View source for Grid energy storage

{{Short description|Large scale electricity supply management}}
{{good article}}
{{Redirect|Grid storage|data storage with grid computing|Grid-oriented storage}}
{{Use dmy dates|date=October 2020}}
[[File:20241206 Grid energy storage.svg|thumb|Energy from fossil or nuclear power plants and renewable sources is stored for use by customers.]]
[[File:20241201 Power grid storage energy flow - daily.svg|thumb|Diagram showing flow of energy between energy storage facilities and power grids, as a function of time over a 24 hour period]]

'''Grid energy storage''', also known as '''large-scale energy storage''', are technologies connected to the [[electrical power grid]] that [[Energy storage|store energy]] for later use. These systems help balance supply and demand by storing excess electricity from [[Variable renewable energy|variable renewables]] such as [[Solar power|solar]] and inflexible sources like [[nuclear power]], releasing it when needed. They further provide [[Ancillary services (electric power)|essential grid services]], such as helping to [[Black start|restart the grid]] after a [[power outage]]. 

{{As of|2023}}, the largest form of grid storage is [[pumped-storage hydroelectricity]], with [[Battery energy storage system|utility-scale batteries]] and behind-the-meter batteries coming second and third.{{sfn|Cozzi|Petropoulos|Wanner|2024|p=68}} [[Lithium-ion battery|Lithium-ion batteries]] are highly suited for shorter duration storage up to 8 hours. [[Flow battery|Flow batteries]] and [[Compressed-air energy storage|compressed air energy storage]] may provide storage for medium duration. Two forms of storage are suited for long-duration storage: [[green hydrogen]], produced via [[electrolysis]] and [[thermal energy storage]].{{Sfn|IPCC AR6 WG3 Ch6|2022|pp=653, 656}}

Energy storage is one option to making grids more flexible. An other solution is the use of more [[Dispatchable generation|dispatchable power plants]] that can change their output rapidly, for instance [[peaking power plant]]s to fill in supply gaps. [[Demand response]] can shift load to other times and [[Interconnector|interconnections]] between regions can balance out fluctuations in renewables production.{{Sfn|IPCC AR6 WG3 Ch6|2022|p=651}} 

The price of storage technologies typically [[Experience curve|goes down with experience]]. For instance, lithium-ion batteries have been getting some 20% cheaper for each doubling of worldwide capacity.{{Sfn|Schmidt|Staffell|2023|p=92}} Systems with under 40% variable renewables need only short-term storage. At 80%, medium-duration storage becomes essential and beyond 90%, long-duration storage does too. The economics of long-duration storage is challenging, and alternative flexibility options like demand response may be more economic.

{{toclimit|3}}

==Roles in the power grid==
Any [[grid (electricity)|electrical power grid]] must match electricity production to consumption, both of which vary significantly over time. Energy derived from [[Solar power|solar]] and [[Wind power|wind sources]] varies with the weather on time scales ranging from less than a second to weeks or longer. [[Nuclear power]] is less flexible than [[Fossil fuel|fossil fuels]], meaning it cannot easily match the variations in demand. Thus, [[low-carbon electricity]] without storage presents special challenges to [[Electric utility|electric utilities]].{{Sfn|Schmidt|Staffell|2023|p=8}}

Electricity storage is one of the three key ways to replace flexibility from [[Fossil fuel|fossil fuels]] in the grid. Other options are [[demand-side response]], in which consumers change when they use electricity or how much they use. For instance, households may have [[Dynamic pricing#Time-based utility pricing|cheaper night tariffs]] to encourage them to use electricity at night. Industry and commercial consumers can also change their demand to meet supply. Improved [[Electric power transmission|network interconnection]] smooths the variations of renewables production and demand. When there is little wind in one location, another might have a surplus of production. Expansion of [[Transmission line|transmission lines]] usually takes a long time.{{Sfn|Schmidt|Staffell|2023|pp=10-11}}
{| class="wikitable"
|+Potential roles of energy storage in the grid{{sfn|Schmidt|Staffell|2023|pp=74-76}}{{sfn|Cozzi|Petropoulos|Wanner|2024|p=36}}
!
!Consumption
!Network
!Generation
|-
! Short-term flexibility
|Increased use [[Rooftop solar power|rooftop solar]], cost reductions from [[Dynamic pricing#Time-based|time-based]] rates
|[[Transmission congestion|Congestion]] relief
|Renewables integration (smoothing, [[arbitrage]])
|-
! Essential grid services
|Backup power during outages
|[[Frequency regulation]]
|[[Black start]]
|-
! System reliability and planning
|Creation of [[mini-grids]]
|Savings in transmission and [[Electric power distribution|distribution network]]
|Meeting [[peak demand]]
|}
Energy storage has a large set of roles in the electricity grid and can therefore provide many different services. For instance, it can [[arbitrage]] by keeping it until the [[Electricity pricing|electricity price]] rises, it can help make the grid more stable, and help reduce investment into transmission infrastructure.{{Sfn|Armstrong|Chiang|2022|pp=6-7}} The type of service provided by storage depends on who manages the technology, whether the technology is based alongside generation of electricity, within the network, or at the side of [[Electric energy consumption|consumption]].{{sfn|Cozzi|Petropoulos|Wanner|2024|p=36}} 

