A Brief Review of Geothermal Energy Extraction With Natural Circulation Systems

Enhanced geothermal system (EGS) provides a feasible way for extracting geothermal energy from hot dry rock (HDR), whereas seismic risk it might induce and enormous cost impair its prospects. An ideal way of harvesting geothermal energy receives increasingly attention: extracting heat from a geothermal reservoir without drawing water out of the ground, which sparks increasing attempts in this respect. With this aim, long gravity-assisted heat pipe was used to extract thermal energy from geothermal reservoirs due to its extremely high heat transport ability. Jiang et al. designed a super-long gravity heat pipe, by which a thermal power as high as 200 kW was achieved from a 3,000 m depth HDR successfully. Zhang et al. realized a maximum geothermal extraction power of 174 kW by a 3,000 m length gravity-assisted heat pipe. The pioneering work proves the feasibility of the ideal way for harvesting geothermal energy other than EGS, compared with which, employing gravity-assisted heat pipes to extract geothermal energy doesn’t consume power, more importantly, it can keep the system away from scaling, the toughest issue in a conventional geothermal exploitation system. The article will review the latest advances in this area. Different technologies employing natural circulation inclusive of gravity-assisted heat pipe to exploit geothermal energy will be thoroughly investigated and compared, several representatives will be put on the first place. Besides the conventional gravity-assisted heat pipe, two modified ones are presented: one installed with an insulated coaxial pipe inside the heat pipe, the other with a tube for draining the condensate back to the evaporation section. They will be compared in details in the aspects of working principles, performance, configurations, etc. Key factors involved will also be discussed, covering working medium, filling ratio and thermophysical properties of the reservoir. Scientific issues pertaining to the long gravity-assisted heat pipe will be clarified to the utmost, such as the natural circulation process, falling liquid film along a long pipe wall, flow pattern and transition, etc. Meanwhile, their drawbacks are also going to be taken into account to figure out countermeasures. Finally, experiences and lessons of deep geothermal energy exploitation will be summarized. The alternative method employing natural circulation offers new insights for obtaining thermal energy from HDR, whereas the lack of theoretical and practical investigations delays its development. It is expected that the paper can benefit to the tentative research in this field.