Solar Energy

Solar energy is energy given off by the sun. It consists of light, heat, and other forms of electromagnetic radiation (see Radiation). Solar energy is produced by nuclear reactions that take place inside the sun.

Every 40 minutes, the sun delivers as much energy to the earth's surface as all the people on the earth use in a year. People directly use only a fraction of the solar energy that reaches the earth. Scientists are developing new ways to capture solar energy and to put it to use where--and when--it is needed.

How Solar Energy Affects the Earth

The sun is the chief source of energy for all life on the earth. Life depends on the sun for heat and light. It also depends on the sun for food. Plants use solar energy to produce food during photosynthesis. Some of the plants are eaten by animals, which, in turn, are eaten by other animals.

The sun is our chief source of energy. Plants need sunlight to grow, and animals depend on plants for food and on the sun for warmth. Solar energy heats homes and greenhouses; produces wind power; and generates water power through evaporation and rainfall. Coal and petroleum are stored solar energy from very long ago and cannot be renewed once they are used.
From The World Book^™ Multimedia Encyclopedia ©1998 World Book, Inc., 525 W. Monroe, Chicago, IL 60661. All rights reserved.
World Book illustration by Oxford Illustrators Limited.

Energy from the sun also sets the earth's weather in motion. For example, precipitation occurs when water evaporated by the sun condenses and falls back to the earth. Wind occurs because the sun's rays are more direct--and thus stronger--at the equator than they are at the poles. The strong rays in tropical regions warm the air there, causing it to rise. Cooler air from the polar regions then flows under the warm tropical air. These movements create air currents that circulate around the earth. The currents are influenced by the earth's rotation, the surface features of the continents, and variations in the amount of moisture in the atmosphere. Similarly, ocean currents are formed by wind and by the sun's warming of tropical waters, under which cold polar waters flow.

Stored Solar Energy

Precipitation, wind, and ocean currents all can be viewed as stored solar energy. Much of the precipitation that falls on land flows into rivers. Hydroelectric power stations built along the rivers collect the energy of the moving water. People use wind to power sailboats and windmills. Large groups of windmills called wind farms have been set up to generate electricity in areas where the wind is steady and strong. Scientists and engineers are developing methods of using the energy of ocean waves. They also are exploring ways to harness the heat energy in ocean water.

Solar energy also is stored in plants and animals. This energy can be used in a variety of ways. For example, trees can be burned as firewood. Such crops as corn, sugar cane, and sugar beets can be fermented to produce alcohol, a fuel similar to gasoline. Petroleum, coal, and natural gas developed from the remains of plants that lived millions of years ago. Thus, these fuels contain solar energy that has been stored in the earth for ages. As supplies of these fuels diminish, people are working to increase the direct use of the sun's energy.

Capturing Direct Solar Energy

People have devised a number of ways to make direct use of the sun's energy. These uses include heating water, heating and cooling buildings, generating electricity, and cooking food.

Solar heating. Many people in warm climates heat water with simple, inexpensive batch heaters. A batch heater consists mainly of an insulated tank with several layers of clear glass covering the side of the tank that faces south. Manufacturers blacken the outside of the tank because black absorbs more sunlight than any other color. The black surfaces convert the sunlight to heat that warms the water. The glass prevents most of the heat from escaping from the tank. The hot water rises to the top of the tank and flows from there directly to a faucet.

A solar-heated home has large south-facing windows that let in heat from the sun. The walls and floor absorb the heat during the day and release it at night. A wood-burning stove provides heat on cloudy days. Overhangs shade the windows in summer when the sun is high. Sunlight also heats collectors on the roof. Liquid inside the collectors flows to a heat exchanger in the basement, where water is heated for household use.
A flat-plate collector has a black plate that absorbs heat from sunlight. When the plate gets hot, it heats a liquid that flows in channels inside the collector. Glass or plastic sheets and insulation prevent heat loss.
From The World Book^™ Multimedia Encyclopedia ©1998 World Book, Inc., 525 W. Monroe, Chicago, IL 60661. All rights reserved.
World Book illustrations by Oxford Illustrators Limited.

