Performance Optimization of Double U-Tube Borehole Heat Exchanger for Thermal Energy Storage

Abstract

This paper presents an optimization study of the thermal performance of a double U-tube borehole heat exchanger (BHE) with two independent circuits that can be used in borehole thermal energy storage. The study applies the Taguchi method and utility concept to obtain the optimum parameters for two objective functions: maximum heat transfer rate and thermal effectiveness of the BHE. A validated numerical heat transfer model with a fully implicit method is applied to compute the transient heat transfer in the BHE. The Taguchi optimization results revealed that the optimal factors (denoted with letters and numbers showing their levels) for achieving the maximum heat transfer rate and thermal effectiveness are A₁B₃C₂D₁E₃F₃G₃H₃ and A₃B₃C₂D₃E₃F₃G₁H₁, respectively. This resulted in an optimal heat transfer rate of 120 W/m and a thermal effectiveness of 69.3%. Using the utility concept method, a single set of optimal parameters (denoted by their levels as A₃B₃C₃D₂E₃F₃G₂H₃) is obtained to maximize the performance of the BHE. These parameters yielded an optimum heat transfer rate of 87.3 W/m and thermal effectiveness of 54.6%. Finally, analysis of variance (ANOVA) showed that ground thermal conductivity, the inlet temperature of the working fluid, and borehole depth are the most influential parameters affecting the performance of the BHE. The study provides crucial information for performance improvement, enhanced energy savings, reduced environmental impact, and optimization of a hybrid ground source heat pump system that can be integrated with borehole thermal energy storage.