Arabic Bulgarian Chinese Croatian Czech Danish Dutch English Estonian Finnish French German Greek Hebrew Hindi Hungarian Icelandic Indonesian Italian Japanese Korean Latvian Lithuanian Malagasy Norwegian Persian Polish Portuguese Romanian Russian Serbian Slovak Slovenian Spanish Swedish Thai Turkish Vietnamese
Arabic Bulgarian Chinese Croatian Czech Danish Dutch English Estonian Finnish French German Greek Hebrew Hindi Hungarian Icelandic Indonesian Italian Japanese Korean Latvian Lithuanian Malagasy Norwegian Persian Polish Portuguese Romanian Russian Serbian Slovak Slovenian Spanish Swedish Thai Turkish Vietnamese

definition - Compressed_air_energy_storage

definition of Wikipedia

   Advertizing ▼


Compressed air energy storage


Compressed Air Energy Storage (CAES) is a way to store energy 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.[1]



Compression of air generates heat; the air is warmer after compression. Expansion requires heat. If no extra heat is added, the air will be much colder after expansion. If the heat generated during compression can be stored and used during expansion, 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, 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, but a German project ADELE is planned to enter development in 2013[2]. The theoretical efficiency of adiabatic storage approaches 100% with perfect insulation, but in practice round trip efficiency is expected to be 70%.[3] 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 intercoolers (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 to power a generator which can be accomplished with a natural gas fired burner for utility grade storage or with a heated metal 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.[4] A General Electric 7FA 2x1 combined cycle plant, one of the most efficient natural gas plants in operation, uses 6.6 MJ (LHV) of gas per kW–h generated,[5] 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 by constant heat exchange to the environment. They are only practical for low power levels, without very effective heat exchangers. The theoretical efficiency of isothermal energy storage approaches 100% for 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 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's mining locomotives[6] and trams.[7] 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'[8] or air engines driving electrical generators to produce electricity.

The storage vessel is often an underground cavern created by solution mining (salt is dissolved in water for extraction)[9] or by utilizing an abandoned mine. 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 cars and air-driven locomotives, and also by the use of high-strength carbon-fiber air storage tanks.


City-wide compressed air energy systems have been built since 1870.[10] Cities such as Paris, France; Birmingham, England; Rixdorf, Germany; Offenbach, Germany; Dresden, 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.[11] 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.[10] The systems were the main source of house-delivered energy in these days and also powered the machines of dentists, seamstresses, printing facilities and bakeries.

  • 1978— The first utility-scale compressed air energy storage project was the 290 megawatt Huntorf plant in Germany using a salt dome.
  • 1991— A 110 megawatt plant with a capacity of 26 hours was built in McIntosh, Alabama (1991). The Alabama facility's $65 million cost works out to $550 per Kilowatt hour of capacity, using a 19 million square foot solution mined salt cavern to store air at up to 1100 psi. Although the compression phase is approximately 82% efficient, the expansion phase requires combustion of natural gas at one third the rate of a gas turbine producing the same amount of electricity.[12][13][14]
  • November 2009— The US Department of energy awards $24.9 million in matching funds for phase one of a 300 MW, $356 million Pacific Gas and Electric CAES installation utilizing a saline porous rock formation being developed near Bakersfield in Kern County, California. Goals of the project is to build and validate an advanced design.[15]
  • December, 2010— DOE provides $29.4 million in funding to conduct preliminary work on a 150 MW salt-based CAES project being developed by Iberdrola USA in Watkins Glen, New York. The goal is to incorporate smart grid technology to balance intermittent renewable energy sources.[16][15]
  • 2013 (projected)— The first adiabatic CAES project, a 200 megawatt facility called ADELE, is planned for construction in Germany.

  Storage thermodynamics

In order to achieve a near thermodynamic reversible process so that most of the energy is saved in the system and can be retrieved, and losses are kept negligible, a near reversible isothermal process or an isentropic process is desired.

