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Discussion Starter · #1 ·
While doing some digging on greenhouse gas emissions I had another thought. Water Vapour is a effectively a greenhouse gas (it's a vapur not a gas but amounts to the same effect). FCEVs emit water to the tune of around 80-100g/km mostly in the form of vapour when up to temperature. Water in itself is not a dangerous pollutant, but it is an emission which has a greenhouse effect. I have no idea what the magnitude is and haven't got time to try doing the sums at the moment, but describing FCEVs as Zero Emission Vehicles seems misleading, they emit water vapour and possibly have a significant greehouse effect.

As I've pointed out before I'm in favour of any technology that reduces the pollutants we throw into our atmosphere and FCEVs are definitely better than ICE vehicles from the perspective of air quality particularly in urban areas, but if they are going to add to the climate change issues then a rethink might be in order. If the hydrogen is generated from cracking water using solar energy then you could argue that it's just moving the water around the cycle but effectively it's changing state from a non greenhouse, to a greenhouse state, so that worries me.

I'd be interested to know others views - the effect may be completely negligable because on a very quick calculation even if every car in the world currently was a FCEV the amount of water emitted into the atmosphere every year would be about 0.0001% of the mass contained in our atmosphere.

Thoughts?
 

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How much water vapour does a human being breathe out while walking a kilometre?
 

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While doing some digging on greenhouse gas emissions I had another thought. Water Vapour is a effectively a greenhouse gas (it's a vapur not a gas but amounts to the same effect). FCEVs emit water to the tune of around 80-100g/km mostly in the form of vapour when up to temperature. Water in itself is not a dangerous pollutant, but it is an emission which has a greenhouse effect. I have no idea what the magnitude is and haven't got time to try doing the sums at the moment, but describing FCEVs as Zero Emission Vehicles seems misleading, they emit water vapour and possibly have a significant greehouse effect.
heh. No, not really. The reason that water vapour emissions are irrelevant is that it is not the quantity of such emissions that enter the atmosphere, it is the conditions of the atmosphere that matter. Water vapour will condense out at its dew point.

Funnily enough - and is exactly why 'climate models' have so far rarely tried to incorporate cloud cover - it is a chaotic and astable system. The warmer it is the more water vaopur you find in the lower atmosphere. However this then rises and causes more condensate (cloud over) higher in the atmosphere, which increases albedo, which lowers temperature.

Overall, at current global conditions, generally speaking more water vapour content in the atmosphere leads to a stronger self-regulating effect on temperature, than it has a destabilising effect.

However, it is also worth bearing in mind that more water vapour in the atmosphere means there is more energy in the atmosphere. Energy need not only be heat energy. Another form is humidity levels. As it happens, average global humidity does appear to have gone up over the last decade or two (or so I have been lead to believe by data Phil Jones sent me some years ago), which actually means there has been even more energy added to the atmosphere than just the average global temperature measures indicate.

If humidity goes down with increase in temperature then it means 'global warming' may simply be a manifestation of transforming forms of energy. However, as both humidity AND temperature have increased, so it strengthens the argument that a net input of energy has occurred into the atmosphere.
 

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Discussion Starter · #5 ·
Interesting @donald - I did some more reading and as you say the current models don't seem to include the effects of clouds properly. The main effect seems to be when the water turns to ice in clouds when they reflect more energy back but also there seems to be an effect from the increased humidity trapping heat so overall corrected models seem to be predicting a significantly higher temperature increase overall. When I get some time I'll do some more reading - I still think we need a higher energy density solution for commercial vehicles than batteries and hydrogen seems to be the best option at present.
 

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Discussion Starter · #6 ·
Presumably condenser coils are out of the question? I had this vague recollection that the Clarity (maybe Mirai) had a water tank and stored it up ...
Not sure about condenser coils and a tank because the mass of the water will be 9x higher than the mass of hydrogen plus it's quite aggressively corrosive I understand until it's ionised which it would do by stripping atoms out of the tank material. I suspect a lot of the water will be exhausted in a liquid form anyway so probably not a major problem apart from the local humidity effect and maybe more ice formation in winter?.
 
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Most fuel cells systems (PEM based) will indeed condense the exhaust and the water produced is reused in the cells. The reason is that PEM relies on water as an ion carrier and coolant, and it has to be pure. No 'antifreeze' allowed here, so as there is always pure water in the system PEM systems also need heaters to ensure it doesn't freeze solid below 0 C. This is an energy waste that doesn't seem to be accounted for in real world overall FC efficiency (a bit like EV winter heating, but more so).

