Thermal performance evaluation of a vertical closed-loop ground heat exchanger according to rock type in Korea

Abstract

Geothermal heat pump systems have a variety of uses including air conditioning and heating for buildings and greenhouses, hot water supplies, and crop drying applications. The installation environment and conditions of operation can have a large influence on the performance of the system and need to be considered. In this study, we applied the line-source method to different types of rocks and borehole depths in order to analyze the characteristics of effective thermal conductivity. We conducted thermal performance tests for a geothermal heat exchanger in accordance with initial ignoring times (5, 7, 9, and 12 h) and analyzed the stability of effective thermal conductivity and the relationship between the heat injection and slope of k in λeff = Q/4πHK according to variations in appropriate heat injection levels. The analysis of effective thermal conductivity according to the rock type and borehole depth for 40 boreholes showed that the value of effective thermal conductivity was generally high at the borehole depth of 200 m. This seemed to be due to the increase of underground space for energy storage. Consequently, we determined that the aquifer had the largest effect on the effective thermal conductivity.