Thermal conductivity enhancement on phase change materials for thermal energy storage: A review

Phase change energy storage technology, which can solve the contradiction between the supply and demand of thermal energy and alleviate the energy crisis, has aroused a lot of interests in recent years. Due to its high energy density, high temperature and strong stability of energy output, phase change material (PCM) has been widely used in thermal energy systems. The aim of this review is to provide an insight into the thermal conduction mechanism of phonons in PCM and the morphology, preparation method as well as thermal conductivity of composite PCMs. Phonon thermal conduction mechanism is suggested to cover three forms: phonon-phonon scattering, phonon-defect scattering and phonon-boundary scattering. Then the microcosmic factors affecting the thermal conductivity of composite PCMs are analyzed. The research progress of adding three-dimensional, two-dimensional, one-dimensional and zero-dimensional structure additives to PCM is reviewed. Besides summarizing the preparation method and microstructure, the thermal conductivity of the composite PCMs are also analyzed from the aspect of phonon thermal conductivity mechanism. Meanwhile, some novel materials, including metal organic frameworks (MOFs), titanium dioxide foam, highly graphitized network carbon, graphene foam and hexagonal boron nitride (HBN) nanoparticles, are proposed for the additives of heat storage materials. Ongoing research and development studies indicate that the challenges of the improving the thermal conductivity of PCM focus on the aspects of clarifying the phonon scattering mechanism in PCM, increasing the number of thermal conductivity chains and broadening the thermal transmission channels. Booming progress illustrates that the exploration of high performance PCM is an extremely valuable and scalable option for storing industrial waste heat and solar energy, especially for constant temperature storage and utilization. To bring the phase change heat storage solution into a broader market, more intensive studies in fields of phonon thermal conductivity mechanism, development of high performance composite PCMs and efficient and compact phase change heat storage system are still required.