Effect of long term outdoor exposure on antisoiling coatings for solar reflectors

Results of the outdoor exposure campaign of two newly developed anti-soiling coatings for CSP mirrors are presented in this paper. Samples of coated mirrors were exposed for two years under realistic outdoor conditions and the application of two different cleaning techniques was investigated. Mirror samples were analyzed during exposure and their reflectance and cleanliness were measured. The performance of the anti-soiling coated mirror samples was compared to conventional uncoated mirrors. Depending on the selected combination of coating and cleaning technique, an advantage or disadvantage was observed over the exposure duration. One coating presented an overall cleanliness gain of 0.008 with pressurized water cleaning, due to its superior anti-soiling behavior, while presenting a negative gain of -0.009 with contact cleaning, due to its poor durability. The other coating showed a worse anti-soiling behavior, with a negative gain of -0.005 for pressurized water cleaning, due to its poor washability, and a positive gain of 0.005 with contact cleaning

UV degradation of primary mirrors in outdoor exposure and accelerated aging

The objective of this work is to quantify de degradation originated by UV radiation and temperature in silvered-glass reflectors exposed in three locations with high solar irradiance and to replicate the degradation mechanisms observed in outdoors through accelerated aging tests

Lifetime prediction model of reflector materials for concentrating solar thermal energies in corrosive environments

Concentrated solar thermal technologies play an essential role in the energetic transition which is currently facing our society. The energy generation in this technology vastly depends on the optical behaviour of the reflector materials of the solar field. Corrosion of solar reflectors might be an issue in locations with high corrosive environments because an excessive corrosion of the solar mirror could be catastrophic for the profitability of the concentrated solar thermal plant. This research is focusing on modelling the durability of four different solar reflector materials exposed outdoors by accelerated aging tests. For this purpose, ten locations suitable for concentrating solar thermal applications were classified depending on their corrosive aggressiveness. Commercial, free-lead and low-cost reflectors samples were exposed in all the sites to determine the influence of the corrosion in its durability. Corrosion defects appeared in the solar reflectors during outdoor exposure were properly reproduced by CASS test. Novel lifetime prediction models were developed for all the solar reflectors depending on the corrosive aggressiveness of the place. Number and thickness of the paint coatings employed in the solar mirrors were identified as one of the most important parameters to improve the energy generation of a CSP plant in corrosive environments. A reduction of the capital invested in the solar mirror purchase is expected for sites with low corrosivity

Advanced Analysis of Corroded Solar Reflectors

The corrosion of the reflective layer is one of the main degradation mechanisms of solar reflectors. However, the appropriate assessment of the corroded reflector samples is not accomplished by the current analysis techniques. On the one hand, the reflectance measurement protocol of non-damaged solar reflectors for concentrating solar thermal technologies is widely addressed in the SolarPACES reflectance guideline. However, this methodology is not adequate for reflectors whose surface is partially corroded by many kind of corrosion agents. In this work, a new measurement technique to properly assess corroded samples was developed. To check the usefulness of the method, several damaged samples (subjected to two accelerated aging tests) were evaluated with the conventional technique and with the improved one. The results showed that a significant discrepancy is observed between the two methods for heavily corroded samples, with average reflectance differences of 0.053 ppt. The visualization of the reflector images illustrated that the improved method is more reliable. On the other hand, both the corrosion products formed and the corrosion rates were identified after each corrosive test. The chemical atmosphere significantly affects the products formed, whereas the corrosion rates are influenced by the test conditions and the reflector Quality

Water Saving in CSP Plants by a Novel Hydrophilic Anti-Soiling Coating for Solar Reflectors

In this work, results of the outdoor exposure campaign of a newly developed hydrophilic anti-soiling coating for concentrated solar thermal power (CSP) mirrors are presented. The material was exposed for nearly two years under realistic outdoor conditions and the influence of two different cleaning techniques was evaluated. Mirror samples were analyzed during exposure and their reflectance and cleanliness were measured. The performance of the anti-soiling coated mirror samples was compared to conventional uncoated silvered-glass mirrors. The coatings showed appropriate anti-soiling and easy-to-clean behavior, with a mean cleanliness gain of 1 pp and maximum values under strong soiling conditions of up to over 7 pp. Cleanliness of the coated samples stayed higher throughout the whole campaign before and after cleaning, resulting in lower soiling rate compared to the reference material. Taking into account these values and supposing a threshold for cleaning of 96%, the number of cleaning cycles could be decreased by up to 11%. Finally, the coated material showed negligible degradation, not exceeding the degradation detected for the reference material

Uncertainty Study of Reflectance Measurements for Concentrating Solar Reflectors

The solar reflector is one of the main components of concentrated solar thermal systems. Therefore, accurate knowledge of its solar-weighted, near-specular reflectance is highly important. Currently, this parameter cannot be properly measured with a single commercial instrument. There is a great interest in having a suitable procedure that can guarantee the accuracy of reflector quality analysis, which already led to the publication of an international measurement guideline (title “Parameters and method to evaluate reflectance properties of reflector materials for concentrating solar power technology”). Still, more research work is needed to improve the state of the art. At present, both the specular reflectance and the spectral hemispherical reflectance are measured by using commercial portable reflectometers and spectrophotometers, respectively, to gain enough information. This article concentrates on the evaluation and calculation of the type-B (nonstatistical) uncertainties associated with these employed instruments and, therefore, leads to a more accurate definition of the measurement uncertainty. Considering type-B uncertainty, the expanded uncertainties of measurements for most of the reflector types are UB,ref = 0.006 for monochromatic specular reflectance and UB,spec = 0.016 for solar-weighted hemispherical reflectance

Advanced Cyclic Accelerated Aging Testing of Solar Reflector Materials

Realistic lifetime prediction and testing procedures for solar mirrors have been demanded by investors, plant developers and material manufacturers during the last years. It has been proven that most of the commonly used accelerated aging tests, which were adopted from other industries, cannot be correlated to outdoor exposure. This work studies different accelerated aging test sequences and analyzes the produced degradation. The results made it possible to discover the most demanding environmental conditions for the three tested mirror types. The degradation of the mirrors was strongly affected by the share of the Copper Accelerated Salt Spray (CASS, ISO9227) testing time in the cycle. The CASS test was combined with several other aging tests and it was concluded that especially the combination with the UV/humidity test according to ISO16474-3 was harmful for the protective coatings of the tested silvered-glass mirrors. The conclusions from the testing campaign presented in this paper are helpful to design suited comparative accelerated aging testing procedures

Degradation types of reflector materials used in concentrating solar thermal systems

Reflector materials for concentrating solar thermal (CST) technologies must preserve high quality properties (in terms of solar specular reflectance) over the complete lifetime of the solar system. However, these solar components are exposed during their operation to a combination of ambient agents that provokes a number of different defects that might severely curtail their quality. To avoid significant failures, durability studies are being conducted both in outdoor and accelerated aging conditions. The identification of the possible degradation of solar reflectors is thus a key issue for the feasibility of a CST system. This review paper presents a survey of the durability tests most commonly used (mechanical abrasion/erosion, humidity, temperature, thermal cycling, UV radiation and chemical agents), as well as the main degradation types reported in the literature for different reflectors materials (erosion, abrasion, stains on the glass, metal corrosion, tarnishing, cracking, delamination, decohesion, blistering, yellowing, discoloration, haze, agglomeration, diffusion, chalking, etc.). In addition, some defects recently identified are revealed in this publication for the first time. The results shown comprise a useful tool for the interpretation of further durability studies