Providing short-term flexibility is a key role for energy storage. On the generation side, it can help with the integration of [[variable renewable energy]], storing it when there is an oversupply of wind and solar and electricity prices are low. More generally, it can exploit the changes in prices of electricity over time in the [[wholesale market]], charging when electricity is cheap and selling when it is expensive. It can further help with [[grid congestion]] (where there is insufficient capacity on [[transmission lines]]). Consumers can use storage to use more of their self-produced electricity (for instance from [[rooftop solar power]]).{{sfn|Cozzi|Petropoulos|Wanner|2024|p=36}}{{sfn|Schmidt|Staffell|2023|pp=74-76}}

Storage can also be used to provide [[ancillary services|essential grid services]]. On the generation side, storage can smooth out the variations in production, for instance for solar and wind. It can assist in a [[black start]] after a [[power outage]]. On the network side, these include [[frequency regulation]] (continuously) and [[frequency response]] (after unexpected changes in supply or demand). On the consumption side, storage can help to improve the [[Electric power quality|quality of the delivered electricity]] in less stable grids.{{sfn|Cozzi|Petropoulos|Wanner|2024|p=36}}{{sfn|Schmidt|Staffell|2023|pp=74-75}}

Investment in storage may make some investments in the transmission and [[distribution network]] unnecessary, or may allow them to be scaled down. Additionally, storage can ensure there is sufficient capacity to meet [[peak demand]] within the electricity grid. Finally, in off-grid home systems or [[Mini-grid|mini-grids]], electricity storage can help provide [[energy access]] in areas that were previously not connected to the electricity grid.{{sfn|Cozzi|Petropoulos|Wanner|2024|p=36}}
==Forms==
{{multiple image | total_width=450
|image1= 20240706 Energy storage - renewable energy - battery - 100 ms.gif  |caption1= Energy from sunlight or other renewable energy is converted to potential energy for storage in devices such as electric batteries. The stored potential energy is later converted to electricity that is added to the power grid, even when the original energy source is not available.
| image2=20241208 Energy demand and generation - daily flow.svg |caption2= In daytime, different renewable sources provide different amounts of power to the grid. At night, energy is provided by batteries that were charged during the day when renewable energy exceeded customer demand.
}}

Electricity can be stored directly for a short time in capacitors, somewhat longer electrochemically in [[Electric battery|batteries]], and much longer chemically (e.g. hydrogen), mechanically (e.g. pumped hydropower) or as heat.{{Sfn|Schmidt|Staffell|2023|p=33}} The first pumped hydroelectricity was constructed at the end of the 19th century around [[Alps|the Alps]] in Italy, Austria, and Switzerland. The technique rapidly expanded during the 1960s to 1980s [[History of nuclear power#Development and early opposition to nuclear power|nuclear boom]], due to nuclear power's inability to quickly adapt to changes in electricity demand. In the 21st century, interest in storage surged due to the rise of [[Sustainable energy|sustainable energy sources]], which are often weather-dependent.<ref>{{Cite journal |last1=Mitali |first1=J. |last2=Dhinakaran |first2=S. |last3=Mohamad |first3=A. A. |date=2022 |title=Energy storage systems: a review |url=https://linkinghub.elsevier.com/retrieve/pii/S277268352200022X |journal=Energy Storage and Saving |volume=1 |issue=3 |pages=166–216 |doi=10.1016/j.enss.2022.07.002 |issn=2772-6835|url-access=subscription |doi-access=free }}</ref> Commercial batteries have been available for over a century,<ref>{{Cite journal |last=Gür |first=Turgut M. |date=2018-10-10 |title=Review of electrical energy storage technologies, materials and systems: challenges and prospects for large-scale grid storage |url=https://pubs.rsc.org/en/content/articlelanding/2018/ee/c8ee01419a |journal=Energy & Environmental Science |language=en |volume=11 |issue=10 |pages=2696–2767 |doi=10.1039/C8EE01419A |issn=1754-5706|url-access=subscription }}</ref> their widespread use in the power grid is more recent, with only 1 GW available in 2013.{{Sfn|Cozzi|Petropoulos|Wanner|2024|p=32}} 

=== Batteries ===
{{Main|Battery energy storage system}}
[[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>]]
==== Lithium-ion batteries ====
Lithium-ion batteries are the most commonly used batteries for grid applications, {{As of|2024|lc=y}}, following the application of batteries in electric vehicles (EVs). In comparison with EVs, grid batteries require less [[energy density]], meaning that more emphasis can be put on costs, the ability to charge and discharge often and lifespan. This has led to a shift towards [[Lithium iron phosphate battery|lithium iron phosphate batteries]] (LFP batteries), which are cheaper and last longer than traditional lithium-ion batteries.{{Sfn|Cozzi|Petropoulos|Wanner|2024|p=45}} 

Costs of batteries are declining rapidly; from 2010 to 2023 costs fell by 90%.{{Sfn|Cozzi|Petropoulos|Wanner|2024|p=18}} {{As of|2024}}, utility-scale systems account for two thirds of added capacity, and home applications (behind-the-meter) for one third.{{Sfn|Cozzi|Petropoulos|Wanner|2024|p=20}} Lithium-ion batteries are highly suited to short-duration storage (<8h) due to cost and degradation associated with high [[State of charge|states of charge]].{{Sfn|Cozzi|Petropoulos|Wanner|2024|p=45–46}} 

