Compression can be done with electrically powered turbo-compressors and expansion with turbo 'expanders' or air motors that drive electrical generators to produce electricity.[9]​.

El sistema de almacenamiento de un CAES (Almacenamiento de Energía de Aire Comprimido) es uno de las características más interesantes de esta tecnología, y es estrictamente relacionado con su viabilidad económica, densidad de energía y flexibilidad. Hay unas cuantas categorías de barcos de almacenamiento del aire, basados en las condiciones termodinámicas del almacenamiento, y en la tecnología escogida:.

Constant Volume Storage

This storage system uses a volume (a room or cavern) with rigid walls to store large amounts of air. From a thermodynamic point of view the system is understood as constant volume and variable pressure. Something that can cause operation problems in connected compressors and turbines. Pressure variations have to be kept within a safe limit.[10]​.

The storage ship is often an underground cavern created by solution mining (salt is dissolved in water for extraction) or by using an abandoned mine; Use of porous rock formations (which have holes through which liquids or air can pass) such as those in which natural gas reservoirs are found has also been studied.[11]​[12]​.

In some cases, a pipe at surface level was also tested as a storage system, providing good results.

Constant Pressure Storage

In this case the storage vessel is maintained at a constant pressure, while the gas is contained in a vessel of variable volume. Many types of storage ship have been proposed, but the operating conditions follow the same principle, the storage ship is placed hundreds of meters underwater, the hydrostatic pressure of the water column above the storage ship allows to maintain the pressure at the desired level.

This configuration leaves:

On the other hand, the cost of this storage system is higher, due to the need to place the storage ship at the bottom of the chosen water reservoir (often the sea or ocean) and due to the cost of the ship itself.[13]​.

Plants operate on a daily cycle, charging at night and releasing during the day. Heating compressed air that uses natural gas or geothermal heat to increase the amount of energy that is extracted has been studied by the Pacific Northwest National Laboratory[12]​.

Compressed air energy storage can also be employed on a smaller scale as exploited by air cars and air-driven locomotives, and also by the use of high-strength carbon-fiber air storage tanks. Even so, to retain the energy stored in compressed air, this tank would have to be thermally isolated from the environment; Furthermore, the stored energy will escape in the form of heating since compressing the air raises its temperature.

Transmission

City-wide compressed air power systems have been built since 1870.[14] Cities such as Paris, France; Birmingham, England; Dresden, Rixdorf and Offenbach, Germany and Buenos Aires, Argentina installed such systems. Victor Popp built the first systems to power clocks by sending a pulse of air every minute to change his aiming weapons. They quickly evolved to deliver power to homes and industry.[15] As of 1896, the Paris system had 2.2 MW of generation distributed at 550 kPa over 50 km of air tubes for motors in light and heavy industry. Usage was measured by meters.[14]​ The systems were the main source of home-delivered energy in those days and also powered the machines of dentists, seamstresses, printing facilities and bakeries.

Para conseguir un proceso reversible termodinámico cercano de modo que la mayoría de la energía está salvado en el sistema y puede ser recuperado, y las pérdidas están mantenidas insignificantes, un proceso isotermo reversible cercano o un isentropico el proceso está deseado.[2]​.

Isothermal storage

In an isothermal compression process, the gas in the system is maintained at a constant temperature throughout. This necessarily requires extraction of heat from the gas, which would otherwise experience a temperature increase due to the energy that has been added to the gas by the compressor. This heated extraction can be achieved by heat exchangers (intercooling) between subsequent stages in the compressor. To avoid wasted energy, the intercoolers have to be optimized for high heat transfer and low pressure drop. Naturally this is only an approximation to an isothermal compression, since heating and compression occur in discrete phases. Some smaller compressors can approach isothermal compression even without intercooling, due to the relatively high surface area to volume ratio of the compression room and the resulting improvement in heat dissipation from the compressor body.

To obtain a perfect isothermal storage process, the process has to be reversible. This requires that heat transfer between the environment and the gas occur over a small infinitesimal temperature difference. In that case, there is no energy loss in the heat transfer process, and so the work of compression can be completely recovered as work of expansion: 100% storage efficiency. However, in practice there is always a temperature difference in any heat transfer process, so the energy efficiency of the process begins to decrease with each difference in each process, this implies that the final storage efficiency will be less than 100%.

