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discovered_02_2013

FOCUS// The HZDR Research Magazine WWW.Hzdr.DE 04 05 the transition from liquid to vapor. However, this does not happen all at once, but gradually instead. Over a stretch of more than half a kilometer (> 500 yards), the focused solar radiation adds energy to the liquid that no longer increases its temperature, but instead transforms the liquid increasingly into steam. Only after about three-quarters of a kilometer has the water been completely converted to steam. The sun's rays further heat this steam over the remaining about 200 meters. The higher the temperature becomes, the more efficiently the turbines operate and the higher the effectiveness and thus the yield of electricity. The devil is in the intervening half kilometer, where increasingly more steam is developed in the water-steam mix. "Instabilities can arise due this two-phase flow for various reasons," explains Alexander Hoffmann. In the simplest case, they occur when the sun penetrates the cloud cover again after a period of time. More focused solar energy reaches the tubing then, which vaporizes more water, and thereby shortens the stretch over which the water/steam mixture is travelling. The engineers can imagine a whole series of conditions under which these kinds of instabilities can arise and stress the material, and thereby reduce its durability and operating life. Even worse: it is possible for instabilities to arise that have not yet been observed at all. Nuclear know-how for solar power plants To better understand the processes, you can simulate the flow conditions of the steam-water mixture on a computer. To develop software for this would far exceed the scope of a doctoral dissertation. However, the Gesellschaft für Anlagen- und Reaktorsicherheit GRS (Association of Plant and Reactor Safety) in Garching, near Munich, Germany, has already developed and successfully used simulation software for an entirely unrelated, but very similar problem. Two- phase flows of steam and water can circulate, namely, in the cooling circuits of a nuclear power plant. HZDR engineers rely on this ATHLET software from GRS. They are very familiar with the simulation program as well as the flow behavior in nuclear power plants. Alexander Hoffmann based his research precisely on this transfer of know-how. In applying the software, he does not want to find out just whether there are as-yet unobserved instabilities. He primarily would like to investigate under what conditions reliable operation is feasible. Furthermore, one could also use the software for improved control over the entire solar power plant. Using this knowledge, which researchers at DLR will also in part be testing in practical experiments, the parabolic trough power plants should be able to be operated not just better, but also more economically. CONTACT _Institute of Resource Ecology at HZDR Alexander Hoffmann alexander.hoffmann@hzdr.de _German Aerospace Center (DLR) Institute of Solar Research Dr.-Ing. Tobias Hirsch tobias.hirsch@dlr.de A DIFFERENT KIND OF POWER PLANT: Parabolic trough power plants use focusing mirrors to concentrate sunlight onto an absorber tube. The heat energy that is captured within the tube is then used to produce steam to power a turbine. Photo: DLR

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