The study of materials life-time or its accelerated aging, especially for absorbers or receivers used in concentrating solar technologies is the subject addressed in Working package WP14 (JRC).
The solar spectrum is mainly in the visible spectral range and has a penetration depth of about 20-50 nm in many metals and opaque materials Thus highly concentrated solar beams are ideally suited for surface heating and/or to provide a controllable mean of delivering large flux densities to solid surfaces, where the resulting thermal energy can cause phase changes, atomic migrations, and chemical reactions on a surface without greatly perturbing the bulk properties; alternatively, the photons may directly interact with species on the surface (as black and/or selective coatings, catalytic coatings, etc.).
The degradation mechanisms of CSP receivers depend on three main factors: 1) The material selection (with its chemical, mechanical, thermal and optical characteristics), 2) The cooling media selection (with its chemical and thermal properties affecting the material corrosion and thermal stress); and 3) The operational conditions (in temperature, concentrated solar flux and cycling), which determines the stress in operation. The combination of the three selections is associated to a degradation mechanism (mainly corrosion and fatigue). Corrosion usually implies the oxidation of the material and typically a loss of material and structural properties.
On the other hand, fatigue is the progressive and localized structural damage that occurs when a material is subjected to cyclic loading. The maximum stress values are less than the ultimate tensile stress limit, and may be below the yield stress limit of the material. Fatigue is strongly associated to the concentrated solar technologies where the daily and cloudy transients may cause fatigue failures.
For materials exposed to highly concentrated and transient solar radiation the degradation of their thermo-physical properties may be quite different than with equivalent thermally aged regular materials. This results from the predominantly superficial effect of concentrated solar radiation. The differences or similarities between these two types of degradations (solar vs. thermal) are rather unknown and the adaptation of the existing concentrating solar facilities to facilitate testing with controlled solar and/or temperature cycles, will allow both to give answer to this question and to facilitate the selection of the most durable materials for a certain application and operational conditions, among a set of candidate materials. Parabolic dishes and solar furnaces, that can obtain concentration factors of up to 10,000 resulting in fluxes of 1000 W/cm2, are the appropriate facilities for these kind of tests as described in JRC.
Expected WP14 (JRC) results are:
- guidelines on the characterization and testing for accelerating aging and comparative durability prediction for selected materials
- recommendations about common procedures issues such as achievable lifetimes, and
- description of minimal laboratory capabilities, including metrology and quality control, to conduct such tests