An experimental study of a direct expansion ground‐coupled heat pump system in heating mode

In this paper, an experimental performance evaluation of a direct expansion ground-coupled heat pump (DX-GCHP) system in heating mode is presented. The DX-GCHP uses R134a as the refrigerant, and consists of three single U-tube copper ground heat exchangers (GHEs) placed in three 30 m vertical boreholes. During the on–off operations from December 25, 2007, to February 6, 2008, the heat pump supplied hot water to fan-coil at around 50.4°C, and its heating capacity was about 6.43 kW. The energy-based heating coefficient of performance (COP) values of the heat pump and the whole system were found to be on average 3.55 and 3.28 at an evaporating temperature of 3.14°C and a condensing temperature of 53.4°C, respectively. The second law efficiency on the DX-GCHP unit basis was around 0.36. The exergetic COP values of the heat pump and the whole system were obtained to be 0.599 and 0.553 (the reference state temperature was set equal to the average outdoor temperature of −1.66°C during the tests), respectively. The authors also discussed some practical points such as the heat extraction rate from the ground, refrigerant charge and two possible new configurations to simultaneously deal with maldistribution and instability of parallel GHE evaporators. This paper may reveal insights that will aid more efficient design and improvement for potential investigators, designers and operators of such DX-GCHP systems. Copyright © 2009 John Wiley & Sons, Ltd.

[1]  Daniel Pahud,et al.  Comparison of the thermal performance of double U-pipe borehole heat exchangers measured in situ , 2001 .

[2]  Z. Fang,et al.  A finite line‐source model for boreholes in geothermal heat exchangers , 2002 .

[3]  René Rieberer,et al.  Direct-Expansion Ground-Coupled Heat Pumps , 2003 .

[4]  Arif Hepbasli,et al.  Exergetic performance assessment of a ground‐source heat pump drying system , 2007 .

[5]  Steve P. Kavanaugh,et al.  Thermal Conductivity of Cementitious Grouts and Impact On Heat Exchanger Length Design for Ground Source Heat Pumps , 1999 .

[6]  John R. Thome,et al.  Prediction of Two-Phase Pressure Gradients of Refrigerants in Horizontal Tubes , 2002 .

[7]  Yusuf Ali Kara,et al.  Experimental performance evaluation of a closed‐loop vertical ground source heat pump in the heating mode using energy analysis method , 2007 .

[8]  J. R. Goulburn,et al.  Deep ground coil evaporators for heat pumps , 1978 .

[9]  Abdeen Mustafa Omer,et al.  Ground-source heat pumps systems and applications , 2008 .

[10]  Yasuhiro Hamada,et al.  Field performance of an energy pile system for space heating , 2007 .

[11]  Ursula Eicker,et al.  Potential of geothermal heat exchangers for office building climatisation , 2009 .

[12]  G. Hellström.  Ground heat storage : thermal analyses of duct storage systems , 1991 .

[13]  J. R. Goulburn,et al.  Domestic heat pump with deep hole ground source evaporator , 1983 .

[14]  Arif Hepbasli,et al.  Experimental study of a closed loop vertical ground source heat pump system , 2003 .