Towards optimal mixtures of working fluids: Integrated design of processes and mixtures for Organic Rankine Cycles

Abstract

Organic Rankine Cycles transform low-temperature heat from sustainable sources into electrical power. Exploiting the full potential of a low-temperature heat source requires the optimal combination of Organic Rankine Cycles and working fluid. Today, working fluids are commonly pure components. However, mixtures can significantly improve the process efficiency due to their favorable temperature-glide during evaporation and condensation. In this work, we present a method for the integrated design of Organic Rankine Cycles and working fluid mixtures, so-called 1-stage Continuous-Molecular Targeting Computer-aided mixture and blend design (CoMT-CAMbD). In 1-stage CoMT-CAMbD, the physically-based perturbed-chain statistical associating fluid theory (PC-SAFT) equation of state is used to model both, the equilibrium and the transport properties of the mixture. A CAMbD formulation enables us to consider the molecular structure of the mixture components as well as its composition as degrees of freedom during process optimization. A detailed sizing of the equipment allows us to optimize not only thermodynamic but also economic objectives. 1-stage CoMT-CAMbD is demonstrated for the design of an Organic Rankine Cycle for waste heat recovery. The method identifies the optimal working fluid mixture from several million possible mixtures jointly with the corresponding optimal process and equipment, e.g., the mixture propane/diethyl ether maximizing the net power output (Pnet=295kW) or propene/propionaldehyde minimizing the specific investment cost (SIC=3479€ /kW). The presented method allows us to rigorously analyze the potential of optimal mixtures compared to pure components for varying heat source and cooling medium of the process and systematically exploit the potential of working fluid mixtures for Organic Rankine Cycles.