What Is Solar Energy?
Every day the sun radiates, or sends out, an enormous amount of energy. It radiates more energy in one second than people have used since time began.
Where does all this energy come from? It comes from within the sun itself. Like most stars, the sun is a big gas ball made up mostly of hydrogen and helium gas. The sun makes energy in its inner core in a process called nuclear fusion.
Only a small part of the solar energy that the sun radiates into space ever reaches the earth, but that is more than enough to supply all our energy needs. Every day enough solar energy reaches the earth to supply our nation’s energy needs for one and a half years!
It takes the sun’s energy just a little over eight minutes to travel the 93 million miles to earth. The solar energy travels to earth at a speed of 186,000 miles per second, the speed of light.
Today people use solar energy to heat buildings and water and to generate electricity.
Heating with solar energy is not as easy as you might think. Capturing sunlight and putting it to work is difficult because the solar energy that reaches the earth is spread out over a large area. The sun does not deliver that much energy to any one place at any one time. How much solar energy your town receives depends on the time of day, the season of the year, the cloudiness of the sky, and how close you are to the earth’s equator.
A solar collector is one way to capture sunlight and change it into usable heat energy. A closed car on a sunny day is like a solar collector. As sunlight passes through the car’s windows, it is absorbed by the seat covers, walls, and floor of the car. The absorbed light changes into heat. The car’s windows let light in, but they don’t let all the heat out. A closed car can get hot!
Solar Space Heating
Space heating means heating the space inside a building. Today many homes use solar energy for space heating. A passive solar home is designed to let in as much sunlight as possible. It is like a big solar collector. Sunlight passes through the house’s windows and heats the walls and floors inside the house. The light can get in, but the heat is trapped inside. A passive solar home does not depend on mechanical equipment, such as pumps and blowers, to heat the house.
An active solar home, on the other hand, uses special equipment to help heat the house. An active solar house may use special collectors that look like boxes covered with glass. These collectors are mounted on the roof top facing south to take advantage of the winter sun. Dark-colored metal plates inside the boxes absorb sunlight and change it into heat. (Black absorbs sunlight more than any other color.) Air or water flows through the collector and is warmed by the heat. The warmed-up air or water is then distributed to the rest of the house just as it would be with an ordinary furnace system.
Solar Hot Water Heating
Solar energy also can be used to heat water. Heating water for bathing, dishwashing, and clothes washing is the second biggest home energy cost.
A solar water heater works in pretty much the same way as solar space heating. A solar collector is mounted on the south side of a roof where it can capture the most sunlight. The sunlight is converted to heat energy and used to heat water in a tank. The hot water is then piped to faucets throughout a house just as it would be with an ordinary water heater. Today over 1.5 million homes in the United States use solar water heaters.
Besides using solar energy to heat homes or water, solar energy also can be used to produce electricity. Two ways to make electricity from solar energy are photovoltaics and solar thermal systems.
Photovoltaic comes from the words photo meaning “light” and volt, a measurement of electricity. Sometimes photovoltaic cells are called PV cells or solar cells for short. You are probably already familiar with photovoltaic cells. Solar-powered toys, calculators, and roadside telephone call boxes all use solar cells to convert sunlight into electricity.
Solar cells are made up of silicon, the same substance that makes up sand. Silicon is the second most common substance on earth. Solar cells can supply energy to anything that is powered by batteries or electrical power.
Electricity is produced when sunlight strikes the solar cell, causing the electrons to move around. The action of the electrons starts an electric current. The conversion of sunlight into electricity takes place silently and instantly. There are no mechanical parts to wear out.
You won’t see photovoltaic power plants today, though. Compared to other ways of making electricity, photovoltaic systems are expensive. It costs about 27 cents a kilowatt-hour to produce electricity from solar cells. Most people pay their electric companies eight cents a kilowatt-hour for the electricity they use. Today photovoltaic systems are mainly used to generate electricity in remote areas that are a long way from electric power lines.
Solar Thermal Electricity
Like solar cells, solar thermal systems use solar energy to produce electricity. But instead of sunlight, solar thermal systems use the sun’s heat to do it.
Most solar thermal systems use a solar collector with a mirrored surface to concentrate sunlight onto a receiver that heats a liquid. The super-heated liquid is used to make steam that drives a turbine generator to produce electricity in the same way that coal, oil, or nuclear power plants do.
Until recently, the most successful thermal power plant was the LUZ plant in the Mojave desert of California. This solar power plant used mirrored solar collectors shaped like water troughs to collect sunlight. LUZ made electricity as cheaply as most coal or oil-fired electric power plants. Then in 1992, LUZ had to shut down because of falling oil prices and an over-budget construction project at LUZ’s homebase.
Despite its problems, solar energy has great potential for the future. Solar energy is free for the taking and its supplies are unlimited. Solar energy does not pollute or otherwise damage the environment. It cannot be controlled by any one nation or industry. If we can improve the technology to harness the sun’s enormous power, we may never face energy shortages again.