Optimization of double-stage latent heat storage unit in whole cycle with entransy analysis

Abstract

Double-stage latent heat storage (LHS) unit is believed as a valid design to improve performance by separating LHS unit into two portions filled with different phase-change materials (PCMs). In this paper, double-stage LHS unit is optimized with entransy analysis. Different from those in references, optimization is conducted for LHS unit in whole cycle, i.e. charging and discharging process. Charging rate and inlet temperature of heat transfer fluid (HTF) are constrained constant. All the heat stored in charging process is released in discharging process. The criterion formulas of optimum melting temperature match is derived. The performance, i.e. discharging rate and entransy dissipation of double-stage LHS unit optimized in whole cycle, is compared with those of single-stage LHS unit and those of double-stage LHS unit optimized in only charging process. It is concluded that optimum melting temperature match exists in double-stage LHS unit in whole cycle. Performance is enhanced in double-stage LHS unit optimized in whole cycle. Constraint of discharging rate equal to charging rate is not favored in entransy analysis. Comparing with those in single-stage LHS unit, discharging rate of double-stage LHS unit optimized in whole cycle is always larger and entransy dissipation is always smaller. The difference of entransy dissipation between that of double-stage LHS unit optimized in whole cycle and that of single-stage LHS unit increases with coefficient c. It is also concluded that the double-stage LHS unit optimized in whole cycle is not the same as that optimized in only charging process. Comparing with those in double-stage LHS unit optimized in only charging process, entransy dissipation of double-stage LHS unit optimized in whole cycle is smaller, but discharging rate is also smaller. The difference of entransy dissipation between that of double-stage LHS unit optimized in whole cycle and that of double-stage LHS unit optimized in only charging process decreases with coefficient c. Optimization results are discussed in typical heat storage cases. The results are helpful to optimal design and performance improvement of LHS unit.