Evaluation of the long-term performance of the deep U-type borehole heat exchanger on different geological parameters using the Taguchi method

In recent years, deep geothermal energy has become a solid option for building heating worldwide. For the densely populated urban area, a novel closed-loop system of the deep U-type borehole heat exchanger (DUBHE) has attracted attention in northern China. In order to achieve the evaluation of geological parameters on the long-term performance of DUBHE, thermal conductivity, geothermal gradient, specific heat capacity, groundwater flow direction, and Darcy velocity are selected in this paper and all the parameters are set in three levels. And orthogonal test L₁₈(3⁵) is established by adopting the Taguchi method for the reduction of simulation time cost. Three evaluation indexes are introduced to investigate the impacts of geological parameters on the performance of DUBHE and obtain the optimal set, including average outlet temperature T_out, maximum cumulative heat extraction amount Q_total and soil temperature decay rate D_soil. Results show that the geothermal gradient has the most significant effect on T_out and Q_total, and its contribution degree is 70.14% and 62.10% respectively. As for the D_soil index, the most influential parameter is specific heat capacity and the corresponding contribution ratio is 92.41% determined by the analysis of variance (ANOVA) method. For the optimized combination by adopting matrix analysis, the T_out reaches 33.92 °C, the Q_total is 284.08 TJ and the D_soil of the optimized scenario is 12.66% after the long-term operation. In addition, according to the orthogonal test results, this study intuitively discusses the potential of carbon emission reduction for the DUBHE under the typical geological conditions in northern China. The study results presented in this work can guide the system design of DUBHE and serve as the reference for the decision-maker in the application of deep geothermal heating technology.