Glass anti-reflection coatings have been the low-hanging fruit for improving PV module efficiencies, but reaching for it requires the right mix of low-cost approaches and smart science.
Researchers at the Hong Kong University of Science and Technology (HKUST) think they may have cracked it. Dr. Zhiyong Fan and his group have developed a novel coating made of silicone (the material used to make soft contact lenses) with tiny nano-cones on its surface, which can be directly applied onto glass or even onto a silicon wafer for much cheaper than other similar nano-textured films.
Without an anti-reflection coating, the glass surface of a PV module can reflect 4-5% of the light that falls on it, causing module efficiency and energy output to drop. While traditional anti-reflection (AR) coatings are simple to apply, they are engineered to work only in a narrow range of the sun’s spectrum. More importantly, such coatings become less effective when the sun rays hit the module at a large angle – such as in the morning or evening.
The key to capturing this light is creating a textured surface on the AR coating (as opposed to the standard smooth/flat films). However, dirt can be more easily lodged in the ‘bumps’ (think hills and valleys) that form the texture if they are too big, making the soiling losses outweigh any gains due to the texturing. To prevent soiling, it is critical to keep the features of the texture very small – in the order of a few nanometres (1 nanometer is 1/100,000,000th of a metre). Such fine texturing typically requires expensive, time-consuming and material-intensive processes like photo-lithography or chemical etching, often on the glass itself. All this completely changes the cost equation and makes textured AR coatings infeasible.
Fan and his team have managed to walk the fine line by creating a thin layer of silicone with nanometre-sized cones on its surface using a mould-patterning technique. The process used to create the mould involves multiple electro-chemical processes, but once a mould is created, preparing the film simply involves pouring on the liquid silicone, drying it and peeling it off the mould. Then, thanks to the special type of silicone used, this layer can be applied onto any glass easily even without an adhesive.
When applied to a small thin-film cadmium-telluride module (see top image), the coating halved the reflection losses and increased the energy output not only at noon (when sunlight hits the module directly) but also in the morning and evenings, where the suns rays hit the module at a larger angle (see in-set image in the graphic below). The net result – a neat 7% improvement in the energy output throughout the day.
Click to enlarge (image credit: Zhiyong Fan via SPIE)
In addition to improving the energy output, the film is also super-hydrophobic (strongly repels water), making it self-cleaning and very effective against soiling, which is a major contributor to maintenance costs in large-scale solar plants.
The researchers at the so-called ‘Fan lab’ at HKUST aim to continue exploring other silicone and non-silicone materials so they can tailor the films to different types of glass used with various PV cell technologies.
However, the researchers seem confident that the film will attract fans in the PV industry, with details soon to be published about a high-throughput fabrication method to produce the films cheaply and at a large scale.
Whether the production costs will make it a feasible alternative to existing sol-gel based AR coatings and whether the anti-soiling properties remain stable throughout the lifetime of commercial modules (typically 20-25 years) remains to be seen.
Top Image Credit: Siu-Fung Leung et al
© 2014 Solar Choice Pty Ltd
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