Numerical simulation on the performance of energy pile considering effect of unsaturated soil condition

Abstract

This study presents a numerical simulation model of energy pile in unsaturated soil condition. Conjugated heat transfer through a vertical ground heat exchanger was simulated with three dimensional finite elemental analysis using transient conductive heat transfer model. Intermittent operation mode was modeled to be a forced convective heat boundary conditions which represented the convective heat transfer of circulating fluid. To take into account the unsaturated ground condition, non-homogeneous soil thermal conductivity and heat capacity, which vary with depth above the ground water table, were applied to the ground elements using a three phase model. The developed model was validated with an analytical infinite line source model. The results of comparison analysis show that there is a limitation of analytical model to predict the performance of intermittent operation mode because of big difference due to steady-state borehole assumption of analytical solution within few hours. However, the numerical results showed relatively good agreement with the analytical solution after few hours. Parametric studies with various ground water table conditions were also conducted to investigate the influence of unsaturated soil conditions on the performance of the ground heat exchanger. Simulation results considering unsaturated soil showed up to 40 percent drop of mean heat exchange rate compared to that for saturated soil condition. These results indicate that taking the unsaturated soil condition into account can provide more accurate design and performance evaluation for operation of vertical ground heat exchangers.