To estimate the work/expansion of compression in an isothermal process, it can be assumed that compressed air obeys the ideal gas law.

From a process from an initial state A to a final state B, with absolute temperature constant, one finds the work required for compression (negative) or done by expansion (positive), to be.

Where, and so, . Here, it is the absolute pressure,

is the volume of the ship, is the amount of gas substance (mol) and is the ideal gas constant.

Example.

How much energy can be stored in a 1 bar (0.1 MPa) m³ storage vessel at a pressure of 70 bars (7.0), if the ambient pressure is 1 bar (0.10 MPa). In this case, the process work is.

The negative sign means that work is done on the gas by the environment. Process irreversibilities (as in heat transfer) will result in less energy being recovered from the expansion process than is required for the compression process. If the environment is at a constant temperature, for example, the thermal resistance in the intercoolers will mean that compression occurs at a temperature slightly higher than ambient temperature, and expansion will occur at a temperature slightly lower than ambient temperature. So a perfect isothermal storage system is impossible to achieve.

Adiabatic (isentropic) storage

An adiabatic process is one where there is no heat transfer between the fluid and the surroundings: the system is insulated against heat transfer. If the process is also internally reversible (smooth, slow and frictionless, to the ideal limit) then it is also isentropic.

An adiabatic storage system does away with intercooling during the compression process, and simply allows the gas to heat up during compression, and also cool down during expansion. This is attractive, since the energy losses associated with heat transfer are avoided, but the downside is that the storage ship has to be insulated against heat loss. It should also be mentioned that actual turbines and compressors are not isentropic, but instead have an isentropic efficiency of around 85%, with the result that round-trip storage efficiency for adiabatic systems is also considerably less than perfect.

Thermodynamics of large storage system

Energy storage systems often use large underground caverns. This is the preferred system design, due to the very large volume, and thus the large amount of energy that can be stored with only a small pressure change. The cavern space can be easily insulated, compressed adiabatically with small temperature change (approaching a reversible isothermal system) and heat loss (approaching an isentropic system). This advantage is in addition to the low cost of building the gas storage system, using underground walls to assist in containing the pressure.

Recently there have been developed undersea insulated air bags, with thermodynamic properties similar to large underground cavern storage.[27]​.

Para almacenamiento de aire del uso en vehículos o aeronave para aire o tierra prácticos transporte, el sistema de almacenamiento de la energía tiene que ser compacto y ligero. Densidad de energía y la energía concreta son la ingeniería denomina aquello define estos desearon calidades.

Concrete energy, energy density and efficiency

As explained in the thermodynamics of gas storage section above, compressing the air heats it and expanding it cools it. Therefore, practical air engines require heat exchangers to avoid excessively high or low temperatures and still do not achieve ideal constant temperature conditions, or ideal thermal insulation.

However, as stated above, it is useful to describe the maximum storable energy using the isothermal case, which works out to approximately 100 kJ/m [ ln(PA/PB)].

Thus if 1.0 m³ of ambient air is very slowly compressed to a 5 L bottle at 20 MPa (200 bar), the potential energy stored is 530 kJ. A highly efficient air motor can transfer this to kinetic energy if it runs very slowly and directs the air to expand from its initial 20 MPa pressure down to 100 kPa (the bottle completely "empty" at ambient pressure). Achieving high efficiency is a technical challenge both due to heat loss to the ambient and unrecoverable internal gas heat.[28]​ If the bottle above is emptied at 1 MPa, the extractable energy is approximately 300 kJ at the motor shaft.

A standard 20 MPa, 5 L steel bottle has a mass of 7.5 kg, a higher one 5 kg. High-tensile strength fibers such as carbon-fiber or Kevlar can weigh down 2 kg in this measurement, compatible with legal safety codes. One cubic meter of air at 20 °C has a mass of 1,204 kg at standard pressure and temperature.[29] Thus, the theoretical concrete energies are approximately 70 kJ/kg in the motor shaft for a simple steel bottle to 180 kJ/kg for a fiber-advanced-wound one, while practical achievable concrete energies for the same containers would be 40 to 100 kJ/kg.

