Experimental Charging Characteristics of Medium-Temperature Cascaded Packed Bed Latent Heat Storage Systems

Metallic phase change materials are attractive candidates for use in thermal energy storage systems, but their high cost per unit volume may prohibit their usage. To reduce the cost, layers of metallic phase change materials may be cascaded with layers of other cheaper phase change materials in a thermal energy storage system. Experimental charging characteristics of four medium-temperature packed bed latent heat thermal energy storage systems at different flow-rates (4, 6 and 8 ml/s), and set heater charging temperatures (260, 280, 300 °C) are compared in a 7 l storage tank using sunflower oil as the heat transfer fluid. The first system utilises eutectic solder (Sn63Pb37) capsules as the only phase change material. The second system, cascaded thermal energy storage 1, comprises of eutectic solder capsules at the top and erythritol capsules at the bottom in equal volumes. In the third system, cascaded thermal energy storage 2, eutectic solder capsules are at the top, while adipic acid capsules are at the bottom in equal volumes. The fourth system, cascaded thermal energy storage 3, is a cascade of eutectic solder capsules at the top, adipic acid capsules in the middle and erythritol capsules at the bottom in equal volumes. Cascaded system 3 is more stratified showing the highest average stratification number of about 0.54. It also shows higher average energy and exergy storage rates of between 660 W and 157 W, respectively, under the different charging conditions. The single PCM system shows the worst thermal performance in terms of the average stratification number which is around 0.38. However, cascaded system 1 with the lower melting temperature at the bottom (erythritol) shows lower energy and exergy rates compared to the single PCM suggesting that lower melting temperature PCMs are not very efficient in cascaded systems. Cascaded system 2 with the higher melting point adipic acid at the bottom shows improved performance when compared to the single PCM system and cascaded system 1. Substituting layers of adipic acid and/or erythritol for a partial volume of the metallic eutectic solder (Sn63Pb37) reduces the cost and improves the charging performance of the storage system as shown by the 3 PCM system.