View source for Grid energy storage

{{Short description|Large scale electricity supply management}}
{{Redirect|Grid storage|data storage with grid computing|Grid-oriented storage}}
{{Use dmy dates|date=October 2020}}
[[File:grid energy storage.png|thumb|Simplified electrical grid with energy storage]]
[[File:grid storage energy flow.png|thumb|Simplified grid energy flow with and without idealized energy storage for the course of one day]]

'''Grid energy storage''' (also called '''large-scale energy storage''') is a collection of methods used for [[energy storage]] on a large scale within an [[grid (electricity)|electrical power grid]]. Electrical energy is stored during times when electricity is plentiful and inexpensive (especially from [[Variable renewable energy|intermittent power]] sources such as [[renewable electricity]] from [[wind power]], [[tidal power]] and [[solar power]]) or when demand is low, and later returned to the grid when demand is high, and electricity prices tend to be higher.

{{As of|2020}}, the largest form of grid energy storage is dammed [[hydroelectricity]], with both conventional hydroelectric generation as well as [[pumped-storage hydroelectricity]].<ref>{{Cite web|url=https://www.pnnl.gov/sites/default/files/media/file/Final%20-%20ESGC%20Cost%20Performance%20Report%2012-11-2020.pdf |title=Grid Energy Storage Technology Cost and Performance Assessment |website=US Department of Energy |access-date=23 December 2021}}</ref><ref>{{Cite web|url=https://www.pnnl.gov/ESGC-cost-performance |title=Energy Storage Cost and Performance Database |website=US Department of Energy |access-date=23 December 2021}}</ref>

Developments in [[electric battery|battery]] storage have enabled commercially viable projects to store energy during peak production and release during peak demand, and for use when production unexpectedly falls giving time for slower responding resources to be brought online. [[Green hydrogen]], which is generated from [[electrolysis of water]] via electricity generated by renewables or relatively lower carbon emission sources, is a more economical means of long-term renewable energy storage in terms of [[capital expenditure]]s than pumped-storage hydroelectricity or batteries.<ref name="Schrotenboer" /><ref name="Lipták" />

Two alternatives to grid storage are the use of [[peaking power plant]]s to fill in supply gaps and [[demand response]] to shift load to other times.
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==Benefits==
Any [[grid (electricity)|electrical power grid]] must match electricity production to consumption, both of which vary drastically over time.  Any combination of energy storage and demand response has these advantages:
* fuel-based power plants (i.e. coal, oil, gas, nuclear) can be more efficiently and easily operated at constant production levels
* electricity generated by intermittent sources can be stored and used later, whereas it would otherwise have to be transmitted for sale elsewhere, or shut down
* peak generating or transmission capacity can be reduced by the total potential of all storage plus deferrable loads (see [[demand side management]]), saving the expense of this capacity
* more stable pricing – the cost of the storage or demand management is included in pricing so there is less variation in power rates charged to customers, or alternatively (if rates are kept stable by law) less loss to the utility from expensive on-peak wholesale power rates when peak demand must be met by imported wholesale power
* emergency preparedness – vital needs can be met reliably even with no transmission or generation going on while non-essential needs are deferred

Energy derived from solar, tidal and wind sources inherently varies on time scales ranging from minutes to weeks or longer&nbsp;– the amount of electricity produced varies with time of day, moon phase, season, and random factors such as the weather.  Thus, renewables in the absence of storage present special challenges to electric utilities.  While hooking up many separate wind sources can reduce the overall variability, solar is reliably not available at night, and tidal power shifts with the moon, so slack tides occur four times a day.

How much this affects any given utility varies significantly. In a [[summer peak]] utility, more solar can generally be absorbed and matched to demand.  In [[winter peak]] utilities, to a lesser degree, wind correlates to heating demand and can be used to meet that demand.  Depending on these factors, beyond about 20–40% of total generation, grid-connected [[intermittent energy source|intermittent source]]s such as [[solar power]] and [[wind power]] tend to require investment in grid interconnections, grid energy storage or demand-side management.

In an [[grid (electricity)|electrical grid]] without energy storage, generation that relies on energy stored within fuels (coal, biomass, natural gas, nuclear) must be scaled up and down to match the rise and fall of electrical production from intermittent sources (see [[load following power plant]]).  While hydroelectric and natural gas plants can be quickly scaled up or down to follow the demand, wind, coal and nuclear plants take considerable time to respond to load.  Utilities with less natural gas or hydroelectric generation are thus more reliant on demand management, grid interconnections or costly pumped storage.

The French consulting firm Yole Développement estimates the "stationary storage" market could be a $13.5 billion opportunity by 2023, compared with less than $1 billion in 2015.<ref>{{cite web|last1=Smit|first1=Debra|title=Jay Whitacre and the edible battery|date=24 August 2015|url=http://www.ozy.com/rising-stars/jay-whitacre-and-the-edible-battery/60826|website=Ozy|access-date=2016-06-15|ref=Ozy|url-status=live|archive-url=https://web.archive.org/web/20160608062631/http://www.ozy.com/rising-stars/jay-whitacre-and-the-edible-battery/60826|archive-date=8 June 2016}}</ref>

===Demand side management and grid storage===
[[File:Energy-Units-01.png|thumb|A sense of units and scale for electrical energy production and consumption]]

The demand side can also store electricity from the grid, for example charging a [[battery electric vehicle]] stores energy for a vehicle and [[storage heater]]s, [[District heating#Heat accumulators and storage|district heating storage]] or [[ice storage air conditioning|ice storage]] provide thermal storage for buildings.<ref>{{cite web |url=https://www.energy.gov/sites/prod/files/2014/09/f18/Grid%20Energy%20Storage%20December%202013.pdf |title=Grid Energy Storage |date=December 2013 |publisher=[[U.S. Department of Energy]] |page=28 |access-date=2017-02-13 |url-status=live |archive-url=https://web.archive.org/web/20170228214713/https://energy.gov/sites/prod/files/2014/09/f18/Grid%20Energy%20Storage%20December%202013.pdf |archive-date=28 February 2017}}</ref> At present this storage serves only to shift consumption to the off-peak time of day, no electricity is returned to the grid.

The need for grid storage to provide peak power is reduced by demand side [[Dynamic pricing#Time-based utility pricing|time of use pricing]], one of the benefits of [[smart meter]]s. At the household level, consumers may choose less expensive off-peak times to wash and dry clothes, use dishwashers, take showers and cook. As well, commercial and industrial users will take advantage of cost savings by deferring some processes to off-peak times.

Regional impacts from the unpredictable operation of wind power has created a new need for interactive [[demand response]], where the utility communicates with the demand. Historically this was only done in cooperation with large industrial consumers, but now may be expanded to entire grids.<ref>{{cite web |url=http://www.raponline.org/wp-content/uploads/2016/05/synapse-hurley-demandresponseasapowersystemresource-2013-may-31.pdf |title=Demand Response as a Power System Resource |author1=Doug Hurley |author2=Paul Peterson |author3=Melissa Whited |publisher=RAP Energy Solutions, Synapse Energy Economics |date=May 2013 |page=13 |access-date=2017-02-13 |url-status=live |archive-url=https://web.archive.org/web/20170430205055/http://www.raponline.org/wp-content/uploads/2016/05/synapse-hurley-demandresponseasapowersystemresource-2013-may-31.pdf |archive-date=30 April 2017}}</ref> For instance, a few large-scale projects in Europe link variations in wind power to change industrial food freezer loads, causing small variations in temperature. If communicated on a grid-wide scale, small changes to heating/cooling temperatures would instantly change consumption across the grid.

A report released in December 2013 by the [[United States Department of Energy]] further describes the potential benefits of energy storage and demand side technologies to the electric grid: "Modernizing the electric system will help the nation meet the challenge of handling projected energy needs—including [[Climate change mitigation|addressing climate change]] by integrating more energy from renewable sources and enhancing efficiency from non-renewable energy processes. Advances to the electric grid must maintain a robust and resilient electricity delivery system, and energy storage can play a significant role in meeting these challenges by improving the operating capabilities of the grid, lowering cost and ensuring high reliability, as well as deferring and reducing infrastructure investments. Finally, energy storage can be instrumental for emergency preparedness because of its ability to provide backup power as well as grid stabilization services".<ref name=energy-department-releases-grid-energy-storage-report>{{cite news|url=https://www.energy.gov/articles/energy-department-releases-grid-energy-storage-report|date=12 December 2013|title=Energy Department Releases Grid Energy Storage Report|url-status=live|archive-url=https://web.archive.org/web/20170513022854/https://energy.gov/articles/energy-department-releases-grid-energy-storage-report|archive-date=13 May 2017}}</ref> The report was written by a core group of developers representing [[Office of Electricity Delivery and Energy Reliability]], [[ARPA-E]], [[Office of Science]], [[Office of Energy Efficiency and Renewable Energy]], [[Sandia National Laboratories]], and [[Pacific Northwest National Laboratory]]; all of whom are engaged in the development of grid energy storage.<ref name= energy-department-releases-grid-energy-storage-report />

===Energy storage for grid applications===
Energy storage assets are a valuable asset for the [[electrical grid]].<ref>{{cite journal |last1=Lai |first1=Chun Sing |last2=Locatelli |first2=Giorgio |last3=Pimm |first3=Andrew |last4=Wu |first4=Xiaomei |last5=Lai |first5=Loi Lei |title=A review on long-term electrical power system modeling with energy storage |journal=Journal of Cleaner Production |date=September 2020 |volume=280 |pages=124298 |doi=10.1016/j.jclepro.2020.124298|doi-access=free |hdl=11311/1204822 |hdl-access=free }}</ref> They can provide benefits and services such as [[load management]], [[power quality]] and [[uninterruptible power supply]] to increase the efficiency and supply security. This becomes more and more important in regard to the [[energy transition]] and the need for a more efficient and sustainable energy system.

Numerous energy storage technologies ([[pumped-storage hydroelectricity]], [[electric battery]], [[flow battery]], [[flywheel energy storage]], [[supercapacitor]] etc.) are suitable for grid-scale applications, however their characteristics differ. For example, a pumped-hydro station is well suited for bulk load management applications due to their large capacities and power capabilities. However, suitable locations are limited and their usefulness fades when dealing with localized [[power quality]] issues. On the other hand, flywheels and capacitors are most effective in maintaining [[power quality]] but lack storage capacities to be used in larger applications. These constraints are a natural limitation to the storage's applicability.

