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Coatings 2019, 9(11), 739; https://doi.org/10.3390/coatings9110739
- Negligible reduction of the reflectance properties in the initial status.
- Appropriate durability over time, which means that the coating must keep its optical properties after being exposed to the weather agents (abrasion, temperature, humidity, pollutants and radiation).
- Appropriate behavior in reducing the dust accumulation on the reflector surface under real outdoor conditions.
2. Materials and Methods
- The overall cleanliness factor of samples for S2 is higher than for S1. This is due to the fact that the cleaning with brush is more effective and able to restore the cleanliness basically to its initial values. This fact will be confirmed by the values in the clean state in the following section.
- The cleanliness is higher in periods with a high quantity of rain. Rain usually acts as a natural cleaning mechanism and prevents the samples from excessive soiling.
- The cleanliness gain of the coated samples for both structures is higher when the soiling is stronger and thus the overall cleanliness is lower. The stronger the soiling is, the better the coatings can fulfill their purpose of avoiding the soiling (this can be extracted from the combinations of both graphs).
- The cleanliness gain remains positive for the whole exposure campaign, proving a positive effect of the coatings for all occurring soiling situations. There is only one single event where the coating shows a negative value for S2. This was an event of exceptionally strong soiling due to light rain in combination with a dusty atmosphere (mean cleanliness of 0.66). These events lead to a very inhomogeneous soiling pattern on the samples (Figure 3) and to a high standard deviation of the measurements. With these high deviations, the probability of the occurrence of aberrations is increased. The rain event in this case was too weak to contribute a sufficient quantity to be detected by the rain gauge measurements.
- The highest cleanliness gain Δξ detected was as high as 7.2 pp for S2 after another medium to strong soiling event (mean cleanliness 0.88 on that day).
- The mean cleanliness gain before cleaning is 1.4 pp for S1 and 0.9 pp for S2. When only situations of stronger soiling are evaluated, the mean cleanliness gain is even higher. Looking only at events with a high soiling level (cleanliness below 0.9), the mean cleanliness gain rises up to 2.4 pp (S1) and 1.5 pp (S2).
- The cleanliness after cleaning remains higher for S2 during the whole exposure campaign and always reaches values around 1. The values even reach values slightly higher than 1 due to the measurement uncertainty of the reflectometer. Therefore, the cleaning method using the brush is much more effective than the high pressure water method.
- The cleaning method with pressurized water is not able to restore the initial cleanliness values over the whole exposure campaign (ξ < 1). Remaining soiling stays on sample surfaces decreasing the reflectance of the samples on S1. The effect is cumulative, showing decreasing values after cleaning in certain periods, especially after the first reset when strong soiling appears (compare Figure 2). This tendency is more pronounced for the uncoated reference material. That also influences the before cleaning values in Figure 2, where the difference between S1 and S2 is especially strong in periods of lower soiling after the accumulation of soiling on S1.
- After the two resets (extensive cleaning, marked in the chart of the cleanliness factor), the initial cleanliness is also nearly restored for S1, indicating negligible degradation.
- The cleanliness gain Δξ after cleaning is lower than before cleaning. This underlines the fact that Δξ is more pronounced with stronger soiling. Nevertheless, it stays in the positive regime for the whole campaign and thus proves the advantage of the anti-soiling coatings and indicates the positive effect on the “easy-to-clean” property.
- The mean cleanliness gain is also higher for S2 because of the lower cleanliness of S1.
- The anti-soiling effect of the applied coatings leads to a higher cleanliness of the reflectors throughout the exposure campaign. Taking into account the whole campaign, an accumulated cleanliness gain of 1.0 and 0.7 pp is reached, for pressurized water and brush cleaning respectively. When only the samples in the soiled state (before cleaning) are evaluated, the accumulated cleanliness gain increases to 1.4 and 0.9 pp, respectively. These values are even higher for situations of stronger soiling: for a cleanliness below 90%, the accumulated cleanliness gain increases to 2.4 and 1.5 pp respectively. The detected maximum momentary cleanliness gain reaches over 7 pp for a single soiling event.
