Deserts seem an obvious location for capturing the vast amounts of solar energy entering the Earth’s atmosphere each day. The deserts in the Middle East represent huge swathes of uninhabited space with high solar radiation levels; yet there are large, modernised cities nearby that require huge amounts of energy.
There is one significant problem with placing solar panels in the desert. The deposition of dust and sand particles on the surface of the glass panels that protect the photovoltaic (PV) cells rapidly reduces the efficiency of the system, sometimes within hours of the panels being put in position. In addition to the daily movement of dust by the wind, dust storms, sometimes with fierce intensity, can bury panels under a thick layer of particles and damage the structural integrity of the system. Cleaning solar panels regularly under these conditions becomes time consuming and costly.
Harvesting the desert’s solar energy remains compelling, however, so scientists have spent years trialling different methods for reducing the impact that dust has on the efficacy of solar panels. Adding a dust-repellent coating to the glass covering the PV cells is one possible solution to improve PV efficiency.
Mohammed Bahattab and his team at KACST in Riyadh, together with co-workers at the Fraunhofer Institute for Silicate Research in Würzburg, Germany, are refining the design of dust-repellent coatings for PV applications. Their coatings may also work for other purposes, such as covering window glass for buildings in arid regions.
“The complexities of desert dust may surprise people,” says Bahattab. “Sand grains, of course, come in a wide range of sizes and shapes, but we also need to consider all manner of microscopic, airborne particles within the dust mixture. The origin of the dust determines the size and type of particles involved. Different parts of the country will experience a wide variety of dust deposition under different weather conditions. This compounds the problem when you want to create effective dust-repellent surfaces.”
The researchers were mindful that their coating must not interfere with the optical properties of the PV cells under the coated glass. Maintaining transparency was vital. An increase in the refractive index of the glass, for example, could impact the ability of the PV cells to harvest solar radiation to their highest potential. It was also vital for the coating to be anti-reflective so that it absorbs as much light as possible.
The team used a technique called sol-gel processing to design novel protective coatings. This method involves creating solid films that incorporate small molecules of certain desired sizes and shapes. Bahattab’s team made a colloidal solution of silicon dioxide (SiO2) nanoparticles of varying sizes for their PV coating. They designed the solution so that it solidified after baking in an oven at 500°C.
The researchers created two sol-gel coatings that they used to dip-coat glass panels and form an antireflective, anti-dust layer to protect the PV cells. The first film incorporated uniform SiO2 nanoparticles of approximately 8 nanometers in diameter. This resulted in a fine-grained surface topography to the coating, slightly rougher than plain glass. The topography of the second film’s surface was more corrugated; the team’s cross-sectional analysis showed that circular protrusions were created by the addition of larger SiO2 particles (up to 60 nanometers in diameter) enclosed in the fine-grained SiO2 matrix.
“We wanted to examine the dust-repelling capabilities of both these manufactured films in comparison to plain glass, in both laboratory and outdoor settings,” says Bahattab. “In the lab, we replicated conditions similar to those found in very arid regions, using three different grades of standardized Arizona test dust, ultrafine, fine and medium grain sizes, and an electric fan or compressed airflow to represent the wind. We also experimented with small amounts of water, replicating rainfall. In outdoor tests, we chose six locations across Saudi Arabia that experience the wide range of desert weather conditions we have in the kingdom.”
The two new coatings performed well in all tests, with a significant anti-soiling effect in comparison with bare glass panels. Initial results suggest anti-dust coatings could increase desert solar panel efficiency by 10 percent. The fine-grained coating appeared to work most effectively, although the addition of larger particles in the other coating only made a minor difference to transmittance (the ability of a material to transmit light energy of different wavelengths). In the lab, the fine-grain Arizona dust presented the biggest challenge, suggesting that the microstructures of certain dust particles are more problematic than others.
The outdoor tests were carried out at six sites for one year, and only minor rainfall occurred during that time – the site in Al Ahsa in eastern Saudi Arabia, for example, received 36 millimeters of rain over the whole year. The laboratory test results coincided reasonably well with outdoor results, suggesting the lab scenarios are a good replication of actual desert environments in Saudi Arabia.
“Our outdoor trials will help us determine the best formulation for coatings under specific weather conditions. Our analysis of this data is ongoing,” says Bahattab. “In general, it seems that the patterns made across the glass by dust particles influence PV cell efficiency more than levels of contamination.”
A further advantage of the coatings is that PV cells will be protected from damaging ultraviolet rays, a component of natural solar radiation that cannot penetrate the coated glass.
“We expect our dust-repellent coating materials will contribute to solving many problems caused by dust in arid regions,” says Bahattab. “As well as enhancing solar cell efficiency and reducing cleaning time, these coatings could be used on glass surfaces in buildings. The process is easily scaled up for widespread distribution. We are now creating a pilot solar plant at the kingdom’s Solar Village [dedicated to solar energy research and development] to trial our coatings on a larger scale.”
- Bahattab, M.A., Alhomoudi, I.A., Alhussaini, M.I., Mirza, M., Hegmann, J. Glaubitt, W., Lobmann, P. Anti-soiling surfaces for PV applications prepared by sol-gel processing: Comparison of laboratory testing and outdoor exposure. Solar Energy Materials and Solar Cells 157, 422-428 (2016).
- Conference papers from the Second International Conference on Dust, Ilam University, Iran, April 2018. | article