Heat extraction from hot dry rock by super-long gravity heat pipe: A field test

Recent theoretical studies dealing with the novel scheme for earth-deep hot dry rock geothermal energy extraction based on the use of a super-long heat pipe indicate its superior technical viability. The present work reports on a field test using a 3000-m heat pipe to extract hot dry rock geothermal energy conducted with the aim of demonstrating its practical feasibility. The in-house developed heat pipe uses deionized water as working fluid and produces steam with a maximum temperature of ∼90 °C, when the average underground formation temperature around the subsurface heat pipe is 95.6 °C. During the 30-day continuous heat mining process, the heat extraction rate achieved an average value of 190 kW with no obvious downward trend. After one month of continuous heat production, a one-week heat recovery can restore the system temperature to just slightly lower than the initial value. The thermal response of the system to different condensing temperatures was also tested. The results show that the heat extraction rate can be increased by decreasing the condensing temperature, but this improvement is insignificant when the condensing temperature is 55 °C or lower. Furthermore, the experimental results show overall good agreement with the model predictions, which indicates that the heat pipe is performing close to the desired operating conditions considered in the simulation model, in particular: no liquid accumulation, no local dry-out, and no vapor-liquid entrainment. This performance is a major departure from that of the typical heat pipes, and it makes the super-long gravity heat pipe a leading technological candidate for earth-deep geothermal heat extraction.