Compressed Air Cars

ZPM's car starts at $15000

Compressed-Air Powered cars could take you over 800 miles on a single fill-up, at speeds of up to 96 mph. They should refuel in less than 3 minutes, and at speeds over 35 mph emit about half the CO2 of a Toyota Prius.

• ZPM (Zero Pollution Motors) will begin taking reservations in mid-2009 for US deliveries of their compressed air vehicle in 2010. ZPM's cars will start at $15000. and wants to produce a 6-seater, 75-hp model for $17,800.

• India's largest automaker, Tat Motors, anticipates having about 6000 compressed-air cars on the road this year.

The biggest questions appears to be safety and the efficiency of compressing the air in the first place.

StartChange turned to our resident expert, Bill Thomson, for a realistic appraisal of compressed-air powered cars for everyday use. Bill is an Electrical Engineer with thirty years expertise in alternative energies.

"A technology that potentially competes with lead acid storage batteries if no other battery technology is practical.  There are large efficiency problems, although at least one company claims 90% efficiency -- I find that claim VERY suspect.  When air is compressed, a lot of heat is generated.  If that heat is not saved in the air tank (as hot air) so that it can be reclaimed when the air is used, the efficiency is worst than lead acid batteries.

On the other hand, air conditioning is easier, since a compressed air engine needs to be heated instead of cooled!  Might make the car more economic in hot climates.

http://en.wikipedia.org/wiki/Compressed_air_vehicle
The crux of the matter is the poor energy density of compressed air.

From the link above: Compressed air has a low energy density. In 300 bar containers, about 0.1 MJ/L and 0.1 MJ/kg is achievable, comparable to the values of electrochemical lead-acid batteries. While batteries can somewhat maintain their voltage throughout their discharge and chemical fuel tanks provide the same power densities from the first to the last litre, the pressure of compressed air tanks falls as air is drawn off. A consumer-automobile of conventional size and shape typically consumes 0.3-0.5 kWh at the drive shaft[4] per mile of use, though unconventional sizes and may perform with significantly less."