View source for Energy storage

{{Short description|Captured energy for later usage}}
{{Use mdy dates|date=September 2018}}
[[File:Stwlan.dam.jpg|thumb|The Llyn Stwlan dam of the [[Ffestiniog Power Station|Ffestiniog]] [[Pumped-storage hydroelectricity|Pumped Storage Scheme]] in Wales. The lower power station has four water turbines which can generate a total of 360 MW of electricity for several hours, an example of artificial energy storage and conversion.]]
{{Power engineering}}
{{Sustainable energy}}
'''Energy storage''' is the capture of [[energy]] produced at one time for use at a later time<ref>{{cite web |last1=Clarke |first1=Energy |title=Energy Storage |url=https://www.clarke-energy.com/energy-storage/ |website=Clarke Energy |access-date=5 June 2020 |archive-date=July 28, 2020 |archive-url=https://web.archive.org/web/20200728083046/https://www.clarke-energy.com/energy-storage/ |url-status=live }}</ref> to reduce imbalances between energy demand and energy production. A device that stores energy is generally called an [[Accumulator (energy)|accumulator]] or [[Battery (electricity)|battery]]. Energy comes in multiple forms including radiation, [[chemical energy|chemical]], [[gravitational potential energy|gravitational potential]], [[Electric potential energy|electrical potential]], electricity, elevated temperature, [[latent heat]] and [[kinetic energy|kinetic]]. Energy storage involves converting energy from forms that are difficult to store to more conveniently or economically storable forms.

Some technologies provide short-term energy storage, while others can endure for much longer. Bulk energy storage is currently dominated by hydroelectric dams, both conventional as well as pumped. [[Grid energy storage]] is a collection of methods used for energy storage on a large scale within an electrical power grid.

Common examples of energy storage are the [[rechargeable battery]], which stores chemical energy readily convertible to electricity to operate a mobile phone; the [[Hydroelectricity|hydroelectric]] dam, which stores energy in a [[reservoir]] as gravitational [[potential energy]]; and [[Ice storage air conditioning|ice storage]] tanks, which store ice frozen by cheaper energy at night to meet peak daytime demand for cooling. [[Green hydrogen]], from the [[electrolysis of water]], is a more economical means of long-term renewable energy storage in terms of [[capital expenditure]]s than [[pumped-storage hydroelectricity]] or batteries.{{Citation needed|date=December 2023|reason=Citations previously provided do not support claim}}<ref name="Schrotenboer" />{{Failed verification|date=December 2023|reason=No storage system cost comparison in paper}}<ref name="Lipták" />{{Failed verification|date=December 2023|reason=Storage system costs never mentioned in article}} [[Fossil fuel]]s such as coal and gasoline store ancient energy derived from sunlight by organisms that later died, became buried and over time were then converted into these fuels. [[Food]] (which is made by the same process as fossil fuels) is a form of energy stored in [[chemical]] form.
{{toclimit|3}}

== History ==
In the 20th century grid, electrical power was largely generated by burning fossil fuel. When less power was required, less fuel was burned.<ref name="IEEE2019"/> [[Hydropower]], a mechanical energy storage method, is the most widely adopted mechanical energy storage, and has been in use for centuries. Large hydropower [[dam]]s have been energy storage sites for more than one hundred years.<ref>{{Cite journal|last1=Hittinger|first1=Eric|last2=Ciez|first2=Rebecca E.|date=2020-10-17|title=Modeling Costs and Benefits of Energy Storage Systems|journal=Annual Review of Environment and Resources|language=en|volume=45|issue=1|pages=445–469|doi=10.1146/annurev-environ-012320-082101|doi-access=free|issn=1543-5938}}</ref> Concerns with air pollution, energy imports, and [[Climate change|global warming]] have spawned the growth of renewable energy such as solar and wind power.<ref name="IEEE2019">{{cite book | chapter=Optimizing microgrid using demand response and electric vehicles connection to microgrid | date=2019-07-30 | doi=10.1109/SGC.2017.8308873 | isbn=978-1-5386-4279-5 | title=2017 Smart Grid Conference (SGC) | last1=Liasi | first1=Sahand Ghaseminejad | last2=Bathaee | first2=Seyed Mohammad Taghi | pages=1–7 | s2cid=3817521 }}</ref> Wind power is uncontrolled and may be generating at a time when no additional power is needed. [[Solar power]] varies with cloud cover and at best is only available during daylight hours, while demand often peaks after sunset (''see'' [[duck curve]]). Interest in storing power from these intermittent sources grows as the [[renewable energy industry]] begins to generate a larger fraction of overall energy consumption.<ref name="Renewable and Sustainable Energy Reviews 2017 pp. 292–312">{{cite journal | title=Power to Gas projects review: Lab, pilot and demo plants for storing renewable energy and CO2 | journal=Renewable and Sustainable Energy Reviews | volume=69 | date=2017-03-01 | issn=1364-0321 | doi=10.1016/j.rser.2016.11.130 | pages=292–312 | last1=Bailera | first1=Manuel | last2=Lisbona | first2=Pilar | last3=Romeo | first3=Luis M. | last4=Espatolero | first4=Sergio | url=http://zaguan.unizar.es/record/75731 | archive-url=https://web.archive.org/web/20200310124229/http://zaguan.unizar.es/record/75731 | url-status=dead | archive-date=2020-03-10 }}</ref>

[[Off-the-grid|Off grid electrical]] use was a niche market in the 20th century, but in the 21st century, it has expanded. Portable devices are in use all over the world. Solar panels are now common in the rural settings worldwide. Access to electricity is now a question of economics and financial viability, and not solely on technical aspects. [[Electric vehicles]] are gradually replacing combustion-engine vehicles. However, powering long-distance transportation without burning fuel remains in development.

== Methods ==
[[File:Available storage technologies, their capacity and discharge time.jpg|thumb|right|400px|Comparison of various energy storage technologies]]

=== Outline ===
The following list includes a variety of types of energy storage:

{{Div col}}
* Fossil fuel storage
* Mechanical
** [[Spring (device)|Spring]]
** [[Compressed air energy storage]] (CAES)
** [[Fireless locomotive]]
** [[Flywheel energy storage]]
** [[#Solid_mass_gravitational|Solid mass gravitational]]
<!-- NOTE: [[Gravitational potential energy storage]] redirects to this article itself, i.e. Energy storage -->
** [[Hydraulic accumulator]]
** [[Pumped-storage hydroelectricity]] (a.k.a. pumped hydroelectric storage, PHS, or pumped storage hydropower, PSH)
** [[Thermal expansion]]
* Electrical, electromagnetic
** [[Capacitor]]
** [[Supercapacitor]]
** [[Superconducting magnetic energy storage]] (SMES, also superconducting storage coil)
* Biological
** [[Glycogen]]
** [[Starch]]
* Electrochemical (battery energy storage system, BESS)
** [[Flow battery]]
** [[Rechargeable battery]]
** [[UltraBattery]]
* Thermal
** [[Storage heater|Brick storage heater]]
** [[Cryogenic energy storage]], liquid air energy storage (LAES)
** [[Liquid nitrogen engine]]
** [[Eutectic system]]
** [[Ice storage air conditioning]]
** [[Molten salt heat storage|Molten salt storage]]
** [[Phase-change material]]
** [[Seasonal thermal energy storage]]
** [[Solar pond]]
** [[Steam accumulator]]
** [[Thermal energy storage]] (general)
* Chemical
** [[Biofuels]]
** [[Hydrate|Hydrated salts]]
** [[Hydrogen peroxide]]
** [[Power to gas]] ([[methane]], [[Hydrogen storage]], [[oxyhydrogen]])
{{Div col end}}

=== Mechanical ===
Energy can be stored in water pumped to a higher elevation using [[pumped storage]] methods or by moving solid matter to higher locations ([[gravity battery|gravity batteries]]). 
Other commercial mechanical methods include [[Compressed-air energy storage|compressing air]] and [[Flywheel energy storage|flywheels]] that convert electric energy into internal energy or kinetic energy and then back again when electrical demand peaks.

==== Hydroelectricity ====
{{Main|Hydroelectricity}}
[[Hydroelectric dam]]s with reservoirs can be operated to provide electricity at times of peak demand. 
Water is stored in the reservoir during periods of low demand and released when demand is high. 
The net effect is similar to pumped storage, but without the pumping loss.

While a hydroelectric dam does not directly store energy from other generating units, it behaves equivalently by lowering output in periods of excess electricity from other sources. 
In this mode, dams are one of the most efficient forms of energy storage, because only the timing of its generation changes. 
Hydroelectric turbines have a start-up time on the order of a few minutes.<ref name="Huggins2010">{{cite book|first=Robert A|last=Huggins|title=Energy Storage|url={{google books |plainurl=y |id=Nn5y9gQeIlwC|page=60}}|date=September 1, 2010|publisher=Springer|isbn=978-1-4419-1023-3|pages=60}}</ref>

==== Pumped hydro ====
[[File:Adam Beck Complex.jpg|thumb|300px|The [[Sir Adam Beck Hydroelectric Generating Stations|Sir Adam Beck Generating Complex]] at [[Niagara Falls, Ontario|Niagara Falls, Canada]], which includes a large [[Pumped-storage hydroelectricity|pumped storage hydroelectricity reservoir]] to provide an extra 174 MW of electricity during periods of peak demand.]]
{{Main|Pumped-storage hydroelectricity}}

Worldwide, [[pumped-storage hydroelectricity]] (PSH) is the largest-capacity form of active [[grid energy storage]] available, and, as of March 2012, the [[Electric Power Research Institute]] (EPRI) reports that PSH accounts for more than 99% of bulk storage capacity worldwide, representing around 127,000 [[Megawatt|MW]].<ref name="EconomistPSH" /> PSH [[Energy conversion efficiency|energy efficiency]] varies in practice between 70% and 80%,<ref name="EconomistPSH" /><ref name="thier" /><ref name="Levine" /><ref name="yang" /> with claims of up to 87%.<ref name="heco" />

At times of low electrical demand, excess generation capacity is used to pump water from a lower source into a higher reservoir. When demand grows, water is released back into a lower reservoir (or waterway or body of water) through a [[turbine]], generating electricity. Reversible turbine-generator assemblies act as both a pump and turbine (usually a [[Francis turbine]] design). Nearly all facilities use the height difference between two water bodies. Pure pumped-storage plants shift the water between reservoirs, while the "pump-back" approach is a combination of pumped storage and conventional [[hydroelectric plant]]s that use natural stream-flow.

==== Compressed air ====
[[Image:Compressed Air Loco.jpg|thumb|300px|A [[Fireless locomotive|compressed air locomotive]] used inside a mine between 1928 and 1961.]]
{{Main|Compressed air energy storage|Salt dome}}

Compressed air energy storage (CAES) uses surplus energy to compress air for subsequent electricity generation.<ref name="NYT-2010.07.28">Wild, Matthew, L. [https://www.nytimes.com/2010/07/28/business/energy-environment/28storage.html Wind Drives Growing Use of Batteries] {{Webarchive|url=https://web.archive.org/web/20191205102332/https://www.nytimes.com/2010/07/28/business/energy-environment/28storage.html |date=December 5, 2019 }}, ''[[The New York Times]]'', July 28, 2010, pp. B1.</ref> Small-scale systems have long been used in such applications as propulsion of mine locomotives. The compressed air is stored in an  [[underground reservoir]], such as a [[salt dome]].

