would hydrogen cars cause cloudy skies?

January 10, 2005 10:31 AM

Weird theoretical question filter: At the moment, gas exhaust from cars rises, and produces a layer of smog above large cities. If everyone used hydrogen fuel cells, this exhaust would be water vapor. Does this mean that it would often be rainy or cloudy due to this water exhaust? If not, what would happen?
posted by unreason to Science & Nature (18 answers total)

Actually, I remember reading somwhere.. or being told, or something, that the water vapour could be worse for global warming than fossil fuels.
posted by eurasian at 11:17 AM on January 10, 2005

I'll ignore that comment unless it has some supporting documentation. The atmostphere is full of gaseous H20.

I think the most you could say about this effect is that the cars would increase the relative humidity by a certain amount. That doesn't mean rain or clouds, unless all other conditions including temperature, pressure, and ambient humidity all play along. Also - I think one of the reasons smog is so noticeable is that it sinks, collects in valleys and such. Water vapor might not do that at all.
posted by scarabic at 11:28 AM on January 10, 2005

I don't know the numbers off-hand, but I'd guess that the amount of water vapor produced would be negligible compared to the amount already in the atmosphere.

Also, keep in mind that water vapor is one of the products of burning fossil fuels. Replacing fossil fuels with hydrogen cells would result in an increase in the net amount of water vapor produced to generate the same amount of energy, but it's not as if the current amount of water vapor we produce is zero, either.

In a previous discussion in the blue on hydrogen as a fuel source, someone asked about how the water produced would affect ocean levels. Here is my response. That's not quite the same as how it affects the atmosphere, but it gives you an idea of how negligible the water produced is compared to all the water that's already out there. (And also note that that calculation is for all of humanity's energy needs, not just that used by motor vehicles.)

Actually, I remember reading somwhere.. or being told, or something, that the water vapour could be worse for global warming than fossil fuels.

I find that difficult to believe, and would be curious to see the argument.
posted by DevilsAdvocate at 11:51 AM on January 10, 2005

Water vapor rises as a heat exchange, forming clouds. Since cities generate waste heat in greater proportion to surrounding areas, cloudier skies would make sense, on average, in or downwind of urban areas. At the very least, those areas would probably be more humid.
posted by AlexReynolds at 11:55 AM on January 10, 2005

A significant question is how hot will the water be coming out of the exhaust? Will there be significant condensation? Where will it happen, in the surface layer or further up in the atmosphere?

Of the candidates out there for cars, only the solid-oxide fuel cell (SOFC) runs hot: 700-1000 C. The polymer-based systems (running on hydrogen, methane or alcohol) all run at slightly above ambient: 50-100 C. There are other types of cells, but they tend to be more candidates for stationary use rather than for transportation.
If a polymer-type cell ends up being the main technology, cars will emit either a cold vapour or (hottish) liquid water. So, perhaps naively, I would expect high low-temperature fuel cell use to greatly increase local humidity and, perhaps have a significant effect on the local microclimate. I don't know anything about climatology except what I hear on the evening news, but I'd bet that thermal inversions and hazy days are still in a fuel-cell powered future. On the other hand, that "smog" will really just be fog, with no ozone, NOX or particulate matter (PM10s) present, which are the things that kill people.

A hot cell, like an SOFC (or one of the more exotic ones), produces hot vapour, which would escape to the middle atmosphere. I suspect we'll be talking about the rain-shadow of cities in that cases, much as we talk about the rain-shadow of lakes now.

Incidentally, I cannot see heating waste water to steam---that would be more of an energy burden than powering the vehicle probably.
posted by bonehead at 11:59 AM on January 10, 2005

Actually, I remember reading somewhere.. or being told, or something, that the water vapour could be worse for global warming than fossil fuels.

It would, if we could significantly increase the water concentration in the atmosphere. It doesn't look like the present human-level activity can do that. The water cycle corrects much more quickly (days) than the carbon cycle does (centuries to millennia?).
posted by bonehead at 12:04 PM on January 10, 2005

Also, keep in mind that water vapor is one of the products of burning fossil fuels.

