DEVELOPMENT OF A GROUND SOURCE HEAT PUMP SYSTEM WITH GROUND HEAT EXCHANGER UTILIZING THE CAST-IN-PLACE CONCRETE PILE FOUNDATIONS OF A BUILDING

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 employs the cast-in-place concrete pile foundations of a building as heat exchangers in order to reduce the initial boring cost. In this system, eight 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 ¥79/W (approx. US$0.79/W) for this system, whereas it is ¥300/W (US$3/W) for existing standard borehole systems. Therefore, this system is expected to be commercially viable.