Assessment of methods for performance comparison of pure and zeotropic working fluids for organic Rankine cycle power systems

Jesper Graa Andreasen, Martin Ryhl Kærn, Fredrik Haglind*

*Corresponding author for this work

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    Abstract

    In this paper, we present an assessment of methods for estimating and comparing the thermodynamic performance of working fluids for organic Rankine cycle power systems. The analysis focused on how the estimated net power outputs of zeotropic mixtures compared to pure fluids are affected by the method used for specifying the performance of the heat exchangers. Four different methods were included in the assessment, which assumed that the organic Rankine cycle systems were characterized by the same values of: (1) the minimum pinch point temperature difference of the heat exchangers; (2) the mean temperature difference of the heat exchangers; (3) the heat exchanger thermal capacity (Ū A); or (4) the heat exchanger surface area for all the considered working fluids. The second and third methods took into account the temperature difference throughout the heat transfer process, and provided the insight that the advantages of mixtures are more pronounced when large heat exchangers are economically feasible to use. The first method was incapable of this, and deemed to result in optimistic estimations of the benefits of using zeotropic mixtures, while the second and third method were deemed to result in conservative estimations. The fourth method provided the additional benefit of accounting for the degradation of heat transfer performance of zeotropic mixtures. In a net power output based performance ranking of 30 working fluids, the first method estimates that the increase in the net power output of zeotropic mixtures compared to their best pure fluid components is up to 13.6%. On the other hand, the third method estimates that the increase in net power output is only up to 2.56% for zeotropic mixtures compared to their best pure fluid components.

    Original languageEnglish
    Article number1783
    JournalEnergies
    Volume12
    Issue number9
    Number of pages25
    ISSN1996-1073
    DOIs
    Publication statusPublished - 2019

    Keywords

    • Heat exchanger
    • Low grade heat
    • Method comparison
    • Organic Rankine cycle system
    • Thermodynamic optimization
    • Zeotropic mixture

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