Devices called flat-plate collectors are used in cool climates to heat water and the air inside buildings. A flat-plate collector is an insulated box covered by one or more layers of clear glass or plastic. Inside the box is a plate of black metal or black plastic. The plate absorbs sunlight and converts it to heat, which becomes trapped under the glass. Air, water, or some other fluid circulates through tubes welded to the plate and absorbs heat from the plate. The heated fluid then flows to a heat exchanger, where it transfers its heat to water. The heated water may be used at once to heat the building or stored in a tank and pumped to faucets.

Many buildings use passive solar energy systems for heating air. In most cases, these buildings have large south-facing windows to trap heat. During the day, sunlight passes through the windows and heats walls and floors made of stone or brick. At night, the walls and floors release the heat. Additional heat may be stored by placing water or special phase-change materials inside the walls. These phase-change materials melt at about room temperature. As they melt, the materials store large amounts of heat. The materials later release the heat as they become solid again. In buildings with passive solar energy systems, special insulating shades or shutters help keep heat from escaping through the windows at night.

Solar air conditioning. Most solar air conditioning systems use solar collectors and special materials called desiccants that can absorb large amounts of water. The air conditioning process begins when fans force air from outdoors through a desiccant, which removes moisture from the air. The air then flows through a revolving wheel that acts as a heat exchanger and removes heat. Next, the air passes over a surface soaked with water. As the water comes into contact with the dry air, it evaporates, and absorbs more heat from the air. The cooled air then passes through the building. After the air leaves the building, the solar collectors reheat it. The desiccant is dried out by blowing the reheated air through it, and the process begins again.

Creating electricity. Direct solar energy can also be used to create electricity. Two basic types of devices used for this purpose are solar cells and high-temperature collectors.

Solar cells, also called photovoltaic cells, consist of thin slices of semiconductor materials (see Semiconductor). When the sun shines on a solar cell, electric current flows from one side of the cell to the other. Solar cells power most artificial satellites and many small electronic devices.

High-temperature collectors, also called solar furnaces, generate large amounts of electricity. In one type of high-temperature collector, many flat or slightly curved mirrors focus the sun's rays on a target, such as a piece of metal. A fluid such as water is pumped inside the target, where it is heated. The steam or gas that results from heating the fluid carries heat energy to turbines that generate electricity.

Solar cooking can be done by using parabolic (dish-shaped) reflectors to focus sunlight on the food or on a pot that contains the food. Another method of solar cooking uses a solar oven. A solar oven is an insulated box with a window and several reflective inner surfaces. The oven heats up when the window is pointed toward the sun.

History

People have harnessed the sun's energy since ancient times. By the 400's B.C., the Greeks knew that glass spheres filled with water could concentrate the sun's rays and start fires. By the 200's, the Greeks and the Chinese used curved mirrors to light fires with sunlight.

From the A.D. 900's until about 1300, the Anasazi Indians of what is now the Southwestern United States built dwellings with massive south-facing stone or adobe walls. These walls absorbed heat during the day and released it at night.

Modern flat-plate collectors evolved from early experiments with hot boxes. Horace Benedict de Saussure, a Swiss scientist, built the first hot box in 1767. He used a glass-covered wooden box with cork insulation to collect solar heat energy. William J. Bailey, an American engineer, developed the first modern flat-plate collector in California in 1909.

In 1940, George Fred Keck, an American architect, built the first modern passive solar home in a Chicago suburb. Most of the south wall was covered by windows made of two panes of glass with a thin layer of air sealed between them. Engineers at Bell Telephone Laboratories created an efficient solar cell in 1954.

During the 1970's and 1980's, shortages of oil and natural gas led to the development of more efficient solar technology. In some areas, new solar homes have become an economical alternative to new homes heated with gas or electricity. As people use direct solar energy more and more, the mass production of solar equipment--and the development of better equipment--will reduce the cost of using this clean energy source.

Contributor: Daniel Tamil Bauch, B.C.E., Director, Aerius Design Group.

Additional Resources

Brooke, Bob. Solar Energy. Chelsea Hse., 1992.

Rickard, Graham. Solar Energy. Gareth Stevens, 1991. Younger readers.

Spence, Margaret. Solar Power. Gloucester Pr., 1993. Younger readers.