Effect of critical temperature and category of zeotropic mixture working fluids on the thermal performance in subcritical ORC

Abstract

Several thermodynamic properties are crucial in selecting the optimal working fluids and significantly affect the performance of the Organic Rankine cycle (ORC); one of them is the liquid-vapor critical temperature. Four different thermodynamic models were used; they differed by the definition of maximal (TEva) and minimal (TCond) temperatures. Three different scenarios were investigated in all models by picking components from different working fluid classes to prepare zeotropic mixtures. The main focus of our study was on the investigation of the effect of the critical temperature (Tcr) of the mixture and the class of the components (wet/wet, wet/dry, and dry/dry) on thermal efficiency (ηth) and net-work output (Wnet). Results indicate that occasionally, the zeotropic mixtures with higher critical temperatures (but not always) outperform other mixtures under given conditions. Consequently, it cannot be asserted that the higher thermal efficiency and net- work output are monopolized by the highest critical temperature of all mixed working fluids. Selecting a certain mixing ratio and the appropriate model (evaporator and condenser temperatures set as on the bubble and dew points) that matches the ORC application can assist in obtaining the highest thermal efficiency and net-work output.