Investigation of heat pump performance in heterogeneous ground

Seasonal performance of a heat pump in heterogeneous ground is investigated using a resistance-capacity model. Heat pump and borehole heat exchanger models are based on the existing ground-coupled heat pump system. Simulation results show that the application of average undisturbed ground temperature instead of a ground temperature profile has no effect on the obtained seasonal values of the coefficient of performance (COP). On the other hand, a different orientation of the ground thermal conductivity affects the seasonal coefficient of performance. The highest seasonal COP value of 4.86 is found in the case of a homogenous distribution of thermal conductivity while the largest drop in the COP value is observed in the ground profile with the thermal conductivity which steeply increases with depth. The difference between the two cases of 3% in the cooling season and 1% in the heating season is much smaller than the differences reported in previous studies where internal borehole heat transfer is not accounted for. Small relative difference indicate that the homogenous ground approach slightly overestimates the heat pump performance in a purely conductive underground.