(Producing electricity without pollution is a global challenge)
By Daniel Gorelick
Washington — Most power plants are elaborate water-boiling factories; generating electricity often relies on boiling water and using the steam to rotate a turbine.
Most electricity results from a process called electromagnetic induction, in which a moving magnet generates electricity. In 1831, British scientist Michael Faraday observed this phenomenon when he spun a copper disk between the poles of a magnet and found that the copper disk generated electricity (the disk has to move to generate electricity). This led to the invention of the electrical generator, where magnets rotate around a material like copper that conducts electricity. Generators convert mechanical energy (rotating magnets) into electricity (current flowing through a wire). This is the opposite of a motor, which converts electricity into mechanical energy.
The mechanical energy that drives a generator can come from water, wind or steam. At a hydropower plant, water flows across a turbine propeller. The turbine, turned by the flowing water, drives a generator and produces electricity. Wind turbines are turned by the wind. A steam turbine works similarly: boil water, produce steam.
THE BEST WAY TO BOIL WATER
Fossil fuels like coal burn easily. In coal-fired power plants, coal is burned in large furnaces. The heat from this combustion is used to boil water, which drives a steam turbine and generates electricity. In natural gas plants, burning gas generates steam directly, without the need to boil water. Petroleum is used to power automobiles, ships, airplanes and home heating systems, but is not often used to generate electricity.
Nuclear power plants have a far more complicated method of boiling water inside their core. Uranium atoms are bombarded by neutrons. The uranium breaks apart, releasing gamma ray radiation, neutrons and heat in a fission reaction. The heat is used to boil water and produce steam, which drives a turbine and generates electricity. The neutrons produced in a fission reaction can split more uranium atoms, producing a self-sustaining chain reaction, generating more heat.
Geothermal power plants harness the heat stored in the earth. In dry steam plants, the steam from underground wells drives a turbine. In binary cycle plants, hot but not steaming geothermal fluid (generally less than 200 degrees Celsius) is drawn up to the surface and used to heat fluid ? usually butane or pentane ? that boils at a lower temperature than water. The geothermal fluid may not be hot enough to boil water, but it can boil butane or pentane, producing steam and driving a turbine to generate electricity.
In concentrated solar power plants, mirrors or lenses use sunlight to heat water (or another fluid) and generate steam. The first commercial solar power concentrating plant was built outside Seville, Spain, in 2005. Hundreds of mirrors that track sunlight — called heliostats ? reflect sunlight onto a central tower. The concentrated beam of sunlight heats fluid and generates steam, driving a turbine and producing electricity.
Biofuels are liquid fuels derived from plants. Like their fossil fuel counterparts, biofuels are readily combustible and can be used to power automobiles or to drive a steam turbine and generate electricity. Processing facilities use chemicals to extract sugar from plant matter. The sugar is combined with micro-organisms and allowed to ferment, producing ethanol or other alcohols (a similar process to the one used to brew beer). Ethanol can be blended with gasoline and used as an automobile fuel that produces less carbon monoxide and smog than gasoline. Ethanol can also be burned to generate steam, powering a generator that produces electricity.
Photovoltaic solar panels convert energy from the sun into electricity directly and require no mechanical energy. Photons of light hit electrons (often in silicon), bumping them into a higher energy state and generating electricity. Initially used to power satellites in outer space, solar power is now used to generate electricity on Earth. Photovoltaic panels do not use electromagnetic induction.
POWER AND POLLUTION
All of these methods for generating power have drawbacks. Either they release toxic byproducts, or they are expensive. Coal is inexpensive and abundant, but burning coal produces toxic and environmentally damaging byproducts: carbon dioxide, a greenhouse gas; sulfur dioxide, which forms acid rain and damages respiratory systems; nitrogen oxides, which form ozone and smog; fly ash, which contains environmental toxins such as arsenic.
In 2007, almost half of the electricity generated in the United States was from coal, according to the U.S. Department of Energy. “Coal is an abundant resource in the world,” Energy Secretary Steven Chu said in January 2009 during his confirmation hearing. “It is imperative that we figure out a way to use coal as cleanly as possible.”
Burning natural gas produces less carbon dioxide and toxic particles than burning coal — 43 percent fewer carbon dioxide emissions than coal for each unit of energy produced, according to the Union of Concerned Scientists. But burning natural gas still produces pollutants like carbon monoxide and nitrogen oxides. Natural gas contains methane, a more potent greenhouse gas than carbon dioxide. Carbon monoxide and carbon dioxide are also byproducts of petroleum refining and combustion.
Nuclear power plants produce no greenhouse gases and no chemicals that form ozone or smog. But the byproduct of the fission reaction is radioactive waste, which must be properly stored and contained to prevent it from damaging plants and animals. Radioactive waste from nuclear power plants remains toxic for thousands of years. Today there is no widely accepted long-term storage method.
Both types of solar power plants produce no greenhouse gases or pollution, but the cost of materials and installation make solar power more expensive than using fossil fuels to generate power. Manufacturing photovoltaic panels requires toxic chemicals, and electricity generation is inefficient. Commercial solar panels convert less than a third of the incoming sunlight into electricity.
The story is similar for wind and geothermal power. Both produce few, if any, byproducts (dry steam releases some pollutants and greenhouse gases from beneath the earth into the atmosphere), but both are generally more expensive than using fossil fuels. Wind and geothermal power plants can be installed only in locations where wind or geothermal energy is abundant.
There is only a finite amount of coal, petroleum and natural gas in the Earth’s crust. Because it takes hundreds of millions of years for fossil fuels to form, they are effectively nonrenewable. Uranium ore, the fuel for nuclear fission reactions, is present at low abundance in the earth and is generated when stars explode, meaning that Earth’s uranium supply is nonrenewable. According to the International Atomic Energy Agency, “There is enough uranium known to exist to fuel the world's fleet of nuclear reactors at current consumption rates for at least a century.”
Hydropower, wind, geothermal, solar and biofuel are renewable energy sources. They are constantly replenished and will never run out.
The sun will shine for billions of years. Wind is a form of solar energy, caused by the heating of the atmosphere by the sun, the rotation of the Earth, and irregularities in the Earth's surface. Water continually flows, and the Earth constantly generates heat. Plants, harvested to produce biofuels, take only a few months to grow again.
Read more about biofuels ( http://www.america.gov/st/env-english/2008/February/20080225140244lcnirellep0.8307154.html ) and about solar ( http://www.america.gov/st/env-english/2008/February/20080228144750lcnirellep0.1695825.html ), wind ( http://www.america.gov/st/energy-english/2008/February/20080226181115lcnirellep0.4339105.html ) and geothermal ( http://www.america.gov/st/env-english/2008/February/20080206161409lcnirellep0.5360224.html ) energy at America.gov.
(This is a product of the Bureau of International Information Programs, U.S. Department of State. Web site: http://www.america.gov)