Eismann, Ralph

Eismann, Ralph


Gerade angezeigt 1 - 8 von 8
  • Publikation
    A thermal-hydraulic model for the stagnation of solar thermal systems with flat-plate collector arrays
    (MDPI, 30.01.2021) Eismann, Ralph; Hummel, Sebastian; Giovanetti, Federico [in: Energies]
    Stagnation is the transient state of a solar thermal system under high solar irradiation where the useful solar gain is zero. Both flat-plate collectors with selective absorber coatings and vacuum-tube collectors exhibit stagnation temperatures far above the saturation temperature of the glycol-based heat carriers within the range of typical system pressures. Therefore, stagnation is always associated with vaporization and propagation of vapor into the pipes of the solar circuit. It is therefore essential to design the system in such a way that vapor never reaches components that cannot withstand high temperatures. In this article, a thermal-hydraulic model based on the integral form of a two-phase mixture model and a drift-flux correlation is presented. The model is applicable to solar thermal flat-plate collectors with meander-shaped absorber tubes and selective absorber coatings. Experimental data from stagnation experiments on two systems, which are identical except for the optical properties of the absorber coating, allowed comparison with simulations carried out under the same boundary conditions. The absorber of one system features a conventional highly selective coating, while the absorber of the other system features a thermochromic coating, which exhibits a significantly lower stagnation temperature. Comparison of simulation results and experimental data shows good conformity. This model is implemented into an open-source software tool called THD for the thermal-hydraulic dimensioning of solar systems. The latest version of THD, updated by the results of this article, enables planners to achieve cost-optimal design of solar thermal systems and to ensure failsafe operation by predicting the steam range under the initial and boundary conditions of worst-case scenarios.
    01A - Beitrag in wissenschaftlicher Zeitschrift
  • Publikation
    Cost reduction of solar thermal plants by advanced thermalhydraulic design methods
    (IOP Publishing, 04.09.2019) Eismann, Ralph; Scartezzini, Jean-Louis [in: CISBAT]
    04B - Beitrag Konferenzschrift
  • Publikation
    CoolShift – Cooling of buildings by chiller-assisted nocturnal radiation and convection
    (IOP Publishing, 04.09.2019) Koch, Manuel; Dott, Ralf; Eismann, Ralph; Scartezzini, Jean-Louis [in: CISBAT]
    A novel approach for cooling an office building is numerically evaluated. PVT collectors are used for nocturnal radiative and convective cooling. A TABS ceiling serves as thermal storage. If the free cooling power of the PVT collectors is too low, it is boosted by a chiller raising the collector temperature. While the energy efficiency improves compared to a conventional daytime chiller cooling system, the room temperature cannot always be kept in the desired band. Furthermore, the PVT collectors stay cooler than comparable PV modules during the day, increasing the electricity generation.
    04 - Beitrag Sammelband oder Konferenzschrift
  • Publikation
    Pipe Network Analysis for Solar Thermal Plants
    (ISES Conference Proceedings, 10.09.2018) Eismann, Ralph; Haeberle, Andreas [in: EuroSun, 12th International Conference on Solar Energy for Buildings and Industry]
    Efficiency, cost-effectiveness and operational safety of solar plants must be based on the knowledge of pressure loss as well as flow distribution and temperature distribution of branched collector arrays. Therefore, pipe network analysis is an essential step during the dimensioning process. This article presents an explicit, fast converging numerical scheme especially suited for collector fields. The effect of pipe dimensions and pipe routing on flow, temperature and pressure-distribution and collector field efficiency is demonstrated by application of a dedicated software tool, which is provided as an open source code.
    04 - Beitrag Sammelband oder Konferenzschrift
  • Publikation
    An Explicit and Continuous Friction Factor Correlation for Helical Tubes with Arbitrary Roughness
    (Global Digital Central, 11.04.2018) Eismann, Ralph; Adams, Robert [in: Frontiers in Heat and Mass Transfer]
    Convergence of numerical schemes for pipe network analysis requires continuous modelling of pressure losses in the transition region between laminar and turbulent regions. Several existing correlations for the friction factor of straight pipes and helical tubes are presented. Based on these correlations a new explicit correlation for helical tubes with arbitrary surface roughness is derived. The friction factor is expressed as a continuous function of the Reynolds number covering laminar, transitional, and turbulent flow regions. Potential sources of error are also discussed, including the effects of tube deformation caused by the bending process.
    01A - Beitrag in wissenschaftlicher Zeitschrift
  • 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
  • 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