Fossil fuels will remain an important energy source for the foreseeable future, but they will eventually run out and the world will have to switch to what environmental visionaries have been dreaming about since the original Earth Day: endlessly renewable power from wind and sun.

Wind has the edge. It's fast catching up with oil and gas in cost efficiency with the help of experiments such as the one at Ames Research Center. By comparing what they learn from the wind tunnel's smooth airflow with data from the turbulent breezes at the National Renewable Energy Laboratory's test range near Golden, Colorado, engineers expect to build a new generation of superefficient wind turbines with blades well over 200 ft. (60 m) across. Efficiency doesn't help when the wind isn't blowing; you need to store energy generated during gales for use when the air is still. The best way to do that, says Robert Williams, of Princeton University's Center for Energy and Environmental Studies, is to use the excess to compress air and force it into subterranean aquifers, caves or salt domes.

Then, when the wind dies, the compressed air can be pulled out to help drive the turbines. "The technology was originally developed in the 1960s," says Williams, "to let nuclear power plants store excess electricity during off-peak hours." Now it could permit countries rich in wind resources�including China, the U.S., Denmark and Germany�to take advantage of a free, unlimited and nearly pollution-less source of electricity.

On the solar-power front, the visions of 1970s-era environmentalists can still be realized, at least in part, if manufacturers could find a way to produce silicon-based photovoltaic cells more efficiently and thus drive down their high cost. One strategy is to reduce the thickness of a solar cell from the current standard�about that of a piece of cardboard�to one-hundredth of that size. Such thin-film cells, whose development is furthest along in Japan, will use less raw material and will be far easier to manufacture with the extraordinary purity required to make them efficient enough to be economical. Beyond that, their light weight will make installation easy, permitting them to be applied as building fa�ades or even over windows. Because a given solar cell is sensitive to just a few colors of the many that make up sunlight, researchers are working on multilayered cells, which will trap most of the colors of the rainbow.

Since the sun doesn't shine with equal power everywhere, even a building slathered with solar cells will need another source of electricity. One possibility: a system that uses both solar cells and a two-way fuel cell. During daylight, when the solar cells are operating, excess electricity could be run through a fuel cell to produce hydrogen from water. At night, the fuel cell could use the hydrogen to produce electricity again.

Ideally, every factory, building, home and vehicle would have its own clean, renewable power source, eliminating oil wells, coal mines, power plants and power lines�and all the environmental disruption they cause. For now, though, the world has a more urgent mission: to stop the planet from overheating, and do it in a hurry. Thanks to the fuel-cell cars and more advanced wind turbines and solar cells that are close to fruition, the global-warming challenge seems a little less daunting than it did just a few years ago.

— With reporting by Richard Woodbury/Golden and Regine Wosnitza/Stuttgart