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

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.

The Description of “Temperature Self-Limiting” in Jordanian Solar Generators with Natural Cooling

“…According to Eismann et al [19], stagnation situations are any case where a solar collector is needed.…”

“…The different methods used to monitor or mitigate inactiveness conditions at the collector are depicted in the system situation. (i.e., operational), or solar array level [19] The design provided [20] describes a modern strategy of aluminum heat pipes with limited copper usage. The highest Inactiveness temperature of this new kind of pipe is 140 Celsius.…”

“…Eismann et al [19] outlined the impacts of inactiveness factors that were developed and tested. As seen in Figure 2, some suggested approaches really aren't appropriate for all systems and functions.…”

“…Several methods were tested and applied to control inactiveness in the experiential reg system; the appropriate solution must be simple to execute, inexpensive, and run continuously without any technical issues [13][14][15][16][17][18][19][20][21].…”

“…The thermal shock could reveal the entry solution on hot days, causing turbulence, vibrating, and destroying the power piping arrangement. Managing and controlling inactiveness encompasses all measures that can be applied to stop the emergence of a no-flow event [13][14][15][16][17][18][19][20][21][22][23][24]. Inactiveness is a common issue with solar systems worldwide, particularly in hot, dry areas, and it is an outcome of poor system design or cost-cutting by a manufacturer.…”