Heat extraction from hot dry rock by super-long gravity heat pipe: Selection of working fluid

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 the SLGHP thermal performance. The thermal performance of SLGHP using several typical working fluids is also evaluated via numerical simulation. Comparison between the numerical predictions and the fast-screening evaluations presents good consistency, indicating the validity of the proposed working fluid selection criterion. Both approaches reveal that ammonia would be preferable to water and methanol as the SLGHP working fluid for low source (hot dry rock) temperature and small diameter heat pipe; while water would be the best choice when the source temperature is high and the heat pipe has a large diameter. This selection criterion can act as a convenient and reliable tool for the decision process on the best-suited working fluid for an SLGHP tailored for actual geothermal energy exploitation use.