Several studies have developed interest and investigated the suitability or selection of the optimal energy storage for certain applications. Literature surveys comprise the available information of the state-of-the-art and compare the storage's uses based on current existing projects.<ref>{{Cite journal |last1=Palizban|first1=Omid |last2=Kauhaniemi|first2=Kimmo |date=May 2016 |title=Energy storage systems in modern grids—Matrix of technologies and applications |journal=Journal of Energy Storage|volume=6|pages=248–259|doi=10.1016/j.est.2016.02.001}}</ref><ref>{{Cite journal |last1=Luo|first1=Xing |last2=Wang|first2=Jihong |last3=Dooner|first3=Mark |last4=Clarke|first4=Jonathan |date=1 January 2015|title=Overview of current development in electrical energy storage technologies and the application potential in power system operation |journal=Applied Energy |volume=137 |pages=511–536 |doi=10.1016/j.apenergy.2014.09.081|doi-access=free }}</ref> Other studies take a step further in evaluating energy storage with each other and rank their fitness based on [[multiple-criteria decision analysis]].<ref>{{Cite journal |last1=Daim|first1=Tugrul U. |last2=Li|first2=Xin |last3=Kim|first3=Jisun |last4=Simms |first4=Scott |date=June 2012 |title=Evaluation of energy storage technologies for integration with renewable electricity: Quantifying expert opinions |journal=Environmental Innovation and Societal Transitions |volume=3|pages=29–49 |doi=10.1016/j.eist.2012.04.003 }}</ref><ref>{{Cite book |last1=Pham|first1=Cong-Toan |last2=Månsson|first2=Daniel |date=November 2015 |chapter=Suitability analysis of Fuzzy Logic as an evaluation method for the selection of energy storage technologies in Smart Grid applications |volume=2015 International Symposium on Smart Electric Distribution Systems and Technologies (EDST) |pages=452–457 |doi=10.1109/SEDST.2015.7315251|title=2015 International Symposium on Smart Electric Distribution Systems and Technologies (EDST) |isbn=978-1-4799-7736-9 |s2cid=42921444 |chapter-url=http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-173722 }}</ref> Another paper proposed an evaluation scheme through the investigation and modelling of storage as equivalent circuits.<ref>{{Cite journal |last1=Pham |first1=Cong-Toan |last2=Månsson|first2=Daniel |date=October 2017 |title=On the physical system modelling of energy storages as equivalent circuits with parameter description for variable load demand (Part I) |journal=Journal of Energy Storage |volume=13 |pages=73–84 |doi=10.1016/j.est.2017.05.015|url=http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-212218 }}</ref><ref>{{Cite journal |last1=Pham|first1=Cong-Toan |last2=Månsson|first2=Daniel |date=August 2018 |title=Optimal energy storage sizing using equivalent circuit modelling for prosumer applications (Part II) |journal=Journal of Energy Storage |volume=18 |pages=1–15 |doi=10.1016/j.est.2018.04.015|s2cid=64857425 }}</ref> An indexing approach has also been suggested in a few studies, but is still in the novel stages.<ref>{{Cite journal |last1=Raza|first1=Syed Shabbar |last2=Janajreh|first2=Isam |last3=Ghenai|first3=Chaouki |date=December 2014 |title=Sustainability index approach as a selection criteria for energy storage system of an intermittent renewable energy source  |journal=Applied Energy |volume=136 |pages=909–920 |doi=10.1016/j.est.2018.04.015|s2cid=64857425 }}</ref> In order to gain increased economic potential of grid connected energy storage systems, it is of interest to consider a portfolio with several services for one or more applications for an energy storage system. By doing so, several revenue streams can be achieved by a single storage and thereby also increasing the degree of utilization.<ref>{{Cite journal|title=A MILP model for optimising multi-service portfolios of distributed energy storage|pages=554–566|journal=Applied Energy|volume=137|doi=10.1016/j.apenergy.2014.08.080|hdl=10044/1/39706|date=January 2015|last1=Moreno|first1=Rodrigo|last2=Moreira|first2=Roberto|last3=Strbac|first3=Goran|url=http://spiral.imperial.ac.uk/bitstream/10044/1/39706/2/AE_MainBody_v110614.pdf|hdl-access=free}}</ref> To mention two examples, a combination of [[Frequency response (electrical grid)|frequency response]] and reserve services is examined in,<ref>{{Cite journal|title=A closed-loop analysis of grid scale battery systems providing frequency response and reserve services in a variable inertia grid|pages=961–972|journal=Applied Energy|volume=236|doi=10.1016/j.apenergy.2018.12.044|date=15 February 2019|last1=Lee|first1=Rachel|last2=Homan|first2=Samuel|last3=Mac Dowell|first3=Niall|last4=Brown|first4=Solomon|s2cid=116444177 |url=http://eprints.whiterose.ac.uk/140156/3/Manuscript_clean.pdf}}</ref> meanwhile load peak shaving together with power smoothing is considered in.<ref>{{Cite journal|title=Load peak shaving and power smoothing of a distribution grid with high renewable energy penetration|pages=1372–1379|journal=Renewable Energy|volume=86|doi=10.1016/j.renene.2015.09.050|date=February 2016|last1=Reihani|first1=Ehsan|last2=Motalleb|first2=Mahdi|last3=Ghorbani|first3=Reza|last4=Saad Saoud|first4=Lyes|doi-access=free}}</ref>

==Forms==
===Air===

====Compressed air====
{{Main|Compressed-air energy storage}}
One grid energy storage method is to use off-peak or renewably generated electricity to [[compressed air|compress air]], which is usually stored in an old [[mining|mine]] or some other kind of geological feature. When electricity demand is high, the compressed air is heated with a small amount of [[natural gas]] and then goes through [[turboexpander]]s to generate electricity.<ref>{{citation|doi=10.1016/S0262-4079(07)62476-2|title=Storing energy from the wind in compressed-air reservoirs|journal=New Scientist|volume=195|issue=2623|pages=44–47|year=2007|last1=Pendick|first1=Daniel}}</ref>

Compressed air storage is typically around 60–90% efficient.<ref>{{cite web|url=http://cleantechnica.com/2013/02/21/lightsail-gets-5-5-million-for-compressed-air-energy-storage/#gsc.tab=0|title=LightSail Gets $5.5M From Total, Thiel, Khosla, Gates for Compressed Air Energy Storage|work=[[CleanTechnica]]|date=21 February 2013}}</ref>

====Liquid air====
{{Main|Liquid air|Cryogenic energy storage}}
Another electricity storage method is to compress and cool air, turning it into liquid air,<ref>{{cite web|title=The Resurgence of Liquid Air for Energy Storage|url=http://www.technologyreview.com/news/514936/liquefied-air-could-power-cars-and-store-energy-from-sun-and-wind/|work=MIT Technology Review|access-date=2013-06-07|author=Kevin Bullis|date=20 May 2013}}</ref> which can be stored, and expanded when needed, turning a turbine, generating electricity, with a storage efficiency of up to 70%.<ref>{{cite web|url=http://www.extremetech.com/extreme/137231-british-company-efficient-energy-storage|title=British company offers efficient energy storage using 'liquid air'|work=ExtremeTech|url-status=live|archive-url=https://web.archive.org/web/20121214130342/http://www.extremetech.com/extreme/137231-british-company-efficient-energy-storage|archive-date=14 December 2012}}</ref>

A commercial liquid-air energy storage plant is under construction in the North of England,
<ref name="BBC22Oct2019">{{cite news |title=How liquid air could help keep the lights on |work=BBC News |date=22 October 2019 |url=https://www.bbc.co.uk/news/business-50140110 |access-date=23 October 2019}}</ref>
<ref name="HV21Oct2019">{{cite web |title=Highview Power to Develop Multiple Cryogenic Energy Storage Facilities in the UK and to Build Europe's Largest Storage System |url=https://www.highviewpower.com/news_announcement/highview-power-to-develop-multiple-cryogenic-energy-storage-facilities-in-the-uk-and-to-build-europes-largest-storage-system/ |publisher=Highview power |access-date=23 October 2019}}</ref>
<ref name="BBC-20201107">{{cite news |last1=Roger |first1=Harrabin |title=UK energy plant to use liquid air |date=6 November 2020 |url=https://www.bbc.co.uk/news/business-54841528 |publisher=BBC News |access-date=7 November 2020}}</ref>
<ref name="HP-20201107">{{cite web |title=Highview Power Breaks Ground on 250MWh CRYOBattery Long Duration Energy Storage Facility |url=https://highviewpower.com/news_announcement/highview-power-breaks-ground-on-250mwh-cryobattery-long-duration-energy-storage-facility/ |website=Company News and Announcements |publisher=Highview Power |access-date=7 November 2020}}</ref>
with commercial operation planned for 2022.<ref name="EIT-202009">
{{cite journal |url=https://highviewpower.com/wp-content/uploads/2020/09/TEIT-Sept-2020.pdf|author1=Junior Isles |title=Really cool storage |journal=The Energy Industry Times |date=September 2020 |volume= 13 |issue= 5 |page=15 |access-date=7 November 2020 |issn=1757-7365}}</ref>
The energy storage capacity of 250MWh of the plant will be nearly twice the capacity of the world's largest existing lithium-ion battery, the [[Hornsdale Power Reserve]] in South Australia.<ref name="Economist-20201130">{{cite news |title=Powering the future: Electrical energy can be captured as liquid air |url=https://www.economist.com/science-and-technology/2019/11/30/electrical-energy-can-be-captured-as-liquid-air |access-date=8 November 2020 |newspaper=The Economist |date=Nov 30, 2019}}</ref>

====Compressed carbon dioxide====
{{main|Compressed carbon dioxide energy storage}}
Gaseous carbon dioxide can be compressed to store energy at grid scale. The gas is well suited to this role because, unlike air, it liquifies at ambient temperatures. Liquid {{CO2}} can be stored indefinitely in high-pressure cylinders, for use when needed. 
<ref name="Verge-20221012">{{cite web |author1=Justine Calma |title=Meet the {{CO2}} battery cozying up with a wind energy giant |url=https://www.theverge.com/2022/10/12/23399567/co2-battery-wind-solar-renewable-energy-storage |publisher=The Verge |access-date=16 April 2023 |date=12 Oct 2022 |quote=t faces stiff competition, but the {{CO2}} battery has some unique strengths that could accelerate the transition to clean energy}}</ref>
<ref name="Maccarini">{{cite web |author1=Mr. Simone Maccarini |title=The Carbon Dioxide for energy storage applications |url=https://netl.doe.gov/sites/default/files/netl-file/21LEAP_S5_Maccarini.pdf |website=DoE |publisher=THERMOCHEMICAL POWER GROUP, UNIVERSITY OF GENOA (ITALY) |access-date=16 April 2023 |date=2021}}</ref>

The main proponent of the technology is start-up company Energy Dome, which in 2022 built a 2.5MW/4MWh demonstrator plant in Sardinia. The company claim a round trip efficiency of 75% and a projected cost of EUR220/kWh of storage capacity, which is half that of Li-ion batteries.
<ref name="ESN-20230412">{{cite web |author1=Andy Colthorpe |title=Energy Dome: Turning a greenhouse gas into a cheaper form of energy storage than lithium-ion batteries |url=https://www.energy-storage.news/energy-dome-turning-a-greenhouse-gas-into-a-cheaper-form-of-energy-storage-than-lithium-ion-batteries/ |website=Editors blog |publisher=Energy Storage News |access-date=16 April 2023 |date=12 April 2023}}</ref>
<ref>{{cite web | url=https://www.youtube.com/watch?v=wftcfc901Fo&t=68s | title=Energy Dome: This new battery uses CO₂ to store wind and solar power | website=[[YouTube]] }}</ref>
<ref name="Ansaldo">{{cite web |title={{CO2}} battery licensed by Energy Dome |url=https://www.ansaldoenergia.com/offering/solutions-for-the-transition/energy-storage/co2-battery |website=Power generation: news and insights |publisher=Ansaldo Energia |access-date=16 April 2023}}</ref>

===Batteries===
{{Main|Battery storage power station}}
[[File:Light-plant-Fig1198-Page989-Ch45-Hawkins-Electrical-Guide.png|thumb|A 900 watt direct current light plant using 16 separate lead acid battery cells (32 volts) from 1917.<ref name="Hawkins1917">{{cite book|first=Nehemiah |last=Hawkins|title=Hawkins Electrical Guide ...: Questions, Answers & Illustrations; a Progressive Course of Study for Engineers, Electricians, Students and Those Desiring to Acquire a Working Knowledge of Electricity and Its Applications; a Practical Treatise|url={{google books |plainurl=y |id=yyEVAAAAYAAJ|page=989}}|year=1917|publisher=T. Audel & Company|pages=989–}}</ref>]]
[[File:Battery-cost-learning-curve.png|thumb| [[Learning curve]] of lithium-ion batteries: the price of batteries declined by 97% in three decades.<ref>{{Cite journal |last1=Ziegler |first1=Micah S. |last2=Trancik |first2=Jessika E. |date=2021 |title=Re-examining rates of lithium-ion battery technology improvement and cost decline |url=http://xlink.rsc.org/?DOI=D0EE02681F |journal=Energy & Environmental Science |language=en |volume=14 |issue=4 |pages=1635–1651 |doi=10.1039/D0EE02681F |s2cid=220830992 |issn=1754-5692|doi-access=free |arxiv=2007.13920 }}</ref><ref>{{Cite web |title=The price of batteries has declined by 97% in the last three decades |url=https://ourworldindata.org/battery-price-decline |access-date=2022-04-26 |website=Our World in Data}}</ref>]]
Battery storage was used in the early days of [[direct current]] electric power. Where AC grid power was not readily available, isolated lighting plants run by [[wind turbine]]s or internal combustion engines provided lighting and power to small motors. The battery system could be used to run the load without starting the engine or when the wind was calm. A bank of lead–acid batteries in glass jars both supplied power to illuminate lamps, as well as to start an engine to recharge the batteries. Battery storage technology is typically around 80% to more than 90% efficient for newer lithium-ion devices.<ref>{{cite web |url=http://www.greentechmedia.com/articles/read/Aquion-Energys-Disruptive-Battery-Tech-Picks-Up-35M-in-VC |author=Eric Wesoff |date=2 April 2013 |title=Aquion Energy's Disruptive Battery Tech Picks Up $35M in VC |work=greentechmedia.com |url-status=live |archive-url=https://web.archive.org/web/20130806181853/http://www.greentechmedia.com/articles/read/Aquion-Energys-Disruptive-Battery-Tech-Picks-Up-35M-in-VC |archive-date=6 August 2013}}</ref><ref>{{Cite web |url=https://cleantechnica.com/2015/05/09/tesla-powerwall-powerblocks-per-kwh-lifetime-prices-vs-aquion-energy-eos-energy-imergy/ |title=Tesla Powerwall & Powerpacks Per-kWh Lifetime Prices vs Aquion Energy, Eos Energy, & Imergy |date=9 May 2015 |author=Zachary Shahan |website=CleanTechnica |access-date=2018-03-19}}</ref>

Battery systems connected to large solid-state converters have been used to stabilize power distribution networks. Some grid batteries are co-located with renewable energy plants, either to smooth the power supplied by the intermittent wind or solar output, or to shift the power output into other hours of the day when the renewable plant cannot produce power directly (see [[Battery storage power station#Installation examples|Installation examples]]). These hybrid systems (generation and storage) can either alleviate the pressure on the grid when connecting renewable sources or be used to reach self-sufficiency and work "off-the-grid" (see [[Stand-alone power system]]).

