The Air Car
The Air Car has gotten the press excited on and off over the years.
The French design, which has received the most publicity, uses compressed air as an energy delivery mechanism. It has the potential for providing a clean-burning solution, but as usual it takes net energy to compress the air. No free lunch, unless wind or solar energy are involved to run the air compressors. And even there, we require energy to make the windmills and solar conversion devices.
As a sanity check here are two ways to calculate the energy value of 1 liter of compressed air. Remember that the gold standard is 1 GJ/30 liters for gasoline (or 33,000,000 joules/liter).
First, if you compress air completely you actually get liquid. So we take the energy value of liquid nitrogen (air consists of 70% nitrogen by volume).
- Energy Density/Specific Energy of liquid nitrogen = 320 KJ/l or 320,000 joules/liter
- Heat of Vaporization of liquid nitrogen = 161 KJ/l or 161,000 joules/liter (to double-check the above value)
Looking at specific energy, this is at best 100 times less energy content than gasoline. On the plus side, the transfer to mechanical power is better than for gasoline (burning gas generates much wasted heat). Granted that advantage, we still have to generate the compressed air by using energy, and to top it off, we also have much worse energy density (i.e. energy per volume) than gasoline. You understand why consumers and corporation like gasoline (little energy overhead to extract a free lunch).
This is not all bad; after all, humans on bicycles and battery powered vehicles can get around prestty good right now. The bottom line: compressed air or liquid nitrogen power is similar to battery power, very good for light-weight vehicles and short excursions.
Specific Energy of various storage devices1 :
1from U.Washington cryogenic auto lab.
added: Fixed up some of the numbers. Some of my initial skepticism over the Air Car or cryogenic-based cars was due to being a consumer of many liquid nitrogen cannisters/day during my basic research days. As anyone who has used these before, the cannisters were big and heavy enough to need special carts to haul them around. And then they were only used to cryogenically pump out semiconductor vacuum systems. Thinking that the energy I wasted was enough to drive over a 100 miles (as The Air Car claims go) initially raised my suspicions. But then again, I usually rode in on a bike and could do a century bike ride no problem on energy formed out of carbohydrates and fat. Ergo, we will always find creative ways to get around with an energy source at our disposal.