Compressed Air Car - Disadvantages

Disadvantages

The principal disadvantage is the indirect use of energy. Energy is used to compress air, which - in turn - provides the energy to run the motor. Any conversion of energy between forms results in loss. For conventional combustion motor cars, the energy is lost when chemical energy in fossil fuels is converted to mechanical energy, most of which goes to waste as lost heat. For compressed-air cars, energy is lost when chemical energy is converted to electrical energy, when electrical energy is converted to compressed air, and then when the compressed air is converted into mechanical energy.

• When air expands in the engine it cools dramatically and must be heated to ambient temperature using a heat exchanger. The heating is necessary in order to obtain a significant fraction of the theoretical energy output. The heat exchanger can be problematic: while it performs a similar task to an intercooler for an internal combustion engine, the temperature difference between the incoming air and the working gas is smaller. In heating the stored air, the device gets very cold and may ice up in cool, moist climates.
• This also leads to the necessity of completely dehydrating the compressed air. If any humidity subsists in the compressed air, the engine will stop due to inner icing. Removing the humidity completely requires even additional energy that cannot be reused and is lost.
• Conversely, when air is compressed to fill the tank it heats up. If the stored air is not cooled as the tank is filled, then when the air cools off later, its pressure decreases and available energy decreases. The tank may require an internal heat-exchanger in order to cool the air quickly and efficiently while charging, since without this it may either take a long time to fill the tank, or less energy is stored.
• Refueling the compressed air container using a home or low-end conventional air compressor may take as long as 4 hours, though specialized equipment at service stations may fill the tanks in only 3 minutes. To store 14.3 kWh @300 bar in 300 liter reservoirs (90 m3 of air @ 1 bar), requires about 30 kWh of compressor energy (with a single-stage adiabatic compressor), or approx. 21 kWh with an industrial standard multistage unit. That means a compressor power of 360 kW is needed to fill the reservoirs in 5 minutes from a single stage unit, or 250 kW for a multistage one. However, intercooling and isothermal compression is far more efficient and more practical than adiabatic compression, if sufficiently large heat exchangers are fitted. Efficiencies of up to 65% may be achieved, however this is lower than the Coulomb's efficiency with lead acid batteries.
• The overall efficiency of a vehicle using compressed air energy storage, using the above refueling figures, is around 5-7%. For comparison, well to wheel efficiency of a conventional internal-combustion drivetrain is about 14%,
• Early tests have demonstrated the limited storage capacity of the tanks; the only published test of a vehicle running on compressed air alone was limited to a range of 7.22 km.
• A 2005 study demonstrated that cars running on lithium-ion batteries out-perform both compressed air and fuel cell vehicles more than threefold at the same speeds. MDI claimed in 2007 that an air car will be able to travel 140 km in urban driving, and have a range of 80 km with a top speed of 110 km/h (68 mph) on highways, when operating on compressed air alone, but in as late as mid 2011, MDI has still not produced any proof to that effect.
• A 2009 University of Berkeley Research Letter found that "Even under highly optimistic assumptions the compressed-air car is significantly less efficient than a battery electric vehicle and produces more greenhouse gas emissions than a conventional gas-powered car with a coal intensive power mix." However, they also suggested, "a pneumatic–combustion hybrid is technologically feasible, inexpensive and could eventually compete with hybrid electric vehicles."