View source for Compressed-air energy storage

{{Short description|Method for matching variable production with demand}}
{{More citations needed|date=November 2024}}
{{Use mdy dates|date=April 2012}}
[[File:Bouteille air démarrage.JPG|thumb|A pressurized air tank used to start a diesel generator set in Paris Metro]]
'''Compressed-air-energy storage''' (CAES) is a way to [[energy storage|store energy]] for later use using [[compressed air]]. At a [[public utility|utility]] scale, energy generated during periods of low demand can be released during [[peak load]] periods.<ref name="NYT-2010.07.28">{{cite news |last=Wald |first=Matthew L..|url=https://www.nytimes.com/2010/07/28/business/energy-environment/28storage.html |title=Wind Drives Growing Use of Batteries |newspaper=[[The New York Times]] |date=July 28, 2010 |page=B1}}</ref>

The first utility-scale CAES project was in the Huntorf power plant in [[Elsfleth|Elsfleth, Germany]], and is still operational {{As of|2024|lc=y}}.<ref>{{Cite journal |last1=Zhang |first1=Xinjing |last2=Gao |first2=Ziyu |last3=Zhou |first3=Bingqian |last4=Guo |first4=Huan |last5=Xu |first5=Yujie |last6=Ding |first6=Yulong |last7=Chen |first7=Haisheng |date=2024 |title=Advanced Compressed Air Energy Storage Systems: Fundamentals and Applications |journal=Engineering |volume=34 |pages=246–269 |doi=10.1016/j.eng.2023.12.008 |issn=2095-8099|doi-access=free }}</ref> The Huntorf plant was initially developed as a load balancer for [[Fossil fuel power station|fossil-fuel-generated electricity]], but the global shift towards [[renewable energy]] renewed interest in CAES systems,<ref name="The role of compressed air energy storage (CAES) in future sustainable energy systems." /> to help highly intermittent energy sources like [[photovoltaics]] and [[wind power|wind]] satisfy fluctuating electricity demands.<ref name="The role of compressed air energy storage (CAES) in future sustainable energy systems.">{{cite journal | last1=Lund | first1=Henrik | last2=Salgi | first2=Georges | title=The role of compressed air energy storage (CAES) in future sustainable energy systems | journal=Energy Conversion and Management | publisher=Elsevier BV | volume=50 | issue=5 | year=2009 | issn=0196-8904 | doi=10.1016/j.enconman.2009.01.032 | pages=1172–1179| bibcode=2009ECM....50.1172L }}</ref>

One ongoing challenge in large-scale design is the management of thermal energy, since the compression of air leads to an unwanted [[Joule-Thompson effect|temperature increase]] that not only reduces operational efficiency but can also lead to damage. The main difference between various architectures lies in thermal engineering. On the other hand, small-scale systems have long been used for propulsion of [[Fireless locomotive|mine locomotives]]. Contrasted with traditional batteries, systems can store energy for longer periods of time and have less upkeep.

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== Types ==
Compression of air creates heat; the air is warmer after compression. Expansion removes 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, then the efficiency of the storage improves considerably.<ref name="NYTimes-2012.10.01">{{cite news |last=Gies |first=Erica |url=https://www.nytimes.com/2012/10/02/business/energy-environment/a-storage-solution-is-in-the-air.html |title=Global Clean Energy: A Storage Solution Is in the Air |newspaper=[[International Herald Tribune]] |date=October 1, 2012 |via=NYTimes.com}}</ref> There are several ways in which a CAES system can deal with heat. Air storage can be [[adiabatic]], diabatic, [[isothermal]], or near-isothermal.

=== Adiabatic ===
Adiabatic storage continues to store the energy produced by compression and returns it to the air as it is expanded to generate power. This is a subject of an ongoing study, with no utility-scale plants as of 2015. The theoretical [[Energy conversion efficiency|efficiency]] of adiabatic storage approaches 100% with perfect insulation, but in practice, round trip efficiency is expected to be 70%.<ref name="BINE1">{{cite web| title = German AACAES project information| url = http://www.bine.info/fileadmin/content/Publikationen/Englische_Infos/projekt_0507_engl_internetx.pdf| access-date = February 22, 2008| archive-date = August 26, 2018| archive-url = https://web.archive.org/web/20180826211224/http://www.bine.info/fileadmin/content/Publikationen/Englische_Infos/projekt_0507_engl_internetx.pdf| url-status = dead}}</ref> Heat can be stored in a solid such as concrete or stone, or in a fluid such as hot oil (up to 300&nbsp;°C) or molten salt solutions (600&nbsp;°C). Storing the heat in hot water<!-- 100&nbsp;°C ? --> may yield an efficiency around 65%.<ref name="es2022jan20">{{cite web |last1=Colthorpe |first1=Andy |title=Why Goldman Sachs thinks advanced compressed air is worthy of US$250m investment |url=https://www.energy-storage.news/why-goldman-sachs-thinks-advanced-compressed-air-is-worthy-of-us250m-investment/ |website=Energy Storage News |archive-url= https://web.archive.org/web/20220403123901/https://www.energy-storage.news/why-goldman-sachs-thinks-advanced-compressed-air-is-worthy-of-us250m-investment/ |archive-date=3 April 2022 |date=20 January 2022 |url-status=live}}</ref>

[[Packed bed]]s have been proposed as thermal storage units for adiabatic systems. A study <ref>{{cite journal | last1=Barbour | first1=Edward | last2=Mignard | first2=Dimitri | last3=Ding | first3=Yulong | last4=Li | first4=Yongliang | title=Adiabatic Compressed Air Energy Storage with packed bed thermal energy storage | journal=Applied Energy | publisher=Elsevier BV | volume=155 | year=2015 | issn=0306-2619 | doi=10.1016/j.apenergy.2015.06.019 | pages=804–815| s2cid=28493150 | doi-access=free | bibcode=2015ApEn..155..804B | hdl=20.500.11820/31a2a7f9-5fc6-4452-8bd8-b08614bebae2 | hdl-access=free }}</ref> numerically simulated an adiabatic compressed air energy storage system using packed bed thermal energy storage. The efficiency of the simulated system under continuous operation was calculated to be between 70.5% and 71%.

Advancements in adiabatic CAES involve the development of high-efficiency thermal energy storage systems that capture and reuse the heat generated during compression. This innovation has led to system efficiencies exceeding 70%, significantly higher than traditional Diabatic systems.

=== Diabatic ===
Diabatic storage dissipates much of the heat of compression with [[intercooler]]s (thus approaching isothermal compression) into the atmosphere as waste, essentially wasting the energy used to perform the work of compression. Upon removal from storage, the temperature of this compressed air is ''the one indicator'' of the amount of stored energy that remains in this air. Consequently, if the air temperature is too low for the [[energy recovery]] process, then the air must be substantially re-heated prior to expansion in the [[turbine]] to power a [[Electrical generator|generator]]. This reheating can be accomplished with a [[Natural gas|natural-gas]]-fired burner for [[utility]]-grade storage or with a heated metal mass. As recovery is often most needed when renewable sources are quiescent, the fuel must be burned to make up for the ''wasted'' heat. This degrades the efficiency of the storage-recovery cycle. While this approach is relatively simple, the burning of fuel adds to the cost of the recovered electrical energy and compromises the ecological benefits associated with most [[renewable energy]] sources. Nevertheless, this is thus far{{As of when|date=May 2024}} the only system that has been implemented commercially.

The [[McIntosh, Alabama]], CAES plant requires 2.5&nbsp;MJ of electricity and 1.2&nbsp;MJ [[lower heating value]] (LHV) of gas for each MJ of energy output, corresponding to an energy recovery efficiency of about 27%.<ref>{{cite report | title=History of First U.S. Compressed-Air Energy Storage (CAES) Plant (110 MW 26h) |volume=2: Construction | website=EPRI Home | url=https://www.epri.com/research/products/TR-101751-V2 |date=May 7, 1994 |df=mdy-all }}</ref> A [[General Electric]] 7FA 2x1 [[combined cycle]] plant, one of the most efficient natural gas plants in operation, uses 1.85&nbsp;MJ (LHV) of gas per MJ generated,<ref>{{cite web|url=http://www.westgov.org/wieb/electric/Transmission%20Protocol/SSG-WI/pnw_5pp_02.pdf |title=Natural Gas Combined-cycle Gas Turbine Power Plants |date=August 8, 2002 |access-date=2008-01-04 |url-status=dead |archive-url=https://web.archive.org/web/20080411124518/http://www.westgov.org/wieb/electric/Transmission%20Protocol/SSG-WI/pnw_5pp_02.pdf |archive-date=April 11, 2008 |df=mdy }}</ref> a 54% [[thermal efficiency]].

