Advances in the effects of multiple factors on the thermal performance of deep borehole heat exchangers: A systematic review

As a safe and efficient source of clean energy, medium-deep geothermal energy has been widely used in building heating. The medium and deep borehole heat exchanger (MDBHE) is an essential apparatus for extracting medium-depth geothermal energy, which can be classified into the medium-deep coaxial borehole heat exchanger (MDCBHE) and the medium-deep U-type borehole heat exchanger (MDUBHE). During the heating period, multiple factors can affect the thermal performance of MDBHE. Therefore, to enhance the performance of MDBHE, it is crucial to conduct research on multiple factors. Nevertheless, most extant studies are predicated on a limited number of influencing factors, and more comprehensive and systematic research needs to be done on the factors of MDBHE. Based on this, the paper comprehensively reviews the current research on the effects of multiple factors on the heat extraction of MDBHE from three perspectives: operational conditions, wellbore structural properties, and geothermal properties, and clarifies the mechanisms of their influences. In addition, the effects of each factor in different studies are compared to obtain the optimal operating or design values. The heating characteristics of different MDBHE types are discussed, and the feasibility of using super-long gravity heat pipes for heating is also analyzed. Furthermore, an outlook for future research is presented in terms of the analysis of coupled system performance and economics, the influence of meteorological parameters on the system, the types of working fluids in operation, and multi-energy complementarity. This paper can guide the design and practice of MDBHE.