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Name: Caroline
Status: educator
Grade: 6-8
Location: PA

A student asked a question about the emissions from a hydrogen fuel cell. He asked "If water is what is emitted during the winter wouldn't it (the water) freeze on the roads making them icy?" I thought this was a great question and I am completely stumped on finding he answer for him (he and I have both searched and have found nothing that references any problem of this sort).

Hi Caroline,

Water is indeed the "exhaust" of a typical fuel cell. But water in the exhaust is nothing new. Half of the exhaust of any normal car motor is also water (the remainder being largely CO2), and there has never been a problem with this water freezing on roads!

The reason that a fuel cell's water "exhaust" is not going to cause ice to form on roads, is that just as in a normal car exhaust, the water in a fuel cell's exhaust is mainly in the form of water vapor, where it dissipates harmlessly in the atmosphere. This is something you can already observe when starting a car on a cold morning: the "fog" of water droplets that condense when the water vapor in the exhaust hits the cold air, soon evaporate again to water vapor and dissipates, leaving very little liquid water on the ground. A fuel cell is nowhere close to 100% efficient and generates a lot of waste heat, and therefore operates at an elevated temperature. This heat results in its exhaust being mainly water vapor, not liquid water.

Bob Wilson

Fuel cells operate at very high temperatures, and thus emit water vapor, not liquid water. Eventually, when the water vapor cools, it could end up on the road, but the mass of water vapor is not large enough to 'flood' the road or create an ice slick.

Hope this helps,
Burr Zimmerman

A good point, but there are a lot of qualifications: 1) The temperature of the roadway would have to be less than 0 C. for ice to form. In many regions the ground temperature exceeds the air temperature by several degrees. 2) The freezing of water vapor from a hydrogen fuel cell may not be that much different that the freezing of water vapor from a conventional engine exhaust. A fuel cell would not be exhausting massive amounts of water vapor greater than a convention internal combustion engine -- maybe even less.

The value of your student's question is not so much what the answer might be, but rather that the question was asked!!

Vince Calder


As I understand the fuel cell chemical reactions, the water produced is in the form of a gas. In temperatures below the freezing point of water, the water will condense as tiny droplets in the air, not as ice on the road.

Warren Young

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