===== Electric vehicles =====
{{Main|Vehicle-to-grid|}}
[[File:Nissan Leaf aan Amsterdamse laadpaal.jpg|thumb|Batteries from old electric cars, such as this [[Nissan Leaf]], can be reused but as of 2024 it is not known whether grid storage or behind the meter storage will be the best application.<ref>{{Cite web |date=2024-04-30 |title=Nissan and Ecobat to give used EV batteries a second life beyond the car |url=https://uk.nissannews.com/en-GB/releases/nissan-and-ecobat-to-give-used-ev-batteries-a-second-life-beyond-the-car |access-date=2024-11-21 |website=Official Great Britain Newsroom |language=en-GB}}</ref>|alt=A Nissan Leaf charging]]
The [[electric vehicle]] fleet has a large overall battery capacity, which can potentially be used for grid energy storage. This could be in the form of [[vehicle-to-grid]] (V2G), where cars store energy when they are not in use, or by [[repurposing]] batteries from cars at the end of the vehicle's life. Car batteries typically range between 33 and 100 kWh;<ref>{{Cite journal |last1=Xu |first1=Chengjian |last2=Behrens |first2=Paul |last3=Gasper |first3=Paul |last4=Smith |first4=Kandler |last5=Hu |first5=Mingming |last6=Tukker |first6=Arnold |last7=Steubing |first7=Bernhard |date=2023-01-17 |title=Electric vehicle batteries alone could satisfy short-term grid storage demand by as early as 2030 |url=http://dx.doi.org/10.1038/s41467-022-35393-0 |journal=Nature Communications |volume=14 |issue=1 |page=119 |doi=10.1038/s41467-022-35393-0 |pmid=36650136 |issn=2041-1723|pmc=9845221 |bibcode=2023NatCo..14..119X }}</ref> for comparison, a typical upper-middle-class household in Spain might use some 18 kWh in a day.<ref>{{Cite journal |last1=García-Vázquez |first1=Carlos Andrés |last2=Espinoza-Ortega |first2=Hernán |last3=Llorens-Iborra |first3=Francisco |last4=Fernández-Ramírez |first4=Luis M. |date=2022-11-01 |title=Feasibility analysis of a hybrid renewable energy system with vehicle-to-home operations for a house in off-grid and grid-connected applications |url=https://linkinghub.elsevier.com/retrieve/pii/S2210670722004371 |journal=Sustainable Cities and Society |volume=86 |pages=104124 |doi=10.1016/j.scs.2022.104124 |bibcode=2022SusCS..8604124G |issn=2210-6707|url-access=subscription }}</ref> By 2030, batteries in electric vehicles may be able to meet all short-term storage demand globally.<ref>{{Cite journal |last1=Xu |first1=Chengjian |last2=Behrens |first2=Paul |last3=Gasper |first3=Paul |last4=Smith |first4=Kandler |last5=Hu |first5=Mingming |last6=Tukker |first6=Arnold |last7=Steubing |first7=Bernhard |date=2023-01-17 |title=Electric vehicle batteries alone could satisfy short-term grid storage demand by as early as 2030 |journal=Nature Communications |language=en |volume=14 |issue=1 |pages=119 |doi=10.1038/s41467-022-35393-0 |pmid=36650136 |issn=2041-1723|pmc=9845221 |bibcode=2023NatCo..14..119X }}</ref>

{{As of|2024}}, there have been more than 100 V2G pilot projects globally.<ref name=":0">{{Cite journal |last1=Aguilar Lopez |first1=Fernando |last2=Lauinger |first2=Dirk |last3=Vuille |first3=François |last4=Müller |first4=Daniel B. |date=2024-05-16 |title=On the potential of vehicle-to-grid and second-life batteries to provide energy and material security |url=http://dx.doi.org/10.1038/s41467-024-48554-0 |journal=Nature Communications |volume=15 |issue=1 |page=4179 |doi=10.1038/s41467-024-48554-0 |pmid=38755161 |issn=2041-1723|pmc=11099178 |bibcode=2024NatCo..15.4179A }}</ref> The effect of V2G charging on battery life can be positive or negative. Increased cycling of batteries can lead to faster degradation, but due to better management of the [[state of charge]] and gentler charging and discharging, V2G might instead increase the lifetime of batteries.<ref name=":0" /><ref>{{Cite journal |last1=Bhoir |first1=Shubham |last2=Caliandro |first2=Priscilla |last3=Brivio |first3=Claudio |date=2021-12-01 |title=Impact of V2G service provision on battery life |url=https://linkinghub.elsevier.com/retrieve/pii/S2352152X21008781 |journal=Journal of Energy Storage |volume=44 |pages=103178 |doi=10.1016/j.est.2021.103178 |bibcode=2021JEnSt..4403178B |issn=2352-152X|doi-access=free }}</ref> Second-hand batteries may be useable for stationary grid storage for roughly 6 years, when their capacity drops from roughly 80% to 60% of the initial capacity. [[Lithium iron phosphate battery|LFP batteries]] are particularly suitable for reusing, as they degrade less than other lithium-ion batteries and recycling is less attractive as their materials are not as valuable.<ref name=":0" />

