Editing Energy storage (section)

== 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
** [[Biofuel]]s
** [[Hydrate|Hydrated salts]]
** [[Hydrogen peroxide]]
** [[Power-to-gas]] ([[methane]], [[hydrogen storage]], [[oxyhydrogen]])
{{Div col end}}

=== Mechanical ===
[[File:20240706 Energy storage - renewable energy - battery - 100 ms.gif |thumb |Energy from sunlight or other renewable energy is converted to potential energy for storage in devices such as electric batteries. The stored potential energy is later converted to electricity that is added to the power grid, even when the original energy source is not available. In pumped hydro systems, energy from the source is used to lift water upward against the force of gravity, giving it potential energy that is later converted to electricity provided to the power grid.]]
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|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|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">{{Cite news |last=Wald |first=Matthew L. |date=2010-07-27 |title=Wind Drives Growing Use of Batteries |url=https://www.nytimes.com/2010/07/28/business/energy-environment/28storage.html |access-date=2025-04-24 |work=The New York Times |language=en-US |issn=0362-4331}}</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]], [[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">{{Cite web |title=Wayback Machine |url=http://www.xtrac.com/pdfs/Torotrak_Xtrac_CVT.pdf |archive-url=https://web.archive.org/web/20120311124324/http://www.xtrac.com/pdfs/Torotrak_Xtrac_CVT.pdf |archive-date=March 11, 2012 |access-date=2025-04-24 |website=www.xtrac.com |url-status=dead }}</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|publisher=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=The Scotsman|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&nbsp;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&nbsp; 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<ref>{{Cite web |title=Microsoft datacenter batteries to support growth of renewables on the power grid |url=https://news.microsoft.com/source/features/sustainability/ireland-wind-farm-datacenter-ups/ |access-date=2025-04-24 |website=Source |language=en-US}}</ref><ref>{{Cite web |title=What is an Uninterruptible Power Supply - Definition from TechTarget.com |url=https://www.techtarget.com/searchdatacenter/definition/uninterruptible-power-supply |access-date=2025-04-24 |website=Search Data Center |language=en}}</ref>]]
{{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 crystals 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]].

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" />

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
 |publisher    = [[Fraunhofer Society]]
 |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 | bibcode = 2015RSERv..45..207V }}{{Dead link|date=March 2024 |bot=InternetArchiveBot |fix-attempted=yes }}</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" />