How it works: Photovoltaic cells, in modules (solar panels) or thin film, convert sunlight to electricity. Solar thermal power uses sunlight to heat water, which turns to steam, which runs turbines.
Top companies: First Solar, Nanosolar, SunPower, United Solar Ovonic
Pros: Sunlight is free; state and federal subsidies available to users; works on individual roofs.
Cons: Sun only shines half the time, or less; systems are expensive--payback in reduced energy cost can take more than seven years; requires large amounts of land; dependent on silicon prices, which fluctuate.
How it works: Wind turns blades that power generators and produce electricity.
Top companies: General Electric, Nordex (Germany), Siemens (Germany), Vestas (Denmark)
Pros: Wind is free; European companies are bringing advanced technologies to the U.S.; electricity generated by wind can cost the same as coal or natural gas.
Cons: Wind is unreliable; requires large amounts of land; best winds blow in the least populated areas; requires backup power system.
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How it works: Drills down into the earth's superheated core; the steam or hot water that escapes turns a turbine.
Top companies: C. Rokas SA (Greece), Ormat Technologies, Polaris Geothermal (Canada)
Pros: Available 24/7; requires less land than solar or wind; technology is inexpensive and easy to mass produce.
Cons: Geothermal reservoirs are hard to locate; drilling could cause earthquakes (A recent project in Switzerland was called off because of tremors).
How it works: Battery-powered electric motors to completely or partially (hybrid) power vehicles.
Top companies: General Motors (Volt), Tesla Motors, Toyota, Zero Motorcycles
Pros: Reduces or eliminates dependence on oil; cuts carbon emissions; less expensive to operate than internal combustion engines.
Cons: Batteries are large and expensive; range for all-electric cars is only 50 miles; requires system of recharging stations that electric grid may not be able to handle.
How it works: Agricultural products to power engines instead of fossil fuels.
Top companies: Archer Daniels Midland, Bunge Ltd., Novozymes (Denmark)
Pros: Cuts dependence on oil; conventional engines easy to convert; can use waste materials, including wood chips, restaurant oils or manure.
Cons: Conversion into energy is expensive; creates some pollution; can cut into world's food supply.
Julie Bennett is a freelance writer.