Experimental study on thermal response of a PCM energy pile in unsaturated clay

Abstract

Energy piles are a new renewable energy technology that is suitable for use as a heat exchanger in ground source heat pump systems. In this study, hollow steel balls macro-encapsulated phase change materials (PCMs) were used for the development of concrete pile, the resulting pile is referred to as the “PCM energy pile.” A laboratory-scale PCM energy pile in unsaturated clay was constructed, and the thermal responses of the pile and surrounding soil subjected to different thermal operation modes were examined and compared with those of a traditional concrete energy pile. The test results showed that the soil zone experiencing temperature changes around the PCM pile is approximately twice the pile diameter. The PCM energy pile showed less temperature change but a much higher heat transfer power than that of the traditional energy pile. However, the heat transfer power was significantly influenced by the operation modes, and a reasonable flow rate was required to optimize the efficiency of the PCM energy pile. Water content in the soil decreased gradually because of water seepage and evaporation. The test results proved that the proposed PCM energy pile was an effective solution for improving the heat exchange capacity and saving underground space resources.