This study determined the required lengths of borehole heat exchangers (BHEs) in ground-source heat pump systems for heating/cooling a building (with 300 m² of floor area) across Japan’s four main islands through a simulation approach. Hourly thermal loads were estimated in 10 km gridded cells based on the outside temperature and humidity. Three-dimensional estimates of ground thermal conductivity from our previous study at the depths of the BHEs were used. A 5-year system operation was simulated in a total of 4059 cells with 81 combinations of individual lengths and total numbers of BHEs to determine the shortest total length required to achieve sustainable use and targeted performance. The optimal combination of individual length and total number varied regionally due to climate conditions and locally among adjacent cells due to geological conditions. The total required lengths ranged widely from 78 to 1782 m. However, the lengths were less than 400 m in 85% of the cells. Additionally, cost-effectiveness in 69% of the cells was shown by reducing the total lengths to half or less of those in the practical method. The reduction could potentially increase the feasibility of heat pump system use in Japan. The total lengths were dependent on the heating/cooling loads approximately as secondary-polynomial functions, but the relations with the ground thermal conductivity were not clear.

中文翻译：

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在日本各地根据气候和地质条件确定多个钻孔换热器所需的总长度

这项研究确定了地源热泵系统中用于加热/冷却建筑物（300 m ²通过模拟方法遍及日本四个主要岛屿的建筑面积）。根据外部温度和湿度，估算10 km网格单元中的每小时热负荷。使用了我们先前研究中BHE深度处的地面热导率的三维估算。在总共4059个单元中模拟了5年的系统运行，其中81个单独的长度和BHE总数相结合，以确定实现可持续使用和目标性能所需的最短总长度。个体长度和总数的最佳组合因气候条件而在区域内变化，而由于地质条件而在相邻单元之间局部地变化。所需的总长度范围从78到1782 m。但是，在85％的单元中，长度小于400 m。此外，通过将总长度减少到实际方法中的总长度的一半或更少，可以显示69％的电池具有成本效益。减少可能会增加在日本使用热泵系统的可行性。总长度大约取决于加热/冷却负荷，作为二次多项式函数，但与地面导热系数的关系尚不清楚。