A study on optimal composition of zeotropic working fluid in an Organic Rankine Cycle (ORC) for low grade heat recovery

Abstract

The Organic Rankine Cycle (ORC) is an interesting heat recovery alternative for low grade heat at the present time. In an ORC, the irreversibility of heat transfer in the cycle could be reduced by the application of zeotropic working fluid. In this work, the potential of zeotropic working fluid application in an ORC for low grade heat recovery was investigated. The types of fluid, the composition and the operating conditions that achieved the maximum net work output were determined through flowsheet modeling and optimization in Aspen Plus v.8.4 simulation software. The working fluids considered in this study were the components found in the 400-series refrigerant blends from REFPROP database. Among the group of pure working fluids in this study, the ORC using R-227ea provided the best performance in terms of net work output. In case of zeotropic working fluid, the optimum performance fluid was the blend of R-218/227ea/C318/245fa (32.1/13.4/38.8/15.7). Besides, when including the environmental factor into consideration, the optimum low-GWP blend consisting of R-290/152a/600a/601a (35.1/38.1/22.4/4.4) was determined. The possible binary and ternary blends of the resulting constituents were also investigated. In addition, the thermo-economic analysis of each working fluid was carried out and discussed.