==== Other battery types ====
In redox [[flow battery|flow batteries]], energy is stored in liquids, which are placed in two separate tanks. When charging or discharging, the liquids are pumped into a cell with the electrodes. The amount of energy stored (as set by the size of the tanks) can be adjusted separately from the power output (as set by the speed of the pumps).{{sfn|Cozzi|Petropoulos|Wanner|2024|p=46}} Flow batteries have the advantages of low capital cost for charge-discharge duration over 4 h, and of long durability (many years). Flow batteries are inferior to [[lithium-ion batteries]] in terms of [[Energy efficiency (physics)|energy efficiency]], averaging efficiencies between 60% and 75%. [[Vanadium redox battery|Vanadium redox batteries]] is most commercially advanced type of flow battery, with roughly 40 companies making them {{As of|2022|lc=y}}.<ref>{{Cite journal |last=Tolmachev |first=Yuriy V. |date=2023-03-01 |title=Review—Flow Batteries from 1879 to 2022 and Beyond |url=https://iopscience.iop.org/article/10.1149/1945-7111/acb8de |journal=Journal of the Electrochemical Society |volume=170 |issue=3 |pages=030505 |bibcode=2023JElS..170c0505T |doi=10.1149/1945-7111/acb8de |issn=0013-4651}}</ref> 

[[Sodium-ion battery|Sodium-ion batteries]] are a possible alternative to lithium-ion batteries, as they are less [[flammable]],<ref>{{Cite news |title=Grid-scale storage is the fastest-growing energy technology |url=https://www.economist.com/the-world-ahead/2024/11/20/grid-scale-storage-is-the-fastest-growing-energy-technology |access-date=2025-01-04 |work=The Economist |issn=0013-0613}}</ref> and use cheaper and less critical materials. They have a lower energy density, and possibly a shorter lifespan. If produced at the same scale as lithium-ion batteries, they may become 20% to 30% cheaper.{{Sfn|Cozzi|Petropoulos|Wanner|2024|p=46}} [[Iron-air batteries]] may be suitable for even longer duration storage than flow batteries (weeks), but the technology is not yet mature.{{Sfn|Cozzi|Petropoulos|Wanner|2024|p=47}}  

{| class="wikitable sortable" style="width: auto; text-align: center; table-layout: fixed;"
|+Technology comparison{{Sfn|Cozzi|Petropoulos|Wanner|2024|p=47}}
|-
! scope="col" | Technology
! scope="col" | Less than 4h
! scope="col" | 4h to 8h
! scope="col" | Days
! scope="col" | Weeks
! scope="col" | Seasons
|-
! scope="row" | [[Lithium-ion battery|Lithium-ion]]
| {{Yes}}
| {{Yes}}
| {{No}}
| {{No}}
| {{No}}
|-
! scope="row" | [[Sodium-ion battery|Sodium-ion]]
| {{Yes}}
| {{Yes}}
| {{No}}
| {{No}}
| {{No}}
|-
! scope="row" | [[Vanadium redox battery|Vanadium flow]]
| {{Maybe}}
| {{Yes}}
| {{Yes}}
| {{No}}
| {{No}}
|-
! scope="row" | [[Metal–air electrochemical cell|Iron-air]]
| {{No}}
| {{No}}
| {{Maybe}}
| {{Yes}}
| {{No}}
|}

=== Electrical ===
Storage in [[Supercapacitor|supercapacitors]] works well for applications where a lot of power is needed for short amount of time. In the power grid, they are therefore mostly used in short-term frequency regulation.{{Sfn|Schmidt|Staffell|2023|pp=54-55}} 

===Hydrogen and chemical storage===

{{Main|Hydrogen economy|Hydrogen storage}}

{{See also|Combined cycle hydrogen power plant}}Various [[power-to-gas]] technologies exist that can convert excess electricity into an easier to store chemical. The lowest cost and most efficient one is [[hydrogen]]. However, it is easier to use synthetic [[methane]] with existing infrastructure and appliances, as it is very similar to natural gas.{{Sfn|Letcher|2022|p=606}}

{{As of|2024}}, there have been a number of demonstration plants where hydrogen is burned in [[Gas turbine|gas turbines]], either co-firing with natural gas, or on its own. Similarly, a number of [[Coal-fired power station|coal plants]] have demonstrated it is possible to co-fire [[ammonia]] when burning coal. In 2022, there was also a small pilot to burn pure ammonia in a gas turbine.{{Sfn|Remme|Bermudez Menendez|2024|pp=54-55}} A portion of existing gas turbines are capable of co-firing hydrogen, which means there is, as a lower estimate, 80 GW of capacity ready to burn hydrogen.{{Sfn|Remme|Bermudez Menendez|2024|p=57}}

==== Hydrogen ====
[[Hydrogen]] can be used as a long-term storage medium.{{Sfn|Smith|2023|p=5}} [[Green hydrogen]] is produced from the [[electrolysis of water]] and converted back into electricity in an [[internal combustion engine]], or a [[fuel cell]], with a round-trip efficiency of roughly 41%.{{Sfn|Smith|2023|p=14}} Together with thermal storage, it is expected to be best suited to seasonal energy storage.{{Sfn|IPCC AR6 WG3 Ch6|2022|p=653}} 