Contrary to electric vehicle applications, batteries for stationary storage do not suffer from mass or volume constraints. However, due to the large amounts of energy and power implied, the cost per power or energy unit is crucial. The relevant metrics to assess the interest of a technology for grid-scale storage is the $/Wh (or $/W) rather than the Wh/kg (or W/kg). The electrochemical grid storage was made possible thanks to the development of the electric vehicle, that induced a fast decrease in the production costs of batteries below $300/kWh. By optimizing the production chain, major industrials aimed to reach $150/kWh by the end of 2020, but actually achieved $140/kWh. The rate of decline in battery prices has consistently outpaced most estimates, reaching $132/kWh in 2021.<ref>{{Cite web |title=Report: EV battery costs hit another low in 2021, but they might rise in 2022 |url=https://www.greencarreports.com/news/1134307_report-ev-battery-costs-might-rise-in-2022 |access-date=2022-09-08 |website=Green Car Reports |language=en}}</ref> These batteries rely on a [[lithium-ion battery|lithium-ion]] technology, which is suited for mobile applications (high cost, high density). Technologies optimized for the grid should focus on low cost per kWh.   Lithium iron phosphate batteries are increasingly being used in both vehicles and grid storage because of their low cost, scale and acceptable energy density for many applications.<ref>{{Cite web |last=Alamalhodaei |first=Aria |date=2021-07-28 |title=What Tesla's bet on iron-based batteries means for manufacturers |url=https://techcrunch.com/2021/07/28/what-teslas-bet-on-iron-based-batteries-means-for-manufacturers/ |access-date=2022-09-08 |website=TechCrunch |language=en-US}}</ref>

====Grid-oriented battery technologies====
[[Sodium-ion battery|Sodium-ion batteries]] are a cheap and sustainable alternative to lithium-ion, because sodium is far more abundant and cheaper than lithium, but it has a lower power density. However, they are still on the early stages of their development.

Automotive-oriented technologies rely on solid electrodes, which feature a high energy density but require an expensive manufacturing process. Liquid electrodes represent a cheaper and less dense alternative as they do not need any processing.

=====Molten-salt/liquid-metal batteries=====
These batteries are composed of two molten metal alloys separated by an electrolyte. They are simple to manufacture but require a temperature of several hundred degree Celsius to keep the alloys in a liquid state. This technology includes [[molten-salt battery#Zebra|ZEBRA]], [[NaS battery|sodium-sulfur batteries]] and [[liquid metal battery|liquid metal]].<ref>{{cite web|url=http://web.mit.edu/newsoffice/2009/liquid-battery.html|title=Liquid battery big enough for the electric grid?|date=19 November 2009|author=David L. Chandler, MIT News Office|work=MIT News |url-status=live |archive-url=https://web.archive.org/web/20100213233043/http://web.mit.edu/newsoffice/2009/liquid-battery.html |archive-date=13 February 2010}}</ref> Sodium sulphur batteries are being used for grid storage in Japan and in the United States.<ref>{{cite press release |url=http://www.appalachianpower.com/news/releases/viewrelease.asp?releaseID=281 |title=Appalachian Power Dedicates Mega Battery; New Technology Provides Extra Power, Reliability |publisher=Appalachian Power |date=20 July 2006 |url-status=dead |archive-url=https://web.archive.org/web/20061022221031/http://www.appalachianpower.com/news/releases/viewrelease.asp?releaseID=281 |archive-date=22 October 2006}}</ref> The electrolyte is composed of solid beta alumina. The liquid metal battery, developed by the group of Pr. [[Donald Sadoway]], uses molten alloys of magnesium and antimony separated by an electrically insulating molten salt. It is being brought to market by MIT spinoff company [[Ambri (company)|Ambri]], which is currently contracted to install a first 250MWh system for TerraScale data centre company near Reno, Nevada.<ref>{{cite web|url=https://www.energy-storage.news/news/ambris-liquid-metal-battery-to-be-used-at-desert-data-centre-in-nevada|title=Ambri's liquid metal battery to be used at desert data centre in Nevada|date=26 November 2020|author=Andy Colthorpe|work=Energy Storage News}}</ref><ref>{{cite web |author=Eric Wesoff |url=http://www.greentechmedia.com/articles/read/Sadoways-MIT-Liquid-Metal-Battery-Startup-Adds-15M-and-Khosla-Ventures-as/ |title=Sadoway's MIT Liquid Metal Battery Startup Adds $15M and Khosla Ventures as Investor |archive-url=https://web.archive.org/web/20120925010842/http://www.greentechmedia.com/articles/read/Sadoways-MIT-Liquid-Metal-Battery-Startup-Adds-15M-and-Khosla-Ventures-as |archive-date=25 September 2012 |work=greentechmedia.com |date=24 May 2012}}</ref>

=====Flow batteries=====
In rechargeable [[flow battery|flow batteries]], that store energy in liquids, such solutions of transition metal ions in water at room temperature. Flow batteries have the advantages of low capital cost for charge-discharge duration over 2-4 h, and of long durability (many years). Flow batteries are inferior to [[lithium-ion batteries]] in terms of [[Energy efficiency (physics)|energy efficiency]].<ref>Flow batteries from 1879 to 2022 and beyond. 2023. Journal of The Electrochemical Society. 170/030505. Y.V. Tolmachev. doi: 10.1149/1945-7111/acb8de.</ref> Flow batteries are currently deployed for storing energy from intermittent renewable sources, such as wind and solar.<ref>{{Cite web|url=http://www.memagazine.org/backissues/membersonly/oct05/features/rerere/rerere.html|archive-url=https://web.archive.org/web/20090115212122/http://www.memagazine.org/backissues/membersonly/oct05/features/rerere/rerere.html |url-status=dead |title="Renewable. Rechargeable. Remarkable.", Feature Article, September 2005<!-- Bot generated title -->|archive-date=15 January 2009}}</ref> 

[[Vanadium redox battery|Vanadium redox batteries]] is most technologically and commercially advanced type of flow battery.<ref>{{cite web|url=https://redtenergy.com/solutions/vanadium_redox_flow_machines/|title=Grid-Scale storage with vanadium redox flow batteries|website=REDT Energy Storage|archive-url=https://web.archive.org/web/20140515145126/http://www.redtenergy.com/news/redt-wins-%C2%A336m-decc-award-energy-storage|archive-date=15 May 2014|url-status=dead}}</ref><ref>1.	An Overview of the Design and Optimized Operation of Vanadium Redox Flow Batteries for Durations in the Range of 4–24 Hours. 2023. Batteries. 9/4. V.V. Viswanathan, A.J. Crawford, E.C. Thomsen, N. Shamim, G. Li, Q. Huang, et al. doi: 10.3390/batteries9040221.</ref> Currently there are dozens of Vanadium Redox Flow batteries installed at different sites including; [[Huxley Hill Wind Farm, Tasmania|Huxley Hill wind farm]] (Australia), Tomari Wind Hills at [[Hokkaidō]] (Japan), as well as in non-wind farm applications. A 12&nbsp;MW·h flow battery was to be installed at the [[Sorne Hill wind farm]] ([[Republic of Ireland|Ireland]]).<ref>{{cite web|url=http://www.leonardo-energy.org/drupal/node/959|title=Wind farm with battery storage in Ireland|publisher=Leonardo Energy |url-status=dead |archive-url=https://web.archive.org/web/20071102075502/http://www.leonardo-energy.org/drupal/node/959 |archive-date=2 November 2007}}</ref>{{update inline|reason='Was to be installed'? And was it? From what I can tell it was not installed. And hence this is an empty and outdated claim.... |date=November 2022}} These storage systems are designed to smooth out transient wind fluctuations.

====Examples====
In Puerto Rico a system{{explain|date=April 2020}} with a capacity of 20 megawatts for 15 minutes (5 megawatt hour) stabilizes the frequency of electric power produced on the island. A 27 megawatt 15-minute (6.75 megawatt hour) nickel-cadmium battery bank was installed at Fairbanks Alaska in 2003 to stabilize voltage at the end of a long transmission line.<ref>{{cite journal|year=2005|title=The United States of storage|journal=IEEE Power and Energy Magazine|volume=3|issue=2|pages=31–9|doi=10.1109/MPAE.2005.1405868|vauthors=Gyuk I, Kulkarni P, Sayer JH, etal|s2cid=34193246}}</ref>

In 2014, the [[Tehachapi Energy Storage Project]] was commissioned by [[Southern California Edison]].<ref>{{Cite web|title=SCE Unveils Largest Battery Energy Storage Project in North America|url=https://newsroom.edison.com/releases/sce-unveils-largest-battery-energy-storage-project-in-north-america|last=International|first=Edison|website=Edison International|language=en|access-date=2020-05-10}}</ref>

In 2016, a [[zinc-ion battery]] was proposed for use in grid storage applications.<ref>{{Cite web|url=http://www.kurzweilai.net/a-cheap-long-lasting-sustainable-battery-for-grid-energy-storage|title=A cheap, long-lasting, sustainable battery for grid energy storage {{!}} KurzweilAI|date=2016-09-16|website=www.kurzweilai.net|language=en-US|access-date=2017-02-02|url-status=live|archive-url=https://web.archive.org/web/20161228050817/http://www.kurzweilai.net/a-cheap-long-lasting-sustainable-battery-for-grid-energy-storage|archive-date=28 December 2016}}</ref>

In 2017, the [[California Public Utilities Commission]] installed 396 refrigerator-sized stacks of Tesla batteries at the Mira Loma substation in [[Ontario, California]]. The stacks are deployed in two modules of 10&nbsp;MW each (20&nbsp;MW in total), each capable of running for 4 hours, thus adding up to 80&nbsp;MWh of storage. The array is capable of powering 15,000 homes for over four hours.<ref>{{Cite news |url=http://www.zmescience.com/science/news-science/socal-tesla-batteries |title=Rows of Tesla batteries will keep Southern California's lights on during the night |last=MICU |first=ALEXANDRU |date=30 January 2017 |work=ZME Science |access-date=2 February 2017 |url-status=live |archive-url=https://web.archive.org/web/20170201211008/http://www.zmescience.com/science/news-science/socal-tesla-batteries/ |archive-date=1 February 2017}}</ref>

[[Battery storage power station#BYD in Hongkong|BYD]] proposes to use conventional consumer battery technologies such as [[Lithium iron phosphate|lithium iron phosphate (LiFePO4) battery]], connecting many batteries in parallel.