To improve the efficiency of Diabatic CAES systems, modern designs incorporate heat recovery units that capture waste heat during compression, thereby reducing energy losses and enhancing overall performance.

===Isothermal===
Isothermal compression and expansion approaches attempt to maintain [[operating temperature]] by constant heat exchange to the environment. In a reciprocating compressor, this can be achieved by using a finned piston <ref>{{Cite journal|last1=Heidari|first1=Mahbod |last2=Mortazavi|first2=Mehdi |last3=Rufer|first3=Alfred |date=2017-12-01|title=Design, modeling and experimental validation of a novel finned reciprocating compressor for Isothermal Compressed Air Energy Storage applications|url=http://www.sciencedirect.com/science/article/pii/S0360544217315529 |journal=Energy|language=en |volume=140|pages=1252–1266 |doi=10.1016/j.energy.2017.09.031 |bibcode=2017Ene...140.1252H |issn=0360-5442}}</ref> and low cycle speeds.<ref>{{Cite journal|last1=Mohammadi-Amin|first1=Meysam |last2=Jahangiri|first2=Ali Reza|last3=Bustanchy|first3=Mohsen |date=2020|title=Thermodynamic modeling, CFD analysis and parametric study of a near-isothermal reciprocating compressor|url=https://linkinghub.elsevier.com/retrieve/pii/S245190492030144X |journal=Thermal Science and Engineering Progress|language=en |volume=19|pages=100624|doi=10.1016/j.tsep.2020.100624|bibcode=2020TSEP...1900624M |s2cid=225574178 }}</ref> Current{{When|date=May 2024}} challenges in effective [[heat exchanger]]s mean that they are only practical for low power levels. The theoretical efficiency of isothermal energy storage approaches 100% for perfect heat transfer to the environment. In practice, neither of these perfect thermodynamic cycles is obtainable, as some heat losses are unavoidable, leading to a near-isothermal process. Recent developments in isothermal CAES focus on advanced thermal management techniques and materials that maintain constant air temperatures during compression and expansion, minimizing energy losses and improving system efficiency.

=== Near-isothermal ===
Near-isothermal compression (and expansion) is a process in which a gas is compressed in very close proximity to a large incompressible thermal mass such as a heat-absorbing and -releasing structure (HARS) or a water spray.<ref>{{Cite journal|last1=Guanwei|first1=Jia |last2=Weiqing|first2=Xu |last3=Maolin|first3=Cai |last4=Yan|first4=Shi |date=2018-09-01|title=Micron-sized water spray-cooled quasi-isothermal compression for compressed air energy storage|url=http://www.sciencedirect.com/science/article/pii/S0894177718304862 |journal=Experimental Thermal and Fluid Science|language=en |volume=96|pages=470–481 |doi=10.1016/j.expthermflusci.2018.03.032|bibcode=2018ETFS...96..470G |s2cid=126094265 |issn=0894-1777}}</ref> A HARS is usually made up of a series of parallel fins. As the gas is compressed, the heat of compression is rapidly transferred to the thermal mass, so the gas temperature is stabilized. An external cooling circuit is then used to maintain the temperature of the thermal mass. The isothermal efficiency (Z)<ref>{{cite web |url=http://www.fluidmechanics.co.uk/wp-content/uploads/2015/07/Calculating-Isothermal-Efficiency-V1.2.pdf |title=Calculating Isothermal Efficiency |date=2015 |website=www.fluidmechanics.co.uk |access-date=July 4, 2015 |archive-date=February 14, 2019 |archive-url=https://web.archive.org/web/20190214190805/http://www.fluidmechanics.co.uk/wp-content/uploads/2015/07/Calculating-Isothermal-Efficiency-V1.2.pdf |url-status=dead }}</ref> is a measure of where the process lies between an adiabatic and isothermal process. If the efficiency is 0%, then it is totally adiabatic; with an efficiency of 100%, it is totally isothermal. Typically with a near-isothermal process, an isothermal efficiency of 90–95% can be expected.

=== Hybrid CAES systems ===
Hybrid Compressed Air Energy Storage (H-CAES) systems integrate [[Renewable energy|renewable energy sources]], such as wind or solar power, with traditional CAES technology. This integration allows for the storage of excess renewable energy generated during periods of low demand, which can be released during peak demand to enhance grid stability and reduce reliance on [[Fossil fuel|fossil fuels]]. For instance, the Apex CAES Plant in Texas combines wind energy with CAES to provide a consistent energy output, addressing the intermittency of renewable energy sources.

=== Other ===
One implementation of isothermal CAES uses high-, medium-, and low-pressure pistons in series. Each stage is 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 gas|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]]<ref>{{cite web  |author=Douglas Self   |author-link=Douglas Self   |url=http://www.douglas-self.com/MUSEUM/TRANSPORT/comprair/comprair.htm |title=Compressed-Air Propulsion |access-date=2014-05-11}}</ref> and trams.<ref name="autogenerated1">{{cite web |url=http://www.aircaraccess.com/images/3stage%201.jpg |title=3-stage propulsion with intermediate heating |access-date=2014-05-11 |archive-url= https://web.archive.org/web/20151031053915/http://www.aircaraccess.com/images/3stage%201.jpg |archive-date=October 31, 2015 |url-status=dead }}</ref> Here, the heat of compression is effectively stored in the atmosphere (or sea) and returned later on.{{citation needed|date=April 2021}}

== Compressors and expanders ==
Compression can be done with electrically-powered [[turbocompressor|turbo-compressor]]s and expansion with [[turbo-expander]]s<ref name="DEP1">{{cite web | title=Distributed Energy Program: Compressed Air Energy Storage | website=eere.energy.gov | date=2005-09-14 | url=http://www.eere.energy.gov/de/compressed_air.html | archive-url=https://web.archive.org/web/20060827060137/http://www.eere.energy.gov/de/compressed_air.html | archive-date=2006-08-27 | url-status=dead}}</ref> or [[air engine]]s driving [[electrical generator]]s to produce electricity.

== Storage ==
Air storage vessels vary in the thermodynamic conditions of the storage and on the technology used:

# Constant volume storage ([[In situ leach|solution-mined]] caverns, above-ground vessels, aquifers, automotive applications, etc.)
# Constant pressure storage (underwater pressure vessels, hybrid pumped hydro / compressed air storage)

=== Constant-volume storage ===
This storage system uses a chamber with specific boundaries to store large amounts of air. This means from a thermodynamic point of view that this system is a constant-volume and variable-pressure system. This causes some operational problems for the compressors and turbines, so the pressure variations have to be kept below a certain limit, as do the stresses induced on the storage vessels.<ref name=PRIN_WP>{{cite web |title=Compressed Air Energy Storage: Theory, Resources, And Applications For Wind Power |url=http://www.princeton.edu/pei/energy/publications/texts/SuccarWilliams_PEI_CAES_2008April8.pdf |archive-url= https://wayback.archive-it.org/all/20120119134651/http://www.princeton.edu/pei/energy/publications/texts/SuccarWilliams_PEI_CAES_2008April8.pdf |url-status=dead |archive-date=January 19, 2012 |access-date=June 6, 2015}}</ref>

The storage vessel is often a [[Natural gas storage#Salt formation|cavern]] created by [[solution mining]] (salt is dissolved in water for extraction)<ref>{{cite web |url=http://www.saltinstitute.org/12.html |title=Solution Mining for Salt |access-date=2007-10-26 |url-status=dead |archive-url= https://web.archive.org/web/20071017181924/http://saltinstitute.org/12.html |archive-date=October 17, 2007 |df=mdy-all }}</ref> or by using an [[abandoned mine]]; use of porous and permeable rock formations (rocks that have interconnected holes, through which liquid or air can pass), such as those in which reservoirs of natural gas are found, has also been studied.<ref name=PNNL413>{{cite web |title=Compressed Air Energy Storage |url=http://caes.pnnl.gov/ |publisher=Pacific Northwest National Laboratory |access-date=May 20, 2013 |date=April 2013}}</ref>

In some cases, an above-ground pipeline was tested as a storage system, giving some good results. Obviously, the cost of the system is higher, but it can be placed wherever the designer chooses, whereas an underground system needs some particular geologic formations (salt domes, aquifers, depleted gas fields, etc.).<ref name=PRIN_WP/>

=== Constant-pressure storage ===
In this case, the storage vessel is kept at constant pressure, while the gas is contained in a variable-volume vessel. Many types of storage vessels have been proposed, generally relying on liquid displacement to achieve isobaric operation. In such cases, the storage vessel is positioned hundreds of meters below ground level, and the hydrostatic pressure (head) of the water column above the storage vessel maintains the pressure at the desired level.

