Thermal design and selection of the optimal working fluid for organic Rankine cycles based on the equivalent temperature concept

A new approach, based solely on the inlet conditions of external energy sources, is proposed for selecting the set of optimal working fluids as well as operating parameters of Organic Rankine Cycles (ORC). The approach is composed of two sequential steps. The first step, formulated as an optimization problem, minimizes the total thermal conductance of the system without recourse to the working fluid properties. Here, a second-law analysis based on the equivalent temperature concept constrains the nonlinear optimization problem, where a specific geometrical feature of equivalent temperature on the h-s diagram establishes a link between operating conditions and entropy generation in each component of the system. The second step of the approach, i.e. reconstruction procedure, correlates the optimization results, which are in terms of equivalent temperatures, with the essential thermodynamic characteristics of the cycle. The reconstruction procedure also paves the way for the selection of the most appropriate working fluid among the candidates based on a capital cost criterion. To illustrate the application and the advantages of our approach, two case studies are presented.