Hydrogen can be stored aboveground in tanks or underground in larger quantities. Underground storage is easiest in [[salt cavern]]s, but only a certain number of places have suitable geology.{{Sfn|Armstrong|Chiang|2022|p=150}} Storage in porous rocks, for instance in empty [[Gas field|gas fields]] and some [[aquifer]]s, can store hydrogen at a larger scale, but this type of storage may have some drawbacks. For instance, some of the hydrogen may leak, or react into [[H2S|H<sub>2</sub>S]] or [[methane]].<ref>{{Cite journal |last1=Miocic |first1=Johannes |last2=Heinemann |first2=Niklas |last3=Edlmann |first3=Katriona |last4=Scafidi |first4=Jonathan |last5=Molaei |first5=Fatemeh |last6=Alcalde |first6=Juan |date=2023-08-30 |title=Underground hydrogen storage: a review |url=https://www.lyellcollection.org/doi/10.1144/SP528-2022-88 |journal=Geological Society, London, Special Publications |language=en |volume=528 |issue=1 |pages=73–86 |doi=10.1144/SP528-2022-88 |issn=0305-8719|hdl=10261/352537 |hdl-access=free }}</ref>

====Ammonia ====

Hydrogen can be converted into ammonia in a reaction with [[nitrogen]] in the [[Haber-Bosch process]]. Ammonia, a gas at room temperature, is more expensive to produce than hydrogen. However, it can be stored more cheaply than hydrogen. Tank storage is usually done at between one and ten [[Bar (unit)|times atmospheric pressure]] and at a temperature of {{Convert|-30|C|F}}, in liquid form.{{Sfn|Smith|2023|p=18}} 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> It can be used for power generation directly, or converted back to hydrogen first. Alternatively, it has potential applications as a fuel in [[shipping]].<ref>{{Cite web |date=2020 |title=Green ammonia |url=https://royalsociety.org/news-resources/projects/low-carbon-energy-programme/green-ammonia/ |access-date=2024-11-23 |website=Royal Society |language=en}}</ref>

====Methane====

It is possible to further convert hydrogen into [[methane]] via the [[Sabatier reaction]], a chemical reaction which combines {{CO2}} and H<sub>2</sub>. While the reaction that converts CO from [[gasified coal]] into {{CH4}} is mature, the process to form methane out of {{CO2}} is less so. Efficiencies of around 80% one-way can be achieved, that is, some 20% of the energy in hydrogen is lost in the reaction.{{Sfn|Letcher|2022|p=602}}
=== Mechanical ===

==== Flywheel ====
{{Main|Flywheel storage power system|Flywheel energy storage}}
[[File:Beacon Power New York - 20 MW Flywheel Energy Storage.jpg|thumb|[[Beacon Power]] 20 MW flywheel energy storage plant in [[New York (state)|New York]]]]

Flywheels store energy in the form of mechanical energy. They are suited to supplying high levels of electricity over minutes and can also be charged rapidly. They have a long lifetime and can be used in settings with widely varying temperatures. The technology is mature, but more expensive than batteries and supercapacitors and not used frequently.{{Sfn|IPCC AR6 WG3 Ch6|2022|p=655}}

==== Pumped hydro ====
{{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]]

{{As of|2023}}, pumped-storage hydroelectricity (PSH) was the largest form of grid energy storage globally, with an installed capacity of 181 [[Watt|GW]], surpassing the combined capacity of utility-scale and behind-the-meter battery storage, which totaled approximately 88 GW.<ref>{{Cite web |title=Global installed energy storage capacity by scenario, 2023 and 2030 – Charts – Data & Statistics |url=https://www.iea.org/data-and-statistics/charts/global-installed-energy-storage-capacity-by-scenario-2023-and-2030 |access-date=2024-08-25 |website= |publisher=International Energy Agency |language=en-GB}}</ref> 

PSH is particularly effective for managing daily fluctuations in energy demand. During periods of low demand, water is pumped to a higher-elevation reservoir, and during peak demand, the stored water is released to generate electricity through turbines.{{Sfn|IPCC AR6 WG3 Ch6|2022|p=654}} The system has an efficiency rate of 75% to 85% and can quickly respond to changes in demand, typically within seconds to minutes.<ref>{{Cite journal |last1=Javed |first1=Muhammad Shahzad |last2=Ma |first2=Tao |last3=Jurasz |first3=Jakub |last4=Amin |first4=Muhammad Yasir |date=2020-04-01 |title=Solar and wind power generation systems with pumped hydro storage: Review and future perspectives |url=https://linkinghub.elsevier.com/retrieve/pii/S0960148119318592 |journal=Renewable Energy |volume=148 |pages=176–192 |doi=10.1016/j.renene.2019.11.157 |bibcode=2020REne..148..176J |issn=0960-1481|url-access=subscription }}</ref>

While traditional PSH systems require specific geographical conditions, alternative designs have been proposed. These include using deep [[salt cavern]]s or constructing hollow structures on the [[seabed]], where the ocean serves as the upper reservoir.{{Sfn|IPCC AR6 WG3 Ch6|2022|p=654}} However, PSH construction is often expensive, time-consuming, and can have significant environmental and social impacts on nearby communities.{{Sfn|IPCC AR6 WG3 Ch6|2022|p=654}} Installing [[Floating solar|floating solar panels]] on reservoirs, can increase the efficiency of PSH systems. These panels reduce water evaporation and benefit from cooling by the water surface, which also improves their energy generation efficiency.{{Sfn|IRENA|2020|p=7}}

==== 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. While technically no electricity is stored, the net effect is the similar as pumped storage. The amount of storage available in hydroelectric dams is much larger than in pumped storage. Upgrades may be needed so that these dams can respond to variable demand. For instance, additional investment may be needed in transmission lines, or additional turbines may need to be installed to increase the peak output from the dam.{{Sfn|Armstrong|Chiang|2022|pp=69-70}}

