| Abstract |
A variety of technologies have been developed on the heat extraction or power generation. However, few technologies have been actively developed for exploitation of the super hot formations or the low productive geothermal reservoirs which produce insufficient amount of hot water or steam for power generation or direct use. The downhole coaxial heat exchanger is composed of a well drilled into the hot formations and an inner pipe (See Fig. 1). Heat is extracted using water or low boiling point heat extraction medium. The downhole coaxial heat exchanger system is always applicable when the formation temperature is high enough and is best suited for development of geothermal energy resources as stated above. The authors have noted the possibility of this method and been studying effects of various design parameters on the performance of heat exchanger by means of computer simulation. As a result, it was found that the amount of extracted heat was greatly increased by combining highly insulated inner pipe with the reverse circulation (Morita et al., 1985; Morita et al., 1986). The thermal output of this method is ten times or much higher than previous methods, e.g. using steel pipe as the inner pipe, coaxial heat exchanger for the same geotogical conditions. It is also pointed out by recent theoretical and experimental researches that the heat extraction rate is highly increased-when the convective heat transfer exists in the formation as compared with cases where the heat is transferred by the conduction only (Kimura et al., 1986). The effect of conv~ction in the formations on the thermal output is the most directly and accurately evaluated by field experiments of heat extraction. However, high performance small diameter insulated inner pipes that can be applied to the field experiment has not yet been developed. Accordingly, the authors has initiated design and manufacturing of prototype models of small diameter highly insulated inner pipes for the field heat extraction experiments. The rest of this paper describes manufacturing of insulated inner pipe and the evaluation of its heat insulating performance. |