Last week General Motors (GM) unveiled a hydrogen-fuel-cell-powered version of its Chevrolet Volt concept, a family of electric cars that get a portion of their energy from being plugged into the electrical grid. The first version, announced in January, married plug-in electric drive to a gasoline or ethanol generator that can recharge the battery.

But swapping out the generator for a fuel cell may be a step backward. That is in part because producing the hydrogen needed to power the fuel-cell version could increase rather than decrease energy demand, and it may not make sense economically.

“The possibility that this vehicle would be built successfully as a commercial vehicle seems to me rather unlikely,” says Joseph Romm, who managed energy-efficiency programs at the Department of Energy during the Clinton administration. “If you’re going to the trouble of building a plug-in and therefore have an electric drive train and a battery capable of storing a charge, then you could have a cheap gasoline engine along with you, or an expensive fuel cell.” Consumers will likely opt for the cheaper version, Romm notes.

Still, the Volt is part of a promising trend toward automotive electrification–which could decrease petroleum use and reduce carbon emissions. It is part of GM’s response to an anticipated future in which both petroleum and carbon-dioxide emissions will carry a heavy price, driving consumers to buy vehicles that run on alternative, low-carbon power sources.

The new Volt, announced in Shanghai, replaces the generator with a fuel cell and cuts the battery pack in half, in part to make room for storing hydrogen. The lithium-ion battery pack can be recharged by plugging it in. The fuel cell kicks in immediately when the car is started and provides power at a constant rate at which it is most efficient. If more power is needed, such as for acceleration or high speeds, the battery provides a boost of power, much like what happens in today’s gas-electric hybrid vehicles. When less power is needed, such as when the vehicle is stopped or at low speeds, the battery stores energy to be used later.

By allowing the fuel cell to run at a constant rate, the batteries improve efficiency, cutting down on hydrogen consumption. The battery further improves efficiency by storing energy generated during braking. Compared with earlier prototypes, the new concept also uses a more advanced fuel-cell design (thinner stainless-steel parts were substituted for thick composite parts) and the vehicle is lighter, making it possible to have a 300-mile range using half the hydrogen.

The car emits no harmful emissions from the tailpipe. But because hydrogen fuel today is primarily made from fossil fuels this means the carbon-dioxide emissions are simply happening someplace else, Romm notes. He says that using renewable energy to charge up the battery in the gas-generator version of the Volt makes more sense than using it to make hydrogen. That’s because it’s more efficient to charge a battery than to make hydrogen, compress it, and then convert it back into electricity using a fuel cell.

Nick Zielinski, the chief engineer at GM responsible for advanced vehicle concepts, says that GM released the fuel-cell version in China because hydrogen has a better chance of taking hold there. In China, energy infrastructure is still being developed, and gasoline and electricity may not be available everywhere. “They could develop a hydrogen infrastructure much sooner than we do here. And a fuel-cell vehicle may make more sense than a plug-in-to-grid option because hydrogen may be much more accessible,” Zielinski says. He adds that “hydrogen, when it’s generated in a renewable way, produces no emissions. And that’s where I think we’d like to get to.”

With the Volt, the power source can vary according to the proposed market. In the original version revealed in January, likely to attract customers in the United States, the first 40 miles of driving are powered by energy stored in a battery pack that can be recharged by plugging it in. (See “GM’s New Electric Vehicle.”) That’s enough range for a typical daily commute. For longer trips, a gas- or ethanol-powered generator recharges the battery, allowing for an additional 600 miles of range. In Europe, diesel generators can be used, rather than gasoline generators or fuel cells.

Other major automakers are also developing plug-in fuel-cell and battery-powered vehicles. In January, Ford unveiled a vehicle that runs off stored power in the battery for the first 25 miles before a fuel cell starts recharging the battery, which can add an additional 200 miles of range. As with GM’s Volt, the fuel cell could be replaced by a gas or diesel generator.

Both the fuel-cell and generator versions of the Volt will face challenges, but it’s likely that the fossil-fuel versions will reduce carbon-dioxide emissions more than the fuel-cell versions. The Volt equipped with a gas-powered generator has a large battery pack that will allow most drivers to skip the gas station altogether for daily commuting. And because the generator is very efficient, even for trips longer than 40 miles it may use less gas than ordinary cars. Of course, total carbon emissions will depend on the source of the electricity for charging the battery, but the relatively high efficiency of power plants compared with conventional vehicles will likely lead to lower carbon-dioxide emissions overall.

The main challenge with this vehicle is that a big enough lithium-ion battery pack, made to withstand the abuse of automotive applications, hasn’t yet been created. Zelinski says that no more major breakthroughs in battery-cell technology are needed. All that’s left is to integrate hundreds of these cells to make a big battery pack. That’s going to be challenging, but, he says, “it’s something that can be handled in a straightforward, solid-engineering way.”