| Abstract |
Geothermal heat pumps (GHPs) are highly efficient renewable energy sources for space heating and cooling. Open-loop GHP is a type of GHP system that extracts groundwater from pumping wells, exchanges heat energy with the water, and injects the water into injection wells. Apart from the advantages of energy efficiency, open-loop GHP has a potential environmental impact resulting from groundwater pumping and injection activities in the surrounding area. Understanding environmental impact is essential to avoid interference with existing groundwater applications and the sustainability of the system. The impact on the groundwater temperature depends on the flow rate and the temperature difference of the water reinjected into the aquifer and the natural velocity of groundwater in the aquifer and the subsurface thermal dispersivity. Therefore, groundwater heat transport models were used in this study. This study aims to simulate regional groundwater flow and heat transport in an alluvial fan and assess the environmental impact of pumping and injecting open-loop GHP systems. The study area is located on an alluvial fan of the Nagara River, Gifu City, Japan, with a size of 12 km (NS) × 12 km (EW). It is bounded by mountains ranging from north to northeast side, and the plain area remains. The Nagara River flows in the center of the alluvial fan. The underground temperature in the alluvial fan is influenced by rapid groundwater flow recharged from the river. This alluvial fan is composed of sand and gravel and often intercalates thin fine sand and silt layers. To assess the utilization of open-loop GHP systems, a regional 3D model of groundwater flow with heat transport in the study area was developed using the FEFLOW program. The model is constructed using geological data. The boundary condition of the groundwater flow system is defined by the water table distribution. The ground surface temperatures of the model were obtained from the annual average ambient temperature. The Nagara River temperature data apply the temperature of a boundary condition in the model. The open-loop simulation model consists of a pumping well and an injection well. The operation mode was applied for heating from January to March, and cooling from July to September. The simulation was performed for ten years. Injection temperature is assumed to be three variant values: 3, 5, and 10 °C higher and lower than the temperature of pumping well for the heating and cooling periods, respectively. The flow rate of pumping and injection was 3.33 × 10–3 m3/s. To confirm the results of regional groundwater flow and heat transport simulation, the calculated groundwater tables, and groundwater temperatures were compared with the measured ones at the observational wells. The calculated results are well correlated with the measured data in the observational wells. In the local scale model of open-loop simulation, as the temperature difference of water injected into the aquifer increases, the size of the thermal plumes increases. The temperature change region is defined by a change of more than 1 °C in groundwater temperatures around injection activities. There is changing groundwater temperature around the injection area caused by injection activities, the distance of environmental impact got further toward groundwater flow as temperature difference (3, 5, and 10 °C) between pumping and injection of groundwater increase. |