Analysis of Zeotropic Mixture in a Geothermal Organic Rankine Cycle Power Plant with an Air-cooled Condenser

The purpose of this research is to investigate a zeotropic working fluid mixture in terms of its performance in an organic Rankine cycle (ORC) and the heat transfer characteristics in an air-cooled condenser (ACC). The motivation of this study is that it is well known that the use of a mixture improves the efficiency of an ORC system. However, the behaviour of the mixture in the condensers is not well understood. A standard ORC unit utilising hot spring water to generate electricity is considered for the analysis. A mixture of two fluids, isobutane and pentane, is used as the working fluid. Numerical models of the ORC power plant and ACC are developed and simulations are conducted with the pure fluids and their mixtures with four different compositions. The optimum operating conditions of the system are estimated for each fluid, and they are then applied to a condenser, designed to use pentane, to calculate and compare the heat transfer parameters. The mixtures display a significant increase in the system performance compared to the pure fluids. A highest exergetic efficiency of 24.2% is achieved for the system using a mixture of 80% isobutane/20% pentane. However, with the exception of the mixtures having the isobutane composition between approximately 35% and 60%, all of the mixtures require an additional surface area of the condenser compared to that for pure pentane due to their relatively low heat transfer characteristics. Therefore, the design of the ORC system and ACC involves a compromise between the system efficiency and condenser size when mixtures are employed as the working fluid.