Theoretical basis and experimental facility for parabolic trough collectors at high temperature using gas as heat transfer fluid

The efficiency of any thermal power plant is related to the maximum working temperature that some components can reach. Examples of material constraints are the first stage blades of a gas turbine or the receiver of a solar tower plant, but there are other temperature constraints, such as the ones related with the heat transfer fluid. Nowadays, this happens in the most common solar technology: parabolic trough collectors that use synthetic oil. This fluid must work below 400°C. This limitation affects the power plant efficiency due to a poor Rankine cycle yield. To avoid this problem and go to higher temperatures, a gas can be used as heat transfer fluid, providing at the same time other significant advantages over synthetic oil: non-flammability and no environmental threats. The purpose of this paper is to justify the theoretical basis of the gas use in parabolic troughs and the problems related, and also to describe the test loop built at the PSA (Plataforma Solar de Almería) in order to demonstrate the technical feasibility of this new technology, testing all components required to build a pre-commercial power plant. This paper describes the main features of this technology, the theoretical basis, the validation of the design tools and the results of the operation. Of them, the more important aspects are that high working pressure can reduce pumping power to adequate levels, that good controllability of the technology with hard solar transients can be achieved and that ball joints leaks problems detected appeared as the main inconvenience, but promising solution has been found.