Preliminary Considerations for Power Generation Using a Downhole Coaxial Heat Exchanger System (I)

Studies on the Downhole Coaxial Heat Exchanger (3rd Report)

Abstract

In order to fully utilize the geothermal resources which abound in Japan, technology for the exploitation of such resources as low productivity geothermal reservoirs, super hot formations adjacent to magma bodies and magma itself should be developed. The downhole coaxial heat exchanger (DCHE) system is thought to be the most suitable for the exploitation of these geothermal resources.
The authors have carried out a preliminary study on power generation using a DCHE system. The purpose of this study was to clarify the thermal output levels per DCHE or the effective thermal conductivity of the formation, at which the power generation system becomes economical. In addition, we wanted to clarify the geothermal resources favorable for this system.
The following conclusions have been drawn in this paper:
(1) For specified geothermal conditions, there are optimum operation conditions, e.g. inlet water temperature and flow rates, which give maximum net power output, in the case of power generation using the DCHE system.
(2) when used with the DCHE system, the two-phase turbine type power generation system is superior in cost performance to the multi-flush type power generation system under the conditions specified in this study.
(3) The net thermal output levels of the DCHE under optimum operation conditions were estimated for the four assumed effective thermal conductivities of the formation. The estimated net thermal output levels after 15 years of power generation were 1.4, 4.6, 9.0 and 13.0 MW/DCHE for the effective thermal conductivities of 2.7, 10, 20 and 30 kcal/mh°C, respectively.