Technical review on coaxial deep borehole heat exchanger

This review paper summarizes recent developments regarding geothermal exploitation using coaxial deep borehole heat exchangers (DBHE). Specifically, this study focuses on field tests, analytical and semi-analytical approaches, and numerical simulations. First, field tests and applications of coaxial DBHE are summarized and future work for the field tests is suggested. Then, the ongoing analytical and numerical modeling approaches on coaxial DBHE are evaluated regarding the capability and incapability of describing physical behaviors. Lastly, key factors for the design of coaxial DBHE are summarized and discussed based on collected results. Regarding field tests, future work should focus more on (1) long-term performance; (2) effect of groundwater flow within formation and fractures; (3) technology for larger diameter boreholes; (4) new and cheap materials for insulated inner pipe; (5) treatment of fluid, pipe wall, and different working fluid; (6) economic analysis of coaxial DBHE-based geothermal power plant. As for the analytical methods and numerical simulations, it is important to consider the dependence of fluid and formation properties on pressure and temperature. Besides, verification and calibration of empirical models for working fluids other than water such as CO2 should be performed based on laboratory and field tests. Different borehole properties and pump parameters should be optimized to obtain the maximum thermal power of a coaxial DBHE, and an insulated inner pipe is recommended by many researchers. An intermittent working pattern of the DBHE could be more realistic when modeling a DBHE. To further improve the performance of coaxial DBHE, continuous research to enhance heat transfer and working fluid performance is still important.