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View Diary: Recent DOE Break-Through with Hydrogen Fuel Cells, should make them Affordable (275 comments)

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  •  It is more dangerous. Much. (4+ / 0-)
    Recommended by:
    Sneelock, JeffW, jamess, JohnnySacks

    Let's listen to NASA, one of the biggest users of hydrogen in this country.  I wrote this up a while ago, but to repost:

    Ignition:

        * "Hydrogen-air mixtures can ignite with very low energy input, 1/10th that required igniting a gasoline-air mixture. For reference, an invisible spark or a static spark from a person can cause ignition."
        * " Although the autoignition temperature of hydrogen is higher than those for most hydrocarbons, hydrogen's lower ignition energy makes the ignition of hydrogen-air mixtures more likely. The minimum energy for spark ignition at atmospheric pressure is about 0.02 millijoules"

    Mixtures:

        * "The flammability limits based on the volume percent of hydrogen in air (at 14.7 psia) are 4.0 and 75.0. The flammability limits based on the volume percent of hydrogen in oxygen (at 14.7 psia) are 4.0 and 94.0."
        * "Condensed and solidified atmospheric air, or trace air accumulated in manufacturing, contaminates liquid hydrogen, thereby forming an unstable mixture. This mixture may detonate with effects similar to those produced by trinitrotoluene (TNT) and other highly explosive materials"
        * "Explosive limits of hydrogen in air are 18.3 to 59 percent by volume"
        * "Flames in and around a collection of pipes or structures can create turbulence that causes a deflagration to evolve into a detonation, even in the absence of gross confinement."
        * Deflagration limit of gasoline in air: 1.4-7.6%

    Leaks:

        * "Leakage, diffusion, and buoyancy: These hazards result from the difficulty in containing hydrogen. Hydrogen diffuses extensively, and when a liquid spill or large gas release occurs, a combustible mixture can form over a considerable distance from the spill location."
        * "Hydrogen, in both the liquid and gaseous states, is particularly subject to leakage because of its low viscosity and low molecular weight (leakage is inversely proportional to viscosity). Because of its low viscosity alone, the leakage rate of liquid hydrogen is roughly 100 times that of JP-4 fuel, 50 times that of water, and 10 times that of liquid nitrogen."

    Hydrogen also has a tendency to collect under roofs and overhangs from even minor leaks. Combined with its tendencies to leak and even to enter other pipes and then follow them to where they let out, plus its ability to ignite from very weak ignition sources, building codes for anywhere that hydrogen will be stored tend to be very stringent (see 6.5.4: Buildings). This would apply to garages (both home garages and public garages), parking shelters, eaves, overhangs, etc -- anywhere that more than 1kg of hydrogen is stored (even a small H2 car like the Clarity uses 4.1kg).

    The only comparison in which gasoline could possibly win in terms of safety is that gasoline can pool given a (much more difficult to achieve) leak, while hydrogen, if somehow not ignited by whatever caused the leak, and if not pooling in the engine or even the cab of the vehicle or in any outside structures, will escape. If there were then a delayed ignition source, hydrogen could win in this (rather improbable) safety comparison.

    One final safety note: proponents of hydrogen sometimes still cite the bizarre theory proposed by Addison Bain that hydrogen wasn't the cause of the Hindenburg disaster. This theory holds no water.

    While gasoline leaks are indeed somewhat poisonous and can contaminate water if allowed to leak into it, any hydrogen leak is essentially guaranteed to do significant ozone damage. As covered above, hydrogen leaks two orders of magnitude more readily than gasoline.

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