Energy storage#Storage for electricity

In the 20th century grid, electrical power was largely generated by burning fossil fuel. When less power was required, less fuel was burned. Hydropower, a mechanical energy storage method, is the most widely adopted mechanical energy storage, and has been in use for centuries. Large hydropower dams have been energy storage sites for more than one hundred years.{{Cite journal|last1=Hittinger|first1=Eric|last2=Ciez|first2=Rebecca E.|date=2020-10-17|title=Modeling Costs and Benefits of Energy Storage Systems|journal=Annual Review of Environment and Resources|language=en|volume=45|issue=1|pages=445–469|doi=10.1146/annurev-environ-012320-082101|doi-access=free|issn=1543-5938}} Concerns with air pollution, energy imports, and global warming have spawned the growth of renewable energy such as solar and wind power.{{cite book | chapter=Optimizing microgrid using demand response and electric vehicles connection to microgrid | date=2019-07-30 | doi=10.1109/SGC.2017.8308873 | isbn=978-1-5386-4279-5 | title=2017 Smart Grid Conference (SGC) | last1=Liasi | first1=Sahand Ghaseminejad | last2=Bathaee | first2=Seyed Mohammad Taghi | pages=1–7 | s2cid=3817521 }} Wind power is uncontrolled and may be generating at a time when no additional power is needed. Solar power varies with cloud cover and at best is only available during daylight hours, while demand often peaks after sunset (see duck curve). Interest in storing power from these intermittent sources grows as the renewable energy industry begins to generate a larger fraction of overall energy consumption.{{cite journal | title=Power to Gas projects review: Lab, pilot and demo plants for storing renewable energy and CO2 | journal=Renewable and Sustainable Energy Reviews | volume=69 | date=2017-03-01 | issn=1364-0321 | doi=10.1016/j.rser.2016.11.130 | pages=292–312 | last1=Bailera | first1=Manuel | last2=Lisbona | first2=Pilar | last3=Romeo | first3=Luis M. | last4=Espatolero | first4=Sergio | url=http://zaguan.unizar.es/record/75731 | archive-url=https://web.archive.org/web/20200310124229/http://zaguan.unizar.es/record/75731 | url-status=dead | archive-date=2020-03-10 }} In 2023 BloombergNEF forecast total energy storage deployments to grow at a compound annual growth rate of 27 percent through 2030.{{cite web |title=BloombergNEF |url=https://www.energy-storage.news/worlds-energy-storage-capacity-forecast-to-exceed-a-terawatt-hour-by-2030/#:~:text=In%20BloombergNEF's%202H%202023%20Energy,reach%201.6TWh%20by%202030. |website=Energy Storage News| date=October 18, 2023 }}

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

File:Available storage technologies, their capacity and discharge time.jpg

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

{{Div col}}

• Fossil fuel storage
• Mechanical
• Spring
• Compressed-air energy storage (CAES)
• Fireless locomotive
• Flywheel energy storage
• Solid mass gravitational

• Hydraulic accumulator
• Pumped-storage hydroelectricity (a.k.a. pumped hydroelectric storage, PHS, or pumped storage hydropower, PSH)
• Thermal expansion
• Electrical, electromagnetic
• Capacitor
• Supercapacitor
• Superconducting magnetic energy storage (SMES, also superconducting storage coil)
• Biological
• Glycogen
• Starch
• Electrochemical (battery energy storage system, BESS)
• Flow battery
• Rechargeable battery
• UltraBattery
• Thermal
• Brick storage heater
• Cryogenic energy storage, liquid-air energy storage (LAES)
• Liquid nitrogen engine
• Eutectic system
• Ice storage air conditioning
• Molten salt storage
• Phase-change material
• Seasonal thermal energy storage
• Solar pond
• Steam accumulator
• Thermal energy storage (general)
• Chemical
• Biofuels
• Hydrated salts
• Hydrogen peroxide
• Power-to-gas (methane, hydrogen storage, oxyhydrogen)

{{Div col end}}

File:20240706 Energy storage - renewable energy - battery - 100 ms.gif

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

{{Main|Hydroelectricity}}

Hydroelectric dams 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.{{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}}

File:Adam Beck Complex.jpg at Niagara Falls, Canada, which includes a large 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 MW. PSH energy efficiency varies in practice between 70% and 80%, with claims of up to 87%.

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 plants that use natural stream-flow.

Image:Compressed Air Loco.jpg 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.{{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}} 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.{{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 }}

Compression of air creates heat; the air is warmer after compression. 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. A CAES system can deal with the heat in three ways. Air storage can be adiabatic, diabatic, or isothermal. Another approach uses compressed air to power vehicles.

