In order to enhance the utilization of underground heat including underground water, the comparison between the existing high density polyethylene underground heat exchanger and the newly developed metal heat exchanger was conducted to suggest the potential use of geothermal energy corresponding to the geological characteristics of Jeju Island. When the acquired heat of the underground loop system test condition of the KS B 8292 water-water geothermal heat pump unit was compared by installing heat exchanger of HDPE material and the heat exchanger of STS material under the same bore hole condition, the total heat area of the heat exchanger of STS material was about 15% less than HDPE, but the acquired heat was estimated to be about 4 times or more.

The thermal conductivity for the ground heat exchanger installed in the local facilities of greenhouse area at JangSu-Gun in Jeollabuk-Do was analyzed experimentally. For measuring the thermal conductivity of the ground heat exchanger, a bore hole was punched at the size of diameter 150mm, depth 160m. And a closed-type heat exchanger of high density polyethylene at the size of diameter 40mm was installed vertically. The heated water by an electric heater was circulated through the ground heat exchanger with a pump. The inlet/outlet temperatures and the flow rates of this water are measured and the thermal conductivity was calculated. The experimental results were as follows. The inlet/outlet temperatures of the ground heat exchanger were increased rapidly until 7 hours, then they were increased gently and the difference of inlet/outlet temperature was maintained at about 4.5℃. The thermal conductivity calculated by the line source theory was 2.207W/mK.