Compressed-air energy storage (CAES) plants can bridge the gap between production volatility and load. CAES storage addresses the energy needs of consumers by effectively providing readily available energy to meet demand. Renewable energy sources like wind and solar energy vary. So at times when they provide little power, they need to be supplemented with other forms of energy to meet energy demand. Compressed-air energy storage plants can take in the surplus energy output of renewable energy sources during times of energy over-production. This stored energy can be used at a later time when demand for electricity increases or energy resource availability decreases.<ref>{{cite journal |last1=Keles |first1=Dogan |last2=Hartel |first2=Rupert |last3=Möst |first3=Dominik |last4=Fichtner |first4=Wolf |title=Compressed-air energy storage power plant investments under uncertain electricity prices: an evaluation of compressed-air energy storage plants in liberalized energy markets |journal=The Journal of Energy Markets |date=Spring 2012 |volume=5 |issue=1 |page=54 |id={{ProQuest|1037988494}} |doi=10.21314/JEM.2012.070 }}</ref>

[[Gas compressor|Compression]] of [[air]] creates heat; the air is warmer after compression. [[Thermal expansion|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, efficiency improves considerably.<ref name="NYTimes-2012.10.01" /> A CAES system can deal with the heat in three ways. Air storage can be [[adiabatic]], [[diabatic]], or [[isothermal]]. Another approach uses compressed air to power vehicles.<ref name="Auto.com-2004.03.18" /><ref name="Freep-2004.03.18" />

==== Flywheel ====
[[File:Example of cylindrical flywheel rotor assembly.png|thumb|right|The main components of a typical flywheel.]]
[[File:Flybrid Systems Kinetic Energy Recovery System.jpg|thumb|right|A Flybrid [[Kinetic Energy Recovery System]] [[Flywheel energy storage|flywheel]]. Built for use on [[Formula One|Formula 1 racing cars]], it is employed to recover and reuse kinetic energy captured during braking.]]
{{Main|Flywheel energy storage|Flywheel storage power system}}

Flywheel energy storage (FES) works by accelerating a rotor (a [[flywheel]]) to a very high speed, holding energy as [[rotational energy]]. When energy is added the rotational speed of the flywheel increases, and when energy is extracted, the speed declines, due to [[conservation of energy]].

Most FES systems use electricity to accelerate and decelerate the flywheel, but devices that directly use mechanical energy are under consideration.<ref name="Torotrak">[http://www.xtrac.com/pdfs/Torotrak_Xtrac_CVT.pdf Torotrak Toroidal variable drive CVT] {{webarchive|url=https://web.archive.org/web/20110516084534/http://www.xtrac.com/pdfs/Torotrak_Xtrac_CVT.pdf |date=May 16, 2011 }}, retrieved June 7, 2007.</ref>

FES systems have rotors made of high strength [[carbon-fiber]] composites, suspended by [[magnetic bearing]]s and spinning at speeds from 20,000 to over 50,000&nbsp;revolutions per minute (rpm) in a vacuum enclosure.<ref name="ScienceNews">{{Cite journal| last1 = Castelvecchi| first1 = Davide| title = Spinning into control: High-tech reincarnations of an ancient way of storing energy| doi = 10.1002/scin.2007.5591712010| journal = Science News| volume = 171| issue = 20| pages = 312–313| date = May 19, 2007| url = http://sciencewriter.org/flywheels-spinning-into-control/| access-date = May 8, 2014| archive-url = https://web.archive.org/web/20140606223717/http://sciencewriter.org/flywheels-spinning-into-control/| archive-date = June 6, 2014| url-status = dead}}</ref> Such flywheels can reach maximum speed&nbsp;("charge") in a matter of minutes. The flywheel system is connected to a combination [[electric motor]]/[[electric generator|generator]].

FES systems have relatively long lifetimes (lasting decades with little or no maintenance;<ref name="ScienceNews" /> full-cycle lifetimes quoted for flywheels range from in excess of 10<sup>5</sup>, up to 10<sup>7</sup>, cycles of use),<ref name="Investire">{{Cite web |url=http://www.itpower.co.uk/investire/pdfs/flywheelrep.pdf |title=Storage Technology Report, ST6 Flywheel |access-date=May 8, 2014 |archive-url=https://web.archive.org/web/20130114062530/http://www.itpower.co.uk/investire/pdfs/flywheelrep.pdf |archive-date=January 14, 2013 |url-status=dead }}</ref> high [[specific energy]] (100–130&nbsp;W·h/kg, or 360–500 kJ/kg)<ref name="Investire" /><ref name="pddnet">{{cite web |title=Next-gen Of Flywheel Energy Storage |url=http://www.pddnet.com/article-next-gen-of-flywheel-energy-storage/ |publisher=Product Design & Development |access-date=May 21, 2009 |url-status=dead |archive-url=https://web.archive.org/web/20100710052927/http://www.pddnet.com/article-next-gen-of-flywheel-energy-storage/ |archive-date=July 10, 2010  }}</ref> and [[power density]].

==== Solid mass gravitational {{anchor|Gravitational_potential_energy_storage}} ====
{{Main|Gravity battery}}
Changing the altitude of solid masses can store or release energy via an elevating system driven by an electric motor/generator. Studies suggest energy can begin to be released with as little as 1 second warning, making  the method a useful supplemental feed into an electricity grid to balance load surges.<ref>{{cite news|last1=Fraser|first1=Douglas|title=Edinburgh company generates electricity from gravity|work=BBC News|date=October 22, 2019|url=https://www.bbc.com/news/uk-scotland-scotland-business-50146801|access-date=14 January 2020|archive-date=July 28, 2020|archive-url=https://web.archive.org/web/20200728083135/https://www.bbc.com/news/uk-scotland-scotland-business-50146801|url-status=live}}</ref>

Efficiencies can be as high as 85% recovery of stored energy.<ref name="quartz" />

This can be achieved by siting the masses inside old vertical  mine shafts or in specially constructed towers where the heavy weights are  [[winch]]ed up to store energy and allowed a controlled descent to release it. At 2020 a prototype vertical store is being built in Edinburgh, Scotland <ref>{{Cite web|last=Gourley|first=Perry|date=31 August 2020|title=Edinburgh firm behind incredible gravity energy storage project hails milestone|url=https://www.edinburghnews.scotsman.com/business/edinburgh-firm-behind-incredible-gravity-energy-storage-project-hails-milestone-2955863|access-date=2020-09-01|website=www.edinburghnews.scotsman.com|language=en|archive-date=September 2, 2020|archive-url=https://web.archive.org/web/20200902003909/https://www.edinburghnews.scotsman.com/business/edinburgh-firm-behind-incredible-gravity-energy-storage-project-hails-milestone-2955863|url-status=live}}</ref>

Potential energy storage or gravity energy storage was under active development in 2013 in association with the [[California Independent System Operator]].<ref name="Economist-2012.03.03" /><ref name="Bloomberg-2012.09.06" /><ref name="Kernan" /> It examined the movement of earth-filled [[Hopper car|hopper rail cars]] driven by [[electric locomotive]]s from lower to higher elevations.<ref name="Scientific American-2014.03.25" />

Other proposed methods include:-

* using rails,<ref name="Scientific American-2014.03.25" /><ref>{{cite magazine |author=David Z. Morris |date=May 22, 2016 |title=Energy-Storing Train Gets Nevada Approval |url=http://fortune.com/2016/05/22/energy-storing-train-nevada/ |magazine=Fortune |access-date=August 20, 2018 |archive-date=August 20, 2018 |archive-url=https://web.archive.org/web/20180820140850/http://fortune.com/2016/05/22/energy-storing-train-nevada/ |url-status=live }}</ref> cranes,<ref name="quartz">{{cite news |title=Stacking concrete blocks is a surprisingly efficient way to store energy |author=Akshat Rathi |date=August 18, 2018 |url=https://qz.com/1355672/stacking-concrete-blocks-is-a-surprisingly-efficient-way-to-store-energy/ |work=Quartz |access-date=August 20, 2018 |archive-date=December 3, 2020 |archive-url=https://web.archive.org/web/20201203050354/https://qz.com/1355672/stacking-concrete-blocks-is-a-surprisingly-efficient-way-to-store-energy/ |url-status=live }}</ref> or elevators<ref>{{Cite web |date=2022-05-31 |title=Lift Energy Storage System: Turning skyscrapers into gravity batteries |url=https://newatlas.com/energy/lift-energy-skyscraper-batteries/ |access-date=2022-05-31 |website=New Atlas |language=en-US}}</ref> to move weights up and down;
* using high-altitude solar-powered balloon platforms supporting winches to raise and lower solid masses slung underneath them,<ref>{{cite web | title=StratoSolar gravity energy storage | url=http://www.stratosolar.com/gravity-energy-storage.html | access-date=August 20, 2018 | archive-date=August 20, 2018 | archive-url=https://web.archive.org/web/20180820110224/http://www.stratosolar.com/gravity-energy-storage.html | url-status=live }}</ref> 
* using winches supported by an ocean barge to take advantage of a 4&nbsp;km (13,000&nbsp;ft) elevation difference between the sea surface and the seabed,<ref>{{cite web |last1=Choi |first1=Annette |title=Simple Physics Solutions to Storing Renewable Energy |url=https://www.pbs.org/wgbh/nova/article/storing-renewable-energy/ |website=[[Nova (American TV program)|NOVA]] |publisher=[[PBS]] |date=May 24, 2017 |access-date=29 August 2019 |archive-date=August 29, 2019 |archive-url=https://web.archive.org/web/20190829141630/https://www.pbs.org/wgbh/nova/article/storing-renewable-energy/ |url-status=live }}</ref>

[[File:Fernwärmespeicher Theiss.jpg|thumb|District heating accumulation tower from Theiss near [[Krems an der Donau]] in [[Lower Austria]] with a thermal capacity of 2 GWh]]

=== Thermal ===
{{Main|Thermal energy storage|Molten salt|Seasonal thermal energy storage}}

Thermal energy storage (TES) is the temporary storage or removal of heat.

