As politicians conclude their 10-day environmental summit in Kyoto tomorrow, an unlikely band of scientists and entrepreneurs is reviving a long-forgotten technology that may represent our best hope for solving the problem of global warming: the fuel cell.

Across the globe, car companies including Daimler-Benz, Toyota, Chrysler and Honda are pouring hundreds of millions of dollars into research and development for replacing conventional internal combustion engines with electric motors powered by fuel cells.

"Of all of the technologies that are out there, this is the one the auto industry seems to look at as the best option for getting the kind of super efficiency the governments are looking for," said Henry Lee, director of the Environmental and Natural Resource Program at the John F. Kennedy School of Government.

Daimler has said that it will manufacture 100,000 fuel cell engines a year by 2005, at prices competitive with conventional engines.

What is often left out in the buzz is that fuel cells are hardly a new technology--though the techniques that are making them accessible to the masses are.

Fuel cells have been around since well before the Civil War, but until now, like many other "environmentally-friendly" energy sources, they have been too expensive to be worth using.

Unlike an internal combustion engine, which burns its fuel so inefficiently that only 20 percent of the energy in gasoline gets used, a fuel cell can pull off at least 30 percent efficiency by reacting its special fuels--hydrogen and oxygen gas--electrochemically.

Pure, Clean Air

But efficiency isn't the only reason fuel cell advocates take the gadgets so seriously. In theory, unlike internal combustion engine--which spew greenhouse gases such as carbon dioxide into the atmosphere--a fuel cell's only "pollution" is pure water and electrical current.

In practice, the equation isn't so spotless. Though oxygen is plentiful in the air, pure hydrogen is much harder to come by and must be obtained artificially.

The process of extracting hydrogen from more conventional fuels such as gasoline and methanol inevitably releases some carbon dioxide--but not as much as the internal combustion engine does, and therein lies the rub.

Though the issue is still controversial, many scientists believe that a documented rise in carbon dioxide levels worldwide has caused large-scale climate changes. If the trend continues, it could lead to environmental problems such as the melting of polar ice caps and a rise in ocean levels across the globe.

But despite their advantages, fuel cells still have yet to find their way under the hood of passenger cars. Industry experts attribute the delay to scarcity of fuel, price and size.

Yet there is room for optimism and profit, as Daimler's $350 million investment in Ballard Power Systems, a Canadian manufacturer of high-tech fuel cell, would suggest.

"In the next 10 years we might have some breakthroughs," Lee said.