The largest grid storage batteries in the United States include the 31.5&nbsp;MW battery at Grand Ridge Power plant in Illinois and the 31.5&nbsp;MW battery at Beech Ridge, West Virginia.<ref>[http://www.solarserver.com/solar-magazine/solar-news/current/2015/kw50/invenergys-grand-ridge-energy-storage-facility-wins-2015-best-renewable-project-award.html Invenergy's Grand Ridge energy storage facility wins 2015 Best Renewable Project Award] {{webarchive|url=https://web.archive.org/web/20160110180155/http://www.solarserver.com/solar-magazine/solar-news/current/2015/kw50/invenergys-grand-ridge-energy-storage-facility-wins-2015-best-renewable-project-award.html |date=10 January 2016 }}, Solar Server, 12 December 2015</ref> Two batteries under construction in 2015 include the 400&nbsp;MWh (100&nbsp;MW for 4 hours) [[Southern California Edison]] project and the 52&nbsp;MWh project on Kauai, Hawaii to entirely time shift a 13MW solar farm's output to the evening.<ref>[http://www.utilitydive.com/news/5-battery-energy-storage-projects-to-watch-in-2016/409624/ 5 battery energy storage projects to watch in 2016 ] {{webarchive|url=https://web.archive.org/web/20170129232806/http://www.utilitydive.com/news/5-battery-energy-storage-projects-to-watch-in-2016/409624/ |date=29 January 2017 }}, Utility Dive, Krysti Shallenberger, 30 November 2015</ref> Two batteries are in [[Fairbanks, Alaska]] (40&nbsp;MW for 7 minutes using [[nickel-cadmium battery|Ni-Cd]] cells),<ref>Conway, E. (2 September 2008) [https://archive.today/20121223173018/http://www.telegraph.co.uk/scienceandtechnology/3312118/World%27s-biggest-battery-switched-on-in-Alaska.html "World's biggest battery switched on in Alaska"] ''Telegraph.co.uk''</ref> and in [[Notrees, Texas]] (36&nbsp;MW for 40 minutes using [[lead–acid battery|lead–acid batteries]]).<ref>{{cite web |url=http://www.energystorageexchange.org/projects/11 |title=Duke Energy Notrees Wind Storage Demonstration Project |work=DOE Global Energy Storage Database |url-status=dead |archive-url=https://web.archive.org/web/20141026103945/http://www.energystorageexchange.org/projects/11 |archive-date=26 October 2014 |access-date=13 October 2014 }}</ref><ref>{{cite news |last=Lie |first=Øyvind |url=http://www.tu.no/kraft/2014/10/12/her-er-verdens-kraftigste-batterier |title=Her er verdens kraftigste batterier |newspaper=Tu.no |language=da |trans-title=Here are the world's most powerful batteries |archive-url=https://web.archive.org/web/20141014235435/http://www.tu.no/kraft/2014/10/12/her-er-verdens-kraftigste-batterier |archive-date=14 October 2014 |publisher=[[Teknisk Ukeblad]] |date=12 October 2014 |access-date=13 October 2014}}</ref> A 13&nbsp;MWh battery made of used batteries from Daimler's [[Smart electric drive]] cars is being constructed in [[Lünen]], Germany, with an expected second life of 10 years.<ref>{{cite web|url=http://www.greencarcongress.com/2015/11/20151104-daimler.html|title=Green Car Congress: Daimler and partners deploying world's largest 2nd-life EV battery storage unit for grid support|first=BioAge|last=Media|url-status=live|archive-url=https://web.archive.org/web/20151107214650/http://www.greencarcongress.com/2015/11/20151104-daimler.html|archive-date=7 November 2015}}</ref>

In 2015, a 221&nbsp;MW battery storage was installed in the US, with total capacity expected to reach 1.7&nbsp;GW in 2020.<ref>{{cite web|url=http://reneweconomy.com.au/2016/us-energy-storage-market-grew-243-in-2015-largest-year-on-record-61611|title=US energy storage market grew 243% in 2015, largest year on record|date=4 March 2016|url-status=live|archive-url=https://web.archive.org/web/20160305114017/http://reneweconomy.com.au/2016/us-energy-storage-market-grew-243-in-2015-largest-year-on-record-61611|archive-date=5 March 2016}}</ref>

The UK had a 50&nbsp;MW lithium-ion grid-battery installed in Hertfordshire in 2018.<ref>{{cite web |author=Madelyn Newton |date=10 July 2018 |url=https://www.businessgreen.com/bg/news/3035576/uks-largest-grid-battery-storage-facility-completed-in-hertfordshire |title=UK's 'largest' grid battery storage facility completed in Hertfordshire |url-access=registration}}</ref> In February 2021, construction began on a 50 MW battery storage development in Burwell, Cambridgeshire and a 40 MW site in Barnsley, South Yorkshire.<ref>{{cite web |url=https://www.energyglobal.com/energy-storage/10022021/sms-begins-construction-of-british-battery-storage-projects/|title=SMS begins construction of British battery storage projects | last=Weetch | first=Bella |date=21 February 2021 |website=Energy Global |access-date=1 July 2021}}</ref>

In November 2017 [[Tesla, Inc.|Tesla]] installed a 100&nbsp;MW, 129&nbsp;MWh battery system in South Australia.<ref>{{cite web |url=https://arstechnica.com/cars/2017/12/tesla-beats-deadline-switches-on-gigantic-australian-battery-array/ |title=Tesla beats deadline, switches on gigantic Australian battery array |author=Megan Geuss |date=1 December 2017 |access-date=29 September 2018}}</ref> The [[Australian Energy Market Operator]] stated that this "is both rapid and precise, compared to the service typically provided by a conventional synchronous generation unit".<ref>{{cite web|url=https://arstechnica.com/information-technology/2018/04/australian-energy-market-operator-likes-its-new-tesla-battery-quite-a-bit/|title=Australian Energy Market Operator likes its new Tesla battery quite a bit |author=Megan Geuss |date=11 April 2018 |access-date=29 September 2018}}</ref><ref>{{cite web |publisher=[[Australian Energy Market Operator]] |date=April 2018 |url=http://www.link.aemo.com.au/-/media/Files/Media_Centre/2018/Initial-operation-of-the-Hornsdale-Power-Reserve.pdf |title=Initial operation of the Hornsdale Power Reserve Battery Energy Storage Syetem |access-date=29 September 2018}}</ref>

{| class="wikitable sortable" style="width: auto; text-align: center; table-layout: fixed;"
|+Technology comparison 
|-
! scope="col"| Technology
! scope="col"| Moving Parts
! scope="col"| Ambient Temperature
! scope="col"| Flammable
! scope="col"| Toxic Materials
! scope="col"| In production
! scope="col"| Rare metals
|-
! scope="row"| [[Vanadium redox battery|Vanadium flow]]<ref>{{cite journal |url=https://www.newscientist.com/article/mg21729096.400-flow-batteries-could-back-up-grid-of-the-future.html |title=Flow batteries could back up grid of the future |author=Martin Lamonica |date=20 March 2013 |journal=New Scientist |volume=217 |issue=2909 |pages=22 |url-status=live |archive-url=https://web.archive.org/web/20150506011326/http://www.newscientist.com/article/mg21729096.400-flow-batteries-could-back-up-grid-of-the-future.html |archive-date=6 May 2015|bibcode=2013NewSc.217...22L |doi=10.1016/S0262-4079(13)60735-6 }}</ref>
| {{No|Yes}}
| {{Yes}}
| {{Yes|No}}
| {{No|Yes}}
| {{Yes}}
| {{Yes|No}}
|-
! scope="row"| [[Liquid metal battery|Liquid Metal]]
| {{Yes|No}}
| {{No}}
| {{No|Yes}}
| {{Yes|No}}
| {{No}}
| {{Yes|No}}
|-
! scope="row"| [[Sodium-ion battery|Sodium-Ion]]
| {{Yes|No}}
| {{Yes}}
| {{No|Yes}}
| {{Yes|No}}
| {{No}}
| {{Yes|No}}
|-
! scope="row"| [[Lead–acid battery|Lead–Acid]]<ref>{{cite web |url=http://www.greentechmedia.com/articles/read/Gridtential-Goes-After-Energy-Storage-With-Improved-Lead-Acid-Batteries |title=Gridtential Goes After Energy Storage With Improved Lead–Acid Batteries |work=greentechmedia.com |url-status=live |archive-url=https://web.archive.org/web/20130320094846/http://www.greentechmedia.com/articles/read/Gridtential-Goes-After-Energy-Storage-With-Improved-Lead-Acid-Batteries |archive-date=20 March 2013|year=2013 }}</ref>
| {{Yes|No}}
| {{Yes}}
| {{Yes|No}}
| {{No|Yes}}
| {{Yes}}
| {{Yes|No}}
|-
! scope="row"| [[Sodium–sulfur battery|Sodium–sulfur batteries]]
| {{Yes|No}}
| {{No}}
| {{No|Yes}}
| {{Yes|No}}
| {{Yes}}
| {{Yes|No}}
|-
! scope="row"| [[nickel–cadmium battery|Ni–Cd]]
| {{Yes|No}}
| {{Yes}}
| {{Yes|No}}
| {{No|Yes}}
| {{Yes}}
| {{No|Yes}}
|-
! scope="row"| [[Aluminium-ion battery|Al-ion]]
| {{Yes|No}}
| {{Yes}}
| {{Yes|No}}
| {{Yes|No}}
| {{No}}
| {{Yes|No}}
|-
! scope="row"| [[Lithium-ion battery|Li-ion]]
| {{Yes|No}}
| {{Yes}}
| {{No|Yes}}
| {{Yes|No}}
| {{Yes}}
| {{Yes|No}}
|-
! scope="row"| [[Metal–air electrochemical cell|Fe-air]]
| {{No|Yes}}
| {{No}}
| {{No|Yes}}
| {{Yes|No}}
| {{No}}
| {{Yes|No}}
|}

===Electric vehicles ===
{{Main|Electric vehicle|Vehicle-to-grid}}
[[File:Nissan Leaf aan Amsterdamse laadpaal.jpg|thumb|[[Nissan Leaf]], the world's top-selling highway-capable [[electric car]] as of 2015]]
Companies are researching the possible use of electric vehicles to meet peak demand. A parked and plugged-in electric vehicle could sell the electricity from the battery during peak loads and charge either during night (at home) or during off-peak.<ref>{{cite news |url=http://news.bbc.co.uk/2/hi/science/nature/8523338.stm|title=BBC News – New electric car scheme for California|work=BBC|url-status=live|archive-url=https://web.archive.org/web/20100220053131/http://news.bbc.co.uk/2/hi/science/nature/8523338.stm|archive-date=20 February 2010|date=19 February 2010}}</ref>

