Evaluation of the Shallow Geothermal Potential for a Ground-Source Heat Exchanger: A Case Study in Obama Plain, Fukui Prefecture, Japan
A ground-source heat exchanger (GHE) is an energy system exploiting shallow geothermal energy that is economical, environmentally friendly, and is rapidly increasing in popularity worldwide. Evaluating the available subsurface heat energy through thermal response tests and/or numerical simulations to design appropriate GHE systems (e.g. deciding the depth and number of boreholes for heat exchange) is important. Geological structures, groundwater properties, and subsurface temperatures are essential input data for such numerical simulations.
In the present study, we demonstrate the application of the GHE potential map, a new method based on the regional geological structure, subsurface temperature, and groundwater flow. Our target area is the Obama Plain in the central part of Japan, which faces the Sea of Japan. Subsurface temperature measurements at four stations in the Obama Plain were used for the present evaluation.The results of the GHE numerical simulations show a linear increase in GHE efficiency of about 35% with a subsurface temperature increase from 15 °C to 20 °C. In addition, GHE efficiency approximately triples when the groundwater flow ranges from 0 to 10 m/year. We estimated the specific heat-extraction for each 100 m × 100 m grid cell. The mapping results indicate a high GHE potential for the central part of the Obama Plain, with a value of more than 100 W/m. The specific heat-extraction ranges from 50 to 110 W/m in most areas of the Obama Plain. This value is sufficient for GHE to cover the potential heat-extraction demand for the Obama Plain. Our evaluation method can be applied to other plains in Japan and around the world.
KeywordsShallow geothermal energy Renewable energy Ground-source heat exchanger Heat pump Obama plain Groundwater flow Subsurface temperature
This research was mainly conducted as a part of the integrated research project of “Human-Environmental Security in Asia-Pacific Ring of Fire: Water-Energy-Food Nexus by the Research Institute for Humanity and Nature (RIHN)”. A portion of the analyses in this work was supported by Japan Society for the Promotion of Science (JSPS) KAKENHI Grant Number JP16K00595 and it was conducted as the research project of the Center for Environmental Science in Saitama. We thank Dr. M. Taniguchi, Dr. A. Endo, and Dr. M. Fujii, who lead the NEXUS project of RIHN, for their invaluable advices for our investigation and further analysis. Dr. S. Hachinohe helped in the investigation of subsurface temperature in the Obama Plain. We thank Ms. T. Okamoto, who provided accounting support. The groundwater simulation information was provided by the Obama City local government and the Nippon Koei Corporation.
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