The local non-equilibrium heat transfer in phase change materials embedded in porous skeleton for thermal energy storage

Thermal energy storage with phase change materials (PCMs) is a promising technology to improve energy efficiency in the fields of renewable energy, electronic cooling, buildings, etc. However, the low thermal conductivity of PCMs decreases the heat transfer rate and leads to low energy efficiency. Adding a porous skeleton with high thermal conductivity is a promising method to solve the above problem. However, there are thermal non-equilibrium effects in the PCMs embedded in the porous skeleton due to the significant difference in the thermal diffusivity between porous skeleton and PCMs. In this paper, a novel 3D hexahedron model is proposed showing the microstructure characteristics of a porous skeleton, such as volume fraction, specific surface and pores. The effects of the porous skeleton on non-equilibrium heat transfer and coupling factors between PCMs and the porous skeleton are disclosed. The results show that the strength and duration of local non-equilibrium grow up with the increase of the volume fraction and the connected area fraction of PCMs. For the element model with the max area fraction (EM1), the non-equilibrium effects are obvious when the conditions are below: The volume fraction of PCM is >80 %; the dimensionless time is <0.2; the dimensionless temperature difference is >0.01. The two-temperature model can predict the non-equilibrium heat transfer process, and the coupling factor decreases almost linearly with the increase of the volume fraction and the area fraction of the PCM. This paper provides an insight into the accurate design and scale application of PCMs embedded in the porous skeleton.