The well-established correlation of Hollands et al. (1976), which is applicable to isothermal boundaries and Rayleigh numbers up to 10 5 , underestimates the convective heat loss across the air gap of flat plate solar collectors with tube-and-sheet type absorbers both in normal operation and at stagnation. Two reasons for this discrepancy were identified. 1) The Rayleigh number of the air gap above absorbers with highly selective coatings can be three times as high as the application limit. 2) The absorber is not isothermal during normal operation. Based on a literature study and theoretical considerations the application limit of the correlation was extended to Ra = 3•10 5. By means of an analytically derived correction parameter, the correlation was adapted to non-isothermal boundary conditions. For the cost-and efficiency optimization of flat-plate collectors an accurate analytical model was developed, based on the model of Duffie and Beckman (1991), enhanced by the new correlation for convective heat loss between absorber and cover plate and further correlations presented by Eismann (2015).The model was validated against data of standardized collector tests. It is able to predict both the thermal efficiency and the stagnation temperature within the uncertainty limit of the standardized test method EN 12975-2 (CEN 2006).

No 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 structure of Fe deposited on Al(001) at room temperature has been investigated during growth. It is found that even less than 1 monolayer (ML) of Fe destroys the atomic order at the surface due to the interdiffusion of Fe and Al. After deposition of 3 ± 0.5 ML, short-range order at the surface is restored due to islands which locally have the body-centered cubic structure and are aligned in registry with the Al matrix. These islands coalesce during further Fe deposition, and produce poor long-range order at an Fe dose of 5 ± 0.5 ML. At this coverage, magnetic ordering is found perpendicular to the surface. The in-plane component of the surface magnetization is detected above 8 ± 0.5 ML.

No abstract