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Compressed air energy storage

Compressed air energy storage

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Encyclopedia
Compressed Air Energy Storage (CAES) is a way to store energy
Energy storage
Energy storage is accomplished by devices or physical media that store some form of energy to perform some useful operation at a later time. A device that stores energy is sometimes called an accumulator....

 generated at one time for use at another time. At utility scale, energy generated during periods of low energy demand (off-peak) can be released to meet higher demand (peak load) periods.

Types


Compression of air generates a lot of heat. The air is warmer after compression. Decompression requires heat. If no extra heat is added, the air will be much colder after decompression.
If the heat generated during compression can be stored and used again during decompression, the efficiency of the storage improves considerably.

There are three ways in which a CAES system can deal with the heat.
Air storage can be adiabatic, diabatic
Diabatic
A diabatic process is one in which heat transfer takes place, which is the opposite of an adiabatic process. In quantum chemistry, the potential energy surfaces are obtained within the adiabatic or Born-Oppenheimer approximation...

, or isothermic:
  • Adiabatic storage retains the heat produced by compression and returns it to the air when the air is expanded to generate power. This is a subject of ongoing study, with no utility scale plants as of 2010. Its theoretical efficiency
    Energy conversion efficiency
    Energy conversion efficiency is the ratio between the useful output of an energy conversion machine and the input, in energy terms. The useful output may be electric power, mechanical work, or heat.-Overview:...

     approaches 100% for large and/or rapidly cycled devices and/or perfect thermal insulation, but in practice round trip efficiency is expected to be 70%. Heat can be stored in a solid such as concrete or stone, or more likely in a fluid such as hot oil (up to 300 °C) or molten salt solutions (600 °C).

  • Diabatic storage dissipates the extra heat with intercooler
    Intercooler
    An intercooler , or charge air cooler, is an air-to-air or air-to-liquid heat exchange device used on turbocharged and supercharged internal combustion engines to improve their volumetric efficiency by increasing intake air charge density through nearly isobaric cooling, which removes...

    s (thus approaching isothermal compression) into the atmosphere as waste. Upon removal from storage, the air must be re-heated prior to expansion in the turbine
    Turbine
    A turbine is a rotary engine that extracts energy from a fluid flow and converts it into useful work.The simplest turbines have one moving part, a rotor assembly, which is a shaft or drum with blades attached. Moving fluid acts on the blades, or the blades react to the flow, so that they move and...

     to power a generator
    Electrical generator
    In electricity generation, an electric generator is a device that converts mechanical energy to electrical energy. A generator forces electric charge to flow through an external electrical circuit. It is analogous to a water pump, which causes water to flow...

     which can be accomplished with a natural gas
    Natural gas
    Natural gas is a naturally occurring gas mixture consisting primarily of methane, typically with 0–20% higher hydrocarbons . It is found associated with other hydrocarbon fuel, in coal beds, as methane clathrates, and is an important fuel source and a major feedstock for fertilizers.Most natural...

     fired burner for utility
    Utility
    In economics, utility is a measure of customer satisfaction, referring to the total satisfaction received by a consumer from consuming a good or service....

     grade storage or with a heated metal
    Metal
    A metal , is an element, compound, or alloy that is a good conductor of both electricity and heat. Metals are usually malleable and shiny, that is they reflect most of incident light...

     mass. The lost heat degrades efficiency, but this approach is simpler and is thus far the only system which has been implemented commercially. The McIntosh, Alabama CAES plant requires 2.5 MJ of electricity and 1.2 MJ lower heating value (LHV) of gas for each megajoule of energy output. A General Electric
    General Electric
    General Electric Company , or GE, is an American multinational conglomerate corporation incorporated in Schenectady, New York and headquartered in Fairfield, Connecticut, United States...

     7FA 2x1 combined cycle
    Combined cycle
    In electric power generation a combined cycle is an assembly of heat engines that work in tandem off the same source of heat, converting it into mechanical energy, which in turn usually drives electrical generators...

     plant, one of the most efficient natural gas plants in operation, uses 6.6 MJ (LHV) of gas per kW–h generated, a 54% thermal efficiency comparable to the McIntosh 6.8 MJ, at 53% thermal efficiency.

