Several articles ago, I believe I made mention of how generators and alternators work. I think it was in an article about induction motors and electric motors in general. Spinning generators are certainly common and they are by far the most common way we get our household power. Hydro, nuclear, coal, natural gas, geothermal, tidal, wind…basically any type of power generation that involves something moving…they all use generators. In the case of things like nuclear, coal, natural gas, and geothermal, the moving medium is steam created by heating regular water.
If you understand generators, that’s all there is to most power generation. Of course with nuclear, you might want to understand the nuclear fission process, but that’s separate from the power generation itself. One method of power generation that’s become increasingly common and promoted these days is solar, specifically photovoltaic. It’s important to distinguish photovoltaic power from solar power generally because you can actually use mirrors to concentrate sunlight to generate steam to spin a conventional generator. Photovoltaic generation is solid state, meaning that it has no moving parts anywhere in the process.
Photovoltaic (or solar) power cells are actually quite complicated, combining structural design, chemistry, electromagnetism, and physics. In simple terms, a solar power cell absorbs photons in semiconducting materials. Electrons in these materials are energized enough so that they can move. The structure and materials of the cell constricts how the electrons can move. Because we want to generate useable power, we want the electrons to all move in the same direction so that they can be pushed through whatever circuitry we’re trying to power. The actual nitty gritty science of the semiconducting materials and structure is very complicated.
Solar cells vary widely in their performance and efficiency. This is because not all photons are equal. While a given area of sunlight has in it some amount of energy, only some percentage of the photons will properly interact with the silicon semiconductor. Many of the photons that hit a solar cell will have too much energy and will simply be converted to heat. This is exactly the same kind of heat you feel on your skin if you lay in the sun.
Metal-semiconductor contacts are imprinted onto both sides of the solar cell to funnel the moving electrons into external wires and on to external loads. After the electrons pass through the external loads, they come back to the solar cell and fill in the gap left by other electrons that have moved. If it’s not clear by now, solar cells can only generate direct current power. In grid-tie solar applications, the DC from the solar array must be inverted to grid-level AC power. This is definitely the biggest difference between photovoltaic power and almost all other forms of electricity generation.
To some people it may seem counterintuitive, but solar cells work best in bright, cool conditions. If a cell heats too much, its output will be reduced. When a solar panel is rated, they rate it at a nominal 25 degrees celsius. For each degree of temperature increase, every panel will have a known loss of efficiency. To grossly oversimplify it, a cold electron at rest will have a much greater change in energy state when it’s knocked free than a hot electron already in motion. Therefore a hot panel will produce a lower voltage and thus, less power. On cold winter days, my solar panel at home increases its voltage so much that the controller can use the extra voltage to create more charging current. So my panel which is rated for a short-circuit current of 2.78 amps can actually deliver a charging current to my battery of over 4 amps (in ideal conditions).
I had to simplify this topic considerably because there’s just no way I could go into p/n-type doped silicon and how semiconductors work in a short article. I highly recommend reading up on it though because it is a fascinating combination of electromagnetism and chemistry. Semiconductors have a lot of uses and are very interesting. Maybe I can condense some of it into a future article.