Critics argue that plug-in cars are an environmental hoax: Because they recharge from a power grid that relies on coal-fired plants, they merely pollute somewhere else. It’s a legitimate point, but several studies calculating the environmental impact of battery-powered cars over their life cycle have determined they are still greener than the average gasoline-fueled car, despite their dependence on coal power. And, of course, they also help us kick the oil habit.

If there’s a green obstacle facing this technology, it’s not environmental; it’s financial. The advanced batteries and lightweight carbon-fiber bodywork necessary to give electric cars decent range add thousands of dollars to their price. The Prius has shown that green technologies can eventually achieve economies of scale, but early adopters must be willing to pay a premium.

Will it take off? The main reason this technology may have a bright future is the noisy environmental movement supporting it. Advocates have even convinced the White House that this hobbyist’s invention is the Next Big Thing. Political pressure will force Detroit and Japanese automakers to devise at least a token number of plug-ins by 2010.

Fuel Cells
Larry Burns, vice president of research and development at G.M., was one of the first in the industry to conclude that the internal combustion engine will one day be replaced by the fuel cell. His reasoning is simple: Ninety-eight percent of the auto industry’s vehicles run on petroleum, and that creates volatility, political risk, and greenhouse gas emissions. If the 2.5 billion people who will be added to the world’s population by 2050 drive cars, Burns believes, only hydrogen-fuel-cell electric vehicles will prevent this mass of new automobiles from ruining the planet.

That is a surprising view from Detroit. But Burns is not alone: Bill Ford himself has predicted that “fuel cells will end the 100-year reign of the internal combustion engine.” So what are fuel cells, exactly? Simply put, they are electro­chemical devices that replace internal combustion engines. Instead of gasoline, they use hydrogen as fuel.

Fuel cells are the ultimate clean-car technology, promising zero local pollution and zero global-warming gases. What’s more, they could prove to be the coolest cars you’ve ever seen. G.M.’s Hy-Wire, a hydrogen concept car, compresses the entire fuel-cell apparatus into an 11-inch-thick “skateboard.” On top of that, the car is designed to let customers snap off the body, so they could have a convertible in summer and an S.U.V. in winter. The superquiet fuel cell could power a mobile office or a cabin in the woods, and you could use its perfectly clean steam exhaust to make double-foam lattes.

G.M. is working furiously to develop a practical prototype based on the Hy-Wire’s concepts by the end of 2010. It is developing several fuel-cell models, including a version of its Chevy Equinox, which it expects to have production-ready by then. But take that with a grain of salt, as G.M. has given itself an out: It has promised to make a million of the revolutionary vehicles a year, at prices the company says will appeal to the mass market, but only after a suitable network of hydrogen filling stations is in place. Honda also has an advanced fuel-cell car, dubbed the FCX, which was launched in very limited numbers in the U.S. this year. In typical Japanese fashion, Honda executives are tight-lipped on details but have let slip to the Japanese press that they aim to price the cars below $85,000.

This progress is promising, but fuel-cell technology has had false starts before. DaimlerChrysler spent a billion dollars on the effort in the 1990s but bet on the wrong fuel. Rather than relying on pure hydrogen, Chrysler’s fuel-cell cars extracted hydrogen onboard from ­methanol, which proved a costly dead end. Today, most serious contenders plan to use hydrogen for their fuel cells.

Therein lies another potential snag. Hydrogen may be the most common element in the universe, but you can’t just pull up at your local Exxon station and ask for a tank of it. Until there are hydrogen-refueling stations, nobody will buy a fuel-cell car, but no firm will invest in refueling stations if no one is already driving fuel-cell cars. For its part, Honda is developing a home energy station that lets customers make hydrogen fuel at home from natural gas. Governments also might lend a hand. Arnold Schwarzenegger, California’s hypergreen governor, has proposed a “hydrogen highway” of refueling stations up and down the state. The U.S. Department of Energy also plans to cooperate with energy companies to develop hydrogen-refueling technologies. Royal Dutch Shell and Chevron have both independently developed hydrogen-refueling methods that tap into the existing natural-gas grid, thus helping solve the chicken-or-egg problem.

Will it take off? Fuel cells are the riskiest bet in clean-car technology, but they also have the most potential to change the game. It won’t be easy to replace the vast infrastructure that supports today’s engine technologies with an entirely new one. But fuel cells’ promise of clean, carbon-emission-free personal transportation and an end to oil addiction is so great that this long shot is well worth keeping an eye on.