Numerical investigation on thermal performance of vertically mounted spiral tube Ground Heat Exchangers (GHEs). All of these GHEs were installed in a borehole that was 20 meters in depth. Tube material of GHEs was polyethylene. The borehole was backfilled with sand silica and soil material around the borehole was clay. Aiming to improve thermal performance of uniform pitched spiral GHE, several modifications were made where total number of turns and spiral tube materials of GHEs were same. In some modifications, loops were densified in the down part of the spiral GHEs. The position of the exit straight pipe was changed in some modifications. The spiral's diameter, total number of turns, and borehole’s depth were set at 0.1398 m, 100, and 20 m, respectively. The effect of the ambient atmosphere on upper soil surfaces was neglected. The simulations were carried out with the ANSYS FLUENT software package. Water was used as the working fluid, with a  flow rate of 2 liters/min and an inlet water temperature of 300.15 K. The initial temperature of the soil was 290.85 K. After 72 hours of continuous operation, the water in the uniform pitched spiral tube GHE cooled to 297.2 K. In some modifications, the average heat transfer rate improved and provided a maximum 7.67% greater performance than uniform pitched spiral tube GHE 

performance analysis of U-tube Ground Heat Exchanger (GHE). Numerical simulation was done by using ANSYS Fluent software. For the purpose of enhancement of the thermal performance of U-tube GHE, the traditional design was modified. For the modification, the total length and material volume of the U-tube GHEs are kept same as the conventional U-tube model. Then the U-tube was partitioned in a manner that there is an upper portion and a lower portion. Diameter of the upper part was reduced and of the lower part was increased. Furthermore, a different design in which one of the legs of the U-tube was maintained smaller in diameter and the other leg was maintained large in diameter. All the modified designs are simulated for 72 hours of operation and for a flow rate of 2 liter/minute. For the different modifications, the heat transfer rate per meter borehole is 28.69W/m, 22.53 W/m, 21.32 W/m, 26.33 W/m, 30.34 W/m, 24.61 W/m respectively, whereas, the heat transfer rate per meter borehole for the U-tube is 21.18 W/m. Among the modified configurations, all the configurations give a higher heat transfer rate than the conventional U-tube configuration. Since the material volume of modified U- tube GHE is same as the conventional U-tube design and those models are capable of giving better performance, hence the modified configurations are more attractive than the conventional U-tube design