Stratified Flow Phenomena in Parabolic Trough Systems

Parabolic trough power plants today represent the state of the art in solar thermal electricity generation. A large number of plants have been realized in Spain based on the same principle as the first systems in the U.S. back in the 1980s. The market today is characterized by a strong need for cost reduction as competing technologies like PV showed remarkable improvements in terms of economics. For solar thermal power plants, several paths to higher profitability are open. Line focusing systems with either direct steam generation (DSG) or molten salt as a heat transfer fluid aim at higher process temperatures and thus better cycle efficiency. The same motivation holds for the activities in solar tower plants. Along with these “big” technological innovations it is important to optimize the performance of each single component. Detailed knowledge of physical effects is essential to identify potential for improvement. This paper focuses on the topic of flow stratification in horizontal tubes. Since the diameter to length ratio of absorber tubes in parabolic trough or Linear Fresnel systems is very small, a common assumption is that the temperature and flow are equally distributed over the cross section. In many cases, this assumption is appropriate to simplify calculation procedures. Nevertheless, there are applications for which this assumption is not valid any more. The paper will describe two such examples observed at DLRs test facilities. The first one deals with accurate measurement of the temperature rise within a collector efficiency test bench. It is shown how temperature stratification in the absorber tubes could lead to severe errors in the temperature measurement. The second example is a collector test facility for direct steam generation. During the start-up procedure we observed a damage of two absorber tube glass covers which was caused by large temperature gradients over the pipe cross section.