Editing Compressed-air energy storage (section)

== Types ==
[[File:20240706 Energy storage - renewable energy - battery - 100 ms.gif |thumb |Energy from a source such as sunlight is used to compress air, giving it potential energy. The stored potential energy is later converted to electricity that is added to the power grid, even when the original energy source is not available.]]
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|url-access=subscription}}</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 |url-access=subscription}}</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|url-access=subscription}}</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}}