Guidelines Accelerated Aging Testing of Aluminum Reflectors for Concentrated Solar Power

This testing guideline specifies the recommended accelerated aging conditions to reproduce realistic degradation mechanisms on aluminum reflectors, which have been observed during a 3 year outdoor exposure testing campaign at 9 exposure sites with 9 different aluminum reflector types (7 of them protected with a SiO2 based sol-gel coating, 2 of them protected by a polymeric coating). Accelerated aging parameters have been derived to simulate the exposure after 3 and 10 years at three reference outdoor scenarios: “extreme desert”, “desert” and “coastal”. The parameters have been achieved by averaging the 9 tested material types. However, if the material to be tested deviates significantly in its chemical composition, it is likely that other degradation mechanisms will be accelerated and the correlation to the outdoor reference scenarios will not apply. Fig. 1 shows the materials to be tested under the scope of this guideline. In any case, the indicated correlation to the outdoor reference scenarios can only be considered as an estimate. For some of the 9 tested materials the correlation applied better than for others. The expected uncertainty is 4.4 % (pp) specular reflectance, ρ, at a wavelength, λ, of 660 nm, an incidence angle, θi, of 15° and an acceptance angle, φ, of 12.5 mrad to simulate 3 years of exposure on the desert and coastal sites. For the “extreme desert” site the expected uncertainty is considerably higher because of the strong dependence on specific abrasion effects on site. The simulation of 10 years of exposure is based on extrapolation of the available outdoor data after 3 years

Evaluation of anti-soiling coatings for CSP reflectors under realistic outdoor onditions

Soiling of solar reflectors affects their reflectance and has a direct impact on the power output of concentrated solar power (CSP) plants. One way to minimize the efficiency losses is the implementation of anti-soiling coatings on the reflector surfaces. This method is being studied for the past decade, but has not been successfully commercialized yet. The purpose of the coatings is to reduce soiling and improve the washability of the reflectors. In this work results are presented from an extensive outdoor campaign of two potential anti-soiling coatings under realistic conditions at a representative CSP site in southern Spain. Nearly six years of outdoor data are available, which makes this campaign the longest published on this type of coatings. Regular cleaning and reflectance measurements were performed during the exposure and conclusions about the performance and durability of the coatings are drawn. It is shown that in the initial state the coatings show an advantageous behavior, resulting in higher reflectance during outdoor exposure due to less soiling and better cleaning of the reflectors. The second main finding is that durability is an important issue for the implemented coatings, as their properties degrade over time resulting in lower reflectance values after several years of exposure compared to conventional glass reflectors

Advanced cyclic accelerated aging testing of solar reflector materials

ABSTRACT: Lifetime prediction methods for the components of concentrated solar power (CSP) plants have been in the focus of interest of manufacturers and plant developers for the past years. Recently, an accelerated aging standard for solar mirrors was published by the Spanish AENOR committee [1]. This standard allows performing comparative testing but it is not suited to derive meaningful life-time estimations. Firstly, the testing defined in the standard is not aggressive enough to produce significant degradation on most of the materials and secondly, it has been shown that the passing of these tests does not guarantee a high durability during outdoor exposure [2]. These findings have proven the need for the investigation of more realistic procedures. To provoke the mechanisms that are detected during outdoor exposure, a more realistic application of environmental stresses is being investigated. In this work a series of tests is conducted in which several weathering stresses are combined and applied in a cyclic manner. Some of these cycles have shown to correlate better with real outdoor behavior.N/

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

Standardizing Accelerated Aging Testing Conditions for Silvered-Glass Reflectors

To assure the quality of accelerated aging tests for solar concentrators, their standardization is crucial. It guarantees the employment of adequate testing, measurement and characterization procedures and the comparability between results. A committee of the Spanish AENOR standardization agency is working on the draft “Reflector Panels for Concentrating Solar Technologies”. This work focuses on the evaluation of the procedures defined in this standard. The reflector material from the Japanese company Nishio Glass Mirror Co., which showed severe degradation after 7 years of outdoor exposure in Abu Dhabi, is tested according to the AENOR standard to check if its poor outdoor performance could have been predicted by accelerated aging testing. However, after completion of the accelerated tests, in some cases even for considerably longer test durations than the minimum required by AENOR, no considerable degradation was detected. The results suggest that the proposed testing program by AENOR is not aggressive enough to identify material failure. Ways to improve the current standard are proposed through development of more realistic tests

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

Parallel soiling measurements for 4 mirror samples during outdoor exposure with TraCS

The upgraded version of the Tracking Cleanliness Sensor, the so-called TraCS4, which determines the realtime cleanliness and soiling rate of four mirror samples simultaneously is presented. Using this device, different materials can be intercompared at the site of interest which is the main advantage as the performance of anti-soiling-coatings is usually dependent on local weather conditions. A detailed uncertainty analysis of the TraCS4 device results in a cleanliness measurement uncertainty of about 0.019 cleanliness points. The uncertainty for the intercomparison of different materials is derived to be 0.012 cleanliness points. Exemplary soiling measurements with different anti-soiling and reference coatings at PSA are presented

Accelerated aging test of solar reflectors according to the new AENOR standard – results of a round Robin test

Durability tests of reflector materials for concentrating solar applications are crucial to guarantee the profitability of the plants and to ensure a proper efficiency during their lifetime. A standard including a set of five accelerated aging tests is close to be published by the Spanish standardization entity AENOR, within the sub-committee AEN/CTN 206/SC117. Under the framework of the STAGE-STE project, a Round Robin Test was organized by six partners to evaluate the comparability of results obtained in their respective laboratories after performing these durability tests. According to the results, in general a good agreement among the partners was found, with negligible to slight reflectance losses. In addition, it was noticed that the reference standard used in the reflectance measurements is of high importance. The conclusions of this work will help to improve the standard in future versions.The research leading to this work has received funding from the European Union Seventh Framework Programme (FP7/2007-2013) under grant agreement n° 609837 (STAGE-STE). The authors would like to thank the participants of the working group for solar reflectors of WG 2 of the Spanish sub-committee AEN/CTN 206/SC 117

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