Extraction of Geothermal Energy and Electric Power Generation Using a Large Scale Heat Pipe

Abstract

In order to utilise low enthalpy natural heat sources, a heat pipe using a binary fluid is a good device which can extract heat without using electric power. When the heat flux in a geothermal bore is moderate, a long heat pipe is needed. We developed a 150 m long, large scale heat pipe of 150mm outer diameter, in which liquid feeding tubes with showering nozzles were installed. It has an excellent thermal performance compared with other long vertical evaporators. A demonstration test was carried out in a well with temperatures between 100 and 150 oC at a geothermal site in Kyushu, Japan. The heat pipe extracted 90 kW heat at a working temperature of 80 oC. We also calculated the extraction rate for different lengths of the heat pipe and different temperatures of the geothermal source. Similar pipes can be used for heat extraction of high temperature geothermal fluids.

In addition to heat extraction, we also studied the performance of a turbine which can be installed on top of a heat pipe using a binary fluid. We used a modification of a thermosyphon, with excellent heat and mass transfer characteristics, and incorporated a turbine. The heat transfer in a thermosyphon occurs through the circulation of a binary working fluid through a sequence of evaporation, vapour transfer, condensation and liquid return. It is then possible to convert some of the energy of the vapour stream into electric energy by installing a turbine which is coupled to an electric generator. Simulation showed that a heat pipe turbine, using R-123 as working fluid, can generate about 7.8 kW electric power at a turbine speed of 3000 rpm when the rotor diameter is about 0.8m and the evaporator section of the heat pipe is 150m long. The Heat Pipe Turbine or Thermosyphon Rankine Engine (TSR) is therefore an interesting concept for power generation using geothermal or other low grade heat sources.