Eismann, Ralph

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Eismann
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Ralph
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Eismann, Ralph

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2013 - 20152
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Thema: Thermal efficiency ×

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Gerade angezeigt 1 - 2 von 2
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    Publikation
    Accurate analytical modeling of flat plate solar collectors: Extended correlation for convective heat loss across the air gap between absorber and cover plate
    (Elsevier, 2015) Eismann, Ralph [in: Solar Energy]
    The well-established correlation of Hollands et al. (1976), which is applicable to isothermal boundaries and Rayleigh numbers up to 105, underestimates the convective heat loss across the air gap of flat plate solar collectors with tube-and-sheet type absorbers both in normal operation and at stagnation. Two reasons for this discrepancy were identified. (1) The Rayleigh number of the air gap above absorbers with highly selective coatings can be three times as high as the application limit. (2) The absorber is not isothermal during normal operation. Based on a literature study and theoretical considerations the application limit of the correlation was extended to Ra =3 105. By means of an analytically derived correction parameter, the correlation was adapted to non-isothermal boundary conditions. For the cost- and efficiency optimization of flat-plate collectors an accurate analytical model was developed, based on the model of Duffie and Beckman (1991), enhanced by the new correlation for convective heat loss between absorber and cover plate. The model was validated against data of standardized collector tests. It is able to predict both the thermal efficiency and the stagnation temperature within the uncertainty limit of the standardized test method EN 12975-2 (CEN, 2006).
    01A - Beitrag in wissenschaftlicher Zeitschrift
  • Vorschaubild nicht verfügbar
    Publikation
    Correction for the absorber edge effect in analytical models of flat plate solar collectors
    (Elsevier, 2013) Eismann, Ralph [in: Solar Energy]
    Analytical models based on the theory of Hottel and Whillier (1958) overestimate the efficiency of solar collectors with single plate absorbers because the absorber tubes of real absorbers are usually not bonded up to the edge of the absorber plate. Based on two-dimensional numerical simulations, a correction factor has been derived. This correction factor, multiplied to the standard fin efficiency defined by the analytical model, takes the edge effect into account within 0.5% when compared to the numerical results. The advantage is twofold: The extended analytical model enables more rigorous cost- and efficiency optimization of the absorber, which is the most expensive part of a flat plate solar collector. Since one of the major uncertainties of the analytical model is eliminated by the addition of the correction factor, parametric studies will be more accurate. If the efficiency characteristics, the geometrical data and the material properties of a sufficient number of collectors are known, the enhanced model could be used to derive more accurate correlations for the heat loss coefficients.
    01A - Beitrag in wissenschaftlicher Zeitschrift