Working fluid selection general method and sensitivity analysis of an Organic Rankine Cycle (ORC): application to Ocean Thermal Energy Conversion (OTEC)

A general model of an ORC (Organic Rankine Cycle) that is applicable to various technologies of heat exchangers has been carried out. It has been done considering the entropic mean temperature difference between the heat source and the working fluid as a parameter for heat exchangers. This model allows to compare pure fluids and mixtures in similar operating conditions. Simulations were conducted in the case of OTEC (Ocean Thermal Energy Conversion), for which the inputs are based on experimental measurements on an onshore prototype in Reunion Island. 26 fluids have been compared, considering thermodynamic performances (thermal efficiency and volumic work), technicity, security and environmental impacts. In the end, NH3, R507a and R1234yf appears to be the most suitable ones. In order to have a better understanding of the model and to determine its validity in the whole domain of variation of the parameters for OTEC, a parametric sensitivity analysis has been carried out by ANOVA (ANalysis Of VAriance) associated to polynomial chaos expansion method. The most sensible parameters are heat source temperature, heat exchangers entropic mean temperature difference and turbine isentropic efficiency. The ranking given for the fluids is still valid in the whole range of the parameters for OTEC.