I also have to wonder what the effect of a few million cars emitting water vapor would have. Isn't it a stronger greenhouse gas than CO2?

Hydrogen is just a way for our politicians to say "See? We're doing something!" without rocking the boat.

There should be roughly 500 hydrogen powered vehicles on the road today or may be 1000.

Hybrid vehicles should be 700,000 +.
So far only individual groups were converting HEV to Plugin HEV, now there is talk about Toyota offering Plugin Prius and if it comes to market, it will be a big hit.

So where will Hydrogen powered vehicles stand.
A big question mark.

Have a look at this, Joe. [Wikipedia]

Water vapor is a natural greenhouse gas which, of all greenhouse gases, accounts for the largest percentage of the greenhouse effect. Water vapor levels fluctuate regionally, but in general humans do not produce a direct forcing of water vapor levels.

Assuming hydrogen production somehow becomes widespread and its use as a tranportation fuel practical, what effect would several million H2O-emitting vehicles have?

Isn't [water vapor] a stronger greenhouse gas than CO2?

Yes, it is. However, unlike carbon dioxide, water vapor can condense and fall out of the atmosphere at levels we see on earth. Also, hydrogen powered vehicles can capture the water they create and store it until it needs emptied.

How about they do a H-prize for someone that can show a good distribution method for hydrogen which is safe, cost effective and doesn't leak H2 gas (which will potentially destroy the ozone layer if leaked in any significant quantities).

Why distribute and store hydrogen as a gas? You're thinking of it in old-fashioned terms, just like the groups who tried to cross the Atlantic in huge aircraft. Create the hydrogen at the distribution point and only store enough to meet daily peak demands.

Additionally, they should have the prize show where all the energy will come from to produce the hydrogen for all vehicles in the US.

Wind, wave, hydroelectric, geothermal, solar, nuclear. Increasing demand for electricity to produce hydrogen combined with a carbon tax will lead to more demand for clean electricity production.

I'm just hoping this thing doesn't lead to a system that requires a garage. Having the ability to refuel your car at home might be great for some, but I don't have a regular parking space, let alone a garage, and I don't think the city would like to have hoses full of flammable gas going down the block.

Why distribute and store hydrogen as a gas? You're thinking of it in old-fashioned terms, just like the groups who tried to cross the Atlantic in huge aircraft. Create the hydrogen at the distribution point and only store enough to meet daily peak demands.
Great job, Icelander! You have just demonstrated that we do not need hydrogen at all: If you carry the fuel that you would use to create the hydrogen "at the distribution point", why bother creating the hydrogen? Just use the primary fuel in your application. That way you avoid the low efficiency that hydrogen production inherently suffers from. Your system is also much safer without the hydrogen.

Wind, wave, hydroelectric, geothermal, solar, nuclear. Increasing demand for electricity to produce hydrogen combined with a carbon tax will lead to more demand for clean electricity production.
Again, good idea if you loose the hydrogen. Take the "clean" electricity directly to your application and increase efficiency and safety.

I'm just hoping this thing doesn't lead to a system that requires a garage. Having the ability to refuel your car at home might be great for some, but I don't have a regular parking space, let alone a garage, and I don't think the city would like to have hoses full of flammable gas going down the block.
Again, loose the hydrogen and you are home free...

Just to put it into perspective:

If you exclude manufacturing power use; residential and commercial energy use combined is just barely greater than the energy use of our transportation sector. Transportation uses more energy than manufacturing but not by much. Now when you convert all vehicles to electric you just created a huge load on the power infrastructure. Convert all vehicles to hydrogen and that load is even greater still. You would have to increase capacity by 1/3 across the entire US for electric vehicles. I'm not sure how much more you'd have to increase the capacity by to have all vehicles run on hydrogen but it will be even greater.

