Automakers, engineers, politicians, and many others foresee a hydrogen future for America and the world. In most eyes, hydrogen means fuel cells, providing electricity to propel a vehicle. However, hydrogen can also be used to power a conventional internal-combustion engine, as BMW does with its experimental series of Hydrogen 7 sedans.

Like a battery, a fuel cell is an electrochemical energy conversion device. While a battery eventually becomes discharged, a fuel cell produces electricity as long as fuel (hydrogen or another substance) and oxygen are arriving. Like batteries, fuel cells work in tandem. Each cell produces about 0.7 volt, so many are combined into a stack.

Exhaust emits only plain water vapor
While other alternative-fuel systems promise reductions in harmful emissions, hydrogen fuel cells release no tailpipe or greenhouse-gas emissions at all. In the Chevrolet Equinox Fuel Cell Vehicle, for example, four slots at the rear release water vapor -- the sole emission.

May be produced from readily-available natural gas
Considered the most abundant element, hydrogen can come from various sources: Natural gas, water or organic waste. Compressed natural gas (CNG) already is used in vehicles, including Honda's Civic GX. The California Fuel Cell Partnership estimates that a two-percent increase in the natural gas supply would support 10 million fuel-cell vehicles.

Improvements have been numerous and rapid
Since the first fuel-cell stack in 1997, volumetric power density has increased tenfold. General Motors has developed a series of experimental fuel-cell vehicles, including the Hy-wire (2002), AUTOnomy (2002) and Sequel (2005). GM's latest, the Equinox, can start and run in below-freezing temperatures -- a troubling fuel-cell obstacle. Equinox's fourth-generation propulsion system uses a nickel-metal-hydride battery pack and three 10,000-psi storage tanks. GM claims efficiency double that of gasoline.

Honda created the FCX in 2003 and, two years later, was the first to lease a fuel-cell vehicle. Promising 210-mile range, that FCX got an EPA fuel-economy estimate equivalent to 62 mpg city/51 mpg highway. Honda's new FCX Clarity demonstrates a 400-pound weight cut, 20-percent better fuel economy (equivalent to 68 mpg combined) and 270-mile range. Because the stack is 65-percent smaller, it fits in a tunnel between the front seats. Rather than the ultra-capacitor used to store electrical energy in the original FCX, the Clarity uses lithium-ion batteries.

Real-world trials going on now
Starting in summer 2008, Honda is leasing 200 FCX Clarity sedans, mostly to selected U.S. customers. About 50 go to southern California. For $600 a month, the three-year lessees receive even collision insurance. GM's Project Driveway, meanwhile, makes Chevrolet Equinox Fuel Cell Vehicles available to 100 motorists in southern California, New York City and Washington DC. Range is estimated at 150 to 200 miles.

More than 200 fuel-cell vehicles are running, according to the California Fuel Cell Partnership, having traveled 1.6 million miles. Daimler, Nissan, GM, Ford, Honda, VW, Toyota, Hyundai, and Audi are among the participants. California has 25 hydrogen-refueling stations. Mercedes-Benz plans small-scale production of its B-Class fuel-cell vehicle in 2010.

Safety concerns are exaggerated
High-pressure tanks are nothing new. After all, in the early days of the gasoline automobile, people worried about carrying tanks of flammable gasoline.

Potential for home refueling
Honda, with partner Plug Power, has been developing a Home Energy Station to refuel the vehicle overnight. Running on natural gas, this system can supply high-purity hydrogen, as well as heat a home, provide hot water and generate electricity for residential use. Looking ahead, engineers also are working on vehicles that create their own hydrogen.


That's half the story. Here's the other:

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