Super-long gravity heat pipe geothermal space heating system: A practical case in Taiyuan, China

Using super-long gravity heat pipes (SLGHPs) to exploit deep-earth geothermal energy has indicated its technical superiority and viability; yet there is no practical application so far. The present work reports an SLGHP geothermal space heating system constructed in Taiyuan, China, which has two SLGHPs (2020 m and 2180 m long, respectively) in combination with a single heat pump. The two wells are located only ∼30 m apart, and the well-log temperature at 2000 m depth is around 63 °C for both. Temperatures measured by the optical fiber arranged along the SLGHP outer wall show remarkable uniformity, indicating the good performance of SLGHPs. Testing with the SLGHP system finds that the slightly longer SLGHP extracts 70 % more heat than the other one. Analyses reveal that the heat transfer in the geothermal formations surrounding the shorter SLGHP approximately follows the heat conduction regime while the convection of groundwater contributes less; the high yield of the slightly longer SLGHP is due to an interesting downhole heat transfer enhancement mechanism arising from the interlayer crossflow of groundwater. Further, a numerical model is developed to predict the SLGHP geothermal system's performance in 120-days’ operation. It is found that this system can output 1 MW of heat, sufficing space heating of ∼25,000 m2 buildings. The simulated 20-years’ operation indicates the thermal output degradation is 20.4 % for the shorter SLGHP, whereas it is only 6.8 % for the slightly longer SLGHP; the downhole groundwater crossflow lowers the SLGHP system degradation rate.