  • Isothermal compression and expansion approaches attempt to maintain operating temperature
    Operating temperature
    An operating temperature is the temperature at which an electrical or mechanical device operates. The device will operate effectively within a specified temperature range which varies based on the device function and application context, and ranges from the minimum operating temperature to the...

     by constant heat exchange to the environment. They are only practical for low power levels, without very effective heat exchanger
    Heat exchanger
    A heat exchanger is a piece of equipment built for efficient heat transfer from one medium to another. The media may be separated by a solid wall, so that they never mix, or they may be in direct contact...

    s. The theoretical efficiency of isothermal energy storage approaches 100% for small and/or slowly cycled devices and/or perfect heat transfer to the environment. In practice neither of these perfect thermodynamic cycles are obtainable, as some heat losses are unavoidable.


A different, highly efficient arrangement, which fits neatly into none of the above categories, uses high, medium and low pressure pistons in series, with each stage followed by an airblast venturi pump that draws ambient air over an air-to-air (or air-to-seawater) heat exchanger between each expansion stage. Early compressed air torpedo designs used a similar approach, substituting seawater for air. The venturi warms the exhaust
Exhaust gas
Exhaust gas or flue gas is emitted as a result of the combustion of fuels such as natural gas, gasoline/petrol, diesel fuel, fuel oil or coal. According to the type of engine, it is discharged into the atmosphere through an exhaust pipe, flue gas stack or propelling nozzle.It often disperses...

 of the preceding stage and admits this preheated air to the following stage. This approach was widely adopted in various compressed air vehicles such as H. K. Porter, Inc
H. K. Porter, Inc
H. K. Porter, Inc. manufactured light-duty railroad locomotives in the USA, starting in 1866. The company became the largest producer of industrial locomotives, and built almost eight thousand of them...

's mining
Mining
Mining is the extraction of valuable minerals or other geological materials from the earth, from an ore body, vein or seam. The term also includes the removal of soil. Materials recovered by mining include base metals, precious metals, iron, uranium, coal, diamonds, limestone, oil shale, rock...

 locomotives and tram
Tram
A tram is a passenger rail vehicle which runs on tracks along public urban streets and also sometimes on separate rights of way. It may also run between cities and/or towns , and/or partially grade separated even in the cities...

s. Here the heat of compression is effectively stored in the atmosphere (or sea) and returned later on.

Compression can be done with electrically powered turbo-compressors and expansion with turbo 'expanders' or air engines driving electrical generator
Electrical generator
In electricity generation, an electric generator is a device that converts mechanical energy to electrical energy. A generator forces electric charge to flow through an external electrical circuit. It is analogous to a water pump, which causes water to flow...

s to produce electricity.

The storage vessel is often an underground cavern created by solution mining (salt
Salt
In chemistry, salts are ionic compounds that result from the neutralization reaction of an acid and a base. They are composed of cations and anions so that the product is electrically neutral...

 is dissolved in water for extraction) or by utilizing an abandoned mine
Mining
Mining is the extraction of valuable minerals or other geological materials from the earth, from an ore body, vein or seam. The term also includes the removal of soil. Materials recovered by mining include base metals, precious metals, iron, uranium, coal, diamonds, limestone, oil shale, rock...

. Plants operate on a daily cycle, charging at night and discharging during the day.

Compressed air energy storage can also be employed on a smaller scale such as exploited by air car
Air car
A compressed air car is a car that uses a motor powered by compressed air. The car can be powered solely by air, or combined with gasoline, diesel, ethanol, or an electric plant with regenerative braking.-Engines:...

s and air-driven locomotive
Locomotive
A locomotive is a railway vehicle that provides the motive power for a train. The word originates from the Latin loco – "from a place", ablative of locus, "place" + Medieval Latin motivus, "causing motion", and is a shortened form of the term locomotive engine, first used in the early 19th...

s, and also by the use of high-strength carbon-fiber air storage tanks.

History


City-wide compressed air energy systems have been built since 1870. Cities such as Paris
Paris
Paris is the capital and largest city in France, situated on the river Seine, in northern France, at the heart of the Île-de-France region...

, Birmingham, England, Rixdorf, Offenbach, Dresden in Germany and Buenos Aires, Argentina installed such systems. Victor Popp constructed the first systems to power clocks by sending a pulse of air every minute to change the pointer. They quickly evolved to deliver power to homes and industry. As of 1896, the Paris system had 2.2 MW of generation distributed at 550 kPa in 50 km of air pipes for motors in light and heavy industry. Usage was measured by meters. The systems were the main source of house-delivered energy in these days and also powered the machines of dentists
Dentistry
Dentistry is the branch of medicine that is involved in the study, diagnosis, prevention, and treatment of diseases, disorders and conditions of the oral cavity, maxillofacial area and the adjacent and associated structures and their impact on the human body. Dentistry is widely considered...

