11:35 AM - 11:50 AM
[AGE27-04] Open-Loop System Ground Source Heat Pump on the Nagara River Alluvial Fan: An Assessment of Thermal Environmental Impact
Keywords:Ground Source Heat Pump, Open-Loop System, Thermal Environmental Impact, FEFLOW
Ground Source Heat Pump (GSHP) are renewable energy technologies that use thermal energy from the ground for heating and cooling purposes. GSHP has gained attention to benefits such as low costs of running costs and low emissions of CO2. Many residential areas in Japan are located on alluvial deposits, where groundwater flow is usually present. An open-loop GSHP system or groundwater heat pump (GWHP) is potentially feasible in areas where abundant groundwater is available. Open-loop GSHP systems use groundwater as a heat source. Groundwater is extracted from a pumping well and passed through a heat exchanger or a heat pump before being discharged back into the aquifer through an injection well.
Apart from the advantages, open-loop GSHP has a potential thermal environmental impact resulting from groundwater pumping and injection activities in the surrounding area. Therefore, this study aims to simulate groundwater flow and heat transport in the alluvial fan and assesses the thermal environmental impact of pumping and injecting the open-loop system from the wells. The study area is located on an alluvial fan of the Nagara River, Gifu City, central 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 underground temperature in the alluvial fan is influenced by rapid groundwater flow recharged from the Nagara River. This alluvial fan is composed of sand and gravel and often intercalates thin fine sand and silt layers.
In this study, groundwater flow and heat transport were modeled by DHI FEFLOW to understand regional groundwater flow and heat transport in the Nagara River alluvial fan. Based on the regional groundwater flow and heat transport simulation, a local scale model of an open-loop GSHP was constructed to assess the thermal environmental impact of the open-loop geothermal heat pump system. 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.
The calculated results were compared with measured data of the hydraulic head and groundwater temperature to validate the regional groundwater flow and heat transport simulation. Hydraulic heads in the observation wells show that almost all calculated results match the measured data. This result indicates that the calculated result well represented the distribution of the hydraulic head in the study area. Nevertheless, some discrepancies (approximately 1 – 2 m) occur in 2 wells located near the recharge area. This suggests that the hydraulic conductivity within an alluvial fan is not homogeneous and that in the upper fan area, including the recharge area, is higher than that of the other area. Comparison of the annual underground temperature change between the calculated and measured values are almost consistent with the measured data. However, there are noticeable differences between the measured and calculated data on an average of 1 – 2 °C. The calculated phase differences of annual groundwater temperature change against annual river water temperature change are almost equal to the measured ones. In the local scale model of open-loop simulation, it can be found that there are changing groundwater temperatures around the injection well caused by heat/cold injection. In this study, the temperature change was defined by a change of more than 1 °C at groundwater temperatures. The thermal plume size would have a length of about 10 m in summer and 20 m in winter when water reinjected is at 3 °C of the temperature difference between pumping and injection. These thermal plumes' size becomes further when the temperature difference between pumping and injection gets higher. Injection activities by an open-loop GHP system do not cause a severe temperature change in the areas of 100 m and 200 m from the injection well. The thermal environmental impact and interference effect is avoidable from another open-loop GHP system according to regulation by other countries.
Apart from the advantages, open-loop GSHP has a potential thermal environmental impact resulting from groundwater pumping and injection activities in the surrounding area. Therefore, this study aims to simulate groundwater flow and heat transport in the alluvial fan and assesses the thermal environmental impact of pumping and injecting the open-loop system from the wells. The study area is located on an alluvial fan of the Nagara River, Gifu City, central 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 underground temperature in the alluvial fan is influenced by rapid groundwater flow recharged from the Nagara River. This alluvial fan is composed of sand and gravel and often intercalates thin fine sand and silt layers.
In this study, groundwater flow and heat transport were modeled by DHI FEFLOW to understand regional groundwater flow and heat transport in the Nagara River alluvial fan. Based on the regional groundwater flow and heat transport simulation, a local scale model of an open-loop GSHP was constructed to assess the thermal environmental impact of the open-loop geothermal heat pump system. 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.
The calculated results were compared with measured data of the hydraulic head and groundwater temperature to validate the regional groundwater flow and heat transport simulation. Hydraulic heads in the observation wells show that almost all calculated results match the measured data. This result indicates that the calculated result well represented the distribution of the hydraulic head in the study area. Nevertheless, some discrepancies (approximately 1 – 2 m) occur in 2 wells located near the recharge area. This suggests that the hydraulic conductivity within an alluvial fan is not homogeneous and that in the upper fan area, including the recharge area, is higher than that of the other area. Comparison of the annual underground temperature change between the calculated and measured values are almost consistent with the measured data. However, there are noticeable differences between the measured and calculated data on an average of 1 – 2 °C. The calculated phase differences of annual groundwater temperature change against annual river water temperature change are almost equal to the measured ones. In the local scale model of open-loop simulation, it can be found that there are changing groundwater temperatures around the injection well caused by heat/cold injection. In this study, the temperature change was defined by a change of more than 1 °C at groundwater temperatures. The thermal plume size would have a length of about 10 m in summer and 20 m in winter when water reinjected is at 3 °C of the temperature difference between pumping and injection. These thermal plumes' size becomes further when the temperature difference between pumping and injection gets higher. Injection activities by an open-loop GHP system do not cause a severe temperature change in the areas of 100 m and 200 m from the injection well. The thermal environmental impact and interference effect is avoidable from another open-loop GHP system according to regulation by other countries.