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 optimisation of design is carried out for each part of the device, the system's own defects cannot be avoided. In periods of low solar radiation density, the heat transfer fluid cannot continue to heat the hot water in the tank, resulting in a loss of heat. According to published literature in 2013, the solar collector temperature drops lower than that of the hot water in the storage tank after 15:00, when any remaining solar energy gathered will not be able to be stored resulting in considerable energy loss, as for the single-phase PCM storage. Hence, the authors suggest the use of cascade PCM storage to maximise the energy absorbed and stored for off-radiation use. Different PCMs with various melting temperature were used to storage the energy along the solar collect surface temperature curve. The results of the cascade PCM heat storage were compared with the single-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 the more PCM’s used, the better heat storage performance can be obtained. 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 provides reference for the application of cascade PCM heat storage technology in the field of solar thermal storage.