extractor can be
decreased, reducing the milk flow on the heat exchanger, making the EUI further
descending to 0.4-0.7 kWh/100 l (SCE, 2004). The EUI is related to milk cooled up to
7°C, while for milk cooling in Italy, storage temperatures are lower, about 4°C. This
aspect shows that EUI for milk cooling in Italy and Europe are higher than the United
States. There are also no data concerning neither the incidence of breeding sheep on the
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overall energy demand of national agriculture nor the energy consumption for cooling
sheep milk. Furthermore the bibliography is scarce even for energy auditing studies in
dairy sheep farms.
3.
THE MILK COOLING SYSTEM
Milk is a good growth medium for many microorganisms commonly in the
environment. The contamination could derive both from within the breast (endogenous
flora) and from outside (exogenous flora). Some of these organisms may be harmful to
human health, others may cause alteration of milk constituents (emphasising the
storage and processing issues), and still others are a marker of insufficient hygienic
conditions. The cooling, inducing stasis of the multiplicative effect at low temperature
(microbial growth is inhibited completely at 4°C), is the best way to contain microbial
proliferation, and its effectiveness is linked both to the performance of the tank and to
the initial milk quality. Since cooling is not a sanitary treatment but a stabilisation
method, both during breeding and milking, all procedures limiting the initial
contamination of the milk should be applied (Pazzona, 1999). In fact the quality of raw
milk and all dairy products is the result of activities related to the production process,
from the farm to processing in the dairy company. The safety of the process is mainly
ensured by preventive approaches, such as the application of good hygiene practices
based on risk analysis and critical control points (HACCP, Hazard Analysis and Critical
Control Point) (Vilar et al., 2012).
The cooling is not a sanitation process, because it does not kill microorganisms, but
stops or slows down their growth. The milk derived from dirty animals, environments
and milking systems, even if stored at 4°C, doubles its microbial charge after 24 hours,
which becomes four times higher after 48 hours; on the contrary, storing the milk at 4°C
produced by clean animals and environments, using clean utensils, maintains roughly
unchanged the microbial charge even after 48 hours. During storage at low
temperature, a psychrophile microflora can develop, which is able to produce enzymes
even at 4-5°C. These enzymes, mainly extracellular lipases and proteases, as well as
being heat resistant, cause denaturation of proteins which can lead to release ammonia.
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The consequence is the taste of rancid (lipolysis), bitter (proteolysis and lipolysis),
other unpleasant tastes and a deterioration of the rheologic properties of milk.
Generally such defects appear only after 48 hours storage at 4-5°C and are more
remarkable in the milk highly contaminated after two days storage (Unalat and CRPA,
1992).
Cooling the milk is meant as a decrease of its temperature without taking it to the
freezing point. This involves the removal of a certain amount of heat that must be
transferred and disposed in an external medium. When heat follows the gradient, the
heat flow decreases temperature, which turns the body from hot to cold. A cooling
machine allows to reverse the natural direction of heat flow, thus to transfer heat from
a cold source to a hot body. All this happens, of course, consuming energy.
Among the practical refrigeration systems, the most common one in many applications
(domestic cooling, industrial, air conditioning) is the compression refrigeration system,
which uses substances with special thermodynamic properties as heat carriers, called
Refrigerant Fluids. Nowadays the refrigerants commonly used are the R22
(chlorodifluoromethane), now replaced by R-507 or R-407C, and R-404a (a mixture of
1,1,1 trifluoroethane, and 1,1,1,2 tetrafluoroethane and pentafluoroethane) that,
contrarily to CFCs used in the past, have an ozone depletion rate extremely low
(Pazzona, 1999). Even the R12, used in the oldest cooling systems, will be withdrawn by
2031 and replaced by the refrigerant R-134a.
These fluids are subjected to changes of state (cooling cycle) that take place repeatedly
and under control, within a refrigerating circuit composed of four main parts:
 compressor
 condenser
 lamination device
 evaporator.
The refrigeration cycle consists of a series of thermodynamic transformations charged
to the refrigerant (Fig. 1).
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Fig. 1. Diagram of the refrigeration cycle.
The process can be described starting from the compressor, which compresses the
refrigerant in gas state and low temperature coming from the evaporator (suction line),
so that it can be condensed at high temperature. The compressed and high temperature
refrigerant flows to the condenser (flow line), where it releases heat to the outside air,
it condenses and cools down. The cooled and liquid state refrigerant still tends to be
hotter than the milk. For this reason there is a thermostatic expansion valve (lamination
device), where the refrigerant expands, cooling down to a very low temperature. The
cold refrigerant flows through the evaporator (low pressure circuit), which is in contact
with the milk (direct expansion systems) or with water (indirect expansion systems
with ice builder). The evaporator performs a heat exchange from the milk to the
refrigerant, which turns from liquid state to gas state. Then it comes back to the
condenser, completing the cycle. In fact, when the refrigerant goes back to the
condenser, the heat previously taken from the milk by the evaporator is transferred to
the outside air (SCE, 2004).
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3.1
Compressor
The compressors commonly used in the milk cooling
systems are hermetic reciprocating type, in which the
compressor itself and the electric traction engine are
mounted on the same shaft and sealed inside a case of
welded steel. This type is characterised by a
cylindrical metallic box in which the suction pipe, the
delivery pressure and the electrical connections
emerge outside (Fig. 2). Despite the open and semihermetic types used in refrigerators, the hermetic
Fig. 2. A hermetic reciprocating
compressor.
type has the advantage of being compact, low noisy, protected from the external
environment; if a damage occurs, it can not be repaired, but this negative aspect is
compensated by a high reliability and long life. The performance is influenced by the
design characteristics of the compressor (displacement, valves type) and the working
conditions (rotation speed, suction and discharge pressure, type of refrigerant used).
According to this, manufacturers certify the rated performance as cooling capacity
(kcal/h) and maximum power (kW). Some milk cooling systems use a new generation of
chillers, also known as "Scroll compressors" (Fig. 3). They are cylindrical shape rotary
compressors, characterized by two metal spirals: the first one is attached to the body of
the compressor, while the second one is connected to the engine shaft and rotates
eccentrically inside the first one. The orbital rotation of
the rotating scroll compresses the air in the spaces
between the two spirals, until reaching the desired
pressure at the center of the attached scroll, where it is
expelled through the discharge line. The advantage of
the scroll compressor is mainly due to the energy
savings achieved, estimated about 20%, compared to
reciprocating compressors of the same capacity
(Emerson Climate, 2006).
Fig. 3. Scroll compressor with
pattern of metal spirals.
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3.2
Condenser
The condenser is a heat exchanger that transfers heat to
the external environment (Fig. 4). Milk cooling tanks are
usually provided with air-cooled condensers. The cooling
medium exchanges heat with the refrigerant only as
sensible heat, thus the amount of heat that can be
disposed of, at constant temperature drop and flow rate,
depends on the specific weight and the specific heat of
the refrigerant. Given the low specific heat (1 kJ/kg°C
Fig. 4. Air condenser.
against 4.18 kJ/kg°C of the water) and the limited coefficient of thermal adduction, the
air condensers need high flow rates and large exchange areas. However they are simple
under a designing and operating point of view, considering the availability of virtually
unlimited air. The air-cooled condenser is the finned-pack type, formed by a series of
thin diameter tubes, with the refrigerant flowing in, equipped with fins that extend the
exchanging surface. An electric fan forces the passage of the air independently.
3.3
Lamination device
The lamination device provides the evaporator
with liquid refrigerant at low pressure. This aim
is achieved by pushing the fluid through a
calibrated hole, causing a fast decrease of the
condensing pressure, together with the cooling
of the fluid through partial evaporation. Two
parts characterises the lamination device: the
capillary tube and the thermostatic expansion
valve (Fig. 5). The capillary consists of a very thin
tube, with diameter and length proportioned
according to the power and the specific
Fig. 5. The lamination device: the
capillary on the top and the
thermostatic expansion valve on the
bottom.
operating conditions of the system. The thermostatic expansion valve injects a certain
amount of refrigerant in the evaporator, according to the specific thermal load. It is a
control device, controlled by a temperature sensor, which dispenses the fluid flow rate
as a function of superheat rate of vapors of the refrigerant at the evaporator outlet.
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3.4
Evaporator
The evaporator can remove heat directly from the substance to cool down, or indirectly
when an intermediate fluid is used (pure water or alcoholic solution). The evaporators
mounted in the milk tanks are often provided with the direct expansion type,
characterized by neither fluid recycling, nor a clear separation between the vapor and
the liquid phase of the refrigerant. In the direct expansion systems, the evaporator is
welded on the outer side of the tub containing the milk. It can be formed by a pipe
arranged in a spiral, or by two steel sheets welded among which a channelling of
various shapes (mesh honeycomb, foam) is positioned, in which the refrigerant flows.
The heat exchange surface, the conformation of the path and the flow rate of the
refrigerant determine the heat exchange efficiency of the evaporator: the higher the
flow rate, the higher the quantity of heat exchanged per surface unit.
3.5
Control devices and other system components
The control devices can start and stop the system (primary control), or regulate and
protect it during operation (secondary control). Generally these devices consist of a
sensor and an actuator that turns on the operation of the system. The most important
ones are the thermostats and pressure switches that control the system, both primary
and secondary type, as a function of temperature and pressure detected in different
sections of the circuit. Moreover the complementary parts have the purpose of
improving the performance of the refrigeration system, but they are not essential for its
operation. There are components and parts commonly on both the high pressure circuit
(oil separator, liquid receiver, filter drier, etc..) and low pressure circuit (suction
accumulator).
The oil separator is mounted on the pressure side of the compressor, and is
characterised by a cylindrical box in which takes place the separation of a largest part of
the lubricant contained in the gas; the oil accumulates at the bottom and is pulled back
to the compressor crankcase, thus avoiding deposits on heat transfer surfaces of the
evaporator and the condenser.
The liquid receiver is a container with cylindrical shape, placed upstream of the
expansion valve (not present in plants in capillary), which temporarily collects the
liquid from the condenser. It is a reserve of refrigerant that allows coping with rapid
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changes in heat load on the evaporator; during the shutdowns for maintenance of the
system it hosts all the liquid.
The dehydrator filter has to eliminate the residual moisture in the refrigerant that may
be originated from poor hermetic seal of the circuit or from lubricants and fluids not
properly dehydrated. The humidity must be avoided in systems with halogenated
hydrocarbons, thanks to their poor hygroscopicity. Moisture may cause the formation of
oxides and metal deposits that would negatively affect the performance of components
in the circuit.
The accumulator on the gas suction line preserves compressor from liquid slugging due
to accidental income of refrigerant not evaporated. This may occur for mistakes in
dimensioning or power system malfunctions of the evaporator, particularly when the
thermal load is low during switching off, in systems with capillary tube.
3.6
Refrigeration systems
The refrigeration systems commonly used can be with direct or indirect expansion.
3.6.1 Direct expansion
This is the most common expansion type for sheep milk cooling systems. The
evaporator of the refrigerating unit, honeycomb type, plaque or semi-tubular, is welded
on the outer wall of the tub containing the milk which is in direct thermal contact with
it. The heat exchange takes place directly on the refrigerant and the milk cooling is
contemporary to the operation of the refrigerating unit (Fig. 6).
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Fig. 6. Scheme of a milk cooling system with direct refrigeration system (Pazzona, 1999). Picture
on the right by Frigomilk.
3.6.1 Indirect expansion
With the indirect expansion system, heat is removed from the milk through an
intermediate medium, usually water (Fig. 7). The evaporator, placed on the bottom of
the interspace that surrounds the milk container, is immersed in water, which
exchanges heat with the cooling fluid, producing an ice layer around the coil, generating
a "chilled water storage".
In the most diffused models, called “instant cooling”, a second circuit, in addition to the
refrigerator, allows the constant circulation of cold water during the milk cooling. The
water with a temperature of 0,5-1 °C, is sucked by a pump and sprayed through nozzles
against the outer walls of the milk container, removing heat from the entire surface; the
warm water flows to the ice, fusing it and ensuring the continuous availability of cold
water. In these systems there is no coincidence in time between the refrigerating unit
work and milk cooling: the compressor operates in the interval between milkings to
build ice; when the milk cooling is taking place, only the circulation system works.
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Fig. 7. Pattern and shape of a instant cooling tank. 1) evaporator, 2) reserve of ice water, and 3)
water pump, 4) nozzles, 5) insulating layer, 6) agitator 7) manhole. Photo on the right by Packo.
3.7
Milk cooling systems typologies
The milk cooling systems available on the market include tanks with capacities from
100 to 25,000 litres, whose external shape can be simplified to five main types: vertical
cylindrical, horizontal cylindrical, semi-cylindrical, ellipsoidal (Fig. 8). The emptying
takes place via a hole located at the bottom of the tank and equipped with a tap-tight.
The chiller is equipped with one or more agitators to promote thermal exchange within
the bulk milk and to prevent fat stratification. Rotation should not exceed 50 r/min, in
order to avoid a strong and vigorous mechanical action, which may damage the
constituents of the milk, primarily the fat globules, and cause the formation of air
bubbles in the milk bulk, with potential oxidation and rancidity. The operation of the
agitator is continuous during the milk cooling, while is cyclic during storage, with 2-3
minutes of stirring among 13-15 min break.
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Fig. 8. Refrigeration systems with different tank shapes. From the top right: type vertical
cylindrical tank with top opening, horizontal cylindrical, and ellipsoidal.
3.8
Standard regulation for milk cooling systems
The European standard reference for milk cooling systems is the EN 13732:2009, which
specifies the characteristics for design, construction, operation and testing methods.
This classification is a key element for the evaluation of the system during purchase,
allowing immediate identification of the milk cooling system performance.
3.8.1 Milk cooling system manufacturing
The legislation requires that the tank and all accessories in contact with the milk must
be designed and manufactured in order to prevent the contamination of milk, avoiding
smells or flavours, and can be easily cleaned and disinfected without any corrosion or
degradation of tank surfaces. Stainless steel is the most common material used for its
chemical inertness and mechanical strength. The rated volume of the tank must be
between 90 and 98% of its maximum capacity. The tank must be provided with covers
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for internal inspection and the drawing of the samples. The discharge duct must be
made of stainless steel, with a minimum diameter of 50 mm and equipped with a stop
tap. The outflow of the milk in the tank occurs thanks to the tilt angle that the bottom of
the tank must be provided with, inclined toward the discharge duct.
The agitator must be constructed to avoid any contamination from the outside; it must
be able to be easily cleaned and disinfected, and there must be adequate protection so
that the operator does not come in contact with moving parts.
In indirect expansion systems, the water tank must be of adequate size for the proper
functioning of the control systems and the accumulation of ice-cold water circulation;
the ice formation must be smooth on the whole surface of the evaporator; there must be
the possibility to inspect the ice reserve and the water replacement. The accumulation
of the ice must be sufficient enough to cool down from 35 °C to 4 °C, without any further
actuation of the chilling unit, 60%, 30% or 20% of the rated volume of a tank
respectively for two, four or six milkings.
3.8.2 Equipment for regulation and control
The control switch of the chilling unit must include the following functions: 0, off;
automatic milk cooling; manual milk cooling; harvest; washing. A timer manages the
agitator for predetermined time intervals, independently from other setup. Selecting
the harvest function, a time switch must operate the agitator for not less than two
minutes.
The thermostat must ensure that the cooling process begins as soon as the second or
subsequent milkings are introduced; it must operate adequately with a milk quantity
between 10 and 100% of the rated volume, with a milk temperature between 0 and 35
°C and at an ambient temperature between -20 °C and the maximum operating
temperature, which is related to the milk tank temperature class. The thermostat
automatically controls the milk agitator and the simultaneous operation of the
condensing unit in direct expansion systems, or the agitator and the water freezing
circulation devices in indirect expansion systems. The indirect expansion system has an
independent adjustment device for each condensing unit, which automatically controls
the ice formation and must work with ambient temperature between 5 °C and the
maximum operating temperature, so that for each quantity of milk including between
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10% and 100% of the rated volume, there is enough ice to meet the required
performance.
The cooling and the electric system must meet the standards set by international and
national specifications. Each cooling tank must be equipped with a thermometer, which
detects the temperature of the milk bulk between 10% and 100% of the rated volume.
The graduated rod for measuring the milk quantity stored must be able to measure
volumes between 10 and 100% of the rated volume and must be scaled with a
millimeter definition.
3.8.3 Performance
The operating characteristics of a milk cooling system are defined according to the
number of milkings that the tank may store, the ambient temperature during operation,
the time required for cooling the milk from 35 to 4 °C. According to EN 13732:2003 the
performance must be specified using the following classification:
 number of milkings. It is the number of milkings that can be stored in the tank
before the collection. The number of milkings can be 2, 4 or 6, depending on
whether the milk collection is daily, alternate days or every two days;
 temperature class. This is the maximum operating ambient temperature
guaranteed by the manufacturer for optimal performance of the tank. It is
indicated by a capital letter. The safe operating temperature is the maximum
ambient temperature for the cooling plant to operate;
 cooling time class. It indicates the maximum time necessary for cooling the milk
from 35 to 4 °C. It is indicated with a roman number (Table 3).