Comparison with batteries

Advanced fiber-reinforced bottles are comparable in being rechargeable, leading-acid batteries in terms of energy density. Batteries provide nearly constant voltage over their entire charge level, while pressure varies greatly while using a pressure vessel from full to empty. Technically it is 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 primary acceleration and deceleration objectives of transportation systems, particularly for hybrid vehicles.

Compressed air systems have advantages over conventional batteries that include longer pressure vessel lifetimes and lower material toxicity. Newer battery designs such as those based on Lithium Iron Phosphate chemistry also suffer from these problems. Compressed air costs are potentially lower; Advanced pressure however ships are expensive to develop and safety-test and are currently more expensive than mass-produced batteries.

So with electrical storage technology, compressed air is only as "clean" as the source of the energy it stores. Life cycle assessment addresses the issue of overall emissions from a given energy storage technology combined with a given generation mix in a power grid.

Security

So with more technologies, compressed air has safety concerns, mainly catastrophic tank rupture. Safety codes make this a rare occurrence at the cost of higher weight and additional safety features such as pressure relief valves. Codes may limit legal working pressure to less than 40% of burst pressure for steel bottles (safety factor of 2.5), and less than 20% for fiber-wound bottles (safety factor of 5). Commercial designs adopt the ISO 11439 standard.[30] High pressure bottles are strong enough so that they generally do not rupture in vehicle accidents.

History

Air engines have been used since the century to power mine locomotives, pumps, drill and trams, centralized track, city-level, distribution. Racing cars use compressed air to start their internal combustion engine (ICE), and large diesel engines may have pneumatic starting engines.

Engine

A compressed air engine uses the expansion of compressed air to drive the pistons of an engine, turn the shaft "Shaft (mechanical)"), or to drive a turbine.

A highly effective 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 heats the exhaust from the preceding stage and admits this preheated air to the next stage. Just exhausting the gas from each stage passes cold air which can be as cold as −15 °C (5 °F); Cold air can be used to air condition a car.[8]​.

Additional heat can be supplied by burning fuel when in 1904 for Whitehead torpedoes.[31] This improves the range and acceleration available for a given tank volume at the cost of the additional fuel.

Since around 1990 several companies have claimed to be developing compressed air cars, but none are available. Typically the main claimed advantages are: no roadside pollution, low cost, use of cooking oil for lubrication, and integrated air conditioner.

The time required to replace a depleted tank is important for vehicle applications. "Transfer moves" the pre-compressed air volume from a stationary tank to the vehicle tank almost instantaneously. Alternatively, a stationary or table top-compressor can compress air on demand, possibly requiring several hours.

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 primary power source. Air storage can be used for regenerative braking and to optimize the piston engine cycle which is not equally effective 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 2.9l/100 km (81 mpg, 69 g CO2/km) on the NEDC cycle for a compact frame such as the Peugeot 208. The system is claimed to be much more affordable than competing electric and flywheel KERS "Regenerative Braking") systems and is expected on road cars by 2016.[32]​.

Hybrid systems

Brayton cycle engines compress and expel heat air with a fuel suitable for an internal combustion engine. For example, natural gas or biogas heats compressed air, and then a conventional gas turbine engine or the rear portion of a jet engine expands it to produce work.

Compressed air motors can recharge an electric battery. The apparently deceased promoted his Pne-PHEV or Tire Plug-in Hybrid Electric Vehicle-energy system.[33]​.

Huntorf, Germany in 1978, and McIntosh, Alabama, USA in 1991 commissioned hybrid power plants.[9]​[34]​ Both systems use off-peak power to compress air and burn natural gas in the compressed air during the power generating phase.

The Iowa Stored Energy Park (ISEP) will use aquifer storage rather than cavern storage. The cubic capacity of water in the aquifer results in control of air pressure by the constant hydrostatic pressure of water. A spokesperson for ISEP claims, "You can optimize your equipment for efficiency better if you have a constant pressure."[34] Power output from the McIntosh and Iowa systems is in the range of 2–300 MW.

Additional facilities are under development in Norton, Ohio. FirstEnergy, an Akron, Ohio electric utility obtained development rights to the 2,700 MW Norton project in November 2009.[35]​.

