Modeling the interactions between the performance of ground source heat pumps and soil temperature variations

The adoption of geothermal energy in space conditioning of buildings through utilizing ground source heat pump (GSHP, also known as geothermal heat pump) has increased rapidly during the past several decades. However, one problem in operating GSHPs is that collection or rejection heat from the ground alters the ground temperature, which can adversely affect the coefficient of performance (COP). In turn, the amount of heat that must be exchanged with the ground increases in order to satisfy a given heating or cooling load. This paper presents a novel model to calculate the soil temperature distribution and the COP of GSHP. Different scenarios were simulated to quantify the impact of different factors on the GSHP performance, including seasonal balance between heat collection and heat rejection, daily running mode, and spacing between boreholes. Our results show that greater loads and smaller distances between boreholes cause changes in soil temperature large enough to adversely affect the GSHP performance, even resulting in COPs less than those commonly achieved with air source heat pumps. However, shifting from heating to cooling on a seasonal basis can, in part, mitigate this problem. Long boreholes, additional space between boreholes and intermittent running mode could also improve the performance of GSHP, but large initial investment is required.