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ePaper created 2014-04-30, 16:22:09 | version 1.30.01
discovered 02 .13 FOCUS WWW.Hzdr.DE As usual, the devil is in the details. For the energy turnaround, scientists and politicians have committed to solar power plants, which use rays focused from the sun to generate steam, and therefore operate just like coal-fired or nuclear- powered turbines and generators. The electricity produced in this way is largely carbon-neutral because the steam is generated by solar power instead of fossil fuels. The German Aerospace Center (DLR) is researching a promising variant to this technology close to the Spanish city of Almeria. Concave mirrors there concentrate the sun’s rays onto long tubes in which this concentrated energy converts water directly into steam. In principle, the physics is very simple. Looked at in detail, however, the flow of this mixture of water and steam contained in the absorber tube can be quite unstable. Since these instabilities in two-phase flows can have an impact on the cooling system and thereby place greater demands on the materials, engineers would naturally like to avoid them. And best right from the start. The doctoral dissertation by Alexander Hoffmann at HZDR should help with exactly this problem. For a young engineer, working on a future technology right off in your dissertation is a dream job, of course. Even if the task is considerably more complicated than it appears at first glance. This is because the absorbing tube is no drinking straw – it has an interior diameter of five centimeters (two inches) and is made up of different segments, referred to as collectors, that are about a kilometer long in total. While quite ordinary water flows through this long component, the water is subjected to extreme conditions compared to usual ones found otherwise on Earth. Instabilities in the tube Even right at the inlet of this installation, the water can be about 260 degree C (500 degree F). In order for it to remain in liquid state, the pressure in the tube is eighty times higher than air pressure at sea level. Over the first hundred or so meters of tubing, the focused solar radiation increasingly heats up the water until it begins to turn to steam. At that point, even the high pressure is no longer sufficient to prevent // Experts in reactor safety at HZDR are researching how to make solar power plants more efficient. _TEXT . Roland Knauer WITHOUT ANY LOSSES: At the DUKE test facility, steam is generated directly inside the parabolic troughs’ tubes for subsequent power production. Photo: DLR Simulated flows