A review of advances and applications of geothermal energy extraction using a gravity-assisted heat pipe

Enhanced geothermal system (EGS) provides a feasible way for extracting geothermal energy from hot dry rock (HDR) reservoirs, whereas seismic risk it might induce and enormous cost impair its prospects. For sustainability, circulating fluid in a closed loop to extract thermal energy from a deep geothermal reservoir seems more reasonable and receives increasing attention. With this aim, using a super long gravity heat pipe (SLGHP) to extract geothermal energy from HDR reservoirs was tried by several researchers due to its extremely extraordinary heat transport ability. Jiang et al. designed a SLGHP, by which a thermal power as high as 200 kW was achieved from a 3000 m depth HDR reservoir successfully. The pioneering work proves the feasibility of a safer way for harvesting geothermal energy other than EGS, more importantly, it does not consume power and avoids the toughest issue of pipe scaling in a conventional geothermal production system. The article reviews the latest advances in SLGHPs for geothermal energy extraction. Laboratory experiments and representative in-situ tests are thoroughly analyzed with respect to technologies employing natural circulation inclusive of gravity-assisted heat pipe (GHP) to harvest geothermal energy. Theoretical and numerical work is also reviewed as significant supplements to experimental investigation. Key factors involved are discussed to clarify the challenges of GHPs in engineering for geothermal energy extraction. Low thermal extraction power and economic benefits as well as lack of highly efficient technology to convert heat to electricity are the main barriers of the application of SLGHPs in geothermal energy extraction and utilization. Nonetheless, previous investigation on SLGHPs offers new insights into geothermal energy exploitation from HDR reservoirs.