Solar Electric Energy

Solar energy is energy that arrives to us here on earth from the sun. In your previous SCI courses, you probably dicussed that light was a wave. Just what kind of wave is it? Light is just a narrow slice of the electromagnetic spectrum. Electromagnetic waves, as the name implies, are composed of oscillating electric and magnetic fields. We have just studied how moving electric charge, i.e. current, creates magnetism. We also saw that magnetism can create current. It was Clerk Maxwell who really put all the pieces of the puzzle together in his famous four equations. Among a few other the important things, they say the following:

This is how light can propagate through empty space. Unlike sound, which must propagate through something such as air or water, electromagnetic waves require no medium. The electric and magnetic fields propagate each other in a lovely dance that illuminates the Universe.

Although sunlight is an electromagnetic wave, it can be understood very well from another point of view. Light can be thought of as composed of photons, little "particles of light" which travel at the amazing speed of 186,000 miles per second. These particles carry energy. The photons can be absorbed and their energy converted to another form. The amount of energy reaching the earth from the sun is considerable. Outside the earth's atmosphere, the radiation intensity is about 1400 Watts/m². Intensity is the power per unit area. Hence, an area 1 meter by 1 meter will have 1400 Joules of electromagnetic energy every second impinging on it. At the surface of the earth, that number is reduced roughly by a factor of 2. This is due to the atmosphere and its effects depend upon the location on earth and where the sun is in the sky. Nevertheless, there's a lot of energy there if we can harvest it.

There are two converted forms that are very useful, heat and electricity. Will study direct solar heating in the next module. In this section, we will see how light can be converted into electricity through two different mechanisms.

Solar Thermal Electricity

There are several variations of solar thermal electric generating systems, but they are all similar. Essentially, sunlight is used to heat a working fluid. The fluid is part of the internal workings of an engine that drives an electric generator. The size can vary from a modest 6 kW system for the home to the massive parabolic trough systems. One of the nine Solar Electric Generating Systems (SEGS) operating in Southern California is shown to the left.

One of the smaller scale solar thermal generation technologies is the Dish/Stirling system. Using a single or multiple reflecting dish array, sunlight is focussed onto a container filled with a gas, such as helium. The gas expands and drives the piston of a Stirling engine. The motion of the piston is converted into mechanical power that drives an electric generator. The system is not unlike the alternator system that charges the battery in your car, except that sunlight provids the energy source, rather than gasoline.

Electricity from Photovoltaics

There is more "direct" way to make electricity from the sun. The same semi-conductor technology that gave us the transistor and our present computer chip technology, also provides a way to use the energy of single photon to move an electron ... that is, create an electrical current. The most common photovoltaic (PV) or solar cell is made of thin crystalline layers of silicon. By "doping" the silicon layers with impurity atoms (that reside on either side of silicon on the periodic chart), photons of light will eject electrons from one layer, where they are swept into the adjacent area. The Soltices organization's tutorial on PV physics will provide more details on just how this works. Although PV's have some drawbacks, research has made great advances in the last few years. Photovoltaic systems capable of powering a home are now price effective. Since hurricane Marilyn in 1995, many homes here in the VI have successfully installed such systems. The state of Hawaii has committed significant resources to photovoltaic systems and is one of the leaders in the field.