[[Plug-in hybrid]] or [[electric car]]s could be used<ref name="RSC">{{cite web | title=Sustainable transportation based on electric vehicle concepts: a brief overview | url=https://www.researchgate.net/publication/224880220 | last1=Eberle | first1=Ulrich | first2=Rittmar | last2=von Helmolt | publisher=[[Royal Society of Chemistry]] | date=14 May 2010 | access-date=2010-06-08 | url-status=live | archive-url=https://web.archive.org/web/20131021070726/http://www.researchgate.net/publication/224880220_Sustainable_transportation_based_on_electric_vehicle_concepts_a_brief_overview | archive-date=21 October 2013}}</ref><ref>{{cite web|url=https://www.newscientist.com/article.ns?id=dn7081|title=Charge a battery in just six minutes|url-status=live|archive-url=https://web.archive.org/web/20081015143733/http://www.newscientist.com/article.ns?id=dn7081|archive-date=15 October 2008}}</ref><ref>{{cite web|url=http://www.toshiba.co.jp/about/press/2005_03/pr2901.htm|title=Toshiba : Press Releases 29 March 2005 |work=toshiba.co.jp|url-status=live|archive-url=https://web.archive.org/web/20161230023355/http://www.toshiba.co.jp/about/press/2005_03/pr2901.htm|archive-date=30 December 2016}}</ref> for their energy storage capabilities.  [[Vehicle-to-grid]] technology can be employed, turning each vehicle with its 20 to 50&nbsp;kWh [[battery pack]] into a distributed load-balancing device or emergency power source. This represents two to five days per vehicle of average household requirements of 10&nbsp;kWh per day, assuming annual consumption of 3,650&nbsp;kWh. This quantity of energy is equivalent to between {{convert|40|and|300|mi|km|order=flip|round=10}} of range in such vehicles consuming {{convert|0.16|to|0.5|kWh/mile|kWh/km|order=flip|sigfig=1}}. These figures can be achieved even in home-made [[electric vehicle conversion]]s. Some electric utilities plan to use old plug-in vehicle batteries (sometimes resulting in a giant battery) to store electricity<ref name="woody">Woody, Todd. [http://blogs.business2.com/greenwombat/2007/06/photo_green_wom.html "PG&E's Battery Power Plans Could Jump Start Electric Car Market."] {{webarchive|url=https://web.archive.org/web/20080208091638/http://blogs.business2.com/greenwombat/2007/06/photo_green_wom.html |date=8 February 2008 }} (Blog). ''Green Wombat'', 2007-06-12. Retrieved on 2007-08-19</ref><ref>{{cite web|url=http://www.planetark.com/dailynewsstory.cfm/newsid/44343/story.htm|title=E.on UK Plans Giant Battery to Store Wind Power |publisher=Positive Environment News|last=Planet Ark Environmental Foundation|url-status=live|archive-url=https://web.archive.org/web/20070918030808/http://www.planetark.com/dailynewsstory.cfm/newsid/44343/story.htm|archive-date=18 September 2007}}</ref> However, a large disadvantage of using vehicle to grid energy storage would be if each storage cycle stressed the battery with one complete charge-discharge cycle.<ref name="RSC" /> However, one major study showed that used intelligently, vehicle-to-grid storage actually improved the batteries longevity.<ref>{{cite web|url=https://www.cleanenergynews.co.uk/news/storage/v2g-found-to-improve-the-lifetime-of-electric-vehicle-batteries|title=V2G found to improve the lifetime of electric vehicle batteries|website=Clean Energy News|access-date=5 May 2018|url-status=dead|archive-url=https://web.archive.org/web/20180328102412/https://www.cleanenergynews.co.uk/news/storage/v2g-found-to-improve-the-lifetime-of-electric-vehicle-batteries|archive-date=28 March 2018}}</ref> Conventional (cobalt-based) lithium-ion batteries break down with the number of cycles – newer li-ion batteries do not break down significantly with each cycle, and so have much longer lives. One approach is to reuse unreliable vehicle batteries in dedicated grid storage<ref>{{cite news |url=https://www.forbes.com/sites/peterdetwiler/2014/03/18/the-afterlife-for-electric-vehicle-batteries-a-future-source-of-energy-storage/ |work=Forbes |date=2014-03-18 |title= The Afterlife For Electric Vehicle Batteries: A Future Source of Energy Storage? |first=Peter |last=Kelly-Detwiler}}</ref> as they are expected to be good in this role for ten years.<ref>{{cite news |url=http://news.nationalgeographic.com/news/energy/2012/11/121116-second-life-for-used-electric-car-batteries/ |archive-url=https://web.archive.org/web/20121118014117/http://news.nationalgeographic.com/news/energy/2012/11/121116-second-life-for-used-electric-car-batteries/ |url-status=dead |archive-date=18 November 2012 |work=National Geographic |date=2012-11-12 |title= Second Life for Old Electric-Car Batteries: Guardians of the Electric Grid |first=Josie |last=Garthwaite}}</ref> If such storage is done on a large scale it becomes much easier to guarantee replacement of a vehicle battery degraded in mobile use, as the old battery has value and immediate use.

===Flywheel===
{{Main|Flywheel storage power system|Flywheel energy storage}}
[[File:G2 front2.jpg|thumb|NASA G2 flywheel]]

Mechanical inertia is the basis of this storage method. When the electric power flows into the device, an [[electric motor]] accelerates a heavy rotating disc. The motor acts as a generator when the flow of power is reversed, slowing down the disc and producing electricity. Electricity is stored as the [[kinetic energy]] of the disc. [[Friction]] must be kept to a minimum to prolong the storage time. This is often achieved by placing the flywheel in a vacuum and using [[magnetic bearing]]s, tending to make the method expensive. Greater flywheel speeds allow greater storage capacity but require strong materials such as [[steel]] or [[composite material]]s to resist the [[centrifugal force]]s. The ranges of power and energy storage technology that make this method economic, however, tends to make flywheels unsuitable for general power system application; they are probably best suited to load-leveling applications on railway power systems and for improving [[power quality]] in [[renewable energy]] systems such as the 20MW system in Ireland.<ref>{{cite web |url=https://www.djei.ie/en/News-And-Events/Department-News/2015/March/First-Hybrid-Flywheel-Energy-Storage-Plant-in-Europe-announced-in-Midlands-.html  |title=Energy Storage Plant in Europe announced in Midlands |date=26 March 2015 |publisher=Department of Business, Enterprise and Innovation |url-status=live |archive-url=https://web.archive.org/web/20161128202155/https://www.djei.ie/en/News-And-Events/Department-News/2015/March/First-Hybrid-Flywheel-Energy-Storage-Plant-in-Europe-announced-in-Midlands-.html |archive-date=28 November 2016 |access-date=28 January 2020 }}</ref><ref>{{cite news |url=https://www.theguardian.com/environment/2015/apr/08/new-energy-storage-plant-could-revolutionise-renewable-sector |title=New energy storage plant could 'revolutionise' renewable sector |work=[[The Guardian]] |url-status=live |archive-url=https://web.archive.org/web/20161204075936/https://www.theguardian.com/environment/2015/apr/08/new-energy-storage-plant-could-revolutionise-renewable-sector |archive-date=4 December 2016 }}<!--inaccurate--></ref>

Applications that use flywheel storage are those that require very high bursts of power for very short durations such as [[tokamak]]<ref>{{cite web|title=Joint European Torus facility – Flywheel details|url=http://www.efda.org/2011/10/775-tons-of-steel/|access-date=18 January 2014|url-status=dead|archive-url=https://web.archive.org/web/20140201124209/http://www.efda.org/2011/10/775-tons-of-steel/|archive-date=1 February 2014}}</ref> and [[laser]] experiments where a motor generator is spun up to operating speed and is partially slowed down during discharge.

Flywheel storage is also currently used in the form of the [[Diesel rotary uninterruptible power supply]] to provide [[Uninterruptible power supply#Rotary|uninterruptible power supply]] systems (such as those in large [[Data Center|datacenters]]) for ride-through power necessary during transfer<ref>{{cite web |url=http://www.thewhir.com/web-hosting-news/010810_Terremark_Installs_Space_Saving_Flywheel_UPS_in_New_Data_Center |title=Terremark Installs Space-Saving Flywheel UPS in New Data Center |author=David Hamilton |date=8 January 2010 |website=Web Host Industry Review |access-date=2010-11-16 |url-status=dead |archive-url=https://web.archive.org/web/20100428165248/http://www.thewhir.com/web-hosting-news/010810_Terremark_Installs_Space_Saving_Flywheel_UPS_in_New_Data_Center |archive-date=28 April 2010}}</ref>&nbsp;– that is, the relatively brief amount of time between a loss of power to the mains and the warm-up of an alternate source, such as a [[diesel generator]].

This potential solution has been implemented by EDA<ref>{{cite web|url=http://www.eda.pt|title=EDA – Electricidade dos Açores|url-status=live|archive-url=https://web.archive.org/web/20071128050358/http://www.eda.pt/|archive-date=28 November 2007}}</ref>{{better source needed|date=December 2015}} in the [[Azores]] on the islands of [[Graciosa]] and [[Flores Island (Azores)|Flores]]. This system uses an 18 megawatt-second flywheel to improve [[power quality]] and thus allow increased renewable energy usage. As the description suggests, these systems are again designed to smooth out transient fluctuations in supply, and could never be used to cope with an outage exceeding a couple of days.

Powercorp in Australia have been developing applications using wind turbines, flywheels and low load diesel (LLD) technology to maximize the wind input to small grids. A system installed in Coral Bay, Western Australia, uses wind turbines coupled with a flywheel based control system and LLDs. The flywheel technology enables the wind turbines to supply up to 95 percent of Coral Bay's energy supply at times, with a total annual wind penetration of 45 percent.<ref>{{cite web |url=https://www.energystorageexchange.org/projects/756 |title=Coral Bay PowerStore Flywheel Project |work=DOE Global Energy Storage Database |access-date=2017-08-26 |url-status=live |archive-url=https://web.archive.org/web/20170826071853/https://www.energystorageexchange.org/projects/756 |archive-date=26 August 2017}},</ref>

===Hydrogen===
{{Main|Hydrogen economy|Hydrogen storage}}
{{See also|Combined cycle hydrogen power plant}}
[[Hydrogen]] is being developed as an electrical energy storage medium.<ref name="RSC" /><ref name=RSC2>{{cite web|title=Fuel cell electric vehicles and hydrogen infrastructure: status 2012|url=https://www.researchgate.net/publication/233987484|last1=Eberle|first1=Ulrich|first2=Bernd|last2=Mueller|first3=Rittmar|last3=von Helmolt|publisher=[[Royal Society of Chemistry]]|date=15 July 2012|access-date=2013-01-08|url-status=live|archive-url=https://web.archive.org/web/20140209172012/http://www.researchgate.net/publication/233987484_Fuel_cell_electric_vehicles_and_hydrogen_infrastructure_status_2012?ev=prf_pub|archive-date=9 February 2014}}</ref> Hydrogen is produced, then compressed or liquefied, cryogenically stored at −252.882&nbsp;°C, and then converted back to electrical energy or heat. Hydrogen can be used as a fuel for portable (vehicles) or stationary energy generation. Compared to pumped water storage and batteries, hydrogen has the advantage that it is a high energy density fuel.<ref name="RSC2" /> [[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 [[Electric battery|batteries]].<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><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>

Hydrogen can be produced either by [[steam reforming|reforming natural gas with steam]] or by the electrolysis of water into hydrogen and [[oxygen]] (see [[hydrogen production]]). Reforming natural gas produces [[carbon dioxide]] as a by-product. [[High temperature electrolysis]] and [[high pressure electrolysis]] are two techniques by which the efficiency of hydrogen production may be able to be increased. Hydrogen is then converted back to electricity in an [[internal combustion engine]], or a [[fuel cell]].

The AC-to-AC efficiency of hydrogen storage has been shown to be on the order of 20 to 45%, which imposes economic constraints.<ref name="RSC2" /><ref name=anscombe /> The price ratio between purchase and sale of electricity must be at least proportional to the efficiency in order for the system to be economic. Hydrogen fuel cells can respond quickly enough to correct rapid fluctuations in electricity demand or supply and regulate frequency. Whether hydrogen can use natural gas infrastructure depends on the network construction materials, standards in joints, and storage pressure.<ref>{{Cite web|url=http://www.bartlett.ucl.ac.uk/energy/research/themes/energy-systems/hydrogen/dodds-demoullin-2013-gas-network-conversion|archive-url=http://arquivo.pt/wayback/20160516103851/http://www.bartlett.ucl.ac.uk/energy/research/themes/energy-systems/hydrogen/dodds-demoullin-2013-gas-network-conversion |url-status=dead |title=Conversion of the UK gas system to transport hydrogen|archive-date=16 May 2016}}</ref>

The equipment necessary for hydrogen energy storage includes an electrolysis plant, [[hydrogen compressor]]s or [[Liquid hydrogen|liquifiers]], and storage tanks.

[[Biohydrogen]] is a process being investigated for producing hydrogen using biomass.

[[Micro combined heat and power]] (microCHP) can use hydrogen as a fuel.

Some nuclear power plants may be able to benefit from a symbiosis with hydrogen production. High temperature (950 to 1,000&nbsp;°C) gas cooled nuclear [[generation IV reactor]]s have the potential to electrolyze hydrogen from water by thermochemical means using nuclear heat as in the [[sulfur-iodine cycle]]. The first commercial reactors are expected in 2030.

