30 May 2005
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.