==== Sensible heat thermal ====

Sensible heat storage take advantage of [[sensible heat]] in a material to store energy.<ref>Layered Materials for Energy Storage and Conversion, Editors: Dongsheng Geng, Yuan Cheng, Gang Zhang, Royal Society of Chemistry, Cambridge 2019,</ref>

[[Seasonal thermal energy storage]] (STES) allows heat or cold to be used months after it was collected from waste energy or natural sources. The material can be stored in contained aquifers, clusters of boreholes in geological substrates such as sand or crystalline bedrock, in lined pits filled with gravel and water, or water-filled mines.<ref name=TES_BIES/> Seasonal thermal energy storage (STES) projects often have paybacks in four to six years.<ref name="Hellström" /> An example is [[Drake Landing Solar Community]] in Canada, for which 97% of the year-round heat is provided by solar-thermal collectors on garage roofs, enabled by a borehole thermal energy store (BTES).<ref name="Wong" /><ref name="DistrictEnergy.org-a" /><ref>[http://www.nrcan.gc.ca/media-room/news-release/2012/6586 Canadian Solar Community Sets New World Record for Energy Efficiency and Innovation] {{Webarchive|url=https://web.archive.org/web/20130430221347/http://www.nrcan.gc.ca/media-room/news-release/2012/6586 |date=April 30, 2013 }}, Natural Resources Canada, October 5, 2012.</ref> In Braedstrup, Denmark, [[Solar power in Denmark|the community's solar district heating system]] also uses STES, at a temperature of {{convert|65|C}}. A [[heat pump]], which runs only while surplus wind power is available. It is used to raise the temperature to {{convert|80|C|F}} for distribution. When wind energy is not available, a gas-fired boiler is used. Twenty percent of Braedstrup's heat is solar.<ref name="Solar District Heating" />

==== {{anchor|Latent heat thermal energy storage}} Latent heat thermal (LHTES) ====
Latent heat thermal energy storage systems work by transferring heat to or from a material to change its phase. A phase-change is the melting, solidifying, vaporizing or liquifying. Such a material is called a [[Phase-change material|phase change material]] (PCM). Materials used in LHTESs often have a high [[latent heat]] so that at their specific temperature, the phase change absorbs a large amount of energy, much more than sensible heat.<ref>{{cite journal |last1=Sekhara Reddy |first1=M.C. |last2=T. |first2=R.L. |last3=K. |first3=D.R |last4=Ramaiah |first4=P.V |title=Enhancement of thermal energy storage system using sensible heat and latent heat storage materials |journal=I-Manager's Journal on Mechanical Engineering |date=2015 |volume=5 |page=36 |id={{ProQuest|1718068707}} }}</ref>

A [[steam accumulator]] is a type of LHTES where the phase change is between liquid and gas and uses the [[latent heat of vaporization]] of water. [[Ice storage air conditioning]] systems use off-peak electricity to store cold by freezing water into ice. The stored cold in ice releases during melting process and can be used for cooling at peak hours.

==== Cryogenic thermal energy storage ====
{{main|Cryogenic energy storage}}

Air can be liquefied by cooling using electricity and stored as a cryogen with existing technologies. The liquid air can then be expanded through a turbine and the energy recovered as electricity. The system was demonstrated at a pilot plant in the  UK in 2012.<ref name=LAES2012/>
In 2019, Highview announced plans to build a 50 MW in the North of England and northern Vermont, with the proposed facility able to store five to eight hours of energy, for a 250-400 MWh storage capacity.<ref name=LAES2019/>

==== Carnot battery ====
{{main|Carnot battery}}

Electrical energy can be stored thermally by resistive heating or heat pumps, and the stored heat can be converted back to electricity via [[Rankine cycle]] or [[Brayton cycle]].<ref name="DumontFrate2020"/> This technology has been studied to retrofit coal-fired power plants into fossil-fuel free generation systems.<ref name = Kraemer2019/> Coal-fired boilers are replaced by high-temperature heat storage charged by excess electricity from renewable energy sources. In 2020, [[German Aerospace Center]] started to construct the world's first large-scale Carnot battery system, which has 1,000 MWh storage capacity.<ref name = DLR2020/>

=== Electrochemical ===

==== Rechargeable battery ====
[[File:Datacenter Backup Batteries.jpg|thumb|A rechargeable battery bank used as an [[uninterruptible power supply]] in a data center]]
{{main|Rechargeable battery|Battery storage power station}}

A rechargeable battery comprises one or more [[electrochemical cell]]s. It is known as a 'secondary cell' because its [[electrochemistry|electrochemical]] [[chemical reaction|reactions]] are electrically reversible. Rechargeable batteries come in many shapes and sizes, ranging from [[Button cell#Rechargeable variants|button cell]]s to megawatt grid systems.

Rechargeable batteries have lower total cost of use and environmental impact than non-rechargeable (disposable) batteries. Some rechargeable battery types are available in the same form factors as disposables. Rechargeable batteries have higher initial cost but can be recharged very cheaply and used many times.

Common rechargeable battery chemistries include:
* [[Lead–acid battery]]: Lead acid batteries hold the largest market share of electric storage products. A single cell produces about 2V when charged. In the charged state the metallic lead negative electrode and the [[lead sulfate]] positive electrode are immersed in a dilute [[sulfuric acid]] (H<sub>2</sub>SO<sub>4</sub>) [[electrolyte]]. In the discharge process electrons are pushed out of the cell as lead sulfate is formed at the negative electrode while the electrolyte is reduced to water.
** Lead-acid battery technology has been developed extensively. Upkeep requires minimal labor and its cost is low. The battery's available energy capacity is subject to a quick discharge resulting in a low life span and low energy density.<ref>{{cite journal |last1=Yao |first1=L. |last2=Yang |first2=B. |last3=Cui |first3=H. |last4=Zhuang |first4=J. |last5=Ye |first5=J. |last6=Xue |first6=J. |title=Challenges and progresses of energy storage technology and its application in power systems |journal=Journal of Modern Power Systems and Clean Energy |volume=4 |issue=4 |date=2016 |pages=520–521 |doi=10.1007/s40565-016-0248-x |doi-access=free }}</ref>
* [[Nickel–cadmium battery]] (NiCd): Uses [[nickel oxide hydroxide]] and metallic [[cadmium]] as [[electrode]]s. Cadmium is a toxic element, and was banned for most uses by the European Union in 2004. Nickel–cadmium batteries have been almost completely replaced by nickel–metal hydride (NiMH) batteries.
* [[Nickel–metal hydride battery]] (NiMH): First commercial types were available in 1989.<ref name="Aifantis et al" /> These are now a common consumer and industrial type. The battery has a hydrogen-absorbing [[alloy]] for the negative [[electrode]] instead of [[cadmium]].
* [[Lithium-ion battery]]: The choice in many consumer electronics and have one of the best [[specific energy|energy-to-mass ratios]] and a very slow [[self-discharge]] when not in use.
* [[Lithium-ion polymer battery]]: These batteries are light in weight and can be made in any shape desired.
* [[Aluminium]]-[[sulfur]] battery with [[rock salt crystal]]s as electrolyte: aluminium and sulfur are Earth-abundant materials and are much more cheaper than traditional Lithium.<ref>{{cite web|url=https://news.mit.edu/2022/aluminum-sulfur-battery-0824|title=A new concept for low-cost batteries|date=August 24, 2022|author=David L. Chandler}}</ref>

===== Flow battery =====
{{Main|Flow battery|Vanadium redox battery}}

A [[flow battery]] works by passing a solution over a membrane where ions are exchanged to charge or discharge the cell. [[Electrode potential#Potential difference of a cell assembled of two electrodes|Cell voltage]] is chemically determined by the [[Nernst equation]] and ranges, in practical applications, from 1.0 V to 2.2 V. Storage capacity depends on the volume of solution. A flow battery is technically akin both to a [[fuel cell]] and an [[Electrochemical cell|electrochemical accumulator cell]]. Commercial applications are for long half-cycle storage such as backup grid power.

==== Supercapacitor ====
[[File:Expo 2010 Electric Bus.jpg|thumb|One of a fleet of [[Capa vehicle|electric capabuses]] powered by supercapacitors, at a quick-charge station-bus stop, in service during [[Expo 2010|Expo 2010 Shanghai China]]. Charging rails can be seen suspended over the bus.]]
{{main|Supercapacitor}}

[[Supercapacitor]]s, also called electric double-layer capacitors (EDLC) or ultracapacitors, are a family of [[electrochemical capacitor]]s<ref name="Conway" /> that do not have conventional solid [[dielectric]]s. [[Capacitance]] is determined by two storage principles, double-layer capacitance and [[pseudocapacitance]].<ref name="Halper" /><ref name="Frackowiak1" />

Supercapacitors bridge the gap between conventional capacitors and [[Rechargeable battery|rechargeable batteries]]. They store the most energy per unit volume or mass ([[energy density]]) among capacitors. They support up to 10,000 [[farads]]/1.2 Volt,<ref name="Elton" /> up to 10,000 times that of [[electrolytic capacitor]]s, but deliver or accept less than half as much power per unit time ([[power density]]).<ref name="Conway" />

While supercapacitors have specific energy and energy densities that are approximately 10% of batteries, their power density is generally 10 to 100 times greater. This results in much shorter charge/discharge cycles. Also, they tolerate many more charge-discharge cycles than batteries.

Supercapacitors have many applications, including:
* Low supply current for memory backup in [[static random-access memory]] (SRAM)
* Power for cars, buses, trains, cranes and elevators, including energy recovery from braking, short-term energy storage and burst-mode power delivery

=== Chemical ===

==== Power to gas ====
[[File:World’s 1st Low-Emission Hybrid Battery Storage, Gas Turbine Peaker System.jpg|thumb|220px|right|The new technology helps reduce greenhouse gases and operating costs at two existing peaker plants in [[Norwalk, California|Norwalk]] and [[Rancho Cucamonga, California|Rancho Cucamonga]]. The 10-megawatt battery storage system, combined with the gas turbine, allows the peaker plant to more quickly respond to changing energy needs, thus increasing the reliability of the electrical grid.]]
{{Main|Power to gas}}

[[Power to gas]] is the conversion of [[electricity]] to a gaseous [[fuel]] such as [[hydrogen]] or [[methane]]. The three commercial methods use electricity to reduce [[water splitting|water]] into [[hydrogen]] and [[oxygen]] by means of [[electrolysis]].

In the first method, hydrogen is injected into the natural gas grid or is used for transportation. The second method is to combine the hydrogen with [[carbon dioxide]] to produce [[methane]] using a [[methanation]] reaction such as the [[Sabatier reaction]], or biological methanation, resulting in an extra energy conversion loss of 8%. The methane may then be fed into the natural gas grid. The third method uses the output gas of a [[wood gas generator]] or a [[biogas]] plant, after the [[biogas upgrader]] is mixed with the hydrogen from the electrolyzer, to upgrade the quality of the biogas.

===== Hydrogen =====
{{Main|Hydrogen storage}}
{{See also|Combined cycle hydrogen power plant|Hydrogen fuel cell power plant}}
The element [[hydrogen]] can be a form of stored energy. Hydrogen can produce electricity via a [[hydrogen fuel cell]]. [[Green hydrogen]], from [[electrolysis of water]], is a more economical means of long-term renewable energy storage in terms of [[capital expenditure]]s than [[pumped-storage hydroelectricity]] or [[Rechargeable battery|batteries]].{{Citation needed|date=December 2023|reason=Citations previously provided do not support claim}}<ref name="Schrotenboer">{{Cite journal |last1=Schrotenboer |first1=Albert H. |last2=Veenstra |first2=Arjen A.T. |last3=uit het Broek |first3=Michiel A.J. |last4=Ursavas |first4=Evrim |date=October 2022 |title=A Green Hydrogen Energy System: Optimal control strategies for integrated hydrogen storage and power generation with wind energy |url=https://pure.rug.nl/ws/portalfiles/portal/230184233/1_s2.0_S1364032122006323_main.pdf |journal=Renewable and Sustainable Energy Reviews |language=en |volume=168 |pages=112744 |doi=10.1016/j.rser.2022.112744|s2cid=250941369 }}</ref>{{Failed verification|date=December 2023|reason=No storage system cost comparison in paper}}<ref name="Lipták">{{Cite news |last=Lipták |first=Béla |date=January 24, 2022 |title=Hydrogen is key to sustainable green energy |work=Control |url=https://www.controlglobal.com/home/article/11288951/hydrogen-is-key-to-sustainable-green-energy |access-date=February 12, 2023}}</ref>{{Failed verification|date=December 2023|reason=Storage system costs never mentioned in article}}

At penetrations below 20% of the grid demand, renewables do not severely change the economics; but beyond about 20% of the total demand,<ref name="ZerrahnSchill2018"/> external storage becomes important. If these sources are used to make ionic hydrogen, they can be freely expanded. A 5-year community-based pilot program using [[wind turbine]]s and hydrogen generators began in 2007 in the remote community of [[Ramea, Newfoundland and Labrador]].<ref name="NaturalResourcesCan" /> A similar project began in 2004 on [[Utsira]], a small Norwegian island.

