Recharge! In a polluted world, can nature provide cleaner energy? (Renewable Energy).

What's the number-one cause of air pollution in the U.S.? Electricity production, according to the U.S. Environmental Protection Agency. Combusting (burning) fossil fuels--like oil, coal, natural gas, and gasoline--to generate electricity spews the bulk of all toxic chemicals into the atmosphere. Burning fossil fuels accounts for

* 67 percent of the nation's sulfur-dioxide emissions, which causes acid rain and also worsens respiratory and cardiovascular illnesses;

* 23 percent of nitrogen-oxide emissions, which react with sunlight to create lung-irritating ground-level ozone and smog;

* 40 percent of human-made carbon dioxide, a global warming greenhouse gas.

How to ease both environmental and public health problems? Replace fossil fuels with renewable energy, or unlimited energy powered by sources like the sun, ocean, and wind. Here's a look at three cleaner ways to charge up.

SOLAR ENERGY

POWER SOURCE: The sun is one huge ball of hot gas. Solar energy in the form of visible light and other radiation travels from the sun's surface to Earth in 8.5 minutes--a constant, free source of power.

THE WORKS: Two common ways to convert solar energy into electricity: photovoltaic (PV) and solar-thermal technologies. In PV systems, sunlight strikes cells made mostly of a silicon alloy; a chemical reaction results in the release of electricity. Solar thermal technologies use reflective devices such as mirrors to direct the sun's rays to heat a liquid. The resulting steam turns an electric generator.

THUMBS UP: The sun is expected to shine for another 4 billion years. And solar energy doesn't generate toxic emissions.

THUMBS DOWN: Sunlight is intermittent due to time of day, geography, weather, and seasons. And PV-cell manufacturing involves potentially toxic chemicals. But proper monitoring and waste disposal should limit environmental risks. On a large scale, solar-thermal technologies demand a vast amount of space.

NEWS: EnviroMission, an Australian power company, hopes to erect a 1,000 meter (3,300 foot)-high solar tower in New South Wales. The chimney-like structure--as wide as a soccer field--will stand in the middle of a 7 km (4.3 mi) diameter glass roof. As the sun heats liquid under the glass, steam rises up the tower to be sucked through 32 turbines; the turbines spin to generate enough power for 200,000 homes. "People told me you're a dreamer," says EnviroMission CEO Roger Davey. "Now, it's not if it can be built, but when it can be built." Completion is expected by 2006.

TIDAL ENERGY

POWER SOURCE: The gravity (attracting forces) of the moon and sun tugging on rotating Earth causes ocean tides. This gravity causes oceans to periodically bulge and fall, which in turn makes coastlines expand Harnessing powerful ocean motion could provide a tidal wave of energy!

THE WORKS: Usually engineers erect a barrage (dam) across the opening to a tidal basin. They install gates and turbines along the dam. Inflowing water churns the turbines, which inturn propel a generator to crank out electricity. Then the gates are shut to contain water as tides pull out. When there's an adequate difference in water elevation on each side of the barrage, gates are reopened and water flows out, again spinning the turbines.

THUMBS UP: Because tidal currents are cyclical, they're a highly predictable energy source. Tidal energy doesn't emit pollutants.

THUMBS DOWN: The difference between high and low tides varies from place to place, so tidal power is only practical in coastlines with very high tides. "Of all renewable energy technologies, ocean energy is probably the one in the earliest stages," says Mark Hammonds of the International Energy Agency in Paris. "Many projects have proved to be too costly." Also, giant dams may damage the ecology of coastlines and rivers.

NEWS: This year, a Norwegian town of 11,000 residents will become the first to use underwater mills to leash the kinetic (moving) energy of tidal currents. Each 200-ton tidal-stream turbine stands on the seabed and sports 15-meter-long blades. As tidal currents push against the propeller, the turbine spins. Each turbine is equipped with a generator to produce electricity; a cable sends the power into a grid (power distribution hub) on shore. By 2004, the project aims to expand to 20 mills to power 1,000 homes.

WIND ENERGY

POWER SOURCE: Earth's tilt and rotation, along with varying cloud cover, causes the sun to heat Earth unevenly. Warm air expands and rises because it's lighter than cold air. And as warm air ascends, heavier cold air sinks in its place. This constant air movement is wind--and scientists want to harness it as a plentiful, clean power source.

