An enhanced super-long gravity heat pipe geothermal system: Conceptual design and numerical study

Recently, it was reported a novel method of deep geothermal energy exploitation by using the super-long gravity heat pipe (SLGHP) in a single-well geothermal system. However, the low heat transfer rate from the geothermal formation outside the heat pipe is one main factor limiting the heat extraction rate of the SLGHP system. In this respect, the concept of a novel enhanced super-long heat pipe system (ESLHPS) is proposed, which encompasses a super-long gravity heat pipe and a heat transfer enhanced region. The heat transfer enhanced region, built around the evaporation section of the heat pipe, features near-well fracture reservoir filled with high thermal conductivity phase change composite. The phase change composite remains as a semiliquid mixture during operation, eliminating the thermal contact resistance between the heat pipe and the reservoir. To evaluate the thermal performance of the proposed system, it is employed a numerical model, and the key parameters including those of the heat transfer enhanced region are carefully analyzed. In addition, an insulation layer is set around the heat pipe to make a specific adiabatic section. It is found that the heat transfer enhanced region can significantly improve the thermal performance of the SLGHP system. The overall thermal performance of ESLHPS is found to be dependent on the thermal conductivity, length and radius of the heat transfer enhanced region. The insulation layer can effectively reduce the heat loss of ESLHPS, and the thermal insulation shows the best performance when its length just equals the length of heat pipe minus the optimum length of the heat transfer enhanced region. In addition, the design procedure for the ESLHPS is also proposed, and it leads to a realistic strategy for the design of single-well SLGHP geothermal systems. The results obtained in this study under idealized conditions offer guidance towards the optimization of the system design.