  Isothermal storage

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

To obtain a perfect isothermal storage process, we need the process to be reversible. This requires that the heat transfer between the surroundings and the gas occur over an infinitesimally small temperature difference. In that case, there is no exergy loss in the heat transfer process, and so the compression work can be completely recovered as expansion work: 100% storage efficiency. However, in practise, there is always a temperature difference in any heat transfer process, and so all practical energy storage obtains efficiencies significantly lower than 100%.

To estimate the compression/expansion work in an isothermal process, we can assume that the compressed air obeys the ideal gas law,


From a process from an initial state A to a final state B, with absolute temperature T = T_A = T_B constant, we find that the work required for compression (negative) or done by the expansion (positive), to be

W_{A\to B} & = \int_{V_A}^{V_B} p dV = \int_{V_A}^{V_B} \frac{nRT}{V} dV = nRT\int_{V_A}^{V_B} \frac{1}{V} dV \\
 & = nRT(\ln{V_B}-\ln{V_A}) = nRT\ln{\frac{V_B}{V_A}} = nRT\ln{\frac{p_A}{p_B}} = p_A V_A\ln{\frac{p_A}{p_B}} \\

where pV = p_A V_A = p_B V_B, and so, \frac{V_B}{V_A} = \frac{p_A}{p_B} . Here, p is the absolute pressure, V is the volume of the vessel, n is the amount of substance of gas (mol) and R is the ideal gas constant.


How much energy can we store in a 1m³ storage vessel at a pressure of 7 000 000 Pa (=70 bar), if the ambient pressure is 100 000 Pa (=1 bar) In this case, the process work is

W = p_B v_B \ln \frac{p_A}{p_B} = 70 bar × 1 m³ × ln(1 bar/70 bar) = -29 MJ.

The negative sign means that work is done on the gas by the surroundings. Process irreversibilities (such 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 the compression occurs at a temperature somewhat higher than the ambient temperature, and the expansion will occur at a temperature somewhat 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 furthermore internally reversible (smooth, slow and frictionless, to the ideal limit) then it will additionally be isentropic.

An adiabatic storage system does away with the intercooling during the compression process, and simply allows the gas to heat up during compression, and likewise to cool down during expansion. This is attractive, since the energy losses associated with the heat transfer are avoided, but the downside is that the storage vessel must be insulated against heat loss. It should also be mentioned that real compressors and turbines 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.

  Large storage system thermodynamics

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 easily insulated, compressed adiabatically with little temperature change (approaching a reversible isothermal system) and heat loss (approaching an isentropic system). This advantage is in addition to the low cost of constructing the gas storage system, using the underground walls to assist in containing the pressure.

Recently there have been developed undersea insulated air bags, with similar thermodynamic properties to large underground cavern storage.[17]

  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 is the engineering term that defines these desired qualities.

  Energy density and efficiency

As explained in the thermodynamics of gas storage section above, 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, 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 ambient air is very slowly compressed into a 5 L bottle at 20 MPa (200 bar), 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 heat loss to the ambient and to unrecoverable internal gas heat.[18] 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 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.[19] 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 lead-acid battery in terms of energy density. 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 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.


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.[20] High pressure bottles are fairly strong so that they generally do not rupture in vehicle crashes.

  Vehicle applications


Air engines have been used since the 19th century to power mine locomotives, pumps, drills and trams, via centralized, city-level, distribution.

  A compressed air locomotive by H. K. Porter, Inc., in use at the Homestake Mine between 1928 and 1961.

Racecars use compressed air to start its internal combustion engine (ICE).

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


A 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 −15 °C (5 °F); the cold air may be used for air conditioning in a car.[7]

Additional heat can be supplied by burning fuel as in 1904 for Whitehead's torpedoes.[21] 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 is also capable of running on compressed air, and is thus also a compressed air engine.


