Research on the natural circulation characteristic of deep borehole heat exchanger and the influences on the water circulation resistance

As deep borehole heat exchangers (DBHEs) extract heat from geothermal energy with depth of 2-3 kilometers, the circulation water pressure drop is larger than that of shallow-depth borehole heat exchangers, influenced by the water flow rates. This paper conducted field tests and simulation analysis to study the heat transfer performance and water circulation resistance of DBHE in coupled, where the natural circulation characteristic has been discovered and analyzed quantitatively. Results show that the water temperature and density variation along DBHE forms the driving force of natural circulation. For mechanical flow rate of 6.0 kg/s and inlet water temperature of 20.0 degrees C, the natural circulation flow rate reaches about 2.2 kg/s with transient heat extraction power of 78.5 kW, without energy consumption of water pumps. And the larger inlet water temperature, smaller mechanical water flow rate, higher inner tube thermal conductivity coefficient and larger depth of DBHE all contribute to the larger natural circulation water flow rate. In addition, the natural circulation could effectively decrease the comprehensive water pressure drops of DBHE, which is about 47.3% smaller than the calculated value of traditional models. Thus the natural circulation characteristic has significant influence on the heat transfer performance of DBHE, and also on the energy performance of whole heat pump systems.