The Ergosphere
Thursday, October 27, 2005

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
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
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:


Thanks, I posted somewhat sceptically on this and was hoping someone who actually knew what they were talking about would have a look at the announcement.

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!
Me?  Know what I'm talking about?  You take me for far more than I am, sir.  I'm just a guy who has a CRC Handbook of Chemistry and Physics and can balance a reaction.  This is first-semester chemistry in any university science or engineering program.  Go to the library, grab a book on it, you'll soon be as good as I am.

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.
How do real scientists and engineers think they can get away with this crap anymore? Companies need to do their exergy computations and figure out the 2nd law efficiency of their systems. Enough with this, "Entropy, what's that?" attitude of ignoramus.

For a minor shift of topic to metal-air, have you seen this website for a Chinese zinc-air fuel cell manufacturer?
I'd come across their name before, but as they've decided to make their homepage inaccessible without Shockwave I have not looked beyond.  (I really dislike craptacular excesses like Shockwave animations on what should be informational sites.)
EP - I know we have had a few arguments however you will get none from me on the issue of hydrogen.

Sheer common sense SHOULD tell most people it is far more efficient just to put electricity into batteries to run an electric motor to power a car.

Yet people persist in wanting to use electricity to split water then to compress or liquefy to store H2 then pipe it to service stations, in new pipes, then store it again then put it into a car's storage then to put in into a fuel cell then to produce electricity to run an electric motor.

How can all these steps that involve losses and inefficiencies be better than battery electric cars or pluggable hybrid electric cars?
Hydrogen does have two arguments going for it.  One is filling time:  it pumps into a tank like gasoline, and you don't have to wait hours for a battery to charge.  The other, more realistic one is that it is storable in ways that electricity is not.

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.
EP - An interesting link to read
on tanks. A high pressure 5000 to 10000 psi tank will take some time to fill. It will not be as quick as a petrol tank however will be quicker than current batteries. Also if you have time to read the link a 5000 psi tank will only give enough hydrogen for 200Km. This is well within quite an economical lithium battery pack's capabilities.

A longer range 10000psi tank will be harder to fill and presumably take longer. The current focus is on giving the battery a quick 5 or 10 min rapid charge to give say 20 or 30 km so that you can get home if you end up stuck with a flat battery.

These links provide some practical experience and a new, experimental type of lithium battery that can be almost instantly recharged


"As part of Altair's development of the electrode materials it has produced several batches of the lithium titanate spinel electrode material. This material was used to construct several prototype batteries by Rutgers University Energy Research Group. They then conducted testing of these prototype batteries. The results of these tests have indicated that batteries constructed using Altair's nano-lithium titanate electrode materials could have the following characteristics:

-- Very fast charge rates - currently measured at six minutes to a full charge. In the power tools market this is a major breakthrough compared to the current two to four hours. And in the electric vehicle (EV) market this would allow a recharge in the same amount of time as it currently takes to fill a car with gasoline. Rapid charge rates would provide a significant performance improvement for current EV designs, which presently take several hours to charge and make them only practical for short haul trips."
I read somewhere that, energywise, petroleum is already a negative energy product. That is to say that it takes more energy to produce pertoleum energy products then those petroleum products produce. Why can this still continue? Because 'efficiency' is not the problem, mobility is. Power where and when we need it is the key. Hydrogen has the possibility of being renewable and as mobile as petroleum products. I expect there is a real future there.
Robert McLeod cites a source claiming over 80% well-to-tank efficiency for both gasoline and diesel (though diesel is better).  (He says the source is here, on page 93; I haven't looked.)
Their claim that burning Al or Mg in water makes H2 and that burning the H2 is cost-competitive with gasoline is complete junk. I have a couple of articles on this over at the where I figure what the cost would be per gallon of gasoline equivalent for Mg or Al.

There are a *lot* of "drive your car on water", some even publicly traded.
Would I be simplifying too much if I abbreviated that to "there's a sucker born every minute?" ;-)
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