, seamstresses, printing facilities and bakeries
Bakery
A bakery is an establishment which produces and sells flour-based food baked in an oven such as bread, cakes, pastries and pies. Some retail bakeries are also cafés, serving coffee and tea to customers who wish to consume the baked goods on the premises.-See also:*Baker*Cake...

. The first utility-scale compressed air energy storage project was the 290 megawatt Huntorf plant in Germany (1978). The second was the 110 megawatt McIntosh plant in Alabama (1991). Both of these projects use salt domes for air caverns. Currently under development is the Iowa Stored Energy Park, a 270 megawatt project which will use aquifer-based air storage. A 300 MW project utilizing depleted gas storage is being developed in California, and a 150 MW salt-based project is under development in upstate New York. The first adiabatic CAES project, a 200 megawatt facility called ADELE, is planned for construction in Germany in 2013.

Isothermal storage physics


The isothermal process
Isothermal process
An isothermal process is a change of a system, in which the temperature remains constant: ΔT = 0. This typically occurs when a system is in contact with an outside thermal reservoir , and the change occurs slowly enough to allow the system to continually adjust to the temperature of the reservoir...

 maintains constant temperature. The heat that compression generates must flow to the environment for the temperature to remain constant. In practice this is often not the case, because proper intercooling requires a compact internal heat exchanger
Heat exchanger
A heat exchanger is a piece of equipment built for efficient heat transfer from one medium to another. The media may be separated by a solid wall, so that they never mix, or they may be in direct contact...

 that is optimized for high heat transfer
Heat transfer
Heat transfer is a discipline of thermal engineering that concerns the exchange of thermal energy from one physical system to another. Heat transfer is classified into various mechanisms, such as heat conduction, convection, thermal radiation, and phase-change transfer...

 and low pressure drop. Without an internal heat exchanger, isothermal compression can be approximated only at low flow rates, particularly for small systems. Small compressors have higher inherent heat exchange, due to a higher ratio of surface area to volume. Nevertheless it is useful to describe the limiting case of ideal isothermal compression of an ideal gas
Ideal gas
An ideal gas is a theoretical gas composed of a set of randomly-moving, non-interacting point particles. The ideal gas concept is useful because it obeys the ideal gas law, a simplified equation of state, and is amenable to analysis under statistical mechanics.At normal conditions such as...

:

The ideal gas law
Ideal gas law
The ideal gas law is the equation of state of a hypothetical ideal gas. It is a good approximation to the behavior of many gases under many conditions, although it has several limitations. It was first stated by Émile Clapeyron in 1834 as a combination of Boyle's law and Charles's law...

 for an isothermal process is:
By the definition of work, where A and B are the initial and final states of the system:

where, , and so,
is the absolute pressure
Pressure
Pressure is the force per unit area applied in a direction perpendicular to the surface of an object. Gauge pressure is the pressure relative to the local atmospheric or ambient pressure.- Definition :...

, is the volume
Volume
Volume is the quantity of three-dimensional space enclosed by some closed boundary, for example, the space that a substance or shape occupies or contains....

 of the vessel, is the amount of substance
Amount of substance
Amount of substance is a standards-defined quantity that measures the size of an ensemble of elementary entities, such as atoms, molecules, electrons, and other particles. It is sometimes referred to as chemical amount. The International System of Units defines the amount of substance to be...

 of gas, is the ideal gas constant, is the absolute temperature, is the energy
Energy
In physics, energy is an indirectly observed quantity. It is often understood as the ability a physical system has to do work on other physical systems...

 stored or released.

This amounts to about 2.271 ln(PA/PB) kJ at 0 °C (273.2 K) or 2.478 ln(PA/PB) kilojoules (kJ) at 25 °C (298.2 K), per mole
Mole (unit)
The mole is a unit of measurement used in chemistry to express amounts of a chemical substance, defined as an amount of a substance that contains as many elementary entities as there are atoms in 12 grams of pure carbon-12 , the isotope of carbon with atomic weight 12. This corresponds to a value...

, or
simply 100 ln(PA/PB) kJ/m³ of gas (at 0.1 megapascal = approx. atmospheric pressure).

