PHY 241 Net Research Project for 2/8/99

SOLAR ENERGY

Solar energy is energy that arrives to us here on earth from the sun. It represents a type of energy quite different from anything we will study in Physics 241. You will learn much more in Physics 242 when you study electricity and magnetism. 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. There are two converted forms that are very useful, heat and electricity.

Solar Heat

It is easy to sense photon energy being converted into heat energy. When the sun is shining, even on a cold winter's day, absorption of the photons streaming from the sun will warm a face, a rock or any other dark absorbing surface. (Even white snow absorbs some energy.) Cloudy and hazy days notwithstanding, the energy reaching the earth's surface depends upon where on the earth you live and the time of day. Here in St. Thomas (latitude 18.3^o from the equator), the sun is relatively "high in the sky" and a lot of solar radiation reaches the ground. The amount of radiation depends upon the angle between the horizon and the sun. This angle is called the altitude (it is an angle, not a height!). You can learn about the sun's altitude as well as other important angles from a very nice tutorial by Heather B. Potucek, a senior meteorology major at Plymouth State College. The maximum solar radiation occurs when the sun is directly overhead, when about 1000 watts (joules/sec) of energy strikes every square meter. Think of the heat energy from ten 100-watt light bulbs spread over a square meter. We should be able to use all that energy, and we can!

Homemade examples of the simplest method can be seen all over St. Thomas. Get a 50 gallon drum and paint it black. Fill it full of water and let the sun do the heating. Of course, it takes a bit more engineering to construct a solar water heater which provides continuous hot water to your faucets. You can see a good example on top of many of the buildings at the Pearson Gardens complex. Inside the flat rectangular box is a continuous pipe which takes water from the cylindrical tank just above it. The scale of such systems is limited only by the imagination. The systems at the Pearsons Gardens serve a single individual unit. The St. Rose Hospital in San Antonio, Texas heats most of their water (up to 9,000 gallons) using a similar but larger scale technology.

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. (You will learn all about generators next semester.) 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. (We will study the Stirling engine later this semester!) 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.

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