ONE 0F THE CHALLENGES TO THE hydrogen economy is the development of mass-efficient "hydrogen storage materials." Complex hydrides such as LiAlH4 and NaBH4, both at four-thirty-eighths or about 10% mass efficiency has been mentioned as candidates for this purpose (C&EN,June 9, page 35). As appealing as these materials may be, it nonetheless seems to me that we should not rule out the use of carbon hydride, at about 25% mass efficiency. Unlike other sources of hydrogen, this material is available from methane with zero loss and zero additional cost, and it leaves upon combustion no residue that needs to be recovered and recyded.
OUR NATION'S REAL NEED FOR A viable long-term solution to renewable energy, especially for private transportation, is not well served by articles that paint a fanciful picture of the promise of fuel cells (C&EN, June 9, page 35; June 16, page 16). Economically viable solutions for the following fuel-cell challenges seem highly unlikely in the foreseeable future.
The total cost (without subsidy) of proton exchange membrane (PEM) fuel-cell engines (fuel cells, power conditioning, electric motors, and so on) with mass low enough to be practical in a vehicle is in the range of $3,000-$7,000 per kW-40 times that of the advanced diesel engine.
It is worth noting that PEM fuel cells have been in use and development for 40 years, and costs have not yet begun to drop signifîcantly&endash;not withstanding many assertions to the contrary (such as those in the June 16 article) that use artificial costs from heavily subsidized projects or cite costs of massive, stationary fuel cells that are unsuitable for vehicles.
Safety-approved affordable compressed-gas cylinders achieve 1.5% H2 storage by mass at 34 MPa (5,000 psi). A $25,000 carbon-fiber-wrapped fuel tank achieving 6% H2 storage seems impractical for the small private car, and liquid hydrogen (LH2) doesn't keep long. The huge mass penalty associated with economical H2 storage seems likely to keep the mileage of fuel-cell-powered automobiles (of acceptable range, acceleration, cost, and cargo capacity) below 25 miles per kg of H2 for many decades.
Current U.S. H2 production is enormous &endash; about 2 x 10exp.10 kg per year. Yet the current pretax cost of LH2, delivered in 15,000-gal (4,300-kg) tankers to high-volume customers, is $4.30 per kg, and other methods of H2 distribution are even more expensive. On the other hand, the current U.S. pretax cost of gasoline for the individual consumer at the local station is about 30 cents per kg.
The oniy economically viable sources of H2 in the U.S. are natural gas and coal. The nearly adiabatic partial-oxidation/reformation/shift reactions use 3 kg of natural gas (90% CH4) to produce 1 kg of H2 plus 9.5 kg of CO2. Then more than 3 kg of coal must be burned (releasing another 10 kgs of CO2 to generate the 10 kWh (36 MJ) needed to purify and liquefy 1 kg of H2. The energy efficiency in producing LH2 is under 50%. (This number has not budged in 15 years and will not in the next 50. We're near Camot limits.) The energy content of 1 kg of H2 is equivalent to 2.8 kg (1.1 gal) of gasolme, which contains only 2.3 kg of carbon.
At 80 miles per gallon, the advanced diesel hybrid achieves 7 miles per kg of total CO2. The fuel-cell automobile at 25 miles per kg of hydrogen achieves 1.1-1.3 miles per kg of total CO2. Hence, when miles per kg of CO2 release ("fossil mileage") is more fairly calculated, the total CO2 generated per mile by a hydrogen vehicle is likely to be five times that of a comparable diesel-powered hybrid vehicle for at least four decades. (If we have not been able to raise fuel taxes a nickel in the past two decades, how can we expect to impose a $1.00 per kg surtax on H2 production to support CO2 sequestration?).
It is most interesting to note that, seven years ago, the Department of Energy expected fleets of fuel-cell-powered vehicles to be in use by now. Today, they are projectmg that it will occur seven years from now. Undoubtediy if DOE invests $2 billion (as expected) over the next seven years, many more demonstration vehicles (at $300,000 each) will be on the road, but that really does not accomplish much. I expect to still see that "seven-year" projection for commercial fleets 20 years from now. It's time we start putting some serious money into real options for our future transportation needs.
F. DAVID DOTY
52 C&EN/AUGUST 25,2003