A Study on Relation between Groundwater Flow and the Design of Ground-Coupled HP System with Borehole

Evaluation of heat transfer due to groundwater flow is a central theme for obtaining a more reliable design of the ground-coupled heat pump with borehole (hereinafter referred to as GeoHP system). In practical, the test site in Omachi, Nagano (Japan) showed the heat extraction rate of 209 W/m from GeoHP system with the borehole of 100 m in depth and double U-tubes through the heat recovery test in Feb. 2002. To evaluate such an advantage for GeoHP system, a relation of borehole temperature to heat extraction rate and groundwater flow velocity has been examined in this study, using two-dimensional numerical model. The calculated result clarified the upper bound of heat extraction rate (maximum heat load), which in turn roughly predicts borehole length required for the GeoHP system with groundwater flow. The constraint is the temperature of circulation fluid in heat exchanger (e.g., in U-tube), or the temperature of soil with groundwater around borehole, that is, the temperature must be maintained above the freezing point. The calculated result showed that for the Darcy fluid velocity of groundwater larger than 10^-5 m/s at least, we can save the length of heat exchanger, compared to that of the GeoHP design assuming only the heat conduction as heat transfer in ground. Further, this study has applied the calculation to some data measured in the Omachi test site (in Omachi business office of Chubu Electric Power Co., Inc.). The calculation explained that the main distance of the heat extraction in this site was -30 m to -100 m in depth, assuming uniform flow velocity of groundwater in the order of 10^-4 m/s. Its value agrees with the estimate from the thermal response in the observation well at this site.