A multi-criteria approach for the optimal selection of working fluid and design parameters in Organic Rankine Cycle systems

The selection of the cycle configuration, working fluid and operating parameters is crucial for the economic profitability of Organic Rankine Cycle systems using low to medium temperature heat sources. The aim of this paper is to show a method that improves this selection taking into account several criteria at a time: an original thermodynamic optimization procedure of the system configuration and design parameters which explores all possible configurations, the design options around the optimum values of the objective function, an economic modeling technique validated on real cost data, and the consideration of the off-design behavior. The method is applied to comparatively assess the performance of two working fluids, isobutane and R134a, in the temperature interval between 130 and 180°C. The results show that the optimal cycle configuration is in most cases subcritical for isobutane and supercritical recuperated for R134a. The maximum power output of R134a is higher than isobutane for all the temperatures considered. The analysis of the objective function around the optimum shows the extent of the best range of turbine inlet pressures and enthalpies. These results highlight alternative design conditions to those maximizing the power output which might be preferred for technical and economic reasons.