[S27-1] Geothermal energy: Ground source heat pump, hydrothermal system, and hot dry rocks

[S27-1-01] The efficiency of Borehole heat exchanger system by regional differences

^○Hideki Hamamoto¹, Yuji Miyashita², Philipp Blum³, Alexander Limberg⁴, Makoto Taniguchi⁵ (1.Center for Environmental Science in Saitama, Saitama, Japan, 2.Hot Springs Research Institute of Kanagawa Prefecture, Kanagawa, Japan, 3.Karlsruhe Institute of Technology, Karlsruhe, Germany, 4.Senate Department for the Environment, Transport and Climate Protection, Berlin, Germany, 5.Research Institute for Humanity and Nature, Kyoto, Japan)

invited

Borehole heat exchanger (BHE) is an economically and environmentally friendly technology. Therefore, the BHE is beginning to spread around the world. For design and deployment of the system, it is thus important to evaluate the available subsurface heat energy through thermal response tests and/or numerical simulations and to design appropriate systems (depth and the number of boreholes for heat exchange). Geological structures, groundwater properties, and subsurface temperatures are essential input data for the numerical simulations. The evaluations are roughly separated to two models; wide model and local model.
We demonstrate the wide model at first. We illustrate a new construction method of BHE potential map from regional geological structure and measured subsurface temperature for typical Japanese plains. Our main target areas are central part of Kanto Plain and Obama Plain. Kanto Plain is including of Tokyo metropolitan and Obama Plain is located in the central part of Japan and faces the Sea of Japan. We have conducted measurements of subsurface temperatures 25 stations in central part of Kanto Plain and 4 stations in Obama Plain for the evaluation.
A part of the numerical simulations results shows that BHE efficiency increase 20 % when subsurface temperature rise up 5 degree and the efficiency also increase 30 % when groundwater flow vary from 0 to 15 m/year. We show also the efficiency of BHE difference due to the difference of the subsurface temperatures in the worldwide cities (example for Tokyo, Osaka, Bangkok, and Berlin).
We have also discussed an influence of subsurface warming effect on the BHE efficiency by numerical simulations. We reveal that subsurface warming effect increase BHE efficiency of heating drive (inversely, decrease efficiency of cooling).