Integrated Thermo-Economic Optimization and Working Fluid Selection for ORC Plants Operating with Zeotropic Mixtures

Mixtures as working fluids for Organic Rankine Cycle (ORC) power plants are often suggested as a pos sible replacement for pure fluids because of the better temperature profile matching that can be achieved in heat exchangers and the increase in degrees of freedom that can be attained for the system design. In particular, recent studies have shown that simple blends of linear hydrocarbons enable a higher conver sion efficiency compared to their pure fluid components as well as comparable to that of refrigerants, which are either too expensive or being phased out due to environmental regulations. However, the techno-economic feasibility has yet to be assessed, and previous works either considered a limited set of mixture components or included very simplified procedures to size the ORC components. This workpresents a novel integrated design approach for ORC plants operating with zeotropic mixtures, in which the optimal thermo-economic solution is obtained through the simultaneous optimization of the thermodynamic cycle and the preliminary design of each plant component. Additionally, the molecular structure of the mixture components is directly determined during the optimization process by relying on the homo-segmented PCP-SAFT equation of state. The methodology is exemplified by considering a geothermal application. The plant performance is subsequently compared to that of an ORC system operating with a pure fluid and designed using the same methodology.