Isothermal processes are thermodynamically reversible. To the extent the compression process is isothermal, its efficiency approaches 100%. The above equation represents the maximum energy that can be stored.
To be more accurate, that energy isn't stored into compressed air. Instead the energy given to compress the air is converted into heat that is expelled to the ambient. So the energy is stored into the ambient while the air is just the vehicle to do that. This is true because according to thermodynamics ( for the case of a perfect gas ) the internal energy of the gas is given by E=(df/2)*N*k*T where df are the degrees of freedom ( 3 for a monoatomic gas, 5 for a biatomic), N the number of particles, k the Boltzmann constant.
So a isothermal compression just diminishes the entropy of air while converting work to heat that is stored in the ambient. Inversely that air may be used at a later time to convert heat into work. But the heat must be taken from the ambient or somewhere else.
In practice, no process is perfectly isothermal and the compressors and motors will have heat-related energy losses.

Under adiabatic compression, some of the compression work goes into heating the gas. If this heat is then lost to the surroundings, and the same quantity of heat is not added to the gas upon expansion, efficiency is reduced. Energy storage systems often use large underground caverns. This is the preferred system design, due to the very large volume, and thus the large quantity of energy that can be stored with only a small pressure change. The cavern space can be compressed adiabatically with little temperature change and heat loss.

Energy density and efficiency


Compressing air heats it and expanding it cools it. Therefore practical air engines require heat exchangers in order to avoid excessively high or low temperatures and even so don't reach ideal constant temperature conditions. Nevertheless 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 ambient air is very slowly compressed into a 5 L bottle at 20 MPa, 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 ambient pressure). Achieving high efficiency is a technical challenge both due to nonlinear energy storage and thermodynamic considerations. If the bottle above is emptied to 1 MPa, 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, a superior one 5 kg. High-tensile strength fibers such as carbon-fiber or Kevlar
Kevlar
Kevlar is the registered trademark for a para-aramid synthetic fiber, related to other aramids such as Nomex and Technora. Developed at DuPont in 1965, this high strength material was first commercially used in the early 1970s as a replacement for steel in racing tires...

 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 1.225 kg. Thus, theoretical energy densities 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 energy densities for the same containers would be from 40 to 100 kJ/kg.

Comparison with batteries


Advanced fiber-reinforced bottles are comparable to the rechargeable
Rechargeable battery
A rechargeable battery or storage battery is a group of one or more electrochemical cells. They are known as secondary cells because their electrochemical reactions are electrically reversible. Rechargeable batteries come in many different shapes and sizes, ranging anything from a button cell to...

 lead-acid battery
Lead-acid battery
Lead–acid batteries, invented in 1859 by French physicist Gaston Planté, are the oldest type of rechargeable battery. Despite having a very low energy-to-weight ratio and a low energy-to-volume ratio, their ability to supply high surge currents means that the cells maintain a relatively large...

 in terms of energy density. Advanced battery systems are several times better. Batteries also 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 vessel
Pressure vessel
A pressure vessel is a closed container designed to hold gases or liquids at a pressure substantially different from the ambient pressure.The pressure differential is dangerous and many fatal accidents have occurred in the history of their development and operation. Consequently, their design,...

s and lower material toxicity. Newer battery designs such as those based on Lithium Iron Phosphate
Lithium iron phosphate battery
The lithium iron phosphate battery, also called LFP battery, is a type of rechargeable battery, specifically a lithium-ion battery, which uses LiFePO4 as a cathode material.-History:...

 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
Life cycle assessment
A life-cycle assessment is a technique to assess environmental impacts associated with all the stages of a product's life from-cradle-to-grave A life-cycle assessment (LCA, also known as life-cycle analysis, ecobalance, and cradle-to-grave analysis) is a technique to assess environmental impacts...

 addresses the question of overall emissions from a given energy storage technology combined with a given mix of generation on a power grid.

Safety


As with most technologies, compressed air has safety concerns, mainly catastrophic tank rupture. Safety codes make this a rare occurrence at the cost of higher weight. Codes may limit the legal working pressure to less than 40% of the rupture 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. High pressure bottles are fairly strong so that they generally do not rupture in vehicle crashes.

