The purpose of this study is to evaluate an efficiency of ground source heat pump systems (GSHPs) in a standard residensial on a regional scale by the numerical simulation with the three-dimensional database of the effective thermal conductivity. The database was constructed through the indicator-kriging procedure based on the available borehole and water-well data to cover the depths beyond about 100 m for installation of borehole heat exchangers (BHEs). This study also proposed the methodology to determine required lengths of BHEs for a target value of seasonally averaged coefficients of performance (tCOP) in the GSHPs. The GSHPs simulation was demonstrated by using the Ground Club algorithm in a regular grid of 10 km over the northern area (the latitude > 36°) of Japan. First, this study showed that the COP values were calculated at about 4.0 in the Hokkaido area and about 4.4 in the Tohoku and northern Kanto areas when the BHE was assumed to be a standard length of 100 m. Next, the required median length of BHE for tCOP = 4.25 was estimated at about 80 m in the Tohoku and the northern Kanto areas. The smaller BHE length means that the intial costs for GSHPs could be efficiently reduced. On the other hand, the required median length was about 130 m in the Hokkaido area because the heating loads were relatively high in the cold region. The reduction ratio of CO_2 emissions was estimated at about 45-60 % when the ground source heat pump systems were utilized over the area instead of the conventional electric, oil and gas boiler systems. The reduction ratio was relatively large in the Tohoku and northern Kanto areas, indicating the potential increase of GSHPs in the future.
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