Dams usually have multiple purposes. As well as energy generation, they often play a role in [[flood defense]] and protection of ecosystems, recreation, and they supply water for [[irrigation]]. This means it is not always possible to change their operation much, but even with low flexibility, they may still play an important role in responding to changes in wind and solar production.{{Sfn|Armstrong|Chiang|2022|pp=69}}

==== Gravity ====
{{Main|Energy storage#Solid mass gravitational}}

Alternative [[Gravity battery|methods that use gravity]] include storing energy by moving large solid masses upward against gravity. This can be achieved inside old mine shafts<ref>{{Cite web |date=21 October 2019 |title=How UK's disused mine shafts could be used to store renewable energy |url=http://www.theguardian.com/environment/2019/oct/21/how-uks-disused-mine-shafts-plan-to-store-renewable-energy |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 |author=Akshat Rathi |date=August 18, 2018 |title=Stacking concrete blocks is a surprisingly efficient way to store energy |url=https://qz.com/1355672/stacking-concrete-blocks-is-a-surprisingly-efficient-way-to-store-energy/ |work=Quartz}}</ref>

====Compressed air ====
{{Main|Compressed-air energy storage}}

Compressed air energy storage (CAES) stores electricity by compressing air. The compressed air is typically stored in large underground caverns. The expanding air can be used to drive turbines, converting the energy back into electricity. As [[Adiabatic cooling|air cools when expanding]], some heat needs to be added in this stage to prevent freezing. This can be provided by a low-carbon source, or in the case of advanced CAES, by reusing the heat that is released when air is compressed. {{As of|2023}}, there are three advanced CAES projects in operation in China.{{Sfn|Smith|2023|p=19}} Typical efficiencies of advanced CAES are between 60% and 80%.<ref>{{Cite journal |last1=Zhang |first1=Xinjing |last2=Gao |first2=Ziyu |last3=Zhou |first3=Bingqian |last4=Guo |first4=Huan |last5=Xu |first5=Yujie |last6=Ding |first6=Yulong |last7=Chen |first7=Haisheng |date=2024 |title=Advanced Compressed Air Energy Storage Systems: Fundamentals and Applications |journal=Engineering |volume=34 |pages=246–269 |doi=10.1016/j.eng.2023.12.008 |issn=2095-8099 |doi-access=free}}</ref>

==== Liquid air or {{CO2}} ====
{{Main|Cryogenic energy storage|}}

Another electricity storage method is to compress and cool air, turning it into [[liquid air]], which can be stored and expanded when needed, turning a turbine to generate electricity. This is called liquid air energy storage (LAES).{{Sfn|Smith|2023|p=20}} The air would be cooled to temperatures of {{Convert|-196|C|F}} to become liquid. Like with compressed air, heat is needed for the expansion step. In the case of LAES, low-grade industrial heat can be used for this.{{Sfn|IPCC AR6 WG3 Ch6|2022|p=655}} Energy efficiency for LAES lies between 50% and 70%. {{As of|2023}}, LAES is moving from pre-commercial to commercial.<ref>{{Cite journal |last1=Liang |first1=Ting |last2=Zhang |first2=Tongtong |last3=Lin |first3=Xipeng |last4=Alessio |first4=Tafone |last5=Legrand |first5=Mathieu |last6=He |first6=Xiufen |last7=Kildahl |first7=Harriet |last8=Lu |first8=Chang |last9=Chen |first9=Haisheng |last10=Romagnoli |first10=Alessandro |last11=Wang |first11=Li |last12=He |first12=Qing |last13=Li |first13=Yongliang |last14=Yang |first14=Lizhong |last15=Ding |first15=Yulong |date=2023 |title=Liquid air energy storage technology: a comprehensive review of research, development and deployment |url=https://iopscience.iop.org/article/10.1088/2516-1083/aca26a/meta |journal=Progress in Energy |language=en |volume=5 |issue=1 |pages=012002 |doi=10.1088/2516-1083/aca26a |bibcode=2023PrEne...5a2002L |issn=2516-1083|doi-access=free }}</ref> An alternative is the compression of {{Chem|CO|2}} to store electricity.{{Sfn|Schmidt|Staffell|2023|p=64}}