Energy losses involved in the [[hydrogen storage]] cycle come from the electrolysis of water, liquification or compression of the hydrogen and conversion to electricity.<ref name="PhysOrg.com-news-85074285" />

About 50&nbsp;kW·h (180 MJ) of solar energy is required to produce a kilogram of hydrogen, so the cost of the electricity is crucial. At $0.03/kWh, a common off-peak high-voltage line rate in the [[United States]], hydrogen costs $1.50 per kilogram for the electricity, equivalent to $1.50/gallon for [[gasoline]]. Other costs include the [[High pressure electrolysis|electrolyzer plant]], [[hydrogen compressor]]s or [[Liquid hydrogen|liquefaction]], storage and [[Hydrogen infrastructure|transportation]].{{Citation needed|date=October 2009}}

Hydrogen can also be produced from [[aluminum]] and [[water]] by stripping aluminum's naturally-occurring [[aluminum oxide]] barrier and introducing it to water. This method is beneficial because recycled aluminum cans can be used to generate hydrogen, however systems to harness this option have not been commercially developed and are much more complex than electrolysis systems.<ref name="Aluminum-Hydrogen" /> Common methods to strip the oxide layer include caustic catalysts such as [[sodium hydroxide]] and alloys with [[gallium]], [[Mercury (element)|mercury]] and other metals.<ref name="Woodall" />

[[Underground hydrogen storage]] is the practice of [[hydrogen storage]] in [[cave]]rns, [[salt dome]]s and depleted oil and gas fields.<ref name="Royal Society of Chemistry-a" /><ref name="Hyunder" /> Large quantities of gaseous hydrogen have been stored in caverns by [[Imperial Chemical Industries]] for many years without any difficulties.<ref name="Hyweb.de" /> The European Hyunder project indicated in 2013 that storage of wind and solar energy using underground hydrogen would require 85 caverns.<ref name="Hyunder.eu-b" />

[[Powerpaste]] is a [[magnesium]] and [[hydrogen]] -based fluid gel that releases hydrogen when reacting with [[water]]. It was [[invention|invented]], [[patent]]ed and is being developed by the ''Fraunhofer Institute for Manufacturing Technology and Advanced Materials'' (''IFAM'') of the [[Fraunhofer-Gesellschaft]].  Powerpaste is made by combining magnesium powder with hydrogen to form [[magnesium hydride]] in a process conducted at 350&nbsp;°C and five to six times [[atmospheric pressure]]. An [[ester]] and a [[Salt (chemistry)|metal salt]] are then added to make the finished product.  Fraunhofer states that they are building a production plant slated to start production in 2021, which will produce 4 tons of Powerpaste annually.<ref name="FraunhoferPowerpaste2021">{{cite press release
 |author       = <!--Not stated-->
 |title        = Hydrogen-powered drives for e-scooters
 |url          = https://www.fraunhofer.de/en/press/research-news/2021/february-2021/hydrogen-powered-drives-for-e-scooters.html
 |location     = 
 |publisher    = [[Fraunhofer Society]]
 |agency       = 
 |date         = 2021-02-01
 |access-date  = 2021-02-22
 |archive-date = February 3, 2021
 |archive-url  = https://web.archive.org/web/20210203211045/https://www.fraunhofer.de/en/press/research-news/2021/february-2021/hydrogen-powered-drives-for-e-scooters.html
 |url-status   = live
}}</ref> Fraunhofer has patented their invention in the [[United States]] and [[European Union|EU]].<ref name="FraunhoferPowerpaste2019">{{cite tech report
|first= Lars
|last= Röntzsch
|first2= Marcus
|last2= Vogt
|title= White paper - PowerPaste for off-grid power supply
|number= 
|institution= [[Fraunhofer Society]]
|url= https://www.researchgate.net/publication/331929208_PowerPaste_for_off-grid_power_supply
|date= February 2019
|access-date= February 22, 2021
|archive-date= February 7, 2021
|archive-url= https://web.archive.org/web/20210207214925/https://www.researchgate.net/publication/331929208_PowerPaste_for_off-grid_power_supply
|url-status= live
}}</ref> Fraunhofer claims that Powerpaste is able to store hydrogen energy at 10 times the [[energy density]] of a [[Lithium-ion battery|lithium battery]] of a similar dimension and is safe and convenient for automotive situations.<ref name="FraunhoferPowerpaste2021"/>

===== Methane =====
{{Main|Substitute natural gas}}

[[Methane]] is the simplest hydrocarbon with the molecular formula CH<sub>4</sub>. Methane is more easily stored and transported than hydrogen. Storage and combustion infrastructure (pipelines, [[gas holder|gasometers]], power plants) are mature.

Synthetic natural gas ([[syngas]] or SNG) can be created in a multi-step process, starting with hydrogen and oxygen. Hydrogen is then reacted with [[carbon dioxide]] in a [[Sabatier reaction|Sabatier process]], producing methane and water. Methane can be stored and later used to produce electricity. The resulting water is recycled, reducing the need for water. In the electrolysis stage, oxygen is stored for methane combustion in a pure oxygen environment at an adjacent power plant, eliminating [[nitrogen oxide]]s.

Methane combustion produces carbon dioxide (CO<sub>2</sub>) and water. The carbon dioxide can be recycled to boost the Sabatier process and water can be recycled for further electrolysis. Methane production, storage and combustion recycles the reaction products.

The CO<sub>2</sub> has economic value as a component of an energy storage vector, not a cost as in [[carbon capture and storage]].

==== Power to liquid ====
Power to liquid is similar to power to gas except that the hydrogen is converted into liquids such as [[methanol]] or [[ammonia]]. These are easier to handle than gases, and require fewer safety precautions than hydrogen. They can be used for [[transportation]], including [[aircraft]], but also for industrial purposes or in the power sector.<ref>{{cite journal | last1 = Varone | first1 = Alberto | last2 = Ferrari | first2 = Michele | year = 2015 | title = ''Power to liquid and power to gas: An option for the German Energiewende'' | url = http://publications.iass-potsdam.de/pubman/item/escidoc:896902| journal = [[Renewable and Sustainable Energy Reviews]] | volume = 45 | pages = 207–218 | doi = 10.1016/j.rser.2015.01.049 }}</ref>

==== Biofuels ====
{{Main|Biofuel}}

Various [[biofuels]] such as [[biodiesel]], [[straight vegetable oil|vegetable oil]], [[alcohol fuel]]s, or [[biomass]] can replace [[fossil fuels]]. Various chemical processes can convert the carbon and hydrogen in coal, natural gas, plant and animal [[biomass]] and organic wastes into short hydrocarbons suitable as replacements for existing hydrocarbon fuels. Examples are [[Fischer–Tropsch process|Fischer–Tropsch]] diesel, [[methanol]], [[dimethyl ether]] and [[syngas]]. This diesel source was used extensively in [[World War II]] in Germany, which faced limited access to crude oil supplies. South Africa produces most of the country's diesel from coal for similar reasons.<ref name="EPA-2002.a" /> A long term oil price above US$35/bbl may make such large scale synthetic liquid fuels economical.

===== Aluminum =====

[[Aluminum]] has been proposed as an energy store by a number of researchers. Its [[electrochemical equivalent]] (8.04 Ah/cm3) is nearly four times greater than that of lithium (2.06 Ah/cm3).<ref name="Aluminum-density" /> Energy can be extracted from aluminum by reacting it with water to generate [[hydrogen]].<ref name="Alchemy white paper" /> However, it must first be stripped of its natural [[oxide]] layer, a process which requires pulverization,<ref name="Army" /> chemical reactions with caustic substances, or alloys.<ref name="Woodall" /> The byproduct of the reaction to create hydrogen is [[aluminum oxide]], which can be recycled into aluminum with the [[Hall–Héroult process]], making the reaction theoretically renewable.<ref name="Woodall" /> If the Hall-Heroult Process is run using solar or wind power, aluminum could be used to store the energy produced at higher efficiency than direct solar electrolysis.<ref name="Aluminum Solar Storage" />

==== Boron, silicon, and zinc ====

[[Boron]],<ref name="Cowan" /> [[silicon]],<ref name="Auner" /> and [[zinc]]<ref name="Engineer-Poet" /> have been proposed as energy storage solutions.

==== Other chemical ====
The organic compound ''[[norbornadiene]]''&nbsp;converts to ''[[quadricyclane]]'' upon exposure to light, storing solar energy as the energy of chemical bonds. A working system has been developed in Sweden as a molecular solar thermal system.<ref>{{Cite web|url=https://www.sciencedaily.com/releases/2017/03/170320085445.htm|title=Liquid storage of solar energy: More effective than ever before|website=sciencedaily.com|access-date=March 21, 2017|archive-date=March 20, 2017|archive-url=https://web.archive.org/web/20170320203533/https://www.sciencedaily.com/releases/2017/03/170320085445.htm|url-status=live}}</ref>

=== Electrical methods ===

==== Capacitor ====
{{main|capacitor}}
[[File:Mylar-film oil-filled low-inductance capacitor 6.5 MFD @ 5000 VDC.jpg|thumb|175px|right|This mylar-film, oil-filled capacitor has very low inductance and low resistance, to provide the high-power (70 megawatts) and the very high speed (1.2 microsecond) discharges needed to operate a [[dye laser]].]]

A [[capacitor]] (originally known as a 'condenser') is a [[passivity (engineering)|passive]] [[terminal (electronics)|two-terminal]] [[electronic component|electrical component]] used to store [[energy]] [[electrostatic]]ally. Practical capacitors vary widely, but all contain at least two [[electrical conductor]]s (plates) separated by a [[dielectric]] (i.e., [[insulator (electricity)|insulator]]). A capacitor can store electric energy when disconnected from its charging circuit, so it can be used like a temporary [[Battery (electricity)|battery]], or like other types of [[rechargeable energy storage system]].<ref name="Miller" /> Capacitors are commonly used in electronic devices to maintain power supply while batteries change. (This prevents loss of information in volatile memory.) Conventional capacitors provide less than 360 [[joule]]s per kilogram, while a conventional [[alkaline battery]] has a density of 590 kJ/kg.