Table 3. Performance classes of milk cooling systems.
Temperature
class
Maximum
ambient
temperature (°C)
Safe operating
temperature (°C)
Milk cooling
time class
Cooling time from 35
to 4°C (hours)
A
38
43
0
2,0
B
32
38
I
2,5
C
25
32
II
3,0
III
3,5
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Thus if a performance class 2BII is reported on the identification plate attached on the
tank (Fig. 9), that means a two milkings tank, with temperature class B (the cooling
process takes place properly only below an ambient temperature of 32 °C) and a
cooling time less than 3 hours.
The cooling time class defines the maximum time within which a tank of a particular
performance class must ensure the cooling of all the milk from 35 to 4 °C, with an
ambient temperature between 5 °C and the one corresponding to the temperature class
of the cooling tank.
The milk rate is referred to:
 a two milkings system, empty or containing 50% of its rated volume with milk at
4 °C, in which a quantity of milk at 35 °C equal to 50% of the rated volume is
spilled at once;
 a four milkings system, empty or containing 25, 50 or 75% of its rated volume
with milk at 4 °C, in which a quantity of milk at 35 °C equal to 25% of the rated
volume is spilled at once;
 a six milkings system, empty or containing 16.6, 33.3, 50, 66.6 or 83.2% of its
rated volume with milk at 4 °C, in which a quantity of milk at 35 °C equal to
16.6% of the rated volume is spilled at once.
The operating characteristics and technical specifications of the system shall be
reported on a metallic identification plate, easily visible and permanently attached to
the tank (Fig. 9). The information should include at least:
 manufacturer's name or trade name;
 b) type and serial number;
 rated volume in litres;
 class of service expressed by three symbols indicating respectively the number
of milkings, the temperature class, the cooling time class and optionally in
brackets the fat content of the milk used during the performance test. For
example 2BII (4.5%) indicates a milk cooling system for two milkings, with a
temperature class B corresponding to a maximum operating ambient
temperature of 32 °C and belonging to the II class, corresponding to a cooling
time within 3 hours, and a milk fat content of 4.5%;
 refrigerant used.
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Fig. 9. A identification plate attached to a 430 l tank, 2 milkings, with temperature class B and II
class cooling time.
The regulation states that there must be also the indication about the manufacture year,
the amount of refrigerant in the refrigerating circuit, the compressor power and the
maximum cooling capacity.
During storage of the milk between two cooling sessions, the average milk temperature
must not exceed 4 °C. Furthermore, the thermal insulation of the tank must be
dimensioned in order to ensure that (for the specific temperature class and rated
volume of the tank) the temperature increase of the stored milk is less than 3 °C in 12
hours. During cooling or storage there should be no ice in the milk. The agitator must
not cause overflow of milk from the tank, even when filled to its rated volume. The
HACCP control plans the hygiene and safety equipment for milk cooling, identifies
several critical points to be monitored during cooling. The risk is due mainly to an
increase of the microbial charge and can be prevented through the choice of a proper
initial sizing of the milk tank during purchase, the periodic inspection and maintenance
of the tank and the thermometer (Vilar et al. 2012).
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4.
DIMENSIONING A MILK COOLING SYSTEM AND MANAGEMENT COSTS
The milk produced daily, the collection interval (daily or alternate days) and the
maximum cooling time are the parameters for dimensioning the cooling tank, both in
terms of capacity and performance of the refrigerating unit. The choice between the
direct or indirect expansion system depends on the need for getting a quick cooling and
containing the electricity bills of the farm. In fact, the electric power installed in indirect
systems is 40-50% lower than the direct expansion systems, since the operating time of
the refrigerating unit for producing the ice reserve does not coincide with the cooling
time of the milk, and usually requires a long time (8-10 hours), with a consequent
higher energy consumption, relatively to a cooling session.
The rated volume of the tank (Vn) is calculated according to the daily milk production
and the frequency of collection: in a daily delivery, the tank should be large enough to
contain two milkings, or four milkings in the case of collected on alternate days. The
daily amount of milk must be calculated referring to the period of maximum production
of the cattle or flock (Lmax), increased by 10% to take into account any peaks, and
multiplied by the collection interval, expressed as number of days (n):
V n  L max  1,10  n
Whereas in the farm, especially sheep and goats, the milk production is subjected to
strong seasonal variations, the daily average milk bulk refrigerated (L), as the ratio
between the total annual production and the length of the milking season, is equal to
73-75% of the daily maximum milk production. Therefore the capacity of the tank,
dimensioned as function of Lmax, is usually under-utilised.
The ratio between the amount of milk cooled between two consecutive deliveries and
the rated volume of the tank defines the utilization coefficient (CU):
CU 
Ln
Vn
Marco Cossu – Energy consumption analysis of sheep milk cooling systems
Tesi di Dottorato in Scienze e Biotecnologie dei Sistemi Agrari e Forestali e delle produzioni alimentari
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27
For an optimal use of the system, CU should be close to 1, even if its real annual value is
averagely 0.70. A lower coefficient suggests an excessive over-sizing of the tank, which
lead to an increase of fixed and variable costs. Even the collection interval must be
taken into account when choosing the cooling tank type. It is important to use the milk
cooling system compatibly to its design features: using a two milkings tanks for
alternate days collection means to under-utilise it, increasing the cost for one milk litre
cooling. On the other side, using a four milkings tank as two milkings does not
guarantee, due to the lower power of the compressor, the performance indicated by the
cooling time class.
The cooling time, thus the performance of the cooling tank, depends on the efficiency of
the refrigerating unit and in particular by the power of the compressor. The
performance class plays a very important role during the choice of the system, since
there is a positive correlation between the cooling time and the bacterial growth, which
heavily affects the milk quality. A quick cooling corresponds to a significant decrease of
the microbial growth factor, while the influence of the initial bacterial charge is
statistically less significant. In fact the microbial growth factor is not influenced by the
initial microbial charge of the milk, but from the performance class of the cooling tank:
storing milk with a very high microbial charge (3.65 million cells/ml) for 20 hours in a I
class tank, causes an increase in the microbial growth factor only 11% higher than what
found using a tank with the same performance class, using milk with initial microbial
charge five times lower (Pazzona and Murgia, 1992).
As for the temperature class (A, B or C), it is necessary to consider the climatic condition
in which the cooling tank is supposed to work. A high ambient temperature decreases
the efficiency of heat dissipation by the air-cooled condenser, causing the cooling time
to be longer, increasing the specific electric energy consumption. In the mediterranean
climate the class B is usually the most appropriate, since the corresponding ambient
temperature coincides roughly with average maximum temperatures in summer.
The milk cooling system price, which is important to determine the annual depreciation
and interest rates based on a lifetime of 10 years, is the most relevant element of
assessment when purchasing the system. Excluding the differences in materials and
standard features offered by the manufacturers on the market, the price of the system
increases with the capacity of the tank, but generally is not proportional, thus the price
Marco Cossu – Energy consumption analysis of sheep milk cooling systems
Tesi di Dottorato in Scienze e Biotecnologie dei Sistemi Agrari e Forestali e delle produzioni alimentari
Indirizzo di Scienze e Tecnologie Zootecniche – XXV Ciclo - Università degli Studi di Sassari
28
unit (€/100 litres) is reduced in larger tanks than those with smaller capacity. Within
the same rated volume and performance, the price changes according to the type of
plant and the refrigeration system. The price is higher in two milking tanks compared to
four milkings, due to the higher power of the refrigerating unit installed. Furthermore
the price is higher in indirect expansion systems, compared to the direct expansion, due
to the higher complexity of the refrigerating unit. The presence of auxiliary equipments,
such the automatic washing devices or pre-coolers, can significantly affect the price of
the system and should be assessed on the basis of qualitative and economic advantages,
which must widely cover the higher investment (Pazzona, 2009). The technical and
economic analysis about the purchase of milk cooling systems suggests that the
economic convenience increases together with the quantity of milk produced. On the
other hand the need for initial capital reduces purchases by small livestock farms
(Sant'Anna et al., 2003). The variable costs, meaning by this term those strictly linked to
utilization of the cooling tank, include electricity consumption, detergents for washing,
ordinary and extraordinary maintenance operations. The energy consumption for
cooling is higher in indirect expansion systems, compared to the direct expansion. The
cost for washing increases in tanks with larger rated volumes, and is halved in four
milkings tanks, compared for two milkings with the same capacity. The operating costs,
if referred to the one litre of milk, vary depending on the quantity actually refrigerated
and the CU of the tank. A high CU reduces the unitary cost because the fixed cost
(depreciation, interest, cleaning, maintenance) is spread over a larger milk production
(Pazzona et al., 2009).
Marco Cossu – Energy consumption analysis of sheep milk cooling systems
Tesi di Dottorato in Scienze e Biotecnologie dei Sistemi Agrari e Forestali e delle produzioni alimentari
Indirizzo di Scienze e Tecnologie Zootecniche – XXV Ciclo - Università degli Studi di Sassari
29
5.
ENERGY SAVING IN MILK COOLING
As already mentioned, the main electric users in a dairy farm are characterised by milk
cooling and heating of the water required washing the milking plant. Improving the
efficiency of these operations saves energy and thus reduces the economic impact.
Some accessory devices, such as the pre-cooler and the heat recovery unit, allow a 4050% decrease in energy consumption for milk cooling.
5.1
Pre-cooling
The pre-cooler is a device that allows a fast heat exchange in countercurrent between
the milk and water coming from a well or the public water supply. The milk is partially
cooled down, depending on the temperature of the water.
The design adopted for milk pre-coolers is usually characterised by a plate or a tubular
heat exchanger. The first is formed by a variable number of stainless steel plates with a
shaped surface, generally in a herringbone pattern, arranged in series and strictly
connected to a supporting structure, using a pressure plate and the threaded tie-rods
(Fig. 10).
Fig. 10. Plate heat exchanger (Techno System) and a single plate chiller with herringbone
pattern (Packo).
The milk and the water flowing in countercurrent on contiguous plates and along the
ducts formed by the shaping increasing the turbulence of the flow, improving the heat
Marco Cossu – Energy consumption analysis of sheep milk cooling systems
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30
exchange efficiency between the two fluids. The seal between plates is ensured by a seal
of nitrile rubber disposed along the perimeter of each plate.
The second type of pre-cooler is the coaxial tubular heat exchanger, formed by a coil of
variable length, comprising an inner tube of stainless steel in which the milk flows,
wrapped by a second outer tube that creates a cavity in which the water flows. The
system is located inside a metallic container with cylindrical shape (Fig. 11). The
insertion of the pre-cooler does not require any
modification of the milking plant and does not interfere
with the duration or the milking technique. The heat
exchanger is mounted on the transfer tube from the milking
plant to the cooling tank, downstream of the extractor
pump and the filter. The movement of the milk inside the
exchanger is controlled by the milk pump, thus no auxiliary
pump is required, while the circulating water is controlled
by a solenoid valve whose opening time is adjusted
according to the required flow rate. The performance of the
Fig. 11. Tubular pre-cooler
(Packo).
pre-cooler, meant as the efficiency in moving heat from the milk to the cooling medium,
depend on many factors such as the contact surface, the flow rate, the retention time of
the milk inside the pre-cooler and the temperature difference between the two fluids.
The effectiveness of cooling is expressed as the real temperature decrease of the milk,
compared to the maximum potential decrease. The potential decrease occurs when the
temperature of the milk in output equals the cooling medium temperature at the
entrance of the heat exchanger, and therefore the efficiency is equal to 1:
E
Tmi  Tmo
Tmi  Twi
Where:
Tmi = Inlet milk temperature
Tmo = Outlet milk temperature
Twi = Inlet water temperature in the heat exchanger
Marco Cossu – Energy consumption analysis of sheep milk cooling systems
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Indirizzo di Scienze e Tecnologie Zootecniche – XXV Ciclo - Università degli Studi di Sassari
31
The increase of the contact surface between the two fluids, which depends on the
number and the area of the plates in plate heat exchangers, and on the length, diameter
and number of tubes in tubular ones, determines a higher effectiveness of the heat
exchanger. A high flow rate, with a consequent higher speed of the fluids, increases the
turbulence of the heat exchange. The ratio between the water and milk flow rate and
ranges commonly between 1:1 and 2.5:1; the higher the ratio, the better the efficiency
of the pre-cooler, because of its higher temperature difference between the two fluids.
However improving the flow rate can be difficult due to the limited availability of water,
thus a ratio of 2:1 is generally used. The average temperature difference between the
milk and the cooling medium should be as large as possible. This is achieved by
ensuring an efficient countercurrent movement of the two fluids.
The main advantages related to pre-cooling are the reduction of the cooling costs and
the improvement of the milk quality. The economic benefit is due to the reduction in
energy costs, since the use of pre-cooler saves about 40-50% of the electricity needed
for cooling milk in ordinary conditions. In fact, after the pre-cooler the milk
temperature can decrease up to 15-20 °C, depending on the water temperature. The
pre-cooler allows the milk cooling to be completed with a considerably shortened
cooling time, or to use a cooling tank with a less powerful compressor, so that energy
consumption is lower.
5.2
Instant cooling
The technology is characterised by heat exchangers, in which iced water circulates,
produced by an appropriate refrigeration system that generates and accumulates ice
(Fig. 12). The temperature of the milk can rapidly decrease to 4 °C. The instant cooling
has the purpose to limit the development of bacterial microflora. Furthermore it
prevents any temperature rise of the milk already in the tank, when adding successive
milkings.
Marco Cossu – Energy consumption analysis of sheep milk cooling systems
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32
Fig. 12. Ice storage system for instant cooling. The machine is connected to a tubular heat
exchanger (Packo).
5.3
Heat recovery
The thermal energy extracted from the milk by the cooling tank, together with the heat
produced by the refrigerating unit for compression of the refrigerant, is normally
dispersed into the atmosphere through the air-cooled condenser. The systems for heat
recovery are characterised by heat exchangers, which can use the thermal energy for
heating water required in several other operations of the farm. The heat recovery unit
does not interfere with the refrigeration cycle, but simply changes the type or the
combination of the condensers used to dissipate heat from the refrigerant. One solution
consists on replacing the air-cooled condenser with a water condenser, which allows
moving all the heat from the refrigerant to the water. To prevent the continuous raising
of the temperature inside the recovery system from reducing the efficiency of the
system and overloading the compressor, the hot water must be removed continuously,
or a source of permanent cooling should be installed. Even the increase of the
condensing temperature for recovering more heat decreases the efficiency of the
refrigerating circuit, overcharging the compressor and causing the energy consumption
by 40-50%. In fact the larger amount of energy required with high condensation
temperatures is used only to raise the pressure of the same condensation circuit.
Therefore, in order to preserve the life of the refrigeration system and to contain the
electrical consumption, the condensation temperature should be kept within limits
commonly used in cooling plants without recovery systems (Pazzona, 1981).
Marco Cossu – Energy consumption analysis of sheep milk cooling systems
Tesi di Dottorato in Scienze e Biotecnologie dei Sistemi Agrari e Forestali e delle produzioni alimentari
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33
The technology usually involves plate heat exchangers, placed between the compressor
and the air-cooled condenser. The plates exchange heat with the refrigerant, recovering
thermal energy before being dissipated by the condenser. Other solutions use the
energy recovered from the refrigerant to heat the water, which is stored in a boiler and
distributed, after further heating, to cover other thermal energy demands in the farm,
such as the washing of the milking plant (Fig. 13 and 14).
Fig. 13. Diagram of heat recovery unit in the hot water storage tank. The heat is recovered from
the refrigerant before reaching the condenser, through the plate heat exchangers. The water
heated through the heat exchangers is stored inside a boiler in which an electrical resistance is
installed to reach the final temperature desired (Ecolacteo).
Fig. 14. Heat recovery storage tank, for a cooling tank with high rated volume. To the right a
detail of the control panel, from where hot water temperature can be adjusted, by regulating the
electrical resistance inside the boiler (source Ecolacteo).
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34
6.
STANDARD REGULATION FOR PERFORMANCE TESTS
The performance of milk cooling systems is evaluated through the milk cooling time.
According to the official methods described in the standard EN 13732/2009 regulation,
tests can be performed both with milk and water, and can be either complete or
simplified. Manufacturers usually carry out performance tests in order to establish the
performance class to be reported on the tank identification plate. These tests are
performed in the laboratory, using the following test and environment conditions, also
known as Standard Test Conditions (SC):
 milk/water rate of 50% of the rated volume for a two milkings tank, or 25% for a
four milkings tank;
 ambient temperature constant, equal to the value established by the
temperature class for which the manufacturer guarantees the optimal
performance of the cooling tank;
 initial milk/water temperature of 35 °C;
 brand new cooling tank to be tested.