In fact, some systems struggle to generate enough water to reuse in the stack under certain conditions!

Pure water is not really corrosive, but I say that with a zillion caveats. Rusting chemistry is very interesting. It is quite correct that residual ionics will be the basic cause of corrosion problems, but done properly these can be removed - you just use a water wash in manufacture as a last stage and (if they are soluble ions) they will wash away. It's not quite that simple, as I am sure you can guess, but that's the thing to do.
 

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Discussion Starter · #8 ·
We've been dealing at work with a fuel cell manufacturer and for our product have to use fresh air rather than the exhaust because they've told us the exhaust is highly corrosive and will even eat away stainless steel given time. The explanation was that the water is pure and strips ions out of anything it comes in contact with which gives them serious durability issues. I've very little personal knowledge of fuel cells but they don't sound an ideal solution to me - if the water is contained in the system then they would qualify as ZEVs though.

I hadn't though about cold conditions - this is of course an issue for EVs as well.
 
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...they've told us the exhaust is highly corrosive and will even eat away stainless steel given time. The explanation was that the water is pure and strips ions out of anything it comes in contact with ....
I'm sure something is missing in that, perhaps through over-simplification or translation. Yes, of course water strips ions out of anything that is water soluble. That is what 'dissolving' means. If they use parts made out of salt then, yup, they are going to have problems! If there are no soluble ions to start with (if you wash it thoroughly) then there is nothing to solvate.

Perhaps they are using a special additive which is compatible with the PEM but is a bit reactive?
 

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The hydrogen-powered Toyota Mirai is not the eco-friendly car we've been looking for | VentureBeat | Enterprise | by Dwayne De Freitas

"The vehicle has a reservoir that holds the water but there’s no easy way for a Mirai owner to reclaim it. Instead Toyota has placed a button on the lefthand side of the car’s instrument cluster that releases the water the car has collected. This can be important in winter when the water vessel may freeze and cause problems. If the driver doesn’t eject the water, the Mirai will eventually purge itself. "

Knew I'd seen it somewhere. So not vapour, it dumps liquid water. That will be awesome on a winter's morning, a nice ice rink drive!
 

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Discussion Starter · #11 ·
Hi @donald - I honestly don't know. The explanation was that the water was highly corrosive because of being pure and would wreck our unit very quickly based on their experience. They are apparently facing major materials challenges to get workable solutions, even with their initial low volume high cost units which have more margin available. I've not been directly involved but will ask some questions.

The experience with this project was what triggered me into looking at FCEVs a bit more carefully.
 

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It's a tiny amount. More water would be dropped from a bit of snow melting on an ICE bonnet.
Numbers vary, but 240ml per 4km is in wiki (reference link doesn't work though) and it's in the gallons for all 300 miles according to this:

Fuel cell vehicles: The only cars with potable exhaust?

Can't find a reference on the size of the retention tank, but even a litre of water would make quite a puddle when dumped in one go.
 

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travel one mile at 3.6miles/kWh = 1mile/1MJ

H2 = 286kJ/mol, so at 50% conversion efficiency, need 8 mols H2/mile.

So exhaust will be ~8 mols water/mile = ~150 ml water/mile.

Make of it what you will ....
 

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We've been dealing at work with a fuel cell manufacturer and for our product have to use fresh air rather than the exhaust because they've told us the exhaust is highly corrosive and will even eat away stainless steel given time. The explanation was that the water is pure and strips ions out of anything it comes in contact with which gives them serious durability issues. I've very little personal knowledge of fuel cells but they don't sound an ideal solution to me - if the water is contained in the system then they would qualify as ZEVs though.

I hadn't though about cold conditions - this is of course an issue for EVs as well.
Why would you have a stainless steel exhaust on a FCEV? Surely it would just be plastic, it's only water vapour....
 

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Nice calculation!

Don't you have to feed some of that water back into the system though? Even 75ml per mile over 300 miles is quite a lot of water.
22.5 litres of water (about 5 gallons) spread all the way between London and Newcastle? Sounds pretty tiny.... ;)
 

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The guys on the Cleantech Talk podcast had an interesting take on the water vapour emissions from FCEVs a while back.

They weren't considering it being a greenhouse gas (vapour) per se, instead they were discussing the impact of humidity rising given that global temperatures are rising. They particularly mentioned the health impact of increased humidity in cities such as Tokyo and Los Angeles.
 
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