It would take the entire budget of the US over several years to create the powerplants to support that much energy use if we stick to renewable sources (wind, solar and geothermal). Nuclear is more cost efficient but then you face the "NIMBY" effect. In fact, a reactor was just torn down in Portland, OR not too long ago and you won't see a new nuclear reactor going up to replace it any time soon (not even a "safe" reactor like a pebble bed reactor or similarly safe reactors).

Icelander-

Yes that is what I said. Are you illiterate?

It would take the entire budget of the US over several years to create the powerplants to support that much energy use if we stick to renewable sources (wind, solar and geothermal). Nuclear is more cost efficient but then you face the "NIMBY" effect. In fact, a reactor was just torn down in Portland, OR not too long ago and you won't see a new nuclear reactor going up to replace it any time soon (not even a "safe" reactor like a pebble bed reactor or similarly safe reactors).

Depends on the technology, Patrick. Certainly if we were to produce enough hydrogen to power all transportation things would be prohibitively expensive. For all electric, things would not be quite as bad. For PHEV, the impact is apparently not that large. This from a previous GCC story:
As to the concern about the impact charging PHEVs would have on the grid, Duvall noted that a typical battery charger for a plug-in hybrid will draw about 1,400 watts of power from a 120-volt outlet and be active for about 2-8 hours per day—roughly equivalent to an electric space heater.

EPRI has reviewed the discussion draft and is of the opinion that it addresses the most critical technical challenges to the development and adoption of plug-in hybrid vehicles.Mark Duvall, EPRI
see http://www.greencarcongress.com/2006/05/congressional_w.html

So, at 100% PHEV, we are talking about a rather modest 8 - 15% increase in electrical power demand. Couple that with financial incentives to encourage people to charge overnight (as opposed to during the day), and very little increase would be necessary.

"...It is clean, it is abundant..."

Abundant? Certainly not as H2, but as H20, CH4, etc. etc.

Seems more likely that carbon-neutral liquid fuels (biodiesel, etc) will be the practical winners in the marketplace.

Note the difference between this competition and the Orteig prize of the 1920s: the Oretig prize rewarded an outcome (fly from NY to Paris non-stop), not the means to achieve it. The outcome we want to reward is progress towards a transportation system which reduces GHG emissions and reduces reliance on foreign oil. To reward tinkering with hydrogen economy infrastructure misses the point and skews the incentive.

I didn't believe you about the 250 million fuel cell vehicles Idiot Police. It seems like an insane proposition given what I hear in the news about not being able to produce the hydrogen. I checked the Air Product website though, and sure enough they concurred.

Here's what they said in their FAQ:

"There is a large infrastructure of hydrogen today to meet the needs of industrial applications including metals processing, refining, chemical production, fats and oils production, and electronics processing. About 45 billion kilograms (50 million tons) is produced every year—enough hydrogen to fuel 250 million fuel cell cars. Some of this hydrogen is being used to fuel vehicles."

I found that here:
http://www.airproducts.com/Products/LiquidBulkGases/HydrogenEnergyFuelCells/FrequentlyAskedQuestions.htm

Hey IP,
Before insulting all of us, how about engaging that idle organ between your ears?

Of course refining crude into gasoline involves hydrogen. However, the whole point of the H-prize is to get us off crude. Or did you miss that?

You also seem unclear on the fate of the associated carbon when hydrocarbons are reformed to produce hydrogen. To fix that, a chemistry lesson:
CnHm + nH2O -> nCO + (m/2 + n)H2
CO + H2O -> H2 + CO2
In other words, not exactly carbon-free. Not GHG free. You call that a solution, officer?

I don't think anybody here wants to see us use fossil fuels until those are exhausted (hence the name Green Car Congress). But perhaps you have spent too much time policing the idiots to notice such details.

So go ahead, tell us: Where will the hydrogen come from? Not from crude, that would be no improvement over today's situation. Coal? Natural gas?

Waiting for your wisdom...

We have been injecting fossil water into the atmosphere since we started burning coal. Burn any hydrocarbon (nat gas, oil, coal) and CO2 and H2O is generated.