Applications of cascaded phase change materials in solar water collector storage tanks: A review

Thermal energy storage plays a vital role to solve the problems related to the intermittent nature of solar energy. Phase change materials (PCMs) have shown promising applications in different thermal energy storage systems. Cascaded Thermal Storage (CTS) PCMs (multiple PCMs in a descending order of their melting temperatures), added new advantages to the applications of PCMs in thermal energy storage systems. Solar water collectors (SWCs) that utilize PCMs have been used to achieve economic and environmental benefits via substituting large-scale demand of energy. This literature review focuses on the different developments of PCMs and CTSPCMs and their implementation in SWC storage tanks. The novel and most recent developments of PCMs and CTSPCMs utilized in solar storage tanks with SWCs, such as, multi-storage tank with cascaded PCM, packed bed storage units, different configurations of heat exchanger storage units, and other innovative storage tank designs, are reviewed and discussed. Furthermore, the existing studies that evaluated the PCM heat transfer improvement, thermal storage performance, thermodynamic optimization, and applications of cascaded thermal storage systems are also reviewed. The reviewed results in this work showed that the CTSPCM system can promote a significant improvement in the PCM heat transfer and stored rates of entransy, energy, and exergy, as well as lead to an increased storage energy and exergy efficiencies compared to the single stage PCM system. It is also revealed that: the multi-tank integrated with cascaded PCMs system is the best amongst the studied PCM storage tank systems within SWCs, for augmenting the storage capacity and improving the overall thermal efficiency of the water heating system. A noteworthy observations, important proportionalities, and future trends are also discussed for each technique.