The result of the test is the Standard Cooling Time (SCT), thus the cooling time required
to cool down the milk during SC. The SCT establish the milk cooling time class. However
when testing a milk cooling system in the farm, the Operating Test Conditions (OC) are
often different from the SC in the laboratory. Thus the variables affecting the
performance of the tank, in terms of cooling time, must be taken into account.
6.1
Complete test
The complete test is carried out by monitoring the temperature trend over time of the
milk/water bulk, for the entire cooling session from 35 °C (although the temperature of
the milk is always inferior) to 4 °C. Unfortunately the method requires some hours to be
completed. The main variables influencing the cooling time are:
 ambient temperature;
 milk/water tank stage during the test;
 initial temperature of the milk/water 35°C;
 age of the milk cooling system.
Marco Cossu – Energy consumption analysis of sheep milk cooling systems
Tesi di Dottorato in Scienze e Biotecnologie dei Sistemi Agrari e Forestali e delle produzioni alimentari
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35
After the test, each variable is corrected to the value assumed in SC by using equations.
Therefore all milk cooling systems can be compared each other, even when monitored
in OC different from SC.
6.1.1 Ambient temperature
This parameter affects the efficiency of the refrigeration cycle and the cooling time.
With an ambient temperature higher than the one defined by the temperature class
reported on the plate, the manufacturer does not guarantee the optimal performance of
milk cooling system, while at a lower temperature, the tank works in favourable
conditions. To classify the performance of a milk cooling system, the test should be
conducted with the ambient temperature indicated by the manufacturer, thus the
temperature provided by the temperature class (25 °C for a tank in class C, 32 °C for
tanks in class B or 38 °C for class A tanks). This is possible through the application of
experimental equations (Pazzona, Murgia, 1997). After the test, by estimating the
influence of ambient temperature (Ta) on the cooling time, the following correction
factors can be calculated:
hat = 1.3925 – 0.0203 x + 1.846 · 10-4 x2
hatI = 1.4958 – 0.0219 x + 1.988 · 10-4 x2
Where:
hat = ambient temperature correction coefficient for Ta = 25°C (C class tanks)
hatI = ambient temperature correction coefficient for Ta = 32°C (B class tanks)
x = ambient temperature during test (°C)
These correction coefficients, which are applied to the average ambient temperature
observed during the test, allow to balance the effect of temperature on the cooling time,
when it is equal to the value indicated by the temperature class reported on the cooling
tank.
6.1.2
Milk rate
The performance of the tank should be measured for a milk rate equal to 50% of the
rated volume for two milkings tanks, 25% in four milkings tanks or to 16.6% in a 6
Marco Cossu – Energy consumption analysis of sheep milk cooling systems
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Indirizzo di Scienze e Tecnologie Zootecniche – XXV Ciclo - Università degli Studi di Sassari
36
milkings tank. These milk volumes are difficult to be found inside the tank in OC, so that
even in this case the correction coefficients are necessary. Thus the effect of the milk
rate on the cooling time can be measured, assuming the milk volume exactly equal to
the milk rate of the number of milkings:
hmr = 2.432 – 3.114 x + 0.5086 x2
hmrI = 3.011 – 10.847 x + 11.629 x2
Where:
hmr = correction coefficient for milk rate of two milkings cooling tanks
hmrI = correction coefficient for milk rate of four milkings cooling tanks
x = milk stage as fraction of 1
6.1.3 Initial milk temperature
The milk entering the tank is always below 35 °C, so that the cooling time measured
could be significantly lower than what indicated by the manufacturer with the cooling
time class. The correction coefficient for the initial milk temperature is obtained using
the equation:
hmt = 4.8606 – 0.2055 x + 2.7244 · 10-3 x2
Where:
hmt = correction coefficient for initial milk temperature
x = initial temperature of the milk (°C)
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37
6.1.4 Age of the milk cooling system
The efficiency of the refrigerating circuit decays during the working life of a tank,
having a negative influence on the cooling time, compared to a new tank. The influence
of age on performance can be estimated through the following equation:
ha = 1.0005 – 9.119 · 10-3 x – 2.727 · 10-4 x4
Where:
ha = correction coefficient for the cooling tank age
x = age of the cooling tank (years)
By multiplying together all the correction factors, the total correction coefficient (ht)
can be obtained:
ht = hat · hmr · hmt · ha
This coefficient, multiplied by the Total Cooling Time (TCT), thus the cooling time
measured after the complete test in OC, allows the calculation of the SCT, thus the total
cooling time required to cool down the milk in SC, which are:
 ambient temperature equal to the temperature class of the tank;
 milk rate equal to 50%, 25 or 16.66% respectively for 2,4 or 6 milkings tank;
 initial temperature of milk/water: 35 °C;
 new milk cooling system.
The SCT allows comparing the performance of milk cooling systems tested in several
different OC, because the SCT is always referred to SC.
The factors mainly influencing the performance of the cooling tank are the milk rate and
the initial milk temperature, from which derives a wide variation (Δh) of the
corresponding correction coefficient. The other two factors (ambient temperature and
age), having tighter variations, have a limited influence on the SCT (Fig. 15).
Marco Cossu – Energy consumption analysis of sheep milk cooling systems
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38
Fig. 15. Correction factors and maximum variation (Δh) for the variability range factor plotted in
abscissa. From the left: trend of the correction coefficient relative to the ambient temperature
(from 5 to 38 ° C), milk rate (0.15 to 1.00), initial milk temperature (from 5 ° C to 35 ° C) and age of
the cooling system (from 0 to 30 years).
At the end of the test, the complete cooling curve can be plotted, derived from the OC
during the test (Fig. 16).
40
36
Temperature (°C)
32
28
24
20
16
12
8
4
0
10
20
30
40
50
60
70 80 90 100 110 120 130 140 150 160 170
Cooling time (min)
Fig. 16. Milk cooling curve after a complete test over a 800 l cooling tank, 2BII.
In the present study, the age coefficient was not considered in the calculation of SCT, as
drawn up on the basis of tests performed on tanks less than 10 years old. The cooling
tanks included in the sample of this experimental work were characterized by much
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39
higher age, so that the coefficient is outside its age intervals, inducing an overestimation of the influence of age on the SCT, giving unreliable results.
6.2
Simplified test
Compared to the complete test, the simplified test requires only 60-90 minutes to be
completed, considering that the monitoring is carried out over a tighter temperature
interval. In fact the complete milk cooling curve can be divided into three segments,
Temperature (°C)
within which the cooling rate is approximately constant (Fig. 17).
36
34
32
30
28
26
24
22
20
18
16
14
12
10
8
6
4
1st Segment: 35°C – 24 °C 2nd Segment: 24°C – 14 °C
0
10
20
30
40
50
60
70
Cooling time (min)
3rd Segment: 14°C – 4 °C
80
90
100
110
120
130
Fig. 17. Experimental milk cooling curve divided into its three segments, on a 340 l cooling tank,
two milkings, BII class, with a milk rate of 0.550 and average ambient temperature of 16 °C.
Since the milk temperature in the tank is always less than 35 °C and, in some cases, the
cooling ends when the milk has not reached 4 °C yet, the test is performed only for a
reference segment which lies between 24 and 14 °C, and through an appropriate
correction coefficient applied on the cooling time measured, the entire cooling curve,
thus the SCT, is calculated. Also the segments from 35-24 °C or 14-4 °C can be used for
performing a simplified test. Obviously the SCT calculated with these coefficients is
affected by an error (about 4.5%), which leads to an approximation with a magnitude of
7-8 minutes. However this approximation is acceptable for simply checking the milk
cooling system performance for the estimation of the energy consumption.
Marco Cossu – Energy consumption analysis of sheep milk cooling systems
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40
In the simplified test, in order to calculate the entire cooling curve and the TCT, the
cooling time measured on the specific segment from 24 to 14 °C (Tr) must be multiplied
for the coefficient of the simplified control (hsc), which is equal to 3.2388:
TCT = Tr x hsc (24-14 °C)
The other two correction factors, relating to performance tests carried out by
monitoring the other two portions of the curve are:
hsc (35 – 24°C) = 3.4657
hsc (14 – 4°C) = 2.4310
These coefficients were developed examining the cooling curves acquired during
several performance tests on sheep and cow milk cooling systems (Pazzona and Murgia,
1997).
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41
II.
AIM OF THE STUDY
When purchasing a new milk cooling system, the sheep breeder, often uninformed, does
not take into account the performance class of the system, focusing on other aspects
such as price, the manufacturer's name and the shape of the tank. The milk cooling
systems, despite to what already happens for the milking plants, rarely benefit from a
maintenance program, which provides procedures and timelines specifically developed
for the type of plant and environmental conditions. Thus it is important to develop a
control service, to monitor and give assistance to the breeders in managing the milk
cooling plants. The service could be organized along to what has already been
implemented for milking plants (Pazzona, Murgia et al., 2009). A preliminary survey
carried out by LAORE (Sardinian Agency for technical support in agriculture) in 2010
on several milk cooling systems is due precisely to establish this service, which does not
exist yet. The results of the survey stated the high age of the cooling tanks and lack of
attention to maintenance by the breeder. These two problems are arising due to both
unfavorable economic conditions (maintenance interventions are often expensive), and
for a poor culture regarding the importance of a correct cooling. On the contrary the
milking systems are periodically checked, because the breeder developed an interest in
keeping his plant fully efficient and optimised. Energy is another large cost in the
budget and business of the farm and, as already mentioned, milk cooling is one of the
main factors affecting energy consumption. It is strategic to identify solutions for
producing and saving the energy consumed by the companies. Bibliography is relatively
poor and lacks of analytical data about energy consumption for cooling the milk.
Therefore it is important to perform an investigation, set up as a screening study on
milk cooling systems, providing the first experimental data and bibliographical
references about their energy demand.
The present work is aimed to analyze the energy consumption and performance of milk
cooling systems employed by the sample farms, in order to quantify their impact on the
energy balance of the breeding farm and evaluate the energy costs for sheep milk
cooling. Through the performance test and monitoring energy consumption, this study
will perform a screening survey on a sample of sheep dairy farms in order to:
Marco Cossu – Energy consumption analysis of sheep milk cooling systems
Tesi di Dottorato in Scienze e Biotecnologie dei Sistemi Agrari e Forestali e delle produzioni alimentari
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42
 calculate the energy consumption for sheep milk cooling through the
formulation of the corresponding EUI;
 compare the energy consumption differences between milk cooling systems
belonging to different number of milkings and performance classes, to establish
which cooling tank is more convenient to purchase nowadays from an energetic
point of view, in a modern livestock farm;
 estimate the energy consumption resulting from malfunctioning tanks due to
insufficient maintenance. The carelessness in Sardinian sheep farms predisposes
to failure and malfunction;
 quantify the energy consumption derived from the cooled milk storage overnight
and between milkings. This consumption has to be added to the energy
consumed during the cooling session.
Through the detailed analysis of electricity consumption of the cooling systems in the
sample, the energy consumption for milk cooling will be quantified in order to estimate
its economic impact on the current sheep milk price. The results will provide useful
energy auditing indications about the milk cooling process, as one of the most
important factors affecting electrical consumption in a dairy farm, assessing the impact
on the electricity bill and the economic budget.
Marco Cossu – Energy consumption analysis of sheep milk cooling systems
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43
III.
MATERIALS AND METHODS
The survey on sheep milk cooling systems was carried out with the assistance of
LAORE, which provided support in identifying 22 dairy sheep farms in Sardinia (Italy),
on the basis of the following rated volume classes:
 <400 l
 401-600 l
 601-800 l
 > 800 l
The tests were carried out in the period from May to July 2012. All cooling tank data
were collected when available and readable in the identification plate. Since tests were
performed in the late part of the lactation season, sometimes the milk rate in the tank
was not enough for reliable performance test, thus approximately 30-55% of rated
volume for two milkings, or about 25-30% for four milkings tanks (Pazzona and Murgia,
1997). Consequently, in several tests water was used instead of the milk.
On the sheep milk cooling systems of the sample, the following tests were performed:
 performance test, using water or milk, to determine the SCT;
 monitoring of electricity consumption and the main parameters of electric
current during the milk cooling session.
On a few cooling systems, performance tests and electricity consumption monitorings
were conducted over a period of 24 or 72 hours, in order to study both the performance
of the system during the cooling of the milk/water bulk, both the electrical consumption
in post-cooling, for keeping the milk temperature around 4 °C. The results were
analysed with Pearson correlation test (p value <0.05) using the program Minitab 15, to
assess which variables statistically affected EUI.
Marco Cossu – Energy consumption analysis of sheep milk cooling systems
Tesi di Dottorato in Scienze e Biotecnologie dei Sistemi Agrari e Forestali e delle produzioni alimentari
Indirizzo di Scienze e Tecnologie Zootecniche – XXV Ciclo - Università degli Studi di Sassari
44
1.
MATERIALS FOR PERFORMANCE TESTS
During the performance tests, the milk/water temperature and the environment have
been monitored with a 30 s interval with a datalogger Delta Ohm mod. DO 2003,
equipped with an immersion probe TP472 The PT 100 and a hot wire thermoanemometer AP 471 S1 (Fig. 18).
Delta Ohm DO2003 HVAC Datalogger
Dimensions
72 x 210 x 40 mm
Operating temperature and humidity
From -5 °C to + 50 °C. From 0% to 90% RH
Serial Port
Type: RS232
Up to 2 connectable sensors with SICRAM module
Memory
12.000 Reads
Milk Temperature
TP472 I PT 100 immersion probe
Temperature range: -196 – 500 °C
Accuracy: 0.25 °C
Ambient Temperature
AP 471 S1 Thermo-anemometer probe
Wind speed: 0.1 – 40 m/s
Accuracy: ±0.01 m/s
Temperature: -25 – 80 °C
Accuracy: ±0.8 °C
Fig. 18. Specifications of the temperature datalogger Delta Ohm DO2003 (Delta Ohm, 2012).
Only for the cooling tank with a rated volume of 2500 l, a datalogger Grant 2020 was
used, equipped with an internal probe for the ambient temperature (-50, +150 ° C) and
a temperature probe immersion PT100 (- 50, +250 ° C), because it was provided with a
longer cable.
Marco Cossu – Energy consumption analysis of sheep milk cooling systems
Tesi di Dottorato in Scienze e Biotecnologie dei Sistemi Agrari e Forestali e delle produzioni alimentari
Indirizzo di Scienze e Tecnologie Zootecniche – XXV Ciclo - Università degli Studi di Sassari
45
2.
MATERIALS FOR MONITORING ELECTRICITY CONSUMPTION
Most of the milk cooling systems are powered by a
three-phase 400 V current. The monitoring of the
electricity
consumption
is
usually
performed
through power quality analysers with amperometric
clamps (Fig. 19). The clamps are individually
connected to each of the three phases, allowing to
measure almost any electric parameter of the
alternate current line and analytically defining the
electrical behavior of users, in terms of active and
reactive energy, intensity, voltage, peak power,
power factor etc.. The use of power analysers
requires
and
authorized
electrician,
as
Fig. 19. Professional power
quality analyser with clamps for
connection to generic electric
networks and users (HT).
amperometric clamps must be connected directly at the electric board in the milk room
or at the power cord of the refrigerating system, isolating the single phases. In order to
become independent by the constant presence of an electrician, an investigation on the
market was initiated, looking for a device offering the possibility to measure and store
the same data featured by a power analyzer, without working or manipulating electric
boards or circuits. The device should be added in a “plug&play” mode, through common
three-phase outlets, between the power plug of the cooling tank and the three-phase
power socket on the wall. Thus even unskilled personnel can connect the instrument to
the power line in safe conditions, without modifying the electrical system or power
cables of the monitored user.
However such an instrument does not exist on the market yet, so a custom built
instrument was designed and assembled, for all aims mentioned above. The instrument
basically consists of a portable electric panel IP 65, in which a three-phase power meter
is installed and coupled to a gateway ethernet server, equipped with internal memory
(Fig. 20).
Marco Cossu – Energy consumption analysis of sheep milk cooling systems
Tesi di Dottorato in Scienze e Biotecnologie dei Sistemi Agrari e Forestali e delle produzioni alimentari
Indirizzo di Scienze e Tecnologie Zootecniche – XXV Ciclo - Università degli Studi di Sassari
46
Fig. 20. Assembled instrument for monitoring electricity consumption. The gateway ethernet
server and the three-phase power meter are located in the upper part of the panel. The
instrument is equipped with incoming and outgoing three-phase electrical outlets for connection
between the power plug of the milk cooling system and the electric socket. The panel is inserted
and locked in a suitcase for easy transport and protecion from bumps and splashes.
The gateway ethernet server is an integrated server with preinstalled software,
designed to receive and store data of the main parameters of power consumption of
single-phase and three-phase power loads, detected by the power meter connected to
the gateway. The data collected by the power meter can be displayed and downloaded
to a computer in real time, using any Internet browser. Viewing and downloading data
can also be performed remotely via a network connection to the web (Fig. 21).
