Experimental and numerical study on the thermal performance of ground source heat pump with a set of designed buried pipes

The configuration of pipes is one of the most important factors for the thermal behavior of the Ground Source Heat Pump system (GSHPs). However, few studies have considered the influences of pipe shape in the perspective of turbulent diffusion and pressure loss effects. This paper investigates thermal performance of ground heat exchanger (GHE) with a set of designed U-tubes (i.e., either smooth or with different petals) by using experimental measurements and computational fluid dynamics (CFD) simulation. Thermal performance is based on temperature difference between inlet and outlet of U-tubes as well as the impact on the surrounding soil. First, an experiment (U-tubes buried with a depth of 100 m) was conducted to validate the CFD k-ε RNG model for a smooth U-tube, which agrees well with experiments with suitable boundary conditions and appropriate parameters. Next, CFD simulation tool was mainly employed for other evaluations. Different inlet velocities of U-tubes were considered, and we found the heat transfer per unit borehole depth QL increases with the increase of inlet velocities, e.g., the QL of smooth U-tube was 35 W/m at inlet velocity of 0.2 m/s, and increased by 43% when the velocity was increased to 1.2 m/s. Although U-tubes with different petals would increase the turbulent heat diffusion effects, the flow resistance is also increased due to the increase of pressure loss coefficient, further leading to the declined advection effects on heat exchange. It is shown that the thermal performance of smooth U-tube at different velocities is always better than the petals type. Finally future design of guidelines is given in discussion.