Comparative Numerical Analysis of Thermal Performance in Modified Horizontal U-Tube Ground Heat Exchanger
Abstract
Geothermal energy stands out as a reliable and sustainable solution for heating and cooling buildings, delivering consistent baseload power unaffected by weather or geopolitical factors. Moreover, its subtle design blends seamlessly with the surroundings. This research evaluates the thermal performance of horizontal ground heat exchangers (GHEs), comparing around 17 configurations. A trench with dimensions of 28 m × 3 m × 5 m was modeled, maintaining uniform trench and pipe lengths of 18.35 m and 37 m, respectively, for all configurations. ANSYS FLUENT was used to simulate a 72-hour continuous cooling operation where water was circulated at 2 L/min with an inlet temperature of 303 K, while the soil's initial temperature was 290 K. A time-step size of 60 s was used, with convergence achieved when the energy-equation residuals fell below 10-6. After 72 hours of operation, C10 (inner diameters 40.0/61.6 mm and outer diameters 48.0/69.6 mm) and C16 (inner diameters 45.0/55.9 mm and outer diameters 53.0/63.9 mm) reached similar outlet temperatures, with C16’s average heat transfer rate being marginally lower than that of C10, a difference small enough to be disregarded. Furthermore, the average heat transfer rates per meter trench for C10 and C16 were 38.17 W/m and 38.15 W/m, respectively, indicating effective space utilization and cost efficiency. Additionally, this configuration demonstrated superior thermal performance compared to other modified designs, requiring less pumping power due to reduced pressure loss. Increasing the flow rate from 0.5 to 2 L/min increased the average heat-transfer rate from 378.62 to 700.48 W, representing an 85.01% improvement, although the gain decreased at higher flow rates. Among the investigated pipe materials, HDPE achieved the highest heat-transfer rate of 815.57 W, 16.43% higher than PVC. For the investigated ground materials, sand provided the highest heat-transfer rate of 892.88 W, 27.47% higher than clay. Though U-tubes are generally efficient, diameter-optimized configurations can offer better performance for practical applications.