A community based pilot program using wind turbines and hydrogen generators was started in 2007 in the remote community of [[Ramea, Newfoundland and Labrador]].<ref>{{cite web|first=Morel|last=Oprisan|publisher=IEA Wind – KWEA Joint Workshop|date=April 2007|url=http://www.ieawind.org/wnd_info/KWEA_pdf/Oprisan_KWEA_.pdf|title=Introduction of Hydrogen Technologies to Ramea Island|access-date=2 February 2017 |url-status=dead |archive-url=https://web.archive.org/web/20160730110344/http://www.ieawind.org/wnd_info/KWEA_pdf/Oprisan_KWEA_.pdf|archive-date=30 July 2016}}</ref> A similar project has been going on since 2004 in [[Utsira]], a small Norwegian island municipality.

====Underground hydrogen storage====
[[Underground hydrogen storage]] is the practice of [[hydrogen storage]] in [[cave]]rns, [[salt dome]]s and depleted oil and gas fields.<ref name="RSC" /><ref>{{cite web |author1=Olaf Kruck |author2=Fritz Crotogino |date=14 August 2013 |url=http://www.hyunder.eu/wp-content/uploads/2016/01/D3.3_Benchmarking-of-selected-storage-options.pdf |title=Benchmarking of selected storage options |work=HyUnder}}</ref> Large quantities of gaseous hydrogen have been stored in caverns by [[Imperial Chemical Industries]] (ICI) for many years without any difficulties.<ref>{{cite web |url=http://www.hyweb.de/Knowledge/Ecn-h2a.html |author1=Reinhold Wurster |author2=Werner Zittel |title=Hydrogen Energy |work=HyWeb – The LBST Information Portal on Hydrogen and Fuel Cells |url-status=dead |archive-url=https://web.archive.org/web/20040102122446/http://www.hyweb.de/Knowledge/Ecn-h2a.html |archive-date=2 January 2004}}</ref> The European project [[Hyunder]]<ref>{{cite web |url=http://www.hyunder.eu/ |title=Why storing large scale intermittent renewable energies with hydrogen? |work=HyUnder |url-status=live |archive-url=https://web.archive.org/web/20131111061655/http://www.hyunder.eu/ |archive-date=11 November 2013}}</ref> indicated in 2013 that for the storage of wind and solar energy an additional 85 caverns are required as it cannot be covered by [[Pumped-storage hydroelectricity|PHES]] and [[Compressed air energy storage|CAES]] systems.<ref>{{Cite web|url=http://www.hyunder.eu/images/Presentations%20EUSEW/2%20HyUnder%20EUSEW%20workshop%20Luis%20Correas.pdf|title=Storing renewable energy: Is hydrogen a viable solution?}}</ref>

====Power to gas====
[[Power to gas]] is a technology which converts [[electricity|electrical]] power to a gas [[fuel]]. There are 2 methods, the first is to use the electricity for [[water splitting]] and inject the resulting hydrogen into the natural gas grid. The second less efficient method is used to convert [[carbon dioxide]] and water to [[methane]], (see [[natural gas]]) using [[electrolysis]] and the [[Sabatier reaction]]. The excess power or off peak power generated by wind generators or solar arrays is then used for load balancing in the energy grid. Using the existing natural gas system for hydrogen, fuel cell maker [[Hydrogenics]] and natural gas distributor [[Enbridge]] have teamed up to develop such a [[power to gas]] system in Canada.<ref name="anscombe">{{cite news|last=Anscombe|first=Nadya|title=Energy storage: Could hydrogen be the answer?|url=http://www.solarnovus.com/energy-storage-could-hydrogen-be-the-answer_N5028.html|access-date=3 November 2012|newspaper=Solar Novus Today|date=4 June 2012|url-status=live|archive-url=https://web.archive.org/web/20130819022518/http://www.solarnovus.com/energy-storage-could-hydrogen-be-the-answer_N5028.html|archive-date=19 August 2013}}</ref>

Pipeline storage of hydrogen where a natural gas network is used for the storage of hydrogen. Before switching to [[natural gas]], the German gas networks were operated using [[towngas]], which for the most part consisted of hydrogen. The storage capacity of the German natural gas network is more than 200,000&nbsp;GW·h which is enough for several months of energy requirement. By comparison, the capacity of all German pumped-storage power plants amounts to only about 40&nbsp;GW·h. The transport of energy through a gas network is done with much less loss (<0.1%) than in a power network (8%){{Clarify|for what length of the pipeline or power network are the losses given (presumably 1000km)|date=January 2015}}. The use of the existing [[List of natural gas pipelines|natural gas pipelines]] for hydrogen was studied by NaturalHy<ref>{{cite web |url=http://www.naturalhy.net/docs/Naturalhy_Brochure.pdf |title=Preparing for the Hydrogen Economy by Using the Existing Natural Gas System as a Catalyst |publisher=Naturalhy |date=October 2009 |url-status=dead |archive-url=https://web.archive.org/web/20120118102741/http://www.naturalhy.net/docs/Naturalhy_Brochure.pdf |archive-date=18 January 2012}}</ref>

====The power-to-ammonia concept====
The power-to-[[ammonia]] concept offers a carbon-free energy storage route with a diversified application palette. At times when there is surplus [[low-carbon power]], it can be used to create ammonia fuel. Ammonia may be produced by splitting water into hydrogen and oxygen with electricity, then high temperature and pressure are used to combine nitrogen from the air with the hydrogen, creating ammonia. As a liquid it is similar to propane, unlike hydrogen alone, which is difficult to store as a gas under pressure or to cryogenically liquefy and store at −253&nbsp;°C.

Just like natural gas, the stored ammonia can be used as a thermal fuel for transportation and electricity generation or used in a fuel cell.<ref>{{cite journal|title=Ammonia as a Suitable Fuel for Fuel Cells|first1=Rong|last1=Lan|first2=Shanwen|last2=Tao|date=5 May 2018|journal=Frontiers in Energy Research|volume=2|doi=10.3389/fenrg.2014.00035|doi-access=free}}</ref> A standard 60,000 m³ tank of liquid ammonia contains about 211&nbsp;GWh of energy, equivalent to the annual production of roughly 30 wind turbines. Ammonia can be burned cleanly: water and nitrogen are released, but no CO<sub>2</sub> and little or no nitrogen oxides. Ammonia has [[Ammonia#Applications|multiple uses]] besides being an energy carrier, it is the basis for the production of many chemicals, the most common use is for fertilizer.<ref>{{cite web|last1=Service|first1=Robert F.|date=2018-07-12|title=Ammonia—a renewable fuel made from sun, air, and water—could power the globe without carbon|url=https://www.science.org/content/article/ammonia-renewable-fuel-made-sun-air-and-water-could-power-globe-without-carbon|access-date=2021-04-15|website=Science {{!}} AAAS|language=en}}</ref> Given this flexibility of usage, and given that the infrastructure for the safe transport, distribution and usage of ammonia is already in place, it makes ammonia a good candidate to be a large-scale, non-carbon, energy carrier of the future.

===Hydroelectricity===
====Pumped water====
{{Main|Pumped-storage hydroelectricity}}
[[File:Taiwan Power Ccopany Mingtan Power Station.JPG|thumb|[[Mingtan Pumped-Storage Hydro Power Plant]] dam in [[Nantou County|Nantou]], [[Taiwan]] ]]

In 2008, world pumped-storage generating capacity was 104 [[Watt|GW]],<ref name=eia>{{cite web |url=http://www.eia.gov/cfapps/ipdbproject/iedindex3.cfm?tid=2&pid=82&aid=7&cid=regions&syid=2004&eyid=2008&unit=MK |title=International Energy Statistics |url-status=live |archive-url=https://web.archive.org/web/20111003141252/http://www.eia.gov/cfapps/ipdbproject/iedindex3.cfm?tid=2&pid=82&aid=7&cid=regions&syid=2004&eyid=2008&unit=MK |archive-date=3 October 2011}}</ref> while other sources claim 127&nbsp;GW, which comprises the vast majority of all types of grid electric storage – all other types combined are some hundreds of MW.<ref name="epri">{{cite web |author=Rastler |display-authors=etal |url=http://my.epri.com/portal/server.pt?Abstract_id=000000000001020676 |title=Electric Energy Storage Technology Options: A White Paper Primer on Applications, Costs, and Benefits |website=[[EPRI]] |archive-url=https://web.archive.org/web/20110817202300/http://my.epri.com/portal/server.pt?Abstract_id=000000000001020676 |archive-date=17 August 2011 |format=(Free download) |year=2010 |access-date=30 September 2011}}</ref>

In many places, pumped-storage hydroelectricity is used to even out the daily generating load, by pumping water to a high storage reservoir during off-peak hours and weekends, using the excess base-load capacity from coal or nuclear sources.  During peak hours, this water can be used for [[hydroelectric]] generation, often as a high value rapid-response reserve to cover transient peaks in demand. Pumped storage recovers about 70% to 85% of the energy consumed, and is currently the most cost effective form of mass power storage.<ref>{{cite web |url=http://www.electricitystorage.org/technology/storage_technologies/pumped_hydro/ |title=Pumped Hydro (PH) |website=Electricity Storage Association |access-date=2013-03-26 |url-status=dead |archive-url=https://web.archive.org/web/20130315123945/http://www.electricitystorage.org/technology/storage_technologies/pumped_hydro |archive-date=15 March 2013}}</ref> The chief problem with pumped storage is that it usually requires two nearby reservoirs at considerably different heights, and often requires considerable capital expenditure.<ref>{{cite web |title=Pumped Hydroelectric Energy Storage |url=http://www.doc.ic.ac.uk/~matti/ise2grp/energystorage_report/node6.html |publisher=Imperial College London |url-status=dead |archive-url=https://web.archive.org/web/20071029024441/http://www.doc.ic.ac.uk/~matti/ise2grp/energystorage_report/node6.html |archive-date=29 October 2007}}</ref>

Pumped water systems have high [[Intermittent power source#Terminology|dispatchability]], meaning they can come on-line very quickly, typically within 15 seconds,<ref name=Dinorwig>{{cite web |url=http://www.fhc.co.uk/dinorwig.htm |title=First Hydro Dinorwig Power Station |url-status=dead |archive-url=https://web.archive.org/web/20160512031732/http://www.fhc.co.uk/dinorwig.htm |archive-date=12 May 2016}}</ref> which makes these systems very efficient at soaking up variability in electrical ''demand'' from consumers. There is over 90&nbsp;GW of pumped storage in operation around the world, which is about 3% of ''instantaneous'' global generation capacity. Pumped water storage systems, such as the [[Dinorwig power station|Dinorwig]] storage system in Britain, hold five or six hours of generating capacity,<ref name=Dinorwig/> and are used to smooth out demand variations.

Another example is the 1836&nbsp;MW [[Tianhuangping Pumped Storage Power Station|Tianhuangping Pumped-Storage Hydro Plant]] in China, which has a reservoir capacity of eight million cubic meters (2.1 billion U.S. gallons or the volume of water over [[Niagara Falls]] in 25 minutes) with a vertical distance of 600&nbsp;m (1970&nbsp;feet). The reservoir can provide about 13&nbsp;GW·h of stored [[gravitational energy|gravitational potential energy]] (convertible to electricity at about 80% efficiency), or about 2% of China's daily electricity consumption.<ref>{{Cite web|url=https://www.cia.gov/library/publications/the-world-factbook/print/ch.html|archive-url=https://web.archive.org/web/20080813072716/https://www.cia.gov/library/publications/the-world-factbook/print/ch.html |url-status=dead |title=CIA – The World Factbook – China|archive-date=13 August 2008}}</ref>

A new concept in pumped-storage is utilizing [[wind energy]] or [[solar power]] to pump water. Wind turbines or solar cells that direct drive [[water pump]]s for an energy storing wind or solar [[dam]] can make this a more efficient process but are limited. Such systems can only increase kinetic water volume during windy and daylight periods. A study published in 2013 showed rooftop solar, coupled to existing pumped-storage, could replace the reactors lost at Fukushima with an equivalent capacity factor.<ref>{{Cite journal |last1=Stoll |first1=B L |last2=Smith |first2=T A |last3=Deinert |first3=M R |date=2013-03-01 |title=Potential for rooftop photovoltaics in Tokyo to replace nuclear capacity |journal=Environmental Research Letters |volume=8 |issue=1 |pages=014042 |doi=10.1088/1748-9326/8/1/014042 |bibcode=2013ERL.....8a4042S |s2cid=56317922 |issn=1748-9326|doi-access=free }}</ref>

====Hydroelectric dams====
[[File:FeiCueiReservoir.jpg|thumb|[[Feitsui Dam|Fetsui hydroelectric dam]] in [[New Taipei]], [[Taiwan]] ]]

Hydroelectric dams with large reservoirs can also be operated to provide peak generation at times of peak demand. Water is stored in the reservoir during periods of low demand and released through the plant when demand is higher.  The net effect is the same as pumped storage, but without the pumping loss. Depending on the reservoir capacity the plant can provide daily, weekly, or seasonal load following.

