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 degrees 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 degrees C and a condensing temperature of 53.4 degrees 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 degrees 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 (C) 2009 John Wiley & Sons, Ltd.