Particle Erosion on Solar Mirrors: Construction and First Experimental Stage of an Open Loop Wind Tunnel

Kurzfassung

In this work the development of a device is described, which facilitates the simulation of sand and dust storms in order to cause the same defects on the tested specimen like observed during aging under natural conditions. Concentrating the sunlight into spot or linear formed receivers to use the generated thermal energy is one of the two big categories of solar energy, next to the photoelectric one, to make the power of the sun useable. Both technologies can be used in a concentrating or a non-concentrating version. If the sunlight needs to be concentrated, optical parts like lenses or mirrors come into play. The goal is to concentrate the incoming radiation with the highest possible efficiency. This efficiency can be measured via the solar reflectance and has, for a new CSP-mirror, a value about 0.94 which means that 94 % of incoming light will be rejected. One can imagine that a loss of reflectance directly comes along with a decreasing efficiency, energy yield and of course financial projects of the power plant. The decreasing reflectance depends much on specific local environmental factors, which are initiating different aging mechanisms. There is a high interest to classify and analyze the different mechanisms of reflective materials, in order to asses their lifetime under specific environmental conditions. The fields of application of CSP plants are the arid regions of this world with high values of irradiation but as well with high probability of sand and dust storms. There the most important mechanisms of aging are erosion, soiling and abrasion. While soiling is a reversible effect that can be counteracted by washing, erosion leads to a permanent loss of reflectance due to defects on the surface caused by the high velocity impact of aeolian particles. For abrasion these permanent surface defects are caused by the contact cleaning methods applied at CSP plants. The principle design is that of an open loop wind tunnel, also called Eiffel-tunnel, with the target to create a controlled and homogeneous air flow in the range of about 5 - 40 m/s. Furthermore an important part of this work is to develop a constant and controllable dust injection method for different sorts of sands and dusts, in order to make reproducible and representative material tests. Subsequently to the development process the part systems and the whole system are examined and evaluated with measurement equipment concerning the requirements.