The SRB solar thermal panel

Abstract

tHe srB solar tHerMal paNel Features 16T he mechanical design of the panel [1] is shown schematically in Fig. 1. It consists of a metal frame sealed on both sides by glass windows, allowing solar radiation to enter at either side. The resulting box contains blackened copper heat absorbers which capture the solar energy. The absorbers are laser welded to stainless steel pipes which allow the heat to be extracted by circulating a fluid. These features are common to any flat solar thermal panel. The distinctive feature of the SRB panel is that it is under vacuum. Evacuated flat-plate panels were not produced so far because a vacuum-tight connection between large glass windows and the metal structure of the panel presents an (almost) insurmountable difficulty. The SRB solution to this problem consists of a plasma-sprayed metal coating on the glass perimeter, on which a metal joint may then be fixed by soft soldering. When the panel is under vacuum, the atmospheric pressure applies a force of 10 N/cm -2 on the panel windows, resulting in a weight of many tons for a medium-size panel. This force would lead to implosion of the panel if not properly supported. The spacing between the supports depends on the glass type and thickness. The SRB panel makes use of tempered glass 5 mm thick and longitudinal supports spaced by 14 cm, resulting in an implosion pressure of 2.5 bar.The absorbers are coated with a selective coating, to efficiently absorb the incident sunlight while providing low emission in the infrared range (see optical characteristics). High vacuum is maintained throughout the panel life of 30 years by a getter pump driven by solar energy (see vacuum considerations). Optical characteristicsBoth the pump and the absorbers are blackened electrolytically with a layer of Cr oxide which provides an absorption coefficient of about 0.9 and an infrared emissivity at 300°C below 0.07. This coating is stable when Vacuum considerationsVacuum is the best thermal insulator provided by nature. In a traditional thermal solar panel, conduction and convection through the ambient air results in large thermal losses, which limits the panel temperature and the range of applications. By evacuating the panel, thermal losses are reduced dramatically, thus increasing the panel efficiency.When an SRB panel is exposed to sun light while being in vacuum, its temperature is limited only by emission losses through infrared radiation. If the pressure is progressively increased, the temperature does not change until about 10 -4 mbar, and then it drops quickly due to molecular conduction and becomes constant after reaching 10 -1 mbar. Therefore, the pressure inside the solar panel must be kept below about 10 -4 mbar during the lifetime of the panel to keep the molecular conduction losses negligible with respect to the radiation losses. In any sealed device, after initial evacuation of the atmospheric air, vacuum must be maintained by pumping the gases desorbing from the device components. Since about a century this is done by getters, which...

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