Field Tests of Horizontal Ground Heat Exchangers

Field tests were carried out in Fukuoka City, Japan, to evaluate the applicability of horizontal Ground Heat Exchangers (GHEs) on Geothermal Heat Pump (GHP) systems for the use in greenhouse farming. As the type of horizontal GHEs, slinky coils were employed considering the limited land space in Japanese greenhouse farming. Two types of installations of slinky coils were examined, namely, horizontal and vertical setting in trenches excavated in the shallow ground. The depth and length of each trench were 1.5 m deep and 70 m, respectively. The GHEs were connected to a heat pump and circulation pump to be completed as a GHP system. Using the GHEs, thermal response tests and air-conditioning tests were carried out from summer 2008 to spring 2009 for collecting detailed operation record of the system and ground temperature data during the operation. The obtained field data were examined to evaluate the heat exchange capacity of the horizontal GHEs and to compare the heat exchange performances with those of vertical GHEs. The field tests showed that horizontal installation of slinky coils results in superior performance to vertical installation in terms of energy efficiency, due to the less influence of atmospheric temperature changes. The heat medium temperatures in the air-conditioning test showed that horizontally-installed slinky coils have comparable heat exchange capacity with vertical U-tube GHEs drilled in a formation with favorable thermal conductivity, though installation costs of horizontal GHE are significantly lower than those of vertical GHEs.