A criterion to optimize ORC design performance taking into account real turbine efficiencies

Several studies on Organic Rankine Cycles (ORCs) in the literature search for the optimal cycle parameters and working fluids that maximize the net power output. Only few of these studies evaluate the real turbine efficiency, but this is done only after a thermodynamic optimization is performed assuming a fixed and somewhat arbitrary efficiency value. Instead, a new design optimization procedure of the ORCs is proposed here which embeds a correlation for the design turbine efficiency. The correlation was obtained from published data in the literature and uses the volumetric expansion ratio and the size parameter as predictors of the turbine efficiency. The procedure has a general validity being the correlation applicable to any fluid. Results show how the selection of the working fluid influences the achievable turbine efficiency and how the optimal maximum pressure of the cycle deviates from that obtained in a thermodynamic optimization in which the turbine efficiency is fixed.