Why aren't we using compressed air to power vehicles by now?

The volumetric energy density of compressed air is 17 Wh/l whereas the volumetric energy density of gasoline is 9,000 Wh/l. Even a lithium ion battery has 250 Wh/l. Compressed air is no where's close to being able to store the amount of energy that we expect a modern vehicle to have.

To everybody worried about the compressed air tanks exploding: When these tanks are built right they DO NOT explode. Thats silly to even bring that up. Think about traditional cars and trucks that use gasoline. You are literally sitting on a big *** bomb but you rarely hear about them exploding even when the tank is on fire. The same holds true for compressed air tanks. There are safety valves in place that will rupture instead of the tank exploding like a bomb.

AIR COMPRESSED TANKS IN CARS DO NOT EXPLODE!!!

Compressed air is just a way of storing energy. It doesn't provide energy.

It is more efficient & less expensive to just burn petroleum in the vehicle engine than to burn petroleum in a separate engine, use that power to compress air, and then use the compressed air to power the vehicle.

Every time you transform energy (chemical, potential, mechanical, electrical, etc) the transformation is not 100% efficient. There are losses.

There are also considerable safety risks, like if the large high-pressure tank ruptures. Also range is a factor - in Europe things are close together - in America very spread out.

The problem with air-powered cars is the power-to-weight ratio for the "fuel tank." Air at 30 MPa (4,500 psi) contains about 50 Wh of energy per liter. Gasoline contains about 9411 Wh per liter. This means a compressed air car would need an air storage tank 188 times as large, to get the same range. The problem is, when you add more air storage tank you add significantly more weight--which means it takes more power to get the sucker moving.

So say you take an ordinary U.S. market compact car--the kind that gets 30 mpg and has a 12-gallon tank--and convert that to compressed air. If you want the same 360 mile range, you'd need an air tank capable of holding 2256 gallons of compressed air. That's 301 cubic feet, a spherical tank big enough to hold that would have a 4.1 foot radius--in other words, it would be a ball over 8 feet high. You're not going to fit that into a car. So scale back the range, let's say to equivalent of one gallon of gasoline. You'd still need a 188 gallon tank, a sphere with radius 1.8 feet. This will at least fit in a car, made of steel it'll weigh about 500 lbs. Made of carbon fiber, the weight will be much less and the production cost much more.

Anyhow, the point is that compressed air with the energy of one gallon of gasoline needs a physically large and heavy storage tank, this is the challenge of compressed air cars and the reason they're not already on the road. If you increase the amount of energy stored, you also need to increase the size and weight of the tank--and you can't go very much bigger and still fit in a car, nor go much heavier and still be able to move the car.

It's the limitations of the pressure vessel. in order to store enough energy in the space of a car, to give you a usable commuting range like 40 miles, you need extremely high pressures.

Now you don't want something brittle, like carbon fiber, that would "pop" or more like explode, in the event of an impact. That's dangerous, even if you surround it with cushion and crumple zones, like that how to protect the egg experiment.

You want something with some malleability, that gives you a better chance of holding pressure in an impact. Really steel is the only option. And for a steel tank to hold that much pressure, it has to be extremely heavy.

For instance:

to store 20 KWH of energy (enough for a 40 mile range in an average car), you would need a spherical steel tank, 16 inches in diameter on the inside, holding 4500 psi, with a 4 inch thick wall. That tank would weigh 1450 lbs.

That sounds doable. You could totally build a car around that. Maybe I should take a closer look at compressed air.

You can also compress the air using solar, or wind power, keep it in a storage tank outside the vehicle, then let the pressure equalize into your inboard tank through high pressure lines. It would be a pretty quick fill up process.


Edit:

dirocyn is off by a factor of ten. Compressed air at 4500 psi contains over 550 wh per liter.

http://en.wikipedia.org/wiki/Energy_density
http://www.onlineconversion.com/energy.htm

Compressed air doesn't have enough power. However, my cars do use compressed natural gas, and it works beautifully! It's cheaper than gasoline (95 cents per gallon) and cleaner for the environment, too. My 2 Ford Crown Vics were made in 1998 and 1999, and my F-150 pickup truck is a 1998, so this technology isn't new. Check out my sources below for more info about this cool alternative fuel.

The major problem is inefficiency due to the refridgeration effect.

Compressed air vehicles are just unusual enough that there are many misconceptions about them. In some ways they are like steam engines in their use of pressure to power the engine. In some ways a compressed air vehicle might be considered an electric vehicle. Although it does not have batteries, an electric motor or an electrical connection, electricity would be used to compress air for the vehicle. Windmills could do the same. There would be less environmental benefits with petroleum based air compressors. The advantage of a compressed air vehicle is that it could be recharged very quickly and it is a zero emissions vehicle in its operation.

Yes, compressed air is an "energy carrier" but so is any other "fuel" you use to power vehicles. This one may be inheritantly safer. Gasoline is only chemically stored solar energy. Electricity is energy stored from any number of sources. Hydrogen has to be made and so is essentially storing the energy used to make it.

Modern compressed air cylinders would be made of carbon fiber. Unlike steel tanks which are more likely to rupture and send bits of flying metal around carbon fiber tends to stick together. It may split and release the air but you won't have flying fragments of metal tanks causing further harm to bodies and vehicles.

The power density of compressed air increases with the compression. To store more energy high pressures, liquid air and hybrid designs are considered. Regenerative braking is a realistic possibility and has been considered as a hybrid way to give regen braking to an IC engine. There are ways to make the compressed air last longer with regenerative braking that compresses air back to the storage tank. And you could always use a hybrid design and use an IC engine to compress air en-route. Like an IC engine powering a generator the engine would be operating at its optimal efficiency.

There are two major companies developing compressed air engines. MDI in France and Engineair in Australia. The MDI design uses pistons and the Engineair design is a rotary engine. MDI has licensed its technology to an American subsidiary and there has been a splinter group of employees starting another company. Compressed air vehicles have traditionally been used in places, like some mines, where any other type of engine may ignite gasses in the air.

Small vehicles can meet crash standards through the use of a roll cages and special air bags. There are several that have already done so and we are beginning to see some compressed air vehicles on the market with over a 100 mile range (see below.) But the main design issue that must be overcome with any compressed air vehicle is the loss of energy when the air is compressed and again when it is expanded in the engine. The compression causes a great deal of heat and the expansion a large cooling effect. The heat loss and overcooling represent significant energy losses. Compressed air vehicles may be very useful in hot climates as air conditioning would be a free by product.

As the first reviewer says, compressed air is not an energy source, but a means of energy transformation. So is fuel cell. However the energy storage capacity of fuel cell is much higher than compressed air. Therefore the former is exploited in clean energy technology.

This indicates that chemical energy is more effective than mechanical energy in human scale. Also compare the destructive power of dynamite with catapult. But in astronomical scale, mechanical energy can be devastative such as the asteroid that wiped out the dinosaurs on Earth.

You would need a huge tank to store enough air in to get any kind of driving distance. So it's not practical. But the idea is being developed as a hybrid concept. Basically the stopping action in a car would compress air and then that air would be used to help get the car moving.

everyone has missed the biggest issue. When you use the air in the tank and let the pressure drop across the power turbine, the air get cold, real cold, like down to -250F. SO, you have to let the pressure down in stages and the reheat the air before letting the pressure down again. The air cars have a diesel fuel heater to reheat the air.