File:Example of cylindrical flywheel rotor assembly.png

File:Flybrid Systems Kinetic Energy Recovery System.jpg flywheel. Built for use on 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.{{Cite web |title=Wayback Machine |url=http://www.xtrac.com/pdfs/Torotrak_Xtrac_CVT.pdf |archive-url=http://web.archive.org/web/20120311124324/http://www.xtrac.com/pdfs/Torotrak_Xtrac_CVT.pdf |archive-date=2012-03-11 |access-date=2025-04-24 |website=www.xtrac.com}}

FES systems have rotors made of high strength carbon-fiber composites, suspended by magnetic bearings and spinning at speeds from 20,000 to over 50,000 revolutions per minute (rpm) in a vacuum enclosure.{{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}} Such flywheels can reach maximum speed ("charge") in a matter of minutes. The flywheel system is connected to a combination electric motor/generator.

FES systems have relatively long lifetimes (lasting decades with little or no maintenance; full-cycle lifetimes quoted for flywheels range from in excess of 10⁵, up to 10⁷, cycles of use),{{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 }} high specific energy (100–130 W·h/kg, or 360–500 kJ/kg){{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 }} and power density.

{{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.{{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}}

Efficiencies can be as high as 85% recovery of stored energy.

This can be achieved by siting the masses inside old vertical mine shafts or in specially constructed towers where the heavy weights are winched up to store energy and allowed a controlled descent to release it. At 2020 a prototype vertical store is being built in Edinburgh, Scotland{{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}}

Potential energy storage or gravity energy storage was under active development in 2013 in association with the California Independent System Operator. It examined the movement of earth-filled hopper rail cars driven by electric locomotives from lower to higher elevations.

Other proposed methods include:-

• using rails,{{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 }} cranes,{{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 }} or elevators{{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}} to move weights up and down;
• using high-altitude solar-powered balloon platforms supporting winches to raise and lower solid masses slung underneath them,{{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 }}
• 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,{{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 |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 }}

File:Fernwärmespeicher Theiss.jpg in Lower Austria with a thermal capacity of 2 GWh]]

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

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

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

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

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

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

{{Main|Cryogenic energy storage}}

Air can be liquefied by cooling using electricity and stored as a cryogen with existing technologies. The liquid air can then be expanded through a turbine and the energy recovered as electricity. The system was demonstrated at a pilot plant in the UK in 2012.

In 2019, Highview announced plans to build a 50 MW in the North of England and northern Vermont, with the proposed facility able to store five to eight hours of energy, for a 250–400 MWh storage capacity.

{{Main|Carnot battery}}

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

File:Datacenter Backup Batteries.jpg in a data center{{Cite web |title=Microsoft datacenter batteries to support growth of renewables on the power grid |url=https://news.microsoft.com/source/features/sustainability/ireland-wind-farm-datacenter-ups/ |access-date=2025-04-24 |website=Source |language=en-US}}{{Cite web |title=What is an Uninterruptible Power Supply - Definition from TechTarget.com |url=https://www.techtarget.com/searchdatacenter/definition/uninterruptible-power-supply |access-date=2025-04-24 |website=Search Data Center |language=en}}]]

{{Main|Rechargeable battery|Battery storage power station}}

A rechargeable battery comprises one or more electrochemical cells. It is known as a 'secondary cell' because its electrochemical reactions are electrically reversible. Rechargeable batteries come in many shapes and sizes, ranging from button cells to megawatt grid systems.

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

Common rechargeable battery chemistries include:

• Lead–acid battery: Lead acid batteries hold the largest market share of electric storage products. A single cell produces about 2V when charged. In the charged state the metallic lead negative electrode and the lead sulfate positive electrode are immersed in a dilute sulfuric acid (H₂SO₄) electrolyte. In the discharge process electrons are pushed out of the cell as lead sulfate is formed at the negative electrode while the electrolyte is reduced to water.
• Lead–acid battery technology has been developed extensively. Upkeep requires minimal labor and its cost is low. The battery's available energy capacity is subject to a quick discharge resulting in a low life span and low energy density.{{cite journal |last1=Yao |first1=L. |last2=Yang |first2=B. |last3=Cui |first3=H. |last4=Zhuang |first4=J. |last5=Ye |first5=J. |last6=Xue |first6=J. |title=Challenges and progresses of energy storage technology and its application in power systems |journal=Journal of Modern Power Systems and Clean Energy |volume=4 |issue=4 |date=2016 |pages=520–521 |doi=10.1007/s40565-016-0248-x |doi-access=free }}
• Nickel–cadmium battery (NiCd): Uses nickel oxide hydroxide and metallic cadmium as electrodes. Cadmium is a toxic element, and was banned for most uses by the European Union in 2004. Nickel–cadmium batteries have been almost completely replaced by nickel–metal hydride (NiMH) batteries.
• Nickel–metal hydride battery (NiMH): First commercial types were available in 1989. These are now a common consumer and industrial type. The battery has a hydrogen-absorbing alloy for the negative electrode instead of cadmium.
• Lithium-ion battery: The choice in many consumer electronics and have one of the best energy-to-mass ratios and a very slow self-discharge when not in use.
• Lithium-ion polymer battery: These batteries are light in weight and can be made in any shape desired.
• Aluminium-sulfur battery with rock salt crystals as electrolyte: aluminium and sulfur are Earth-abundant materials and are much more cheaper than traditional Lithium.{{cite web|url=https://news.mit.edu/2022/aluminum-sulfur-battery-0824|title=A new concept for low-cost batteries|date=August 24, 2022|author=David L. Chandler}}

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

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

File:Expo 2010 Electric Bus.jpg powered by supercapacitors, at a quick-charge station-bus stop, in service during Expo 2010 Shanghai China. Charging rails can be seen suspended over the bus.]]

