Analytical considerations on optimization of cascaded heat transfer process for thermal storage system with principles of thermodynamics

Cascaded Thermal Storage (CTS) technique is an efficient solution for storing solar thermal energy of high quality. In this paper, heat transfer rate and thermodynamic irreversibility are combined for optimizing cascaded PCM thermal storage system based on entransy and entropy. Optimal solutions for temperature of CTS unit are obtained based on entransy and entropy, which cannot only present a benchmark for similar research, but also be used for guiding phase change material (PCM) selection of multi-PCM thermal storage device. Limitations for entransy and entropy optimizations are put forward, and comprehensively analyzed. A parametric study is performed for the optimal thermal performance. The optimal PCM temperature based on entropy is geometric while that based on entransy is linearly distributed. By increasing stage number or heat transfer parameter, optimal performances for optimizations of entransy and entropy are promoted. Results show that thermal efficiency in entransy optimization is greater than that in entropy optimization while the exergy efficiency in entropy optimization is superior to that of entransy optimization. Stage number should be adjusted by balancing earnings and costs for cascaded design of thermal storage system. Heat transfer enhancement is essential for performance promotion of cascaded system and selection of optimization principles. (C) 2018 Elsevier Ltd. All rights reserved.