Marco Cossu – Energy consumption analysis of sheep milk cooling systems
Tesi di Dottorato in Scienze e Biotecnologie dei Sistemi Agrari e Forestali e delle produzioni alimentari
Indirizzo di Scienze e Tecnologie Zootecniche – XXV Ciclo - Università degli Studi di Sassari
47
Schneider PowerLogic EGX 300 Integrated Gateway Server
Dimensions
91 x 72 x 68 mm
Mounting
DIN rail mount
Operating temperature and humidity
From -25 °C to + 70 °C. From 5% to 95% RH
Ethernet Port
Port type: 10/100 Mb base TX
Protocol: HTTP, Modbus TCP/IP, FTP, SNTP, SMTP
Serial Port
Number of ports: 1
Type: RS485
Up to 64 connectable system devices
Web Pages
HTML Format customisable web pages
512 MB internal storage memory (more than 43,000
reads)
Standards
EN 55022 Electromagnetic interference standard
EN 60950 Standard compliance for safety
Fig. 21. Specifications of the Integrated Gateway Server Schneider EGX300.
The gateway ethernet server was connected to a power meter Schneider PowerLogic
PM9C. This device can measure the main electrical parameters observed on three-phase
power currents (Fig. 22).
Schneider PowerLogic PM9C Power Meter
Dimensions
72 x 90 x 66 mm
Mounting
DIN rail mount
Operating temperature
From -5 °C to + 55 °C
Serial Port
Type: RS485
Type of measurement
On single-phase or three-phase AC systems
Accuracy
Current and voltage: ± 0.5% on reading
Power accuracy: ± 2%
Power factor: ± 2% from Pf 0.8 leading to 0.5 lagging
Active energy: Class 2 as defined by IEC 62053-2
Fig. 22. Specifications of the three-phase Power meter Schneider PM9C.
Both the gateway ethernet server and the three-phase power meter were installed on a
single framework with DIN modules (Fig. 23).
Marco Cossu – Energy consumption analysis of sheep milk cooling systems
Tesi di Dottorato in Scienze e Biotecnologie dei Sistemi Agrari e Forestali e delle produzioni alimentari
Indirizzo di Scienze e Tecnologie Zootecniche – XXV Ciclo - Università degli Studi di Sassari
48
Fig. 23. The upper part panel of the assembled instrument: from the left is installed the gateway
ethernet server Schneider PowerLogic EGX 300 for the storage and transmission of data; the data
LAN socket for PC or external modem connection; a service outlet single phase 230 V power plug
for various users; the power meter Schneider PM9C for measuring parameters of three-phase
current.
For technical and safety reasons, the instrument is also equipped with warning lights
for detecting current on a single-phase, and switch load disconnectors, to feed the
power meter and the gateway server independently from the user to monitor (Fig. 24).
The equipment is certified according to European standards EN 60439-1 and IEC
60439-3.
Marco Cossu – Energy consumption analysis of sheep milk cooling systems
Tesi di Dottorato in Scienze e Biotecnologie dei Sistemi Agrari e Forestali e delle produzioni alimentari
Indirizzo di Scienze e Tecnologie Zootecniche – XXV Ciclo - Università degli Studi di Sassari
49
Fig. 24. General wiring diagram of the assembled device.
Legend:
1. Phase 1 light presence indicator
2. Phase 2 light presence indicator
3. Phase 3 light presence indicator
4. Device power switch with 3 toroidal transformers
5. Three phase energy source
6. General power switch for measured device
7. Three phase Power Meter Schneider PM9C
8. Gateway server power supply
9. Ethernet Gateway Server Schneider PowerLogic EGX300
10. LAN data output
11. Service power supply plug
Both the monitoring of power consumption and the performance test are performed
simultaneously (Fig. 25).
Marco Cossu – Energy consumption analysis of sheep milk cooling systems
Tesi di Dottorato in Scienze e Biotecnologie dei Sistemi Agrari e Forestali e delle produzioni alimentari
Indirizzo di Scienze e Tecnologie Zootecniche – XXV Ciclo - Università degli Studi di Sassari
50
Fig. 25. Assembled instrument for monitoring electricity consumption (left) and datalogger
DeltaOhm DO2003 (right) during a performance test.
The assembled instrument connects to the PC via LAN port and allows viewing and
downloading data via Internet browser. Once connected to the web home page, the
instrument can be configured.
The assembles device can display and record the following electrical parameters:
 active and reactive energy (kWh and kVARh);
 active and reactive power (kW and kVAR);
 total apparent power (kVA);
 current on phases (A);
 voltage on phases (V);
 voltage differential between phases (V);
 average power factor (cos φ).
All parameters are measured with an interval of 5 min and can be displayed graphically
via the Internet browser (Fig. 26).
Marco Cossu – Energy consumption analysis of sheep milk cooling systems
Tesi di Dottorato in Scienze e Biotecnologie dei Sistemi Agrari e Forestali e delle produzioni alimentari
Indirizzo di Scienze e Tecnologie Zootecniche – XXV Ciclo - Università degli Studi di Sassari
51
Fig. 26. Screen of the program installed in the gateway ethernet server, where to check and
download collected data. The data visualization requires no software installed on the computer,
but only a web browser.
3.
SPREADSHEET FOR CALCULATING THE STANDARD COOLING TIME
The application of equations for calculating SCT requires time, but after a performance
test, there is often a need for having an immediate response. This is important for the
breeder, who can quickly know about the conditions of his own milk cooling system and
the potential need for maintenance operations. For this reason the Department of
Agricultural Engineering of Sassari University (Italy) developed a spreadsheet that
allows a quick calculation of the SCT by entering the cooling plate data of the cooling
tank and the results of a complete or simplified test. It immediately calculates the SCT,
highlighting any performance anomaly of the cooling system.
The spreadsheet features the correction coefficients provided for the calculation of the
cooling times (see introduction, chapter 6.1). It is in Italian language, as it was primarily
developed for technicians performing technical support on milk cooling systems in
Sardinia and it was used for the calculations of SCT in the sample tanks.
The first part features information about the cooling tank; such data may be used for
the construction of databases about milk cooling systems or for statistical purposes.
The spreadsheet (Fig. 27) consists on three parts:
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Tesi di Dottorato in Scienze e Biotecnologie dei Sistemi Agrari e Forestali e delle produzioni alimentari
Indirizzo di Scienze e Tecnologie Zootecniche – XXV Ciclo - Università degli Studi di Sassari
52
 main information about the dairy farm (location, consistency of the flock, etc.);
 specifications of the milk cooling system, provided by the identification plate;
 parameters used for the simplified test, identified by colored boxes in yellow,
which must be filled to calculate the SCT and build the cooling curve.
The variables to be included are:
 age of the tank;
 temperature interval for the simplified test
(1 = from 35 to 24 °C; 2 = from 24 to 14 °C; 3 = from 14 to 4 °C);
 milk/water tank rate;
 average ambient temperature;
 cooling time in the milk/water temperature range after the simplified test.
Through the identification plate data and the cooling time obtained at the end of a
complete or simplified performance test, the spreadsheet calculates the TCT (from 35 to
4 °C) and the SCT, warning with appropriate colors the operating condition of the
cooling tank. There are specifically:
 as many cells as the combinations of number of milkings, temperature class and
performance class (i.e. two milkings class B II, four milkings class B II, etc.);
 cells have a green color if the milk cooling system succeeded a SCT below the
maximum time allowed by the cooling time class specified by the manufacturer,
which indicates its proper operation; on the other hand, a red color occurs when
the cooling tank is not working properly, that is when SCT is higher than the
maximum indicated by cooling time class, suggesting the intervention of a
maintenance technician.
After calculation, the spreadsheet provides the SCT for all possible performance classes;
only the specific cell regarding the performance class of the milk cooling system
monitored must be considered.
The last part of the spreadsheet is a section providing information for consultation and
compilation of the sheet. It describes the procedure and sequence for filling data. There
is also a table included, with the temperature and cooling time classes, in order to
highlight the different information for each class.
Marco Cossu – Energy consumption analysis of sheep milk cooling systems
Tesi di Dottorato in Scienze e Biotecnologie dei Sistemi Agrari e Forestali e delle produzioni alimentari
Indirizzo di Scienze e Tecnologie Zootecniche – XXV Ciclo - Università degli Studi di Sassari
53
Fig 27. Spreadsheet for the calculation of STC.
Marco Cossu – Energy consumption analysis of sheep milk cooling systems
Tesi di Dottorato in Scienze e Biotecnologie dei Sistemi Agrari e Forestali e delle produzioni alimentari
Indirizzo di Scienze e Tecnologie Zootecniche – XXV Ciclo - Università degli Studi di Sassari
54
3.1
Spreadsheet calculation method
Once data relating to the test conditions and the TCT obtained through a complete or
simplified performance test are inserted, the spreadsheet automatically performs the
following tasks:
 with a simplified test, the spreadsheet multiplies the cooling time observed after
the simplified control (Tr) for the corresponding total correction coefficient hsc as
a function of the milk temperature interval observed, to obtain the TCT under
the specific OC, that is to say:
TCT = Tr · hsc
Where:
hsc (35 - 24 °C) = 3.4657
hsc (24 - 14 °C) = 3.3296
hsc (14 - 4 °C) = 2.4310
The application of the formula is not necessary when a complete test is
performed, since the TCT is obtained experimentally and can then be inserted
directly into the corresponding cell;
 determines the total correction coefficient ht (see “introduction 6.1”) for each
variability factor (ambient temperature, milk rate, initial milk temperature) and
multiplies it by Tr, thus calculating the SCT reported to standard test conditions:
Tr · ht = SCT
 allows to build graphically the complete cooling curve from the test data (Fig.
28).
Marco Cossu – Energy consumption analysis of sheep milk cooling systems
Tesi di Dottorato in Scienze e Biotecnologie dei Sistemi Agrari e Forestali e delle produzioni alimentari
Indirizzo di Scienze e Tecnologie Zootecniche – XXV Ciclo - Università degli Studi di Sassari
55
Milk/Water Temperature (°C)
36
34
32
30
28
26
24
22
20
18
16
14
12
10
8
6
4
Test milk cooling curve
0
20
40
60
80
100
120
140
160
180
200
220
240
260
Cooling time (min)
Fig. 28. Example of a milk cooling curve obtained through the spreadsheet.
Marco Cossu – Energy consumption analysis of sheep milk cooling systems
Tesi di Dottorato in Scienze e Biotecnologie dei Sistemi Agrari e Forestali e delle produzioni alimentari
Indirizzo di Scienze e Tecnologie Zootecniche – XXV Ciclo - Università degli Studi di Sassari
56
IV.
RESULTS
1.
SURVEY ON SHEEP MILK COOLING EQUIPMENTS
The first survey about milk cooling systems was carried out in 1992 by the Institute of
Agricultural Mechanics of Sassari University (Italy). The activities of the survey were
conducted on 344 milk cooling equipments, in cattle and sheep dairy farms in Sardinia
(Italy), over an estimated total number in the early 90s less than a thousand milk
cooling systems. The census was based on data obtained by filling in a form with
technical information about the tank and suggestions on how to use the tank. The
survey had the aim to define the milk cooling equipment typologies in Sardinia, the
performance of the systems, how to use, cleaning and maintenance procedures applied
by farmers.
The survey detected both direct and indirect expansion systems, with the
predominance of tanks with rated volume between 500 and 1000 l, which characterized
38% of the total operating tanks. In cattle farms the most common rated volume ranged
between 600 and 1200 l, while in sheep farms was commonly below 600 l. The study
also showed that the increase of the rated volume of the tanks leads to an increase of
the power of the refrigerating unit, less than proportional. Furthermore it stated that
the 2 milking systems had a compressor power averagely 40% higher than the four
milkings. The most common refrigerants were the R12 and R22, spread equally
between the surveyed systems. Only 16% of the milk cooling systems had performance
comparable to I class, 10% II class, 5.5% III class, while 68.5% of the systems were over
the limit of III class (Murgia and Pazzona, 1992).
At the end of 2010, LAORE agency concluded a survey to check the situation and status
of sheep milk cooling equipments over a huge sample of more than 2,500 dairy farms.
The aim was to gain updated information for establishing a monitoring service and
technical support on sheep milk cooling systems. The Division of Agricultural
Engineering of Sassari University supported LAORE in identifying information to collect
and data processing, while the agency was responsible for the data collection.
The database included the following information from the survey:
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Indirizzo di Scienze e Tecnologie Zootecniche – XXV Ciclo - Università degli Studi di Sassari
57
 location and name (not supplied at the Division of Agricultural Engineering for
privacy reasons);
 number of lactating animals;
 presence of milk cooling system;
 presence of the identification plate;
 manufacturer;
 type and serial number of the tank;
 year of manufacture;
 rated volume (litres);
 number of milkings;
 temperature class;
 cooling time class;
 compressor power (kW);
 refrigerant;
 date of the last maintenance check.
Over 2,577 companies included in the database, about 40% had missing data on the
identification plate, and among these, 92% were without it. Therefore all survey results
were elaborated considering only the cooling systems with complete information on the
identification plate. Moreover 143 farms had no milk cooling system: this implies that
the quality of the milk is not guaranteed and that the products are not made in full
compliance with food safety standards (Table 4).
Table 4. Sheep milk cooling systems with incomplete or missing information on the identification
plate.
Tanks with missing data
Number
%
Farms surveyed
2577
100%
Tanks with missing data
1015
39%
unreadable
23
2%
missing
933
92%
incomplete or partially unreadable
59
6%
143
6%
For identification plate:
Farms without milk cooling system
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Tesi di Dottorato in Scienze e Biotecnologie dei Sistemi Agrari e Forestali e delle produzioni alimentari
Indirizzo di Scienze e Tecnologie Zootecniche – XXV Ciclo - Università degli Studi di Sassari
58
In order to adequately describe the current situation regarding rated volume, the
cooling tanks have been divided into six classes of rated volume. The most common
volume class was between 501 and 750 litres (35% of plants), followed by the class
between 251 and 500 litres (31%), then 751 to 1,000 litres (21%). Only a small
proportion had a capacity over 1001 litres (10%) or less than 250 litres (3%) (Fig. 29).
9%
1% 3%
< 250
21%
31%
251 - 500
501 - 750
751 - 1000
1001 - 1500
>1501
35%
Fig. 29. Percentages of rated volume classes of milk cooling systems included in the 2010 survey.
Consequently, in dairy sheep farms there is a predominance of tanks with medium and
medium-low rated volume, according to the average number of lactating sheep. The
situation is basically unchanged compared to the volumes reported in the survey of
1992 by the Istituto di Meccanica Agraria dell’Università di Sassari. The refrigerant R22
is the most diffused, while is no longer allowed in new refrigerating systems. Due to the
advanced age of most systems, nowadays 80% of the systems in Sardinian dairy farms
still use the R22, while only 11.7% charges the R404a. There is also an 8.27% of
installations still using the R12 already not allowed, found in the oldest cooling systems.
It was found that approximately 82% of farms use a two milkings cooling system. There
are no 6 milkings tanks in database. As for the temperature class, 93% of the farms
bought a B class tank, most suitable in mediterranean climate conditions, 6% use a C
class tank and only 1% of the sample had a A class tank. The most diffused time cooling
class was the II class and only 4% are I class tanks. III class refers to 14% of the sample,
while the class "0" is almost absent (Fig. 30).
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59
Temperature Class
6%
Cooling time Class
A
1%
93%
0
0% 4%
14%
82%
B
C
I
II
III
Fig. 30. Temperature and time cooling class of the cooling systems in the survey.
The most diffused manufacturers of milk cooling systems in Sardinia are basically three:
Frigomilk, Prominox and Japy (Table 5).
Table 5. Manufacturers of milk cooling systems in Sardinian sheep dairy farms. Percentages are
calculated over 2,509 installations.
Manufacturers
%
Number
Frigomilk
33.8%
849
Prominox
33.6%
843
Japy
5.1%
127
Bovo
2.6%
64
Manus
2.3%
58
Milkline
1.5%
37
Packo
1.2%
31
Bonsaglia
1.2%
29
Sfoggia
1.1%
28
Others
<1%
443
The compressor power was related to the rated volume of the tank, calculating the
specific power/volume ratio in kW/100 l. The values ranged from 0.212 to 0.379
kW/100 l. The cooling tanks of the survey averagely have a specific power of 0.34
kW/100 l. Comparing the specific power to the number of milkings and the time cooling
class, the resulting values are shown in table 6.
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60
Table 6. Power/volume ratio in kW/100 l as a function of the number of milkings and the
performance class of plants. The class "0" and the class "I" of four milkings systems are not
reported due to lack of equipments in the database.
Time cooling class
Two milkings
kW/100 l
Four milkings
kW/100 l
I
0.396
-
II
0.386
0.244
III
0.355
0.204
The specific power installed in two milkings tanks, even if varying with the capacity, is
averagely 0.379 kW/100 l, about 69% higher than compared to four milkings tanks
with the same capacity (0.224 kW/100 l). In fact the four milkings tanks cool down, for
each milking, a milk volume which is exactly the half compared to two milkings tanks
with the same rated volume.
