Field experiment on performance of water source heat pump using underground rainwater tank as heat source

Research on heat sources for various heat pump (HP) systems is ongoing worldwide to achieve carbon neutrality and mitigate global warming. In this study, a rainwater (RW) tank HP (RWHP) using an underground RW tank as a heat source and heat sink was installed in an actual building and evaluated for its potential to provide cooling during the summer season. The water temperature of RW tank, the inlet and outlet fluid temperature, and the power consumption were measured. Using the collected data, the thermal stability of the RW tank and the performance of the RWHP were analyzed. In addition, the effect of the water level of the RW tank and precipitation was analyzed. As a result, the RW tank functioned as the heat sink for the HP during the cooling season. If the water level in the RW tank and the moisture content of the surrounding ground are high because of precipitation, the water temperature in the tank can be maintained at a relatively stable level. The coefficient of performance (COP) and system COP (COPsys) ranged 2.41-3.38 and 1.80-2.50, respectively. When the rainwater temperature in the RW tank increased, the COP and COPsys tended to decrease. Through this study, the data necessary for optimization through future mathematical modeling was collected. Also, the basic mechanisms of the temperature distribution of the RW tank and the performance of the RWHP were identified. Additionally, the operation method and means for improving the system were determined to increase the energy efficiency and broaden the applicability of the system.