Hydrogen Powered Vehicles

[cassiopia42]

I am not a scientist, but I consider myself fairly smart and I'm university educated. Ever since the possibility of hydrogen powered cars was mentioned I have found myself very nervous about it. To start with, hydrogen is an exceptionally flammable gas (as evidenced by the Hindenburg disaster). Transporting such a fuel, using it in many vehicles, creating fuelling stations stands to be potentially very dangerous. In addition (and this scares me much more) people seem very exited because the technology is considered to be 'zero-emissions' and produces only water vapour. Do they not realize that water vapour is a greenhouse gasses? Would the artificial creation of so much of it upset the environment or change the delicate balance of water? Even if it came from water that already existed we can't say when it would have naturally turned into a gas. Imagine billions of drivers just blithely releasing puffs of water vapour into the environment. Am I wrong to be so worried?

 

I would appreciate any clarification you can give me.

 

Thank you,

 

Cassiopia42

[Moontanman]

I am not a scientist, but I consider myself fairly smart and I'm university educated. Ever since the possibility of hydrogen powered cars was mentioned I have found myself very nervous about it. To start with, hydrogen is an exceptionally flammable gas (as evidenced by the Hindenburg disaster). Transporting such a fuel, using it in many vehicles, creating fuelling stations stands to be potentially very dangerous. In addition (and this scares me much more) people seem very exited because the technology is considered to be 'zero-emissions' and produces only water vapour. Do they not realize that water vapour is a greenhouse gasses? Would the artificial creation of so much of it upset the environment or change the delicate balance of water? Even if it came from water that already existed we can't say when it would have naturally turned into a gas. Imagine billions of drivers just blithely releasing puffs of water vapour into the environment. Am I wrong to be so worried?

 

I would appreciate any clarification you can give me.

 

Thank you,

 

Cassiopia42

 

 

You have nothing to worry about, hydrogen is no more dangerous than gasoline, a pretty good argument can be made that gasoline vapor is more dangerous than hydrogen gas. Hydrogen rises from the ground into the air gasoline pools on the ground because it is heavier than air and gasoline vapor is just as explosive as hydrogen gas when both are mixed with oxygen. Also gasoline is a known carcinogen, spilling even a small amount pollutes an enormous amount of soil or water, hydrogen is benign and can be breathed with no harm as long as it is mixed with oxygen and kept from a spark :unsure:

 

As for the water that is created when you burn hydrogen the air can only hold so much water before it rains out. CO2 stays in the air but gasoline when burned in an internal combustion engine creates lots of bad stuff other than CO2 as well.

 

Hydrogen is a superior fuel and is no more dangerous than gasoline.

[CraigD]

Welcome to hypography, Cassiopia! :)

Am I wrong to be so worried?

In short, yes.

 

While there are many practical difficulties with using hydrogen as an energy-storing fuel - so many that I believe, personally, that it will never be a practical replacement for fuels such as gasoline in high-power applications like automobiles - its dangers are comparable to gasoline, and it environmental impact, regardless of how it is used, much smaller.

 

To start with, hydrogen is an exceptionally flammable gas (as evidenced by the Hindenburg disaster).

Despite its popular reputation, hydrogen is not exceptionally flammable compared to gases such as propane, methane, and gasoline vapor. Like these gasses, the amount of it that can burn is limited to the amount of oxygen in contact with it, and even when oxygen is present, at least a small volume of high temperature (about 500 C), such as a spark, must be present to ignite the gas mixture. Burning it releases substantially less heat than many other common combustible gases - about 1/3 the amount of methane, the greatest part of the piped gas used in many houses.

 

It's worth noting that, while spectacular, the Hindenburg disaster was not very fatal compared to modern aircraft crashes, killing only 45 of its 98 passengers and crew, and 1 bystander. Many passengers and crew walked away from the accident uninjured. In short, the disaster was more like a structure fire than an explosion. (source: wikipedia article Hindenburg disaster)

 

Hydrogen does pose some special risk not posed by gasoline, a major one being that its flame is nearly invisible (the only reason the Hindenburg's flames were so visible is because of the vessels fabric skin and other material burning along with the hydrogen). Alcohol, used as a fuel in uncommon applications such as portable stoves, heaters, and race cars, poses a similar risk, however, and has not caused notable catastrophes. It's safer than many common fuels because of it's buoyancy, which results in leaks escaping into the air rather than accumulating near people and structures.

 

Transporting such a fuel, using it in many vehicles, creating fuelling stations stands to be potentially very dangerous.