The power/volume ratio depends on the manufacturer. For the first three
manufacturers (Frigomilk, Prominox and Japy) it can be noticed that, with the same
number of milkings, temperature and time cooling class, the installed power per unit of
rated volume change (Table 7).
Table 7. Power/volume ratio in kW/100 l for the first 3 manufacturers and for all classes. Missing
values are due to the absence of data in the database.
Manufacturer
Frigomilk
Prominox
Japy
Time cooling
class
I
Two milkings
Four milkings
0.391
-
II
0.378
0.243
III
0.339
-
I
-
-
II
0.421
0.242
III
0.360
0.181
I
-
-
II
0.453
0.309
III
-
-
Since there is a frequent lack of identification plates, if the tank belongs to the
manufacturers listed above, the installed power can be estimated with good accuracy
using the given manufacturer power/volume ratio. For example in figure 31 are
illustrated the trends and the equations allowing to trace the installed power just
knowing the rated volume of the two milkings class B II tank, for the three most
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61
important manufacturers. The installed power increases less than the volumes, which
implies that the power/volume ratio decreases with higher rated volumes (Fig. 32).
6,0
y = 0,0036x + 0,0962
5,5
FRIGOMILK
2
R = 0,9313
2
y = 0.0036x + 0.0962
PROMINOX
Power (kW)
4,5
4,0
R = 0.9313
y = 0,0044x - 0,1197
2
R = 0,86612
5,0
y = 0.0044x + 0.1197
R = 0.8661
y = 0,0026x + 1,0208
2
R = 0,6872
2
JAPY
y = 0.0026x + 0.0208
R = 0.6872
3,5
3,0
2,5
2,0
1,5
1,0
0,5
0,0
0
100
200
300
400
500
600
700
800
900
Rated volume (l)
1000
1100
1200
1300
1400
1500
1600
Fig. 31. Trends of installed power for 2 milkings class B II cooling tanks, in function of the capacity,
for the three most diffused manufacturers.
FRIGOMILK
0,60
PROMINOX
0,55
JAPY
Power/Volume ratio
0,50
0,45
0,40
0,35
0,30
0,25
0,20
0,15
0,10
100
200
300
400
500
600
700
800
900 1000
Rated volume (l)
1100
1200
1300
1400
1500
1600
Fig. 32. Trends of the power/volume ratio on the rated volume using equations in figure 31. The
ratio decreases till reaching a plauteau above 1000 l.
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62
Even the age of the cooling system and the dates of latest maintenance checks were
included in database. Such information can estimate obsolescence, conditions and
efficiency of the system. Approximately 52.7% of the tanks surveyed are less than 15
years old, which can be estimated as the average life of a milk cooling system; 35.8%
are between 16 and 20 years old, while 11.5% are more than 21 years old. Thus 47.5%
of the tanks are technologically obsolete and should be replaced.
Besides high average age, it was found that most farms never made a maintenance
check on their cooling system (77% of total surveyed), while only 23% (581 farms)
made at least one (Table 8). The maintenance checks are important because they
ensure the correct performance over time, from which the quality of the milk depends
on, especially in old refrigerating systems.
Table 8. Number and relative percentage of farms that had at least one maintenance check on the
refrigeration system.
Maintenance
Number of farms
%
No check
1996
77%
Last check in:
581
23%
2010-2009
186
32.0%
2008-2006
243
41.8%
2005-2000
90
15.5%
1999-1990
61
10.5%
1989-1980
1
0.2%
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2.
MILK COOLING SYSTEMS SAMPLE
The 22 milk cooling systems monitored for performance test and electricity
consumption were distributed according to the rated volume classes identified in the
“Materials and Methods” section:
 < 400 l: 3 tanks
 401-600 l: 8 tanks
 601-800 l: 7 tanks
 > 800 l: 4 tanks
The cooling tanks were chosen mainly in the classes with intermediate volume
(between 400 and 800), as it is the most representative volume commonly found in
Sardinian sheep dairy farms. Since four milkings systems are scarce and difficult to find,
only 3 cooling tanks were included in the sample. In addition, many farmers who
claimed to have four milkings tanks actually possessed two milkings tanks used as they
were four milkings.
The main technical characteristics of the monitored tanks are shown in table 9,
including the consistency of lactating flock. The tanks found by visiting the farms with a
random method, reflects the real situation described by the preliminary survey in 2010,
which states the predominance of BII class tanks, for both two and four milkings. There
was only one BIII class system, while no I class tanks were found. Tanks with no
identification plate were later classified with a time cooling II class or III, according to
the results of the performance test.
The manufacturers found were Frigomilk, Prominox and Japy. The most diffused
refrigerant is still R22, since the average age of the cooling systems currently operating
is high (averagely 19 years old). Some farms still use the R12, while only two use the
R404a. The power/volume ratio stood 0.387 kW/100 l for two milkings tanks, and
0.281 kW/100 l for four milkings tanks, and these data are only slightly higher (+5%)
than what found by the survey in 2010, outlining a good adequacy of the sample to the
tank population of the survey. Only the tank N°11 had a power/volume ratio lower than
the sample mean, because of its high rated volume, comparable to cow milk cooling
systems.
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Table 9. Technical specifications of the milk cooling systems monitored. The systems are arranged
in the table according to the chronological order in which they were visited. Missing data in
performance class are due to lack of information on the identification plate.
Tank
N°
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
Flock
(N° Manufacturer
sheep)
400
250
380
150
150
300
320
500
400
550
250
80
210
310
115
500
200
320
150
230
400
550
Frigomilk
Frigomilk
Prominox
Frigomilk
Frigomilk
Prominox
Japy
Frigomilk
Prominox
Frigomilk
Frigomilk
Frigomilk
Japy
Prominox
Prominox
Prominox
Frigomilk
Prominox
Japy
Frigomilk
Prominox
Frigomilk
Model
G4
G4
RFT1055
G4
G5
RFT 650
CV320
G4
G4
G9
G4
CV420
RFT650
RFT330
CM1030
G4
RFT520
CV320
G4
RFT1030
G4
Performance
Rated
class
Age
N°
Volume
Time
(years)
milkings Temp.
(l)
cooling
class
class
15
15
8
18
15
24
18
14
20
16
14
25
18
37
35
16
10
15
22
25
22
16
800
800
1055
430
430
650
320
650
440
800
2500
430
420
650
330
1030
430
520
320
600
1030
800
2
2
2
2
4
2
4
2
2
2
2
2
2
2
2
2
2
2
2
4
2
2
B
B
B
B
B
B
B
B
B
B
B
B
B
II
II
II
II
II
III
II
II
II
II
II
II
II
Refrigerant
Power
(kW)
Power/Volume
ratio (kW/100 l)
R22
R22
R404a
R22
R22
R22
R22
R22
R22
R22
R22
R12
R22
R12
R22
R22
R404a
R22
R22
R12
R22
R22
2.90
2.90
4.53
1.69
0.95
2.20
1.23
1.80
1.69
2.90
4.92
1.69
2.28
2.20
1.10
5.39
1.82
2.80
1.48
1.49
3.43
2.90
0.36
0.36
0.43
0.39
0.22
0.34
0.38
0.28
0.38
0.36
0.20
0.39
0.54
0.34
0.33
0.52
0.42
0.54
0.46
0.25
0.33
0.36
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3.
PERFORMANCE TESTS
The performance tests have been performed in two ways, depending on the control
medium adopted: when water was used, it was usually possible to heat it to 35 °C in
order to perform a complete test; by using the milk, the initial temperature was always
lower than 35 °C, and the test was simplified.
Through the spreadsheet, by performing a complete test, the TCT was measured
directly and used to calculate SCT, coupled to the graphical curve (Fig. 33). To obtain
these results, the water rated volume (0.500), the average ambient temperature (26.2 °
C) and the TCT (173.0 min) have to be inserted in the parameter section. The procedure
allows the automatic calculation of the following correction coefficients, based on the
formulas mentioned in introduction, section 6.1:
 correction coefficient for the ambient temperature of 26.2 °C, for a B class tank:
hat = 3.011 - 10.847x 11.629 + x2 = 1.0585
 correction coefficient for the rate of milk/water in the tank of 0.500 in a two
milkings tank:
hmr = 2.432 to 3.114 x + 0.5086 x2 = 1.0022
The coefficient is equal to 1 because the milk rate (0,500) coincides with the one
adopted under SC, so that in the specific case it will not affect the SCT;
 correction coefficient for initial milk/water temperature (35 °C):
hmt = 4.8606 to 0.2055 x + 2.7244 · 10-3 x2 = 1.0001
Even in this case, the coefficient is 1, because the initial water temperature is 35
° C, equal to SC;
 correction coefficient for the age:
ha = 1.0005 - 9.119 · 10-3 x - 2.727 · 10-4 x4 = 1.0005
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coefficient is equal to 1 because it is not considered in all tests.
The multiplication of all coefficients provides the corrective coefficient total:
ht = hat · hmr · hmt · ha = 1.0585 · 1.0022 · 1.0001 · 1.0005 = 1.0614
This coefficient, multiplied by TCT (173 min), allows the calculation of SCT (183.6 min).
When performance test was simplified, also TCT must be calculated (Fig. 34). Therefore
the simplified control type must be specified (from 35 to 24 °C, from 24 to 14 °C, or
from 14 to 4 °C) to calculate STC. In the present case a simplified test from 14 to 4 °C
was performed, which required 50.5 min (Tr) to be completed. The correction
coefficient to calculate TCT follows the procedures as already described in
“Introduction 6.2”. The correction coefficient hsc (14 - 4 °C) for simplified test from 14 to
4 ° C is of 2.4310. Multiplying this factor by Tr results in TCT:
hsc x Tr = TCT = 122.8 min
The procedure for calculating SCT continues according to the procedure of the previous
example concerning a complete test.
The spreadsheet was then confirmed as an important and smart tool both for the aim of
this study, both for technical and performance tests carried out by technical support
personnel in dairy farms.
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Marco Cossu – Energy consumption analysis of sheep milk cooling systems
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Fig 33. Extract from the spreadsheet for calculating STC. Example of results for a complete test of a cooling tank with rated volume of 800 l,
Class 2BII. Test performed with water. In yellow are: the TCT in minutes (tempo refrigerazione Tr), the total correction coefficient for
calculating the SCT (Coeff. Globale ηtot), reported in the cell corresponding to class B II. The SCT cell for 2BII class is green (2 munte Classe B –
II), indicating good operating conditions. The age correction coefficient was not used because it is not reliable on systems which are more
than 10 years old.
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Fig 34. Extract from the spreadsheet for calculating SCT. Example of results for a simplified test (14-4 ° C), of a cooling tank with a rated volume
of 600 l and time cooling class unknown. Test performed with milk. Assuming a temperature class B, the test allowed to determine the cooling
time class as II, classifying it as 2 B II.
The results of all performance tests are shown in Table 10. Usually the SCT confirmed
the performance class reported on the tank, or allowed to assign it to tanks without
identification plate. For determining a certain performance class, a tolerance of 20
minutes was applied on SCT, considering the age factor (which partially depends on the
performance degradation) and the error committed by the correction coefficients.
Tanks N° 7-13-16-18 showed a behavior similar to I class, although provided with a
plate indicating II class. In fact these systems have benefited from some maintenance
checks (with yearly frequency for the tank 16) that maintained their optimal
performance, bringing them close to the I class. In addition, five plants (tanks N° 8-9-1214-21), equal to 22% of the sample, showed SCT characterized by a delay of more than
20 minutes compared to the average performance of III class tanks (SCT lower than 210
min), and were then classified as malfunctioning III class tanks. The owners of such
cooling tanks were recommended for an immediate intervention of a technician.
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Table 10. Results of performance tests on the sample. Tanks with SCT beyond III class limits are indicated in bold and classified ass malfunctioning
systems.
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In table 11 the average performance of the monitored cooling tanks is shown. The
average SCT of the sample was close to the limit for II class (173 min vs 180 min). There
were no significant differences in SCT between two and four milkings systems with the
same time cooling class. The Japy milk cooling systems seemed to be characterized by
the lowest SCT (averagely 163.8 min), even if the value is calculated over 3 systems
only. Prominox tanks had an average SCT of 169 min, while Frigomilk systems were
characterised by a SCT close to the limit of II class tanks, averagely around 178 min.
The 5 systems showing a performance close to I class tanks, based on information
obtained by interviewing the breeder, had at least one maintenance check. Their
average SCT amounted to 155 min. Malfunctioning tanks instead accused an average
delay of 78 minutes compared to the limits of the III class tanks.
Table 11. Mean and standard deviation of SCT of the sample, sorted by number of milkings and
manufacturer (only well functioning systems are used for calculation). It is also reported the
average performance of the cooling tanks with SCT close to I class and malfunctioning systems.
N° tank
Mean (min)
σ2
Sample mean
17
173.1
13.3
two milkings
14
172.4
13.4
four milkings
3
176.1
15.4
Frigomilk
9
178.1
9.2
Prominox
5
168.5
15.8
Japy
3
163.8
16.4
II class tanks with SCT close to I class
5
154.9
3.4
Malfunctioning tanks
5
288.1
29.5
Manufacturers
Average delay from III class
78.0
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4.
ELECTRICITY CONSUMPTION MONITORING
The electricity consumption monitoring during performance test was used to quantify
the energy absorbed by the milk cooling systems, and then put it in relation to the SCT.
In Table 12 is shown an example of electrical monitoring.
Table 12. Power consumption monitoring in a Prominox cooling tank, 1030 l, two milkings, Class
B II, with milk rate 0.306. Current and voltage is reported for every phase.
Time
(min)
Active
energy
(kWh)
Reactive
energy
(kVARh)
Apparent
power
(kVA)
Active
power
(kW)
Reactive
Current on phase (A) Voltage on phase (V)
power
(kVAR)
Power
factor
(cos φ)
1
2
3
1
2
3
0
5
10
0.00
0.33
0.68
0.00
0.23
0.47
4.84
4.83
5.17
3.96
3.95
4.27
2.79
2.79
2.91
6.45
6.45
6.98
8.93
8.93
9.43
6.49
6.49
6.99
223
222
222
220
219
219
222
222
222
0.82
0.82
0.83
15
20
1.06
1.41
0.72
0.96
5.37
5.12
4.49
4.25
2.95
2.85
7.28
6.93
9.71
9.32
7.31
6.91
222
222
220
219
222
222
0.84
0.83
25
30
1.78
2.13
1.20
1.45
5.25
5.21
4.35
4.29
2.94
2.96
7.12
7.07
9.51
9.46
7.12
7.09
223
222
219
219
222
222
0.83
0.82
35
40
2.49
2.85
1.70
1.94
5.23
5.16
4.30
4.26
2.97
2.91
7.07
6.99
9.49
9.39
7.11
6.97
222
222
219
220
221
221
0.82
0.83
45
50
3.19
3.53
2.18
2.42
5.05
5.06
4.13
4.14
2.91
2.91
6.80
6.79
9.21
9.25
6.79
6.85
223
223
220
219
222
222
0.82
0.82
55
60
3.88
4.22
2.66
2.92
5.01
5.12
4.09
4.15
2.88
3.00
6.75
6.90
9.18
9.30
6.79
7.00
222
222
219
219
221
222
0.82
0.81
65
70
4.56
4.90
3.15
3.40
4.98
4.96
4.07
4.01
2.87
2.91
6.73
6.65
9.06
9.09
6.71
6.70
222
222
220
220
223
222
0.82
0.81
75
80
5.23
5.55
3.63
3.87
4.89
4.79
3.98
3.85
2.85
2.86
6.58
6.42
8.98
8.85
6.57
6.43
222
221
220
220
222
222
0.81
0.80
85
90
5.86
6.16
4.12
4.36
4.72
4.71
3.72
3.66
2.91
2.97
6.26
6.24
8.73
8.70
6.34
6.36
222
222
220
220
223
223
0.79
0.78
95
100
6.45
6.75
4.60
4.84
4.53
4.52
3.51
3.51
2.87
2.85
5.97
5.96
8.42
8.43
6.05
6.00
222
222
221
221
223
223
0.77
0.78
MEAN
4.98
4.04
2.90
6.69
9.11 6.72
222
220
222
0.81
The current absorbed is not equal on every phase. In milk cooling systems, the current
absorption is usually equal on two phases (1 and 3), while the third phase (in this chase
N°2) always shows higher values. In fact, while the compressor splits its absorption
equally among the three phases, the other users of the tank (the condenser fan and
agitator) commonly need single-phase current, so their power consumption is charged
to one phase only. The voltage on individual phase is obviously equal to 220-230 V,
while the voltage difference between phases is 380-400 V.