To amplify Devil's Advocate's point, water vapour production would about double (perhaps triple, depending on efficiencies). That is doubtfully no more significant that what is produced now.
posted by bonehead at 12:08 PM on January 10, 2005

If the water produced by such an engine were sufficiently clean couldn't it be collected by the car, purified and stored for home usage? Ie: your car becomes one big Britta filter?

Sorry for the threadjack ... the thought just popped into my head.
posted by pookzilla at 12:40 PM on January 10, 2005

Would some kind of condenser help with this? Instead of releasing the vapour outside the vehicle you could store it in a reservoir for later use (maybe have it top up the washer jet tank or something).

On preview: Pookzilla has the same idea.
posted by Edame at 12:49 PM on January 10, 2005

If the water produced by such an engine were sufficiently clean couldn't it be collected by the car, purified and stored for home usage? Ie: your car becomes one big Britta filter?

I'd be surprised if the economics of this made it very likely to be attractive. Firstly, the cost of tap water is exceedingly low so you're not saving any money, second, you would be carrying water around in your car so fuel costs would increase to some extent (enough to balance out the trivial reduced water consumption anyway) plus there's the hassle of emptying whatever you're storing the water in, which I imagine wouldn't appeal to many car owners without some other personal benefit. And this is all if the water is pure enough to drink, and even if it is, persuading people that it's potable would be a whole other story.

On preview, Edame's use of the water would make more sense.
posted by biffa at 12:53 PM on January 10, 2005

Another threadjacking: If it were cold enough (like Canadian winter cold), would this car leave behind it a trail of ice an inch or two wide?
posted by grateful at 1:28 PM on January 10, 2005

The issue with using a condensor is that you still need to dissipate the heat energy somewhere, and adding a condenser will decrease the efficiency of the system. (You can't get a 100% efficient heat exchanger, there will be energy costs involved in installing a radiator, either at the front of the car, resulting in increased drag, or through the use of a fan, which uses power)

The amount of water vapour released is very low (no, grateful, you won't leave a trail of water an inch or two wide) It won't look much different then when you car starts up in cold weather right now, and you see the white "smoke" (small water droplets) coming from tailpipes.
posted by defcom1 at 1:47 PM on January 10, 2005

Link I should have posted before making statement.

Sorry, I really should have googled a bit before making that statement. Water vapour DOES matter in the greenhouse effect. However, I'm not sure if the quanitity we will produce with all these fuel-cell cars and the like will make any impact.
posted by eurasian at 1:49 PM on January 10, 2005

Water does matter to the "greenhouse" effect, and would matter if we managed to vaporize enough of it fast enough. Assuming we switched to a completely hydrogen-based economy tomorrow (morning, 9 AM sharp GMT), the annual human output would still be dwarfed by what evaporates from the Pacific on a sunny afternoon.

Here's a link that estimates some of the numbers. interestingly, the authors are concerned that hydrogen-burning aircraft could be a real problem---I hadn't heard that one before.
posted by bonehead at 2:27 PM on January 10, 2005

Wouldn't most of the hydrogen being used be produced by electrolysis of water? So using the fuel would just return water to the atmosphere that was taken out to make the fuel.
posted by squidlarkin at 3:43 PM on January 10, 2005

The thing about nitrous oxides and ozone and such that currently come out of car exhausts is that they're toxic in ppm and there isn't much around at all from natural sources.

In case you somehow missed it, there is quite a bit of water on Earth already, and it's not toxic. Output from vehicles could not possibly make a noticeable impact in that system.
posted by ikkyu2 at 3:55 PM on January 10, 2005

Water vapor is a big part of the exhaust in gasoline engines.
posted by euphorb at 5:56 PM on January 10, 2005

Wouldn't most of the hydrogen being used be produced by electrolysis of water?

Not if it's to be a replacement for fossil fuels. Conservation of energy: the energy generated by burning hydrogen to create water is equal to the energy it took to hydrolyze the water to hydrogen and oxygen in the first place. And where does that energy come from? Back to square one. If the hydrogen is generated from hydrolysis of water, then it's just a form of storing energy, not a means of generating energy.

It's useful as an energy source if we can find other sources of hydrogen.
posted by DevilsAdvocate at 6:25 PM on January 10, 2005