We Must Look at Complete Life-Cycles
A new study published in Environmental Research Letters pours some cold water on those who think that compressed air cars are the future. The technology does sound very good on paper: Simpler than fuel cell vehicles and electric cars and there are no tailpipe emissions. But if you dig a bit deeper than this and look at the whole picture, the picture becomes less rosy…
The problem is that compressing air isn’t very efficient (a lot of the electrical energy used to run the compressor is lost as heat). From the ERL study:
Even under highly optimistic assumptions the compressed-air car is significantly less efficient than a battery electric vehicle and produces more greenhouse gas emissions than a conventional gas-powered car with a coal intensive power mix. However, a pneumatic-combustion hybrid is technologically feasible, inexpensive and could eventually compete with hybrid electric vehicles. […]
The life-cycle analysis of the compressed-air car, however, showed that the CAC fared worse than the BEV in primary energy required, GHG emissions, and life-cycle costs, even under our very optimistic assumptions about performance. Compressed-air energy storage is a relatively inefficient technology at the scale of individual cars and would add additional greenhouse gas emissions with the current electricity mix. In fact, the BEV outperforms the compressed-air car in every category. Uncertainty in technology specifications is considerably higher for CACs than for BEVs, adding a risk premium.
And while batteries are still being improved and new chemistries are being tried, compressed air tanks are probably not going to improve at the same rate. Advanced materials can probably help store air at a higher PSI than we do now, but the limits are probably closer than with batteries (and hypercapacitors).
This doesn’t mean that R&D on air cars should be stopped, or that they will never be a good idea. But it looks like they’ll have a limited part to play in the green(er) transportation landscape of the future.
For more on how air cars work, check out: HowStuffWorks: How the Air Car Works.
by Michael Graham Richard, Ottawa, Canada, http://www.crosspollinate.com, Treehugger, A Discovery Channel, November 23, 2009.