Lake or ocean storage

Deep water in lakes and the ocean can provide pressure without requiring high-pressure ships or drilling into salt caverns or aquifers.[36] Air goes to inexpensive, flexible containers like plastic bags down into deep lakes or off sea shores with steep drop-offs. Obstacles include the limited number of suitable locations and the need for high-pressure piping between the surface and the containers. Since containers would be very economical, the need for large pressure (and large depth) may not be as important. A key benefit of systems built on this concept is that head and flow pressures are a constant function of depth. Carnot Inefficiencies can thus be reduced in the power plant. Carnot Efficiency can be increased by using multi-stage flow and charge and by using economical sinks and heat points such as cold water from rivers or hot water from solar ponds. Ideally, the system has to be very clever—for example, by cooling air before pumping over summer days. It must be engineered to avoid inefficiency, such as wasted pressure changes caused by inadequate piping diameter.[37]​.

A nearly isobaric solution is possible if the compressed gas is used to power a hydroelectric system. However, this solution requires large pressure tanks located above land (as well as underwater air bags). Also, hydrogen gas is the preferred fluid, since other gases suffer from substantial hydrostatic pressures and even relatively modest depths (such as 500 meters).

E.on, one of Europe's leading power and gas companies, has provided €1.4 million (£1.1 million) in funding to develop undersea air storage bags.[38]​[39]​ Hydrostor in Canada is developing a commercial "underwater" storage accumulator system for compressed air energy storage, starting at the 1 to 4 MW scale.[40]​.

There is a plan for some form of compressed air energy storage in undersea caves around Northern Ireland.[41]​.

A number of near isothermal compression methods are being developed. The fluid mechanics have a system with a heat absorbing and releasing structure (HARS) attached to a reciprocating piston.[42]​ The light sail injects a spray of water into a reciprocating cylinder.[43]​ SustainX uses an air-water foam mixture inside a compressor. All of these systems ensure that the air is compressed with high thermal diffusivity compared to the compression speed.

Typically these compressors can run at 1000 rpm. To ensure high thermal diffusivity the median distance a gas molecule is from a heat absorbing surface is approximately 0.5mm.

These near isothermal compressors can also be used as near isothermal expanders and are being developed to improve the travel efficiency of the CASO round.

Compression can be done with electrically powered turbo-compressors and expansion with turbo 'expanders' or air motors that drive electrical generators to produce electricity.[9]​.

El sistema de almacenamiento de un CAES (Almacenamiento de Energía de Aire Comprimido) es uno de las características más interesantes de esta tecnología, y es estrictamente relacionado con su viabilidad económica, densidad de energía y flexibilidad. Hay unas cuantas categorías de barcos de almacenamiento del aire, basados en las condiciones termodinámicas del almacenamiento, y en la tecnología escogida:.

Constant Volume Storage

This storage system uses a volume (a room or cavern) with rigid walls to store large amounts of air. From a thermodynamic point of view the system is understood as constant volume and variable pressure. Something that can cause operation problems in connected compressors and turbines. Pressure variations have to be kept within a safe limit.[10]​.

The storage ship is often an underground cavern created by solution mining (salt is dissolved in water for extraction) or by using an abandoned mine; Use of porous rock formations (which have holes through which liquids or air can pass) such as those in which natural gas reservoirs are found has also been studied.[11]​[12]​.

In some cases, a pipe at surface level was also tested as a storage system, providing good results.

Constant Pressure Storage

In this case the storage vessel is maintained at a constant pressure, while the gas is contained in a vessel of variable volume. Many types of storage ship have been proposed, but the operating conditions follow the same principle, the storage ship is placed hundreds of meters underwater, the hydrostatic pressure of the water column above the storage ship allows to maintain the pressure at the desired level.

This configuration leaves:

On the other hand, the cost of this storage system is higher, due to the need to place the storage ship at the bottom of the chosen water reservoir (often the sea or ocean) and due to the cost of the ship itself.[13]​.

Plants operate on a daily cycle, charging at night and releasing during the day. Heating compressed air that uses natural gas or geothermal heat to increase the amount of energy that is extracted has been studied by the Pacific Northwest National Laboratory[12]​.

Compressed air energy storage can also be employed on a smaller scale as exploited by air cars and air-driven locomotives, and also by the use of high-strength carbon-fiber air storage tanks. Even so, to retain the energy stored in compressed air, this tank would have to be thermally isolated from the environment; Furthermore, the stored energy will escape in the form of heating since compressing the air raises its temperature.

