A thermal-hydraulic model for the stagnation of solar thermal systems with flat-plate collector arrays
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Publication date
30.01.2021
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01A - Journal article
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Energies
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Volume
14
Issue / Number
3
Pages / Duration
1-39
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MDPI
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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.
Keywords
Solar thermal, Flat-plate collector, Stagnation, Steam range, Two-phase mixture model, Thermal-hydraulic model
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ISBN
ISSN
1996-1073
Language
English
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Yes
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Published
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Citation
Eismann, R., Hummel, S., & Giovanetti, F. (2021). A thermal-hydraulic model for the stagnation of solar thermal systems with flat-plate collector arrays. Energies, 14(3), 1–39. https://doi.org/10.3390/en14030733