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Energies 2016, 9(6), 448; doi:10.3390/en9060448

Thermo-Economic and Heat Transfer Optimization of Working-Fluid Mixtures in a Low-Temperature Organic Rankine Cycle System

Clean Energy Processes (CEP) Laboratory, Department of Chemical Engineering, Imperial College London, South Kensington Campus, London SW7 2AZ, UK
This paper is an extended version of our paper published as O. A. Oyewunmi, C. N. Markides, Effect of working-fluid mixtures on organic Rankine cycle systems: Heat transfer and cost analysis. In Proceedings of the 3rd International Seminar on ORC Power Systems, ASME-ORC 2015, Brussels, Belgium, 12–14 October 2015. ISBN:978-2-9600059-2-9.
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Author to whom correspondence should be addressed.
Academic Editor: Sylvain Quoilin
Received: 25 April 2016 / Revised: 23 May 2016 / Accepted: 27 May 2016 / Published: 9 June 2016
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Abstract

In the present paper, we consider the employment of working-fluid mixtures in organic Rankine cycle (ORC) systems with respect to thermodynamic and heat-transfer performance, component sizing and capital costs. The selected working-fluid mixtures promise reduced exergy losses due to their non-isothermal phase-change behaviour, and thus improved cycle efficiencies and power outputs over their respective pure-fluid components. A multi-objective cost-power optimization of a specific low-temperature ORC system (operating with geothermal water at 98 °C) reveals that the use of working-fluid-mixtures does indeed show a thermodynamic improvement over the pure-fluids. At the same time, heat transfer and cost analyses, however, suggest that it also requires larger evaporators, condensers and expanders; thus, the resulting ORC systems are also associated with higher costs. In particular, 50% n-pentane + 50% n-hexane and 60% R-245fa + 40% R-227ea mixtures lead to the thermodynamically optimal cycles, whereas pure n-pentane and pure R-245fa have lower plant costs, both estimated as having ∼14% lower costs per unit power output compared to the thermodynamically optimal mixtures. These conclusions highlight the importance of using system cost minimization as a design objective for ORC plants. View Full-Text
Keywords: organic Rankine cycles (ORC); low-grade heat; working-fluid mixtures; multi-objective optimization; thermo-economic analysis; low-pressure expanders; heat transfer coefficients; ORC capital costs organic Rankine cycles (ORC); low-grade heat; working-fluid mixtures; multi-objective optimization; thermo-economic analysis; low-pressure expanders; heat transfer coefficients; ORC capital costs
This is an open access article distributed under the Creative Commons Attribution License which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. (CC BY 4.0).

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Oyewunmi, O.A.; Markides, C.N. Thermo-Economic and Heat Transfer Optimization of Working-Fluid Mixtures in a Low-Temperature Organic Rankine Cycle System. Energies 2016, 9, 448.

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