Development of soiling process characterisation methods for solar mirrors, for analysing mirror cleaning processes

Abstract

Concentrated Solar Power has the potential to provide power for the developing global economies towards a sustainable future. This solar radiation-based technology, reflects the radiation received by a solar mirror onto a receiver device which absorbs heat. Maintenance is required to keep the solar mirrors clean, and remove airborne particulate matter that settles on the mirror, which has an impact on the solar collector efficiency. Constant research to optimize cleaning methods and cleaning–strategies is paramount. An artificial soiling test rig and soiling methods were developed, which are capable of simulating repeatable soiling events and to specific soiling load. These features are necessary to simulate cleaning cycles with a period of several days. The developed test rig has a capability to provide a minimum soiling load of 0.25g/m² and has a constant error of 16%. Repeatable soiling tests were carried out up to 10 times. Extensive soiling experiments with two soiling materials (silt material and ground taken material from Almeria, Spain) and numerical simulation have revealed the exponential nature of the soiling process. An empirical model was formulated, which calculates specular reflectance, and includes material intrinsic parameters and soiling load data. This model highlighted the fact that compared to a linear model, between 7-20% lower soiling load is predicted, which potentially has a positive influence on cleaning cycles and therefore the costs attributed to them. A simulation series of a 10day cleaning cycles, which includes repeatable soiling and condensation events, used the artificial soiling test rig and a cooling plate located in a dry chamber. The adhesion effect (particle caking and capillary aging) were analysed by a centrifuge and the coverage ratio of the mirror samples before and after the experiments were calculated. It was noted that the repeatable soiling test (soiling and condensation) had a visible difference compared to the one-off soiling and condensation test series. The experimental modelling work will help to improve the considerable maintenance effort involved in mirror cleaning in solar field operations.