Heat extraction from hot dry rock by super-long gravity heat pipe: Effect of key parameters

Extracting hot dry rock energy by super-long gravity heat pipe in a single-well is a novel technical scheme proposed recently. The potential superiority of this scheme is related to its environmental friendliness, economic and technologic feasibility, but the key parameters affecting its performance are not well understood. To this purpose, a detailed sensitivity analysis using a dedicated numerical simulation model is conducted for a 4000 m long gravity heat pipe with water as the working fluid. The analysis considers a wide range of working conditions with the aim of better understanding the effects of operating and structural design variables. It is found that the thermal performance is strongly enhanced with the increasing heat pipe diameter (100 mm–500 mm), which is attributed not only to the increase of heat transfer surface but also to the decrease of fluid flow resistance inside the heat pipe. Lowering the condensation temperature improves the thermal performance of heat pipe, whereas the effect gradually reduces and becomes insignificant when the condensation temperature is less than ∼50 °C. Thermal insulation shows the best positive effect when the insulation section reaches the position where the heat pipe temperature is just equal to the formation temperature. The low thermal conductivity of hot dry rock is confirmed to be a key bottleneck restraining the performance of the heat pipe system. The heat extraction rate can be greatly improved if the local equivalent thermal conductivity is enhanced in the region of 5 m in radius around the heat pipe. The acquired knowledge allows to develop a design strategy for practical super-long heat pipe systems taking into consideration the operating parameters and local geothermal conditions.