Accelerated Aging of a Solar Absorber Material Subjected to Highly Concentrated Solar Flux

Abstract

To support the fast growth of Concentrating Solar Power (CSP) technologies, R&D efforts need to be continued to minimize costs. Reducing manufacturing, processing and maintenance costs is key to reach a competitive low production price for electricity.From this perspective, one issue that becomes increasingly significant is the service lifetime of solar absorber materials. In solar tower receivers, absorber materials are subjected to very intense solar flux and cyclic thermal stresses. The chemical and physical aging mechanisms such as corrosion and oxidation, or the possible apparition of cracks or delamination are responsible for the change of the material's surface and bulk properties over time. This causes the thermal performance of the material to decline more or less rapidly.Taking a two-layer solar absorber material (metal + coating) that is commonly used in power tower receivers (SOLAR TWO, GEMASOLAR, SOLHYCO), we have designed several accelerated aging tests by analyzing the thermal behavior of the material in permanent and variable regimes. The experimental tests consist in subjecting the material to cyclic irradiance of variable intensity, amplitude and period to determine the fastest and most realistic tests. A Solar Accelerated Aging Facility (SAAF) was built for this purpose. It is made of a 2-meter diameter parabola concentrating the solar radiation (up to 16,000 times) onto a material sample that is cooled by air or water in direct contact with its rear face. A flux sensor coupled to a shutter allows us to apply the desired solar irradiance while an infrared pyrometer monitors surface temperature of the sample.Following different constant-irradiance solar aging treatments, the properties that are characteristic of the aging are estimated. The normal solar absorptance of each sample is evaluated by a solar optical fiber reflectometer. The thermophysical properties of the paint such as the thermal diffusivity, effusivity, conductivity and the thermal contact resistance between the paint layer and the metal substrate are estimated by inverse method using the measurements of an impulse photothermal experiment. In addition, untreated samples are characterized in order to provide rare data on the ‘not aged’ material.Careful attention should be paid to the preliminary vitrification heat treatment carried out to give the black paint its optimal absorption properties before being used. The results of the aging tests show a slight deterioration of the properties corresponding to the first aging signs of the material. The solar absorptance and the thermal effusivity of the paint coating are the most affected properties during the tests.