The Pesky Thing Called Physics

The gun blogosphere is pretty quiet today, so I thought I’d take some time to take a look at a new technology. While I’m a gun blogger normally, by training and profession I’m an engineer, so this will be a rare occasion when I get to use that skill in the blogosphere.

Instapundit points to a technology that claims to be able to make hydrogen from magnesium and water. This is interesting, but I consider this another example of a company cashing in on the alternate fuel craze. The big reason not to get excited about this is the second law of thermodynamics will always demand that you lose. You will need to put more energy into cleaving the water molecule into hydrogen and oxygen that you will ever get back from putting it back together either by burning it in a conventional internal combustion (IC) engine or fuel cell. In short, this system is essentially turning magnesium into an energy source, with hydrogen as an intermediary, to be turned into a usable form of energy in a fuel cell or IC engine. Interesting idea, but is magnesium cheap enough and sufficiently energy dense to be a practical motor fuel?

Magnesium seems to cost about $2.75 per kilogram. A kilogram of magnesium contains about 24.7 megajoules of energy. To compare to gasoline, we really need to measure energy by volume, which is 43 megajoules for a liter of of magnesium, compared to 34.6 megajoules for a liter of gasoline. Magnesium wins on energy density!

A liter of magnesium has a mass of 1.74 kilograms, and a cost of $4.79 based on recent pricing. It would take 1.24 liters of gasoline to have the same amount of energy that’s in a liter of magnesium. Gasoline in my area is about $2.60 a gallon, which comes out to about 69 cents per liter. Therefore the energy equivalent for gasoline comes out to cost 86 cents, compared to magnesium’s $4.79. So magnesium costs about five and a half times what gasoline does if you use equivalent energy.! If you do the unit conversions, to get the same amount of energy that’s in a gallon of gas, it would cost you $14.48. A little pricey for driving the kids to soccer practice, wouldn’t you say?

Also consider this is based on the current price of magnesium. No doubt common use of magnesium as a motor fuel would drive the price through the roof. Magnesium mining also requires energy, and is not exactly environmentally friendly.

So unless Ecotality has found a way to get around the second law of thermodynamics, this technology is a dead end. I’m sure the government would gleefully throw lots of grants (tax dollars) in their direction, but I certainly wouldn’t invest.

My source material was largely this Wikipedia article on energy density, plus a little Googling for prices. Someone go ahead and check my math if you want.

UPDATE: Glenn updated the article with a bit from a chemist who reveals where the energy is going in the process; making the elemental magnesium in the first place, and then recycling the oxides back into free metal.  If you submerge magnesium in water, it will react with the water as a matter of course, releasing hydrogen.   Didn’t know that.  My background isn’t in chemistry.   But the physics still says you lose.

16 Responses to “The Pesky Thing Called Physics”

  1. Jim W at FSU Law says:

    Maybe it doesnt actually use magnesium as a fuel but keeps it submerged in a tank of water and lets it get oxidized. The magnesium dissolved into the water and forms a somehat basic solution, taking the place of the hydrogen ions. You would probably want the solution to be somewhat acidic to speed things along.

    You then reverse the process by running electricity through the plate to make the metal take its electrons back, basically electroplating the magnesium with the dissolved magnesium ions. Realistically though, all you are doing is charging a battery. A battery that generates hydrogen gas for fuel instead of electricity, but it’s the same basic thing.

    I imagine you would need a fairly enormous amount of magnesium for this and a ton of electricity to reverse the oxidation of the spent cells. And the recharging process would probably be slow. And then there is the issue of all this hydrogen gas floating around over a tank full of acid with a warm plate of submerged magnesium under it. Overall I think it is probably a dumb idea.

  2. Sebastian says:

    Even if they are oxidizing the magnesium via a reaction other than combustion, I think the energy output would still be the same per unit. In other words, it still wouldn’t beat gasoline on cost.

  3. AughtSix says:

    The Wikipedia entry lists the energy per kg for Mg burned in air. I would imagine that to use Mg to generate H, you’d oxidize the Mg in water to release Hydrogen gas.

