Fuel cells start to look real
More 1
2
3
4
5
6
7
8
Vehicles
Sidebars
Direct hydrogen
The most obvious solution is to use hydrogen directly as the fuel. In this scenario, service stations could install hydrogen tanks next to their pumps or perhaps, miniature electrolysis factories that produce the gas from water. This choice would remove the need for an onboard fuel reformer. It would also avoid producing carbon dioxide and other greenhouse gases in the reforming process, though some would be generated during the most common industrial hydrogen production method, which uses natural gas as a feedstock. (Note that the electricity for electrolysis usually comes from powerplants that burn hydrocarbons or split atoms.)
"The beauty of hydrogen production," said McCormick, "is that it's not dependent on one particular feedstock or processing route. You could use electrolysis of water or you could crack natural gas or petroleum."
Skeptics scoff, saying that establishing a national hydrogen network could cost hundreds of billions of dollars. Currently, only a few hydrogen filling stations exist. Experts also note that refilling a vehicle with hydrogen requires careful safety procedures. For example, the pump has to be electrically well grounded (you don't want a Hindenburg disaster at your local service station). Proponents say that hydrogen availability will become less of an issue when automakers finally opt for hydrogen-fueled cars, but as with most things in this field, it's a question of what's going to come first, the donkey or the cart.
And while hydrogen packs more energy by weight than any other fuel (about three times more than gasoline), it is hard to store much of it in a fuel tank. This might not be much of a problem in a bus, but it is in a car. A reasonably sized, commercially available pressure vessel, operating at typical pressures of about 24 MPa (3500 psi) and fully filled with hydrogen will take a car barely 190 km (120 mi) - not far enough for most drivers. Also, with hydrogen being the lightest and smallest of molecules, it is relatively difficult to contain, which poses safety problems.
Both DaimlerChrysler and GM have boosted range significantly and ameliorated packaging problems in recent concept cars by using liquid hydrogen storage, but the cryogenic technology to keep fuel at -253°C (-423°F) - just 20°C (36°F) above absolute zero - remains problematic for reasons of extra weight, greater complexity, and safety.
Most experts hold that, in the long run, direct hydrogen fueling will be the way to go. "It looks like direct hydrogen is the most likely final result," said Ben Knight, Vice President of Honda R&D America Inc. "It's the most sustainable fuel in the long term, compressed gas storage is well understood, and it's the only set up that clearly beats standard hybrid-electrics regarding total system efficiency. We also believe that direct hydrogen can allow cars to be refueled quickly, and could eventually provide enough range. Nevertheless, we're preparing to handle all the possible fuels."
One possibility for solving the onboard hydrogen storage problem is simply to pack more of the gas into vehicles. Stronger pressure vessels could contain more hydrogen at higher pressures. Progress is being made in this area. Researchers at IMPCO Technologies Inc.'s Advanced Technology Center in Irvine, CA, for example, will soon start safety and performance validation testing on the company's new in-tank regulator for hydrogen storage. The patented H2R 5000 flow regulator is a crash-resistant device that can operate at 34 MPa (5000 psi). They are also working to develop concepts to extend IMPCO's ultra-lightweight TriShield hydrogen storage tank technology to 69 MPa (10,000 psi). Working with technical assistance from the U.S. Department of Energy and more than $3.5 million of federal research funding, IMPCO has completed development of a commercially viable high-performance hydrogen storage cylinder featuring 7.5% hydrogen storage by weight. This technology is expected to permit a 645-km (400-mi) maximum driving range at a total vessel mass less than 68 kg (150 lb). Early this year, the company will also start commercialization of a pressure vessel technology that stores 8.5% hydrogen by weight. Together with researchers at Lawrence Livermore National Laboratory and Thiokol Propulsion, a division of Alcoa Automotive, IMPCO engineers recently demonstrated a prototype storage tank technology capable of holding 11.3% hydrogen by weight.
Alternatively, vehicles could be designed to incorporate specially shaped conformal tanks in their bodies. "It's not clear just how big a pressure bottle (or how many) you can hide away in a car," said Staley, who adds that Thiokol is working on conformal tanks with funding from the Department of Energy.
Another option is to use materials that absorb hydrogen into their crystal structure (metal hydrides) or incorporate it chemically (chemical hydrides), to hold the fuel until needed. Energy Conversion Devices of Troy, MI, has reported good progress in metal-hydride technology based on magnesium (see sidebar). According to Staley, "In metal hydrides, the amount of stored hydrogen per unit weight is still low. And, it's like having a bunch of bricks in there, not unlike having batteries in a car, not to mention the extra weight for the thermal packaging."
GM's McCormick concurred: "Though it's somewhat promising technology, we're not expecting any breakthroughs over the next few years. You not only have to consider the weight percent of stored hydrogen, you've got to look into the energetics of the entire chemical reaction (even if it is close to an adsorption process). That concerns the heat involved when you put the hydrogen on the hydride and when you take it off. That bonding energy has to be managed; it enters into the total energy equation and lowers the net energy gained."
In chemical hydride storage schemes, stable, relatively benign compounds such as boron, sodium, and calcium hydride are processed with water in a catalytic fuel reactor to generate pure hydrogen. A sodium hydride storage system has been developed by Powerball Industries of West Valley City, UT, while sodium borohydride is being used by Millennium Cell Inc. of Eatontown, NJ. Questions remain about total system weight and what to do with the other reaction byproducts, which often need to be recycled for reuse.
Some say that carbon nanostructures - graphite fibers with intricate, high-surface-area configurations - offer promise. Certain carbon nanostructure materials have been shown to absorb more than one-fifth their weight in hydrogen. It seems clear, however, that both hydride and carbon nanostructure storage technologies remain immature at this stage.