This configuration allows:
* Improvement of the energy density of the storage system because all the air contained can be used (the pressure is constant in all charge conditions, full or empty, so the turbine has no problem exploiting it, while with constant-volume systems, if the pressure goes below a safety limit, then the system needs to stop).
* Removal of the requirement of throttling prior to the expansion.
* Avoidance of mixing of heat at different temperatures in the Thermal Energy Storage system, which leads to irreversibility.<ref name=Energies>{{cite journal | title = Comparative Analysis of Isochoric and Isobaric Adiabatic Compressed Air Energy Storage | date = February 2, 2023 | last1=Pottie |first1=Daniel    | journal = Energies | volume = 16 | issue = 6 | page = 2646 | doi = 10.3390/en16062646 | doi-access = free }}</ref>
* Improvement of the efficiency of the turbomachinery, which will work under constant-inlet conditions.
* Use of various geographic locations for the positioning of the CAES plant (coastal lines, floating platforms, etc.).<ref name=OCAES>{{cite web |title=Ocean Compressed Air Energy Storage (OCAES) Integrated with Offshore Renewable Energy Sources |date=May 17, 2013 |url=http://repository.lib.ncsu.edu/ir/bitstream/1840.16/8878/1/etd.pdf |access-date=June 6, 2015|last1=Dong |first1=Lim }}</ref>
 
On the other hand, the cost of this storage system is higher due to the need to position the storage vessel on the bottom of the chosen water reservoir (often the ocean) and due to the cost of the vessel itself.<ref name=OCAES />

A different approach consists of burying a large bag buried under several meters of sand instead of water.<ref>{{Cite news|date=25 September 2009|title=Dansk projekt vil lagre vindenergi under 25 m sand|work=Ingeniøren|url=https://ing.dk/artikel/dansk-projekt-vil-lagre-vindenergi-under-25-m-sand-102625| language=da }}</ref>

Plants operate on a [[Load management|peak-shaving]] daily cycle, charging at night and discharging during the day. Heating the compressed air using natural gas or geothermal heat to increase the amount of energy being extracted has been studied by the [[Pacific Northwest National Laboratory]].<ref name=PNNL413 />

Compressed-air energy storage can also be employed on a smaller scale, such as exploited by [[air car]]s and air-driven [[locomotive]]s, and can use high-strength (e.g., [[carbon-fiber]]) air-storage tanks. In order to retain the energy stored in compressed air, this tank should be thermally isolated from the environment; otherwise, the energy stored will escape in the form of heat, because compressing air raises its temperature.

== Environmental Impact ==
CAES systems are often considered an environmentally friendly alternative to other large-scale energy storage technologies due to their reliance on naturally occurring resources, such as [[Salt cavern|salt caverns]] for air storage and ambient air as the working medium. Unlike [[Lithium-ion battery|lithium-ion batteries]], which require the extraction of finite resources such as lithium and cobalt, CAES has a minimal environmental footprint during its lifecycle.

However, the construction of CAES facilities presents unique challenges. Underground air storage requires geological formations such as [[Salt dome|salt domes]], which are geographically limited. Inappropriate siting or mismanagement during construction can lead to disruptions in local ecosystems, [[land subsidence]], or groundwater contamination.

On the positive side, CAES systems integrated with [[renewable energy]] sources contribute to a significant reduction in [[Greenhouse gas|greenhouse gas emissions]] by enabling the storage and dispatch of clean energy during peak demand. Additionally, repurposing depleted [[Natural gas storage|natural gas fields]] or other geological formations for air storage can mitigate environmental impacts and extend the usefulness of existing infrastructure.

=== Economic Considerations ===
The cost of implementing CAES systems depends heavily on the geological conditions of the site, the scale of the facility, and the type of CAES process used ([[Adiabatic process|adiabatic]], diabatic, or [[Isothermal process|isothermal]]). Initial capital expenditures are significant, often ranging from $500 to $1,200 per kW for large-scale systems. These costs primarily include the development of underground storage caverns, [[Gas compression|compression]] and expansion equipment, and thermal energy storage units (for advanced systems).

Despite the high upfront costs, CAES facilities have long operational lifespans, often exceeding 30 years, with low maintenance and operational costs compared to [[Battery storage power station|lithium-ion battery storage]] systems, which require periodic replacements. This long-term cost efficiency makes CAES particularly attractive for [[electric utility]] companies and [[Electrical grid|grid operators]].

=== Policy and Regulation ===
Market trends suggest growing interest in CAES technology due to increasing [[Intermittent energy source|renewable energy integration]] and the need for [[Grid energy storage|grid-scale energy storage]]. Government incentives and declining costs of advanced components, such as high-efficiency compressors and turbines, are further enhancing the economic feasibility of CAES.

Government policies and regulatory frameworks are critical in determining the pace of CAES adoption and development. Countries like [[Energy in Germany|Germany]] and the [[Energy in the United States|United States]] have implemented various incentives, including tax credits and grants, to promote [[Energy storage|energy storage technologies]]. For instance, the [[United States Department of Energy|U.S. Department of Energy]]’s Energy Storage Grand Challengeincludes CAES as a key focus area for research and development funding.

One of the significant regulatory hurdles for CAES is the permitting process for underground air storage facilities. [[Environmental impact assessment|Environmental impact assessments]], land use approvals, and safety standards for [[Pressure vessel|high-pressure storage]] systems can delay or increase costs for CAES projects. For example, projects sited near urban areas often face additional scrutiny due to concerns about [[noise pollution]], air quality, and potential risks associated with high-pressure air storage.

Internationally, efforts are underway to standardize the design, operation, and safety protocols for CAES systems. Organizations like the [[International Energy Agency]] (IEA) and regional bodies such as the [[Energy policy of the European Union|European Union]] have been instrumental in developing frameworks to support the integration of CAES into modern [[Electrical grid|energy grids]]. As renewable energy adoption accelerates, policies aimed at addressing [[Intermittent energy source|intermittency challenges]] will likely prioritize grid-scale solutions like CAES.

== History ==
Citywide compressed air energy systems for delivering mechanical power directly via compressed air have been built since 1870.<ref name=chambers>{{cite encyclopedia| title = Chambers's Encyclopaedia: A Dictionary of Universal Knowledge| date = 1896| publisher=W. & R. Chambers, LTD| pages =  252–253| url = https://archive.org/details/bub_gb_4pwMAAAAYAAJ| chapter=Pneumatic Dispatch |chapter-url=https://archive.org/details/bub_gb_4pwMAAAAYAAJ/page/n270}}</ref> Cities such as [[Paris]], France; [[Birmingham, England|Birmingham]], England; [[Dresden, Germany|Dresden]], [[Rixdorf]], and [[Offenbach, Germany|Offenbach]], 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 their pointer arms. They quickly evolved to deliver power to homes and industries.<ref>{{cite book | last1=Veldhoen | first1=L. | last2=van den Ende | first2=J. | title=Technische mislukkingen: de Planta-affaire, instortende bruggen, vliegdekschepen van ijs | publisher=Donker | year=1995 | isbn=978-90-6100-400-4 | language=nl |author2-link=Jan van den Ende}}</ref> As of 1896, the Paris system had 2.2&nbsp;MW of generation distributed at 550&nbsp;kPa in 50&nbsp;km of air pipes for motors in light and heavy industry. Usage was measured in cubic meters.<ref name=chambers/> The systems were the main source of house-delivered energy in those days and also powered the machines of [[dentistry|dentists]], [[seamstress]]es, printing facilities, and [[bakery|bakeries]].