THE WORKS: While the tower and rotor blades of a wind turbine may look like a giant fan, the structure works like a fan operating in reverse. The kinetic energy of wind turns the blades, which in turn spin a shaft connected to an electric generator. Turbine size and wind strength determine power output. And to create enough electricity for one town, a group of several wind turbines form a wind-power plant called a wind farm. Electricity from these turbines flows into a local utility grid for consumer use.

THUMBS UP: Wind energy is cost-effective and emits no air pollutants. In 1990, wind power offset the emission of more than 2.5 billion pounds of C[O.sub.2] and 15 million pounds of other air pollutants in California, which would normally require a forest of up to 175 million trees to absorb C[O.sub.2] from the air.

THUMBS DOWN: Wind can be fickle. Ideal wind-farm sites need an average annual wind speed of at least 13 miles per hour. But windy places are often remote from big cities with high electric power demand.

NEWS: In the last decade, wind energy has become the world's fastest-growing alternate electricity source. The boom has mostly occurred in Europe; about 30 million Europeans use wind-generated power. "Wind energy grew by 10 percent in the U.S. in 2002," says Randall Swisher, executive director of the American Wind Energy Association. "With steady supportive policies, wind power could grow at a sustained pace closer to that of Europe." The U.S. Department of Energy's "Wind Powering America" initiative aims to power at least 5 percent of the nation by 2020.

Go With the Flow

Hammerfest, Norway, will soon be the world's first town to harness electricity from tidal currents that spin underwater turbines. The windmill-like structures stand on the seabed.

Thanks to the gravitational pull of the moon and sun, oceans rise and fall in a predictable cycle of 12 hours 25 minutes.

THE MILLS

Each tidal turbine weighs 200 tons. And the seabed's maximum depth of 50 meters (164 feet) is well below busy ship traffic.

The average current velocity (speed) at this site is 1.8 meters per second, with a maximum speed of 2.5 m/s (8.2 ft/s).

The blades automatically rotate to face the tide as currents change.

The project is driven by the need for cleaner alternatives to satisfy growing energy demands in Norway. The sun is an unreliable source--it sets entirely for two months each winter.

TOWER OF POWER: An Australian solar tower may soon be the world's tallest structure. Its power will offset 700,000 tons of greenhouse gases per year from fossil fuels.

CATCH THE WAVE: Norwegian underwater tidal turbines are designed to be maintenance free for three years. But divers can easily plunge in for checkups.

BIG FAN: Wind turbines come in all different tower heights and rotor sizes. The world's largest is in Ontario, Canada, at 117 m (384 ft) high, with a 39 m (128 ft)-blade.

SHREDDER: The world's fastest-growing energy source in the world has its drawbacks: Wind farms require a vast amount of land. The spinning rotor blades are noisy, and birds are often shredded when flying into them.

U.S. Energy Sources
(2001)

Renewable Energy         6.4%
Nuclear Electric Power   8.5%
Fossil Fuels            85.1%

Source:
Energy
Information
Administration

Note: Table made from pie graph.

Did You Know?

* When completed, EnviroMission's solar tower will become the world's tallest human-made structure--more than double the height of Malaysia's 80-story Petronas Towers, the world's tallest building.

* Harnessing tides for energy dates back to around 787 A.D. A tide mill--generally built along the coasts of Spain, France, and England--gathered the flood (incoming) tide through a sluice into a storage pond. The water was emptied through a water wheel, which churned mechanisms to mill grain, during ebb (outgoing) tide.

* North Dakota has enough wind energy to supply 36 percent of the electricity for the mainland U.S. But there is no technology to distribute the energy nationwide.

Cross-Curricular Connection

Social Studies: How did electricity alter everyday functions? For example, have students go back in time to research how early American settlers preserved food before the invention of the refrigerator.

Critical Thinking: Imagine a "no electricity" day. Have students think of solutions to continue with their daily routine and how the alternatives would benefit the environment.

Resources

A general overview of solar energy is available at the U.S. Department of Energy: www.eia.doe.gov/kids/renewable/solar.html

To learn more about wind energy, check out the American Wind Energy Association's Web site: www.awea.org/

Visit Hammerfest Strom AS, the company responsible for the Norwegian underwater turbines, at www.e-tidevannsenergi.com/