History


Air engines have been used since the 19th century to power mine
Mining
Mining is the extraction of valuable minerals or other geological materials from the earth, from an ore body, vein or seam. The term also includes the removal of soil. Materials recovered by mining include base metals, precious metals, iron, uranium, coal, diamonds, limestone, oil shale, rock...

 locomotives, pumps, drills and tram
Tram
A tram is a passenger rail vehicle which runs on tracks along public urban streets and also sometimes on separate rights of way. It may also run between cities and/or towns , and/or partially grade separated even in the cities...

s, via centralized, city-level, distribution.

Racecars use compressed air to start its internal combustion engine
Internal combustion engine
The internal combustion engine is an engine in which the combustion of a fuel occurs with an oxidizer in a combustion chamber. In an internal combustion engine, the expansion of the high-temperature and high -pressure gases produced by combustion apply direct force to some component of the engine...

 (ICE).

Many people have been working on the idea of compressed air vehicles with renewed interest since the 1990 oil price shock.

Engine


A compressed air engine uses the expansion of compressed air to drive the pistons of an engine, turn the axle
Axle
An axle is a central shaft for a rotating wheel or gear. On wheeled vehicles, the axle may be fixed to the wheels, rotating with them, or fixed to its surroundings, with the wheels rotating around the axle. In the former case, bearings or bushings are provided at the mounting points where the axle...

, or to drive a turbine
Turbine
A turbine is a rotary engine that extracts energy from a fluid flow and converts it into useful work.The simplest turbines have one moving part, a rotor assembly, which is a shaft or drum with blades attached. Moving fluid acts on the blades, or the blades react to the flow, so that they move and...

.

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
    Heat engine
    In thermodynamics, a heat engine is a system that performs the conversion of heat or thermal energy to mechanical work. It does this by bringing a working substance from a high temperature state to a lower temperature state. A heat "source" generates thermal energy that brings the working substance...

     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
Heat exchanger
A heat exchanger is a piece of equipment built for efficient heat transfer from one medium to another. The media may be separated by a solid wall, so that they never mix, or they may be in direct contact...

. 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 −15 °C; the cold air may be used for air conditioning
Air conditioning
An air conditioner is a home appliance, system, or mechanism designed to dehumidify and extract heat from an area. The cooling is done using a simple refrigeration cycle...

 in a car.

Additional heat can be supplied by burning fuel as in 1904 for Whitehead's torpedoes. This improves the range and speed available for a given tank volume at the cost of the additional fuel.

As an alternative to pistons or turbines, the Quasiturbine
Quasiturbine
The Quasiturbine or Qurbine engine is a proposed pistonless rotary engine using a rhomboidal rotor whose sides are hinged at the vertices. The volume enclosed between the sides of the rotor and the rotor casing provide compression and expansion in a fashion similar to the more familiar Wankel...

  is also capable of running on compressed air, and is thus also a compressed air engine.

Cars


Since about 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
Lubrication
Lubrication is the process, or technique employed to reduce wear of one or both surfaces in close proximity, and moving relative to each another, by interposing a substance called lubricant between the surfaces to carry or to help carry the load between the opposing surfaces. The interposed...

, 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
Gas compressor
A gas compressor is a mechanical device that increases the pressure of a gas by reducing its volume.Compressors are similar to pumps: both increase the pressure on a fluid and both can transport the fluid through a pipe. As gases are compressible, the compressor also reduces the volume of a gas...

 can compress air on demand, possibly requiring several hours. The cost of driving such car is typically projected to be around €0.75 per 100 kilometres (62.1 mi) with a complete refill at a service station costing about US$3.

Cryogenic systems


A special CAES system has been created that uses liquid air
Liquid air
Liquid air is air that has been cooled to very low temperatures so that it has condensed to a pale blue mobile liquid. To protect it from room temperature, it must be kept in a vacuum flask. Liquid air can absorb heat rapidly and revert to its gaseous state...

 as an energy carrier. This system is called Highview Power Storage's CryoEnergy System (CES).

Hybrid systems


Brayton cycle
Brayton cycle
The Brayton cycle is a thermodynamic cycle that describes the workings of the gas turbine engine, basis of the airbreathing jet engine and others. It is named after George Brayton , the American engineer who developed it, although it was originally proposed and patented by Englishman John Barber...

 engines compress and heat air with a fuel suitable for an internal combustion engine
Internal combustion engine
The internal combustion engine is an engine in which the combustion of a fuel occurs with an oxidizer in a combustion chamber. In an internal combustion engine, the expansion of the high-temperature and high -pressure gases produced by combustion apply direct force to some component of the engine...

