As batteries get old they degrade in their ability to hold a charge. Wouldn't compressed-air hybrids be better

Question

http://green.yahoo.com/blog/ecogeek/66/air-car-ready-for-mass-production.html

I intend to buy one. 90% of our driving is short hops around town. We seldom go 100 miles before going back home again (100 miles is the advertised range for one "fill-up" in an air-car).

But instead of electric hybrids, wouldn't gas-compressed air be a superior platform for a hybrid car? After all, batteries are heavy and have to be replaced every few years and are comprised of hazardous chemicals and heavy metals. A carbon-fiber compressor tank is lightweight, never has to be replaced, holds more energy in reserve than an equal-weight/size bank of batteries and is made of carbon, an inert non-polluting material!

Answer 1

That is a really good idea. I like it.

Answer 2

Really? The energy density is greater?

Or flywheel hybrids.  Do you know their energy density>

Answer 3

Even electric power is a better energy-transfer and storage medium than compressed air.  

For one thing, you have to deal with the water content of the compressed air, which means that you need a hefty refrigeration system at your compressor.  (I have a tiny compressor in the garage I use for air tools, and the water in that thing is an eternal problem, even with a water extractor.)  

For another, you're dealing with high-pressure hoses and pressure vessels.  Have you ever dealt with a broken air brake hose?  It's a grand experience.  

While gasoline does indeed have a higher energy density than compressed air, a structural failure in a gas tank will just dump gasoline in the parking lot.   A failure of a compressed air tank will blow out walls and kill people.  It happens now and again.  

And there are substantial energy losses involved in compressing air for motive purposes.  There's the heating of the air, which can't be avoided.  

And the air compressor machinery itself must be designed to heroic proportions if you're going to drive anything like a modern car.  If, for example, you expect your fifty horsepower car to run for five hours, you'd need a 60 x 50 = 3000 horsepower air compressor to 'charge' it in five minutes.   That'd make an interesting scene at your ecologically-correct filling station.

Answer 4

one of the problems with air powered cars, other than range, is the pressure to which the air has to be compressed to make it useful as a propellant. that is on the order of 3000-4500psi, and that will strain the tanks, even the carbon fiber tanks used in some air cars. an on board compressor is nice, especially if powered by a small gas or diesel engine, as it will extend the range and keep the tanks charged, but as noted there are other issues as well. things like;

the tanks most be constantly inspected to avoid failure

the tanks, even ones made from carbon fiber, are large, somewhat heavy, and cant be used as a structural member

you have to worry about hose failure, compressor failure, tank failure, etc.

perhaps one day an air powered car will be viable, but right now, no. i do however like the idea behind air powered cars, and i hope the manufacturers get the time needed to develop the car fully.

Answer 5

Compressed air is inherently dangerous.  If a battery was involved in a crash, it would just leak liquids.  If a compressed air vessel was involved in a crash, it has far more potential to add destructive energy to the situation.  If the compressed gas is hydrogen, then the situation gets even worse, because when it mixes with oxygen in the air, it becomes explosive.

Answer 6

No.

a) Compressed air cars rely on a mechanical process (pushing a piston) which creates friction and loses energy as heat.  They are therefore less efficient than electric motors (~90% efficiency).

b) Lithium ion batteries are generally expected to last at least 10 years, even in their current young stage of development.

http://www.altairnano.com/documents/Lith...

c) Lithium ion batteries are both recyclable and non-hazardous.

d) As has been mentioned, the extremely high pressures of compressed air tanks put a lot of stress on the materials.  Your claim that they never need to be replaced is inaccurate.