Monday, November 19, 2012

What a Solar Cell Is in Physical Terms

I have written at great length about the potential for solar power, but I have never taken the time to describe what a solar cell is in physical terms. It is an important detail to mention because the physical form of a solar cell is part of the reason why I am convinced that solar cells will be all over the place in the future.

Appearances can be deceiving. If you look at the solar cell of a pocket calculator or similar device, it can look like a cut square piece of metal. Look at a solar panel set up on a roof or a lawn, and it may look like a black piece of glass. These superficial views are not too far off the mark, but they don’t get at the heart of the physical properties of a solar cell that make it work.

Most solar cells are made from silicon, a common material that is not a metal, but has some of the properties of a metal. A solar cell is made of at least two layers of silicon. The two layers are adjusted with other minerals to make them different from each other in their electrical properties, but these other minerals are present in such small amounts that this is a detail you can’t really see. Conductors, such as wires, are added to each surface to carry away the generated electrical current.

The front layer of silicon has to be thick enough to soak up the sun. The thought of a lump or sheet of metal-like material is more than is actually needed. When you think of sheet metal, you probably think of something at least one millimeter thick, but the surface layer of a solar cell does not have be much. A thickness of 16 micrometers, the thickness of household aluminum foil, is enough to take up almost all the light that comes along. Most of the thickness of a solar cell, then, may come from the layers of attached glass or aluminum that protect the cell from breaking.

If most of the physical material of a solar cell is intended for protection against physical impact, then solar panels can be integrated into other devices intended for physical protection. Solar-cell roofing tiles may be the most obvious possibility here, but there are others. Bricks, walkways, even windows could, in principle, incorporate solar panels. For windows, these would be solar cells made thinner so that a third of the light could pass through.

If you think of the weight of aluminum foil, you realize that solar cells are perhaps not too heavy to build into vehicles. With the right design, solar panels could be integrated into the upward-facing surfaces of a vehicle. A vehicular solar array would not generate nearly enough electricity to operate the vehicle, but if it could add just one or two percent to the daytime range of an electric vehicle, or extend the battery life by a similar amount, that would be a welcome addition. (Here’s a story of a record-setting motorcycle powered by a solar array. The array is not on the motorcycle itself, so you’ll have to use your imagination for that part.)

Silicon is one of the most abundant minerals. The use of solar cells is limited mainly by the cost of manufacturing. With current manufacturing, solar cells are efficient enough for use only in installations that are built to optimize the solar power result. It helps, for example, if solar panels can be tilted toward the sun, pointing toward the equator at an angle matching the latitude of the installation. When manufacturing costs go down by one order of magnitude, though, solar cells will be cost-effective in less than optimal installations. And when costs fall below that level, we will start to see solar panels everywhere. I am not saying this will happen soon. But looking at the long run, it seems inevitable.