The Basics: Photovoltaic cells are semiconductors that absorb sunlight and convert it into electricity.
Why Now?: Portable and relatively compact; the most efficient power alternative for remote areas.
Why Not?: A shortage of processed silicon, currently the main material in solar cells, makes the technology relatively expensive; alternatives are being explored.
Market Potential: Commercial revenues for solar technologies are expected to grow from $11 billion in 2005 to $51 billion by 2015.
Major Players: British Petroleum, Kyocera, Sharp Corporation, Q-Cells AG, SunPower, Suntech Power
The Basics: Modern wind turbinessome 300 feet high, with blades 200 feet in diameterconvert wind currents into mechanical energy and electricity, which is transferred to power grids.
Why Now?: Highly efficient; ten-story, industrial-size wind turbines can produce one kilowatt-hour of power for less than five cents (retail electricity today costs about ten cents per kilowatt-hour).
Why Not?: The high costs of steel and carbon fiber used in turbines make wind power relatively expensive. Notsuitable for low-wind areas (best in coastal and mountain regions).
Market Potential: Experts estimate that commercial revenues for wind-power technologies will grow from $12 billion in 2005 to $49 billion by 2015.
Major Players: Enercon, GE, Vestas
The Basics: Biofuels are alcohol- or vegetable-oil-based; they include methanol (made from manure), ethanol (corn or grasses), and biodiesel (soybeans, corn, etc.).
Why Now?: Clean-burning, relatively inexpensive to produce, and can be sourced domestically. Considered viable replacements for gasoline.
Why Not?: Some biofuel factories run on pollution-heavy coal or natural gas. Reliance on corn-based ethanol would require millions of acres of cornfields.
Market Potential: Observers see commercial revenues for biofuels, led significantly by ethanol, growing from $16 billion in 2005 to $53 billion by 2015.
Major Players: Altra Inc. (ethanol), Celunol Corp. (cellulosic ethanol), Imperium Renewables (biodiesel)
The Basics: Batterylike fuel cells convert the chemical reaction between pure oxygen and hydrogen into electricity, which can then be used to power cars, buses, and trains.
Why Now?: Efficient, quiet, and clean (byproducts are heat and water). Unlike batteries, fuel cells won't degrade over time.
Why Not?: Isolating pure hydrogen gas, which doesn't occur naturally, can be costly. Most fuel-cell tech is still in R&D mode, so mass-market adoption is probably decades away.
Market Potential: Commercial revenues for fuel-cell technologies are predicted to grow from $1.2 billion in 2005 to $15 billion by 2015.
Major Players: Ballard Power Systems of Canada, Toyota Motor Company
The Basics: Molecular biologists and genetic engineers produce organisms that actually create energy sources in the form of hydrogen molecules (H2) or ethanol.
Why Now?: Could be an inexhaustible resource: Imagine meeting the world's energy needs by growing fuel in petri dishes.
Why Not?: Expensive; perhaps decades away from practical use. And then there are the ethical concerns associated with creating new forms of life...
Market Potential: Because development is still in the research stage, future potential is not yet known.
Major Players: Genome pioneer J. Craig Venter, through his new Rockville, Marylandbased company Synthetic Genomics