- Lower soiling rates are detected for the anti-soiling coated samples, with a minimum value of 0.52 pp/day for the coated samples on S1, and a maximum of 0.60 pp/day for the uncoated samples on S2.
- The “easy-to-clean” properties of the anti-soiling coatings facilitate the cleaning process and thus help to recover the initial cleanliness of the reflectors.
- Fewer cleaning cycles have to be performed for the coated mirrors to reach the same mean cleanliness compared to uncoated mirrors. For a constant threshold cleanliness of 0.96, the number of cleaning cycles in Tabernas, Spain can be reduced by 7% to 12% for brush cleaning and pressurized water, respectively, considering a constant soiling rate and a fixed cleaning frequency of 2 weeks.
- The coatings show excellent durability during the course of the whole campaign and showing no signs of degradation.
- The investigated site represents an environment with relatively little dust in the atmosphere, leading to low soiling rates. The performance of the coatings is expected to be better for high soiling sites because the coating has demonstrated a higher effectiveness with stronger soiling.
Conflicts of Interest
- Mills, D. Advances in solar thermal electricity technology. Sol. Energy 2004, 76, 13–19. [Google Scholar]
- Bouaddi, S.; Fernández-García, A.; Ihlal, A.; Ait El Cadia, R.; Álvarez-Rodrigo, L. Modeling and simulation of the soiling dynamics of frequently cleaned reflectors in CSP plants. Sol. Energy 2018, 166, 422–431. [Google Scholar]
- Wolfertstetter, F.; Wilbert, S.; Dersch, J.; Dieckmann, S.; Pitz-Paal, R.; Ghennioui, A. Integration of soiling-rate measurements and cleaning strategies in yield analysis of parabolic trough plants. J. Sol. Energy Eng. 2018, 140, 041008–041011. [Google Scholar] [CrossRef]
- Bouaddi, S.; Fernández-García, A.; Sansom, C.; Sarasua, J.A.; Wolfertstetter, F.; Bouzekri, H.; Sutter, F.; Azpitarte, I. A review of conventional and innovative-sustainable methods for cleaning reflectors in concentrating solar power plants. Sustainability 2018, 10, 3937. [Google Scholar] [CrossRef]
- Sarver, T.; Al-Qaraghuli, A.; Kazmerski, L. A comprehensive review of the impact of dust on the use of solar energy: History, investigations, results, literature, and mitigation approaches. Renew. Sustain. Energy Rev. 2013, 22, 698–733. [Google Scholar] [CrossRef]
- Costa, S.C.S.; Diniz, A.S.A.C.; Kazmerski, L.L. Dust and soiling issues and impacts relating to solar energy systems: Literature review update for 2012–2015. Renew. Sustain. Energy Rev. 2016, 63, 33–61. [Google Scholar] [CrossRef]
- Atkinson, C.; Sansom, C.L.; Almond, H.J.; Shaw, C.P. Coatings for concentrating solar systems—A review. Renew. Sustain. Energy Rev. 2015, 45, 113–122. [Google Scholar] [CrossRef]
- Midtdal, K.; Jelle, B.P. Self-cleaning glazing products: A state-of-the-art review and future research pathways. Sol. Energy Mater. Sol. Cells 2013, 109, 126–141. [Google Scholar] [CrossRef]
- Polizos, G.; Sharma, J.K.; Smith, D.B.; Tuncer, E.; Park, J.; Voylov, D.; Sokolov, A.P.; Meyer, H.M.; Aman, M. Anti-soiling and highly transparent coatings with multi-scale features. Sol. Energy Mater. Sol. Cells 2018, 188, 255–262. [Google Scholar] [CrossRef]
- Piliougine, M.; Cañete, C.; Moreno, R.; Carretero, J.; Hirose, J.; Ogawa, S.; Sidrachde-Cardona, M. Comparative analysis of energy produced by photovoltaic modules with anti-soiling coated surface in arid climates. Appl. Energy 2013, 112, 626–634. [Google Scholar] [CrossRef]
