Think of a system with air over water in a free surface. A swimming-pool hall, to give a practical example. Once it reaches steady-state, there is a certain partial pressure of water vapour in the air. This is independent of the amount of water in the pool (or the area of the free surface), depending only on the temperature. Spray more water in, allow to reach steady state again at the same temperature, and the humidity will be the same, and the pool level higher.
Local effects, possibly. If you drive past a big nuclear power plant, there’s often a lot of thick mist near the cooling towers (not radioactive fog, just clean water). But mixing ensures the effect is very local. With a whole city’s worth of cars and heating, maybe it contributes to a chimney-type convection, since humid air is less dense and tends to rise. But probably the effect due to humidity is less than that due to heat.
Thanks for drawing my attention to this previous thread, which has some interesting angles.
I think you’ve misread Gaspode’s contribution. He makes the point that if you stop watering the lawn and allow the grass to die (change of land use), there will be a net carbon flux out of the soil. He does not say that a lawn, in steady state, is continuing to increase the organic content of the soil.
So I don’t think, on the basis of the post you referenced, that he does in fact disagree with me.
And let me clarify the point (and perpetuate a hijack). I don’t disagree with Hibernicus, but I’m not sure I agree either.
One thing I do agree with is that a lawn most definitely is not a carbon sink, and is almost certainly a source of methane. What is debatable however is whether the effect of the carbon release that will inevitably result from paving a lawn area will be greater or less than the effect of the continued methane production from the lawn.
Lawn clippings will often produce methane (though this isn’t unavoidable), but a lawn will also help maintain soil carbon levels. Concreting your lawn will inevitably cause a decrease in soil carbon and consequent increase in atmospheric CO2. However the nature of concrete may be such that it causes much of the CO2 to be trapped as CaCO3, which would be a good thing.
I honestly couldn’t say if the methane produced by anaerobic decay would counter the greenhouse gas release caused by loss from the soil carbon pool, but my gut instinct says it wouldn’t. The soil carbon pool averages about 4x the atmospheric pool and even under agriculture losses of 60% of SOM have been reported. This means more than doubling the atmospheric greenhouse gas levels by converting lawn to paving. Even with methane being about 30 times more effective as a greenhouse gas than CO2 its relatively short atmospheric life suggests to me that any reduction in methane production would never compensate for the long-term increase in atmospheric CO2 levels that paving a lawn would inevitably cause. In short paving a lawn would lead to increased atmospheric CO2 levels.
But that’s all just an educated guess and I could well be wrong.
If all the extra water runs into the sea, then that gives 0.005% extra water here, which can absorb 0.005% more carbon dioxide from the atmosphere. So, “to do a Greenpeace” (i.e. to inflate a tiny number into a massive effect): “FUEL CELLS WILL HALT GLOBAL WARMING!”
(Note: the seas will also absorb 0.005% extra oxygen, so an alternative headline might be “Fuel Cells Will Suffocate Us All!” :))
This might be true if all your reactions were stoiciometric, but it’s not generally true. The main problem with IC engines is that they burn too rich (or unevenly) and produce unburned hydrocarbon exhaust and “bad” NOx. Temperature is also a huge factor in the generation of some pollutants. I believe these factors are easier to regulate in a fuel cell.
The amount of unburned and partially-burned HCs in vehicle exhausts (say, 100 ppm) is not sufficient to make an important difference to the amount of water as a product of combustion. (NO[sub]x[/sub] production makes no difference at all). Also, to ensure complete combustion in a finite time, you need a lean mix, not a stoichiometric mix. Your other points are correct though.
I did not intend for my comments to respond directly to the OP about water production. Instead, I had misread Trucido’s comments to say that fuel cells would produce the exact same amounts of all pollutants as IC engines, and I was responding to that point. On re-reading, I see that he did not say that. Apologies for the unintentional hijack.
One real advantage of fuel cells is their extremely low level of controlled pollutants. HC’s are low due to the combustion efficiency of the process, and NOx is pretty much non-existant because of the extremely low temperatures involved (NOx has an equilibrium reaction rate curve that is very strongly dependent on temperatures above 1600 F, which is much too high for a fuel cell).
As to the OP…does anyone have any idea how much water would actually be produced, if automotive power was to be completely provided by hydrogen combustion? Let me just do a back-of-the envelope calculation here… Taking iso-octane as C8H18 (for the gasoline replacement case only), there are already quite a few hydrogens which are currently being burned.
OK, carbon has an LHV of 33.8 MJ/kg, and hydrogen as H2 has an LHV of 120 MJ/kg. With molecular weights of 12 and 2 kg/kgmole respectively, this gives us molar LHVs of 2.77 MJ/kgmole and 60 MJ/kg*mole.
Now, let’s take 1 mole of C8H18. This has 8 moles of carbon atoms in it and 9 moles of H2 molecules in it, so the heating value of the carbon is:
8 mole2.77 MJ/kgmole = 22.16 MJ/kg. If we take the molar heating value of the hydrogen at 60 MJ/kg*mole, then it seems our answer is:
60 MJ/kg* mole / 22.16 MJ/kg = 2.71 moles H2 extra required to replace the heat lost from the carbon missing.
So what does our imaginary molecule look like with the same LHV as isooctane? What we get is:
LHV[sub]C8H18[/sub] is close to LHV[sub]H23.41[/sub]
Since it takes 2 H’s per molecule of water, in the iso-octane case, 1 mole of C8H18 yields 9 moles of H2O in its combustion. In the case of the imaginary molecule, we get 11.7 moles of H2O resulting from its combustion. So, we would end up with 30% more water overall from automotive transport. This, of course, ignores a great many things.
Ignoring for the moment any difference in conversion efficiency between fuel cells and IC engines:
Gasoline (assume C[sub]8[/sub]H[sub]18[/sub], with LCV of 43.5 MJ/kg)
1 g-mol = (8*12)+18 = 114 g
=> LCV = 4.959 MJ/g-mol
combustion produces 9 g-mol of water per 1 g-mol of iso-octane
=> 1.814 g-mol of H[sub]2[/sub]O per MJ
Hydrogen (assume H[sub]2[/sub], with LCV of 120.0 MJ/kg)
1 g-mol = 2g
=> LCV = 0.24 MJ/g-mol
combustion produces 1 g-mol of water per 1 g-mol of hydrogen
=> 4.167 g-mol of H[sub]2[/sub]O per MJ
So the fuel cell would produce 2.23 times the amount of water for the same amount of fuel heating value.
Anthracite, I think what’s happening here is you’re implicitly overstating the heating value of iso-octane by assuming it’s the sum of the HVs of the C and H. Actually it takes energy to break the existing bonds to allow combustion, so the HV is less. Also you mixed up molecular hydrogen and atomic hydrogen in your analysis.
The same page at www.fuelcells.org referenced earlier, also has an analysis of IC vs fuel cells for water production in terms of a per mile rate. It depends on the efficiencies, of course, but came out with fuel cells making slightly less water per mile travelled.
I’m still confused, though.
-Some of the water condenses out of the car exhaust. So? It is still likely to be vapor soon. I pour a bucket of water on the hot pavement … it ain’t gonna be vapour in an hour?
-hibernicus, The equilibrium point: We have essentially been spraying a fine mist of water out of the backs of our cars for years, and continue to do so. Is the “steady state in the pool room” a good analogy, or should the analogy be that you now introduce a bunch of kids with spray misters into the room? Still, the analogy obviates the culminative effects …
Odd to consider water as a safety hazard? I’ll mention that to the next Life Guard I see! And we can then forget about worrying about CO2 emissions … heck we breathe that stuff out with regularity every day!