Many existing hydroelectric dams are fairly old (for example, the [[Hoover Dam]] was built in the 1930s), and their original design predated the newer intermittent power sources such as wind and solar by decades. A hydroelectric dam originally built to provide [[baseload power]] will have its generators sized according to the average flow of water into the reservoir. Uprating such a dam with additional generators increases its peak power output capacity, thereby increasing its capacity to operate as a virtual grid energy storage unit.<ref name="burec_hydro">{{Cite web|url=http://www.usbr.gov/power/edu/pamphlet.pdf|title=Hydroelectric Power|publisher=[[United States Bureau of Reclamation]]|access-date=2008-10-13|url-status=dead|archive-url=https://web.archive.org/web/20081021182534/http://www.usbr.gov/power/edu/pamphlet.pdf|archive-date=21 October 2008}}</ref><ref name="sccpa_hoover">{{Cite web|url=http://www.scppa.org/hoover.htm |title=SCPPA Hoover Project Page |publisher=[[Southern California Public Power Authority]] |access-date=2008-10-13 |url-status=dead |archive-url=https://web.archive.org/web/20080927000656/http://www.scppa.org/hoover.htm |archive-date=27 September 2008 }}</ref> The [[United States Bureau of Reclamation]] reports an investment cost of $69 per kilowatt capacity to uprate an existing dam,<ref name="burec_hydro"/> compared to more than $400 per kilowatt for oil-fired peaking generators. While an uprated hydroelectric dam does not directly store excess energy from other generating units, it behaves equivalently by accumulating its own fuel – incoming river water – during periods of high output from other generating units. Functioning as a virtual grid storage unit in this way, the uprated dam is one of the most efficient forms of energy storage, because it has no pumping losses to fill its reservoir, only increased losses to evaporation and leakage.

A dam which impounds a large reservoir can store and release a correspondingly large amount of energy, by controlling river outflow and raising or lowering its reservoir level a few meters. Limitations do apply to dam operation, their releases are commonly subject to government [[water right|regulated water rights]] to limit downstream effect on rivers. For example, there are grid situations where baseload thermal plants, nuclear or wind turbines are already producing excess power at night, dams are still required to release enough water to maintain adequate river levels, whether electricity is generated or not. Conversely there's a limit to peak capacity, which if excessive could cause a river to flood for a few hours each day.<ref>{{cite web|url=http://www.rethinkingwater.ca/water_use_plans.html|title=Rethinking our Water Ways - 5.3 Water Use Plans|website=www.rethinkingwater.ca|access-date=5 May 2018|url-status=live|archive-url=https://web.archive.org/web/20171005190830/http://www.rethinkingwater.ca/water_use_plans.html|archive-date=5 October 2017}}</ref>

===Superconducting magnetic energy===
{{Main|Superconducting magnetic energy storage}}

Superconducting magnetic energy storage (SMES) systems store energy in the [[magnetic field]] created by the flow of [[direct current]] in a [[Superconductivity|superconducting]] coil which has been [[cryogenics|cryogenically]] cooled to a temperature below its superconducting critical temperature. A typical SMES system includes three parts: superconducting coil, power conditioning system and cryogenically cooled refrigerator. Once the superconducting coil is charged, the current will not decay and the magnetic energy can be stored indefinitely. The stored energy can be released back to the network by discharging the coil. The power conditioning system uses an [[inverter (electrical)|inverter]]/[[rectifier]] to transform [[alternating current]] (AC) power to direct current or convert DC back to AC power. The 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 are highly efficient; the round-trip efficiency is greater than 95%. The high cost of superconductors is the primary limitation for commercial use of this energy storage method.

Due to the energy requirements of [[refrigeration]], and the limits in the total energy able to be stored, SMES is currently used for short duration energy storage. Therefore, SMES is most commonly devoted to improving [[power quality]].  If SMES were to be used for [[public utility|utilities]] it would be a [[day|diurnal]] storage device, charged from [[base load power plant|base load]] power at night and meeting [[peaking power plant|peak loads]] during the day.

There are significant [[Superconducting magnetic energy storage#Technical challenges|technical challenges]] yet to be solved for superconducting magnetic energy storage to become practical.

===Thermal===
{{Main|Thermal energy storage}}

In Denmark the direct storage of electricity is perceived as too expensive for very large scale usage, albeit significant usage is made of existing Norwegian Hydro.  Instead, the use of existing hot water storage tanks connected to district heating schemes, heated by either electrode boilers or heat pumps, is seen as a preferable approach.  The stored heat is then transmitted to dwellings using [[district heating]] pipes.

[[Molten salt]] is used to store heat collected by a [[solar power tower]] so that it can be used to generate electricity in bad weather or at night.<ref>[http://www.sandia.gov/Renewable_Energy/solarthermal/NSTTF/salt.htm Advantages of Using Molten Salt] {{webarchive|url=https://web.archive.org/web/20110605094349/http://www.sandia.gov/Renewable_Energy/solarthermal/NSTTF/salt.htm |date=5 June 2011 }} Tom Mancini, Sandia National Laboratories, Albuquerque, NM Accessed December 2007</ref>

Building heating and cooling systems can be controlled to store thermal energy in either the building's mass or dedicated thermal storage tanks. This thermal storage can provide load-shifting or even more complex [[ancillary services]] by increasing power consumption (charging the storage) during off-peak times and lowering power consumption (discharging the storage) during higher-priced peak times.<ref>{{cite journal |last1=Lee |first1=Zachary E. |first2=Qingxuan |last2=Sun |first3=Zhao |last3=Ma |first4=Jiangfeng |last4=Wang |first5=Jason S. |last5=MacDonald |first6=K. Max |last6=Zhang |title=Providing Grid Services With Heat Pumps: A Review |journal=Journal of Engineering for Sustainable Buildings and Cities |date=Feb 2020 |volume=1 |issue=1 |doi=10.1115/1.4045819 |s2cid=213898377 |url=https://escholarship.org/uc/item/4w97v0wb |doi-access=free }}</ref> For example, off-peak electricity can be used to make [[ice]] from water, and the ice can be stored.  The stored ice can be used to cool the air in a large building which would have normally used electric AC, thereby shifting the electric load to off-peak hours. On other systems stored ice is used to cool the intake air of a [[gas turbine]] [[electrical generator|generator]], thus increasing the on-peak generation capacity and the on-peak efficiency.

A [[pumped-heat electricity storage]] system uses a highly reversible heat engine/heat pump to pump heat between two storage vessels, heating one and cooling the other. The UK-based engineering company Isentropic that is developing the system claims a potential electricity-in to electricity-out round-trip efficiency of 72–80%.<ref name=Isentropic>{{cite web|title=Isentropic's PHES Technology|url=http://www.isentropic.co.uk/our-phes-technology|url-status=dead|archive-url=https://web.archive.org/web/20141010160418/http://www.isentropic.co.uk/our-phes-technology|archive-date=10 October 2014}}</ref>

A [[Carnot battery]] is a type of energy storage systems that stores electricity in heat storage and converts the stored heat back to electricity via thermodynamics cycles. This concept has been investigated and developed by many research projects recently.<ref name="DumontFrate2020">{{cite journal | last1=Dumont | first1=Olivier | last2=Frate | first2=Guido Francesco | last3=Pillai | first3=Aditya | last4=Lecompte | first4=Steven | last5=De paepe | first5=Michel | last6=Lemort|first6=Vincent | title=Carnot battery technology: A state-of-the-art review | journal=Journal of Energy Storage|volume=32|year=2020|pages=101756 | issn=2352-152X|doi=10.1016/j.est.2020.101756| s2cid=225019981 }}</ref> One of the advantage of this type of system is that the cost at large-scale and long-duration of thermal storage could be much lower than other storage technologies.

===Physical battery; Gravitational potential energy storage with solid masses===
{{Main|Energy storage#Solid mass gravitational}}

Alternatives include storing energy by moving large solid masses upward against gravity. This can be achieved inside old mine shafts<ref>{{Cite web|url=http://www.theguardian.com/environment/2019/oct/21/how-uks-disused-mine-shafts-plan-to-store-renewable-energy|title=How UK's disused mine shafts could be used to store renewable energy|date=21 October 2019|website=The Guardian}}</ref> or in specially constructed towers where heavy weights are [[winch]]ed up to store energy and allowed a controlled descent to release it.<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}}</ref><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}}</ref> In [[rail energy storage]], rail cars carrying large weights are moved up or down a section of inclined rail track, storing or releasing energy as a result;<ref name="Scientific American-2014.03.25">Massey, Nathanael and [[ClimateWire]]. [http://www.scientificamerican.com/article/energy-storage-hits-the-rails-out-west/ Energy Storage Hits the Rails Out West: In California and Nevada, projects store electricity in the form of heavy rail cars pulled up a hill] {{webarchive|url=https://web.archive.org/web/20140430120908/http://www.scientificamerican.com/article/energy-storage-hits-the-rails-out-west/ |date=30 April 2014 }}, ''[[Scientific American|ScientificAmerican.com]]'' website, 25 March 2014. Retrieved 28 March 2014.</ref>
In disused oil-well potential energy storage, weights are raised or lowered in a deep, decommissioned oil well.

==Economics== <!-- Although related to the rest of the article, i think this section, especially the second paragraph, deserves an article in itself -->

The [[levelized cost of storage|levelized cost of storing electricity]] depends highly on storage type and purpose; as subsecond-scale [[Utility frequency|frequency regulation]], minute/hour-scale peaker plants, or day/week-scale season storage.<ref>{{cite web |url=http://www.utilitydive.com/news/some-energy-storage-already-cost-competitive-new-valuation-study-shows/409641/|title=Some energy storage already cost competitive, new valuation study shows|date=24 November 2015|work=Utility Dive |access-date=15 October 2016 |url-status=live |archive-url=https://web.archive.org/web/20161018235841/http://www.utilitydive.com/news/some-energy-storage-already-cost-competitive-new-valuation-study-shows/409641/ |archive-date=18 October 2016}}</ref><ref>{{cite web |title=Lazard's Levelized Cost of Storage Analysis |url=https://www.lazard.com/media/2391/lazards-levelized-cost-of-storage-analysis-10.pdf |access-date=2 February 2017 |url-status=live |archive-url=https://web.archive.org/web/20170202184057/https://www.lazard.com/media/2391/lazards-levelized-cost-of-storage-analysis-10.pdf |archive-date=2 February 2017}}</ref><ref>{{cite journal |last1=Lai |first1=Chun Sing |last2=McCulloch |first2=Malcolm D. |title=Levelized cost of electricity for solar photovoltaic and electrical energy storage |journal=Applied Energy |date=March 2017 |volume=190 |pages=191–203 |doi=10.1016/j.apenergy.2016.12.153 |s2cid=113623853 |url=http://bura.brunel.ac.uk/handle/2438/22670 }}</ref>

Using battery storage is said to have a levelized cost of $120<ref>{{cite web |url=https://www.forbes.com/sites/chipregister1/2015/01/13/the-battery-revolution-a-technology-disruption-economics-and-grid-level-application-discussion-with-eos-energy-storage/2/ |title=The Battery Revolution: A Technology Disruption, Economics and Grid Level Application Discussion with Eos Energy Storage |author=Chip Register |date=13 January 2015 |work=Forbes |url-status=live |archive-url=https://web.archive.org/web/20161111013625/http://www.forbes.com/sites/chipregister1/2015/01/13/the-battery-revolution-a-technology-disruption-economics-and-grid-level-application-discussion-with-eos-energy-storage/2/ |archive-date=11 November 2016}}</ref> to $170<ref>{{cite web|url=http://www.eosenergystorage.com/technology-and-products/|archive-url=https://web.archive.org/web/20140206103219/http://www.eosenergystorage.com/technology-and-products/|url-status=dead|archive-date=2014-02-06|title=Eos Energy Storage – Technology and Products|work=eosenergystorage.com}}</ref> per MWh. This compares with open cycle gas turbines which, as of 2020, have a cost of around $151–198 per MWh.<ref>{{Cite web|url=https://www.lazard.com/perspective/levelized-cost-of-energy-and-levelized-cost-of-storage-2020/|title = Levelized Cost of Energy and of Storage}}</ref>

Generally speaking, energy storage is economical when the [[marginal cost]] of electricity varies more than the costs of storing and retrieving the energy plus the price of energy lost in the process. For instance, assume a [[pumped-storage hydroelectricity|pumped-storage reservoir]] can pump to its upper reservoir a volume of water capable of producing 1,200 [[MW·h]] after all losses are factored in (evaporation and seeping in the reservoir, efficiency losses, etc.). If the marginal cost of electricity during off-peak times is $15 per MW·h, and the reservoir operates at 75% efficiency (i.e., 1,500&nbsp;MW·h are consumed and 1,200&nbsp;MW·h of energy are retrieved), then the total cost of filling the reservoir is $22,500. If all of the stored energy is sold the following day during peak hours for an average $40 per MW·h, then the reservoir will see revenues of $48,000 for the day, for a [[gross profit]] of $25,500.

