The University of Yamanashi has been conducting research on the development of direct expansion geothermal heat pumps. In the direct expansion type geothermal heat pump, the refrigerant flows in a gas-liquid two-phase flow with evaporation and condensation in the ground heat exchanger and heat pump, and its behavior is complicated, so it is not easy to accurately determine the heat exchange performance. Therefore, the theoretical COP was obtained from the changes of refrigerant temperature and pressure before and after the ground heat exchanger and the indoor unit, and the factors of performance degradation were investigated by comparing the COP with that obtained for the indoor unit.

As a result, the average COP during the operation period was 11.5 in the cooling operation and 5.8 in the heating operation.