. For example, natural gas
Natural gas
Natural gas is a naturally occurring gas mixture consisting primarily of methane, typically with 0–20% higher hydrocarbons . It is found associated with other hydrocarbon fuel, in coal beds, as methane clathrates, and is an important fuel source and a major feedstock for fertilizers.Most natural...

 or biogas
Biogas
Biogas typically refers to a gas produced by the biological breakdown of organic matter in the absence of oxygen. Organic waste such as dead plant and animal material, animal dung, and kitchen waste can be converted into a gaseous fuel called biogas...

 heat compressed air, and then a conventional gas turbine
Gas turbine
A gas turbine, also called a combustion turbine, is a type of internal combustion engine. It has an upstream rotating compressor coupled to a downstream turbine, and a combustion chamber in-between....

 engine or the rear portion of a jet engine
Jet engine
A jet engine is a reaction engine that discharges a fast moving jet to generate thrust by jet propulsion and in accordance with Newton's laws of motion. This broad definition of jet engines includes turbojets, turbofans, rockets, ramjets, pulse jets...

 expands it to produce work.

Compressed air engines can recharge an electric battery
Battery (electricity)
An electrical battery is one or more electrochemical cells that convert stored chemical energy into electrical energy. Since the invention of the first battery in 1800 by Alessandro Volta and especially since the technically improved Daniell cell in 1836, batteries have become a common power...

. The apparently defunct Energine promoted its Pne-PHEV or Pneumatic Plug-in Hybrid Electric Vehicle-system).

Existing hybrid systems


Huntorf, Germany in 1978, and McIntosh, Alabama
McIntosh, Alabama
McIntosh is a town located in Washington County, Alabama, along U.S. Highway 43. It is 12½ miles south of Wagarville and north of Mobile. It was named for Alexander McIntosh, a prominent Creek chief of the nineteenth century...

 in 1991 (USA) commissioned hybrid power plants. Both systems use off-peak energy for air compression. The McIntosh plant achieves its 24-hour operating cycle by burning a natural gas/compressed air mix.

Future hybrid systems


The Iowa Stored Energy Park
Iowa Stored Energy Park
The Iowa Stored Energy Park will use electrical energy generated from a large wind farm located in Iowa. This wind power will be used to store compressed air in an underground geologic structure...

 (ISEP) will use aquifer
Aquifer
An aquifer is a wet underground layer of water-bearing permeable rock or unconsolidated materials from which groundwater can be usefully extracted using a water well. The study of water flow in aquifers and the characterization of aquifers is called hydrogeology...

 storage rather than cavern storage. The displacement of water in the aquifer results in regulation of the air pressure by the constant hydrostatic pressure of the water. A spokesperson for ISEP claims, "you can optimize your equipment for better efficiency if you have a constant pressure." Power output of the McIntosh and Iowa systems is in the range of 2–300 MW.

Additional facilities are under development in Norton, Ohio
Norton, Ohio
Norton is a city in Summit and Wayne counties in the U.S. state of Ohio. The population was 11,523 at the 2000 census.The Summit County portion of Norton is part of the Akron Metropolitan Statistical Area, while the small portion in Wayne County is part of the Wooster Micropolitan Statistical...

. FirstEnergy
FirstEnergy
FirstEnergy Corp. , is a diversified energy company headquartered in Akron, Ohio. Its subsidiaries and affiliates are involved in the generation, transmission, and distribution of electricity, as well as energy management and other energy-related services...

, an Akron, Ohio electric utility obtained development rights to the 2,700 MW Norton project in November, 2009.

Lake or ocean storage


Deep water in lakes and the ocean can provide pressure without requiring high-pressure vessels or drilling into salt caverns or aquifers. The air goes into inexpensive, flexible containers such as plastic bags below in deep lakes or off sea coasts with steep drop-offs. Obstacles include the limited number of suitable locations and the need for high-pressure pipelines between the surface and the containers. Since the containers would be very inexpensive, the need for great pressure (and great depth) may not be as important. A key benefit of systems built on this concept is that charge and discharge pressures are a constant function of depth. Carnot
Carnot heat engine
A Carnot heat engine is a hypothetical engine that operates on the reversible Carnot cycle. The basic model for this engine was developed by Nicolas Léonard Sadi Carnot in 1824...

 inefficiencies can thereby be reduced in the power plant. Carnot efficiency can be increased by using multiple charge and discharge stages and using inexpensive heat sources and sinks such as cold water from rivers or hot water from solar pond
Solar pond
A solar pond is a pool of saltwater which acts as a large-scale solar thermal energy collector with integral heat storage for supplying thermal energy...

s. Ideally, the system must be very clever—for example, by cooling air before pumping on summer days. It must be engineered to avoid inefficiency, such as wasteful pressure changes caused by inadequate piping diameter.

