L. David Roper (roperld@vt.edu)
One often hears that powering vehicles with hydrogen fuel cells will eliminate pollution; that their only emission is water. In this situation water may be a very dangerous pollutant.
When hydrogen combines with oxygen to produce water in a fuel cell the hydrogen is taken from a tank carried in the vehicle and the oxygen is taken from the air (as it is in gasoline-combustion vehicles). The reaction is
2 H2 + O2 -> 2 H2O .
So, for every 2 hydrogen atoms taken from the fuel tank, 1 oxygen atom is taken from the air. An oxygen atom is about 16 times more massive than a hydrogen atom, so the water molecule is 9 times [(16 + 2) / 2] more massive than the 2 hydrogen atoms used to make it.
Where is that water going to go when it is produce by the engine of a vehicle? It will come out as liquid or steam, depending on the "exhaust" temperture.
Depending on the weather conditions produced steam may rise and form clouds around the road, or it may form fog at the road surface, or it may condense and drop on the road. Even in warm weather water on the road will be dangerous. In freezing weather it will make the road downright treacherous, especially if most or all vehicles on the road are powered by hydrogen fuel cells.
The steam could be condensed and stored as very hot water in a tank in the vehicle. Then the vehicle will eventually have to carry 9 times as much mass as the hydrogen fuel tank carried when full, and the tank will be dangerously hot. Carrying the water around in the vehicle will greatly reduce the energy efficiency of the vehicle. The water tank would have to be emptied when the hydrogen tank is filled, or more often if it is not large enough to hold all the water produced by the total amount of hydrogen in the fuel tank. Pure water is good to have, but it sounds like something with which most vehicle drivers will not want to bother.
Suppose the fuel cell uses methane instead of hydrogen. Then the reaction is CH4 + 2 O2 -> 2 H2O + CO2 . In this case, since a carbon atom has 14 times the mass of a hydrogen atom, the water tank will eventually carry 2 times [{2 x (16 + 2)} / (14 + 4)] as much mass as the fuel tank did when full. How about the carbon dioxide that is released into the atmosphere? Wasn't the use of fuel cells supposed to eliminate that greenhouse gas being released into the atmosphere? Of course, the fuel tank itself will have to carry 4.5 times [(14 + 4) / 4] as much mass as a pure hydrogen fuel tank would.
Some will argue that the gasoline internal combustion engine also produces water. That is true, but see GasolineVsHyFuelCell.pdf, which shows that there are big differences in the amount and temperature of the water between gasoline combustion and hydrogen-fuel-cell energy production.
The least polluting scenario using fuel cells for transportation would be to have fixed hydrogen fuel cells producing electricity, with the pure water captured for use instead of dumped on the road or into the atmosphere or carried around in tanks on the vehicles, and then use hybrid or electric vehicles.
Don't forget that water vapor is a more powerful greenhouse gas than carbon dioxide. Also, don't forget the pollution that occurs in the process of producing the hydrogen to be used as fuel; that needs to be studied very carefully.
Two modes of transportation that might make good use of fuel cells are railroads and ships. In the railroad case, the pure water might be cooled and then used for drinking, cooking and cleaning or poured out on the sides of the tracks to moisturize the surrounding land. In the water transportation case it can just be cooled and then dumped into the water already there, helping to clean up the pollution.
http://www.planetark.com/dailynewsstory.cfm/newsid/28876/story.htm: Iceland's Hydrogen Buses Zip to Oil-Free Economy::
http://www.grist.org/comments/soapbox/2005/07/19/mckibben-hydrogenbus/: Heaven Help Bus: A visit to Iceland spurs dreams of a hydrogen future:
. However, it depend on the temperature at which the H2O is emitted. If it drips out as liquid water it falls on the pavement, which would turn to ice in low atmospheric temperatures.
As I understand the fuel-cell cars, they have a battery, similar to our 2016 Toyota RAV4 Hybrid and the Prius, that is charged by the fuel cell and allows braking/slowing regeneration. So the fuel cell is not always working during a drive. The question is what is the temperature that the H2O is emitted when the fuel cell is starting and stopping. Maybe some battery energy is used to heat the H2O so that it is always emitted as vapor instead of dripping on the pavement to freeze into ice.