Even after all this time I still get letters like this one:

After comments by a friend, I Googled "Air Car" and I read/seen reports on a compressed air car being developed in France (and also in Australia [different inventors]).

Do you think the physics behind it are plausible?

Oh, well, what the heck. Here's how I answered him:

Depends on what you require. If you're talking about a vehicle capable of traveling a reasonably long distance between recharges, then it would have to have a quite large tank under really very high pressure.

So how safe do you want the system to be? In a wreck, if the tank gets punctured you're going to get a very dramatic explosion which would almost certainly kill everyone in the vehicle. I wouldn't want to be there when it happened. (Several potential failure modes, all quite lethal.)

And it wouldn't take all that much to make the tank rupture; it's going to be near its limit on tensile strength, and it will gain a lot of its strength from its shape. Just denting it could be enough.

If you make the tank really rugged, it's going to weigh a hell of a lot. That adds considerable mass to the vehicle which has to be moved around, so the system is less efficient, which cuts way back on your range and increases your net energy usage per kilometer travelled.

Meanwhile, if the energy to run this isn't coming from petroleum, then just where is it coming from? Air is free, but compressed air isn't.

Apparently, the compressors run on electricity. If there are just a few of these vehicles, it is a negligible increase in electric power consumption. But if you transition the fleet so that a respectable percentage of the vehicles are powered this way, then it means you need a lot more electrical generation capacity. What powers the electric plants behind the compressors?

Probably coal.

Which brings up the entire question of what you're trying to accomplish. If you're a "greenhouse gas" fetishest, you've actually lost more than you've gained. More CO2 will be released this way than would be if you were using petroleum, because there are more energy conversion steps and there are losses on each step.

As with all of these ivory-tower ideas, it's not a question of whether you could build one, or two, or a hundred. It's a question of what would be involved in building 25 million of them, and what side effects there would be if you did. Scaling is the real problem. A "solution" that doesn't scale isn't a solution.

Does the physics work? Sure. But I'm not so sure about the economics, nor am I sure about the safety. How much of an increase in traffic fatalities are we willing to accept for whatever dubious benefits this system claims to offer?

[I've disabled comments on this post because anything written about alternate energy invariably flushes out lots of cockroaches who don't want to hear "no" from spoilsports like me. Alternate Energy, big-L Libertarianism, Objectivism, and Macs; the unholy quartet of hot buttons guaranteed to bring swarms of outraged comments. Believe me, I know this from experience.]

Posted by: Steven Den Beste in System at 03:08 PM | No Comments | Add Comment
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