A wild card in the energy game is pneumatic energy storage and may be a potential player particularly with automotive applications. Air Car. With new technological breakthroughs such as composite pressure tanks created by Airbus that crack instead of explode, it may have transport potential.
Pneumatic systems take compressed air into a tank at Psi’s up to 4350PSI, and then release the air as needed through a valve into pneumatic engine that uses the compressed air to drive pistons that turn that pressure into work. New advances in recapturing the heat generated through compression, and then releasing that heat as the pressure is released has allowed much of the energy to be conserved and used to drive the engine. The benefits of these pressure engines include 1.) being very light, and 2.) typical problems with exhaust heat loss, or compression ratios and so forth are a distant thought. As a result they have very high efficiencies of turning compressed air into motion. Not surprisingly some do not have pistons at all but work off of principles related to pneumatic tools. These completely eliminate the need for transmissions of any kind and bulky engine support. Futher reducing weight.
This is the basic Energy Flow, assuming a commercial pumping station.
Fuel 46.4 MJ -->Industrial Turbine 60% efficient (46.4*.60)-->27.84 MJ rotational energy--> Industrial Compressor 90% efficient (27.8*.9)-->25.05 MJ Compressed Energy--> Air Tank 90% efficient (25.05*.9)--> 22.55MJ stored energy-->pneumatic motor 80% efficient (22.55*.80)--> Work output 18.04 MJ
This would be 38% efficient at transforming chemical energy into pressure and then into motion. On a standard 300 mile tank it would go an extra 583 miles. Once again if we assumed pneumatic air was very cheap it would look like.
Air Pump Nozzle 46.4 MJ--> Vehicle Air Tank 90% efficient (46.4*.9)--> 41.76 MJ stored energy -->pneumatic motor 80% efficient (41.76*.80)--> Work output 33.4 MJ
From this perspective we would see an efficiency of 72%. On a 300 mile standard tank we would see it go an extra 780 miles.
There is always a catch, once again to get these uber numbers of efficiency we need storage capacity. To store that kind of energy in compressed air we need alot of storage. To put this into perspective a kg of gasoline has 46.4 MJ of energy, a kg of compressed air has .5 MJ of energy. So the equivalent amount of energy of a gallon of gasoline (132 MJ) would be 581 pounds of air compressed at 4350 psi in a 174 gallon tank. Puts a whole new perspective on how nature crammed so much energy into a 6 pound gallon of gas. To be fair though due to the ease of extracting most of the energy from compressed air systems you could go 108 miles on that (1 gallon of gas) amount of pressurized air.
So an option with the pneumatic model is to hybridize it. Due to the challenge of storing that amount of pressurized air, you could store about 30 miles worth, and then carry a small engine to produce pressurized air for you at its optimum rpm to supplement air usage. The energy flow for the supplemented portion would look like this.
Fuel 46.4 MJ -->small constant speed 4 stroke 40% efficient (46.4*.40)-->18.56 MJ crank energy--> small Compressor 70% efficient (18.56*.7)-->13 MJ Compressed Energy--> Air Tank 90% efficient (13*.9)--> 11.7 MJ stored energy-->pneumatic motor 80% efficient (11.7*.80)--> Work output 9.35 MJ
Which is just about what a standard engine does. So a pnuematic hybrid for small trips would be very efficient taking compressed air from a station, but for long trips the efficiency would approach standard efficiency because of its reliance on onboard pressure generation. Once again for aviation applications weight is as always an issue, however the very efficient power flow of pressurized air could be attractive for smaller and lighter applications.