Optimization of the particle flow in a wind tunnel for accelerated aging test of concentrating solar power materials

Abstract

The performance of concentrated solar power (CSP) plants is heavily dependent on the direct normal irradiation (DNI) and the specular reflectance of the solar reflecting material. The harsh outdoor conditions of the sites, where CSP plants are usually located lead to natural degradation processes of the materials, caused by a variety of environmental stresses, such as UV radiation, industrial pollution, humidity, temperature changes, mechanical stresses and erosion due to airborne particles. In order to simulate erosive outdoor conditions in the laboratory, a closed-loop wind tunnel system was implemented at the Plataforma Solar de Almería. This wind tunnel system was designed for samples with a size of 6 x 6 cm² to determine the impact of erosion of different reflector materials at various wind velocities and sand and dust types. However, this system exhibited an inhomogeneous erosive particle distribution over the cross section area of the wind tunnel’s tube. To improve the test quality, this work deals with the optimization of the particle flow in the wind tunnel for improved accelerated aging testing of CSP materials. During the course of this work, recent standards and studies that deal with accelerated erosion testing have been reviewed and the initial wind tunnel setup was analyzed. The testing time was 10 minutes, the average particle concentration varied from 40-60 mg m-3 and the wind velocity was set to 6 – 37 m s-1. It has been found that the particle distribution is highly inhomogeneous and needs optimization in order to perform reproducible simulation results. The defect density showed a strong gradient from the outer tube section to the inner tube section. The difference of specular reflectance of a tested sample between the outer and inner section is 54.3% with a maximum gradient of 8.7 % cm-1 at 35 m s-1. After the optimization process, the results improved significantly but still indicated minor deviations. The difference in specular reflectance could be reduced to 12.9 % over the tube diameter and a gradient of 3.4 % cm-1 at 30 m s-1 could be achieved. Even though the particle distribution improved significantly for almost all wind velocities, there is still room for further optimization steps, which are discussed in this work.