Parameters optimization of borehole and internal thermal resistance for single U-tube ground heat exchangers using Taguchi method

Abstract

The borehole thermal resistance and internal thermal resistance are both the significant design factors for the ground heat exchanger (GHE). Careful design procedures are required for these two values to decrease GHE’s length and initial cost. In this study, the effects of eight parameters (flow rate, pipe size, borehole diameter, pipe-pipe distance, layered soil, grout thermal conductivity, pipe thermal conductivity, and borehole depth) on the value of borehole thermal resistance and internal thermal resistance are investigated. In order to implement the analysis, L32 orthogonal array has been established. An improved 3D numerical model with the four-resistance model is applied to calculate corresponding thermal resistances in all cases. Furthermore, the Taguchi method is carried out to obtain the optimal scenarios of parameters combination. Results using the optimal parameters set reveal 67.64% increase in the borehole thermal resistance and 148.29% decrease in the internal thermal resistance. The heat transfer performance can be enhanced by 9.63% at least, up to a maximum of 77.07%. The analysis of variance (ANOVA) technique is carried out lastly to figure out the relative importance of tested parameters. The results indicate that the pipe-pipe distance has the most significant impact on the borehole thermal resistance, whereas the flow rate and layered soil almost play no role in it. The pipe size and flow rate are important driving parameters in determining the internal thermal resistance while the layered soil and borehole diameter’s impact can be nearly neglected.