Multi-factor optimization of coaxial borehole heat exchanger based on Taguchi and matrix method

The medium-deep coaxial borehole heat exchanger (MDCBHE) utilizes mid-deep geothermal energy for building heating via a heat transfer process. The heat extraction performance of MDCBHE is influenced by multiple factors. Previous studies mainly analyzed the short-term influence of major affecting factors on a single evaluation parameter. Long-term effect of multiple factors on multiple evaluation parameters has not been well investigated. Therefore, this study constructed a full-scale three-dimensional model to study the heat extraction characteristic of MDCBHE for a geothermal energy heating system. The Taguchi method was employed to analyze the influence degree and contribution rate of 9 influencing factors on 4 assessment indexes including heat exchange per meter (Q_L−5a), heat pump unit performance coefficient (COP_−5a), rock temperature decay rate (D_soil−5a) and buried pipe heat transfer attenuation rate (R_d−5a). The results show that the inlet water temperature has the most significant effect on Q_L−5a and COP_−5a, with a contribution rate of 35.27 % and 53.70 %, respectively. Rock and soil density and the thermal conductivity have the greatest influence on D_soil−5a and R_d−5, with a contribution rate of 43.48 % and 39.58 %, respectively. Finally, the matrix method was used for multi-index optimization to determine the influencing factors on the total contribution rate of the four evaluation parameters. The study results can provide guidance for the location and system design of MDCBHE.