Energy from the sun

Light is a form of energy visible to the human eye. Sometimes light acts like a wave and sometimes it acts like a particle. The particles are called photons. The waves are called electromagnetic waves. 

Light you can see—visible light—is a small part of the electromagnetic spectrum. 

Click here to see a diagram of the electromagnetic spectrum.

The sun is an incandescent light source, which means that its light comes from heat. The temperature of the sun is about 6,000º C or 11,000° F .  About ½ of the energy in the sunlight that reaches the earth's surface is visible.  About 3% is ultraviolet light (the kind that gives you a sunburn) and the rest is infrared. 

The sun's energy travels through space to the earth but does not heat the space. The radiation only turns into heat when the infrared rays are absorbed by something, like a moon, a planet, or your skin.

How solar cells work

Photovoltaic cells are sometimes called solar cells. 

Light can knock electrons out of the atoms of certain substances. Solar cells take advantage of this fact. Each solar cell is like a little sandwich.  The "bread" is made of silicon, which is the main ingredient in sand. 

Silicon mixed with other chemicals can have either a positive or negative charge. The solar cell sandwich is made so that that one piece of bread has a positive charge (called the p-layer) and the other has a negative charge (the n-layer).  The n-layer has a tendency to loose electrons when light strikes it. The p-layer welcomes electrons to its "holes". 

As sunlight strikes the n-layer facing the sun, it knocks millions of electrons out of their places.  They move to the positive charge of the p-layer. This movement is electricity. 

You can think of this process as a row of people standing side to side. The first person pokes the second person in the ribs. The second person pokes the third person in the ribs, and so on. The tenth person gets poked in the ribs too, but it's by the ninth person, not the first one. The "poking elbow" moved down the line, but the first elbow didn't travel all the way to the other end. 

Solar electricity at Impression 5

The electricity in your house is alternating current. It flows very quickly back and forth. Electricity from solar cells is direct current, or one-way flow. It can be used that way or it can be converted to alternating current by a gizmo called an  inverter. It can also be stored for later use, using batteries. 

The electricity generated by the photovoltaic system at Impression 5 is used to run a fan that transports the heated air from the back side of the transpired air collector into the building. The fan only runs when the sun is shining. Is that a good thing? 

Yes, if the sun isn't shining, the solar collector probably isn't getting hot, so there is no heat to move into the building.

Photovoltaic cells have no moving parts. If they are properly installed, there is nothing to wear out. 

Because the sun shines everywhere (at least some of the time), photovoltaic cells can be used anywhere, including lots of places where there are no power lines—like on space ships or remote places on earth. 

Photovoltaic cells are modular. You can use as many or as few as you need to do your job. They are not like coal-fired power plants that have to be a certain size to be economical. Plus, if your needs change, you can add more cells.

How much does solar electricity cost?

Photovoltaic cells were invented in 1954 by Bell Laboratories. They were very expensive at the time. The only people that needed electricity so much that the cost didn't matter were the people running the space program. 

Today, it cost about 20 to 30 cents per kilowatt-hour to generate electricity through photovoltaic cells. On average it cost about 8 cents per kilowatt-hour to generate electricity with conventional power plants in Michigan. In some places and at some times, it is much more expensive. 

Even though it often costs more, some people use photovoltaic energy anyhow. Some people do it because it is good for the environment. If a person builds a new house, he or she has to pay to have electricity lines run to the house. For people who live more than about ¼ mile from the power lines, installing photovoltaic cells can be cheaper.

Worldwide production of photovoltaic electricity is about 60 Megawatts. Most of that is produced by the US, Japan and Europe. 

As the cost of photovoltaic systems goes down, the number of cost-effective uses goes up. As the number of users goes up, the cost goes down. About every five years, the number of solar cells has doubled, and the cost per cell has been cut in half. If that trend continues, solar electricity should be about the same cost as conventional electricity very soon.