Super-long gravity heat pipe for geothermal energy exploitation - A comprehensive review

One salient feature of the heat pipe system is its superior heat transfer efficiency, which allows the transfer of significant amount of heat at near-zero temperature difference. In recent years, the scientific community has shown considerable interest in advancing geothermal heat extraction through the utilization of super-long gravity heat pipes. This review provides a comprehensive overview of the present status of the application of super-long gravity heat pipe technology for geothermal energy extraction, along with the key parameters including working fluid, length and diameter of heat pipe, filling ratio of the working fluid, and other technical factors. The state-of-the-art on experimental and numerical research, field testing, technical aspects of cogeneration, and thermo-economic analysis of the super-long gravity heat pipe geothermal system are extensively discussed. A super-long gravity heat pipe (SLGHP) with a length of 3000 m was developed and successfully tested; the heat pipe used water as working fluid, and extracted geothermal heat at a rate of 190 kW. To enhance the heat extraction capabilities of the SLGHP system, integrating materials with superior thermal conductivity can diminish the thermal resistance within the geothermal formation. Moreover, selecting appropriate working fluids and optimizing the structure of heat pipe can further reduce its thermal resistance. This study also highlights the existing challenges and research gaps that should be addressed in the future to further increase the heat transfer efficiency of SLGHP geothermal systems.