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, 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. The cost of driving such car is typically projected to be around €0.75 per 100 kilometres (62 mi) with a complete refill at a service station costing about US$3.[citation needed]

  Types of systems

  Cryogenic systems

A special CAES system has been created that uses liquid air as an energy carrier. This system is called Highview Power Storage's CryoEnergy System (CES).[22]

  Hybrid systems

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

Compressed air engines can recharge an electric battery. The apparently defunct Energine promoted its Pne-PHEV or Pneumatic Plug-in Hybrid Electric Vehicle-system)[citation needed].[23]

  Existing hybrid systems

Huntorf, Germany in 1978, and McIntosh, Alabama in 1991 (USA) commissioned hybrid power plants.[8][24] 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 (ISEP) will use aquifer 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."[24] Power output of the McIntosh and Iowa systems is in the range of 2–300 MW.[25]

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.[26]

  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.[27] 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 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 ponds. 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.[28]

A nearly isobaric 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 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.[29] [30] 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.[31]

  See also


  1. ^ Wild, Matthew, L. Wind Drives Growing Use of Batteries, New York Times, July 28, 2010, pp.B1.
  2. ^ "ADELE – Adiabatic compressed-air energy storage (CAES) for electricity supply". http://www.rwe.com/web/cms/en/365478/rwe/innovations/power-generation/energy-storage/compressed-air-energy-storage/project-adele/. Retrieved December 31, 2011. 
  3. ^ "German AACAES project information". http://www.bine.info/pdf/publikation/projekt0507englinternetx.pdf. Retrieved February 22, 2008. 
  4. ^ http://my.epri.com/portal/server.pt?Abstract_id=TR-101751-V2
  5. ^ http://www.westgov.org/wieb/electric/Transmission%20Protocol/SSG-WI/pnw_5pp_02.pdf
  6. ^ Compressed-Air Propulsion
  7. ^ a b 3-stage propulsion with intermediate heating
  8. ^ a b "Distributed Energy Program: Compressed Air Energy Storage". United States Department of Energy. http://www.eere.energy.gov/de/compressed_air.html. Retrieved August 27, 2006. 
  9. ^ http://www.answers.com/topic/solution-mining?cat=technology ; http://www.saltinstitute.org/12.html
  10. ^ a b Chambers's Encyclopaedia: A Dictionary of Universal Knowledge. W. & R. Chambers, LTD. 1896. pp. 252–253. http://books.google.com/books?id=4pwMAAAAYAAJ&pg=PA252. Retrieved January 7, 2009. 
  11. ^ Technische Mislukkingen by Lex Veldhoen & Jan van den Ende
  12. ^ (pdf) Compressed Air Storage (CAES), Dresser-Rand Corporation, 2010, brochure form# 85230, http://www.dresser-rand.com/literature/general/85164-10-CAES.pdf 
  13. ^ Wald, Matthew (September 29, 1991), Using Compressed Air To Store Up Electricity, New York Times, http://www.nytimes.com/1991/09/29/business/technology-using-compressed-air-to-store-up-electricity.html?pagewanted=all&src=pm 
  14. ^ CAES:McIntosh Power Plant, PowerSouth Energy Cooperative, 2010, http://www.powersouth.com/mcintosh_power_plant/compressed_air_energy, retrieved April 15, 2012 
  15. ^ a b (pdf) ARRA Energy Storage Demonstrations, Sandia National Laboratories, http://www.sandia.gov/ess/docs/ARRA_StorDemos_4-22-11.pdf, retrieved April 13, 2012 
  16. ^ NYSEG considering Compressed Air Energy Storage, Energy Overviews Publishing, http://epoverviews.com/articles/visitor.php?keyword=Smart%20Grid%20Demonstration, retrieved April 13, 2012 
  17. ^ Energy bags under the sea to be tested in 2011(Cleantechnica website). See in sections below.
  18. ^ Heat loss of practical systems is explained in the #Thermodynamics of heat storage section.
  19. ^ Air – Density and Specific Weight, The Engineering Toolbox
  20. ^ Gas cylinders – High pressure cylinders for the on-board storage of natural gas as a fuel for automotive vehicles
  21. ^ A History of the Torpedo The Early Days
  22. ^ CryoEnergy System
  23. ^ Energine PHEV-system schematic
  24. ^ a b Pendick, Daniel (November 17, 2007). "Squeeze the breeze: Want to get more electricity from the wind? The key lies beneath our feet". New Scientist 195 (2623): 4. http://environment.newscientist.com/channel/earth/mg19526231.700-rocks-could-be-novel-store-for-wind-energy.html. Retrieved November 17, 2007. 
  25. ^ Frequently Asked Questions
  26. ^ http://www.firstenergycorp.com/NewsReleases/2009-11-23%20Norton%20Project.pdf
  27. ^ "Wind plus compressed air equals efficient energy storage in Iowa proposal". Energy Services Bulletin website. Western Area Power Administration. http://www.wapa.gov/es/pubs/ESB/2003/03Aug/esb084.htm. Retrieved April 29, 2008. 
  28. ^ Prior art. Oliver Laing et al. Energy storage for off peak electricity. United States Patent No. 4873828.
  29. ^ "Energy bags and super batteries". Nottingham University. June 18, 2008. http://www.nottingham.ac.uk/news/pressreleases/2008/june/energybagsandsuperbatteries.aspx. 
  30. ^ "The man making 'wind bags'". BBC. March 26, 2008. http://news.bbc.co.uk/1/hi/england/nottinghamshire/7315059.stm. 
  31. ^ "How Hydrostor Aims To Change The Power Game By Storing Energy Under Water". TechCrunch. July 9, 2011. http://techcrunch.com/2011/07/09/hydrostor-power-storage-under-water. 