The first utility-scale [[adiabatic process|diabatic]] compressed-air energy storage project was the 290-megawatt [[Huntorf]] plant opened in 1978 in Germany using a [[salt dome]] cavern with a capacity of {{convert|580|MWh}} and a 42% efficiency.<ref name="IRENA2017">{{cite report |year=2017 |url=http://www.irena.org/DocumentDownloads/Publications/IRENA_Electricity_Storage_Costs_2017.pdf |title=Electricity Storage and Renewables: Costs and Markets to 2030 |page=55 |publisher=[[International Renewable Energy Agency]] |location=Abu Dhabi}}</ref>

A plant that could store up to {{convert|2860|MWh}} (and produce up to {{val|110|u=MW}} for 26 hours) was built in [[McIntosh, Alabama]] in 1991. The Alabama facility's $65 million cost equals $590 per kW of power capacity and about $23 per kW⋅h of storage capacity. It uses a {{convert|19000000|cuft|adj=on|spell=in}} [[In-situ leach|solution-mined]] salt cavern to store air at up to {{convert|1100|psi|abbr=on}}. Although the compression phase is approximately 82% efficient, the expansion phase requires the combustion of natural gas at one-third the rate of a gas turbine producing the same amount of electricity at 54% efficiency.<ref name="IRENA2017" /><ref>{{citation|url=http://www.dresser-rand.com/literature/general/85164-10-CAES.pdf |title=Compressed Air Storage (CAES)|publisher=Dresser-Rand Corporation|id=brochure form# 85230|date=2010|url-status=dead|archive-url= https://web.archive.org/web/20120907175639/http://www.dresser-rand.com/literature/general/85164-10-CAES.pdf |archive-date=September 7, 2012|df=mdy-all}}</ref><ref>{{cite news|newspaper=The New York Times|date=September 29, 1991|last=Wald|first=Matthew |title=Using Compressed Air To Store Up Electricity|url= https://www.nytimes.com/1991/09/29/business/technology-using-compressed-air-to-store-up-electricity.html?pagewanted=all&src=pm }}</ref><ref>{{cite web|access-date=April 15, 2012|date=2010|publisher=PowerSouth Energy Cooperative|url=http://www.powersouth.com/mcintosh_power_plant/compressed_air_energy |title=CAES:McIntosh Power Plant|archive-url= https://web.archive.org/web/20111018130933/http://www.powersouth.com/mcintosh_power_plant/compressed_air_energy |archive-date=October 18, 2011|url-status=dead}}</ref>

In 2012, General Compression completed construction of a two-megawatt near-isothermal project in [[Gaines County, Texas]], the world's third such project. The project uses no fuel.<ref>{{cite web|author=General Compression, Inc.|title=What We Do{{dash}}Texas {{as written|Dispat|chable [sic]}} Wind 1, LLC|url=http://www.generalcompression.com/index.php/tdw1 |access-date=2 March 2013|url-status=dead|archive-url= https://web.archive.org/web/20130523153848/http://www.generalcompression.com/index.php/tdw1 |archive-date=May 23, 2013|df=mdy-all}}</ref> It appears to have stopped operating in 2016.<ref>{{Cite book |last=Hauer |first=Andreas |url=https://books.google.com/books?id=0ERtEAAAQBAJ&dq=Gaines+Texas+Dispatchable+Wind+1%2C+LLC&pg=PT411 |title=Advances in Energy Storage: Latest Developments from R&D to the Market |date=2022-04-27 |publisher=John Wiley & Sons |isbn=978-1-119-76010-8 |language=en}}</ref>

A 60{{nbsp}}MW, 300{{nbsp}}MW⋅h facility with 60% efficiency opened in [[Jiangsu]], China, using a salt cavern (2022).<ref name=Jiangsu>{{cite web |last1=Colthorpe |first1=Andy |title=China's compressed air energy storage industry makes progress |url=https://www.energy-storage.news/chinas-compressed-air-energy-storage-industry-makes-progress/ |website=Energy Storage News |date=1 June 2022}}</ref>

A 2.5{{nbsp}}MW, 4{{nbsp}}MW⋅h [[Compressed carbon dioxide energy storage|compressed {{CO2}}]] closed-cycle facility started operating in [[Sardinia]], Italy (2022).<ref>{{cite web |last1=Murray |first1=Cameron |title=Energy Dome launches world's first multi-megawatt '{{CO2}} Battery' project in Sardinia, Italy |url=https://www.energy-storage.news/energy-dome-launches-4mwh-demonstrator-project-in-italy-plans-series-b/ |website=Energy Storage News |date=8 June 2022}}</ref>

In 2022, [[Zhangjiakou]] connected the world's first 100{{nbsp}}MW storage system to the grid in north China. It uses supercritical thermal storage, supercritical heat exchange, and high-load compression and expansion technologies. The plant can store 400{{nbsp}}MW⋅h with 70.4% efficiency.<ref name=zhang>{{Cite web |last=Blain |first=Loz |date=2022-10-05 |title=China turns on the world's largest compressed air energy storage plant |url=https://newatlas.com/energy/china-100mw-compressed-air/ |access-date=2022-10-10 |website=New Atlas |language=en-US}}</ref> Construction of a 350{{nbsp}}MW, 1.4{{nbsp}}GW⋅h salt cave project started in [[Shandong|Shangdong]]<ref>{{cite web |last1=Murray |first1=Cameron |title=Construction starts on 1.4GWh compressed air energy storage unit in China |url=https://www.energy-storage.news/construction-starts-on-1-4gwh-compressed-air-energy-storage-unit-in-china/ |website=Energy Storage News |date=26 October 2022}}</ref> at a cost of $208 million, operating in 2024 with 64% efficiency,<ref>{{cite web |last1=Murray |first1=Cameron |title='World's largest' compressed air energy storage project connects to the grid in China |url=https://www.energy-storage.news/worlds-largest-compressed-air-energy-storage-project-connects-to-the-grid-in-china/ |website=Energy-Storage.News |date=10 April 2024}}</ref><ref name=shan>{{cite web |title=World's largest compressed air energy storage project comes online in China |url=https://heat-exchanger-world.com/worlds-largest-compressed-air-energy-storage-project-comes-online-in-china/ |website=Heat Exchanger World |date=27 May 2024}}</ref> and construction of a four-hour, 700{{nbsp}}MW, 2.8{{nbsp}}GW⋅h facility started in China in 2024.<ref>{{cite web |last1=Murray |first1=Cameron |title=Installation starts on 'world's largest' compressed air energy storage unit in China |url=https://www.energy-storage.news/installation-starts-on-worlds-largest-compressed-air-energy-storage-unit-in-china/ |website=Energy-Storage.News |date=31 December 2024}}</ref>

== Largest CAES facilities ==
{{See also|List of energy storage power plants|List of largest power stations#Battery storage}}

{| class="wikitable sortable"
|+ 10 largest compressed-air storage power plants by storage capacity
! Name
! [[Project commissioning|Commissioning date]]
! Energy ([[MWh]])
! [[Power (physics)|Power]] ([[Megawatt|MW]])
! Duration (hours) <!-- =Energy/Power -->
! Efficiency %
! Type
! Country
! class="unsortable"| Location/coords
! style="background: #f2f2f2; color: #000080" class="unsortable" | Refs
|-
|[[Yingcheng]] 
|2025
|1500
|300
|5
|70
|Salt cavern
|China
|
|<ref>{{cite web |last1=Maisch |first1=Marija |title=World's largest compressed air energy storage facility commences full operation in China |url=https://www.ess-news.com/2025/01/10/worlds-largest-compressed-air-energy-storage-facility-commences-full-operation-in-china/ |website=Energy Storage |date=10 January 2025}}</ref>
|-
|[[Shandong]] 
|2024
|1400
|350
|
|64
|Salt cavern
|China
|
|<ref name=shan/>
|-
|[[Zhangjiakou]] 
|2022
|400
|100
|4
|70
|
|China
|
|<ref name=zhang/>
|-
|[[Jiangsu]] 
|2022
|300
|60
|5
|60
|Salt cavern
|China
|
|<ref name=Jiangsu/>
<!-- add Huntorf and McIntosh if they still operate -->
|}

