The station dispenses what city officials and the South Coast Air Quality Management District hope will be the fuel of the future: hydrogen.

Riverside's hydrogen station is part of a project sponsored by the AQMD to test the practicability of hydrogen fueling stations and hydrogen-powered cars on California's roads. Five cities — Riverside, Santa Ana, Ontario, Burbank and Santa Monica — each have fueling stations, as well as a fleet of five Toyota Priuses converted to run on hydrogen fuel.

The five stations will bring the number of hydrogen stations in California to more than 20, further establishing the California Hydrogen Highways Network, a plan proposed by the California Environmental Protection Agency to build 50 to 100 hydrogen stations by 2010.

"We're piecing together a basic network so people can feel comfortable driving around … knowing they won't run out of fuel," said Chung Liu, deputy executive officer at the AQMD's Science and Technology Advancement division.

Other small hydrogen demonstration projects, run by car manufacturers such as Toyota, Honda and BMW and a few universities, have also cropped up around California, clustering in the Bay Area and Southern California. In August, Irvine became the first city in the nation to receive a fuel-cell vehicle, leased by UC Irvine's National Fuel Cell Research Center.

Cities around the state are jumping on the hydrogen wagon as well: Fuel-cell buses are running in public transportation systems at Alameda-Contra Costa Transit, SunLine Transit in Thousand Palms and Santa Clara Valley Transportation Authority, according to the California Energy Commission.

Using hydrogen as a fuel has captured the imagination of environmental engineers for decades. Gasoline-powered cars spew carbon monoxide, sulfur dioxide and other pollutants into the air; hydrogen-powered cars emit almost no pollutants.

One of hydrogen's key advantages is that it can be stored in multiple ways, either in gas or liquid form, or combined with other compounds. The AQMD's five-cities project will rely on highly pressurized hydrogen gas that can be dispensed easily and quickly from a pump.

Hydrogen also offers the possibility of lessening the dependence on fossil fuel, which will become increasingly expensive and scarce over time, said Keith Wipke, senior engineer at the National Renewable Energy Laboratory in Colorado.

But significant barriers make hydrogen a risky venture.

The primary goal for environmentalists is producing the hydrogen in a clean way. Unlike oil or coal, hydrogen is not a fuel that exists readily in the environment in a usable way.

One method of isolating hydrogen requires splitting molecules such as water. That process takes energy, usually supplied by power plants — the largest source of pollution in the United States.

"The beauty of hydrogen, though, is that it is the only energy carrier that can be produced from a variety of sources," said Daniel Emmett, executive director of Energy Independence Now, a nonprofit that promotes alternative fuels. "You can use wind, solar or geothermal sources to produce hydrogen — and those have net zero greenhouse emissions."

Riverside officials are planning on eventually building a canopy over the fueling station that can be covered with solar panels to produce energy for the pump's electrolyzer — which splits water into hydrogen and oxygen, said Martin Bowman, fleet operations manager for Riverside. Until the city gets funding for the canopy, however, the electricity will come from the city's power plants.

All of the energy used for Santa Monica's electrolyzer, the only one of the five stations not yet in operation, will come from renewable resources.

In 2003, the five municipalities were approached by the AQMD to participate in the project. City officials agreed, eager to take an opportunity to test the technology and comply with an AQMD mandate that cities in the South Coast Air Basin use some alternative fuel vehicles in all of their fleets.

Given the converted Toyota Prius' range of about 80 miles, city inspectors who make short trips are the most likely to use the cars.

The hydrogen fueling stations are not designed to accommodate more than the small fleet, although they will be open to owners of hydrogen cars around the state. They can only fill up 10 cars a day but can be expanded to handle up to 20.

The bulk of the project is being paid for by the AQMD, at a total cost of about $7 million, but the cities are required to buy the cars. The cost of each Prius is about $25,000; the conversion from a gasoline to a hydrogen engine — which will be paid for by the AQMD — is $86,200 per car.

Gov. Arnold Schwarzenegger signed a bill in July that would set aside $6.5 million for future hydrogen demonstration projects.

The cost of hydrogen fuel per mile in a converted hybrid car runs from 14 to 27 cents, compared with 4 cents per mile for a gasoline-powered hybrid.

The cost of hydrogen fuel isn't optimal, conceded Matt Miyasato, technology demonstration manager at the AQMD. "But we've got to test all of the technologies now. We can't wait for it to become economical; we're doing research."

The cost of producing hydrogen for cars would drop dramatically if it were ever commercially produced by processing plants on a scale similar to gasoline refineries, alternative-energy supporters said.