However, the marginal cost of electricity varies because of the varying operational and fuel costs of different classes of generators.<ref>{{cite journal |last1=Lai |first1=Chun Sing |last2=Jia |first2=Youwei |last3=Xu |first3=Zhao |last4=Lai |first4=Loi Lei |last5=Li |first5=Xuecong |last6=Cao |first6=Jun |last7=McCulloch |first7=Malcolm D. |title=Levelized cost of electricity for photovoltaic/biogas power plant hybrid system with electrical energy storage degradation costs |journal=Energy Conversion and Management |date=December 2017 |volume=153 |pages=34–47 |doi=10.1016/j.enconman.2017.09.076 |url=https://ora.ox.ac.uk/objects/uuid:9336ab00-8710-4002-ae45-da5d7e35d355 }}</ref> At one extreme, [[base load power plant]]s such as [[coal power|coal]]-fired power plants and [[nuclear power]] plants are low marginal cost generators, as they have high capital and maintenance costs but low fuel costs. At the other extreme, [[peaking power plant]]s such as [[gas turbine]] [[Natural gas#Power generation|natural gas]] plants burn expensive fuel but are cheaper to build, operate and maintain. To minimize the total operational cost of generating power, base load generators are dispatched most of the time, while peak power generators are dispatched only when necessary, generally when energy demand peaks.  This is called "economic dispatch".

Demand for [[electricity]] from the world's various grids varies over the course of the day and from season to season. For the most part, variation in electric demand is met by varying the amount of electrical energy supplied from primary sources. Increasingly, however, operators are storing lower-cost energy produced at night, then releasing it to the grid during the peak periods of the day when it is more valuable.<ref>[http://www.eia.doe.gov/emeu/aer/pdf/pages/sec8_8.pdf Energy Information Administration / Annual Energy Review 2006] {{webarchive|url=https://web.archive.org/web/20080625051521/http://www.eia.doe.gov/emeu/aer/pdf/pages/sec8_8.pdf |date=25 June 2008 }}, Table 8.2a</ref> In areas where hydroelectric dams exist, release can be delayed until demand is greater; this form of storage is common and can make use of existing reservoirs. This is not storing "surplus" energy produced elsewhere, but the net effect is the same – although without the efficiency losses.  Renewable supplies with variable production, like [[wind power|wind]] and [[solar power]], tend to increase the net variation in electric load, increasing the opportunity for grid energy storage.

It may be more economical to find an alternative market for unused electricity, rather than try and store it. [[HVDC|High Voltage Direct Current]] allows for transmission of electricity, losing only 3% per 1000&nbsp;km.

The United States Department of Energy's International Energy Storage Database provides a free list of grid energy storage projects, many of which show funding sources and amounts.<ref>{{cite web |url=http://www.energystorageexchange.org/projects |title=Projects |work=DOE Global Energy Storage Database |url-status=dead |archive-url=https://web.archive.org/web/20141115123502/http://www.energystorageexchange.org/projects |archive-date=15 November 2014|access-date=13 November 2013 }}</ref>

===Load leveling===
The demand for electricity from consumers and industry is constantly changing, broadly within the following categories:
* Seasonal (during dark winters more electric lighting and heating is required, while in other climates hot weather boosts the requirement for air conditioning)
* Weekly (most industry closes at the weekend, lowering demand)
* Daily (such as the morning peak as offices open and [[air conditioners]] get switched on)
* Hourly (one method for estimating television viewing figures in the United Kingdom is to measure the power spikes during advertisement breaks or after programmes when viewers go to switch a kettle on<ref>{{cite web|url=http://news.bbc.co.uk/1/hi/special_report/for_christmas/_new_year/christmas_television/37945.stm|title=BBC News – Christmas Television – The great TV ratings war|work=BBC|url-status=live|archive-url=https://web.archive.org/web/20090112005412/http://news.bbc.co.uk/1/hi/special_report/for_christmas/_new_year/christmas_television/37945.stm|archive-date=12 January 2009}}</ref>)
* Transient (fluctuations due to individual's actions, differences in power transmission efficiency and other small factors that need to be accounted for)

There are currently three main methods for dealing with changing demand:
* Electrical devices generally having a working [[voltage]] range that they require, commonly 110–120&nbsp;V or 220–240&nbsp;V.  Minor variations in load are automatically smoothed by slight variations in the voltage available across the system.
* Power plants can be run below their normal output, with the facility to increase the amount they generate almost instantaneously.  This is termed 'spinning reserve'.
* Additional generation can be brought online.  Typically, these would be hydroelectric or gas turbines, which can be started in a matter of minutes.

The problem with standby gas turbines is higher costs; expensive generating equipment is unused much of the time. Spinning reserve also comes at a cost; plants running below maximum output are usually less efficient. Grid energy storage is used to shift generation from times of peak load to off-peak hours. Power plants are able to run at their peak efficiency during nights and weekends.

Supply-demand leveling strategies may be intended to reduce the cost of supplying peak power or to compensate for the intermittent generation of wind and solar power.

===Portability===
This is the area of greatest success for current energy storage technologies.  Single-use and rechargeable batteries are ubiquitous, and provide power for devices with demands as varied as digital watches and cars.  Advances in battery technology have generally been slow, however, with much of the advance in battery life that consumers see being attributable to efficient power management rather than increased storage capacity. Portable [[consumer electronics]] have benefited greatly from size and power reductions associated with [[Moore's law]]. Unfortunately, Moore's law does not apply to hauling people and freight; the underlying energy requirements for transportation remain much higher than for information and entertainment applications. Battery capacity has become an issue as pressure grows for alternatives to [[internal combustion engine]]s in cars, trucks, buses, trains, ships, and aeroplanes.  These uses require far more [[energy density]] (the amount of energy stored in a given volume or weight) than current battery technology can deliver. Liquid [[hydrocarbon]] fuel (such as [[gasoline]]/[[petrol]] and [[Diesel fuel|diesel]]), as well as alcohols ([[methanol]], [[ethanol]], and [[butanol]]) and [[lipids]] ([[straight vegetable oil]], [[biodiesel]]) have much higher energy densities.

There are synthetic pathways for using electricity to reduce carbon dioxide and water to liquid hydrocarbon or alcohol fuels.<ref name="great_lakes_wind_potential">{{Cite web|url=http://greengold.org/wind/documents/107.pdf|archive-url=https://web.archive.org/web/20090325012758/http://greengold.org/wind/documents/107.pdf|archive-date=25 March 2009 |title=A Great Potential: The Great Lakes as a Regional Renewable Energy Source|first=David|last=Bradley|date=6 February 2004|access-date=2008-10-04}}</ref> These pathways begin with electrolysis of water to generate hydrogen, and then reducing carbon dioxide with excess hydrogen in variations of the reverse [[water gas shift reaction]]. Non-fossil sources of carbon dioxide include [[fermentation (biochemistry)|fermentation]] plants and [[sewage treatment]] plants. Converting electrical energy to carbon-based liquid fuel has potential to provide portable energy storage usable by the large existing stock of motor vehicles and other engine-driven equipment, without the difficulties of dealing with hydrogen or another exotic [[energy carrier]]. These synthetic pathways may attract attention in connection with attempts to improve [[energy security]] in nations that rely on imported petroleum, but have or can develop large sources of renewable or nuclear electricity, as well as to deal with possible future [[Export land model|declines in the amount of petroleum]] available to import.

Because the transport sector uses the energy from petroleum very inefficiently, replacing petroleum with electricity for mobile energy will not require very large investments over many years.{{citation needed|date=January 2015}}

===Reliability===
Virtually all devices that operate on electricity are adversely affected by the sudden removal of their power supply.  Solutions such as UPS ([[Uninterruptible power supply|uninterruptible power supplies]]) or backup generators are available, but these are expensive.  Efficient methods of power storage would allow for devices to have a built-in backup for power cuts, and also reduce the impact of a failure in a generating station.  Examples of this are currently available using [[fuel cell]]s and flywheels.

==See also==
{{Portal|Energy}}
{{div col|colwidth=18em}}
* [[Battery electric vehicles]]
* [[Battery-to-grid]] 
* [[Cost of electricity by source]]
* [[Distributed generation]]
* [[Energy demand management]]
* [[Energy storage]]
* [[Energy storage as a service]] (ESaaS)
* [[Fuel cell vehicle|Fuel-cell vehicle]]
* [[Grid-tied electrical system]]
* [[Hybrid electric vehicle]] 
* [[Hydrogen economy]]
* [[List of energy storage projects]]
* [[peak demand|Off-peak]]
* [[Power-to-X]]
* [[Rechargeable battery]]
* [[Solar vehicle]]
* [[Electric boat|Solar-powered boats]]
* [[United States Department of Energy International Energy Storage Database|U.S. Department of Energy International Energy Storage Database]], a list of grid energy storage projects
* [[Vanadium redox battery]], dispatchable grid energy storage
* [[Vehicle-to-grid]]
* [[Virtual power plant]]
* [[Wind farm]]
{{div col end}}

==References==
{{reflist|30em}}
* [https://web.archive.org/web/20100711124926/http://kn.theiet.org/magazine/issues/1009/windless-day-1009.cfm Saving For a Windless day] by Sean Davies in The E&T Magazine Vol 5 Issue 9 from the [http://www.IET.org www.IET.org]

==Further reading==
* {{cite book|first=Richard |last=Baxter|title=Energy Storage: A Nontechnical Guide|url={{google books |plainurl=y |id=yrd67_MISbkC}}|year=2006|publisher=PennWell Books|isbn=978-1-59370-027-0}}

==External links==
* [https://www.gov.uk/government/publications/benefits-of-long-duration-electricity-storage UK Government report on the Benefits of long-duration electricity storage (Aug 2022)] 
* [http://www.abb.com/cawp/seitp202/0B6D46A05BBC3A27C1256FF2002FD2A0.aspx A large grid-connected nickel-cadmium battery]
* [http://terraverde.wordpress.com/2007/10/07/the-renewable-electron-economy-part-vii-stationary-energy-storage%E2%80%A6key-to-the-renewable-grid/ Stationary Energy Storage…Key to the Renewable Grid]
* [https://web.archive.org/web/20130927211329/http://www.sbc.slb.com/SBCInstitute/Publications/ElectricityStorage.aspx Electricity Storage FactBook]

{{Electricity grid modernization|state=expanded}}
{{Electricity generation}}
{{emerging technologies|energy=yes}}

[[Category:Grid energy storage| ]]
[[Category:Power engineering]]