== Projects ==
In 2009, the [[United States Department of Energy|US Department of Energy]] awarded $24.9 million in matching funds for phase one of a 300{{nbsp}}MW, $356 million [[Pacific Gas and Electric Company]] installation using a saline porous rock formation being developed near [[Bakersfield, California|Bakersfield]] in [[Kern County, California]]. The goals of the project were to build and validate an advanced design.<ref name="sandia1">{{citation |title=ARRA Energy Storage Demonstrations |url=http://www.sandia.gov/ess/docs/ARRA_StorDemos_4-22-11.pdf |publisher=Sandia National Laboratories |access-date=April 13, 2012}}</ref>

In 2010, the US Department of Energy provided $29.4 million in funding to conduct preliminary work on a 150-MW salt-based project being developed by [[Iberdrola|Iberdrola USA]] in [[Watkins Glen, New York]]. The goal is to incorporate [[smart grid]] technology to balance renewable [[intermittent energy source]]s.<ref name="sandia1" /><ref>{{citation |title=NYSEG considering Compressed Air Energy Storage |url=http://epoverviews.com/articles/visitor.php?keyword=Smart%20Grid%20Demonstration |archive-url=https://web.archive.org/web/20180729200608/http://epoverviews.com/articles/visitor.php?keyword=Smart%20Grid%20Demonstration |publisher=Energy Overviews Publishing |access-date=April 13, 2012 |archive-date=July 29, 2018 |url-status=dead}}</ref>

The first [[adiabatic]] project, a 200-megawatt facility called ADELE, was planned for construction in Germany (2013) with a target of 70% efficiency by using {{cvt|600|C|F}} air at 100 bars of pressure.<ref>{{cite web |title=Adiabatic Compressed-air Energy Storage For Electricity Supply |url=https://wecanfigurethisout.org/ENERGY/Web_notes/Round_Pegs/Power_Cycles_and_Energy_Storage_Supporting_Files/ADELE%20%E2%80%93%20Adiabatic%20Compressed-Air%20Energy%20Storage%20for%20Electricity%20Supply.pdf |archive-url= https://web.archive.org/web/20220522223415/https://wecanfigurethisout.org/ENERGY/Web_notes/Round_Pegs/Power_Cycles_and_Energy_Storage_Supporting_Files/ADELE%20%E2%80%93%20Adiabatic%20Compressed-Air%20Energy%20Storage%20for%20Electricity%20Supply.pdf |archive-date=22 May 2022 |date=January 2010 |url-status=live}}</ref> This project was delayed for undisclosed reasons until at least 2016.<ref>{{cite web |title=Adele CAES Pilot: Site Selected but Project Delayed |quote=The RWE/GE Led Consortium That Is Developing an Adiabatic Form of Compressed Air Energy Storage Is to Establish Its Commercial Scale Test Plant at Stassfurt. the Testing Stage, Originally Slated for 2073, Is Not Now Expected to Start before 2016 |url=http://business.highbeam.com/4364/article-1G1-297718155/adele-caes-pilot-site-selected-but-project-delayed |url-status=dead |archive-url=https://web.archive.org/web/20160309080750/https://business.highbeam.com/4364/article-1G1-297718155/adele-caes-pilot-site-selected-but-project-delayed |archive-date=2016-03-09 |publisher=Modern Power Systems |via=highbeam.com}}</ref>

Storelectric Ltd planned to build a 40-MW [[100% renewable energy]] pilot plant in [[Cheshire]], UK, with 800&nbsp;MWh of storage capacity (2017).<ref>{{citation |title=Storelectric website |url=http://www.storelectric.com/}}</ref>

Hydrostor completed the first commercial A-CAES system in [[Goderich, Ontario]], supplying service with 2.2MW / 10MWh storage to the Ontario Grid (2019). It was the first A-CAES system to achieve commercial operation in decades.<ref>{{cite web |title=Grid-connected advanced compressed air energy storage plant comes online in Ontario |url=https://www.energy-storage.news/news/grid-connected-advanced-compressed-air-energy-storage-plant-comes-online-in |website=Energy Storage News |archive-url= https://web.archive.org/web/20191223045810/https://www.energy-storage.news/news/grid-connected-advanced-compressed-air-energy-storage-plant-comes-online-in |archive-date=23 December 2019 |language=en |date=28 November 2019 |url-status=live}}</ref>

The European-Union-funded RICAS (adiabatic) project in Austria was to use crushed rock to store heat from the compression process to improve efficiency (2020). The system was expected to achieve 70–80% efficiency.<ref name=":0">{{Cite web |last=Lee |first=Lisa-Ann |date=March 29, 2017 |title=Creating renewable energy storage out of hot air |url=http://newatlas.com/ricas-2020-aa-compressed-air-energy-storage/48661/ |access-date=2017-04-11 |website=newatlas.com |language=en}}</ref>

Apex planned a plant for [[Anderson County, Texas]], to go online in 2016.<ref>{{cite web | first=PAUL |last=STONE | title=Anderson County getting energy center | website=Palestineherald.com | date=2012-07-11 | url=https://www.palestineherald.com/news/local_news/anderson-county-getting-energy-center/article_51ebfc40-54a0-5195-936b-cb0029210ce9.html}}</ref> This project has been delayed until at least 2020.<ref>{{Cite web|date=2013-10-08 |title=Bethel Energy Center |language=en-US |work=APEX CAES |url=http://www.apexcaes.com/project |access-date=2017-07-06}}</ref>

Canadian company [[Hydrostor]] planned to build four Advance plants in [[Toronto]], Goderich, Angas, and Rosamond (2020). Some included partial heat storage in water, improving efficiency to 65%.<ref>{{Cite web |title=Toronto A-CAES Facility – Hydrostor |url=https://www.hydrostor.ca/toronto-a-caes-facility/ |access-date=2020-09-06 |language=en-CA}}</ref>

As of 2022, the Gem project at [[Rosamond, California|Rosamond]] in [[Kern County, California]], was planned to provide 500 MW / 4,000 MWh of storage. The Pecho project in [[San Luis Obispo, California]], was planned to be 400 MW / 3,200 MWh. The [[Broken Hill]] project in [[New South Wales]], Australia was 200 MW / 1,600 MWh.<ref name="hs2022">{{cite web |last1=Colthorpe |first1=Andy |title=Why Goldman Sachs thinks advanced compressed air is worthy of US$250m investment |url=https://www.energy-storage.news/why-goldman-sachs-thinks-advanced-compressed-air-is-worthy-of-us250m-investment/ |website=Energy Storage News |archive-url= https://web.archive.org/web/20220527135344/https://www.energy-storage.news/why-goldman-sachs-thinks-advanced-compressed-air-is-worthy-of-us250m-investment/ |archive-date=27 May 2022 |date=20 January 2022 |url-status=live}}</ref>

In 2023, [[Alliant Energy]] announced plans to construct a 200-MWh compressed {{CO2}} facility based on the Sardinia facility in [[Columbia County, Wisconsin]]. It will be the first of its kind in the United States.<ref name="Schulz 2023 m338">{{cite web | last=Schulz | first=Joe | title=Wisconsin will house an energy storage facility that's the first of its kind in the US | website=Wisconsin Public Radio | date=2023-09-25 | url=https://www.wpr.org/wisconsin-will-house-energy-storage-facility-thats-first-its-kind-us | access-date=2023-09-28}}</ref>

Compressed air energy storage may be stored in undersea caves in [[Northern Ireland]].<ref>{{cite news |date=October 11, 2015 |title=Can energy be stored in air-filled caves? |url=https://www.bbc.com/news/av/science-environment-34498979/controversial-compressed-air-underwater-cave-energy-project |access-date=July 29, 2018 |work=BBC News}}</ref>

== Storage thermodynamics ==
{{More citations needed section|date=October 2023}}
In order to achieve a near-[[Reversible process (thermodynamics)|thermodynamically-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.<ref name="NYTimes-2012.10.01" />

=== Isothermal storage ===
In an [[isothermal process|isothermal]] compression process, the gas in the system is kept at a constant temperature throughout. This necessarily requires an exchange of heat with the gas; otherwise, the temperature would rise during charging and drop during discharge. This heat exchange can be achieved by heat exchangers (intercooling) between subsequent stages in the compressor, regulator, and tank. To avoid wasted energy, the intercoolers must be optimized for high [[heat transfer]] and low pressure drop. Smaller compressors can approximate isothermal compression even without intercooling, due to the relatively high [[Surface-area-to-volume ratio|ratio of surface area to volume]] of the compression chamber and the resulting improvement in heat dissipation from the compressor body itself.

