Eismann, Ralph

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Eismann, Ralph

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  • Vorschaubild
    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
  • Vorschaubild
    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
  • Vorschaubild
    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
  • Vorschaubild
    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
  • Vorschaubild
    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