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Sustain to gain: November 2009 Archives

Can pneumatic cars, aka compressed-air cars, contribute to sustainable mobility? The answer is a clear no, according to our model that was published last week in Environmental Research Letters.

Compressed-air cars have been heralded as the saviour of mobility by a small but vocal community for the last decade. Focal point of this community are regular press releases of Motor Development Internationa (MDI), a French company with an US branch, announcing every few years the soon deployment of their compressed-air cars that have wonderful properties such that range above 1000 km, rapid refilling, and low costs of charging. There are also other small business such as Energine and K'airmobiles that claim to develop compressed-air cars. The press releases were positively reflected in a few newspaper articles in the Anglo-Saxon world, and reverberated then in a small but enthusiastic community.

Of course, there is some truth in the compressed-air story. Tram lines at the end of the 19th century were running on compressed air (the compression was powered by coal, so not so much the clean solution). In principle, it is valid technology, but not so much in practice.

Our paper shows that the MDI claims are nonsense. Let me shortly rephrase the argument here. In a compressed-air car (CAC) air is the storage medium. A compressor is needed to store air at high pressure in a tank. The air motor is then driven by the expansion of air. In that regard, a CAC is more a less equivalent to an electric car – but with compressed-air as storage medium instead of a battery. Now the point is: a battery is just significantly more efficient than the compressed-air storage. Thermodynamics are not to the advantage of the CAC. This translates into very poor economic and environmental performance. Furthermore, huge tanks would be needed (780l tank for a cruising range of 115 km).

So if you want to run your car on electricity – use battery as carrier technology. If lithium becomes a scarce resource, hydrogen produced by electrolysis would still be the more efficient option.

The ERL paper was immediately perceived in a range of auto-blogs, starting with the New York Times, receiving hundreds of comments. Many comments were up to the point. Others were very sceptical of the paper, e.g. "I'm a strong believer that this article is a bunch of BS. Or more specifically, oil-industry-speak. India auto-maker, TaTa Motors has many orders backed up for a compressed-air car that they build. London is waiting for 500 taxis. It will run for up 300 miles on one filling of the carbonfibre tank. The compressor comes with the vehicle and you install it in your garage. It runs on your local household current."

Now, it's nice to learn that I am a big fan of the oil industry. There is too much opinion on this topic, not sufficient analysis, on both sides. The enthusiasts do not care to do even some back-on-the-envelope calculations, and the critics of the CAC come up with talking points like "Air cars are hot air", which is not up to the point either: air cars are more than mere mind games – and actually run on cold air (look up some thermodynamics).

If you want to test your own set of assumptions of the CAC, you can download a spreadsheet model here. On that side, you can also find a link collection to blogs' discussion of the CAC paper.

F Creutzig, A Papson, L Schipper, D Kammen (2009) Economic and environmental evaluation of compressed-air cars. Environ. Res. Lett. 4 044011.