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A Thermal-Hydraulic Model for the Stagnation of Solar Thermal Systems with Flat-Plate Collector Arrays

Author

Listed:
  • Ralph Eismann

    (Institute of Sustainability and Energy in Construction, School of Architecture, Civil Engineering and Geomatics, University of Applied Sciences Northwestern Switzerland FHNW, 4132 Muttenz, Switzerland)

  • Sebastian Hummel

    (Technische Hochschule Nürnberg Georg Simon Ohm, 90121 Nuremberg, Germany)

  • Federico Giovannetti

    (Institute for Solar Energy Research Hamelin (ISFH), 31860 Emmerthal, Germany)

Abstract

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.

Suggested Citation

  • Ralph Eismann & Sebastian Hummel & Federico Giovannetti, 2021. "A Thermal-Hydraulic Model for the Stagnation of Solar Thermal Systems with Flat-Plate Collector Arrays," Energies, MDPI, vol. 14(3), pages 1-39, January.
    • Handle: RePEc:gam:jeners:v:14:y:2021:i:3:p:733-:d:490261
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    References listed on IDEAS

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    1. Adelard, L. & Pignolet-Tardan, F. & Mara, T. & Lauret, P. & Garde, F. & Boyer, H., 1998. "Sky temperature modelisation and applications in building simulation," Renewable Energy, Elsevier, vol. 15(1), pages 418-430.
    2. Jinho Choi & Eduardo Pereyra & Cem Sarica & Changhyup Park & Joe M. Kang, 2012. "An Efficient Drift-Flux Closure Relationship to Estimate Liquid Holdups of Gas-Liquid Two-Phase Flow in Pipes," Energies, MDPI, vol. 5(12), pages 1-13, December.
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    Cited by:

    1. Agnieszka Jachura & Robert Sekret, 2021. "Life Cycle Assessment of the Use of Phase Change Material in an Evacuated Solar Tube Collector," Energies, MDPI, vol. 14(14), pages 1-18, July.
    2. Sina Jafari & Ali Sohani & Siamak Hoseinzadeh & Fathollah Pourfayaz, 2022. "The 3E Optimal Location Assessment of Flat-Plate Solar Collectors for Domestic Applications in Iran," Energies, MDPI, vol. 15(10), pages 1-17, May.

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