Transmission

City-wide compressed air power systems have been built since 1870.[14] Cities such as Paris, France; Birmingham, England; Dresden, Rixdorf and Offenbach, Germany and Buenos Aires, Argentina installed such systems. Victor Popp built the first systems to power clocks by sending a pulse of air every minute to change his aiming weapons. They quickly evolved to deliver power to homes and industry.[15] As of 1896, the Paris system had 2.2 MW of generation distributed at 550 kPa over 50 km of air tubes for motors in light and heavy industry. Usage was measured by meters.[14]​ The systems were the main source of home-delivered energy in those days and also powered the machines of dentists, seamstresses, printing facilities and bakeries.

Para conseguir un proceso reversible termodinámico cercano de modo que la mayoría de la energía está salvado en el sistema y puede ser recuperado, y las pérdidas están mantenidas insignificantes, un proceso isotermo reversible cercano o un isentropico el proceso está deseado.[2]​.

Isothermal storage

In an isothermal compression process, the gas in the system is maintained at a constant temperature throughout. This necessarily requires extraction of heat from the gas, which would otherwise experience a temperature increase due to the energy that has been added to the gas by the compressor. This heated extraction can be achieved by heat exchangers (intercooling) between subsequent stages in the compressor. To avoid wasted energy, the intercoolers have to be optimized for high heat transfer and low pressure drop. Naturally this is only an approximation to an isothermal compression, since heating and compression occur in discrete phases. Some smaller compressors can approach isothermal compression even without intercooling, due to the relatively high surface area to volume ratio of the compression room and the resulting improvement in heat dissipation from the compressor body.

To obtain a perfect isothermal storage process, the process has to be reversible. This requires that heat transfer between the environment and the gas occur over a small infinitesimal temperature difference. In that case, there is no energy loss in the heat transfer process, and so the work of compression can be completely recovered as work of expansion: 100% storage efficiency. However, in practice there is always a temperature difference in any heat transfer process, so the energy efficiency of the process begins to decrease with each difference in each process, this implies that the final storage efficiency will be less than 100%.

To estimate the work/expansion of compression in an isothermal process, it can be assumed that compressed air obeys the ideal gas law.

From a process from an initial state A to a final state B, with absolute temperature constant, one finds the work required for compression (negative) or done by expansion (positive), to be.

Where, and so, . Here, it is the absolute pressure,

is the volume of the ship, is the amount of gas substance (mol) and is the ideal gas constant.

Example.

How much energy can be stored in a 1 bar (0.1 MPa) m³ storage vessel at a pressure of 70 bars (7.0), if the ambient pressure is 1 bar (0.10 MPa). In this case, the process work is.

The negative sign means that work is done on the gas by the environment. Process irreversibilities (as in heat transfer) will result in less energy being recovered from the expansion process than is required for the compression process. If the environment is at a constant temperature, for example, the thermal resistance in the intercoolers will mean that compression occurs at a temperature slightly higher than ambient temperature, and expansion will occur at a temperature slightly lower than ambient temperature. So a perfect isothermal storage system is impossible to achieve.

Adiabatic (isentropic) storage

An adiabatic process is one where there is no heat transfer between the fluid and the surroundings: the system is insulated against heat transfer. If the process is also internally reversible (smooth, slow and frictionless, to the ideal limit) then it is also isentropic.

An adiabatic storage system does away with intercooling during the compression process, and simply allows the gas to heat up during compression, and also cool down during expansion. This is attractive, since the energy losses associated with heat transfer are avoided, but the downside is that the storage ship has to be insulated against heat loss. It should also be mentioned that actual turbines and compressors are not isentropic, but instead have an isentropic efficiency of around 85%, with the result that round-trip storage efficiency for adiabatic systems is also considerably less than perfect.

Thermodynamics of large storage system

Energy storage systems often use large underground caverns. This is the preferred system design, due to the very large volume, and thus the large amount of energy that can be stored with only a small pressure change. The cavern space can be easily insulated, compressed adiabatically with small temperature change (approaching a reversible isothermal system) and heat loss (approaching an isentropic system). This advantage is in addition to the low cost of building the gas storage system, using underground walls to assist in containing the pressure.

