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

[S27-1-02] Assessment of efficiency and potential of a ground source heat pump system under geological complexity in Japan

^○Yoshitaka Sakata¹, Takao Katsura¹, Katsunori Nagano¹, Atsunao Marui² (1.Hokkaido University, Sapporo, Japan, 2.National Institute of Advanced Industrial Science and Technology, Tsukuba, Japan)

invited

This paper presents an assessment of the efficiency and the potency of a ground source heat pump system (GSHP) in Japan. This research is fundamental on the 3D geo-database which consists of over forty-thousand water well drilling reports. Indicator kriging interpolation is applied to estimate the soil/rock types in any location. The accuracy of kriging interpolation is validated in the leave-one-out cross validation. The geodatabase translates the probability estimates of soil/rock types to the effective thermal conductivity and transmissivity as matched with the in-situ measurements. The database also provides the groundwater flow simulation results by AIST to evaluate heat advection effects in the Holocene, Pleistocene and Neogene layers. This study performs the GSHP simulation by using the Ground Club, which was developed based on the cylindrical heat source theory to calculate ground temperatures rapidly with accuracy. The Ground Club is also useful to assess the advection effects of groundwater flow in the multi-layered formations. The assumed GSHP is composed of a commercial heat pump and a vertical borehole heat exchanger (BHE) in a standard residence. The total external area of the residence is 313 m2 and the U-values are ranged from 0.46 W/m2/K in the cold region to 0.87 W/m2/K in the warm regions. The heat demands are variable according to the hourly data of the outdoor temperatures and the insolation amounts. In each location, the system efficiency values on a third year are estimated under different cases of assumed BHE lengths, so that minimum lengths of BHEs for the required efficiency are determined. The simulation is demonstrated all over the country with a grid of 0.25 km in space. The results are averaged into a grid of 1 to 10 km in space for a national map. The map contributes to the assessment of the efficiency and the possibility of the GSHP under the regional situations of geology and groundwater in Japan.