A numerical thermal approach to study the accelerated aging of a solar absorber material

Solar power tower receivers are exposed to highly-concentrated solar flux. The strong flux variations that they are exposed to during their service life enhance the physical–chemical aging mechanisms and cause the decrease of the material’s thermal performance. A material that is commonly used for these applications was selected for our study. It is used for the absorber tubes of a solar power tower receiver. This material is made of an alloy (Inconel 625) that is coated with a silicone-based paint coating (Pyromark 2500 black) to increase the solar radiation absorption capacity. With the aim of determining the optimal conditions to accelerate the aging of this two-layer material (metal + paint coating), an axisymmetric 2-D model reproducing its thermal behavior was developed. Several thermal indicators (temperature, thermal gradients, temporal gradient), which are representative of the thermal aging factors and the material’s thermal performance, are analyzed in different configurations of boundary conditions. One configuration was defined according to the normal working conditions of a particular solar power plant application; the others are associated with various boundary conditions with the potential to increase or modify the thermal stress factors to accelerate the aging mechanisms. Other aging factors such as humidity, pollutants, and dust are not investigated in this paper. Several simulations were run in permanent and variable regimes. The most influencing boundary conditions and material properties were highlighted by a sensitivity study. To design relevant aging tests, particular attention should be paid to the incident solar power and the cooling characteristics of the material. The surface total absorptance, the thermal conductivities and the thermal contact resistance between the paint and the metal layers are the parameters that most affect the material’s thermal behavior. The evolution of those material properties characterizes the aging and should be monitored. Irradiance cycles were simulated for their potential to increase the thermal fatigue. Depending on the average, the amplitude and the period of the cycle, the evolution of the thermal indicators were analyzed. They were compared to select the most appropriate aging tests that are to be performed. From there, two aging strategies were determined. To put them in practice, an experimental Solar Accelerated Aging Facility (SAAF) was built and is described in this paper. It enables to perform accelerated aging experiments with a 2-m-diameter parabola concentrating the sun radiation up to 16,000 times. A preliminary experiment to validate the model confirmed that the simulation results are in good agreement with the experimental. 2012 Elsevier Ltd. All rights reserved.