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The reactive energy is usually inferior to active energy. However, the reactive energy
consumption is high in all systems of the sample, and often exceeds active energy, when
considering old systems with no or poor maintenance. The trend of cumulative
electricity consumption is structured like a curve, but it can be assumed having a linear
behaviour. The active power shows slight fluctuations (Fig. 35). The average power
consumption stood 4.04 ± 0.27 kW, with deviations of 6.6% around the mean.
8,0
7,0
4,50
Active power
4,00
6,0
3,50
5,0
3,00
4,0
2,50
3,0
2,00
2,0
Active power (kW)
Active energy consumed (kWh)
5,00
Active energy
1,50
1,0
1,00
0,0
0
10
20
30
40
50
Time (min)
60
70
80
90
0,50
100
Fig. 35. Cumulative energy consumption and active power absorbed by the milk cooling system
whose characteristics are reported in Table 13.
The results about the monitoring of electricity consumption of all the installations are
reported in table 13. The last column also shows the energy consumption in a
hypothetical performance test under SC. Calculation was derived from the average
active power absorbed during the control, multiplied by the SCT.
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Table 13. Results of power consumption monitoring. Active and reactive energy are brought back to consumption needed for a complete
performance test. Malfunctioning tanks are shown in bold.
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Data about energy consumption was characterised by high variability, due to the
different test conditions found while testing the cooling tanks. The only way to compare
data is to report the energy consumption converted to values which could be found in
SC, thus the standard conditions used by manufacturers to test new milk cooling
systems. This consumption was calculated multiplying the average active power and
the SCT. The calculation showed that consumption increased with the rated volume of
the tank (No. 3,11,16) and when performance problems occurred.
As for the reactive energy, 45% of the systems are characterised by compressors with
power factor (cos φ) higher than 0.80. None of the plants reached the value of 0.90,
while 60% consumed an amount of reactive energy equal or higher than 75% of the
active energy, putting the breeder in a condition of potential penalty in the electricity
bill. In some cases the consumption of reactive energy was higher active energy, when
the cos φ dropped below 0.70.
Moreover, considering the power indicated on the identification plate, the active power
consumed is always lower: the power absorption is averagely 75% of what indicated in
the plate and the ratio changes with the manufacturer (Table 14). The Frigomilk tanks
showed an active power absorbed averagely 75% of the power indicated on the
identification plate, followed by Prominox with 78% and Japy with 70%. In particular
the power/volume ratio amounted to 0.24, 0.31 and 0.33 kW/100 l respectively for
Frigomilk, Prominox and Japy. Consequently the operating power/volume ratio is
different from what deduced from the identification plate. The active power and the
plate power differ averagely 25% with a correlation of 99.0% (p <0.01) in Frigomilk
and 99.4% (p <0.01) in Prominox.
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Table 14. Relations between identification plate power, active power and rated volume.
Malfunctioning tank data are shown in bold.
Tank
N°
Manufacturer
1
Frigomilk
Plate power Average active Power/volume
(kW)
power (kW)
ratio
2.90
2.16
0.36
Active
power/volume
ratio (kW/100l)
Active power/plate
power ratio
0.27
0.74
2
Frigomilk
2.90
1.97
0.36
0.25
0.68
3
Prominox
4.53
3.60
0.43
0.34
0.79
4
Frigomilk
1.69
1.17
0.39
0.27
0.69
5
Frigomilk
0.95
0.83
0.22
0.19
0.87
6
Prominox
2.20
1.90
0.34
0.29
0.86
7
Japy
1.23
0.83
0.38
0.26
0.68
8
Frigomilk
1.80
1.62
0.28
0.25
0.90
9
Prominox
1.69
1.22
0.38
0.28
0.72
10
Frigomilk
2.90
2.18
0.36
0.27
0.75
11
Frigomilk
4.92
3.56
0.20
0.14
0.72
12
Frigomilk
1.69
1.17
0.39
0.27
0.69
13
Japy
2.28
1.70
0.54
0.41
0.75
14
Prominox
2.20
1.94
0.34
0.30
0.88
15
Prominox
1.10
0.94
0.33
0.28
0.85
16
Prominox
5.39
4.04
0.52
0.39
0.75
17
Frigomilk
1.82
1.20
0.42
0.28
0.66
18
Prominox
2.80
2.01
0.54
0.39
0.72
19
Japy
1.48
1.02
0.46
0.32
0.69
20
Frigomilk
1.49
1.05
0.25
0.18
0.71
21
Prominox
3.43
2.19
0.33
0.21
0.64
22
Frigomilk
2.90
2.30
0.36
0.29
0.79
MEAN
0.75
The relation of inverse proportion between the power/volume ratio and SCT is linear,
and depends on the manufacturer (Fig. 36).
0,6
Power/Volume ratio
(kW/100 l)
0,5
PROMINOX
FRIGOMILK
Power/Volume
Active power/Volume
0,4
R2 = 0.755
R2 = 0.900
0,3
R2 = 0.937
R2 = 0.706
0,2
0,1
0,0
150
155
160 165 170 175 180 185
StandardCooling TimeSCT(min)
190
150
155
160 165 170 175 180
StandardCooling timeSCT(min)
185
190
Fig. 36. Correlation between the power/volume ratio and the SCT, for the different
manufacturers. Only data of two milkings systems are displayed. The brand Japy does not appear
for lack of data.
Marco Cossu – Energy consumption analysis of sheep milk cooling systems
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77
The two variables are correlated only within the specific manufacturer, showing a
negative correlation with a coefficient of 90% (p <0.01) for the Frigomilk and 75% (p
<0.05) for the Prominox, while no significant correlation was observed when
considering systems without distinction among manufacturer (29.3% p> 0.05).
5.
ENERGY UTILIZATION INDEX
The results of performance tests and energy consumption monitoring allow the
calculation of the energy utilization index (EUI) in kWh/100 l of cooled milk, both in OC
and SC (Table 15).
Table 15. EUI (kW/100 l) for OC (operating conditions) and SC (standard condition), in
comparison with the active powers and cooling time. Malfunctioning tanks are shown in bold.
Tank
N°
Manufacturer
1
Frigomilk
N°
Average active
milkings power (kW)
2
2.16
TCT (min)
SCT (min)
156.5
173.6
Operating EUI Standard EUI
(kWh/100l) (kWh/100l)
1.381
1.563
2
Frigomilk
2
1.97
173.0
183.6
1.422
1.510
3
Prominox
2
3.60
136.1
189.4
2.020
2.154
4
Frigomilk
2
1.17
91.6
159.4
1.635
1.451
5
Frigomilk
4
0.83
134.8
182.6
2.065
2.338
6
Prominox
2
1.90
122.8
175.1
1.530
1.702
7
Japy
4
0.83
204.2
158.5
2.686
2.752
8
Frigomilk
2
1.62
261.5
239.2
1.976
1.989
9
Prominox
2
1.22
179.5
237.8
1.923
2.191
10
Frigomilk
2
2.18
126.5
180.3
1.533
1.634
11
Frigomilk
2
3.56
116.7
189.4
0.876
0.898
12
Frigomilk
2
1.17
213.1
276.3
2.398
2.506
13
Japy
2
1.70
121.0
150.6
1.998
2.034
14
Prominox
2
1.94
213.1
338.4
3.462
3.365
15
Prominox
2
0.94
104.9
174.2
1.625
1.651
16
Prominox
2
4.04
102.0
151.1
2.181
1.977
17
Frigomilk
2
1.20
188.1
178.9
1.745
1.660
18
Prominox
2
2.01
106.5
152.6
2.173
1.961
19
Japy
2
1.02
121.6
182.2
1.972
1.939
20
Frigomilk
4
1.05
189.8
187.2
2.080
2.191
21
Prominox
2
2.19
225.5
348.8
2.636
2.773
22
Frigomilk
2
2.30
120.0
171.1
1.643
1.640
EUI are given only referring to a unit of milk in the tank (100 l) and not to the
consistency of the flock, which would be another variability factor, since the milk
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78
production changes during the lactating season, as well as the size of the flock. This
would make the EUI unreliable for quantifying the energy needed for milk cooling,
while it should be as constant as possible throughout the year. The EUI expressed in
kWh/100 l is independent from the productivity of the flock and simplifies calculation
when of the annual consumption for cooling a unit of milk in an energy auditing study.
EUI was calculated by multiplying the average power absorbed and the SCT, for a
milk/water rate of 50% of the rated volume for a two milkings tank or 25% for a four
milkings tank. Although the OC were often much different from the SC, the EUI
calculated under SC (Operating EUI) differed only by 5.9% compared to those observed
in SC (Standard EUI). The Student's t-test performed on the average of the two EUI does
not show a statistical difference between them (p-value <0.05).
EUI appeared to be heterogeneous among manufacturers and depending on the number
of milkings. The four milkings tanks showed values consistently higher than two
milkings tanks: while two milkings systems showed EUI standard (calculated only on
systems functioning properly) equal to 1.795 kWh/100 l, four milkings tanks stood
2.427 kWh/100 l.As for two milkings tanks, differences can be noticed among the
manufacturers: the Japy showed the highest value (1.986 ± 0.018 kWh/100 l), followed
by Prominox (1.889 ± 0.209 kWh/100 l) and Frigomilk (1.576 ± 0.083 kWh/100 l). The
latter turns out to be the manufacturer with the lowest energy consumption, with an
EUI 16% lower than Prominox tanks and 20% than Japy. EUI Standard is basically
related to the average active power absorbed and the SCT. Within the same cooling time
class, the EUI increases with the decrease of the SCT (Fig. 37).
Standard EUI (kWh/100l)
2,4
R2 =0.9464
R2 = 0.7378
2,2
2,0
1,8
1,6
1,4
1,2
1,0
150
160
170
180
190
Standard Cooling Time SCT (min)
200
0,10
0,15
0,20
0,25
0,30
0,35
0,40
0,45
ActivePower/Volumeratio (kW/100l)
Fig. 37. Correlations between Standard EUI, SCT and active power/volume ratio. Data are
reported for well functioning 2 milkings systems only.
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5.1
EUI calculation for performance classes
Since the lack of I and III class tanks for measuring their performance and power
consumption, the evaluation of EUI of the these time cooling classes has been estimated,
so that the power consumption for milk cooling can be calculated for any time cooling
class on the market.
The calculation procedure used the power/volume data in table 6 and 7, multiplied by
the average II class SCT, reduced or increased by 30 minutes (i.e. the difference among
SCT of the cooling time classes), for estimating the average SCT respectively for I and III
class cooling tanks (Table 16). The two milkings tanks in I class are characterized by the
lowest EUI with 1.521 kWh/100 l, 15% lower than tanks in II class and 21% lower than
those belonging to III class.
Table 16. Standard EUI in kWh/100 l for the manufacturers of the sample. Underlined data is
experimental, while the other data is calculated. Tank N°11 was not used for calculating EUI
because of the high rated volume compared to the sample mean. Missing EUI was not estimated
for lack of experimental or bibliographical data. In parenthesis CO2 emissions expressed in
g/kW·100 l, where 1 kWh corresponds to 443 gCO2/kWh (ENEL, 2009).
Cooling
time class
Average
SCT
(min)
Two milkings
Average
SCT
(min)
Four milkings
I
II
III
142.4
172.4
202.4
1.521 (674)
1.795 (795)
1.938 (859)
176.1
206.1
2.427 (1075)
2.480 (1099)
I
II
III
I
II
III
I
II
III
148.1
178.1
208.1
168.5
198.5
163.1
-
1.356 (601)
1.576 (698)
1.652 (732)
1.889 (837)
1.903 (843)
1.986 (880)
-
-
-
SAMPLE
Milk Cooling Systems
MANUFACTURERS
Frigomilk
Prominox
Japy
Marco Cossu – Energy consumption analysis of sheep milk cooling systems
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80
When plotting data in table 16, it can be observed that the SCT increases through the
time cooling classes (Fig. 38).
Standard EUI (kWh/100 l)
2,4
Class 0
Class I
Class II
Class III
2,2
2,0
1,8
1,6
1,4
1,2
1,0
100
110
120
130
140
150
160
170
180
Standard cooling time SCT (min)
190
200
210
220
Fig. 38. Approximate EUI trend through time cooling classes.
5.2
Error check for monitoring and calculation
In order to evaluate the error of the experimental procedures and calculation, 4 tanks
with identical specifications were included in the sample (tank N° 1-2-10-22). These
systems provided similar performance and power input. Specifications are:
 brand: Frigomilk
 model: G4
 rated volume: 800 l
 maximum power: 2.9 kW
 number milkings: 2
 performance class: B II
 age: 15-16 years
 milk/water rate: from 0.350 to 0.510
 average ambient temperature: from 20.1 to 30.3 °C
 initial milk/water temperature: from 25.7 to 35.0 °C
Performance data and EUI are shown in Table 17.
Marco Cossu – Energy consumption analysis of sheep milk cooling systems
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81
Table 17. Main results of the four identical milk cooling systems.
Tank N°
Average active
power (kW)
TCT (min)
SCT (min)
Operating EUI
(kWh/100l)
Standard EUI
(kWh/100l)
1
2
10
22
2.16
1.97
2.18
2.30
156.5
173.0
126.5
120.0
173.6
183.6
180.3
171.1
1.347
1.443
1.417
1.650
1.494
1.531
1.511
1.647
Mean
σ2
2.15
0.14
144.0
25.0
177.2
5.8
1.464
0.130
1.546
0.069
The average Operating EUI was found to be 1.464 ± 0.130 kWh/100 l, which differs by
5.3% from the average Standard EUI, equal to 1.546 ± 0.069 kWh/100 l. As for the
active power absorbed and Operating EUI, the standard deviation was respectively 6.7
and 9.3%. The standard deviation can be considered small, although they derived from
systems monitored under different OC. The SCT and Standard EUI showed smaller
standard deviations, respectively 3.3 and 4.5%. These deviations can be considered
acceptable for the accuracy expected from the performed tests.
Marco Cossu – Energy consumption analysis of sheep milk cooling systems
Tesi di Dottorato in Scienze e Biotecnologie dei Sistemi Agrari e Forestali e delle produzioni alimentari
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6.
ELECTRICITY CONSUMPTION FOR COOLED MILK STORAGE
Two cooling systems with a high rated volume (N°3 and N°11, respectively 1055 and
2500 l) were monitored using the milk, during normal daily operation, respectively for
about 24 and 72 h, in order to estimate the energy consumption for the conservation of
the cooled milk at a temperature of 4-5 °C, between cooling sessions and during the
night (Fig. 39). The energy consumed for keeping the temperature of the cooled milk
around 4°C is basically due to the agitator, the temporary ignitions of the compressor
and electronic thermostats. Electronic thermostats, constantly turned on in the
monitored systems, continuously check the milk temperature and automatically switch
on or off the compressor and the agitator. Therefore the thermostat results in a steady
power consumption baseline, visible in both tanks monitored. The short peaks in the
graph are due to temporary ignitions of the compressor to bring the cooled milk
temperature back to the value set in the thermostat. In the graph A there are regular
curves next to the compressor ignitions, corresponding to the activity of the agitator,
which seemed to be always contemporary to compressor’s ignition for the second
system (graph B).
In cooling system A, the energy consumption between two milkings during the night
was 0.85 kWh, with two ignitions of the refrigerating unit, for periods shorter than 5
min, corresponding to about 0.21 kWh/100 l (93 gCO2/100 l). Between the second
milking and the end of the monitoring, the energy consumption amounted to 0.39 kWh,
corresponding to 0.06 kWh/100 l (26 gCO2/100 l).
In the cooling system B, the energy consumption in the interval between two milkings
was 1.60 kWh during the first night, 1.19 kWh during the day after and 1.85 kWh during
the second night, corresponding respectively to about 0.80, 0.26 and 0.18 kWh/100 l
(354, 115 and 80 gCO2/100 l), averagely with two compressors ignitions for each
interval. The most reliable consumption is the one referring to the conditions in which
the cooling tank has a milk rate around 50%, i.e. 0.06 and 0.18 kWh respectively for
cooling system A and B. Accordingly the power consumption for storing the cooled milk
can be estimated as an average between the two values, thus approximately 0.12
kWh/100 l (53 gCO2/100 l).
Marco Cossu – Energy consumption analysis of sheep milk cooling systems
Tesi di Dottorato in Scienze e Biotecnologie dei Sistemi Agrari e Forestali e delle produzioni alimentari
Indirizzo di Scienze e Tecnologie Zootecniche – XXV Ciclo - Università degli Studi di Sassari
83
Fig. 39. Power consumption monitoring of two milk cooling systems. A: tank Prominox 1030 l, 2 milkings, monitored for 24 h over 2 cooling sessions.
B: tank Frigomilk 2500 l, 2 milkings, monitored for 72 h over 4 cooling sessions.
Marco Cossu – Energy consumption analysis of sheep milk cooling systems
Tesi di Dottorato in Scienze e Biotecnologie dei Sistemi Agrari e Forestali e delle produzioni alimentari
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84
7.
