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Working fluid design for organic rankine cycle systems (ORC):

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Abstract

The Organic Rankine Cycle is an energy conversion cycle similar to the conventional Rankine cycle which runs on a working fluid other than water. The selection of a working fluid is a critical part of designing an Organic Rankine Cycle (ORC) system. The number of fluid types actually used in commercial ORC power plants do not justify the number of fluid selection studies present in scientific literature. Hence the objective of this work is to develop a tool which simultaneously optimizes the energy conversion process and selects the optimum working fluid for a given heat source. It is based on a framework that uses a continuous-molecular targeting approach which allows for an integrated working fluid and system design. The process is modeled in Cycle Tempo, a modern graphical tool for thermodynamic analysis and optimization of systems for the production of electricity, heat and refrigeration. The system is simultaneously optimized with the pure component parameters of PCP-SAFT equation of state using a state-of-the-art optimization suite. The working fluid is selected by comparison of the pure component parameters of the PCP-SAFT equation of state with real fluids. A preliminary turbine model implemented directs the tool to generate suitable fluids for practically realistic systems. The tool has been tested for a waste heat recovery system for heavy-duty truck engines based on an ORC turbogenerator. The choice of working fluid is restricted to only the siloxane class which not only adheres to the technical, environmental, and toxicological requirements typical of the automotive sector but also allows for the implementation of a preliminary radial turbine model, whose shaft can be lubricated by the working fluid itself. The turbine has been modeled by applying the methodology of using non-dimensional parameters. Future work will be devoted to implement detailed component models and extending the scope of fluid selection to other organic fluid classes.

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