A thermal storage system using multiple phase change materials (PCMs)has much better heat transfer and thermodynamic properties than that using single PCM.The phase change temperatures of multiple PCMs have an important effect on the performance of the thermal storage system.To obtain generalized results

the analysis method for thermodynamic exergy is employed

with the type of heat exchanger and heat transfer process neglected.The theoretical equation for the optimum phase change temperature of each PCM is derived.The optimum exergy efficiency is calculated for different number of PCMs.The results indicate that the exergy efficiency could be up to 80% with 4 or more PCMs

which is about 50% higher than that using single PCM.With thermal energy at 573.15 K as example

the optimum temperatures and the maximum exergy efficiencies for one to four PCMs are calculated

and the optimum combination of multiple PCMs is given on the basis of theoretical results.