MALFUNCTIONING SYSTEMS
Some milk cooling systems of the sample (N° 8-9-12-14-21) after the performance test
showed SCT beyond the tolerance limit of III class (above 230 minutes). Therefore they
have been classified as III class cooling systems with performance problems. The
diagnosis of the causes of such malfunctions requires, as suggested to the farmer, the
intervention of a technician. However, the high SCT is often due to refrigerant leakage.
The refrigerant should be periodically refilled, since the small losses from the
refrigerating circuit are physiological and more frequent with the advancing age of the
cooling system. The wear and the environmental conditions can slowly damage the aircooled condenser or the compressor, affecting the efficiency of the refrigerating system
(Fig. 40).
Fig. 40. Example of an air-cooled condenser in a bad shape in its lower part. At the bottom the coil
of the refrigerant is completely frozen, indicating inefficiency of the refrigerating circuit.
To estimate the incidence of performance problems on power consumption, the SCT of
malfunctioning systems was compared to common SCT of III class tanks, thus around
205 min (Table 18).
Marco Cossu – Energy consumption analysis of sheep milk cooling systems
Tesi di Dottorato in Scienze e Biotecnologie dei Sistemi Agrari e Forestali e delle produzioni alimentari
Indirizzo di Scienze e Tecnologie Zootecniche – XXV Ciclo - Università degli Studi di Sassari
85
Table 18. EUI (kWh/100 l) for the malfunctioning cooling tanks of the sample. Standard EUI has
been corrected through the spreadsheet, assuming a SCT within the III class of 205 min (within
210 min).
Active power
Average
Tank
active power
(N°)
(kW)
8
9
12
14
21
1.62
1.22
1.17
1.94
2.19
Measured data
SCT (min)
Standard EUI
(kWh/100 l)
239.2
237.8
276.3
338.4
348.8
1.985
2.202
2.534
3.414
2.425
Corrected data
Average III class
SCT (min)
Standard EUI
(kWh/100 l)
205.0
1.735
205.0
1.930
205.0
1.935
205.0
2.063
205.0
1.432
Average energy saving
%
-13%
-12%
-24%
-40%
-41%
26%
If the systems had worked properly, considering the SCT of III class cooling tanks, their
EUI would decrease averagely by 26%, with peak values up to 40-41%. The energy
saving would stand averagely on 0.693 kWh/100 l, equal to 307 gCO2/kWh·100 l. In fact
the last two tanks showed SCT consistently beyond the expected values (133-143
minutes above the limit of III class tanks).
Marco Cossu – Energy consumption analysis of sheep milk cooling systems
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8.
ANNUAL ENERGY COST FOR SHEEP MILK COOLING
On the basis of the Standard EUI, it is possible to estimate the annual cost for the
operation of milk cooling on the dairy farm and the impact on the average price of
sheep milk in Sardinia, currently around 0.70 €/l (ISMEA, 2012). Sheep milk production
of the sample farms was estimated, based on the number of lactating sheep and the
lactation curve of the “Sarda” sheep, considering a standard lactating curve of 220 days
(Fig. 41).
Sarda ewe lactation curve
y (t )  at b e ct
Milk production (g/day)
1200
where:
t= time (weeks)
a= 934
b= 0,181
c= 0,041
1000
800
600
400
200
0
1
3
5
7
9
11 13 15 17 19 21 23 25 27 29 31 33 35 37 39
Week
Fig. 41. Lactation curve of “Sarda” sheep (Pulina and Nudda, 2001).
The EUI was added with the energy averagely required for maintaining the cooled milk
(0.12 kWh/100 l) and multiplied by the milk produced on annual basis, in order to
estimate the annual energy consumption for milk cooling. The potential energy saving
achievable was calculated only on malfunctioning tanks, considering the Standard EUI
indicated in Table 19. The electricity price (fixed costs items in the bill not included) can
also be more expensive than what indicated, depending on the power company
providing the energy and the tariff chosen by the farmer.
The average electricity consumption is 1244 kWh/year per farm, equal to a CO2
emission of 550 kgCO2/year, with an average annual cost of € 274 (Table 19). The ratio
between the electricity cost and the milk produced gives the annual electricity cost for
milk cooling, estimated about 0.44 €cent/l, equal to 0.63% of the average current price
of sheep milk (0,70 €/l). The malfunctioning tanks would have a lower energy
Marco Cossu – Energy consumption analysis of sheep milk cooling systems
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consumption if a maintenance check is performed, able to save averagely 26% energy,
up to 46%, compared to average consumption of the sample.
Table 19. Average annual milk production of monitored farms, with annual power consumption
for milk cooling. Indicative price charged for electricity: 0.22 €/kWh (ENEL, 2012). The energy
saving is reported only for malfunctioning cooling systems (in bold), by using EUI corrected
through the spreadsheet for a SCT=205 min.
Tank
Manufacturer
N°
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
Frigomilk
Frigomilk
Prominox
Frigomilk
Frigomilk
Prominox
Japy
Frigomilk
Prominox
Frigomilk
Frigomilk
Frigomilk
Japy
Prominox
Prominox
Prominox
Frigomilk
Prominox
Japy
Frigomilk
Prominox
Frigomilk
Lactating Estimated annual
Annual electricity Annual energy
Standard EUI
sheep
milk production
consumption
cost for milk
(kWh/100l)
(N°)
(l)
(kWh)
cooling(€)
400
250
380
150
150
300
50
500
400
550
250
80
210
310
115
500
200
320
150
230
400
550
MEAN
83,141
52,908
83,230
32,854
36,871
52,054
8,676
85,031
85,310
106,728
72,009
14,696
38,094
79,919
18,858
128,902
33,914
48,058
26,249
47,579
82,747
88,112
59,361
1,563
1,510
2,154
1,451
2,338
1,702
2,752
1,989
2,191
1,634
0,898
2,506
2,034
3,365
1,651
1,977
1,660
1,961
1,939
2,191
2,773
1,640
1399
862
1892
516
906
948
249
1793
1971
1873
733
386
820
2785
334
2704
604
1000
540
1100
2394
1551
1244
308
190
416
114
199
209
55
395
434
412
161
85
180
613
73
595
133
220
119
242
527
341
274
Energy
saving
(%)
12%
11%
22%
37%
46%
26%
If the whole sample had been characterised by I class tanks, the energy consumption
would have decreased by 16%, with an average energy saving of 207 kWh per farm
(about 92 kgCO2/year), up to 28%. On the other hand it would rise by 11% for a
hypothetic sample of III class cooling systems (Table 20). Some values in the III class
column of table 20 indicate a small energy saving because the calculated EUI index is
estimated, so all evaluations should be considered as approximate.
Marco Cossu – Energy consumption analysis of sheep milk cooling systems
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Table 20. Estimation of annual energy consumption and potential energy saving for a hypothetic
sample belonging to the other performance classes. Calculations only on two milkings wellfunctioning systems. Experimental EUI derive from table 15 and added with the energy
consumption for cooled milk storage (0.12 kWh/100 l). Standard EUI of the other performance
classes derive from table 16: the specific EUI of the manufacturer has been used when available,
otherwise the general EUI of the sample. Annual energy consumption is calculated by multiplying
the EUI and the annual milk production of table 19.
Experimental conditions
I class sample
III class sample
Tank
N°.
Experimental
Standard EUI +
Storage
(kWh/100 l)
Annual Energy
consumption
(kWh)
1
1.563+0.12
1399
1.356+0.12
1227
12%
1.652+0.12
1711
-22%
2
1.510+0.12
862
1.356+0.12
781
9%
1.652+0.12
1089
-26%
3
2.154+0.12
1892
1.521+0.12
1366
28%
1.903+0.12
1713
9%
4
1.451+0.12
516
1.356+0.12
485
6%
1.652+0.12
676
-31%
6
1.702+0.12
948
1.521+0.12
854
10%
1.903+0.12
1071
-13%
10
1.634+0.12
1873
1.356+0.12
1575
16%
1.652+0.12
2196
-17%
13
2.034+0.12
821
1.521+0.12
625
24%
1.938+0.12
784
4%
15
1.651+0.12
334
1.521+0.12
309
7%
1.903+0.12
388
-16%
16
1.977+0.12
2704
1.521+0.12
2115
22%
1.903+0.12
2653
2%
17
1.660+0.12
604
1.356+0.12
501
17%
1.652+0.12
698
-16%
18
1.961+0.12
1000
1.521+0.12
789
21%
1.903+0.12
989
1%
19
1.939+0.12
540
1.521+0.12
431
20%
1.938+0.12
540
0%
22
1.640+0.12
1551
1.356+0.12
1301
16%
1.652+0.12
1813
-17%
Mean
1,880
1157
1,565
951
16%
1,913
1256
-11%
Standard EUI
Annual
Standard EUI
Annual
I class +
energy
III class +
energy
Energy
Energy
storage
consumption saving (%)
storage
consumption saving (%)
(kWh/100 l)
(kWh)
(kWh/100 l)
(kWh)
Marco Cossu – Energy consumption analysis of sheep milk cooling systems
Tesi di Dottorato in Scienze e Biotecnologie dei Sistemi Agrari e Forestali e delle produzioni alimentari
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89
V.
DISCUSSION
The milk cooling system sample reflects the actual situation of the ewes sector in
Sardinia: the predominance of the performance class BII is actually the best choice in
the mediterranean area, where the average temperature during the milking season
rarely exceed 32 °C. The preponderance of the II class cooling time is probably a
compromise between the need for a good cooling speed and the purchase price of the
cooling system. The lack of maintenance increases the probability of failure or
malfunctions: 77% of the farms in the sample never checked the condition of the milk
cooling system, mainly due to economic reasons, even if the lack of maintenance often
leads to more expensive interventions when a serious failure occurs. The regular
maintenance allows also the prevention of performance problems hard to be diagnosed
by the farmer, who does often not suspect the delay in the cooling time due to
malfunctioning. Only the performance test allowed the identification of the problem. In
addition, these performance problems, which seem quite common in Sardinian sheep
farms, like diagnosed in 22% of the sample, do not seem correlated with the age of the
tanks but with maintenance (even if years cause a gradual wearing of the refrigerating
circuit), showing that even old cooling systems can work properly. The importance of
maintenance is clearly visible in the few systems that received at least one refill of
refrigerant. The refilling of the refrigerant after the maintenance check brings the
refrigerating circuit back to a good efficiency level; in some cases (tank N°4-7-13-1618) milk cooling systems (all belonging to II class) got close to the performance of the I
class (Table 10). Energy consumption observed during performance can be described
like a curve, which is anyway very close to a straight line, so that consumption can be
considered constant over time, despite the slight power fluctuations of the compressor
(Fig. 35). This is important in energy auditing studies, since a linear absorption allows
considering the EUI linear over time, in order to make easier calculations for estimation
of annual consumption.
The monitoring of electricity consumption shows an average difference of 75%
between the compressor power actually absorbed during the cooling session and the
power reported on the identification plate (Table 14). This difference is due to the
Marco Cossu – Energy consumption analysis of sheep milk cooling systems
Tesi di Dottorato in Scienze e Biotecnologie dei Sistemi Agrari e Forestali e delle produzioni alimentari
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power value indicated on the plate, which is related to the maximum absorption the
compressor may develop typically during ignition. However during operation the
power absorption is always lower. Consequently the power indicated on the
identification plate overestimates the power required for operation by averagely 25%.
The difference between the active power absorbed and the plate power slightly changes
according to the manufacturer (75% for the Frigomilk, 78% and 70% for Prominox and
Japy) but is not influenced by the rated volume of the cooling tank. This is due to the
electrical behavior of asynchronous engines used in compressors, which are
technologically identical, regardless of the manufacturer. The average ratio between the
active power and plate power of 25% may be considered as a correction factor to be
applied for estimating the electrical power absorption of refrigerating systems. In
addition, the inverse relationship between the power/volume ratio and SCT suggests
that the cooling time decreases with a higher compressor power considering the rated
volume of the cooling tank constant (Fig. 36).
The screening of the sample farms shows that the energy consumption for milk cooling
is one of the main elements influencing the annual energy consumption of dairy farming
(Institut de l'Elevage, 2009). The energy consumption for sheep milk cooling is higher
than the cow milk: considering an average energy consumption of 1.795 kWh/100 l for
II class tanks (Table 16), the cow milk cooling (on two milkings systems) ranges from
0.9 to 1.1 kWh/100 l (Leggett et al. 1997, Murgia et. al., 2008), approximately 54% less
than sheep milk. This is due to the high rated volumes of milk cooling systems in cattle
farms: the larger the volume, the lower the power/volume ratio (Fig. 32), requiring a
lower compressor power (considering constant rated volume and performance class),
decreasing the power consumption. In fact tank N°11, which was the largest system of
the sample (2,500 l), showed a EUI of 0.898 kWh/100 l, which corresponds to the
average energy consumption for cow milk cooling, and much lower than the overall EUI
of the other cooling tanks in the sample, mainly because of its high rated volume over
which energy is consumed. This is confirmed by EUI for four milkings systems, which
are significantly higher than those for two milkings: energy consumption for two
milkings tanks is distributed on a milk bulk equal to 50% of the rated volume, while in
four milkings is distributed only on 25% of volume, resulting in a higher EUI.
Marco Cossu – Energy consumption analysis of sheep milk cooling systems
Tesi di Dottorato in Scienze e Biotecnologie dei Sistemi Agrari e Forestali e delle produzioni alimentari
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91
The EUI regarding the tanks around 1000 l (tank N° 3 and N° 16) were respectively
2.154 and 1.977 kWh/100 l (Table 15), higher than what measured in experimental
tests carried out on a 1000 l milk cooling system, whose index ranged between 1.55 and
1.66 kWh/100 l (Jovanovic, 1980). However both tanks belong to Prominox, which
showed to have the highest power/volume ratio among the manufacturers of the
sample.
The milk cooling system N°19, with the lowest rated volume in the sample (320 l)
showed a EUI 1.932 kWh/100 l, similar to results of an other test on a 150 l cooling
tank, with a power consumption ranging from 1.8 to 1.9 kWh/100 l (Vogelauer, 1980).
However the few data reported in literature allow to state that the energy consumption
of milk cooling equipments remained basically unchanged, since the construction and
electrical technologies used to build the tanks did not benefited from substantial
evolutions in the last 30 years. Thus the obsolescence time of milk cooling systems can
be estimated around 15-20 years, compared to 10 commonly reported as technical life
(Pazzona, 1999). However the electrical behavior of the refrigerating system
deteriorates over time with a decrease of the power factor (cos φ) and a consequent
excessive consumption of reactive energy. When the power supplied to the power
meter exceeds 16.5 kW, the local electric service provider monitors the power factor
and reactive energy consumed by the farm. The service tolerates reactive energy
consumption up to 50% of the total active energy consumed. If the consumption of
reactive energy ranges from 50 to 75% of active energy, the provider delivers a penalty
in the electricity bill equal to 0.0323 €/kVARh. This penalty increases if the reactive
energy is more than 75% of active energy, with 0.0421 €/kVARh (AEEG, 2004).
Excessive consumption of reactive power by the milk cooling system does not
automatically imply a penalty in the electricity bill for the farmer, because reactive
energy data is read in the power meter of the farm. Therefore, excessive reactive energy
consumption occurs only when several electric users and wiring in the dairy farm show
abnormal reactive power consumption. The reduction of reactive energy consumption
can be reached following some suggestions such as:
 using properly sized engines and electrical transformers;
 avoiding the use of engines with low loads, or powered by voltages higher than
nominal values;
Marco Cossu – Energy consumption analysis of sheep milk cooling systems
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92
 avoiding the use of malfunctioning engines.
When these suggestions are not enough, it is necessary to call a specialized technician,
who can measure the loads of the users and fix the electrical circuit to balance the
energy withdrawal; power factor correctors (capacitive elements or capacitors) are
often used for the purpose. Their positioning and sizing is crucial for adjusting the
reactive power consumed by the user. In most cases the cost for rebalancing the system
is characterized by a quick payback time by saving penalties; moreover with the proper
set-up a better operation of the engine is guaranteed.
The difference of 5.9% between the Standard EUI and Operating EUI (Table 15) is not
statistically significant. In fact the initial milk temperature, which is the largest
variability factor, does not influence the Operating EUI, since both Standard and
Operating EUI are calculated considering a initial milk temperature of 35 °C. However
the variability of the ambient temperature still remains, but has a small impact on the
SCT (Fig. 15). This allows considering Standard EUI as representative of the operating
electricity consumption for milk cooling in dairy farms. When the identification plate is
available on a milk cooling system, a precise information about electricity consumption
can be obtained by multiplying the plate maximum power output for 0.75 (Table 14)
and the number of operating hours, or the SCT derived from the performance test.