Recently there have been developed undersea insulated air bags, with thermodynamic properties similar to large underground cavern storage.[27]​.

Para almacenamiento de aire del uso en vehículos o aeronave para aire o tierra prácticos transporte, el sistema de almacenamiento de la energía tiene que ser compacto y ligero. Densidad de energía y la energía concreta son la ingeniería denomina aquello define estos desearon calidades.

Concrete energy, energy density and efficiency

As explained in the thermodynamics of gas storage section above, compressing the air heats it and expanding it cools it. Therefore, practical air engines require heat exchangers to avoid excessively high or low temperatures and still do not achieve ideal constant temperature conditions, or ideal thermal insulation.

However, as stated above, it is useful to describe the maximum storable energy using the isothermal case, which works out to approximately 100 kJ/m [ ln(PA/PB)].

Thus if 1.0 m³ of ambient air is very slowly compressed to a 5 L bottle at 20 MPa (200 bar), the potential energy stored is 530 kJ. A highly efficient air motor can transfer this to kinetic energy if it runs very slowly and directs the air to expand from its initial 20 MPa pressure down to 100 kPa (the bottle completely "empty" at ambient pressure). Achieving high efficiency is a technical challenge both due to heat loss to the ambient and unrecoverable internal gas heat.[28]​ If the bottle above is emptied at 1 MPa, the extractable energy is approximately 300 kJ at the motor shaft.

A standard 20 MPa, 5 L steel bottle has a mass of 7.5 kg, a higher one 5 kg. High-tensile strength fibers such as carbon-fiber or Kevlar can weigh down 2 kg in this measurement, compatible with legal safety codes. One cubic meter of air at 20 °C has a mass of 1,204 kg at standard pressure and temperature.[29] Thus, the theoretical concrete energies are approximately 70 kJ/kg in the motor shaft for a simple steel bottle to 180 kJ/kg for a fiber-advanced-wound one, while practical achievable concrete energies for the same containers would be 40 to 100 kJ/kg.

Comparison with batteries

Advanced fiber-reinforced bottles are comparable in being rechargeable, leading-acid batteries in terms of energy density. Batteries provide nearly constant voltage over their entire charge level, while pressure varies greatly while using a pressure vessel from full to empty. Technically it is 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 primary acceleration and deceleration objectives of transportation systems, particularly for hybrid vehicles.

Compressed air systems have advantages over conventional batteries that include longer pressure vessel lifetimes and lower material toxicity. Newer battery designs such as those based on Lithium Iron Phosphate chemistry also suffer from these problems. Compressed air costs are potentially lower; Advanced pressure however ships are expensive to develop and safety-test and are currently more expensive than mass-produced batteries.

So with electrical storage technology, compressed air is only as "clean" as the source of the energy it stores. Life cycle assessment addresses the issue of overall emissions from a given energy storage technology combined with a given generation mix in a power grid.

Security

So with more technologies, compressed air has safety concerns, mainly catastrophic tank rupture. Safety codes make this a rare occurrence at the cost of higher weight and additional safety features such as pressure relief valves. Codes may limit legal working pressure to less than 40% of burst pressure for steel bottles (safety factor of 2.5), and less than 20% for fiber-wound bottles (safety factor of 5). Commercial designs adopt the ISO 11439 standard.[30] High pressure bottles are strong enough so that they generally do not rupture in vehicle accidents.

History

Air engines have been used since the century to power mine locomotives, pumps, drill and trams, centralized track, city-level, distribution. Racing cars use compressed air to start their internal combustion engine (ICE), and large diesel engines may have pneumatic starting engines.

Engine

A compressed air engine uses the expansion of compressed air to drive the pistons of an engine, turn the shaft "Shaft (mechanical)"), or to drive a turbine.

A highly effective 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 heats the exhaust from the preceding stage and admits this preheated air to the next stage. Just exhausting the gas from each stage passes cold air which can be as cold as −15 °C (5 °F); Cold air can be used to air condition a car.[8]​.

Additional heat can be supplied by burning fuel when in 1904 for Whitehead torpedoes.[31] This improves the range and acceleration available for a given tank volume at the cost of the additional fuel.

