A rigorous approach for characterising the limiting optimal efficiency of working fluids in organic Rankine cycles

Organic Rankine cycles (ORCs) are a standard and simple layout to convert thermal energy to mechanical energy and, subsequently, electrical energy. On the one hand, it is essential to select the proper working fluids. On the other hand, criteria to choose optimal operation conditions are also necessary. This contribution presents a rigorous and general approach based on the Helmholtz energy function to analyse the optimal performance of an organic Rankine cycle irrespective of the working fluid. The new approach is applied for the n-alkane series and a set of refrigerants using the perturbed-chain statistical association fluid theory (PC-SAFT) equation of state as a calculation tool. The study reveals that a dry global optimum cannot be achieved without partial condensation, and guidelines to avoid this fact are shown. Also, it is parametrically demonstrated that the optimum behaviour of an organic Rankine cycle is not directly connected to the isentropic behaviour of the fluids. However, it is proved that a slightly dry fluid is more suitable for a good performance than a very dry fluid, because the optimum range of expansion lies in a feasible range of temperature, avoiding problems with the limit of the solid phase.