Numerical performance analysis of modified vertical U-tube ground heat exchangers by adjusting the impact of leg spacing

• Vertical U-tube ground heat exchanger (GHE) is modified by varying leg spacing between inlet and outlet. • Modified GHEs also include the combination of straight and spiral parts in inlet or outlet pipes. • Heat transfer rate increases by increasing leg spacing for upper sections without changing leg spacing of lower section. • Combined straight and spiral pipe GHE is promising to enhance thermal performance. Geothermal energy is a reliable and renewable form of energy that offers steady cooling and heating of buildings without relying on sunlight or wind, avoiding price fluctuations. Ground heat exchangers (GHEs) are designed to utilize geothermal energy for building heating and cooling. The present study conducted a numerical investigation on the thermal performance of Modified Vertical U-tube GHEs. Enhancement of the thermal performance of various configurations of GHEs by adjusting the leg spacing and incorporating spiral configuration of U-tube GHEs. The upper section's leg spacing varies to reduce thermal interference and the lower section's leg spacing is kept constant. Some designs incorporated a combination of spiral and straight U-tubes. All GHE models were considered a 20-meter depth. Performances of these GHEs are evaluated with polyethylene, high-density polyethylene, and low-density polyethylene tubes and boreholes filled with silica sand. The surrounding soil consisted of clay. In some configurations, the lower section was modeled with spiral geometry after 10 meters in depth. The design parameters for the spiral section were 70 mm and 100 mm diameter. The simulations were conducted using the ANSYS FLUENT 22.2 software program for cooling operation. The outlet temperature was lower for spiral configurations than for the other configurations after being simulated for 72 hours. Compared to the traditional U-tube configuration, the heat transfer rate achieved by each arrangement is significantly higher. Results indicated that the combination of spiral shape with traditional shape led to a 31.2% increase in average heat transfer rate. Leg spacing variation opens up the opportunity to combine multiple geometries in a single borehole, also providing the solution to reduce the thermal interference between the inlet and outlet pipe.