Measured thermal characteristics of a phase change energy pile in unsaturated clay

The use of a phase change material (PCM) encapsulated in a steel ball in place of the coarse aggregate in concrete can improve the energy density and heat transfer in the energy pile which will reduce the underground space needed for the heat transfer. Cooling-heating loads are used in a traditional concrete energy pile and a PCM energy pile in a container containing unsaturated clay to experimentally study the thermal response of the piles and surrounding soil. The results show that the temperature influence range in the soil surrounding the phase change pile extends out to about 1.5 times the pile diameter during the cooling-heating processes with a larger temperature difference between the PCM energy pile inlet and outlet than with the traditional concrete energy pile, which indicates a larger heat transfer rate. The temperature differences in both the PCM pile and the traditional pile during heating are less than during cooling which shows that the heat transfer rates during cooling are larger than during heating for the same flow conditions. The results also show that the PCM increases the uneven temperature distribution during the cooling in the vertical and horizontal directions in the pile. In addition, irreversible settling of unsaturated clay is observed at the soil surface due to temperature induced soil consolidation and drainage.