Review on transportable phase change material in thermal energy storage systems

Thermal energy storage systems provide a means to store energy for use in heating and cooling applications at a later time. The storage of thermal energy allows renewable sources of energy to be stored if the time of demand does not coincide with the time of production. It also enables access to off-peak electricity tariffs offered during times of low electricity demand. Storage systems can be charged during the low-cost tariff period and provide heating or cooling later when required. This benefits consumers with lower electricity costs and power generators with demand levelling. Thermal energy storage systems predominantly store heat as sensible heat in a substance. However, during a phase change heat energy can be stored as latent heat. Phase change material (PCM) thermal storage systems can store a greater amount of thermal energy per unit volume than sensible heat storage systems. Historically a drawback of using PCMs as a storage medium has been the low rates of heat transfer. Heat transfer enhancement techniques studied have included the use of additional metallic material and increasing heat transfer surface area such as fins to improve heat transfer rates of the PCM. Although these techniques are effective, they add significant cost and reduce the compactness factor of the thermal energy storage system. Recent research has been conducted on heat transfer enhancement that makes use of moving or transporting the PCM. This method is not only effective for increasing the heat transfer; it is less expensive and maintains a high compactness factor for the thermal energy storage system. This review paper presents the different heat transfer enhancement techniques reported in the literature. It also summarises the research conducted on phase change storage systems where the PCM is moved in the storage system.