There’s no question right now that fuel efficiency is in. If gas prices are high enough to bring the much-maligned Geo Metro back to the price range it was originally sold at over a decade ago, you know consumers are taking every action they can to shore up their fuel economy. But the concern is not purely economic; more and more, awareness of the effects of heat-trapping carbon emissions are driving consumers towards better and cleaner choices, especially in the realm of automobiles.

Ford Escape hybrids continue to sell at well above their book prices, and last month, the average time from delivery for sale for a Toyota Prius was a shade under 17 hours. With hybrids selling so well, people are searching everywhere for the next big step toward the zero-emission vehicle. And one of the most hyped contenders in this arena is the hydrogen fuel-cell vehicle.

The technology behind fuel cells is staggeringly simple and immediately appealing. Take some hydrogen, take some oxygen, then put special membrane between them. The hydrogen crosses the membrane to combine with the oxygen, generating an electrical current in the process. And the only chemical emission is regular old water.

Fuel cells have provided power to the space program for decades, and a fairly large number of prototype hydrogen vehicles are already in existence. In fact, if you’re lucky enough to live in certain parts of California, you’ll be able to lease a fuel-cell hydrogen vehicle from Honda, starting sometime this summer. Plus the design is undergoing further refinement everyday. Two of the biggest advances in recent months have been made by BMW; a gorgeous concept sedan that reduces carbon monoxide as it drives, and a fuel tank that can apparently 310 miles on 22lbs of hydrogen.

But sadly, that appears to be the end of the good news about the fuel cell car. As you may have read, BMW has no plans for a production model of that concept sedan, most likely because the cost would be prohibitively high. The documentary Who Killed the Electric Car? estimates that the average cost of a prototype hydrogen fuel cell vehicle is in the millions; not bad for a proof-of-concept, but far too costly for the assembly line. Moreover, hydrogen gas is extremely expensive to produce, both economically and in terms of greenhouse gas emissions. In most cases, it must be electrolyzed from water, in a complete reversal of the reaction used to power a fuel cell car—anyone familiar with the Laws of Thermodynamics can tell you why that’s a losing proposal.

But potentially, the most lethal blow to the hydrogen fuel cell vehicle is that it simply can’t compete against other vehicles in the existing marketplace. Unlike plug-in electrics, it can’t be refueled at home, and unlike hybrids, it cannot refuel at a gas station or augment its range with gasoline. Storing and transporting hydrogen in a useful, liquid state requires keeping it extremely compressed, something which existing gas carriers would require extensive modifications to do.

In short, hydrogen cars do offer the potential for clean, long range transport, but at such an immense cost that widespread adaptation of the technology simply isn’t a feasible option in the foreseeable future.