Second law analysis of zeotropic mixtures as working fluids in organic Rankine cycles

To improve the thermal performance of the basic organic Rankine cycle (ORC) several modifications are proposed in literature. One of these modifications is the use of zeotropic mixtures as working fluids. Zeotropic mixtures as working fluids have the ability to better match the heating (cooling) temperature profiles of the heat source (heat sink). As a result the irreversibilities associated with a finite temperature thermal heat transfer are reduced. In a previous study a first law mixture selection method was proposed, considering most of the commonly used hydrocarbons and siloxane substances as components in various mixture concentrations. This paper extends the previous study by comparing the basic ORC and the ORC with zeotropic mixtures as working fluids based on a second law analysis. The zeotropic mixtures under study are: R245fa-pentane, R245fa-R365mfc, isopentane-isohexane, isopentane-cyclohexane, isopentane-isohexane, isobutane-isopentane and pentane-hexane. The exergetic efficiency, defined as the ratio of shaft power output and input waste heat exergy, is used as optimization criterion. Furthermore, the irreversibilities associated with the different components of the ORC are assessed under different mixture compositions. Next, the optimization potential when using zeotropic mixtures is thoroughly discussed. The results show that the evaporator amounts for the highest exergy loss. However, the best performance is achieved when the condenser heat profiles are matched. A relative increase of exergetic efficiency between 7.1% and 14.2% is found for a waste heat source at 150 °C. The ORC with isobutane-isopentane as working fluid has the highest second law efficiency (32.05%) under optimal mixture concentration and evaporation pressure