At the moment, hydrogen seems to be the most practical way to power vehicles that do not emit carbon dioxide, the principal greenhouse gas. But despite considerable research, many hurdles remain. Here are some questions and answers:
Q: Where does hydrogen come from?
A: Hydrogen, an odorless, colorless gas, is the lightest and most plentiful element in the universe. It is found in water and in most organic matter, but it is usually bound with other elements. For that reason, it is called an energy carrier, meaning that energy has to be expended to extract it.
Q: But can't it be burned in an engine?
A: Fuels like coal, natural gas or oil can be taken from the ground and used with relatively little energy-intensive processing. But hydrogen must be turned into a fuel — and because it takes so much energy to produce, it should not be thought of as a direct replacement for fuels like gasoline. Rather, it is energy in a portable form, somewhat akin to electricity.
Q: Then why bother?
A: Because aside from being plentiful, it is very clean. Cars running on hydrogen produce very little tailpipe pollution and no greenhouse gases — they leave only a trail of water. In this way, cars running on fuel cells — which produce electricity by a chemical reaction of hydrogen and oxygen — would produce no tailpipe emissions.
Q: What is hydrogen used for today?
A: It has several uses. About 8.2 million tonnes of hydrogen are produced annually in the US, mostly for chemical production, petroleum refining and metal treatment.
Q: Can nuclear power be a source of inexpensive hydrogen?
A: Yes. Kenneth Schultz, operations director of the energy group at General Atomics, a research and development firm in San Diego, said that by 2025 the nuclear industry could be able to produce hydrogen for US$1.50 to US$2 a kilogram. But Amory Lovins, chairman and chief scientist at the Rocky Mountain Institute, a nonprofit energy research organization in Snowmass, Colorado, described both the process and the price as "aspirational."
Q: How will we use hydrogen in a motor vehicle?
A: There are several options. In the most common prediction, compressed hydrogen gas is stored in a reinforced tank aboard a car, then fed to a fuel cell. The cell is not an engine, but a small chemical factory that converts the pressurized gas to electricity, which runs an electric motor. The only emission is water.
Liquid hydrogen offers excellent energy density per volume, but it would have to be carried in cryogenic tanks because hydrogen doesn't liquefy until it reaches -97°C.
Metal hydrides offer an opportunity to store hydrogen in solid form, but such a system is heavy and therefore reduces the vehicle's travel range.
Hydrogen can also be burned in internal combustion engines. BMW and Ford, among others, have programs exploring hydrogen combustion, and BMW has a small fleet of bi-fuel 12-cylinder Hydrogen 7 vehicles on the road. No carbon dioxide is produced when hydrogen is burned.
Q: Is hydrogen safe? Isn't it very explosive?
A: That's the question on everybody's minds. Hydrogen definitely has safety challenges. It is extremely flammable (as the 1937 Hindenburg accident demonstrated), and burns without a visible flame while radiating very little heat. Because it is the lightest element, it can easily leak out of very small holes, pipe joints and even some metals. Once leaked, it can concentrate in a sealed space like a garage roof, presenting a fire hazard because hydrogen in a confined space can catch fire more easily than gasoline.
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