Design of the BHP System Considering the Heat Transport of Groundwater Flow

Abstract

It is well known that groundwater exists abundantly in Japan. Various ways to use groundwater as a heat source of BHP system are imaginable. However, the direct pumping of groundwater would cause some troubles such as ground subsidence. Therefore, this study focused on the closed-loop-type BHP system (a conventional ground-coupled heat pump system with borehole), and designed its system, considering the heat transport of groundwater flow. Such a design remarkably saves the depth of borehole (i.e., the cost of drilling) compared to that assuming only the heat conduction under the ground.
In order to evaluate the transport rate of heat from groundwater to the borehole (heat exchanger), the relations of borehole temperature to the heat extraction rate and groundwater flow-velocity have been examined by using a two-dimensional numerical model in this study. The calculated result clarified the upper bound of heat extraction rate (i.e., maximum heat load). Its upper bound is limited by the freezing point either of circulation fluid in U-tube or groundwater. Using the upper bound, we can roughly predict the borehole length required in the BHP system. In the results, the case larger than 10-5 m/s in Darcy flow-velocity of groundwater was able to save the length of heat exchanger compared to that of the BHP system design assuming only the heat conduction.
The applicability of the calculated results was confirmed in a test site of BHP system in Omachi business office of Chubu Electric Power Co., Inc., Japan. The BHP system has installed double U-tubes into a borehole of 100 m in depth. In the test site, the flow-velocity of groundwater was estimated to be 10-4 m/s in Darcy flow velocity. In the steady state of heat extraction test, the BHE system supplied heat to the ground facility at least up to 20 kW (209 W/m). The results showed good agreement with the calculated results.