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Session: Poster session
Session starts: Tuesday 13 October, 13:30

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Peter Collings (University of Glasgow)
Zhibin Yu (University of Glasgow)

Abstract:
Whereas single-component working fluids exhibit phase changes at a constant temperature, working fluid mixtures containing two or more components can exhibit a change in temperature across a phase change, known as “glide”. These fluid mixtures are termed “zeotropic”. Zeotropic mixtures can possess several advantages when used as working fluids in ORC systems. Similarly to supercritical cycles, they can match the temperature profiles on the hot and cold sides of the evaporator, resulting in less of a need for superheating, and increasing second law efficiency. The major advantage of a zeotropic fluid over a supercritical one is that this effect can also be felt in the condenser, which can greatly reduce the need for coolant for a given cycle efficiency. This is of particular benefit in desert areas, where cooling water is not readily available. In general, zeotropic cycles can be made to exhibit significant temperature glide at far lower pressures than an equivalent supercritical cycle. A temperature glide can also result in a greater amount of energy being transferred in a recuperator, should one be included in the system, by increasing the difference in temperature between the cold fluid leaving the pump, and the hot fluid leaving the expander, which results in a greater transfer of energy for a given cycle configuration, especially when this enables a certain amount of phase change to occur in the recuperator. This paper uses a numerical simulation in MATLAB to analyse the effects of varying the composition of a zeotropic mixture of R245fa and R134a on the overall performance of a cycle, both with and without a recuperator installed.