Capacitors store [[energy]] in an [[electric field|electrostatic field]] between their plates. Given a [[potential difference]] across the conductors (e.g., when a capacitor is attached across a battery), an [[electric field]] develops across the dielectric, causing positive charge (+Q) to collect on one plate and negative charge (-Q) to collect on the other plate. If a battery is attached to a capacitor for a sufficient amount of time, no current can flow through the capacitor. However, if an accelerating or alternating voltage is applied across the leads of the capacitor, a [[displacement current]] can flow. Besides capacitor plates, charge can also be stored in a dielectric layer.<ref>{{cite journal|last1=Bezryadin|first1=A.|last2=et.|first2=al.|title=Large energy storage efficiency of the dielectric layer of graphene nanocapacitors|journal=Nanotechnology|date=2017|volume=28|issue=49|pages=495401|doi=10.1088/1361-6528/aa935c|pmid=29027908|arxiv=2011.11867|bibcode=2017Nanot..28W5401B|s2cid=44693636}}</ref>

Capacitance is greater given a narrower separation between conductors and when the conductors have a larger surface area. In practice, the dielectric between the plates emits a small amount of [[leakage (electronics)|leakage current]] and has an electric field strength limit, known as the [[breakdown voltage]]. However, the effect of recovery of a dielectric after a high-voltage breakdown holds promise for a new generation of self-healing capacitors.<ref>{{cite journal|last1=Belkin|first1=Andrey|last2=et.|first2=al.|title=Recovery of Alumina Nanocapacitors after High Voltage Breakdown|journal=Sci. Rep.|volume=7|issue=1|pages=932|date=2017|doi=10.1038/s41598-017-01007-9|pmid=28428625|pmc=5430567|bibcode=2017NatSR...7..932B}}</ref><ref>{{cite journal|last1=Chen|first1=Y.|last2=et.|first2=al.|date=2012|title=Study on self-healing and lifetime characteristics of metallized-film capacitor under high electric field.|journal=IEEE Transactions on Plasma Science|volume=40|issue=8|pages=2014–2019|doi=10.1109/TPS.2012.2200699|bibcode=2012ITPS...40.2014C|s2cid=8722419}}</ref> The conductors and [[Lead (electronics)|lead]]s introduce undesired [[Equivalent series inductance|inductance]] and [[Equivalent series resistance|resistance]].

Research is assessing the quantum effects of [[Nanoscopic scale|nanoscale]] capacitors<ref>{{cite journal|last1=Hubler|first1=A.|last2=Osuagwu|first2=O.|title=Digital quantum batteries: Energy and information storage in nanovacuum tube arrays|journal=Complexity|date=2010|volume=15|issue=5 |pages=48–55|doi=10.1002/cplx.20306|doi-access=free}}</ref> for digital quantum batteries.<ref name="TechReview-2009.12.21" /><ref name="Complexity-Vol14.Iss3" />

==== Superconducting magnetics ====
{{main|Superconducting magnetic energy storage}}

Superconducting magnetic energy storage (SMES) systems store energy in a [[magnetic field]] created by the flow of [[direct current]] in a [[Superconductivity|superconducting]] coil that has been cooled to a temperature below its [[Superconductivity#Superconducting phase transition|superconducting critical temperature]]. A typical SMES system includes a superconducting [[inductor|coil]], power conditioning system and refrigerator. Once the superconducting coil is charged, the current does not decay and the magnetic energy can be stored indefinitely.<ref name="Hassenzahl" />

The stored energy can be released to the network by discharging the coil. The associated inverter/rectifier accounts for about 2–3% energy loss in each direction. SMES loses the least amount of [[electricity]] in the energy storage process compared to other methods of storing energy. SMES systems offer round-trip efficiency greater than 95%.<ref name="Cheung-Cheung-De Silvia-Juvonen-Singh-Woo" />

Due to the energy requirements of refrigeration and the cost of [[superconducting wire]], SMES is used for short duration storage such as improving [[power quality]]. It also has applications in grid balancing.<ref name="Hassenzahl" />

== Applications ==

=== Mills ===

The classic application before the [[industrial revolution]] was the control of waterways to drive water mills for processing grain or powering machinery. Complex systems of [[reservoir]]s and [[dam]]s were constructed to store and release water (and the [[potential energy]] it contained) when required.<ref>{{cite book|url=https://archive.org/details/encyclopediaofte10newy/page/1400/mode/2up|title=Encyclopedia of technology and applied sciences.|date=2000|publisher=Marshall Cavendish|isbn=076147126X|volume=10|location=New York|page=1401|quote=Simple waterwheels were used in the Balkans of Europe in 100 B.C.E for powering flour mills. Elaborate Irrigation systems had been built In Egypt and Mesopotamia a thousand years before that, and it is very likely that these systems contained simple waterwheels. Waterwheels powered by a stream running underneath were common in the Roman Empire during the third and fourth centuries C.E. After the fall of the Western Roman Empire, water technology advanced further in the Middle East than in Europe, but waterwheels were commonly used to harness water as a source of power in Europe during the Middle Ages. The Doomsday Book of 1086 C.E. lists 5624 water powered mills in the southern half of England. The designs of more efficient waterwheels were brought back to Europe from the Middle East by the Crusaders and were used for grinding grain and for powering furnace bellows.|access-date=31 December 2020}}</ref>

=== Homes ===
{{Main|Home energy storage}}

Home energy storage is expected to become increasingly common given the growing importance of distributed generation of renewable energies (especially photovoltaics) and the important share of energy consumption in buildings.<ref name="Silva-Hendrick" /> To exceed a self-sufficiency of 40% in a household equipped with photovoltaics, energy storage is needed.<ref name="Silva-Hendrick" /> Multiple manufacturers produce rechargeable battery systems for storing energy, generally to hold surplus energy from home solar or wind generation. Today, for home energy storage, Li-ion batteries are preferable to lead-acid ones given their similar cost but much better performance.<ref>{{Cite journal|last1=de Oliveira e Silva|first1=Guilherme|last2=Hendrick|first2=Patrick|date=June 1, 2017|title=Photovoltaic self-sufficiency of Belgian households using lithium-ion batteries, and its impact on the grid|journal=Applied Energy|volume=195|pages=786–799|doi=10.1016/j.apenergy.2017.03.112|bibcode=2017ApEn..195..786D |url=https://www.sciencedirect.com/science/article/pii/S0306261917303495|url-access=registration}}</ref>

[[Tesla Motors]] produces two models of the [[Tesla Powerwall]]. One is a 10 kWh weekly cycle version for backup applications and the other is a 7 kWh version for daily cycle applications.<ref name="businessinsider">{{cite news |first=Matthew |last=Debord |url=http://www.businessinsider.com/here-comes-teslas-missing-piece-battery-announcement-2015-4 |title=Elon Musk's big announcement: it's called 'Tesla Energy' |work=[[Business Insider]] |date=May 1, 2015 |access-date=June 11, 2015 |archive-date=May 5, 2015 |archive-url=https://web.archive.org/web/20150505021205/http://www.businessinsider.com/here-comes-teslas-missing-piece-battery-announcement-2015-4 |url-status=live }}</ref> In 2016, a limited version of the Tesla Powerpack 2 cost $398(US)/kWh to store electricity worth 12.5 cents/kWh (US average grid price) making a positive [[Tesla Powerwall#Return on investment calculations|return on investment]] doubtful unless electricity prices are higher than 30 cents/kWh.<ref name="Price-Powerpack-2016">{{cite web|url= https://electrek.co/2016/11/14/tesla-powerpack-2-price/|title= Tesla slashes price of the Powerpack system by another 10% with new generation|date= May 15, 2017|work= Electrek|access-date= November 14, 2016|archive-date= November 14, 2016|archive-url= https://web.archive.org/web/20161114235216/https://electrek.co/2016/11/14/tesla-powerpack-2-price/|url-status= live}}</ref>

RoseWater Energy produces two models of the "Energy & Storage System", the HUB 120<ref>{{Cite web|url=https://www.svconline.com/the-wire/rosewater-energy-group-debut-hub-120-cedia-2017-409670/409670|title=RoseWater Energy Group to Debut HUB 120 at CEDIA 2017|date=August 29, 2017|access-date=June 5, 2019|archive-url=https://web.archive.org/web/20190605152235/https://www.svconline.com/the-wire/rosewater-energy-group-debut-hub-120-cedia-2017-409670/409670|archive-date=June 5, 2019|url-status=dead}}</ref> and SB20.<ref>{{Cite web|url=https://rosewaterenergy.com/products/|title=Rosewater Energy - Products|access-date=June 5, 2019|archive-date=June 5, 2019|archive-url=https://web.archive.org/web/20190605152514/https://rosewaterenergy.com/products/|url-status=live}}</ref> Both versions provide 28.8 kWh of output, enabling it to run larger houses or light commercial premises, and protecting custom installations. The system provides five key elements into one system, including providing a clean 60&nbsp;Hz Sine wave, zero transfer time, industrial-grade surge protection, renewable energy grid sell-back (optional), and battery backup.<ref>{{Cite web|url=https://www.commercialintegrator.com/ci/rosewater_energy_the_cleanest_greenest_60k_power_supply_ever/|title=RoseWater Energy: The Cleanest, Greenest $60K Power Supply Ever|date=October 19, 2015|website=Commercial Integrator|access-date=June 5, 2019|archive-date=June 5, 2019|archive-url=https://web.archive.org/web/20190605152220/https://www.commercialintegrator.com/ci/rosewater_energy_the_cleanest_greenest_60k_power_supply_ever/|url-status=live}}</ref><ref>{{Cite web|url=https://www.cepro.com/cep/how_rosewaters_giant_home_battery_is_different_from_teslas/|title=How RoseWater's Giant Home Battery is Different from Tesla's|date=October 19, 2015|website=CEPRO|access-date=July 12, 2021|archive-date=July 12, 2021|archive-url=https://web.archive.org/web/20210712003902/https://www.cepro.com/cep/how_rosewaters_giant_home_battery_is_different_from_teslas/|url-status=live}}</ref>

[[Enphase Energy]] announced an integrated system that allows home users to store, monitor and manage electricity. The system stores 1.2 kWh of energy and 275W/500W power output.<ref>{{Cite web|title = Enphase plug-and-play solar energy storage system to begin pilot program|url = http://www.gizmag.com/enphase-ac-battery-for-pv-storage/39990|website = www.gizmag.com|access-date = December 20, 2015|date = October 29, 2015|last = Delacey|first = Lynda|archive-date = December 22, 2015|archive-url = https://web.archive.org/web/20151222234650/http://www.gizmag.com/enphase-ac-battery-for-pv-storage/39990/|url-status = live}}</ref>

Storing wind or solar energy using [[thermal energy storage]] though less flexible, is considerably cheaper than batteries. A simple 52-gallon electric water heater can store roughly 12 kWh of energy for supplementing hot water or space heating.<ref>{{cite web|url=http://www.popsci.com/need-high-power-home-battery-use-your-water-heater|title=Your Water Heater Can Become A High-Power Home Battery|website=popsci.com|date=April 7, 2016 |access-date=May 16, 2017|archive-date=May 5, 2017|archive-url=https://web.archive.org/web/20170505233536/http://www.popsci.com/need-high-power-home-battery-use-your-water-heater|url-status=live}}</ref>

For purely financial purposes in areas where [[net metering]] is available, home generated electricity may be sold to the grid through a [[grid-tie inverter]] without the use of batteries for storage.

