The Effectiveness of Borehole Heat Exchanger Depth on Heat Transfer Rate, Study with Numerical Method Using a CFD 3D Simulation

Abstract

Excess solar thermal energy is available, while in winter, when thermal energy is needed for heating systems, its quantity is usually not sufficient. There are different options to cope with the seasonal offset of thermal energy supply and demand. One of these options is borehole thermal energy storages (BTES). Borehole thermal energy storages coupled with ground source heat pumps have been widely developed and researched. The major disadvantage of (BTES) is the initial capital cost required to drill the boreholes. Geothermal energy piles were developed to help offset the high initial cost of these systems. This study investigates thermal performance of vertical ground heat exchangers with constant inlet water temperatures and deferent borehole depths. The performances of three models of U-tube with depth of 100m, 60m, and 30m are evaluated by numerical method using a CFD 3D simulation. The simulation results show that heat transfer rates decrease in the heating mode for 100m depth, and show that the best borehole depth regarding to heat transfer rate efficiency is 60m depth borehole. However for heat storage capacity the model of 100m depth is the best. The results show that increasing the depth of borehole heat exchangers lower the heat exchange efficiency with the ground. By comparing with 100 m depth, the heat transfer rates per unit borehole depth lower of 3.1% in 60 m depth. According to all results, it is highly recommended to construct medium depth around 60 m depth of borehole with U-shaped pipe configuration, due to higher efficiency in heat transfer rate.