Monitoring of the performance of a closed-loop ground source heat pump system

This study reviews the performance of a ground source heat pump (GSHP) system with two vertical borehole heat exchangers (BHEs) installed at a nurse building. The BHEs are drilled with the depths of 300 and 464 m, located at a crystalline granite bedrock with low hydraulic conductivity. The conventional depth of BHE in Korea is around 150 m, but the BHEs is drilled with increase depths. Because of the limited available area for the geothermal field, fewer number of drilling with longer length is required. The loop length is designed to cover the heating and cooling load for afloor. The initial ground temperature is distributed with the geothermal gradient around 25°C/100 km. Thermal response tests (TRTs) were conducted at each borehole and obtained effective ground thermal conductivity and borehole thermal resistance respectively. The monitoring system was set up for collecting temperatures of inlet and outlet and flow rates of the circulation fluid. The performance monitoring have been continued from the summer season in 2017 and one cycle data of a heating and cooling season were analysed. According to calculation the heat exchange rate (HER), the load pattern and period were identified. The average HER is 30 50 W/m during peak time at cooling season, but it is shown 10 25 W/m during peak time at heating season. Even though the measured HER is satisfied within the initial design loads, the difference between that of cooling and heating season is almost double. The imbalance may cause the decrease of system performance after long-term operation. More monitoring period is required for the evaluation of the performance, and this study would help to design economic GSHP.

Acknowledgements: This study was supported financially by a grant from the research project (No. PJ01339203) of National Institute of Agricultural Sciences, Rural Development Administration, Republic of Korea.