Numerical analyses of energy screw pile filled with phase change materials

Abstract

Embedding heat exchangers into a screw pile can form a cost-effective energy pile with a fast installation capability. However, better solutions to handle thermal waves and thermal interferences among energy piles are still required. This work aims to solve the issues by proposing a novel concept of an energy screw pile filled with mixtures of phase change materials (PCM) such as paraffin and “solid” grouting. Numerical simulations are conducted to investigate the utilisation of PCM energy screw piles on reducing fluid and ground temperatures, considering different constituents in PCM-solid mixtures, moisture conditions, phase change temperatures Tpc and operation schemes. Results demonstrate that a mixture with higher effective thermal conductivity λeff slows down the PCM phase transition and results in a lower fluid temperature feeding the ground source heat pump (GSHP). Dry mixtures with higher PCM content benefit the fluid temperature reduction, while a lower PCM content in a wet mixture is also satisfactory. Higher specific heat capacity allows pile back-filling to absorb more heat and reduce the thermal radius of influence, while over-enhancing its λeff might worsen thermal interference. The selection of the PCM phase change temperature Tpc should comply with the GSHP system operation scheme and the project aim.