Simulation of Ground Source Heat Pump Systems with On/Off Control: Evaluating Part-Load Degradation in Single and Double U-Tube Borehole Configurations
As global demand for sustainable heating and cooling solutions intensifies, ground source heat pump (GSHP) systems are increasingly vital for decarbonizing thermal energy. This study presents a simulation model of GSHP systems integrating single and double U-tube borehole heat exchangers (sBHE and dBHE) with a fixed capacity heat pump operating under on/off control and part-load cycling. Using an implicit finite-difference scheme and thermal resistance network, the model captures unsteady heat transfer in the ground and borehole over time. A minute-based simulation framework enables detailed evaluation of system behaviour, particularly in response to fluctuating building loads. Ten cases are examined under identical material, geometric, and boundary conditions, while varying the borehole configuration, depth, and fluid mass flow rate. The novel aspect of this study is the inclusion of an adjusted part-load factor (PLF) that accounts for off-cycle power use, minimum runtime constraints, and degradation of efficiency under part load. PLF adjustment consistently lowers the performance of a GSHP system with reference to the theoretical case without PLF adjustment. On average, the heating coefficient of performance (COP) declines by 6.7-9.5%, the cooling COP decreases by 7.4-9.9%. The effective heat transfer between the heat pump and the borehole is reduced, with seasonal-average fluid temperature difference and corresponding heat exchange per unit borehole length decreasing by approximately 8–11% in heating and 5–8% in cooling. Meanwhile, total compressor work increases by 4-6.9% and overall runtime rises by 5.9-6.8%, reflecting the longer operational periods required under part-load cycling conditions with PLF adjustment.