When one obtains perfect isothermal storage (and discharge), the process is said 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 practice, there is always a temperature difference in any heat transfer process, and so all practical energy storage obtains efficiencies lower than 100%.

To estimate the compression/expansion work in an isothermal process, it may be assumed that the compressed air obeys the [[ideal gas law]]:
: <math>pV = nRT = \text{constant}.</math>
For a process from an initial state ''A'' to a final state ''B'', with [[absolute temperature]] <math>T = T_A = T_B</math> constant, one finds the work required for compression (negative) or done by the expansion (positive) to be
: <math>\begin{align}
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}}  = p_A V_A \ln{\frac{p_A}{p_B}} = p_B V_B \ln{\frac{p_A}{p_B}}, \\
\end{align}</math>

where <math>pV = p_A V_A = p_B V_B</math>, and so <math>\frac{V_B}{V_A} = \frac{p_A}{p_B}</math>.

Here <math>p</math> is the absolute [[pressure]], <math>V_A</math> is the (unknown) [[volume]] of gas compressed, <math>V_B</math> is the volume of the vessel, <math>n</math> is the [[amount of substance]] of gas (mol), and <math>R</math> is the [[ideal gas constant]].

If there is a constant pressure outside of the vessel, which is equal to the starting pressure <math>p_A</math>, the positive work of the outer pressure reduces the exploitable energy (negative value). This adds a term to the equation above:
: <math>W_{A \to B} = p_A V_A \ln{\frac{p_A}{p_B}} + (V_A - V_B) p_A = p_B V_B \ln{\frac{p_A}{p_B}} + (p_B - p_A) V_B.</math>

{{anchor|Example}}
; Example

How much energy can be stored in a 1&nbsp;m<sup>3</sup> storage vessel at a pressure of {{convert|70|bar|MPa}}, if the ambient pressure is {{convert|1|bar|MPa}}? In this case, the process work is

: <math>W = p_B V_B \ln\frac{p_A}{p_B} + (p_B - p_A) V_B</math> = 
: = 7.0&nbsp;MPa × 1&nbsp;m<sup>3</sup> × ln(0.1&nbsp;MPa/7.0&nbsp;MPa) + (7.0&nbsp;MPa − 0.1&nbsp;MPa) × 1&nbsp;m<sup>3</sup> = −22.8&nbsp;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, then 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 the 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 (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 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 [[Steam turbine#Isentropic efficiency|isentropic efficiency]] of around 85%. The result is that round-trip storage efficiency for adiabatic systems is also considerably less than perfect.

=== Large storage system thermodynamics ===
Energy storage systems often use large 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 gas is 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. The cavern space can be insulated to improve efficiency.{{citation needed|date=April 2021}}

Undersea insulated airbags that have similar thermodynamic properties to large cavern storage have been suggested.<ref>{{cite web | last=Kraemer | first=Susan | title=Energy Bags Under the Sea to be Tested to Store Off-Shore Wind | website=CleanTechnica | date=2011-04-18 | url=https://cleantechnica.com/2011/04/18/energy-bags-under-the-sea-to-be-tested-to-store-off-shore-wind/}}</ref>

== Vehicle applications ==
{{Main|Compressed-air vehicle}}

=== Practical constraints in transportation ===
In order to use air storage in vehicles or aircraft for practical land or air transportation, the energy storage system must be compact and lightweight. [[Energy density]] and [[specific energy]] are the engineering terms that define these desired qualities.

==== Specific energy, energy density, and efficiency ====
As explained in the thermodynamics of the gas storage section above, compressing air heats it, and expansion cools it. Therefore, practical air engines require heat exchangers in order to avoid excessively high or low temperatures, and even so do not reach ideal constant-temperature conditions or ideal thermal insulation.

Nevertheless, as stated above, it is useful to describe the maximum energy storable using the isothermal case, which works out to about 100 kJ/m<sup>3</sup> [&nbsp;ln(''P''<sub>''A''</sub>/''P''<sub>''B''</sub>)].

Thus if 1.0&nbsp;m<sup>3</sup> of air from the atmosphere is very slowly compressed into a 5&nbsp;L bottle at {{convert|20|MPa|bar|abbr=on}}, then the potential energy stored is 530&nbsp;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&nbsp;MPa pressure down to 100&nbsp;kPa (bottle completely "empty" at atmospheric pressure). Achieving high efficiency is a technical challenge both due to heat loss to the ambient and to unrecoverable internal gas heat.<ref>Heat loss of practical systems is explained in the [[#Storage|Storage]] section.</ref> If the bottle above is emptied to 1&nbsp;MPa, then the extractable energy is about 300&nbsp;kJ at the motor shaft.

A standard 20-MPa, 5-L steel bottle has a mass of 7.5&nbsp;kg, and a superior one 5&nbsp;kg. High-[[Tensile strength|tensile-strength]] fibers such as [[carbon-fiber|carbon fiber]] or [[Kevlar]] can weigh below 2&nbsp;kg in this size, consistent with the legal safety codes. One cubic meter of air at 20&nbsp;°C has a mass of 1.204&nbsp;kg at [[standard temperature and pressure]].<ref>{{cite web | title=Air Density and Specific Weight | website=engineeringtoolbox.com | date=2006-03-02 | url=http://www.engineeringtoolbox.com/air-desity-specific-weight-d_600.html | archive-url=https://web.archive.org/web/20121122112912/http://www.engineeringtoolbox.com/air-desity-specific-weight-d_600.html | archive-date=2012-11-22 | url-status=dead }}</ref> Thus, ''theoretical'' specific energies are from roughly 70&nbsp;kJ/kg at the motor shaft for a plain steel bottle to 180&nbsp;kJ/kg for an advanced fiber-wound one, whereas practical ''achievable'' specific energies for the same containers would be from 40 to 100&nbsp;kJ/kg.

==== Safety ====
As with most technologies, compressed air has safety concerns, mainly catastrophic tank rupture. Safety regulations make this a rare occurrence at the cost of higher weight and additional safety features such as pressure relief valves. Regulations may limit the legal working pressure to less than 40% of the rupture pressure for steel bottles (for a [[safety factor]] of 2.5) and less than 20% for fiber-wound bottles ([[safety factor]] 5). Commercial designs adopt the [[ISO 11439]] standard.<ref>{{cite ISO standard |csnumber=44755 |title=ISO 11439:2013 — Gas cylinders — High pressure cylinders for the on-board storage of natural gas as a fuel for automotive vehicles |date=June 2013}}</ref> High-pressure bottles are fairly strong so that they generally do not rupture in vehicle crashes.

=== Comparison with batteries ===
Advanced fiber-reinforced bottles are comparable to the [[rechargeable battery|rechargeable]] [[lead–acid battery]] in terms of energy density. Batteries provide nearly-constant voltage over their entire charge level, whereas the pressure varies greatly while using a pressure vessel from full to empty. It is technically challenging to design air engines to maintain high efficiency and sufficient power over a wide range of pressures. Compressed air can transfer power at very high flux rates, which meets the principal acceleration and deceleration objectives of transportation systems, particularly for [[hybrid vehicles]].

