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=== Mechanical === [[File:20240706 Energy storage - renewable energy - battery - 100 ms.gif |thumb |Energy from sunlight or other renewable energy is converted to potential energy for storage in devices such as electric batteries. The stored potential energy is later converted to electricity that is added to the power grid, even when the original energy source is not available. In pumped hydro systems, energy from the source is used to lift water upward against the force of gravity, giving it potential energy that is later converted to electricity provided to the power grid.]] Energy can be stored in water pumped to a higher elevation using [[pumped storage]] methods or by moving solid matter to higher locations ([[gravity battery|gravity batteries]]). Other commercial mechanical methods include [[Compressed-air energy storage|compressing air]] and [[Flywheel energy storage|flywheels]] that convert electric energy into internal energy or kinetic energy and then back again when electrical demand peaks. ==== Hydroelectricity ==== {{Main|Hydroelectricity}} [[Hydroelectric dam]]s with reservoirs can be operated to provide electricity at times of peak demand. Water is stored in the reservoir during periods of low demand and released when demand is high. The net effect is similar to pumped storage, but without the pumping loss. While a hydroelectric dam does not directly store energy from other generating units, it behaves equivalently by lowering output in periods of excess electricity from other sources. In this mode, dams are one of the most efficient forms of energy storage, because only the timing of its generation changes. Hydroelectric turbines have a start-up time on the order of a few minutes.<ref name="Huggins2010">{{cite book|first=Robert A|last=Huggins|title=Energy Storage|url={{google books |plainurl=y |id=Nn5y9gQeIlwC|page=60}}|date=September 1, 2010|publisher=Springer|isbn=978-1-4419-1023-3|pages=60}}</ref> ==== Pumped hydro ==== [[File:Adam Beck Complex.jpg|thumb|The [[Sir Adam Beck Hydroelectric Generating Stations|Sir Adam Beck Generating Complex]] at [[Niagara Falls, Ontario|Niagara Falls, Canada]], which includes a large [[Pumped-storage hydroelectricity|pumped storage hydroelectricity reservoir]] to provide an extra 174 MW of electricity during periods of peak demand]] {{Main|Pumped-storage hydroelectricity}} Worldwide, [[pumped-storage hydroelectricity]] (PSH) is the largest-capacity form of active [[grid energy storage]] available, and, as of March 2012, the [[Electric Power Research Institute]] (EPRI) reports that PSH accounts for more than 99% of bulk storage capacity worldwide, representing around 127,000 [[Megawatt|MW]].<ref name="EconomistPSH" /> PSH [[Energy conversion efficiency|energy efficiency]] varies in practice between 70% and 80%,<ref name="EconomistPSH" /><ref name="thier" /><ref name="Levine" /><ref name="yang" /> with claims of up to 87%.<ref name="heco" /> At times of low electrical demand, excess generation capacity is used to pump water from a lower source into a higher reservoir. When demand grows, water is released back into a lower reservoir (or waterway or body of water) through a [[turbine]], generating electricity. Reversible turbine-generator assemblies act as both a pump and turbine (usually a [[Francis turbine]] design). Nearly all facilities use the height difference between two water bodies. Pure pumped-storage plants shift the water between reservoirs, while the "pump-back" approach is a combination of pumped storage and conventional [[hydroelectric plant]]s that use natural stream-flow. ==== Compressed air ==== [[Image:Compressed Air Loco.jpg|thumb|A [[Fireless locomotive|compressed air locomotive]] used inside a mine between 1928 and 1961]] {{Main|Compressed air energy storage|Salt dome}} Compressed-air energy storage (CAES) uses surplus energy to compress air for subsequent electricity generation.<ref name="NYT-2010.07.28">{{Cite news |last=Wald |first=Matthew L. |date=2010-07-27 |title=Wind Drives Growing Use of Batteries |url=https://www.nytimes.com/2010/07/28/business/energy-environment/28storage.html |access-date=2025-04-24 |work=The New York Times |language=en-US |issn=0362-4331}}</ref> Small-scale systems have long been used in such applications as propulsion of mine locomotives. The compressed air is stored in an [[underground reservoir]], such as a [[salt dome]]. Compressed-air energy storage (CAES) plants can bridge the gap between production volatility and load. CAES storage addresses the energy needs of consumers by effectively providing readily available energy to meet demand. Renewable energy sources like wind and solar energy vary. So at times when they provide little power, they need to be supplemented with other forms of energy to meet energy demand. Compressed-air energy storage plants can take in the surplus energy output of renewable energy sources during times of energy over-production. This stored energy can be used at a later time when demand for electricity increases or energy resource availability decreases.