[Here is a LiveScience article on cars that run "mostly" on air](http://www.livescience.com/technology/081103-air-car.html). I also hear students talking about this - how the world will change when cars run on air. This is misleading. These cars do not really run on air. It is not like you put them outside and BOOM - instant energy.
A better way to say this is that these cars run on energy STORED in compressed air. How do you store energy in compressed air? You need a compressor that typically runs on electricity. Where does that energy come from? Probably coal or natural gas.
It doesn't really matter where the energy comes from. It matters that people realize these "air cars" are not free energy.
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PV = energy, 101.325 joules/liter-atmosphere. A commercial gas cylinder is about 170 atm, so call it 0.017 MJ/liter gas storage. 0.1 MJ/liter gas storage is 14,500 psi - and that is insane for filling, storage, and use.
Gasoline's enthalpy of combustion is -48 kJ/g, density is 0.68 g/cm, 30% of liberated heat goes to move the car. One liter of gasoline for internal combustion:
(680 gm)(-48 kJ/gm)(0.3) = 9.8 MJ/liter usable energy storage
A 14 gallon gas tank with 25 mpg is 350 miles range. If a factor of 580 (98 best case) degradation in stored usable energy is your thing... That would be an air car with 0.6 mile range (or 3.6 mile range best case). A 350 mile range air car would carry 13,700 gallons of 14,500 psi air (at 100% efficiency).
A tanker truck is 8000 gallons. It's gonna be a big air car.
As usual, Uncle Al's analysis is quite insightful. The practicalities are difficult. Plus, I certainly wouldn't want to be around a gas cylinder (sphere?) with it at 14,500 psi if that thing was involved in a crash. That'd be one heck of a BANG!
There are, though, some advantages to using a stored energy system for an automobile. One is that internal combustion engines are rather inefficient, so by producing the actual energy at a large, more efficient plant, the theoretical efficiency may be higher (that is, if the storage mechanism doesn't have too bad of an efficiency). Another advantage is that it gets exhaust gases out of urban areas.
Dave
P.S. At what point do atmospheric gases deviate from the ideal gas law? Can you even compress air to 14,500 psi without hitting this limit? Plus, there's the issue of heating of the air as it's being compressed (and cooling as it expands), resulting in some energy/efficiency loss. PV=nRT
A compressed air car is ineluctably stupid for uselessness and immense energy waste as heat of compression and forced cooling. A HYDROGE*N car is stupid in kind. Calculate H2 "molecules"/volume in diesel (open bucket) and the pressure achieving that in hydrogen gas. Heat of compression! Low pressure adsorbate tanks (Hystore alloys, nanotubes, hyperporous metal-oxide frameworks) suffer heat of binding energy.
The ELECTRI*C car disappoints. Hybrids are a good compromise. Either way, batteries do not like being roasted (American South during summer ) or frozen (American North during winter). They get hot during charge and discharge. Your $10K battery pack lives no more than eight years (declining capacity after four years). This is nuisance for a hybrid and death for an all electric. When 20 million people plug into the local grid every night the grid browns out every night.
America has 1000+ years of coal, much of it within 100 feet of the surface. Coke coal with heat from burning coal (chemical byproduct stream) and blow in steam to make synthesis gas, CO + 2H_2 (endothermic). Syn gas over Cu/Zn0 catalyst makes methanol (terrible stuff - 50 wt-% inert oxygen). Methanol over ZSM-5 zeolite catalyst makes C8 aromatic gasoline with 100+ octane number. Yields are near quantitative each step. Save petroleum for petrochemicals.
If you don't like CO2 emissions go 100% nuclear for central power generation. Civilian recycle spent fuel: Clean beta-waste for safe storage. Beta-contaminated (trans)uranic stram for refueling. It's not rocket science (at least not the way NASA fails at it).