Compressed air systems have advantages over conventional batteries, including longer lifetimes of [[pressure vessel]]s and lower material toxicity. Newer battery designs such as those based on [[Lithium iron phosphate battery|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{{When|date=May 2024}} are more expensive than mass-produced batteries.

As with electric storage technology, compressed air is only as "clean" as the source of the energy that it stores. [[Life cycle assessment]] addresses the question of overall emissions from a given energy storage technology combined with a given mix of generation on a power grid.

=== Engine ===
{{Main|Pneumatic motor}}

A pneumatic motor or compressed-air engine uses the expansion of compressed air to drive the pistons of an engine, turn the [[axle]], or to drive a [[turbine]].

The following methods can increase efficiency:
* A continuous expansion turbine at high efficiency
* Multiple expansion stages
* Use of waste heat, notably in a hybrid [[heat engine]] design
* Use of environmental heat

A highly efficient arrangement uses high, medium, and low pressure pistons in series, with each stage followed by an airblast venturi that draws ambient air over an air-to-air [[heat exchanger]]. This warms the exhaust of the preceding stage and admits this preheated air to the following stage. The only exhaust gas from each stage is cold air, which can be as cold as {{convert|−15|°C|°F}}; the cold air may be used for [[air conditioning]] in a car.<ref name=autogenerated1 />

Additional heat can be supplied by burning fuel, as in 1904 for the [[Whitehead torpedo]].<ref>{{cite journal|last=Kirby |first=Geoff|url=http://www.btinternet.com/~philipr/torps.htm |title=Navies in Transition: A History of the Torpedo; The Early Days |journal=Journal of the Royal Navy Scientific Service |volume= 27 |issue= 1|date=1999-09-18 |access-date=2014-05-11 |url-status=dead |archive-url= https://archive.today/20120530070555/http://www.btinternet.com/~philipr/torps.htm |archive-date=May 30, 2012 |df=mdy }}</ref> This improves the range and speed available for a given tank volume at the cost of the additional fuel.

==== Cars ====
{{Main|Compressed-air vehicle|Compressed-air car}}

Since about 1990, several companies have claimed to be developing compressed-air cars, but none is available. Typically, the main claimed advantages are no roadside pollution, low cost, use of cooking oil for [[lubrication]], and integrated air conditioning.

The time required to refill a depleted tank is important for vehicle applications. "Volume transfer" moves pre-compressed air from a stationary tank to the vehicle tank almost instantaneously. Alternatively, a stationary or on-board [[Gas compressor|compressor]] can compress air on demand, possibly requiring several hours.

==== Ships ====
Large [[marine diesel engine]]s have started using compressed air, typically stored in large bottles between 20 and 30 bar, acting directly on the pistons via special starting valves to turn the crankshaft prior to beginning fuel injection. This arrangement is more compact and cheaper than an electric starter motor would be at such scales and able to supply the necessary burst of extremely high power without placing a prohibitive load on the ship's electrical generators and distribution system. Compressed air is commonly also used, at lower pressures, to control the engine and act as the spring force acting on the cylinder exhaust valves, and to operate other auxiliary systems and power tools on board, sometimes including pneumatic [[PID controller]]s. One advantage of this approach is that, in the event of an electrical blackout, ship systems powered by stored compressed air can continue functioning uninterrupted, and generators can be restarted without an electrical supply. Another is that pneumatic tools can be used in commonly-wet environments without the risk of electric shock.

==== Hybrid vehicles ====
{{Main|Hydraulic hybrid vehicle}}

While the air storage system offers a relatively low power density and vehicle range, its high efficiency is attractive for hybrid vehicles that use a conventional internal combustion engine as the main power source. The air storage can be used for [[regenerative braking]] and to optimize the cycle of the piston engine, which is not equally efficient at all power/RPM levels.

[[Robert Bosch GmbH|Bosch]] and [[PSA Peugeot Citroën]] have developed a hybrid system that uses hydraulics as a way to transfer energy to and from a compressed nitrogen tank. An up-to-45% reduction in fuel consumption is claimed, corresponding to 2.9 L / 100&nbsp;km (81 mpg, 69 g {{CO2}}/km) on the [[New European Driving Cycle]] (NEDC) for a compact frame like [[Peugeot 208]]. The system is claimed to be much more affordable than competing electric and flywheel [[KERS]] systems and is expected on road cars by 2016.<ref>{{cite web|url=http://www.greencarcongress.com/2013/01/psabosch-20130122.html |title=PSA Peugeot Citroën and Bosch developing hydraulic hybrid powertrain for passenger cars; 30% reduction in fuel consumption in NEDC, up to 45% urban; B-segment application in 2016 |publisher=Green Car Congress |date=2013-01-22 |access-date=2014-05-11}}</ref>

=== History of air engines ===
[[File:Compressed Air Loco.jpg|thumb|A compressed air locomotive by [[H. K. Porter, Inc.]], in use at the [[Homestake Mine (South Dakota)|Homestake Mine]] between 1928 and 1961]]
Air engines have been used since the 19th century to power [[mining|mine]] [[Fireless locomotive|locomotives]], pumps, drills, and trams, via centralized, city-level distribution. [[Racecar]]s use compressed air to start their [[internal combustion engine]] (ICE), and large [[diesel engine]]s may have starting [[pneumatic motor]]s.

== Types of systems ==
=== Hybrid systems ===
[[Brayton cycle]] engines compress and heat air with a fuel suitable for an [[internal combustion engine]]. For example, burning 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 engine]]s can recharge an [[Battery (electricity)|electric battery]]. The apparently-defunct [[Energine]] promoted its Pne-PHEV or Pneumatic Plug-in Hybrid Electric Vehicle-system.{{Citation needed|date=April 2008}}<ref>{{cite web |url=http://www.energine.com/phev/e_phev_03.php |title=Energine PHEV-system schematic |publisher=Energine.com |access-date=2014-05-11 |url-status=dead |archive-url= https://web.archive.org/web/20140513011738/http://www.energine.com/phev/e_phev_03.php |archive-date=May 13, 2014 |df=mdy-all }}</ref>

==== Existing hybrid systems ====
Huntorf, Germany in 1978, and [[McIntosh, Alabama]], U.S. in 1991 commissioned hybrid power plants.<ref name="DEP1"/><ref name='new.scientist.com/vol.195 no.2623 p.&nbsp;45'>{{cite journal|title=Squeeze the breeze: Want to get more electricity from the wind? The key lies beneath our feet |journal=New Scientist |date=November 17, 2007 |first=Daniel|last=Pendick |volume=195|issue=2623|page=4 |url=https://www.newscientist.com/channel/earth/mg19526231.700-rocks-could-be-novel-store-for-wind-energy.html |access-date=November 17, 2007 }}</ref> Both systems use off-peak energy for air compression and burn natural gas in the compressed air during the power-generating phase.

==== Future hybrid systems ====
The Iowa Stored Energy Park (ISEP) would have used [[aquifer]] storage rather than cavern storage. The ISEP was an innovative, 270-megawatt, $400 million compressed air energy storage (CAES) project proposed for in-service near Des Moines, Iowa, in 2015. The project was terminated after eight years in development because of site geological limitation, according to the U.S. Department of Energy.