<ref>{{cite journal |last1=Keles |first1=Dogan |last2=Hartel |first2=Rupert |last3=Möst |first3=Dominik |last4=Fichtner |first4=Wolf |title=Compressed-air energy storage power plant investments under uncertain electricity prices: an evaluation of compressed-air energy storage plants in liberalized energy markets |journal=The Journal of Energy Markets |date=Spring 2012 |volume=5 |issue=1 |page=54 |id={{ProQuest|1037988494}} |doi=10.21314/JEM.2012.070 }}</ref> [[Gas compressor|Compression]] of [[air]] creates heat; the air is warmer after compression. [[Thermal expansion|Expansion]] requires heat. If no extra heat is added, the air will be much colder after expansion. If the heat generated during compression can be stored and used during expansion, efficiency improves considerably.<ref name="NYTimes-2012.10.01" /> A CAES system can deal with the heat in three ways. Air storage can be [[adiabatic]], [[Adiabatic|diabatic]], or [[isothermal]]. Another approach uses compressed air to power vehicles.<ref name="Auto.com-2004.03.18" /><ref name="Freep-2004.03.18" /> ==== Flywheel ==== [[File:Example of cylindrical flywheel rotor assembly.png|thumb|right|The main components of a typical flywheel]] [[File:Flybrid Systems Kinetic Energy Recovery System.jpg|thumb|right|A Flybrid [[Kinetic Energy Recovery System]] [[Flywheel energy storage|flywheel]]. Built for use on [[Formula One|Formula 1 racing cars]], it is employed to recover and reuse kinetic energy captured during braking.]] {{Main|Flywheel energy storage|Flywheel storage power system}} Flywheel energy storage (FES) works by accelerating a rotor (a [[flywheel]]) to a very high speed, holding energy as [[rotational energy]]. When energy is added the rotational speed of the flywheel increases, and when energy is extracted, the speed declines, due to [[conservation of energy]]. Most FES systems use electricity to accelerate and decelerate the flywheel, but devices that directly use mechanical energy are under consideration.<ref name="Torotrak">{{Cite web |title=Wayback Machine |url=http://www.xtrac.com/pdfs/Torotrak_Xtrac_CVT.pdf |archive-url=https://web.archive.org/web/20120311124324/http://www.xtrac.com/pdfs/Torotrak_Xtrac_CVT.pdf |archive-date=March 11, 2012 |access-date=2025-04-24 |website=www.xtrac.com |url-status=dead }}</ref> FES systems have rotors made of high strength [[carbon-fiber]] composites, suspended by [[magnetic bearing]]s and spinning at speeds from 20,000 to over 50,000 revolutions per minute (rpm) in a vacuum enclosure.<ref name="ScienceNews">{{Cite journal| last1 = Castelvecchi| first1 = Davide| title = Spinning into control: High-tech reincarnations of an ancient way of storing energy| doi = 10.1002/scin.2007.5591712010| journal = Science News| volume = 171| issue = 20| pages = 312–313| date = May 19, 2007| url = http://sciencewriter.org/flywheels-spinning-into-control/| access-date = May 8, 2014| archive-url = https://web.archive.org/web/20140606223717/http://sciencewriter.org/flywheels-spinning-into-control/| archive-date = June 6, 2014| url-status = dead}}</ref> Such flywheels can reach maximum speed ("charge") in a matter of minutes. The flywheel system is connected to a combination [[electric motor]]/[[electric generator|generator]]. FES systems have relatively long lifetimes (lasting decades with little or no maintenance;<ref name="ScienceNews" /> full-cycle lifetimes quoted for flywheels range from in excess of 10<sup>5</sup>, up to 10<sup>7</sup>, cycles of use),<ref name="Investire">{{Cite web |url=http://www.itpower.co.uk/investire/pdfs/flywheelrep.pdf |title=Storage Technology Report, ST6 Flywheel |access-date=May 8, 2014 |archive-url=https://web.archive.org/web/20130114062530/http://www.itpower.co.uk/investire/pdfs/flywheelrep.pdf |archive-date=January 14, 2013 |url-status=dead }}</ref> high [[specific energy]] (100–130 W·h/kg, or 360–500 kJ/kg)<ref name="Investire" /><ref name="pddnet">{{cite web |title=Next-gen of Flywheel Energy Storage |url=http://www.pddnet.com/article-next-gen-of-flywheel-energy-storage/ |publisher=Product Design & Development |access-date=May 21, 2009 |url-status=dead |archive-url=https://web.archive.org/web/20100710052927/http://www.pddnet.com/article-next-gen-of-flywheel-energy-storage/ |archive-date=July 10, 2010 }}</ref> and [[power density]]. ==== Solid mass gravitational {{anchor|Gravitational_potential_energy_storage}} ==== {{Main|Gravity battery}} Changing the altitude of solid masses can store or release energy via an elevating system driven by an electric motor/generator. Studies suggest energy can begin to be released with as little as 1 second warning, making the method a useful supplemental feed into an electricity grid to balance load surges.