Heat transfer characteristics of coaxial tubes type deep borehole heat exchanger in water-rich geothermal reservoir

According to the operation principle of the coaxial tubes type deep borehole heat exchanger (DBHE), a three-dimensional unsteady state heat transfer model coupled inside and outside of the borehole was established, based upon hydrogeological conditions of water-rich hot reservoirs with the buried depth of 1000-3000 m in Bohai Basin. The transient analytical solutions were obtained by applying Laplace and Fourier transform methods to calculate the vertical temperature profiles in the inlet (outlet) pipe and the grout of the DBHE and the excess temperature in aquifers. The mathematical model and the analytical solutions were validated by the experimental data determined from a demonstration project and the numerical simulation of the finite volume method (FVM). Based on the dual-continuum spatial coupling approach, the influence was performed to examine the seepage process of underground water on the heat transfer performance of the DBHE in water-rich hot reservoirs. The simulated calculation indicates that the average heat exchange capacity increment of the DBHE is up to 55 kW, when the quantity of the circulating water is stable at 30 m3/h and the Darcy velocity of underground water increases from 0 to 5×10-6 m/s in water-rich hot reservoirs. However, the average heat exchange capacity increases 34 kW meanwhile the circulating pump power consumption increases 20.6 kW, ignoring the seepage process, when the quantity of the circulating water is enhanced from 30 m3/h to 60 m3/h. Studies have shown that as the seepage velocity increases, the heat transfer mechanism in the thermal reservoir changes, thereby enhancing the heat transfer process of the deep well heat exchanger; at the same time, it reduces the influence of the circulating water flow on the heat transfer performance of the deep well heat exchanger. © 2021, Chemical Industry Press Co., Ltd. All right reserved.