Last Modified: Sun Feb 21st, 2010 4:26 PM
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Description of the Fuel
Really, the composition doesn't matter for this application. The important thing is that it is a gas, all gasses behave in the same way when they are compressed; given the opportunity, they will release the energy "stored" in the compression, and do work on their environment in order to expand and thereby return to a lower energy state.
For air powered cars, air is used in place of other gasses or mixtures simply because it is free and virtually unlimited. The energy does not come from the "fuel" itself or from any chemical reactions as it does for other alternative fuels, but from the compression of the air.
The amount of work that a volume of compressed air can do through isothermal expansion, according to the ideal gas law, is
WB->A = nRT * ln(PA/PB)
n = moles of gas
R = ideal gas constant (8.314472 J / K * mol)
T = absolute temperature (K)
PA = tank pressure
PB = atmospheric pressure
This equation is applied to isothermal expansion, in which the ideal gas law
PV = nRT
holds true, and T is a constant (hense isothermal), so PV is also a constant, throughout the expansion (to achieve this the expanding gas must be in contact with a fluid heat reservoir). However, this same ideal gas law can be used to show that increasing the temperature of a fixed volume of air (like the compressed tank on the air car) will increase the pressure, which will yield a higher amount of work that the compressed air is capable of doing according to the equation above.
Both of the major companies currently developing air cars are using auxiliary standard-fuel-based engines to pre-heat the air before it expands, thereby increasing the pressure and work done by the expanding air.
Current Use / Application
Compressed air is currently used as an energy storage method in a very wide range of applications. Here is an incomplete list:
- paintball/airsoft guns or other pneumatic air guns
- scuba diving, used to inflate buoyancy devices
- "canned air" for dusting electronics
- air brake systems
- starting engines (diesel and race car engines)
In all of these examples, compressed air is stored in some sort of tank, and released to do work on its environment in one way or another.
Fuel Production Methods
The cars currently being developed have built-in electric air compressors, which can be plugged in and charge up the air tank over the course of four hours. The companies plan to have an infrastructure of compressed air fueling stations developed which would allow drivers to fill their air tanks in about 3 minutes for $2 - $3.
"Fuel Production" is probably the largest source of criticism that these air-powered cars have received. Currently, air compressors are typically 20%-50% efficient in their use of electricity to compress air. Critics question the storage of energy in air, which introduces efficiency losses both in storage and use, when electric cars provide much more efficient ways to store and use energy, without the intermediate stage of storing the energy in compressed air.
Studies of the efficiency of air cars as compared to pure electric cars show that the electric cars are more than three times as efficient in their storage and use of energy from the electric grid as air cars are, leading critics to question what benefits air cars have over pure electric cars.
In theory, air cars need not have any emissions other than air. In reality, there are several types of emissions that need to be considered.
Compressing air generates heat, due to the fact that nobody has created a 100% efficient air compression system. Also, heat is most likely generated by whatever mechanism provides energy to the air compressor.
Emissions from Electricity Production
If the air compressor is powered by the electric grid, the sources of energy used to produce that electricity must also be taken into account. This electricity may be generated from coal, nuclear power plants, hydroelectric dams, wind power, solar power, gasoline, and a number of other energy sources, each of which has emissions of some sort that must be considered.
According to the website Gas 2.0, an air car in the United States would create about .176 pounds of carbon dioxide emissions per mile based on the average mix of electric power sources during refueling, compared to a Toyota Prius Hybrid, which creates about 0.34 pounds of carbon dioxide emissions per mile. Needless to say, an electric car would use about 1/3 of this, due to the fact that electric cars use the energy from the electric grid approximately 3 times as efficiently.
Emissions from Auxiliary Engine
As mentioned previously, the major companies developing these cars use auxiliary engines to heat the air to pressurize it further before expansion. These engines are "flex fuel" engines, which can accept gasoline, biofuels, alcohol or several other odd derivative fuels. Depending on which of these fuels the engine was running on, it could produce a range of emissions.