Cyclic performance of cascaded latent heat thermocline energy storage systems for high-temperature applications

Abstract

The cyclic performance of cascaded latent heat thermocline energy storage systems for high-temperature applications is presented. To investigate this performance, a transient, two-phase numerical model is established based on a concentric-dispersion approach. The storage unit is filled with encapsulated phase change materials (PCMs) having different melting points. Molten salt serves as a heat transfer fluid. The general behavior of the three-PCM cascade system is firstly analyzed. Secondly, the effects of PCMs filling fractions of two and three cascade structures are studied. Lastly, the effects of latent heat and capsule diameter are investigated based on the optimal cascade structure performance of the above analyses. The results show that as the volume fraction of bottom PCM increases than top PCM, the system's performance enhances in terms of heat storage and release. The arrangement wherein the bottom PCM occupies half of the bed height has the highest capacity and total utilization ratios of 83% and 40.5%, respectively. The effect of latent heat has a considerable impact on cyclic performance. The scenario wherein top and bottom PCMs have high latent heat, exhibits the greatest benefits compared to other scenarios. The results also show that the smaller capsule size achieves the best behavior. When the capsule diameter increases from 1.9 cm to 4.2 cm, the total storage capacity decreases by 34.8%; the energy recovered declines by 10%; the total utilization ratio decreases by 13.77%, and the capacity ratio decreases by 1.7%.