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Open AccessArticle

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

1
Institute of Sustainability and Energy in Construction, School of Architecture, Civil Engineering and Geomatics, University of Applied Sciences Northwestern Switzerland FHNW, 4132 Muttenz, Switzerland
2
Technische Hochschule Nürnberg Georg Simon Ohm, 90121 Nuremberg, Germany
3
Institute for Solar Energy Research Hamelin (ISFH), 31860 Emmerthal, Germany
*
Author to whom correspondence should be addressed.
Academic Editor: Maurizio De Lucia
Energies 2021, 14(3), 733; https://doi.org/10.3390/en14030733
Received: 29 December 2020 / Revised: 19 January 2021 / Accepted: 23 January 2021 / Published: 30 January 2021
(This article belongs to the Special Issue Modeling, Design, Development and Testing for Solar System)
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. View Full-Text
Keywords: solar thermal; flat-plate collector; stagnation; steam range; two-phase mixture model; thermal-hydraulic model solar thermal; flat-plate collector; stagnation; steam range; two-phase mixture model; thermal-hydraulic model
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MDPI and ACS Style

Eismann, R.; Hummel, S.; Giovannetti, F. A Thermal-Hydraulic Model for the Stagnation of Solar Thermal Systems with Flat-Plate Collector Arrays. Energies 2021, 14, 733. https://doi.org/10.3390/en14030733

AMA Style

Eismann R, Hummel S, Giovannetti F. A Thermal-Hydraulic Model for the Stagnation of Solar Thermal Systems with Flat-Plate Collector Arrays. Energies. 2021; 14(3):733. https://doi.org/10.3390/en14030733

Chicago/Turabian Style

Eismann, Ralph; Hummel, Sebastian; Giovannetti, Federico. 2021. "A Thermal-Hydraulic Model for the Stagnation of Solar Thermal Systems with Flat-Plate Collector Arrays" Energies 14, no. 3: 733. https://doi.org/10.3390/en14030733

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