Editing Compressed-air energy storage (section)

=== 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>