Operational Response of a Soil-Borehole Thermal Energy Storage System

This study focuses on an evaluation of the subsurface ground temperature distribution during operation of a soil-borehole thermal energy storage (SBTES) system. The system consists of an array of five 9m-deep geothermal heat exchangers, configured as a central heat exchanger surrounded by four other heat exchangers at a radial spacing of 2.5m. In addition to monitoring the temperature of the fluid entering and exiting each heat exchanger, 5 thermistor strings were embedded in boreholes inside and outside of the array to monitor changes in ground temperature with depth. After 75days of heat injection at a constant rate of 20W/m, corresponding to 11.5 GJ of thermal energy, the average ground temperature in the array increased by 7 degrees C. However, depending on the storage volume definition, only 2.43-4.86 GJ of thermal energy was stored attributable to heat losses. After a 4-month rest period the heat storage was observed to decrease by 60% owing to further heat losses. The trends in subsurface temperatures during heat injection were consistent with results from a simplified heat injection simulation using the system thermal conductivity estimated from a line source analysis. Although the heat injection rate of 20W/m is smaller than that expected in actual SBTES systems (35-50W/m), an energy balance analysis indicates the number of boreholes in the array was too few to effectively concentrate the heat injected within the subsurface. Nonetheless, the results provide an experimental reference point between a single borehole and a larger SBTES system.