Relation Between Mean Fluid Temperature and Outlet Temperature for Single U-Tube Boreholes

Aminhossein Jahanbin, Claudia Naldi and Enzo Zanchini
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Aminhossein Jahanbin: Department of Industrial Engineering, Alma Mater Studiorum University of Bologna, Viale Risorgimento 2, 40136 Bologna, Italy
Claudia Naldi: Department of Industrial Engineering, Alma Mater Studiorum University of Bologna, Viale Risorgimento 2, 40136 Bologna, Italy
Enzo Zanchini: Department of Industrial Engineering, Alma Mater Studiorum University of Bologna, Viale Risorgimento 2, 40136 Bologna, Italy

Energies, 2020, vol. 13, issue 4, 1-23

Abstract: Ground-coupled heat pump (GCHP) systems usually utilize buried vertical heat exchangers, named borehole heat exchangers (BHEs). The accurate design or simulation of a GCHP system requires the calculation of the time-dependent outlet temperature from the BHEs, T out . However, the most widely employed BHE simulation models yield the time evolution either of the mean temperature of the BHE-ground surface, T sm , or of that of the fluid, T fm . In transient regime, it is not easy to relate T out to either T sm or T fm . In this paper we determine, through 3D finite element simulations, simple expressions of a dimensionless coefficient φ allowing the calculation of T out by means of a simulation model that yields T fm . These expressions hold for single U-tube BHEs, both in quasi-steady and in unsteady working conditions. We validate our 3D simulation code by comparison with an analytical BHE model. Then, we present applications of our expressions of φ to calculate the time-dependent values of T out through a BHE model that yields those of T fm . Finally, we show that the values of φ in quasi-steady working conditions can be used for a simple calculation of the effective borehole thermal resistance.

Keywords: ground-coupled heat pumps; ground heat exchangers; borehole heat exchangers; single U-tube; outlet temperature; effective borehole thermal resistance (search for similar items in EconPapers)
JEL-codes: Q Q0 Q4 Q40 Q41 Q42 Q43 Q47 Q48 Q49 (search for similar items in EconPapers)
Date: 2020
References: View references in EconPapers View complete reference list from CitEc
Citations: View citations in EconPapers (2)

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