From bad to worse
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:
| Oxide | ΔHf kcal/mol |
| H2O | -70.60 |
| MgO | -144.09 |
| Al2O3 | -404.08 |
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, kcal/mol H2 |
Energy output, kcal/mol H2 |
Efficiency, percent |
| Electrolysis | 100.9 | 70.6 | 70 |
| Al oxidation | 134.69 | 70.6 | 52 |
| Mg oxidation | 144.09 | 70.6 | 49 |
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:
FORGET HYDROGEN!
¶ 10/27/2005 09:56:00 PM 
One question - I can't find anywhere in the announcement or on their site that they even mention fuel cells. Is it possible that there's no cell at all? They talk about "...the improved usage of heat (steam) inside the system..." which seems to imply that the steam is used, which would put a steam engine in the car, right? "...the Hydrogen molecules are free, and will be sent into the engine alongside the steam." And they stress how similar the engine will be to existing engines - "The gas tank in conventional vehicles will be replaced by a device called a Metal-Steam combustor..." Just the gas tank?
Surely it's not possible that they're actually talking about a steam engine? In which case this is all a con? I hope I've got the wrong end of the stick, here.
Keep up the good work!
Of course, in the land of the blind, the one-eyed man is king. Though he's often frustrated as he tries to convince others that what is obvious to him is not just his imagination.... That's one of the reasons I lay out all of my work in this blog: I want more people to learn how to see.
You're right, the article doesn't specify what actually uses the hydrogen, and leaves the option open between fuel cells and conventional internal combustion engines (both are mentioned). If the latter, the throughput efficiency would fall to something on the order of 10%. And if you look at the flow diagram they do not appear to have a steam/gas expander on the output of the "metal combustor".
I'd say it was a pity to miss an opportunity to grab more energy, but with such lousy figures I don't think it is salvageable.
For a minor shift of topic to metal-air, have you seen this website for a Chinese zinc-air fuel cell manufacturer?
http://www.powerzinc.com
Metal-water reactions are one step further back, a way to get around the drawbacks of hydrogen. They achieve storability, but at the cost of large inefficiencies. This may actually be acceptable if it becomes cheaper to produce more energy than to store electricity efficiently.
My bet is that efficiency is easier to achieve in storable systems (like some of the variants possible with the zinc cycle) than it is to achieve greater production with a magnesium or aluminum cycle. But I could be wrong. I cheerfully challenge Enginuity (or anyone else) to replace petroleum using hydrogen from metal-water reactions, regenerated from non-fossil energy, cheaper than RE delivered through wires and batteries. Let the best invention win.
Mg: http://freehydrogen.blogspot.com/2005/10/engineuity-aluminum-or-magnesium-wire.html
Al: http://freehydrogen.blogspot.com/2006/01/engineuity-recycled.html
There are a *lot* of "drive your car on water", some even publicly traded.
<< Home
