Note to readers:
I am receiving post after post in fevered rebuttal to this little post on pneumatic cars. The problem is that most of the rebuttals seem to be reacting to something other than what I have written. So I am going to try to set the stage for my essay in such a way that these misunderstandings disappear.
First, I am a student of thermodynamics and know full well the inefficiencies involved in the process of compressing a gas. It is obvious that less energy is lost to the charging of modern electronic batteries than is lost in in the process of storing energy in a high pressure air tank. Compression works against efficiency in lots of very real ways. Not only that, but the release of this energy through decompression is also inefficient. The relationship between heat and pressure is a bitch and always works against such schemes.
But that is not the point of my post. I am attempting a larger system-wide solution that takes into consideration efficiency metrics not limited to the power-plant of a vehicle. I believe that there are aspects to the compressed air vehicle solution that more than out-weigh the obvious inefficiencies when measured purely from an energy storage density perspective.
To wit: Purely compressed air cars are much much much more simple than electric or hybrid cars. That means they can be much much much lighter. Ultimately, it is the weight of a vehicle that most determines its energy demand.
But most importantly, I keep being slammed by people claiming correctly that it would be ridiculous to use electricity to compress air. Absolutely ridiculous! Which is why my proposal so clearly does not require the use of electricity to compress air. My proposal is to only compress air using gas burning turbine compressors installed at gas stations. This strategy is supported by many logical arguments. For one, it takes advantage of the incredible efficiency of transferring energy as refined gas. It takes advantage of an existing extraction, refinement, and delivery system that brings energy to every community in the developed world. Pollution is vastly reduced (order of magnitude reductions) through the use of scrubbers installed on a relatively few turbines as opposed to today's solution requiring each and every vehicle to cary its own exhaust scrubbers.
The combination of super light weight vehicles and the use of an already well established energy delivery infrastructure means it will vastly cut our emissions, cut our energy/mile demand by 70 or 80 percent, more than halve the cost and materials to build vehicles, avoid the natural resistance of change by established industry players (big oil, auto manufactures, gas stations owners, roads and highways, etc.).
And, my plan will not increase demand on our already limited power grid in the way that electric cars will.
It seems obvious to me that it is easier to build high pressure tanks (we know and we have the materials), than it is to build batteries. The best batteries we have today use rare earth materials, our access to which does not scale with projected demands. Batteries are heavy. Batteries have limited life-spans. The best accept fewer than 500 charges. That is less than two years of nightly recharges! Batteries are super expensive. Each Tessla sports car has six thousand eight hundred laptop sized batteries that must be configured within a complex and expensive refrigerated enclosure with sophisticated charging and optimization control computers. Who pays for your knew batteries after two years? Where are they disposed? What is the ecological cost of mining lithium from mile high strip mines in Bolivia, of disposal and recycling, of carrying the weight of batteries around?
Recharge of compressed air tanks at gas stations should take less than a minute. Even if a tank can provide only 50 miles between compressions, the distribution of gas stations means drivers are never far from a quick refill. Tanks can be refilled thousands of times without ever loosing capacity.
Until we have fusion reactors, our only reasonable choice is to use oil for transportation energy. Our choices then center around how efficiently we use oil and how little of its byproducts reach our environment.
Despite its limits, compressed air has the potential to be the best realistic solution to the complex and interdependent set of energy problems facing transportation today and well into the foreseeable future.
The car is super simple. Each wheel has a little compressed air motor (like the ones in the pneumatic wrenches at auto shops) built right into it. So there is no transmission and no central engine and no drive train to get that energy out to the wheels. Just a high pressure tank or series of smaller tanks (hundred thousand lbs per square inch), and high pressure lines out to each wheel. The vehicle is much simpler and much lighter. There is no toxic or combustive chemical fuel on board. A tank of compressed air is heavy but not nearly as heavy as batteries. You can replenish the tank in seconds. Standard fuel is distributed to gas stations same as today... but it never leaves the station. This is important because gas is efficient to distribute, and because solutions that usurp the oil economy status quo simply don't happen.
At the station, you can start with a traditional piston-type compressor and later transition to a high efficiency turbine engine (like a small commercial jet engine) which burn gas or diesel to compress air in large holding tanks. Big central scrubbers and catalytic converters clean the exhaust from the compressor's turbines much more effectively than can be done today in the tail pipe of each and every car. The cars will go about 200 miles per refill. The cars are far cheaper to build... the third-world can afford to go modern! The biosphere can afford the third-world going modern.
A car without a gas or electrical drive system can be built with a lighter structure, which means they can be built at about one fourth the weight. So, the cars will be at least four times as efficient. Compare this with electric cars which are actually a third heavier than their gas counterparts.
This is a plan that could happen. It doesn't disrupt the current hydrocarbon infrastructure. It does away with much of the problems associated with an oil economy. For one, most of the carbon and green house gasses are filtered and sequestered. The cars use much less fuel per passenger mile driven (up to 200 mpg should be achievable). There are several companies already designing and building the necessary components... some tooling up to build the cars.
Because they won't have quite the range of gas powered cars, you will have to go to the compression (gas) station more often. But, once you are there, it takes less time than filling a gas tank. Compare either with how long it takes to recharge a battery. Then there is weight.
Todays cars weigh in at 3000 to 6500 pounds. Most of that weight is structural support for the engine. A four passenger pneumatic car might reasonably weigh less than 800 lbs.
Another huge and frequently ignored contributor to greenhouse gasses and energy use is the production of new cars. The pneumatic car has about 25% the number of parts as in an internal combustion powered car (even fewer than in the super-complex electric/gas hybrids). That will result in huge (50 to 70%) pre-purchase carbon footprint reductions, and far less materials being consumed. Eventually, as we figure out how to get more energy from the sun, we can move away from hydrocarbons powering the compressors. But even in a hydrogen or pure solar economy, it will still be more efficient to run pneumatic cars than to put a mini power plant in each car (as we do today).