Analysis of isentropic mixtures for their use as working fluids in organic Rankine cycles

The selection of appropriate working media for organic Rankine cycles (ORC) is essential for the optimization of their performance. Dry fluids are widely used in existing ORC because, unlike wet fluids, they do not need to be superheated to avoid wet expansion that may cause damage to the expander. However, regeneration is sometimes needed for fluids with a highly dry behavior in order to improve the overall ORC efficiency, at the expense of increasing the total capital costs. On the contrary, isentropic fluids, with a nearly isentropic saturated vapor curve, overcome these two drawbacks. Because the number of single component fluids with isentropic behavior is scarce, we propose the use of isentropic binary mixtures, composed by a wet and a dry component. In this work, we selected several isentropic binary mixtures composed of hydrofluorocarbons, hydrochlorofluorocarbons and hydrocarbons from the Refprop database. The performance of these mixtures was evaluated in a simple saturated ORC model for source temperatures between (350 and 450) K and a fixed sink temperature of 290 K. The results were compared to those yielded by pure isentropic and dry fluids under the same source and sink conditions.