Multifactor optimization of medium and deep U-type borehole heat exchanger design using Taguchi method

Abstract

The medium and deep U-type borehole heat exchanger (MDUBHE) coupled with ground source heat pump systems has recently received extensive attention as a novel closed-loop geothermal energy utilization system. The heat extraction performance of MDUBHE is impacted by multiple factors, while previous studies mainly concentrate on the impacts of single short-term factors. Therefore, long-term multifactor optimization is a great need to improve the heat extraction performance of MDUBHE. In this paper, six impact factors of buried depth, horizontal pipe length, flow rate, inlet temperature, geothermal gradient, and ground thermal conductivity are selected through sensitivity analyses. The Taguchi method is creatively applied to optimize the multiple impact factors of MDUBHE. The percentage contribution of each impact factor on the evaluation parameters QL-a, COP-a, and Rd-15 is quantified using Analysis of Variance. The results indicate that the geothermal gradient has the highest impact on the average per meter heat transfer rate QL-a, with a percentage contribution of 39.21%. The inlet temperature has the highest percentage contribution of 29.77% for COP-a. The ground thermal conductivity is the main impact factor for the average heat loss rate Rd-15 with a percentage contribution of 80.75%. Comprehensively considering the evaluation parameters with same weights, the ground thermal conductivity has the most significant influence on the heat extraction performance of MDUBHE, with a percentage contribution of 46.15%. This study could provide theoretical guidance and technical support for the design and promotion of MDUBHE.