The EUI of milk cooling tanks, considering the rated volumes constant, is characterized
by a variability correlated with factors such as the number of milkings, the performance
class and the manufacturer, which determines the structural characteristics and size of
the milk cooling system (Table 16). The manufacturers change the designing of tanks in
terms of compressors, power/volume ratio, electronic devices, type and thickness of
the insulating materials of the tank wall. Consequently, manufacturers turn out to be
real statistical clusters so that many results developed were processed separately
depending on the manufacturer itself.
Even the refrigerant is a variability factor influencing the performance and the power
consumption of the system, in particular the coefficient of performance (COP), i.e. the
heat (expressed in thermal kWh) removed by every electrical kWh consumed.
Nowadays new refrigerants are available, beyond R22 and R404a, such as propane
(R290) and ethane (R170) which, without damaging ozone content, increase the COP up
Marco Cossu – Energy consumption analysis of sheep milk cooling systems
Tesi di Dottorato in Scienze e Biotecnologie dei Sistemi Agrari e Forestali e delle produzioni alimentari
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93
to 9%, with an annual average energy saving of 350 kWh for an average size cattle farm
(Cleland et al., 2009).
Within the specific manufacturer, the EUI showed a strong correlation with the active
power and SCT (Fig. 36). The more powerful compressor, the higher the active energy
absorbed, so that SCT decreases, but causing a higher energy consumption, thus a
higher EUI. This aspect is responsible for the EUI differences among the manufacturers
(Table 16). The EUI is inversely proportional to SCT, so that Prominox and Japy have
EUI higher than Frigomilk. This also happens among the performance classes: moving
from I class to III class (i.e. for increasing SCT), EUI increases (Fig. 38), although the
power/volume ratio of the performance class decreases. In fact among the performance
classes, SCT change with a rate higher than the power/volume ratio: while the
power/volume changes by 11.5% from I class to III class (Table 6), SCT changes by 40%
(210/150 min).
As for the storage of the cooled milk, the energy consumption was low and generally
decreased with the increasing of the cooled milk, because such consumption in kWh is
distributed over a higher amount of milk. The energy required for cooled milk storage
should be estimated when the milk rate is close to values commonly used during a
performance test. Energy consumption for cooled milk storage may be considered
constant but not negligible and must be added to the energy consumption for milk
cooling, increasing it averagely by 6.6%.
The malfunctioning tanks represent a large percentage (22%) of the sample. The fact
that these cooling tanks have been found randomly suggests that failures and damages
are a common problem in Sardinian milk cooling systems. The economic crisis hitting
the sector, the decrease of milk demand (even internal) and exports of dairy products,
coupled to a milk price inadequate to cover production costs, lead the farms to a
situation in which investing in the short and medium term for the modernisation and
competitiveness is difficult. The cooling systems of the sample had performance
problems showing standard EUI averagely 26% higher than expected values from II
class cooling tanks, with peaks of 41% (Table 18). These high EUI values caused an
electricity consumption averagely 26% higher than well-functioning systems, with
peaks of 46% (tank N°21) more than a typical III class tank (Table 19), comparable to
the average annual energy demand of an italian family of 4 people, which is about 2,700
Marco Cossu – Energy consumption analysis of sheep milk cooling systems
Tesi di Dottorato in Scienze e Biotecnologie dei Sistemi Agrari e Forestali e delle produzioni alimentari
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94
kWh/year (AEEG, 2008). This suggests that strong deviations of EUI observed on a
cooling tank, compared to those indicated in table 16, may be an indicator of an
abnormal operation of the system and thus a potential malfunction.
The I class tanks are characterised by smaller EUI, which allow saving about 207 kWh
during the milking season, and decreasing up to 28% the costs compared to II class
cooling tanks, more diffused in the farms and on the market (Table 20). Besides the
energy saving there are lower emissions of CO2 into the atmosphere, estimated about
92 kg CO2/year for each average size sheep farm, provided with milk cooling systems
up to 1000 litres. The performance problems are not always easy to diagnose, because
the farmer ususally does not notice an increase in SCT. This leads to persistence of the
problem over time, which increases the electricity consumption and cause potential
progressive degradation of the refrigerating system performance.
The performance test and maintenance are expensive and the sheep farmer is reluctant
to request them on a regular basis, unless serious problems affect the cooling system.
Hence the need for establishing a free public customer service by istitutions providing
technical assistance in agriculture. This service could check on annual basis the
performance of both sheep, cow and goat milk cooling systems, as already performed
for milking systems. The current lack of this service cause an economic damage to the
farms, since it leads to an increase in electricity consumption and emissions of CO2.
Furthermore the microbiological quality of the milk worsens, since a high cooling time
further results in an increase of the microbial charge (Pazzona and Murgia, 1992). In
fact the milk temperature and the storage time spent inside the tank influences the
microbial charge of the milk. Even the slow or incomplete milk cooling and prolonged
storage times cause an increase of the microbial charge (Jayarao and Wolfgang, 2003).
The cooling time is critical during the midsummer, where the coliform bacteria and
somatic cells are higher (Elmoslemany et al., 2009, Gillespie et al., 2012). The somatic
cells are an indicator of discomfort for the animal, which is negatively correlated to the
milk production (Pulina et al., 2005). Other factors contributing to an increase in
bacterial contamination in raw milk are the health status of the animal, the staff training
and the protocols followed for cleaning the cooling system and the milking plant
(Perkins et al., 2009).
Marco Cossu – Energy consumption analysis of sheep milk cooling systems
Tesi di Dottorato in Scienze e Biotecnologie dei Sistemi Agrari e Forestali e delle produzioni alimentari
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95
The number of milkings of the cooling system contributes to a growth of the microbial
charge, because it increases the storage time before the collection and processing.
Significant differences in microbial charge between milk delivered daily and every two
or three days were observed, suggesting a difficulty by the cooling tank to contain the
microbial charge when the cooling time and storage time are high, suggesting that two
milkings systems provide a better containment of bacterial growth, compared to those
with 4 or 6 milkings (Tirard-Collet et al., 1991 and Zweifel et al., 2005).
The limit for microbial content in sheep milk in Italy is established with the Presidential
Decree N° 54 in January 14th 1997, which transposed the European Union Directive CEE
92/46, establishing a maximum microbial charge for sheep milk of 1.5·106 cells/ml and
5·105 cells/ml for the raw milk used for cheese production.
The microbial charge of sheep milk is the only quality parameter that must be
controlled by law in Italy. It is an important indicator of the milk quality, from which its
shelf life and price depends on. A mechanism for linking the price of the milk to its
quality, still not widely applied in Sardinia, is considered the only way to give the
farmer the possibility to invest for a better milk quality and the achievement of
excellent nutritional and rheological parameters. The scheme, still under discussion, is
expected to provide a constant monitoring of the individual farm, both on hygienic and
physical-chemical parameters of sheep milk (Table 21). These parameters will be
divided into classes, contributing to an higher or lower final milk price, as already
established for the cow milk. The goal can be reached through good farming practices
and sanitation, both with the control and maintenance of the systems connected to
milking and milk cooling. The official institution of the system of quality payment for
milk allowed the cattle farms in the Piedmont region (Italy) to reach excellent quality of
cow milk, regarding fat and protein content. At the same time excellent results were
obtained regarding the hygienic and sanitary parameters, which in recent years are no
longer additional or facultative requirements, but represent a basic threshold for
market access, establishing limits awarded with a higher price like in the case of the
“high quality milk” (Quaderni della Regione Piemonte, 2007).
Marco Cossu – Energy consumption analysis of sheep milk cooling systems
Tesi di Dottorato in Scienze e Biotecnologie dei Sistemi Agrari e Forestali e delle produzioni alimentari
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96
Table 21. Provisional parameters under discussion for quality payment of sheep milk in Sardinia
(LAORE Sardegna, 2012).
Hygienic parameters
Composition parameters
Total microbial charge
0.5-1.5·106 cells/ml
Proteins
min 5.7%
Somatic Cells Count (CSS)
1.0-1.5·106 cells/ml
Fat
min 6.4%
Even small sheep farms without a high technology level can achieve a good milk quality,
because good hygienic practices are important in any technique used for breeding and
production (Zweifel et al., 2005). In fact the farms increasing their sanitation level
obtain a significant decrease in microbial charge and somatic cells count (DelgadoPertinez et al., 2003).
The strong correlation between price and quality of the milk would benefit both the
processing industry and the producer. The farmer would have a higher profit to invest
in modernisating the farm, including the income diversification through energy
production (Fig. 41) and energy saving technologies. At the same time it would be
possible to replace old plants for both milking and milk cooling over the Sardinian
territory.
Fig. 41. Brand new photovoltaic system installed on the roof of a
milking parlor in a farm of the sample.
For example with a higher profit the farms could replace the old cooling system with a I
class tank, characterised by lower energy consumption and cooling time. However, only
Marco Cossu – Energy consumption analysis of sheep milk cooling systems
Tesi di Dottorato in Scienze e Biotecnologie dei Sistemi Agrari e Forestali e delle produzioni alimentari
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97
the quality payment allows to purchase new cooling systems, since the energy saving
achievable by operating with a I class cooling tank instead of a II class system (Table
20) is not enough to justify neither the high initial cost, nor the adoption of technologies
to save energy, usually convenient only in cow farms provided with high rated volume
tanks. In fact some economic feasibility calculations on small and medium sized
Sardinian sheep farms, estimated 1.5-1,6% the weight of the annual electricity bill on
the overral budget of the farm (ARA, 2008).
The use of I class cooling systems contributes to a modest energy saving, but also a
reduction of CO2 emissions, estimated in 91 kgCO2/year per farm, while the CO2
emission for sheep milk cooling was estimated 550 kgCO2/year for the whole sample.
The greenhouse gas emissions assumed an international interest because of their
impact on the environment. Dairy farming is considered, inside the agricultural sector, a
huge source of emissions, but few data can be found in literature about it. FAO (2006)
reported that the sector contributing to the highest global greenhouse gas emissions is
agriculture. With the growing interest in greenhouse gas emissions, there is the need
for expressing the greenhouse gas emissions associated with any product or service.
The term that expresses these quantities is the "Carbon Footprint". This term is
originated from a methodology known as "Ecological Footprint" (Kitzes et al., 2008).
The “Footprint” was defined as the area of biologically productive land required to
produce the resources and absorb the waste generated using the most diffused
technologies. The term Carbon Footprint thus refers to the productive land required to
sequester enough carbon to avoid an increase of CO2 in the atmosphere. Today, the
term given to the Carbon footprint is the net exchange of greenhouse gases per unit of
product or service. This net emission is well determined through an assessment of the
life cycle of the product or service, which includes all sources of emission and
absorption of greenhouse gases during the production process, as well as those
associated with the production of the resources used in the system (Rotz et al., 2009).
Even the electricity consumed for various tasks related to production and delivery of
the milk contributes to a CO2 emission factor into the atmosphere. The milk cooling is a
task representing part of this emission, so that the energy consumption should be
evaluated both for its impact on the budget of the farm (electricity bill) and on the
environment.
Marco Cossu – Energy consumption analysis of sheep milk cooling systems
Tesi di Dottorato in Scienze e Biotecnologie dei Sistemi Agrari e Forestali e delle produzioni alimentari
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98
The study accurately determined the incidence of one milk production factor, thus the
milk cooling, on the sheep milk price, even considering a necessary modenisation of the
sheep farms, including the replacement of the existing cooling systems, due to the high
average age of the tanks, now close to 20 years. This information could be useful for the
imminent definition of the milk price in relation to quality, which must take into
account the additional costs related to a high quality level of the finished products.
Marco Cossu – Energy consumption analysis of sheep milk cooling systems
Tesi di Dottorato in Scienze e Biotecnologie dei Sistemi Agrari e Forestali e delle produzioni alimentari
Indirizzo di Scienze e Tecnologie Zootecniche – XXV Ciclo - Università degli Studi di Sassari
99
VI.
CONCLUSIONS
Energy is a production factor which is assuming a growing interest in agricultural
sector for both economic and environment issues. Agriculture offers a huge potential,
because of the multifunctional activities taking place in the farms and the objectives of
the National Climate 20-20-20 package. The farm can contribute to the achievement of
these objectives in terms of renewable energy production, energy efficiency of the agrifood chain and reduction of CO2 emissions.
The livestock sector can give a valuable contribution, taking into account the connection
between the energy consumption of the equipments and the quality of animal products.
Studies of energy auditing in farms identified in the mechanical milking and milk
cooling the main factors determining power consumption. Since bibliographical data
regarding electricity consumption for sheep milk cooling is scarce, but important for the
Sardinian economy, a study was performed to quantify energy consumption. The EUI
(Energy Utilization Index) has been indicated as the most suitable indicator to relate
energy consumption and milk production. It expresses the amount of energy needed to
complete a given operation, compared to an amount of unit product. In the specific case,
it is expressed in kWh/100 l of cooled sheep milk.
Performance tests and power consumption monitorings were performed on 22 milk
cooling systems used to cool down sheep milk in Sardinia. A portable electrical panel,
assembled specifically to measure the consumption of three-phase current loads, was
used for the power monitoring. At the same time the data from a survey conducted in
2010 by the Agency LAORE were elaborated in order to describe the actual
characteristics of the milk cooling systems operating in the sheep farms in Sardinia, the
italian region with the highest sheep population. All the systems of the sample were
open-type tanks with direct refrigeration system. The results highlighted the major
manufacturers, identified two milkings class BII systems as the most diffused over the
territory and underlined how they are tendentially old and obsolete, neglected in terms
of maintenance.
The milk cooling systems of the sample (95% belonging to BII class) showed an average
EUI of 1.795 kWh/100 l for two milkings systems and 2.427 kWh/100 l for four
Marco Cossu – Energy consumption analysis of sheep milk cooling systems
Tesi di Dottorato in Scienze e Biotecnologie dei Sistemi Agrari e Forestali e delle produzioni alimentari
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100
milkings tanks. Electricity consumption for storing the cooled milk was estimated 0.12
kWh/100 l. EUI is strictly correlated with the SCT and the active power absorbed during
the cooling session. The active power is averagely 75% of the power indicated on the
identification plate. Consequently the use of the rated power deducted from the
identification plate causes a significant error of overestimation while calculating energy
consumption. The application of a correction factor of 0.75 allows to properly calculate
the electricity consumption of the cooling system simply knowing the power written in
the identification plate and the estimated cooling time of the monitored system.
The four milkings cooling tanks always have EUI consistently higher than two milkings
systems. The I class tanks, which guarantee the lowest SCT, are also characterised by
the lowest EUI, with average values of 1.521 kWh/100 l, despite of the highest
power/volume ratio.
Electricity consumption for milk cooling is a marginal cost within the budget of a sheep
dairy farm. However its assessment is important as it provides a variety of technical
data, now scarce in the international bibliography, useful for future studies about
energy auditing in the livestock sector. EUI can also be considered like a preliminary
indicator of the proper operation of the milk cooling system and as parameters to
assess the concordance of its power consumption, compared to the average values
calculated in the study.
The establishment of a assistance service for milk cooling systems assumes a strategic
importance importance; a regular check and maintenance ensure the proper operation
in the long period of the systems, from which the milk quality is strongly related. The
lack of maintenance of which induces failures or malfunctions which may increase the
power consumption up to 41%, increasing the electricity bill.
The survey on Sardinian sheep milk cooling systems highlighted the high age of
operating systems, resulting in the need for a renewal of the equipments used for milk
cooling. However the purchase of new cooling systems can be repayed over the middle
term only establishing a milk quality payment policy, which justifies the adoption of
new and modern refrigeration systems.
Marco Cossu – Energy consumption analysis of sheep milk cooling systems
Tesi di Dottorato in Scienze e Biotecnologie dei Sistemi Agrari e Forestali e delle produzioni alimentari
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101
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Indirizzo di Scienze e Tecnologie Zootecniche – XXV Ciclo - Università degli Studi di Sassari
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Tesi di Dottorato in Scienze e Biotecnologie dei Sistemi Agrari e Forestali e delle produzioni alimentari
Indirizzo di Scienze e Tecnologie Zootecniche – XXV Ciclo - Università degli Studi di Sassari
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VIII. ACKNOWLEDGMENTS
I would like to thank Professor Antonio Pazzona for the professional and moral support
during my stay in the Department, both during the preparation of the thesis and also
the previous projects performed together. Despite its several commitments, he always
found the time for any kind of help and advice. It is an honor to work with a master of
such competence, experience and humaneness. Thank you!
A special thank to the Prof. Lelia Murgia for helping me in setting up the work and her
availability despite the persistent lack of time.
Thanks and greetings to Giannetto Fadda of Laore Agency for supporting me during the
identification and the visit of the farms, and for providing me the statistical data of the
Agency.
A final thank to all my family, my parents and Andrea, who recently left for a welldeserved work at Massachusetts University.
Marco Cossu – Energy consumption analysis of sheep milk cooling systems
Tesi di Dottorato in Scienze e Biotecnologie dei Sistemi Agrari e Forestali e delle produzioni alimentari
Indirizzo di Scienze e Tecnologie Zootecniche – XXV Ciclo - Università degli Studi di Sassari
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