=== Grid electricity and power stations ===
{{Main|Grid energy storage|Battery storage power station}}

==== Renewable energy ====
[[File:Abengoa Solar (7336087392).jpg|thumbnail| Construction of the Salt Tanks which provide efficient [[thermal energy storage]]<ref>Wright, matthew; Hearps, Patrick; et al. [http://media.bze.org.au/ZCA2020_Stationary_Energy_Report_v1.pdf Australian Sustainable Energy: Zero Carbon Australia Stationary Energy Plan] {{Webarchive|url=https://web.archive.org/web/20151124173114/http://media.bze.org.au/ZCA2020_Stationary_Energy_Report_v1.pdf |date=November 24, 2015 }}, Energy Research Institute, [[University of Melbourne]], October 2010, p. 33. Retrieved from BeyondZeroEmissions.org website.</ref> so that electricity can be generated after the sun goes down, and output can be scheduled to meet demand.<ref>[http://www.renewableenergyfocus.com/view/3272/innovation-in-concentrating-thermal-solar-power-csp/ Innovation in Concentrating Thermal Solar Power (CSP)] {{Webarchive|url=https://web.archive.org/web/20150924090041/http://www.renewableenergyfocus.com/view/3272/innovation-in-concentrating-thermal-solar-power-csp/ |date=September 24, 2015 }}, RenewableEnergyFocus.com website.</ref> The 280 MW [[Solana Generating Station]] is designed to provide six hours of storage. This allows the plant to generate about 38% of its rated capacity over the course of a year.<ref>{{cite web|url=http://blogs.phoenixnewtimes.com/valleyfever/2013/10/solana_10_facts_you_didnt_know.php|title=Solana: 10 Facts You Didn't Know About the Concentrated Solar Power Plant Near Gila Bend|author=Ray Stern|work=Phoenix New Times|access-date=December 6, 2015|archive-date=October 11, 2013|archive-url=https://web.archive.org/web/20131011235507/http://blogs.phoenixnewtimes.com/valleyfever/2013/10/solana_10_facts_you_didnt_know.php|url-status=dead}}</ref>]]

[[File:Andasol 3.jpg|thumb|right| The 150 MW [[Andasol solar power station]] in [[Renewable energy in Spain|Spain]] is a [[parabolic trough]] [[solar thermal]] power plant that stores energy in [[Thermal energy storage#Molten salt technology|tanks of molten salt]] so that it can continue generating electricity when the sun is not shining.<ref name="Cartlidge" />]]

The largest source and the greatest store of renewable energy is provided by hydroelectric dams. A large reservoir behind a dam can store enough water to average the annual flow of a river between dry and wet seasons, and a very large reservoir can store enough water to average the flow of a river between dry and wet years. While a hydroelectric dam does not directly store energy from intermittent sources, it does balance the grid by lowering its output and retaining its water when power is generated by solar or wind. If wind or solar generation exceeds the region's hydroelectric capacity, then some additional source of energy is needed.

Many [[renewable energy]] sources (notably solar and wind) produce [[variable renewable energy|variable power]].<ref name="NYTimes-2010.07.28" /> Storage systems can level out the imbalances between supply and demand that this causes. Electricity must be used as it is generated or converted immediately into storable forms.<ref name="Ingebretsen-Johansen" />

The main method of electrical grid storage is [[pumped-storage hydroelectricity]]. Areas of the world such as Norway, Wales, Japan and the US have used elevated geographic features for [[reservoir]]s, using electrically powered pumps to fill them. When needed, the water passes through generators and converts the gravitational potential of the falling water into electricity.<ref name="NYTimes-2010.07.28" /> Pumped storage in Norway, which gets almost all its electricity from hydro, has currently a capacity of 1.4 GW but since the total installed capacity is nearly 32 GW and 75% of that is regulable, it can be expanded significantly.<ref>[https://www.hydropower.org/country-profiles/norway "Norway statistics - International Hydropower Association"] {{Webarchive|url=https://web.archive.org/web/20180914022911/https://www.hydropower.org/country-profiles/norway |date=September 14, 2018 }}. Retrieved on September 13, 2018.</ref>

Some forms of storage that produce electricity include pumped-storage [[hydroelectric dams]], [[Rechargeable battery|rechargeable batteries]], [[thermal energy storage|thermal storage]] including [[Molten salt heat storage|molten salts]] which can efficiently store and release very large quantities of heat energy,<ref name="NYTimes-2014.04.21" /> and [[compressed air energy storage]], [[Flywheel energy storage|flywheels]], [[cryogenic energy storage|cryogenic systems]] and [[superconducting magnetic energy storage|superconducting magnetic coils]].

Surplus power can also be converted into [[power to gas|methane]] ([[sabatier process]]) with stockage in the natural gas network.<ref name="Schmid" /><ref name="NégaWatt" />

In 2011, the [[Bonneville Power Administration]] in the [[northwestern United States]] created an experimental program to absorb excess wind and hydro power generated at night or during stormy periods that are accompanied by high winds. Under central control, home appliances absorb surplus energy by heating ceramic bricks in [[Storage heater|special space heaters]] to hundreds of degrees and by boosting the temperature of modified [[Storage water heater|hot water heater tanks]]. After charging, the appliances provide home heating and hot water as needed. The experimental system was created as a result of a severe 2010 storm that overproduced renewable energy to the extent that all conventional power sources were shut down, or in the case of a nuclear power plant, reduced to its lowest possible operating level, leaving a large area running almost completely on renewable energy.<ref name="NYTimes-2011.11.05" /><ref name="NYTimes-2010.07.07" />

Another advanced method used at the former [[the Solar Project|Solar Two project]] in the United States and the [[Gemasolar Thermosolar Plant|Solar Tres Power Tower]] in Spain uses [[Thermal energy storage#Molten salt technology|molten salt]] to store thermal energy captured from the sun and then convert it and dispatch it as electrical power. The system pumps molten salt through a tower or other special conduits to be heated by the sun. Insulated tanks store the solution. Electricity is produced by turning water to steam that is fed to [[turbine]]s.

Since the early 21st century batteries have been applied to utility scale load-leveling and [[Utility frequency|frequency regulation]] capabilities.<ref name="NYTimes-2010.07.28" />

In [[vehicle-to-grid]] storage, electric vehicles that are plugged into the energy grid can deliver stored electrical energy from their batteries into the grid when needed.

=== Air conditioning ===
{{Main|Ice storage air conditioning}}

[[Thermal energy storage]] (TES) can be used for [[air conditioning]].<ref name="Calmac" /> It is most widely used for cooling single large buildings and/or groups of smaller buildings. Commercial air conditioning systems are the biggest contributors to peak electrical loads. In 2009, thermal storage was used in over 3,300 buildings in over 35 countries. It works by chilling material at night and using the chilled material for cooling during the hotter daytime periods.<ref name="NYTimes-2014.04.21" />

The most popular technique is [[Thermal energy storage#Air conditioning|ice storage]], which requires less space than water and is cheaper than fuel cells or flywheels. In this application, a standard chiller runs at night to produce an ice pile. Water circulates through the pile during the day to chill water that would normally be the chiller's daytime output.

A partial storage system minimizes capital investment by running the chillers nearly 24 hours a day. At night, they produce ice for storage and during the day they chill water. Water circulating through the melting ice augments the production of chilled water. Such a system makes ice for 16 to 18 hours a day and melts ice for six hours a day. Capital expenditures are reduced because the chillers can be just 40% - 50% of the size needed for a conventional, no-storage design. Storage sufficient to store half a day's available heat is usually adequate.

A full storage system shuts off the chillers during peak load hours. Capital costs are higher, as such a system requires larger chillers and a larger ice storage system.

This ice is produced when electrical utility rates are lower.<ref name="DistributedEnergy.com" /> Off-peak cooling systems can lower energy costs. The U.S. [[Green Building Council]] has developed the [[Leadership in Energy and Environmental Design]] (LEED) program to encourage the design of reduced-environmental impact buildings. Off-peak cooling may help toward LEED Certification.<ref>Air-Conditioning, Heating and Refrigeration Institute, Fundamentals of HVAC/R, Page 1263</ref>

Thermal storage for heating is less common than for cooling. An example of thermal storage is storing solar heat to be used for heating at night.

Latent heat can also be stored in technical [[Phase-change material|phase change materials]] (PCMs). These can be encapsulated in wall and ceiling panels, to moderate room temperatures.

=== Transport ===
Liquid [[hydrocarbon fuel]]s are the most commonly used forms of energy storage for use in [[transportation]], followed by a growing use of [[Battery Electric Vehicles]] and [[Hybrid Electric Vehicle]]s. Other energy carriers such as [[hydrogen]] can be used to avoid producing greenhouse gases.

Public transport systems like trams and trolleybuses require electricity, but due to their variability in movement, a steady supply of electricity via renewable energy is challenging. [[Photovoltaic]] systems installed on the roofs of buildings can be used to power public transportation systems during periods in which there is increased demand for electricity and access to other forms of energy are not readily available.<ref>{{cite journal |last1=Bartłomiejczyk |first1=Mikołaj |title=Potential Application of Solar Energy Systems for Electrified Urban Transportation Systems |journal=Energies |date=2018 |volume=11 |issue=4 |page=1 |doi=10.3390/en11040954 |doi-access=free }}</ref> Upcoming transitions in the transportation system also include e.g. ferries and airplanes, where electric power supply is investigated as an interesting alternative.<ref>{{Cite journal|last1=Brelje|first1=Benjamin J.|last2=Martins|first2=Joaquim R.R.A.|date=January 2019|title=Electric, hybrid, and turboelectric fixed-wing aircraft: A review of concepts, models, and design approaches |journal=Progress in Aerospace Sciences|language=en|volume=104|pages=1–19|doi=10.1016/j.paerosci.2018.06.004|bibcode=2019PrAeS.104....1B|doi-access=free}}</ref>

=== Electronics ===
Capacitors are widely used in [[electronic circuit]]s for blocking [[direct current]] while allowing [[alternating current]] to pass. In [[analog filter]] networks, they smooth the output of [[power supply|power supplies]]. In [[LC circuit|resonant circuit]]s they tune [[radio]]s to particular [[frequency|frequencies]]. In [[electric power transmission]] systems they stabilize voltage and power flow.<ref name="Bird" />

== Use cases ==
{{Main|United States Department of Energy International Energy Storage Database}}

The [[United States Department of Energy International Energy Storage Database]] (IESDB), is a free-access database of energy storage projects and policies funded by the [[United States Department of Energy]] Office of Electricity and [[Sandia National Labs]].<ref name="DOE Global Energy Storage Database" />

