Cyclic behaviors of the molten-salt packed-bed thermal storage system filled with cascaded phase change material capsules

A transient, one-dimensional dispersion-concentric model to numerically study the cyclic behaviors of the molten-salt packed-bed thermal energy storage system filled with cascaded phase change material (PCM) capsules is presented. Three different storage systems are investigated which include the non-cascaded system, the cascaded systems with 3 and 5 cascaded phase change temperatures (PCTs). Detailed characteristics of heat transfer between molten salt and the packed PCM capsules are discussed, and various numerical results are presented, including the temperature distributions of molten salt and PCM capsules, the variations in the molten-salt outlet temperature, the accumulated efficiency of each cycle. The results show that the non-cascaded system suffers from a low charging ratio and a long charging time due to the constrains of PCT while the cascaded systems especially with 5 cascaded PCTs are found to have both a fast discharging rate and a fast charging rate. The cyclic process of the cascaded system with 5 cascaded PCTs can reach to a repeatable state after some cycles, at which high accumulated efficiencies can be achieved. It is also found that the practical storage capacity of the storage system with cascaded PCM capsules depends highly on the threshold temperatures to stop the charging/discharging process.