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

Electricity can be directly stored thermally with a [[Carnot battery]]. A Carnot battery is a type of energy storage system that stores electricity in heat storage and converts the stored heat back to electricity via [[thermodynamic cycle]]s (for instance, a turbine). While less efficient than pumped hydro or battery storage, this type of system is expected to be cheap and can provide long-duration storage.<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 |year=2020 |title=Carnot battery technology: A state-of-the-art review |journal=Journal of Energy Storage |volume=32 |pages=101756 |bibcode=2020JEnSt..3201756D |doi=10.1016/j.est.2020.101756 |issn=2352-152X |s2cid=225019981 |hdl-access=free |hdl=2268/251473}}</ref>{{Sfn|Vandersickel|Gutierrez|2023|p=107}} A [[pumped-heat electricity storage]] system is a Carnot battery that uses a [[reversible heat pump]] to convert the electricity into heat.<ref>{{Cite journal |last1=Benato |first1=Alberto |last2=Stoppato |first2=Anna |date=2018-06-01 |title=Pumped Thermal Electricity Storage: A technology overview |url=https://linkinghub.elsevier.com/retrieve/pii/S2451904917303700 |journal=Thermal Science and Engineering Progress |volume=6 |pages=301–315 |doi=10.1016/j.tsep.2018.01.017 |bibcode=2018TSEP....6..301B |issn=2451-9049|url-access=subscription }}</ref> It usually stores the energy in both a hot and cold reservoir. To achieve decent efficiencies (>50%), the temperature ratio between the two must reach a factor of 5.<ref>{{Cite journal |last1=Albertus |first1=Paul |last2=Manser |first2=Joseph S. |last3=Litzelman |first3=Scott |date=2020 |title=Long-Duration Electricity Storage Applications, Economics, and Technologies |url=https://linkinghub.elsevier.com/retrieve/pii/S2542435119305392 |journal=Joule |volume=4 |issue=1 |pages=21–32 |doi=10.1016/j.joule.2019.11.009 |bibcode=2020Joule...4...21A |issn=2542-4351}}</ref>

Thermal energy storage is also used in combination with [[concentrated solar power]] (CSP). In CSP, solar energy is first converted into heat, and then either directly converted into electricity or first stored. The energy is released when there is little or no sunshine.<ref>{{Cite journal |last1=Pelay |first1=Ugo |last2=Luo |first2=Lingai |last3=Fan |first3=Yilin |last4=Stitou |first4=Driss |last5=Rood |first5=Mark |date=2017-11-01 |title=Thermal energy storage systems for concentrated solar power plants |url=https://www.sciencedirect.com/science/article/abs/pii/S1364032117304021 |journal=Renewable and Sustainable Energy Reviews |volume=79 |pages=82–100 |doi=10.1016/j.rser.2017.03.139 |bibcode=2017RSERv..79...82P |issn=1364-0321}}</ref> This means that CSP can be used as a [[Dispatchable generation|dispatchable (flexible)]] form of generation. The energy in a CSP system can for instance be stored in [[Molten-salt battery|molten salts]] or in a solid medium such as sand.<ref>{{Cite web |title=Thermal Storage System Concentrating Solar-Thermal Power Basics |url=https://www.energy.gov/eere/solar/thermal-storage-system-concentrating-solar-thermal-power-basics |access-date=2024-11-17 |website=Energy.gov |language=en}}</ref>

Finally, [[Heating, ventilation, and air conditioning|heating and cooling systems]] in [[buildings]] 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>

==Economics==

=== Costs ===
[[File:Battery-cost-learning-curve.png|thumb|upright=1.5| [[Experience curve]] of lithium-ion batteries: the price of batteries dropped 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 |journal=Energy & Environmental Science |language=en |volume=14 |issue=4 |pages=1635–1651 |arxiv=2007.13920 |doi=10.1039/D0EE02681F |issn=1754-5692 |s2cid=220830992 |doi-access=free}}</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>|alt=log-log graph of cumulative capacity and battery costs showing a near-linear downwards line, starting in 1992.]]
The [[levelized cost of storage|levelized cost of storing electricity]] (LCOS) is a measure of the lifetime costs of storing electricity per [[MWh]] of electricity discharged. It includes investment costs, but also operational costs and charging costs.{{sfn|Schmidt|Staffell|2023|pp=131, 136}} It 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 |bibcode=2017ApEn..190..191L |s2cid=113623853 |url=http://bura.brunel.ac.uk/handle/2438/22670 }}</ref> 

For power applications (for instance around [[ancillary services]] or [[black start]]s), a similar metric is the [[annuitized capacity cost]] (ACC), which measures the lifetime costs per kW. ACC is lowest when there are few cycles (<300) and when the discharge is less than one hour. This is because the technology is reimbursed only when it provides spare capacity, not when it is discharged.{{sfn|Schmidt|Staffell|2023|pp=137, 150-151}} 

The cost of storage is coming down following technology-dependent [[Experience curve effects|experience curves]], the price drop for each doubling in cumulative capacity (or experience). Lithium-ion battery prices fast: the price utitlities pay for them falls 19% with each doubling of capacity. Hydrogen production via electrolysis has a similar learning rate, but it is much more uncertain. Vanadium-flow batteries typically get 14% cheaper for each doubling of capacity. Pumped hydropower has not seen prices fall much with increased experience.{{Sfn|Schmidt|Staffell|2023|p=92}}

===Market and system value===
There are four categories of services which provide economic value for storage: those related to power quality (such as frequency regulation), reliability (ensuring peak demand can be met), better use of assets in the system (e.g. avoiding transmission investments) and [[arbitrage]] (exploiting price differences over time). Before 2020, most value for storage was in providing power quality services. Arbitrage is the service with the largest economic potential for storage applications.{{sfn|Schmidt|Staffell|2023|p=|pp=177, 179}}
{| class="wikitable"
|+Storage requirements based on the share of variable renewable energy (VRE). For energy storage, this is the energy stored at a given time, not the total over the year{{Sfn|Schmidt|Staffell|2023|p=227}}
!VRE share
!Power (% of peak demand)
!Energy storage (% of annual demand)
|-
|50%
|Less than 20%
|0.02%
|-
|80%
|20–50%
|0.03–0.1%
|-
|90%
|25–75%
|0.05–0.2%
|}
In systems with under 40% of variable renewables, only short-term storage (of less than 4 hours) is needed for integration. When the share of variable renewables climbs to 80%, medium-duration storage (between 4 and 16 hours, for instance [[Compressed-air energy storage|compressed air]]) is needed. Above 90%, large-scale [[long-duration storage]] is required.{{Sfn|Schmidt|Staffell|2023|p=241}} The economics of long-duration storage is challenging even then, as the costs are high. Alternative flexibility options, such as demand response, network expansions or flexible generation ([[Geothermal energy|geothermal]] or fossil gas with [[carbon capture and storage]]) may be lower-cost.{{Sfn|Schmidt|Staffell|2023|p=247}}

