Thermal capacity effects in borehole ground heat exchangers

For a typical two-pipe configuration, it is shown that the fluid outlet temperature predicted with and without borehole thermal capacity differs by 1.4, 0.35, and 0.23 °C after 0.1, 0.2 and 1 h, respectively. Annual simulations are also performed over an entire heating season (5600 h) with a 6 min time step. Results show that the outlet fluid temperature is always higher when borehole thermal capacity is included. Furthermore, the difference in fluid outlet temperature prediction with and without borehole thermal capacity increases when the heat pump operates infrequently. The end result is that the annual COP predicted is approximately 4.5% higher when borehole thermal capacity is included.