Still, it was fun to park at a Total gas station outside Berlin--one of a few demonstration hydrogen-car filling stations in Europe, with several others scattered around the world--and fill the sleek machine with liquid hydrogen. A fill-up takes about eight minutes. The retail price was 8 Euros (about $10.60) for a kilogram of hydrogen, which has the approximate energy content of one gallon of gasoline.

Klaus Draeger, a BMW research manager, suggests that hydrogen today is where gasoline was 100 years ago. A century ago, he says, "who could imagine that there would be a world-embracing network of filling stations? Only a few visionaries would imagine that happening." Many of the BMW engineers hovering over the journalists noted that radical infrastructure changes are not a major challenge. And they're probably right. I've seen pictures showing what Berlin looked like 60 years ago. It's hard not to agree that infrastructure can be completely rebuilt in a matter of decades.

But only if it makes sense to do so. In this case, the infrastructure isn't the largest issue. Hydrogen is the largest issue. You can't just dig it out of the ground and burn it. You have to either extract it from hydrocarbon fuels, which defeats the clean-energy purpose, or extract it from water molecules by applying electricity--which means you are either burning the fossil fuel back at the power plant or taking away much-needed renewable electricity from the power grid.

When you extract hydrogen from fossil fuels, you actually end up emitting more carbon dioxide. In fact, driving a car whose hydrogen was extracted from natural gas results in roughly double the carbon-dioxide emissions produced by driving a car that simply burns the natural gas directly. For fossil-fuel extraction of hydrogen to ever make sense from an environmental perspective, the separated carbon dioxide would have to be sequestered underground.

And if you use electricity to split water, you'll need to make sure the electricity doesn't itself come from fossil fuels. The electricity would have to come from a renewable source, like wind or the sun. It's not clear that hydrogen production is the wisest use for renewable energy, except marginally: it can absorb electricity on very windy or very sunny days, when renewable power plants are producing excess supply.

BMW maintains that once we have breakthroughs in renewable supply and hydrogen storage, cars based on Hydrogen 7 technology can fill every driveway--and perhaps even every garage. I want the company to be right. The idea of a high-performance car that essentially emits only water vapor is very alluring. But for now, the Hydrogen 7 appears to be a remarkable engineering achievement for a future that may never arrive.