== Capacity ==
'''Storage capacity''' is the amount of energy extracted from an energy storage device or system; usually measured in [[joule]]s or [[Kilowatt hour|kilowatt-hours]] and their multiples, it may be given in number of hours of electricity production at power plant [[nameplate capacity]]; when storage is of primary type (i.e., thermal or pumped-water), output is sourced only with the power plant embedded storage system.<ref>{{cite web
 | url = http://www.nrel.gov/csp/troughnet/pdfs/nava_andasol_storage_system.pdf
 | title = Thermal Storage Concept for a 50 MW Trough Power Plant in Spain
 | last1 = Herrman
 | first1 = Ulf
 | last2 = Nava
 | first2 = Paul
 | date = February 13, 2016
 | website = www.nrel.gov
 | publisher = [[National Renewable Energy Laboratory|NREL]]
 | archive-url = https://web.archive.org/web/20160402190815/http://www.nrel.gov/csp/troughnet/pdfs/nava_andasol_storage_system.pdf
 | archive-date = April 2, 2016
 | url-status = dead
 | access-date = February 13, 2017 }}</ref><ref>{{cite web
 |url          = http://www.iea-eces.org/files/a6.5.2_2014-11-06_defintion_of_storage_capacity_fraunhofer_umsicht.pdf
 |title        = Electric Storage Devices – "Definition" of Storage Capacity, Power, Efficiency
 |last         = Doetsch
 |first        = Christian
 |date         = November 6, 2014
 |website      = www.iea-eces.org
 |archive-url  = https://web.archive.org/web/20170213095950/http://www.iea-eces.org/files/a6.5.2_2014-11-06_defintion_of_storage_capacity_fraunhofer_umsicht.pdf
 |archive-date = February 13, 2017
 |url-status     = dead
 |access-date  = February 13, 2017
 |df           = mdy-all
}}</ref>

== Economics{{anchor|Economic and technical evaluations}} ==
The economics of energy storage strictly depends on the reserve service requested, and several uncertainty factors affect the profitability of energy storage. Therefore, not every storage method is technically and economically suitable for the storage of several MWh, and the optimal size of the energy storage is market and location dependent.<ref>{{Cite journal|last1=Locatelli|first1=Giorgio |last2=Palerma|first2=Emanuele |last3=Mancini|first3=Mauro |date=April 1, 2015|title=Assessing the economics of large Energy Storage Plants with an optimisation methodology|journal=Energy|volume=83|pages=15–28 |doi=10.1016/j.energy.2015.01.050|doi-access=free|hdl=11311/965814|hdl-access=free}}</ref>

Moreover, ESS are affected by several risks, e.g.:<ref name="Locatelli 114–131">{{Cite journal|last1=Locatelli|first1=Giorgio|last2=Invernizzi|first2=Diletta Colette|last3=Mancini|first3=Mauro|date=June 1, 2016|title=Investment and risk appraisal in energy storage systems: A real options approach|journal=Energy|volume=104|pages=114–131|doi=10.1016/j.energy.2016.03.098|s2cid=62779581 |url=http://eprints.whiterose.ac.uk/97158/1/Paper_Final_Review%20V07.pdf|access-date=July 5, 2019|archive-date=July 19, 2018|archive-url=https://web.archive.org/web/20180719033048/http://eprints.whiterose.ac.uk/97158/1/Paper_Final_Review%20V07.pdf|url-status=live}}</ref>

* Techno-economic risks, which are related to the specific technology;
* Market risks, which are the factors that affect the electricity supply system;
* Regulation and policy risks.

Therefore, traditional techniques based on deterministic [[Discounted cash flow|Discounted Cash Flow]] (DCF) for the investment appraisal are not fully adequate to evaluate these risks and uncertainties and the investor's flexibility to deal with them. Hence, the literature recommends to assess the value of risks and uncertainties through the Real Option Analysis (ROA), which is a valuable method in uncertain contexts.<ref name="Locatelli 114–131"/>

The economic valuation of large-scale applications (including pumped hydro storage and compressed air) considers benefits including: [[Curtailment (electricity)|curtailment]] avoidance, grid congestion avoidance, price arbitrage and carbon-free energy delivery.<ref name="NYTimes-2014.04.21" /><ref name="Loisel et al" /><ref name="NYTimes-2012.01.03" /> In one technical assessment by the [[Carnegie Mellon University|Carnegie Mellon Electricity Industry Centre]], economic goals could be met using batteries if their capital cost was $30 to $50 per kilowatt-hour.<ref name="NYTimes-2014.04.21" />

A metric of energy efficiency of storage is energy storage on energy invested (ESOI), which is the amount of energy that can be stored by a technology, divided by the amount of energy required to build that technology. The higher the ESOI, the better the storage technology is energetically. For lithium-ion batteries this is around 10, and for lead acid batteries it is about 2. Other forms of storage such as pumped hydroelectric storage generally have higher ESOI, such as 210.<ref>{{cite web|url=http://news.stanford.edu/news/2013/march/store-electric-grid-030513.html|title=Stanford scientists calculate the carbon footprint of grid-scale battery technologies|work=Stanford University|date=March 5, 2013|access-date=November 13, 2015|archive-date=December 2, 2015|archive-url=https://web.archive.org/web/20151202225009/http://news.stanford.edu/news/2013/march/store-electric-grid-030513.html|url-status=live}}</ref>

[[Pumped-storage hydroelectricity]] is by far the largest storage technology used globally.<ref>{{Cite web|last=Perishable|title=Global Energy Storage Database {{!}} Energy Storage Systems|url=https://www.sandia.gov/ess-ssl/global-energy-storage-database-home/|access-date=2021-07-09|language=en-US|archive-date=July 9, 2021|archive-url=https://web.archive.org/web/20210709184735/https://www.sandia.gov/ess-ssl/global-energy-storage-database-home/|url-status=live}}</ref> However, the usage of conventional pumped-hydro storage is limited because it requires terrain with elevation differences and also has a very [[Surface power density|high land use for relatively small power]].<ref>{{Cite web|title=Hydropower Special Market Report – Analysis|url=https://www.iea.org/reports/hydropower-special-market-report|access-date=2021-07-09|website=IEA|language=en-GB|archive-date=July 9, 2021|archive-url=https://web.archive.org/web/20210709190936/https://www.iea.org/reports/hydropower-special-market-report|url-status=live}}</ref> In locations without suitable natural geography, underground pumped-hydro storage could also be used.<ref>{{Cite journal|last1=Vilanova|first1=Mateus Ricardo Nogueira|last2=Flores|first2=Alessandro Thiessen|last3=Balestieri|first3=José Antônio Perrella|date=2020-07-18|title=Pumped hydro storage plants: a review|url=https://doi.org/10.1007/s40430-020-02505-0|journal=Journal of the Brazilian Society of Mechanical Sciences and Engineering|language=en|volume=42|issue=8|pages=415|doi=10.1007/s40430-020-02505-0|s2cid=225550878|issn=1806-3691}}</ref> High costs and limited life still make batteries a "weak substitute" for [[Dispatchable generation|dispatchable power sources]], and are unable to cover for [[Variable renewable energy|variable renewable power]] gaps lasting for days, weeks or months. In grid models with high VRE share, the excessive cost of storage tends to dominate the costs of the whole grid — for example, in [[California]] alone 80% share of VRE would require 9.6 TWh of storage but 100% would require 36.3 TWh. As of 2018 the state only had 150 GWh of storage, primarily in pumped storage and a small fraction in batteries. According to another study, supplying 80% of US demand from VRE would require a smart grid covering the whole country or battery storage capable to supply the whole system for 12 hours, both at cost estimated at $2.5 trillion.<ref name="auto">{{Cite web|title=The $2.5 trillion reason we can't rely on batteries to clean up the grid|url=https://www.technologyreview.com/2018/07/27/141282/the-25-trillion-reason-we-cant-rely-on-batteries-to-clean-up-the-grid/|access-date=2021-07-09|website=MIT Technology Review|language=en|archive-date=August 24, 2021|archive-url=https://web.archive.org/web/20210824002106/https://www.technologyreview.com/2018/07/27/141282/the-25-trillion-reason-we-cant-rely-on-batteries-to-clean-up-the-grid/|url-status=live}}</ref><ref name="auto1">{{Cite web|title=Relying on renewables alone significantly inflates the cost of overhauling energy|url=https://www.technologyreview.com/2018/02/26/241113/relying-on-renewables-alone-would-significantly-raise-the-cost-of-overhauling-the-energy/|access-date=2021-07-09|website=MIT Technology Review|language=en|archive-date=August 13, 2021|archive-url=https://web.archive.org/web/20210813052731/https://www.technologyreview.com/2018/02/26/241113/relying-on-renewables-alone-would-significantly-raise-the-cost-of-overhauling-the-energy/|url-status=live}}</ref> Similarly, several studies have found that relying only on VRE and energy storage would cost about 30-50% more than a comparable system that combines VRE with [[Nuclear power plant|nuclear plants]] or plants with [[carbon capture and storage]] instead of energy storage.<ref>{{Cite journal|last1=Zappa|first1=William|last2=Junginger|first2=Martin|last3=van den Broek|first3=Machteld|date=January 2019|title=Is a 100% renewable European power system feasible by 2050?|journal=Applied Energy|language=en|volume=233-234|pages=1027–1050|doi=10.1016/j.apenergy.2018.08.109|s2cid=116855350|doi-access=free|bibcode=2019ApEn..233.1027Z }}</ref><ref>{{Cite journal|last1=Baird|first1=Zachariah Steven|last2=Neshumayev|first2=Dmitri|last3=Järvik|first3=Oliver|last4=Powell|first4=Kody M.|date=2021-12-30|title=Comparison of the most likely low-emission electricity production systems in Estonia|journal=PLOS ONE|language=en|volume=16|issue=12|pages=e0261780|doi=10.1371/journal.pone.0261780|issn=1932-6203|pmc=8717974|pmid=34968401|bibcode=2021PLoSO..1661780B|doi-access=free}}</ref>

== Research ==

=== Germany ===
In 2013, the German government allocated €200M (approximately US$270M) for research, and another €50M to subsidize battery storage in residential rooftop solar panels, according to a representative of the German Energy Storage Association.<ref name="NYTimes-2013.10.13" />

[[Siemens|Siemens AG]] commissioned a production-research plant to open in 2015 at the ''Zentrum für Sonnenenergie und Wasserstoff (ZSW,'' the German Center for Solar Energy and Hydrogen Research in the [[Baden-Württemberg|State of Baden-Württemberg]]), a university/industry collaboration in Stuttgart, Ulm and Widderstall, staffed by approximately 350 scientists, researchers, engineers, and technicians. The plant develops new near-production manufacturing materials and processes (NPMM&P) using a computerized [[SCADA|Supervisory Control and Data Acquisition]] (SCADA) system. It aims to enable the expansion of [[rechargeable battery]] production with increased quality and lower cost.<ref name="Physics.org.a" /><ref name="ZSW-BW.de" />

=== United States ===
In 2014, research and test centers opened to evaluate energy storage technologies. Among them was the Advanced Systems Test Laboratory at the [[University of Wisconsin–Madison|University of Wisconsin at Madison]] in [[Wisconsin|Wisconsin State]], which partnered with battery manufacturer [[Johnson Controls]].<ref name="Milwaukee Journal Sentinel-2014.05.05" /> The laboratory was created as part of the university's newly opened [[Wisconsin Energy Institute]]. Their goals include the evaluation of state-of-the-art and next generation [[Electric vehicle battery|electric vehicle batteries]], including their use as grid supplements.<ref name="Milwaukee Journal Sentinel-2014.05.05" />

The [[New York (state)|State of New York]] unveiled its New York Battery and Energy Storage Technology (NY-BEST) Test and Commercialization Center at [[Eastman Business Park]] in [[Rochester, New York]], at a cost of $23&nbsp;million for its almost 1,700&nbsp;m<sup>2</sup> laboratory. The center includes the Center for Future Energy Systems, a collaboration between [[Cornell University]] of [[Ithaca, New York]] and the [[Rensselaer Polytechnic Institute]] in [[Troy, New York]]. NY-BEST tests, validates and independently certifies diverse forms of energy storage intended for commercial use.<ref name="Democrat and Chronicle-2014.04.30" />

