Optimization of geothermal interaction of a double U-tube borehole heat exchanger for space heating and cooling applications using Taguchi method and utility concept

Optimization results for thermal interaction of a double U-tube borehole heat exchanger (BHE) used along with a 5 ton capacity ground source heat pump system (GSHP), are discussed in this paper. Heat transfer from the BHE is computed using thermal resistance concept for space cooling and heating mode operations. The main objective of optimization is to achieve maximum heat extraction during space heating and maximum heat rejection during space cooling operations. For the purpose of optimization, eight control variables consisting of geometric parameters, thermo-physical parameters and mass flow rate are considered at three levels for a series connected double U-tube BHE of 120 m depth. For Taguchi method L-27 orthogonal array has been employed for the computation of heat transfer and S/N ratios for both modes of operations. Results obtained show that optimum values of 9270 W and 7210 W heat can be rejected to and extracted from the ground. Taguchi results are combined to obtain a single set of optimum levels of control variables using the utility concept. Mass flow rate of water in the BHE is found to be the most influential variable. For variation in the duration of cooling and heating mode operations, the optimum heat that could be rejected and absorbed becomes the same and equal to 6180 W for equal duration. It is observed that the heat transfer calculated from the utility concept is 11.11% and 42.86% less than those values calculated by Taguchi method for cooling and heating mode of operations respectively.