Thermal conductivity enhancement of porous shape-stabilized composite phase change materials for thermal energy storage applications: a review

How to realize the efficient use of the renewable energy sources is a present-day challenge to the technologists and has become an important issue in their large scale applications. Energy storage not only reduces the mismatch between supply and demand but also improves the performance and reliability of energy systems and plays an important role in conserving the energy. Current energy storage techniques mainly include sensible heat storage, latent heat storage and chemical reaction heat storage. The researchers place emphasis on the latent heat storage due to its advantages of high heat storage density, little temperature fluctuation and easily controllable utility system. In principle, phase change materials (PCMs) are used for the latent heat storage to absorb and release large amounts of latent heat during their phase change process. Therefore, PCMs are the key factor for the development of latent energy storage technology and play the crucial role in exploring new energy and improving energy utilization. The solid-liquid transition is more efficient compared with the other transformations due to its high latent heat density and small volume change. However, the leakage of solid-liquid PCMs above the melting point from the thermal storage system still hinders their practical applications. Considerable efforts have been devoted to introducing the porous support and development of shape-stabilized composite PCMs to address this technical issue. During the melting or solidifying processes, the PCMs store or release latent heat, while the support materials confine the melted phase from leaking and keep the whole system in the solid state. Moreover, low thermal conductivity of PCMs may degrade the performance for energy storage and thermal regulation during the melting and freezing cycles and restrict their final applications. Therefore, the necessity to enhance thermal conductivity of porous shape-stabilized composite PCMs is evident. In this paper, the recent researches on the enhancement of conductivity of porous shape-stabilized composite PCMs were reviewed. We studied the thermal conductivity enhancement techniques, which included impregnation of PCMs into porous materials with high thermal conductivity, introducing of high conductivity nano-materials and porous support materials into PCMs, construction of hybrid composite for shape stabilized phase change materials. The evaluation of each thermal conductivity enhancement technique was discussed. Finally, we had provided a brief outlook and future challenges in enhancing thermal conductivity of porous shape-stabilized composite PCMs.