On September 27, 2017, Senators [[Al Franken]] of Minnesota and [[Martin Heinrich]] of New Mexico introduced Advancing Grid Storage Act (AGSA), which would devote more than $1 billion in research, technical assistance and grants to encourage energy storage in the United States.<ref>{{Cite news|url=https://pv-magazine-usa.com/2017/09/28/senators-want-more-than-1-billion-to-promote-energy-storage-answers/|title=Senators want more than $1 billion to promote energy storage answers|work=pv magazine USA|access-date=September 28, 2017|language=en-US|archive-date=September 28, 2017|archive-url=https://web.archive.org/web/20170928180958/https://pv-magazine-usa.com/2017/09/28/senators-want-more-than-1-billion-to-promote-energy-storage-answers/|url-status=live}}</ref>

In grid models with high [[Variable renewable energy|VRE]] share, the excessive cost of storage tends to dominate the costs of the whole grid — for example, in [[California]] alone 80% share of VRE would require 9.6 TWh of storage but 100% would require 36.3 TWh. According to another study, supplying 80% of US demand from VRE would require a smart grid covering the whole country or battery storage capable to supply the whole system for 12 hours, both at cost estimated at $2.5 trillion.<ref name="auto"/><ref name="auto1"/>

=== United Kingdom ===
In the United Kingdom, some 14 industry and government agencies allied with seven British universities in May 2014 to create the SUPERGEN Energy Storage Hub in order to assist in the coordination of energy storage technology research and development.<ref name="HVNPlus" /><ref name="ECNMag.com-2014.05.02" />

== See also ==
{{Portal|Energy|Renewable energy}}
{{Div col}}
* [[Efficient energy use]]
* [[Energy storage as a service]] (ESaaS)
* [[Grid energy storage]]
* [[Hybrid power]]
* [[List of energy storage power plants]]
* [[Outline of energy]]
* [[Power-to-X]]
* [[Power transmission]]
* [[Renewable energy]]
* [[Thermal battery]]
* [[Variable renewable energy]]
{{div col end}}

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<ref name=thier>Jacob, Thierry.[http://www.stucky.ch/en/contenu/pdf/Pumped_storage_in_Switzerland_Dr_Jacob.pdf Pumped storage in Switzerland - an outlook beyond 2000] {{Webarchive|url=https://web.archive.org/web/20110707003324/http://www.stucky.ch/en/contenu/pdf/Pumped_storage_in_Switzerland_Dr_Jacob.pdf |date=July 7, 2011 }} ''Stucky''. Accessed: February 13, 2012.</ref>

<ref name="Wong">Wong, B. (2013). [http://www.geo-exchange.ca/en/UserAttachments/flex1304_5-%20SAIC-%20Bill%20Wong%202013%20-%20Integrating%20Solar%20and%20Heat%20Pumps.pdf ''Integrating solar & heat pumps.''] {{Webarchive|url=https://web.archive.org/web/20160610063730/http://www.geo-exchange.ca/en/UserAttachments/flex1304_5-%20SAIC-%20Bill%20Wong%202013%20-%20Integrating%20Solar%20and%20Heat%20Pumps.pdf |date=June 10, 2016 }}.</ref>

<ref name=yang>Yang, Chi-Jen. [http://www.duke.edu/~cy42/PHS.pdf Pumped Hydroelectric Storage] {{webarchive|url=https://web.archive.org/web/20120905193845/http://www.duke.edu/~cy42/PHS.pdf |date=September 5, 2012 }} ''[[Duke University]]''. Accessed: February 12, 2012.</ref>

<ref name="ZerrahnSchill2018">{{cite journal|last1=Zerrahn|first1=Alexander|last2=Schill|first2=Wolf-Peter|last3=Kemfert|first3=Claudia|title=On the economics of electrical storage for variable renewable energy sources|journal=European Economic Review|volume=108|year=2018|pages=259–279|issn=0014-2921|doi=10.1016/j.euroecorev.2018.07.004|arxiv=1802.07885|s2cid=3484041|doi-access=free}}</ref>

<ref name="ZSW-BW.de">[http://www.zsw-bw.de/en/topics/batteries-and-supercapacitors/produktionsforschung.html Produktionsforschung | Prozessentwicklung und Produktionstechnik für große Lithium-Ionen-Zellen] {{webarchive|url=https://web.archive.org/web/20140512214955/http://www.zsw-bw.de/en/topics/batteries-and-supercapacitors/produktionsforschung.html |date=May 12, 2014 }}, '' Zentrum für Sonnenenergie- und Wasserstoff-Forschung Baden-Württemberg'' website, 2011. {{in lang|de}}</ref>

<ref name="Aluminum-Hydrogen">{{cite web |title=Safe, efficient way to produce hydrogen from aluminum particles and water for in-flight aircraft energy |url=https://phys.org/news/2017-04-safe-efficient-hydrogen-aluminum-particles.html |access-date=July 9, 2018 |archive-date=July 9, 2018 |archive-url=https://web.archive.org/web/20180709065507/https://phys.org/news/2017-04-safe-efficient-hydrogen-aluminum-particles.html |url-status=live }}</ref>

<ref name="Woodall">{{cite web |title = New process generates hydrogen from aluminum alloy to run engines, fuel cells |url = https://phys.org/news/2007-05-hydrogen-aluminum-alloy-fuel-cells.html |access-date = July 9, 2018 |archive-date = December 13, 2020 |archive-url = https://web.archive.org/web/20201213123359/https://phys.org/news/2007-05-hydrogen-aluminum-alloy-fuel-cells.html |url-status = live }}</ref>

<ref name="Aluminum-density">{{cite web |title = Overview of Lithium-Ion Batteries |url = http://large.stanford.edu/courses/2015/ph240/uang2/docs/panasonic-battery.pdf |publisher = [[Panasonic]] |access-date = July 9, 2018 |archive-date = September 21, 2018 |archive-url = https://web.archive.org/web/20180921172623/http://large.stanford.edu/courses/2015/ph240/uang2/docs/panasonic-battery.pdf |url-status = live }}</ref>

<ref name="Army">{{cite web |title = Army discovery may offer new energy source {{!}} U.S. Army Research Laboratory |url = https://www.arl.army.mil/www/default.cfm?article=3036 |website=arl.army.mil |language=en |access-date=July 9, 2018 |archive-url = https://web.archive.org/web/20180709064858/https://www.arl.army.mil/www/default.cfm?article=3036 |archive-date=July 9, 2018 |url-status=dead }}</ref>

<ref name="Aluminum Solar Storage">{{cite web |title = Current Efficiency, Specific Energy Consumption, Net Carbon Consumption - The Aluminum Smelting Process |url = http://www.aluminum-production.com/important_figures.html |website = aluminum-production.com |access-date = July 9, 2018 |archive-date = July 9, 2018 |archive-url = https://web.archive.org/web/20180709064754/http://www.aluminum-production.com/important_figures.html |url-status = live }}</ref>
}}

== Further reading ==

''' Journals and papers '''

* Chen, Haisheng; Thang Ngoc Cong; Wei Yang; Chunqing Tan; Yongliang Li; Yulong Ding. [http://ac.els-cdn.com/S100200710800381X/1-s2.0-S100200710800381X-main.pdf?_tid=a5208c44-dab8-11e3-8ec6-00000aacb362&acdnat=1399997413_3fc617bbbeca49f3fd02e976897508df Progress in electrical energy storage system: A critical review], ''Progress in Natural Science'', accepted July 2, 2008, published in Vol. 19, 2009, pp.&nbsp;291–312, doi: 10.1016/j.pnsc.2008.07.014. Sourced from the [[National Natural Science Foundation of China]] and the [[Chinese Academy of Sciences]]. Published by Elsevier and Science in China Press. Synopsis: a review of electrical energy storage technologies for stationary applications. Retrieved from ac.els-cdn.com on May 13, 2014. <small>(PDF)</small>
* Corum, Lyn. [https://web.archive.org/web/20130531074306/http://www.distributedenergy.com/DE/Articles/9027.aspx The New Core Technology: Energy storage is part of the smart grid evolution], ''The Journal of Energy Efficiency and Reliability'', December 31, 2009. Discusses: Anaheim Public Utilities Department, lithium ion energy storage, iCel Systems, Beacon Power, Electric Power Research Institute (EPRI), ICEL, Self Generation Incentive Program, ICE Energy, vanadium redox flow, lithium Ion, regenerative fuel cell, ZBB, VRB, lead acid, CAES, and Thermal Energy Storage. <small>(PDF)</small>
* {{cite journal | last1 = de Oliveira e Silva | first1 = G. | last2 = Hendrick | first2 = P. | year = 2016 | title = Lead-acid batteries coupled with photovoltaics for increased electricity self-sufficiency in households | journal = Applied Energy | volume = 178 | pages = 856–867 | doi = 10.1016/j.apenergy.2016.06.003 | bibcode = 2016ApEn..178..856D }}
* Whittingham, M. Stanley. [http://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=06184265 History, Evolution, and Future Status of Energy Storage], ''[[Proceedings of the IEEE]]'', manuscript accepted February 20, 2012, date of publication April 16, 2012; date of current version May 10, 2012, published in ''Proceedings of the IEEE'', Vol. 100, May 13, 2012, 0018–9219, pp.&nbsp;1518–1534, doi: 10.1109/JPROC.2012.219017. Retrieved from ieeexplore.ieee.org May 13, 2014. Synopsis: A discussion of the important aspects of energy storage including emerging battery technologies and the importance of storage systems in key application areas, including electronic devices, transportation, and the utility grid. <small>(PDF)</small>

''' Books '''

* GA Mansoori, N Enayati, LB Agyarko (2016), [http://www.worldscientific.com/worldscibooks/10.1142/9699 Energy: Sources, Utilization, Legislation, Sustainability, Illinois as Model State], World Sci. Pub. Co., {{ISBN|978-981-4704-00-7}}
* {{cite book|last1=Díaz-González|first1=Franscisco|title=Energy storage in power systems|date=2016|publisher=John Wiley & Sons|location=United Kingdom|isbn=9781118971321}}

== External links ==
{{Commons category|Energy storage}}
{{Wikiversity}}
* [https://web.archive.org/web/20100302155536/http://www.sandia.gov/ess/index.html U.S. Dept of Energy - Energy Storage Systems] Government research center on energy storage technology.
* [http://www.energystorageexchange.org/ U.S. Dept of Energy - International Energy Storage Database] {{Webarchive|url=https://web.archive.org/web/20131113220029/http://www.energystorageexchange.org/ |date=November 13, 2013 }} The DOE International Energy Storage Database provides free, up-to-date information on grid-connected energy storage projects and relevant state and federal policies.
* [https://scholar.google.com.au/citations?hl=en&user=FYow2b4AAAAJ IEEE Special Issue on Massive Energy Storage]
* [http://www.iea-eces.org/ IEA-ECES] - International Energy Agency - Energy Conservation through Energy Conservation programme.
* [http://www.eia.gov/tools/glossary/index.cfm?id=S Energy Information Administration Glossary]
* [https://regeneration.org/index.php/nexus/energy-storage Energy Storage] Project Regeneration.

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