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.<ref>{{cite web|url= http://www.firstenergycorp.com/NewsReleases/2009-11-23%20Norton%20Project.pdf |title=FirstEnergy Corp. Home |publisher=Firstenergycorp.com |date=2014-03-20 |access-date=2014-05-11}}</ref>

The RICAS2020 project attempts to use an abandoned mine for adiabatic CAES with heat recovery. The compression heat is stored in a tunnel section filled with loose stones,<!--temperature gradient--> so the compressed air is nearly cool when entering the main pressure storage chamber. The cool compressed air regains the heat stored in the stones when released back through a surface turbine, leading to higher overall efficiency.<ref>{{cite web|url= http://www.ricas2020.eu/project-objectives/ |title=Project Objectives|work=RICAS |access-date=20 February 2017}}</ref><ref>{{cite web|url= https://www.tu.no/artikler/forskning-luft-kan-bli-verdens-neste-batteri/376666 |title=FORSKNING: Luft kan bli verdens neste "batteri"|work=[[Teknisk Ukeblad]] |date=February 20, 2017|access-date=20 February 2017 |language=no}}</ref> A two-stage process has theoretical higher efficiency of around 70%.<ref>{{cite journal |last1=Wang |first1=Jidai |last2=Lu |first2=Kunpeng |last3=Ma |first3=Lan |last4=Wang |first4=Jihong |last5=Dooner |first5=Mark |last6=Miao |first6=Shihong |last7=Li |first7=Jian |last8=Wang |first8=Dan |title=Overview of Compressed Air Energy Storage and Technology Development |journal=Energies |date=13 July 2017 |volume=10 |issue=7 |pages=991 |doi=10.3390/en10070991|doi-access=free }}</ref>

=== Underwater storage ===

==== Bag/tank ====
Deep water in lakes and the ocean can provide pressure without requiring high-pressure vessels or drilling.<ref name="caes.aquifer">{{cite web |url=http://www.wapa.gov/es/pubs/ESB/2003/03Aug/esb084.htm |title=Wind plus compressed air equals efficient energy storage in Iowa proposal |access-date=April 29, 2008 |work=Energy Services Bulletin website |publisher=Western Area Power Administration |url-status=dead |archive-url=https://web.archive.org/web/20080509104206/http://www.wapa.gov/es/pubs/esb/2003/03Aug/esb084.htm |archive-date=May 9, 2008 |df=mdy-all }}</ref> The air goes into inexpensive, flexible containers such as plastic bags. Obstacles include the limited number of suitable locations and the need for high-pressure pipelines between the surface and the containers. Given the low cost of the containers, great pressure (and great depth) may not be as important. A key benefit of such systems is that charge and discharge pressures are a constant function of depth. [[Carnot heat engine|Carnot]] inefficiencies 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]]s.<ref>{{cite patent |inventor=Oliver Laing |title=Energy storage for off-peak electricity |country=US |status=Patent |number=4873828}}</ref>

==== Hydroelectric ====
A nearly [[Isobaric process|isobaric]] solution is possible by using the compressed gas to drive a [[hydroelectric]] system. This solution requires large pressure tanks on land (as well as underwater airbags). [[Hydrogen]] gas is the preferred fluid, since other gases suffer from substantial hydrostatic pressures at even relatively modest depths (~500 meters).

European electrical utility company [[E.ON]] has provided €1.4 million (£1.1 million) in funding to develop undersea air storage bags.<ref>{{cite press release|url=http://www.nottingham.ac.uk/news/pressreleases/2008/june/energybagsandsuperbatteries.aspx |title=Energy bags and super batteries|date=June 18, 2008|publisher=Nottingham University|url-status=dead|archive-url= https://web.archive.org/web/20110203142906/http://www.nottingham.ac.uk/news/pressreleases/2008/june/energybagsandsuperbatteries.aspx |archive-date=February 3, 2011|df=mdy-all}}</ref><ref>{{cite news| last=Haran | first=Brady | title=The man making 'wind bags' | website=BBC NEWS | date=2008-03-26 | url=http://news.bbc.co.uk/1/hi/england/nottinghamshire/7315059.stm }}</ref> 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.<ref>{{cite web|url=https://techcrunch.com/2011/07/09/hydrostor-power-storage-under-water |title=How Hydrostor Aims To Change The Power Game By Storing Energy Under Water|date=July 9, 2011|work=[[TechCrunch]] |first=Joseph |last=Puopolo}}</ref>

==== Buoy ====
When excess wind energy is available from offshore [[wind turbines]], a spool-tethered buoy can be pushed below the surface. When electricity demand rises, the buoy is allowed to rise towards the surface, generating power.<ref>{{Cite web |last=Casey |first=Tina |date=2024-06-03 |title=New Compressed Air Energy Storage Systems Vs. Li-ion Batteries |url=https://cleantechnica.com/2024/06/03/new-energy-storage-systems-from-thin-compressed-air-can-compete-with-li-ion-batteries/ |access-date=2024-06-08 |website=CleanTechnica |language=en-US}}</ref>

=== Nearly isothermal compression ===
[[File:Crowley's isothermal compressor & expander.jpg|thumb|upright=1.5|Schematic views of a nearly isothermal compressor and expander. Left view with the piston fully retracted, right view with the piston fully inserted.]]
A number of methods of nearly isothermal compression are being developed. Fluid Mechanics has a system with a heat absorbing and releasing structure (HARS) attached to a reciprocating piston.<ref>{{cite web |url=http://www.fluidmechanics.co.uk/isothermal-compression/ |title=Near Isothermal Compression and Expansion |date=May 28, 2015 |access-date=July 29, 2018}}</ref> Light Sail injects a water spray into a reciprocating cylinder.{{citation needed|date=July 2018}} SustainX uses an air-water foam mix inside a semi-custom, 120-rpm compressor/expander.<ref>{{cite web|url=https://www.smartgrid.gov/files/Final-Technical-Report-SustainX_DE-OE0000231.pdf |title=Technology Performance Report, SustainX Smart Grid Program |author=B R Bollinger |publisher=SustainX Inc |date=April 1, 2015}}</ref> All these systems ensure that the air is compressed with high [[thermal diffusivity]] compared to the speed of compression. Typically these compressors can run at speeds up to 1000&nbsp;rpm. To ensure high thermal diffusivity, the average distance a gas molecule is from a heat-absorbing surface is about 0.5&nbsp;mm. These nearly-isothermal compressors can also be used as nearly-isothermal expanders and are being developed to improve the round-trip efficiency of CAES.

== See also ==
{{Portal|Energy}}
* [[Alternative fuel vehicle]]
* [[Fireless locomotive]]
* [[Grid energy storage]]
* [[Hydraulic accumulator]]
* [[List of energy storage power plants]]
* [[Pneumatics]]
* [[Zero-emissions vehicle]]
* [[Cryogenic energy storage]]
* [[Pneumatic motor|Compressed-air engine]]

==References==
{{Reflist|colwidth=30em}}

== External links ==
{{Commons category|Compressed air energy storage}}
* Compressed Air System of Paris – technical notes [http://chestofbooks.com/crafts/scientific-american/sup6/The-Compressed-Air-System-Of-Paris.html Part 1] [http://chestofbooks.com/crafts/scientific-american/sup6/The-Compressed-Air-System-Of-Paris-Part-2.html Part 2] [http://chestofbooks.com/crafts/scientific-american/sup6/The-Compressed-Air-System-Of-Paris-Part-3.html Part 3] [http://chestofbooks.com/crafts/scientific-american/sup6/The-Compressed-Air-System-Of-Paris-Part-4.html Part 4] [http://chestofbooks.com/crafts/scientific-american/sup6/The-Compressed-Air-System-Of-Paris-Part-5.html Part 5] [http://chestofbooks.com/crafts/scientific-american/sup6/The-Compressed-Air-System-Of-Paris-Part-6.html Part 6 ] (Special supplement, Scientific American, 1921)
* [http://www.sandia.gov/media/NewsRel/NR2001/norton.htm Solution to some of country's energy woes might be little more than hot air] ([[Sandia National Labs]], [[United States Department of Energy|DoE]]).
* MSNBC article, [https://www.nbcnews.com/id/wbna10695864 Cities to Store Wind Power for Later Use], January 4, 2006
* [http://www.economist.com/science/displaystory.cfm?story_id=9539806 Power storage: Trapped wind]
* [https://web.archive.org/web/20071016063430/http://www.businessweek.com/magazine/content/07_41/b4053092.htm Catching The Wind In A Bottle A group of Midwest utilities is building a plant that will store excess wind power underground]
* [https://query.nytimes.com/gst/fullpage.html?res=9D0CEEDE103DF93AA1575AC0A967958260&sec=&spon=&pagewanted=print New York Times Article: Technology; Using Compressed Air To Store Up Electricity]
* [http://saurorja.org/2012/06/18/compressed-air-energy-storage-entropy-and-efficiency/ Compressed Air Energy Storage, Entropy and Efficiency]

{{emerging technologies|energy=yes}}

{{DEFAULTSORT:Compressed-Air Energy Storage}}
[[Category:Energy storage]]
[[Category:Compressed air power|*]]