Like with renewables, storage will "[[Market cannibalisation|cannibalise]]" its own income, but even more strongly. That is, with more storage on the market, there is less of an opportunity to do arbitrage or deliver other services to the grid.{{Sfn|Schmidt|Staffell|2023|p=261}} How markets are designed impacts revenue potential too. The income from arbitrage is quite variable between years, whereas markets that have [[capacity payments]] likely show less volatility.{{Sfn|Schmidt|Staffell|2023|p=215}} 

Electricity storage is not 100% efficient, so more electricity needs to be bought than can be sold. This implies that if there is only a small variation in price, it may not be economical to charge and discharge. For instance, if the storage application is 75% efficient, the price at which the electricity is sold needs to be at least 1.33 higher than the price for which it was bought.{{Sfn|Schmidt|Staffell|2023|p=191}} Typically, electricity prices vary most between day and night, which means that storage up to 8 hours has relatively high potential for profit.{{Sfn|Schmidt|Staffell|2023|p=176}}

==See also==
{{Portal|Energy}}
{{div col|colwidth=18em}}
* [[Distributed generation]]
* [[Energy storage as a service]] (ESaaS)
* [[List of energy storage projects]]
* [[Power-to-X]]
* [[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
* [[Virtual power plant]]
{{div col end}}

==References==
{{reflist|30em}}

=== Cited sources ===
* {{cite book |title=The Future of Energy Storage: An Interdisciplinary MIT Study |url=https://energy.mit.edu/wp-content/uploads/2022/05/The-Future-of-Energy-Storage.pdf |last1=Armstrong |first1=Robert |last2=Chiang |first2=Yet-Ming |publisher=Massachusetts Institute of Technology |year=2022 |isbn=978-0-578-29263-2}}
* {{Cite book |last1=Cozzi |first1=Laura |url=https://iea.blob.core.windows.net/assets/cb39c1bf-d2b3-446d-8c35-aae6b1f3a4a0/BatteriesandSecureEnergyTransitions.pdf |title=Batteries and Secure Energy Transitions |last2=Petropoulos |first2=Apostolos |last3=Wanner |first3=Brent |date=April 2024 |publisher=International Energy Agency}}
* {{Cite book | url=https://iea.blob.core.windows.net/assets/3ece5ee4-7537-4992-8ba6-b83c994c3fd4/GlobalHydrogenReview2024.pdf|title=Global Hydrogen Review 2024|date=October 2024|publisher=International Energy Agency|last1=Remme |first1=Uwe|last2=Bermudez Menendez|first2=Jose Miguel|display-authors=etal}}
*{{cite report |ref= {{harvid|IPCC AR6 WG3 Ch6|2022}}
 |chapter-url= https://ipcc.ch/report/ar6/wg3/downloads/report/IPCC_AR6_WGIII_Chapter06.pdf
 |chapter= Chapter 6: Energy Systems
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 |last1= Clarke |first1= L.
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 |last7= Azevedo |first7= I.M.L.
 |last8= Löschel |first8= A.
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 |title=Climate Change 2022: Mitigation of Climate Change. Contribution of Working Group III to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change
 |doi=10.1017/9781009157926.008
}}
* {{cite book | isbn=978-92-9260-180-5|author-last1=IRENA|year=2020 |title=Innovation landscape brief: Innovative operation of pumped hydropower storage|publisher = International Renewable Energy Agency|location=Abu Dhabi}}
* {{Cite book |title=Storing energy: with special reference to renewable energy sources |date=2022 |publisher=Elsevier |isbn=978-0-12-824510-1 |editor-last=Letcher |editor-first=Trevor M. |edition=Second|location=Amsterdam Oxford Cambridge, MA}}
* {{Cite book |last1=Schmidt |first1=Oliver |title=Monetizing energy storage: a toolkit to assess future cost and value |last2=Staffell |first2=Iain |date=2023 |publisher=Oxford University Press |isbn=978-0-19-288817-4 |location=Oxford, United Kingdom|url=https://academic.oup.com/book/55104}}
* {{cite book |title=Large-scale electricity storage |url=https://royalsociety.org/-/media/policy/projects/large-scale-electricity-storage/large-scale-electricity-storage-policy-briefing.pdf |publisher=Royal Society |year=2023 |isbn=978-1-78252-666-7 |last1=Smith |first1=Chris Llewellyn}}
*{{cite report
|last1 = Vandersickel | first1=Annelies
|last2 = Gutierrez |first2 = Andrea
| title = Task 36 Carnot Batteries Final Report
| url=https://iea-es.org/publications/final-report-task-36/
| publisher = Technology Collaboration Programme Energy Storage, International Energy Agency
| date = 2023
| accessdate = 29 October 2024
}}

==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)] 

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

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