Abstract
Ground source heat pumps (GSHPs) use buried pipes to extract heat from the ground and to release heat to the ground. In the conventional GSHP system, which uses an indirect heat exchange method, vertical systems use two long pipes connected by a U-shaped fitting at the bottom of a hole bored in the ground. These pipes form part of a closed loop, called the ground loop, through which a mixture of water and antifreeze circulate. In contrast, a GSHP that uses a direct expansion method circulates a mixed refrigerant through the ground loop. In our tests of this method, the depth of the borehole was 30 m, and the refrigerant was R410A. The heat exchanger of a ready-made air-conditioner was replaced by an underground heat exchanger. In the direct expansion GSHP, the underground heat exchanger consisted of narrow copper tubes inserted into the bottom end of a long pipe filled with water. The coefficients of performance (COP) were obtained for the direct expansion type of GSHP running in the cooling and the heating mode. The amount of heat transferred was evaluated by obtaining an enthalpy difference at the indoor unit. It was found that the COP in the cooling mode was over 10.
