Practical constraints in transportation In order to use air storage in vehicles or aircraft for practical land or air transportation, the energy storage system must be compact and lightweight.
Energy density and
specific energy are the engineering terms that define these desired qualities.
Specific energy, energy density, and efficiency As explained in the thermodynamics of the gas storage section above, compressing air heats it, and expansion cools it. Therefore, practical air engines require heat exchangers in order to avoid excessively high or low temperatures, and even so do not reach ideal constant-temperature conditions or ideal thermal insulation. Nevertheless, as stated above, it is useful to describe the maximum energy storable using the isothermal case, which works out to about 100 kJ/m3 [ ln(
PA/
PB)]. Thus if 1.0 m3 of air from the atmosphere is very slowly compressed into a 5 L bottle at , then the potential energy stored is 530 kJ. A highly efficient air motor can transfer this into kinetic energy if it runs very slowly and manages to expand the air from its initial 20 MPa pressure down to 100 kPa (bottle completely "empty" at atmospheric pressure). Achieving high efficiency is a technical challenge both due to heat loss to the ambient and to unrecoverable internal gas heat. If the bottle above is emptied to 1 MPa, then the extractable energy is about 300 kJ at the motor shaft. A standard 20-MPa, 5-L steel bottle has a mass of 7.5 kg, and a superior one 5 kg. High-
tensile-strength fibers such as
carbon fiber or
Kevlar can weigh below 2 kg in this size, consistent with the legal safety codes. One cubic meter of air at 20 °C has a mass of 1.204 kg at
standard temperature and pressure. Thus,
theoretical specific energies are from roughly 70 kJ/kg at the motor shaft for a plain steel bottle to 180 kJ/kg for an advanced fiber-wound one, whereas practical
achievable specific energies for the same containers would be from 40 to 100 kJ/kg.
Safety As with most technologies, compressed air has safety concerns, mainly catastrophic tank rupture. Safety regulations make this a rare occurrence at the cost of higher weight and additional safety features such as pressure relief valves. Regulations may limit the legal working pressure to less than 40% of the rupture pressure for steel bottles (for a
safety factor of 2.5) and less than 20% for fiber-wound bottles (
safety factor 5). Commercial designs adopt the
ISO 11439 standard. High-pressure bottles are fairly strong so that they generally do not rupture in vehicle crashes.
Comparison with batteries Advanced fiber-reinforced bottles are comparable to the
rechargeable lead–acid battery in terms of energy density. Batteries provide nearly-constant voltage over their entire charge level, whereas the pressure varies greatly while using a pressure vessel from full to empty. It is technically challenging to design air engines to maintain high efficiency and sufficient power over a wide range of pressures. Compressed air can transfer power at very high flux rates, which meets the principal acceleration and deceleration objectives of transportation systems, particularly for
hybrid vehicles. Compressed air systems have advantages over conventional batteries, including longer lifetimes of
pressure vessels and lower material toxicity. Newer battery designs such as those based on
lithium iron phosphate chemistry suffer from neither of these problems. Compressed air costs are potentially lower; however, advanced pressure vessels are costly to develop and safety-test and at present are more expensive than mass-produced batteries. As with electric storage technology, compressed air is only as "clean" as the source of the energy that it stores.
Life cycle assessment addresses the question of overall emissions from a given energy storage technology combined with a given mix of generation on a power grid.
Engine A pneumatic motor or compressed-air engine uses the expansion of compressed air to drive the pistons of an engine, turn the
axle, or to drive a
turbine. The following methods can increase efficiency: • A continuous expansion turbine at high efficiency • Multiple expansion stages • Use of waste heat, notably in a hybrid
heat engine design • Use of environmental heat A highly efficient arrangement uses high, medium, and low pressure pistons in series, with each stage followed by an airblast venturi that draws ambient air over an air-to-air
heat exchanger. This warms the exhaust of the preceding stage and admits this preheated air to the following stage. The only exhaust gas from each stage is cold air, which can be as cold as ; the cold air may be used for
air conditioning in a car. This improves the range and speed available for a given tank volume at the cost of the additional fuel.
Cars Since about 1990, several companies have claimed to be developing compressed-air cars, but none is available. Typically, the main claimed advantages are no roadside pollution, low cost, use of cooking oil for
lubrication, and integrated air conditioning. The time required to refill a depleted tank is important for vehicle applications. "Volume transfer" moves pre-compressed air from a stationary tank to the vehicle tank almost instantaneously. Alternatively, a stationary or on-board
compressor can compress air on demand, possibly requiring several hours.
Ships Large
marine diesel engines have started using compressed air, typically stored in large bottles between 20 and 30 bar, acting directly on the pistons via special starting valves to turn the crankshaft prior to beginning fuel injection. This arrangement is more compact and cheaper than an electric starter motor would be at such scales and able to supply the necessary burst of extremely high power without placing a prohibitive load on the ship's electrical generators and distribution system. Compressed air is commonly also used, at lower pressures, to control the engine and act as the spring force acting on the cylinder exhaust valves, and to operate other auxiliary systems and power tools on board, sometimes including pneumatic
PID controllers. One advantage of this approach is that, in the event of an electrical blackout, ship systems powered by stored compressed air can continue functioning uninterrupted, and generators can be restarted without an electrical supply. Another is that pneumatic tools can be used in commonly-wet environments without the risk of electric shock.
Hybrid vehicles While the air storage system offers a relatively low power density and vehicle range, its high efficiency is attractive for hybrid vehicles that use a conventional internal combustion engine as the main power source. The air storage can be used for
regenerative braking and to optimize the cycle of the piston engine, which is not equally efficient at all power/RPM levels.
Bosch and
PSA Peugeot Citroën have developed a hybrid system that uses hydraulics as a way to transfer energy to and from a compressed nitrogen tank. An up-to-45% reduction in fuel consumption is claimed, corresponding to 2.9 L / 100 km (81 mpg, 69 g /km) on the
New European Driving Cycle (NEDC) for a compact frame like
Peugeot 208. The system is claimed to be much more affordable than competing electric and flywheel
KERS systems and is expected on road cars by 2016.
History of air engines , in use at the
Homestake Mine between 1928 and 1961 Air engines have been used since the 19th century to power
mine locomotives, pumps, drills, and trams, via centralized, city-level distribution.
Racecars use compressed air to start their
internal combustion engine (ICE), and large
diesel engines may have starting
pneumatic motors. == Types of systems ==