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.

中文翻译：

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多因素对深井换热器热性能影响的研究进展：系统评价


中深层地热能作为一种安全高效的清洁能源，已广泛应用于建筑供暖。中深井换热器（MDBHE）是提取中深井地热能的必备设备，可分为中深同轴井热交换器（MDCBHE）和中深U型井下换热器（MDUBHE）。在加热期间，多种因素会影响 MDBHE 的热性能。因此，为了提高 MDBHE 的性能，对多个因素进行研究至关重要。然而，大多数现存研究都基于有限数量的影响因素，需要对 MDBHE 的因素进行更全面、更系统的研究。基于此，本文从作业条件、井筒结构性质和地热性质3个角度全面综述了当前多因素对MDBHE采热影响的研究，并阐明了其影响机制。此外，还比较了不同研究中每个因子的效应，以获得最佳作值或设计值。讨论了不同 MDBHE 类型的加热特性，并分析了使用超长重力热管加热的可行性。此外，从耦合系统性能和经济分析、气象参数对系统的影响、运行中的工作流体类型以及多能源互补性等方面提出了对未来研究的展望。本文可以指导 MDBHE 的设计和实践。