A major practical problem with hydrogen is that it's difficult to compress it enough for it to be an effective fuel, and that it's difficult to prevent from leaking, making it difficult to store. The few hydrogen autos in existence have, therefore, been fueled mostly by small compressed hydrogen tanks containing hydrogen generated from electricity at the fuel station. These vehicles and fuel stations contain less fuel energy than gasoline vehicles and fuel stations, so even in the event of a worst-case scenario (eg: an intentional bombing), the destructive effect would likely be less than with gasoline. Another safety advantage is that, in a fire, one needs only release the stored gas and keep nearby people away and structures wet as it rises into the air and burns, whereas gasoline spreads and can be hurled as a liquid, so is especially dangerous in a fire.

 

Do they not realize that water vapour is a greenhouse gasses? Would the artificial creation of so much of it upset the environment or change the delicate balance of water?

To the second question, I'm pretty confident, no.

 

It's true that water vapor is a powerful greenhouse gas, but it's also rapidly condensed and precipitated as liquid water - all that's necessary is for its temperature to fall below about 100 C. The Earth's naturally cycles (Evaporates and precipitates) about 500,000 km³ of water each year. If all gasoline were replaced with hydrogen fuel (ignoring the water vapor produced when burning gasoline), an additional 6 km³/year (about 0.001%) would be added to this cycle, an amount much less than yearly variation and changes due to agriculture and forestry). (sources: wikipedia articles energy density, water cycle, and my own humble calculating ;))

 

It might be possible - I'm too lazy undertake an approximate calculation - that many hydrogen powered vehicles in one place (eg: cars on urban roads) would cause an increase in local fog and clouds. My guess is that this would be fairly harmless or even beneficial. If not, it would be fairly easy to prevent.

 

A hydrogen fuel cell's exhaust can be cooled to condense the water into liquid, then either drained away or captured for use. In places where water vapor is troublesome, such as inside a building, or where liquid water is valuable, such as in a spacecraft, this is commonly done (Most manned spacecraft use hydrogen fuel cells to both generate electricity, and provide drinking water). In a vehicle, where liquid water would be more of an annoyance than a benefit, it makes more sense, I think, to allow the water to escape as vapor. However, if you really wanted to, a hydrogen fuel cell powered car could, as a not-too-expensive option, provide its passengers with a supply of pure drinking water.

 

All this is, however, I think, idle speculation, because unless a dramatic technical breakthrough in hydrogen fuel cell technology occurs, they'll remain many times too expensive for use in autos and other currently gas-powered applications. Recent estimates place the cost of a fuel cell-powered car at about US$300,000, about 10 times what the market will bear. (source: this wikipedia article section)

[modest]

Despite its popular reputation, hydrogen is not exceptionally flammable compared to gases such as propane, methane, and gasoline vapor.

 

I think the upper and lower flammability concentration limits would be of concern though. Gasoline ignites in air at concentrations between 1.4 and 7.6 percent, methane between 5% and 15%, but hydrogen's flammability Range is 4% - 75%.

 

http://www.engineeringtoolbox.com/explosive-concentration-limits-d_423.html

 

It burns hotter than methane and has a about the same ignition temperature.

 

Nonetheless, vehicles powered by compressed natural gas are not uncommon and have a good safety record. I would suspect that hydrogen tanks would be roughly as safe so I agree with Craig.

 

~modest

[CraigD]

I think the upper and lower flammability concentration limits would be of concern though. Gasoline ignites in air at concentrations between 1.4 and 7.6 percent, methane between 5% and 15%, but hydrogen's flammability Range is 4% - 75%.

 

http://www.engineeringtoolbox.com/explosive-concentration-limits-d_423.html

 

It burns hotter than methane and has a about the same ignition temperature.

Good points. :thumbs_up

 

Nonetheless, vehicles powered by compressed natural gas are not uncommon and have a good safety record. I would suspect that hydrogen tanks would be roughly as safe so I agree with Craig.

Again speaking to the practical problems with hydrogen as a fuel, while I agree hydrogen and methane gas compressed to the same pressure and volume are about equally safe, the hydrogen provides much less energy, and thus, for a vehicle, less range (ignoring mechanical efficiency, about 25%). Compressed (250 bar) methane is in turn has only about 26% the energy as an equal volume of gasoline.

 

In short, with fuels, energy density drives engineering. Hydrogen, with its 146 MJ/kg, is an ideal fuel where mass is critical, volume not, vs. 46.2 for kerosene (diesel) making it the rocket fuel of choice, while kerosene at 37.3 MJ/L vs. liquid hydrogen at 10.1, is a ground vehicle fuel of choice.

 

Making sure the machine doesn't malfunction and kill you is a mere engineering detail. ;)