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The novel technical scheme of extracting the deep-earth hot dry rock heat by a super-long gravity heat pipe (SLGHP) system in a single-well has aroused widespread interest in the geothermal energy research and engineering community. Due to the unique characteristic of the SLGHP, i.e. the extremely large length-to-diameter ratio, the effect of working fluid on its thermal performance is different from that for the normal heat pipes, thus requiring careful evaluation in the working fluid selection. However, there is no unified evaluation standard, and consequently, it is not available an efficient preferred route for the working fluid selection. The present work proposes a selection criterion to guide the fast screening of the potential candidates for SLGHP working fluid. For this criterion, the temperature gradient of the working fluid along the SLGHP in operation is adopted as an index to evaluate the effect of working fluid on …