Ground-source (geothermal) heat pump (GSHP) systems can achieve a higher coefficient of performance than conventional air-source heat pump (ASHP) systems. However, GSHP systems are not widespread in Japan because of their expensive boring costs. The authors have developed a GSHP system that uses the cast-in-place concrete pile foundations of a building as heat exchangers to reduce the initial boring cost. In this system, some U-tubes are arranged around the surface of a cast-in-place concrete pile foundation.

The heat exchange capability of this system, subterranean temperature changes, and heat pump performance were investigated in a full-scale experiment. As a result, the average values for heat rejection were 186–201 W/m (per pile, 25 W/ m per pair of tubes) while cooling. The average COP of this system was 4.89 while cooling, rendering this system about 1.7 times more effective in energy-saving terms than the more typical ASHP systems. The initial cost of construction per unit for heat extraction and rejection is US$0.79/W (approximately ¥79/W) for this system, whereas it is US$3/W (¥300/W) for existing standard borehole systems. Therefore, this system is expected to be commercially viable.

Units: SI