I've been a critic of hydrogen hype for some time, largely because most non-fossil energy is captured as electricity and it is very inefficient to convert energy from electricity to hydrogen and back. But a newly-publicized scheme promises to make electrolytic hydrogen look good by comparison.
A company called Engineuity (plugged by Isracast and picked up by dozens of bloggers from there) is promoting a roundabout way of making hydrogen on-board vehicles, using the chemical reaction of either of the light metals magnesium or aluminum with water. This is at least somewhat clever, as in a fuel-cell vehicle the reaction of hydrogen with oxygen re-creates water and at least some of the material can be recycled on board (if it is not captured in the reaction products as hydroxides). But the efficiency is even worse than pumping hydrogen into a tank.
One of the features of both aluminum and magnesium is that they burn quite spectacularly; aluminum is a potent component in many solid rocket fuels, and magnesium gives a brilliant light when ignited. Both will burn in water, releasing enough chemical energy in their combination with oxygen that they can tear it loose from hydrogen with plenty left over. This excess energy, aside from creating a safety issue, is exactly the problem for efficiency.
Reviewing some heats of formation of oxides:
We can use this to derive the heat released from the reaction of water with either of the two metals:
|Mg + H2O||->||MgO + H2 + 73.49 kcal/mol||(1)|
|2/3 Al + H2O||->||1/3 Al2O3 + H2 + 64.093 kcal/mol||(2)|
In all cases the hydrogen yields the same 70.6 kcal/mol when reacted with oxygen. All this excess energy produced in the reaction with water comes out in the form of heat rather than chemical energy suitable for a fuel cell. As it makes little sense to add a small, inefficient steam engine to a car with a fuel cell, it appears likely that this heat energy will be discarded, or used for nothing better than cabin heat.
Discarding energy means efficiency is lost. The production of hydrogen by electrolysis of water is roughly 70% efficient. Here's a table for comparison:
|Method|| Energy input,
| Energy output,
A system using a fuel cell of 60% efficiency can get 42% throughput using hydrogen from electrolysis, 31% using hydrogen from aluminum, and a mere 29% using hydrogen from magnesium. Note that these efficiencies do not include the losses involved in the production of the metals from oxide; these will be non-trivial and make the net efficiency even lower.
As we can see, production of hydrogen by combustion of aluminum or magnesium with water is a very inefficient process. Typical batteries have efficiencies ranging from 70% for lead-acid to 90% or so for nickel metal hydride and lithium-ion. As the raw energy supply (electric) is going to be a limiting factor for some time, the message is clear:
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