Cooling tests, numerical modeling and economic analysis of semi-open loop ground source heat pump system

Ground source heat pump (GSHP) systems are serving the heating and cooling demands of buildings worldwide. However, the widespread usage of these systems is limited because of their higher initial costs compared with conventional heating and cooling systems, especially in countries with high drilling costs like Japan. The semi open loop GSHP system was introduced by authors and the results of heating tests and numerical modeling have been published. This system comprises two ungrouted vertical Ground heat exchangers (GHEs) in which groundwater is pumped from one well and injected to another using a water pump. The purpose of the water pumping and injection is to create an artificial groundwater flow around the GHEs to increase the heat advection between the GHEs and the surrounding environment. In this study, cooling tests on the semi-open loop GSHP system were performed and the thermal performance of the system was measured in each test. The developed numerical model was validated using the results of the cooling tests. Then, a sensitivity analysis was performed to evaluate the system performance under different operational and geological conditions during cooling operation. The results showed that in comparison with conventional GSHP operation, cooling coefficient of performance (COP) and system coefficient of performance (COPsys) can be enhanced by 13.1% and 6.6%, respectively, under fast groundwater flow conditions, as expected at the experimental site. In the absence of groundwater flow, the semi-open loop system is estimated to boost the cooling COP and COPsys by 101% and 62%, respectively, for cooling operations. Finally, an economic analysis was performed, considering the capital and running costs of the system and also the additional equipment costs associated with semi -open loop systems. The results of the economic analysis showed that water pumping and injection can reduce GSHP system costs by 22-36%.