Simultaneous Optimal Design of Organic Mixtures and Rankine Cycles for Low-Temperature Energy Recovery

Abstract

Because of pollution issues and a forecasted scarcity of fossil fuels, there exists a strong need to consider alternative and sustainable energy sources. Among all the types of alternative energies, normally heat recovery from low-temperature sources is not considered seriously for this purpose because of the poor performance of the conversion to power method. The conventional Rankine cycle that uses water as the working fluid is the most simple way for heat to power conversion. But for low-temperature applications, the conventional Rankine cycle will not achieve good results. In this work, we propose to replace water with a proper optimal combination of a priori selected set of organic fluids as the working fluid. Therefore, the determination of the right type of organic components and the composition of the mixture become decision variables. Moreover, because strong interactions exist between the selection of the type and composition of the organic components and the operating conditions of the Rankine cycle for a given fixed process flowsheet configuration, a simultaneous solution approach will be sought. Hence, by approaching the design problem in a simultaneous rather than in a sequential manner, improved optimal solutions were achieved. The sequential design problem is posed as a nonlinear optimization problem. The proposed methodology is illustrated using three case studies.