- Syafiq, A.; Pandey, A.K.; Adzman, N.N.; Rahim, N.A. Advances in approaches and methods for self-cleaning of solar photovoltaic panels. Sol. Energy 2018, 162, 597–619. [Google Scholar] [CrossRef]
- Moraes Lopes de Jesus, M.A.; Timò, G.; Agustín-Sáenz, C.; Braceras, I.; Cornelli, M.; de Mello Ferreira, A. Anti-soiling coatings for solar cell cover glass: Climate and surface properties influence. Sol. Energy Mater. Sol. Cells 2018, 185, 517–523. [Google Scholar] [CrossRef]
- Schwarberg, F.; Schiller, M. Enhanced solar mirrors with anti-soiling coating. In Proceedings of the 18th Int Conference on CSP and Chemical Energy Systems, Marrakech, Morocco, 11–14 September 2012. [Google Scholar]
- Polizos, G.; Schaeffer, D.A.; Smith, D.B.; Lee, D.F.; Datskos, P.G.; Hunter, S.R. Enhanced durability transparent superhydrophobic anti-soiling coatings for CSP applications. In Proceedings of the ASME 8th Int Conference on Energy Sustainability, Boston, MA, USA, 30 June–2 July 2014. [Google Scholar]
- Plesniak, A.P.; Pfefferkorn, C.; Hunter, S.R.; Smith, D.B.; Polizos, G.; Schaeffer, D.A.; Lee, D.F.; Datskos, P.G. Low cost anti-soiling coatings for CSP collector mirrors and heliostats. In Proceedings of the SPIE 9175, High and Low Concentrator Systems for Solar Energy Applications IX, San Diego, CA, USA, 7 October 2014. [Google Scholar]
- Ubach, J.; Gómez, E.; Zarrabe, H.; Aranzabe, E. Coated Glass for Solar Reflectors. U.S. Patent EP 3090990 A1, 4 May 2015. [Google Scholar]
- Aranzabe, E.; Azpitarte, I.; Fernández-García, A.; Argüelles, D.; Pérez, G.; Ubach, J.; Sutter, F. Hydrophilic anti-soiling coating for improved efficiency of solar reflectors. In Proceedings of the AIP Conference Proceedings, Santiago de Chile, Chile, 26–29 September 2018; Volume 2033, p. 220001. [Google Scholar]
- Fernández-García, A.; Aranzabe, E.; Azpitarte, I.; Sutter, F.; Martínez-Arcos, L.; Reche-Navarro, T.J.; Pérez, G.; Ubach, J. Durability testing of a newly developed hydrophilic anti-soiling coating for solar reflectors. In Proceedings of the 24th Solar PACES International Conference on CSP and Chemical Energy Systems, Casablanca, Morocco, 2–5 October 2018. [Google Scholar]
- Fernández-García, A.; Sutter, F.; Martínez-Arcos, L.; Sansom, C.; Wolfertstetter, F.; Delord, C. Equipment and methods for measuring reflectance of concentrating solar reflector materials. Sol. Energy Mater. Sol. Cells 2017, 167, 28–52. [Google Scholar] [CrossRef]
- Fernández-García, A.; Sutter, F.; Montecchi, M.; Sallaberry, F.; Heimsath, A.; Heras, C.; Le Baron, E.; Soum-Glaude, A. Parameters and Method to Evaluate Reflectance Properties of Reflector Materials for Concentrating Solar Power Technology—Official Reflectance Guideline Version 3.0; SolarPACES: Tabernas, Spain, March 2018. [Google Scholar]
|Material||Initial ξ [–]||Final ξ [–]||Cleanliness Loss Due to Degradation, Δξ [–]|
|S1||Coated||1.000||0.997 ± 0.006||−0.003 ± 0.006|
|Uncoated||1.000||0.996 ± 0.003||−0.004 ± 0.003|
|S2||Coated||1.000||1.001 ± 0.003||0.001 ± 0.005|
|Uncoated||1.000||0.999 ± 0.002||−0.001 ± 0.004|
|Material||S1 Coated||S1 Uncoated||S2 Coated||S2 Uncoated|
|Average soiling rate [pp/day]||−0.52||−0.59||−0.56||−0.60|
|Time until cleaning [days]||7.72||6.81||7.16||6.63|
|Number of cleanings per year||47.34||53.63||50.99||55.06|
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