Guest,stimulator -

Be very cautious about comparing hydrogen storage to propane storage.

Although the slightly polar character of hydrogen makes it perhaps a little easier to store than helium, despite comparable molecular diameters, either gas passes quite easily through most common materials one might consider using for containement vessels. Even common glass is significantly permeable to both gases. Ordinary metals look more like window screen than like a window pane, since microcracks between grains are superhighways for these tiny molecules.

Although helium passes fairly easily through many common materials, hydrogen has an additional feature that makes it very difficult to store economically. Nearly all common vessel materials contain trace materials that can easily combine with hydrogen when sufficient (usually fairly nominal) pressure is applied.

A particular problem is with trace amounts of carbon found in all steels. At nominal pressure, hydrogen has a tendency to combine with the carbon to form methane molecules within the grain boundaries of the steel. The methane molecule must attempt to remain in the same space previously occupied by the carbon around which it forms. Since the methane molecule is enormously larger than either the carbon or the hydrogen, it "pushes apart" the grains, creating very high tensile internal stresses. When even small external tension is applied, the materials can rupture like dried up marshmallows. The term applied by stress analysts is "hydrogen embrittlement" and significant study of this phenomenon is suggested before attempting to predict what storage pressures might be useful, or what exotic materials might be needed.

"Practical" hydrogen storage for vehicle use has generally resorted to reacting the hydrogen with a combination of metals, to retain the hydrogen in "metal hydrides." The hydrogen can be released from the hydride by simple heating, which can be done more easily than "on the road electrolysis" and with much better flow/release rates.

Unfortunately, the temperatures required to "regenerate" the hydrogen from the metal hydride stores is generally fairly high, and safety concerns about explosive gases close to (and inside) glowing hot plumbing have not been resolved.

While hydrogen combines (burns) with oxygen to produce pure water, if air is used to supply the oxygen, there will be more nitrogen than oxygen present in the combustion chamber, and NO_x pollutants comparable to - and possibly/likely worse than - for common petrol fueled vehicles are likely. With one experimental vehicle for which I've had some first hand reports, since ca. 1966, NO_x emissions have consistently been higher than for even '60s era conventional autos.

While it sounds simple to just "burn the hydrogen" it actually is quite difficult to get controlled burning in an engine. Very minor deviations from "the right side of stochastic mixture" do result in detonation rather than burning. A problem with the experimental vehicle mentioned was that the detonations frequently (in 1967 when I witnessed a test and still in 1999 when I last talked to one of the engineers) were in - or transmitted to - the intake manifold, and aside from blowing the air cleaner through the hood, the engine quits until the full regenerator and intake manifold can be recharged and the hydrogen flow re-established to begin running again.

More advanced vehicle experiments have largely discarded the idea of burning hydrogen in an engine, and most research now is aimed at using hydrogen as fuel for a catalytic "battery" (fuel cell) to run electric traction motors on the vehicle.

Although metal hydride storage provides some volumetric efficiency improvement, it appears to be little used, since the "energy density" obtainable still doesn't come close to what's easily achieved with petro fuels. Most such experimentals now use methane or another petro fuel, with a "cracking reactor" to extract hydrogen onboard, and the fuel cell to convert to electric power. Once the cracking reactor, fuel cells, heat exchangers, pumps, thermal conditioners, regulators, controls, safety devices, and ballast batteries are included, most are not as efficient as internal combustion engines, although they may be somewhat (but not perfectly) cleaner. Most are currently NOT cost-competitive with just using rechargeable batteries.

If an efficient method of generating hydrogen from water can be found, gaseous hydrogen remains incredibly difficult to store on mobile vehicles. The heat of combustion from "burning" hydrogen with pure oxygen does not compete favorably with burning carbon with oxygen, so enormous storage (volumetric) capacity would be required to equal the range/power of a typical small petrol/diesel automobile.

Nearly all "reaction products" of hydrogen with trace elements/compounds expected in a typical operating environment are at least somewhat corrosive (usually acidic) and very strict - and expensive - attention to safe operating life limits and reliable useful life prediciton methods are required.

In other words, a few minor challenges do remain.

John