Optimization of Coaxial Borehole Heat Exchanger for Hot Wet Climates: Experimental and Numerical Approaches

In a previous study (Kahwaji et al., Heat Transfer, Vol. 53, No. 3, 2024, pp. 1474-1500), a numerical analysis was employed to optimize the performance of a ground borehole heat exchanger (BHE). Following the analysis, both experimental and numerical studies are conducted here to investigate the performance, efficacy, and feasibility of BHE to be utilized as a sustainable heat rejection method to reduce power consumption in air-conditioning (A/C) systems employed in hot and humid climates. An experimental setup is established, for the first time in Dubai, with five BHEs made of coaxial tube configuration with insulated inner pipe. The boreholes are drilled with 40 m depth and 0.09 m outside diameter. A water-cooled A/C system is selected and connected to the BHEs to provide a heat rejection load. An identical air-cooled A/C system with the same cooling load is installed for comparison. The time-dependent water-side pressure, temperature, flow rate, and power consumptions are measured under different experimental conditions. Additionally, the soil-side temperature, porosity, and permeability are measured at different depths. For validation, optimization, and deep investigation of the performance of the BHE, a numerical model simulating the system is developed. A couple of correlations and physical findings related to water and soil temperature distributions, heat rejection, and power saving are obtained. The BHE is proved to be effective in saving energy up to similar to 20% at high ambient temperature periods and an average of 6-11.7% during the daytime, demonstrating the efficacy and feasibility of the system.