Hydrogen storage

It's open day at uni melb: I had a talk with a rogue 15-year
old about why hydrogen doesn't work yet.

Remember that, to produce the hydrogen needed by America's cars via electrolysis (splitting water with electricity) requires 8,000 TWh---twice the current total electrical throughput.

Well, the other problem is storage. Hydrogen carries a lot of energy per unit volume, since it is so light. But I think you can take it as impossible that the half-million dollar systems being used to store liquid hydrogen in today's experimental cars will ever go to market. Here's the volumetric and weight densities of hydrogen as a gas or liquid, and absorbed into a metal hydride. When they say 'minimum performance goal', they mean it: without a good storage system, a hydrogen fuel cell is essentially an inefficient battery. Pack your car with kilos of metal and it won't get far; it won't sell.


The real alternatives are the solid metal hydrides, metals which hydrize at certain temperatures and pressures. Magnesium Hydride, Mg H_2, stores the largest density of hydrogen but requires >300 C to let go of it

Again of the reversible hydrides simple magnesium does best. Magnesium is the world's third most abundant metal. Iron titanium comes next for price. Pretty much everything else is an exotic designer alloy as of now: tens of thousands of dollars per kilo.


Here you see the temperature at which the metal hydrides release the hydrogen at standard pressure. There's about a 30% penalty to heat the magnesium (30% of the fuel cell keeps the metal hot) but the same applies to liquid hydrogen. Plus you're driving the Hindenberg!

One possibility is alloying Lanthanum (a room temperature hydrilizer, but energetically poor) with the hot metal magnesium. But still---you look at the above graph and its basically a line, either you get density or temperature. It would nevertheless be possible to build a heat battery from Magnesium, and it sometimes wonder if it might beat big flywheels as energy storage.

We're building just massive flywheels at the moment to convet electricity into kinetic energy and back. They're run on superconducting bearings. But another time...