    2 H20 + Mg -> H2 + Mg(OH)2

    That reaction will give off something less than the 24.7MJ/kg of Mg. I’m not sure how you’d recover that energy. For one kg of Mg, you’d then get 1/6 kg of H2 (Molar mass of Mg is 12, molar mas of H2 is 2)… H2 burned in air has an energy density of 120 MJ per kg. So, you’d get 20MJ of energy per kg of Mg input…. Even if you somehow also got 20MJ out of the previous reaction (the one that generated the H2…. which you aren’t going to — I think it’s going to be around 4 MJ… but that’s just a hunch) And now you’re back where you started.

    However, what if you didn’t have to buy the magnesium each time you used it. What if instead it lasted multiple “recharges.” You plug your car in, and it uses electricity (and a source of hydrogen, I’d assume) to reverse the reaction. H2 + Mg(OH)2 + E -> Mg + 2H20. Then you’re not buying Mg each time, but just the power (and hydrogen) to reduce the magnesium hydroxide to the metal and water. Then magnesium becomes an energy storage system, not an energy source. Much like hydrogen itself, but without the problems of densely storing a gas.

    That’s got to be it.

  4. AughtSix says:

    And the electricity cost for that 24.7MJ/kg of Mg is… that’s 6.861 kWh at the high end (say an urban area where this would most likely catch on) is $.14: for a cost of $.96 plus the cost of 1/6 of a kg of H2. Not absurd, but not great. However, if you’re running of a nuke where the cost of power can be more like a third of that… it’s down around $.30 (you calculated the cost of the equivalent energy from gasoline to be about $.86)

    However, internal combustion engines are horribly inefficient. I don’t know how efficient hydrogen fuel cell engines are, but I wouldn’t be suprised if they’re double the efficiency of a gas engine. (nor would I be too shocked if they were half as efficient)

    So, this system *might* be economically feasible. And, it seems like it would require a lot less infrastructure as compared to switching to purely hydrogen fueled (and hydrogen re-fueled) vehicles.

  5. Steve says:

    Ok, so Bill Hobbs says that Hydrogen combustion results in only water. That’s the oxidation reaction, right. Oxygen’s about 21% of ambient air, and 78% Nitrogen. So what happens to the nitrogen in combustion rxns? Isn’t there some NOx?

  6. Sebastian says:

    Bill’s right. If you burn hydrogen, it reacts with the oxygen in the air. Nitrogen is generally inert, but it can react with hydrocarbons, in the combustion process, I think. When you burn hydrocarbons, ideally, you get water and carbon dioxide. But engines aren’t ideal, so you can get water, carbon dioxide, carbon monoxide, unburnt hydrocarbons, sulfur compounds and oxides of nitrogen. Hydrogen burns cleanly, producing only water. Hydrogen won’t react with any other atmospheric gases in the combustion process.

  7. straightarrow says:

    Don’t smoke around it.

  8. Sebastian says:

    If you follow the links there AughtSix, they talk about a refeuling system that removes the magnesium oxide, adds more water, and puts in new magnesium pellets. I suppose they could be hauling the magnesium oxide off to be reprocessed, though. The problem with the system as a battery is going to be that it will take much more energy to recharge the battery than what you get out of discharging it. Second law of thermodynamics again. If the battery were 80% efficient on its charge, I think that would probably be pretty good. Lead-acid batteries typically have a charge efficiency of about 70%. NiMH batteries are lower, at about 66%.

    If the system is really energy dense though, this could be a great advancement in battery technology. But I’m skeptical the efficiencies are really that spectacular. Fuel cells still waste a lot as heat. If I recall, they tend to be around 50-70% efficient. Better than IC engines, for sure, but they still lose a lot.

  9. Sebastian says:

    Turns out Aughtsix was right about the reaction.

