Comparison of cascade PCM storage with single-phase sensible or latent heat storage for solar thermal energy

Solar water heating (SWH) systems have a wide range of applications in the domestic and industrial sectors. Although the optimization design is carried out for each part of the device, the system's own defect throwing cannot be avoided. In the period of low solar radiation density, the heat transfer fluid cannot continue to heat the hot water in the tank, resulting in a lot of heat loss. From the published literature in 2013, the solar collector temperature is lower than that of the hot water in the storage tank after 15:00, in this case the remaining solar energy will not be able to be storage, resulting in considerable energy loss, the same for the single-phase PCM storage. Hence, the authors suggested the cascade PCM storage to maxim the energy to be absorb and stored for the off radiation use. Different PCMs with various melting temperature will be used to storage the energy along the solar collect surface temperature curve. The results of the cascade PCM heat storage are compared with the sing-phase sensible and latent heat storage. The results show that the combination of cascade PCM technology and SWH system can improve system efficiency and solar energy fraction, and more number of PCM can get better heat storage performance. For a single-phase PCM storage mode, if the melting point of PCM is too high, the liquid fraction is too low and the PCM cannot store much energy; if the melting point is too low, the energy can only be obtained in the low temperature range. This paper can provide reference for the application of cascade PCM heat storage technology in the field of solar thermal storage.