2005 Volume 27 Issue 2 Pages 149-162
One of the major features of geothermal resources is its wide variety of existing forms. Hence, it is necessary to develop many types of heat mining methods and to pursue their possibilities. The authors have proposed the Downhole Coaxial Heat Exchanger (DCHE) system for exploitation of undeveloped geothermal resources such as Hot Wet Rock, Super Hot Rock, magma origin fluid systems and magma. The major features of the DCHE include the utilization of a highly insulated inner pipe, reverse circulation (i.e., cold water down the annulus and hot water up through the inner pipe) and a completely closed system. Through a heat extraction experiment carried out on the Island of Hawaii in 1991, it was demonstrated that a highly efficient DCHE could be constructed. The authors have carried out two case studies on small-scale power generation with a 2, 000 m class DCHE by numerical simulations. In the first case study, the operational behaviors of the DCHE or the power generation system were investigated assuming the temperature profile and the structure of a well in Hijiori, Japan. In this case, two cases where the binary or the Kalina cycles are combined with the DCHE were investigated. In order to estimate the possible order of the net thermal output of the DCHE or the power output of the power generation system, the second case study was carried out for a higher temperature profile than that in Hijiori. In this case, a temperature profile from Toyoha, Japan was assumed. Through this study, it has been indicated that minimizing pumping power for circulating water in the DCHE is very important for realizing functional power generation. Hence, an appropriate DCHE design is required. A power generation plant which allows a wide range of temperature difference between the hot water and the re-injection water is preferable for combination with the DCHE. Also, It has been shown that 70 kWe class power generation might be possible at Toyoha, Japan with a DCHE 2, 000 m deep.