Since around 1990 several companies have claimed to be developing compressed air cars, but none are available. Typically the main claimed advantages are: no roadside pollution, low cost, use of cooking oil for lubrication, and integrated air conditioner.

The time required to replace a depleted tank is important for vehicle applications. "Transfer moves" the pre-compressed air volume from a stationary tank to the vehicle tank almost instantaneously. Alternatively, a stationary or table top-compressor can compress air on demand, possibly requiring several hours.

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 primary power source. Air storage can be used for regenerative braking and to optimize the piston engine cycle which is not equally effective 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 2.9l/100 km (81 mpg, 69 g CO2/km) on the NEDC cycle for a compact frame such as the Peugeot 208. The system is claimed to be much more affordable than competing electric and flywheel KERS "Regenerative Braking") systems and is expected on road cars by 2016.[32]​.

Hybrid systems

Brayton cycle engines compress and expel heat air with a fuel suitable for an internal combustion engine. For example, natural gas or biogas heats compressed air, and then a conventional gas turbine engine or the rear portion of a jet engine expands it to produce work.

Compressed air motors can recharge an electric battery. The apparently deceased promoted his Pne-PHEV or Tire Plug-in Hybrid Electric Vehicle-energy system.[33]​.

Huntorf, Germany in 1978, and McIntosh, Alabama, USA in 1991 commissioned hybrid power plants.[9]​[34]​ Both systems use off-peak power to compress air and burn natural gas in the compressed air during the power generating phase.

The Iowa Stored Energy Park (ISEP) will use aquifer storage rather than cavern storage. The cubic capacity of water in the aquifer results in control of air pressure by the constant hydrostatic pressure of water. A spokesperson for ISEP claims, "You can optimize your equipment for efficiency better if you have a constant pressure."[34] Power output from the McIntosh and Iowa systems is in the range of 2–300 MW.

Additional facilities are under development in Norton, Ohio. FirstEnergy, an Akron, Ohio electric utility obtained development rights to the 2,700 MW Norton project in November 2009.[35]​.

Lake or ocean storage

Deep water in lakes and the ocean can provide pressure without requiring high-pressure ships or drilling into salt caverns or aquifers.[36] Air goes to inexpensive, flexible containers like plastic bags down into deep lakes or off sea shores with steep drop-offs. Obstacles include the limited number of suitable locations and the need for high-pressure piping between the surface and the containers. Since containers would be very economical, the need for large pressure (and large depth) may not be as important. A key benefit of systems built on this concept is that head and flow pressures are a constant function of depth. Carnot Inefficiencies can thus be reduced in the power plant. Carnot Efficiency can be increased by using multi-stage flow and charge and by using economical sinks and heat points such as cold water from rivers or hot water from solar ponds. Ideally, the system has to be very clever—for example, by cooling air before pumping over summer days. It must be engineered to avoid inefficiency, such as wasted pressure changes caused by inadequate piping diameter.[37]​.

A nearly isobaric solution is possible if the compressed gas is used to power a hydroelectric system. However, this solution requires large pressure tanks located above land (as well as underwater air bags). Also, hydrogen gas is the preferred fluid, since other gases suffer from substantial hydrostatic pressures and even relatively modest depths (such as 500 meters).

E.on, one of Europe's leading power and gas companies, has provided €1.4 million (£1.1 million) in funding to develop undersea air storage bags.[38]​[39]​ Hydrostor in Canada is developing a commercial "underwater" storage accumulator system for compressed air energy storage, starting at the 1 to 4 MW scale.[40]​.

There is a plan for some form of compressed air energy storage in undersea caves around Northern Ireland.[41]​.

A number of near isothermal compression methods are being developed. The fluid mechanics have a system with a heat absorbing and releasing structure (HARS) attached to a reciprocating piston.[42]​ The light sail injects a spray of water into a reciprocating cylinder.[43]​ SustainX uses an air-water foam mixture inside a compressor. All of these systems ensure that the air is compressed with high thermal diffusivity compared to the compression speed.

Typically these compressors can run at 1000 rpm. To ensure high thermal diffusivity the median distance a gas molecule is from a heat absorbing surface is approximately 0.5mm.

These near isothermal compressors can also be used as near isothermal expanders and are being developed to improve the travel efficiency of the CASO round.