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Solar Mirrors

Believe it or not, the process of making a mirror can be quite complicated. The standard mirror found in a bathroom is usually made of sheet glass with several coatings of metal applied to the back. The metal is what creates the reflection and usually consists of a layer of tin chloride followed by a layer of silver, then copper. A layer of protective paint is then applied to ensure the thin metal surface is not scratched.

The standard method of producing a mirror will often reflect 90 to 95% of light, which is acceptable for almost all applications. In most solar applications, this level of reflection is also acceptable. There is the possibility of raising reflection to 99.9999% through the application of dielectric metal coatings on mirrors. These mirrors are generally used in military applications, satellites, and for observatories that use reflecting telescopes. The cost is simply too prohibitive for use of such mirrors in solar and the gain would be relatively small in terms of energy efficiency.

Thus, the mirrors used in solar applications come in one of four varieties. They can be either metal or glass and they can be either flat (planar) or curved. The pros and cons of each are explored.

Glass vs. Metal Mirrors

For reflection, there is nothing better than glass. A standard glass mirror usually reflects about 95% of the light that strikes it will metal mirrors top out at around 90%. Glass mirrors are also better at specular reflectance, which means they are better at ensuring light is focused and not reflected back in a diffuse manner. This is the result of smoother surfaces that can be achieved with glass in comparison to metal.

There is a drawback to glass mirrors and that is fragility. In outdoor applications where hail, debris, and other projectiles may be flying around, glass mirrors can be crack or shattered, which completely destroys their reflective qualities for many solar applications. Metal mirrors are somewhat more durable and while they can be dented by large debris, they can typically withstand more impact than glass mirrors.

The choice of glass over metal may be moot with the invention of a reflective acrylic coating by 3M. This would provide the benefits of glass for reflection (94.2%) with the durability of metal. The coating is specially treated to prevent the damage that most acrylic suffers when exposed to UV light and also to prevent scratching that can occur from dust. Mirrors using this acrylic would basically be constructed of a solid metal that is overlaid with the acrylic film.

One thing to note about specular reflectance is that the further away the target is from the mirror, the more diffusion there is likely to be. This means glass mirrors are necessary for power tower designs where the distance to the tower may be several hundred meters. The 3M film is capable of achieving 95% specular reflectance, meaning that is could be used for power tower designs as well.

Planar vs. Curved

Planar mirrors are flat and they reflect light back 180 degrees from its incident angle. Curved mirrors, on the other hand, focus light to a point.  The type of mirror used generally depends on the application. For solar power towers and Fresnel reflectors, planar mirrors are perfect. For parabolic dishes and parabolic troughs, curved mirrors are necessary.

Producing curved mirrors is expensive because creating curved glass and ensuring and even backing is more difficult. Curving metal is much less expensive, but as discussed above, the reflectivity is lower. The 3M film may also eliminate the cost difference between flat and curved mirrors. If metal can be coated with the 3M acrylic to provide mirrors that are as reflective as glass mirrors, then curved mirrors will be exceptionally easy to produce and cost will decrease. This will make the already highly competitive parabolic systems even more cost effective.
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