Thermo-economic analysis of the transcritical organic Rankine cycle using R1234yf/R32 mixtures as the working fluids for lower-grade waste heat recovery

Abstract

The aim of this study is to investigate the thermo-economic performance of the transcritical organic Rankine cycle (TORC) system using R1234yf/R32 mixtures as the working fluids for the lower-grade waste heat recovery (WHR). The components of R32 and R1234yf are selected due to their zero ozone depletion potential, lower global warming potential and complementary thermodynamic properties. The influences of the mass fractions of R1234yf/R32 mixtures, isentropic efficiencies of the expander, condensation temperature, turbine inlet pressure and temperature on performances have been investigated. The results show that R1234yf/R32 at optimal mass fraction is superior to pure R1234yf and pure R32 by 1.46% and 4.88% in thermo-economic performance, respectively. The optimal compositions of mixtures and the optimal temperature entropy diagrams are obtained to fit the various isentropic efficiencies possessed by the different types of expanders. In thermo-economic evaluation, the lower the condensation temperature is, the larger the optimal mass fraction of R32 in the mixtures will be. The increase of optimal expander inlet pressure and temperature are proportional to that of mass fraction of R32 in mixtures. The relationships among mass fraction, optimal expander inlet pressure and temperature and performance are expressed as the correlations for convenient design in lower-grade WHR.