In Situ Experiments of Geothermal Heat Pump System Considering: Groundwater Flow

An advantage of geothermal heat pump system (i.e., ground-coupled heat pump, system with borehole) resulting from groundwater flow has been evaluated in Omachi and Iiyama Service Stations of Chubu Electric Power Co., Inc. In Japan, it is well-known that huge amount groundwater exists with the temperature of 10∼15°C. In order to design a geothermal heat pump with higher reliability, we should consider the ground heat deriving not only from heat conduction but also from groundwater flow. At present, the drilling cost in Japan is two or three times higher than that in Europe and USA. This matter directly raises the initial cost of geothermal heat pump system installation in Japan. Some reports have suggested the advantage of groundwater flow on subsurface heat extraction rate, which may affect on the design of geothermal heat pump. However, field experiments. on this topic have not sufficiently been conducted. In our experiments, the geothermal heat pump system is supplied heat from boreholes of 100 m or 70 m deep, in which U-tubes are installed. As results of Omachi site experiments, the filling material (i. e., water, silica sand or cement) of the borehole affected on thermal response in heat extraction, although a heat-extraction-rate higher than 170 W/m was recorded in a steady state for any filling material. Based on the observation results in the observation wells, the groundwater flow velocity in Omachi-site was estimated to be 1.4 × 10^-4 m/s in Darcy fluid flow velocity. On the other hand, in Iiyama site, the flow velocity of groundwater was in the order of 10^-7 m/s. Though its velocity is remarkably smaller than that of Omachi site, groundwater flow in Iiyama site also caused high efficiency of the heat exchanger in a limited time-period.