<ref>{{cite news|last1=Fraser|first1=Douglas|title=Edinburgh company generates electricity from gravity|publisher=BBC News|date=October 22, 2019|url=https://www.bbc.com/news/uk-scotland-scotland-business-50146801|access-date=14 January 2020|archive-date=July 28, 2020|archive-url=https://web.archive.org/web/20200728083135/https://www.bbc.com/news/uk-scotland-scotland-business-50146801|url-status=live}}</ref> Efficiencies can be as high as 85% recovery of stored energy.<ref name="quartz" /> This can be achieved by siting the masses inside old vertical mine shafts or in specially constructed towers where the heavy weights are [[winch]]ed up to store energy and allowed a controlled descent to release it. At 2020 a prototype vertical store is being built in Edinburgh, Scotland<ref>{{Cite web|last=Gourley|first=Perry|date=31 August 2020|title=Edinburgh firm behind incredible gravity energy storage project hails milestone|url=https://www.edinburghnews.scotsman.com/business/edinburgh-firm-behind-incredible-gravity-energy-storage-project-hails-milestone-2955863|access-date=2020-09-01|website=The Scotsman|language=en|archive-date=September 2, 2020|archive-url=https://web.archive.org/web/20200902003909/https://www.edinburghnews.scotsman.com/business/edinburgh-firm-behind-incredible-gravity-energy-storage-project-hails-milestone-2955863|url-status=live}}</ref> Potential energy storage or gravity energy storage was under active development in 2013 in association with the [[California Independent System Operator]].<ref name="Economist-2012.03.03" /><ref name="Bloomberg-2012.09.06" /><ref name="Kernan" /> It examined the movement of earth-filled [[Hopper car|hopper rail cars]] driven by [[electric locomotive]]s from lower to higher elevations.<ref name="Scientific American-2014.03.25" /> Other proposed methods include:- * using rails,<ref name="Scientific American-2014.03.25" /><ref>{{cite magazine |author=David Z. Morris |date=May 22, 2016 |title=Energy-Storing Train Gets Nevada Approval |url=http://fortune.com/2016/05/22/energy-storing-train-nevada/ |magazine=Fortune |access-date=August 20, 2018 |archive-date=August 20, 2018 |archive-url=https://web.archive.org/web/20180820140850/http://fortune.com/2016/05/22/energy-storing-train-nevada/ |url-status=live }}</ref> cranes,<ref name="quartz">{{cite news |title=Stacking concrete blocks is a surprisingly efficient way to store energy |author=Akshat Rathi |date=August 18, 2018 |url=https://qz.com/1355672/stacking-concrete-blocks-is-a-surprisingly-efficient-way-to-store-energy/ |work=Quartz |access-date=August 20, 2018 |archive-date=December 3, 2020 |archive-url=https://web.archive.org/web/20201203050354/https://qz.com/1355672/stacking-concrete-blocks-is-a-surprisingly-efficient-way-to-store-energy/ |url-status=live }}</ref> or elevators<ref>{{Cite web |date=2022-05-31 |title=Lift Energy Storage System: Turning skyscrapers into gravity batteries |url=https://newatlas.com/energy/lift-energy-skyscraper-batteries/ |access-date=2022-05-31 |website=New Atlas |language=en-US}}</ref> to move weights up and down; * using high-altitude solar-powered balloon platforms supporting winches to raise and lower solid masses slung underneath them,<ref>{{cite web | title=StratoSolar gravity energy storage | url=http://www.stratosolar.com/gravity-energy-storage.html | access-date=August 20, 2018 | archive-date=August 20, 2018 | archive-url=https://web.archive.org/web/20180820110224/http://www.stratosolar.com/gravity-energy-storage.html | url-status=live }}</ref> * using winches supported by an ocean barge to take advantage of a 4 km (13,000 ft) elevation difference between the sea surface and the seabed,<ref>{{cite web |last1=Choi |first1=Annette |title=Simple Physics Solutions to Storing Renewable Energy |url=https://www.pbs.org/wgbh/nova/article/storing-renewable-energy/ |website=[[Nova (American TV program)|NOVA]] |publisher=[[PBS]] |date=May 24, 2017 |access-date=29 August 2019 |archive-date=August 29, 2019 |archive-url=https://web.archive.org/web/20190829141630/https://www.pbs.org/wgbh/nova/article/storing-renewable-energy/ |url-status=live }}</ref> [[File:Fernwärmespeicher Theiss.jpg|thumb|District heating accumulation tower from Theiss near [[Krems an der Donau]] in [[Lower Austria]] with a thermal capacity of 2 GWh]]
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