When applied to glass, it oxidizes any dirt or other material adhering to the glass surface, decrease its “stickiness”, and letting it fall away by gravity (or wind) or be washed away when water is applied to the surface. It does this with minimal reduction in the amount of sunlight that passes through the glass.
Our coating gives the surface to which it is applied a hydrophilic nature, allowing water to spread out evenly over the surface, instead of coalescing into droplets. When water is applied, either from rain or from another source, it spreads out (sheens) instead of forming droplets, and when it runs off the glass surface, it takes any loose dirt away with it.
Studies conducted by DTE, in California, showed that soiling of the tested PV panel caused a 33% reduction in energy output over time. A light rain restored 5% of the power for a short time, and only an extended heavy rain brought the power back up to the normal output level.
The use of the ITF Self-cleaning coating allows any moisture coming in contact with the panel to wash off the surface soiling and restore lost power to the system.
What is “Dirt”?
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Briefly, glass can get “dirty” from both organic and inorganic contaminants. The surface of glass will let, to a degree, water from rain or other sources wash away the inorganic contaminants because they generally do not “stick” to the glass surface. Dirt that sticks to the glass, generally organic and opaque in nature, will lower the efficiency of the installation over time. This dirt is not by its nature water-soluble so it will remain on the surface unless steps are taken to remove it.
Also, inorganic dirt, generally dust and other fine particles, either blown or just settling onto the glass surface, can be held to the glass by a static charge on the surface of the glass.
ITF has prepared a paper detailing the definition of “Dirt” and its consequences as it applies to glass, and the issues surrounding its removal. See the Publications section of this site for a full copy of the paper.
The Coating Itself:
The key technical differentiator between the ITF self cleaning coating and those offered by the Architectural Glass Industry (ref: Pilkington Activ) is the optical properties of the coated glass.
The architectural self-cleaning coatings consist of relatively thick layers of TiO2, also known as "Titania", which due to its refractive index renders the glass more reflective than uncoated glass, and thus the glass treated this way is not usable in a situation when the passage of the maximum amount of light is required.
The ITF coating structure consists of nanocrystals of TiO2 incorporated into a “host” thin film such as SnO2:F (known as Fluorine doped Tin Oxide), Al2O3 or MgO. That the TiO2 is not applied by itself as a layer on the glass is the crucial difference. The coating layer we apply is quite thin, and since we do not form a contiguous TiO2 film, we end up with “islands” of TiO2 dispersed in the host layer. So the reflectance of the surface is not dominated by the refractive index of the TiO2, but rather by the refractive index of the host coating.
In addition, the host layer can be useful if we exploit it for its own properties. SnO2 can be electrically conductive, so it could aid in the elimination of static charge build up. The Al2O3 is very hard (abrasion resistant), so it may improve the durability of the surface. MgO can decrease the surface reflectance, enhancing the optical properties of the glass coverplate.
In summary, unlike the other self-cleaning coatings available, the ITF coating does not pose a problem for solar use, and it has application in polycrystalline PV, thin film PV, Solar Mirrors, Heliostats, and hot water thermal collectors. By making the glass Self-Cleaning, it acts to increase the efficiency and economic viability of all solar systems over time.