Effect of thermal interference on energy piles considering various configurations of heat exchangers

In general, the design of closed-loop vertical ground heat exchangers (GHEXs) is to determine the required borehole length for providing the heating and cooling load of a target building. However, the same design procedure cannot be applied to the cast-in-place energy pile since the configuration of the heat exchange pipe inside the pile is too varied and the size of the borehole is relatively large. In this paper, the effect of configuration and denseness of heat exchange pipe inside the energy pile was experimentally and numerically evaluated comparing with conventional GHEXs. First, field experiments were performed for six cast-in-place energy piles constructed in a test bed. The results showed that the cast-in-place energy piles could provide higher thermal performance per unit borehole length than the other conventional types of GHEXs by increasing the heat exchange area along with much longer heat exchange pipes. However, the over-tight layout diminished the improvement of thermal performance due to thermal interference between each pipe loop. In addition, the long-term thermal behavior of energy piles and changes in ground temperature were estimated with numerical simulations. With the simulation results, a design chart was provided for evaluating the effect of thermal interference on energy pile performance.