Friday, June 11, 2010

It's Electric

The funny thing about “energy” is that it is neither created nor destroyed; it only changes form. So when coal is burned at a power plant the coal’s chemical energy is ultimately converted into electricity. This process is typically only 35% efficient ; wasting approximately 65% of coal’s energy when converting its chemical energy into electricity. Basically the heat from the fire is used to boil water, which is used to turn a turbine, which turns a magnet inside a coil of wires to produce electricity. So heat from burning (considered chemical energy) is converted into mechanical energy by turning the turbine which is then converted into electrical energy simply by moving a magnet through a coil, losing energy at each step along the way.

As the world becomes more energy conscious, we are seeing new technologies that convert mechanical energy into electricity. It’s pretty simple to convert mechanical energy into electricity so I encourage you to use your imaginations and invent stuff. Wind farms already exists (many on the Tug Hill Plateau west of here) that convert steady wind into electricity as do hybrid vehicles that use the rolling wheels while breaking to produce electricity. You can sort of think of a coal-fired power plant as a fancy way to create a steady wind that will turn a turbine to produce electricity.

Solar energy conversion does not rely on mechanically moving a magnet through a coil of wires as with many other electricity producing methodologies. It relies on the sun to induce electron motion inside a semiconductor device. Solar energy conversion is very attractive since it is passive and does not rely on burning carbon. However, it’s relatively expensive and requires sunlight. I’m still amazed at the magic of solar power. Think about what we are asking a solar cell to accomplish. We want to create electrical energy from a piece of semiconductor that has no moving parts and we want it to last more than 20 years being blasted by the sun daily and we want it to be cheap and reliable. I would not last 20 years if you put me in the sun everyday and asked me to reliably produce and I think I’m a pretty sophisticated machine, as far as humans go.

How does a solar cell do it? There are two key components to solar cells, the semiconductor material from which it is made and the special junction between two semiconductor materials which acts as an electrical one-way sign. I will leave detailed physics and electrical engineering explanations of solar technology to other more eloquent sources. Instead I will opt to explain it through interpretative dance…. of course.

Imagine a dark stage with dancers on the floor in fetal positions; this is our un-illuminated solar cell. As you watch the motionless dancers in dark silence you notice a line that divides the stage in half and the line has a large one-way symbol pointing to the right. As the stage lights come up, our dancers awaken and begin to randomly walk around the stage. This represents the motion of electrons in a solar cell once the sun is shining. You marvel at the dancers’ movements under the bright lights of the stage and notice something curious. Although each dancer (a.k.a. electron) has a random motion, they are all beginning to congregate on the right hand side of the stage because after they cross the center line, the one-way (P/N junction) does not let them randomly walk stage left. So the stage is loaded with flying dancers on the right while only a few remain on the left. This represents a charge separation and will produce electricity if “released”. So, the last step in our geek-fest interpretive dance of the solar cell is to picture a tube (representing an electrically conducting wire) placed on the stage that allows the dancers on the right to crawl through and get back to the left where there is more space. Many of them crawl through the tube and as they do, you realize that they represent electricity flowing through a wire and are awestruck by the simple elegance that is a solar cell. As the curtain closes you clap, cheer and exhale greenhouse gases to show your appreciation.

Hope this helps to explain how things work. I figure there is never a wrong time to invoke an interpretive dance analogy when explaining complex physics processes.

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