Ferrara , 21, 22, 23 Settembre 2010 - Congresso:
Risorse geotermiche di media e bassa
temperatura in Italia.
Potenziale, Prospettive di mercato,
Azioni
Sessione POSTER
Organizzazione
con la collaborazione di
Numerical Evaluation of BHE Thermal
Resistance for Ground coupled Heat
Pump Applications
Università degli Studi di
Genova, DIPTEM
M.Fossa
The aim of the study is to numerically evaluate
the borehole (internal) thermal resistance for
various vertical pipe configurations and to
compare the results with literature models.
Tf, ave
Rbhe
Tb
Rground(t)
Tground
Q’
Ansys Multiphysics is employed as Finite
Element solver for steady state 2D conduction
problem
The thermal resistance is evaluated by averaging
either the BHE temperature Tb or the heat
tranfer rate along the BHE circular contour
Different pipe geometries (single and double U pipes,
variable pipe spacing), boundary conditions (imposed
temperature or convective heat coeff.) and
grout/ground thermal conductivities are analysed for
an overall number of cases close to 100
Bhe Resistance [mK/W]
Numerical results
Zeng model (2004)
(Tb  T f )
Rb 
Q 
Temperature distribution
Grout
(kg is a parameter)
Pipe spacing [mm]
Bhe Resistance [mK/W]
Dpipe =32mm
Dpipe =25mm
Hellstrom
equation (1991)
Grout conductivity [W/mK]
Calculation revealed that pipe spacing and grout conductivity play an important role
in BHE resistance, as well as the laminar or turbulent conditions at the fluid side.
Among the literature models, the Zeng one (2004) proved to be robust and
accurate, while the Hellstrom model (1991) seems not to provide reliable results
Acknowlwdgements Special thanks to Dr. Nicola Meli,
who worked at this problem in his thesis