Re:There are other ways to compress the air. (Score 1)

Any time you switch from one form of energy to another, you have losses, their magnitude depending on the efficiency of the conversion, storage, and transmission systems used. In the paper we're commenting about, there was a buried aside noting that while they didn't see compressed air as a particularly efficient primary propulsion mechanism for cars, hybrid electric/pneumatic vehicles hold some promise. Certainly a regenerative braking system can be utilized that compresses air directly, which is probably at least as efficient as generating electricity with it, assuming the propulsive power of the air can be harnessed directly to power the car. As has been pointed out, the study assumed that automotive air bottles would be filled using electricity generated by burning hydrocarbons. But air can be compressed by various carbon-neutral means. The directly-compressive windmill mentioned previously is one; rather than relying on batteries, an isolated windmill can pump up manifolded pressure vessels until they're full. Then an (air-powered) truck can collect them and convey them to a filling station. An air infrastructure like this would not require exotic metals for batteries, or obtrusive power transmission lines. Other means of directly compressing air include harnessing tidal and hydro forces. Electric power, while convenient, is transient - one must use it or lose it. Batteries don't really mitigate that basic fact very much. But compressing air is a way of storing energy that has a lot of shelf-life. Air compression technology hasn't advanced much in the last 50 years or so, but then it hasn't had much attention focused on it either. With a little technological rethinking, such as the development of stronger, lighter pressure vessels, or more efficient pumps, this basically benign and powerful source of portable power might prove a valuable ally in our efforts to build the clean and carbon-neutral vehicles of the near future.