A nearly isobaric
Isobaric
Isobaric may refer to:*in thermodynamics, an isobaric process, i.e. one that is carried out at constant pressure;...

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

The University of Nottingham
University of Nottingham
The University of Nottingham is a public research university based in Nottingham, United Kingdom, with further campuses in Ningbo, China and Kuala Lumpur, Malaysia...

 is one centre of research on seabed–anchored energy bags. 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.

Hydrostor in Canada is developing a commercial system of underwater storage "accumulators" for compressed air energy storage, starting at the 1 to 4 MW scale.

See also



  • Alternative fuel
    Alternative fuel
    Alternative fuels, known as non-conventional or advanced fuels, are any materials or substances that can be used as fuels, other than conventional fuels...

  • Alternative propulsion
  • Battery electric vehicle
    Battery electric vehicle
    A battery electric vehicle, or BEV, is a type of electric vehicle that uses chemical energy stored in rechargeable battery packs. BEVs use electric motors and motor controllers instead of, or in addition to, internal combustion engines for propulsion.A battery-only electric vehicle or...

  • Compressed air battery
    Compressed air battery
    A compressed air battery stores energy in the form of compressed air and uses it to generate electricity.. A compressed air battery uses air to drive a scroll expander or other device, which in turn drives a conventional generator, to produce electricity...

  • Compressed air vehicle (CAV)
  • Compressed air
    Compressed air
    Compressed air is air which is kept under a certain pressure, usually greater than that of the atmosphere. In Europe, 10 percent of all electricity used by industry is used to produce compressed air, amounting to 80 terawatt hours consumption per year....

     (other uses)
  • Compressed natural gas
    Compressed natural gas
    Compressed natural gas is a fossil fuel substitute for gasoline , diesel, or propane/LPG. Although its combustion does produce greenhouse gases, it is a more environmentally clean alternative to those fuels, and it is much safer than other fuels in the event of a spill...

     cylinders.
  • Fireless locomotive
    Fireless locomotive
    A fireless locomotive is a type of locomotive designed for use under conditions restricted by either the presence of flammable material or the need for cleanliness...

  • Grid energy storage
    Grid energy storage
    Grid energy storage refers to the methods used to store electricity on a large scale within an electrical power grid. Electrical energy is stored during times when production exceeds consumption and the stores are used at times when consumption exceeds production...

  • Iowa Stored Energy Park
    Iowa Stored Energy Park
    The Iowa Stored Energy Park will use electrical energy generated from a large wind farm located in Iowa. This wind power will be used to store compressed air in an underground geologic structure...

  • Liquid nitrogen economy
  • Pneumatic motor
    Pneumatic motor
    A pneumatic motor or compressed air engine is a type of motor which does mechanical work by expanding compressed air. Pneumatic motors generally convert the compressed air to mechanical work through either linear or rotary motion...

  • Pneumatics
    Pneumatics
    Pneumatics is a branch of technology, which deals with the study and application of use of pressurized gas to effect mechanical motion.Pneumatic systems are extensively used in industry, where factories are commonly plumbed with compressed air or compressed inert gases...

  • Pumped-storage hydroelectricity
    Pumped-storage hydroelectricity
    Pumped-storage hydroelectricity is a type of hydroelectric power generation used by some power plants for load balancing. The method stores energy in the form of water, pumped from a lower elevation reservoir to a higher elevation. Low-cost off-peak electric power is used to run the pumps...

  • Vehicle-to-grid
    Vehicle-to-grid
    Vehicle-to-grid describes a system in which plug-in electric vehicles, such as electric cars and plug-in hybrids , communicate with the power grid to sell demand response services by either delivering electricity into the grid or by throttling their charging rate.Vehicle-to-grid can be used with...

  • Zero-emissions vehicle
    Zero-emissions vehicle
    A zero-emissions vehicle, or ZEV, is a vehicle that emits no tailpipe pollutants from the onboard source of power. Harmful pollutants to the health and the environment include particulates , hydrocarbons, carbon monoxide, ozone, lead, and various oxides of nitrogen. Although not considered emission...


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