Physical Modelling of Thermal Load on an Energy Pile

Ground Source Heat Pumps (GSHPs) are a clean technology directed towards reducing dependence on the use of fossil fuels for heating and cooling of buildings by using the ground as a heat source or sink. Energy piles are one type of component GSHP that uses a closed loop system in foundation piles to transfer heat from the ground to a building or vice versa. The piles provide structural support while exchanging heat with the surrounding soil, but the thermal behaviour of the soil surrounding thermal piles and the effect of cyclic heating and cooling on the soil properties is not fully understood. This research was aimed at addressing the aspect of inadequate geotechnical input in energy pile design by focusing on the factors that govern heat flow within soils around an energy pile. The relationships between soil type, water content, temperature, overburden pressure, time and thermal cycles were investigated. A novel experimental test rig and laboratory procedures for operating it, were designed and developed to study heat dissipation radially by conduction from a linear heat source within a soil mass modelled to depict a scenario typical to that of an energy pile exchanging heat with the surrounding soil while being subjected to overburden pressure, to simulate practical use of energy piles in an office building. From tests conducted on soil samples in the rig, the results demonstrate a reliable system. An energy pile will perform suitably for cooling an office building, while continuous use of the ground as a heat sink will eventually lead to decreased water content and increased shear strength around the pile. The soil thermal diffusivity is mainly influenced by the soil composition. These and the other findings will be of interest to designers and other researchers as they are of practical importance in energy pile applications.