Computer Simulations of Heat Transfer in Soil for Ground-Source Heat Pumps

　In order to design a ground-source heat pump, the temperature profile and the ice formation and melting during heat extraction and recovery in the ground must be documented. Computer simulations were used for investigations with a direct expansion heat pump on evaluations of heat extraction levels, various heat pump operations, and the fill material of vertical heat exchange tubes. Actual operational data were recorded at the Hokkaido National Industrial Research Institute(HNIRI).
　It became clear from simulations that the tube diameter has little effect on the heat extraction rate and that the effect of the tube length is nearly proportional to the heat extraction rate.
　These results are of usefull applicability to heat extraction from the ground in static water-saturated condition. When heat extraction rate levels are too high, they act to decrease initial soil temperature from the start of one year to the next; therefore it is necessary to inject heat artificially during the heat recovery period.
　Nighttime operation combined with a heat storage system, when electricity rates are low, reduces energy costs while generating a small increase in investment costs.