Debbie Jenovai, age 14, of Des Plaines, I11., for her question:

WHAT IS A SOLAR CELL?

Did you know that nearly all energy comes directly or indirectly from the sun? Well, it is true. The energy reaching the earth is radiant energy.  If man were able to store all of the radiant energy that reaches the earth in three minutes, he would have enough to run all of the world's machines and energy consuming devices for one year.

Unfortunately, no one has developed a way to store all of the world's radiant energy.  Energy running the machines comes from the sun in many ways, the most common sources of this indirect solar energy being wind, fuels and water power.

Also used now is the solar cell, a type of electric device which uses sunlight to generate electricity. The electrical energy obtained from solar cells is used in artificial satellites. One type of solar cell which produces electricity has been made of specially treated silicon crystals. The slabs of silicon crystal are similar to transistors. The crystal has p type and n type semiconductor junctions near the surface.

Sunlight falling on the silicon crystal surface displaces the electrons from the atoms. Since the junction tends to transport electrons across it in one direction only, an electric current will then flow.  Under ideal conditions, nine square feet of silicon crystal in a solar cell will produce 100 watts of electric power.  The first Vanguard satellite in 1958 was equipped with solar cells to operate the radio transmitter.  

Many satellites launched since Vanguard I have used solar cells to energize their delicate electronic equipment. Portable radios have been built which use solar cells. There's even a telephone system operating on electricity from solar cells. During daylight hours, the solar cells charge storage batteries which then supply current for night telephone operation.  Today solar cell panels are being installed on roof tops to provide electricity required to operate appliances and lights.