  10. Nomen Nescio says:

    if the magnesium-hydroxide reprocessing can be done reasonably distributed (say, at this system’s equivalent of gas stations), i suppose it might catch on. then the magnesium wouldn’t actually have to move around very much, other than tagging along in its users’ commutes. (i think it’ll be a while before we see heavy freight haulers running on this. if it happens at all, it’ll be commuter chiclet cars first. that’d still be better than nothing.)

    the inefficiencies are inevitable, of course, but just about any energy *storage* system has the advantage of having the energy *production* centralized in whopping big power plants, which are easier to optimize and clean (and monitor the cleanliness and efficiency of) than a zillion internal combustion engines all over the country. you can’t put the same sort of exhaust scrubbers on a car as you can on a fluidized-bed coal burner plant, much less a nuke plant. there’s transmission losses in high-voltage lines, to be sure, but the “electricity allergy” wackos will just have to live with those; better that than atmospheric pollution and global warming, says *THIS* greenie.

  11. Sebastian says:

    I’m interested in the idea of modular battery packs for cars. If you’re on a long trip, just swap out with another set at a service station that’s already been fully charged. The problem is that most batteries have memory, so they degrade with repeated charging/discharging and age. If someone could come up with a battery that’s very chemically stable, has a high energy density, and doesn’t have any “memory”, it could open the door for a practical, fully electric car.

  12. Brad says:

    Wired Magazine had an article a little while back on the quest to build a better rechargable battery. []

  13. AughtSix says:

    “If you follow the links there AughtSix, they talk about a refeuling system that removes the magnesium oxide, adds more water, and puts in new magnesium pellets. I suppose they could be hauling the magnesium oxide off to be reprocessed, though.”

    You’re right… they are putting in “fresh” magnesium. But it does talk about recycling the Mg(OH)2 (Or maybe it’s MgO–which would mean you’d need less water, and less overall weight…) back into Mg. Of course, that causes complications with your fill up: you’ve got to pay for the Mg you put in, and get credits for the MgO (Mg(OH)2?) that they pump out. Someone’s going to game that system.

    All in all, it doesn’t sound as crazy as it did at first blush. As Nomen said, cars don’t need an energy source, they need a good system of energy storage. This *could* be a reasonable solution to the storage problem. We’ll have to see.

  14. straightarrow says:

    Fully electric cars relying on batteries do nothing for the ecology. And, in fact, may harm our enviroment.

    Instead of power generating stations being able to dial back their output and thus the concomitant pollution and/or not necessarily pleasant by-products of electricity production, they would need to operate at peak for most of 24 hr/day, simply to recharge batteries when motorists are not necessarily motoring,but may be sleeping, working, or lounging while recharging their automobiles. Add to that the horrible problem of what to do with the depleted batteries that will no longer take a charge, and you have a far worse environmental problem than the one you tried to solve. Even allowing for recycling of those depleted batteries, it is an ultimate loss of efficiency and environmental benefit. Reclamation of said batteries requires more energy than making new, once again raising the level of pollutant by-products. Not reclaiming old batteries invokes the hoary problem of safe disposal, which has yet to be satisfactorily solved.

  15. FuelScout says:

    Aughtsix is right. This magnesium system seems a good dense storage system for energy versus compressed hydrogen – add to that the fact that electric motors are close to 4X more efficient than internal combustion engines.
    Why would NASA/JPL be running the science program for this company (and in fact is a significant shareholder!) unless there was merit?
    There is no free energy, but this is an efficient energy carrier system, and they have not seemed to pretend that it is a “miracle” but instead a commercially viable alternative. It could make “hydrogen doable”.

  16. Sebastian says:

    Electric motors are pretty efficient, but you still have the efficiency of the fuel cell as an issue. How much energy does it take to turn the magnesium hydroxide back into elemental magnesium? One has to understand the the process of getting back to elemental magnesium is the problem with this process. I don’t doubt that this is a reasonable way to carry around energy, just once the “battery” is spent, then what? You need more magnesium, which isn’t cheap.

    Look at the process here. This is a large part of why magnesium carries a high price tag, and reversing the reaction that would happen in this “battery” would be roughly the same, once you have a slurry of magnesia.


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