  External links



All translations of Compressed_air_energy_storage

sensagent's content

  • definitions
  • synonyms
  • antonyms
  • encyclopedia

Dictionary and translator for handheld

⇨ New : sensagent is now available on your handheld

   Advertising ▼

sensagent's office

Shortkey or widget. Free.

Windows Shortkey: sensagent. Free.

Vista Widget : sensagent. Free.

Webmaster Solution


A windows (pop-into) of information (full-content of Sensagent) triggered by double-clicking any word on your webpage. Give contextual explanation and translation from your sites !

Try here  or   get the code


With a SensagentBox, visitors to your site can access reliable information on over 5 million pages provided by Sensagent.com. Choose the design that fits your site.

Business solution

Improve your site content

Add new content to your site from Sensagent by XML.

Crawl products or adds

Get XML access to reach the best products.

Index images and define metadata

Get XML access to fix the meaning of your metadata.

Please, email us to describe your idea.


The English word games are:
○   Anagrams
○   Wildcard, crossword
○   Lettris
○   Boggle.


Lettris is a curious tetris-clone game where all the bricks have the same square shape but different content. Each square carries a letter. To make squares disappear and save space for other squares you have to assemble English words (left, right, up, down) from the falling squares.


Boggle gives you 3 minutes to find as many words (3 letters or more) as you can in a grid of 16 letters. You can also try the grid of 16 letters. Letters must be adjacent and longer words score better. See if you can get into the grid Hall of Fame !

English dictionary
Main references

Most English definitions are provided by WordNet .
English thesaurus is mainly derived from The Integral Dictionary (TID).
English Encyclopedia is licensed by Wikipedia (GNU).


The wordgames anagrams, crossword, Lettris and Boggle are provided by Memodata.
The web service Alexandria is granted from Memodata for the Ebay search.
The SensagentBox are offered by sensAgent.


Change the target language to find translations.
Tips: browse the semantic fields (see From ideas to words) in two languages to learn more.

last searches on the dictionary :

2844 online visitors

computed in 0.062s

I would like to report:
section :
a spelling or a grammatical mistake
an offensive content(racist, pornographic, injurious, etc.)
a copyright violation
an error
a missing statement
please precise:



Company informations

My account



   Advertising ▼