{{main|Supercapacitor}}

Supercapacitors, also called electric double-layer capacitors (EDLC) or ultracapacitors, are a family of electrochemical capacitors that do not have conventional solid dielectrics. Capacitance is determined by two storage principles, double-layer capacitance and pseudocapacitance.

Supercapacitors bridge the gap between conventional capacitors and rechargeable batteries. They store the most energy per unit volume or mass (energy density) among capacitors. They support up to 10,000 farads/1.2 Volt, up to 10,000 times that of electrolytic capacitors, but deliver or accept less than half as much power per unit time (power density).

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

Supercapacitors have many applications, including:

• Low supply current for memory backup in static random-access memory (SRAM)
• Power for cars, buses, trains, cranes and elevators, including energy recovery from braking, short-term energy storage and burst-mode power delivery

File:World’s 1st Low-Emission Hybrid Battery Storage, Gas Turbine Peaker System.jpg and Rancho Cucamonga. The 10-megawatt battery storage system, combined with the gas turbine, allows the peaker plant to more quickly respond to changing energy needs, thus increasing the reliability of the electrical grid.]]

{{Main|Power-to-gas}}

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

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

{{Main|Hydrogen storage}}

{{See also|Combined cycle hydrogen power plant|Hydrogen fuel cell power plant}}

The element hydrogen can be a form of stored energy. Hydrogen can produce electricity via a hydrogen fuel cell.

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

Energy losses involved in the hydrogen storage cycle come from the electrolysis of water, liquification or compression of the hydrogen and conversion to electricity.

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

Underground hydrogen storage is the practice of hydrogen storage in caverns, salt domes and depleted oil and gas fields. Large quantities of gaseous hydrogen have been stored in caverns by Imperial Chemical Industries for many years without any difficulties. The European Hyunder project indicated in 2013 that storage of wind and solar energy using underground hydrogen would require 85 caverns.

Powerpaste is a magnesium and hydrogen -based fluid gel that releases hydrogen when reacting with water. It was invented, patented and is being developed by the Fraunhofer Institute for Manufacturing Technology and Advanced Materials (IFAM) of the Fraunhofer-Gesellschaft. Powerpaste is made by combining magnesium powder with hydrogen to form magnesium hydride in a process conducted at 350 °C and five to six times atmospheric pressure. An ester and a metal salt are then added to make the finished product. Fraunhofer states that they are building a production plant slated to start production in 2021, which will produce 4 tons of Powerpaste annually.{{cite press release

|author =

|title = Hydrogen-powered drives for e-scooters

|url = https://www.fraunhofer.de/en/press/research-news/2021/february-2021/hydrogen-powered-drives-for-e-scooters.html

|publisher = Fraunhofer Society

|date = 2021-02-01

|access-date = 2021-02-22

|archive-date = February 3, 2021

|archive-url = https://web.archive.org/web/20210203211045/https://www.fraunhofer.de/en/press/research-news/2021/february-2021/hydrogen-powered-drives-for-e-scooters.html

|url-status = live

}} Fraunhofer has patented their invention in the United States and EU.{{cite tech report

|first= Lars

|last= Röntzsch

|first2= Marcus

|last2= Vogt

|title= White paper – PowerPaste for off-grid power supply

|number=

|institution= Fraunhofer Society

|url= https://www.researchgate.net/publication/331929208_PowerPaste_for_off-grid_power_supply

|date= February 2019

|access-date= February 22, 2021

|archive-date= February 7, 2021

|archive-url= https://web.archive.org/web/20210207214925/https://www.researchgate.net/publication/331929208_PowerPaste_for_off-grid_power_supply

|url-status= live

}} Fraunhofer claims that Powerpaste is able to store hydrogen energy at 10 times the energy density of a lithium battery of a similar dimension and is safe and convenient for automotive situations.

{{Main|Substitute natural gas}}

Methane is the simplest hydrocarbon with the molecular formula CH₄. Methane is more easily stored and transported than hydrogen. Storage and combustion infrastructure (pipelines, gasometers, power plants) are mature.

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

Methane combustion produces carbon dioxide (CO₂) and water. The carbon dioxide can be recycled to boost the Sabatier process and water can be recycled for further electrolysis. Methane production, storage and combustion recycles the reaction products.

The CO₂ has economic value as a component of an energy storage vector, not a cost as in carbon capture and storage.

Power-to-liquid is similar to power to gas except that the hydrogen is converted into liquids such as methanol or ammonia. These are easier to handle than gases, and require fewer safety precautions than hydrogen. They can be used for transportation, including aircraft, but also for industrial purposes or in the power sector.{{cite journal | last1 = Varone | first1 = Alberto | last2 = Ferrari | first2 = Michele | year = 2015 | title = Power to liquid and power to gas: An option for the German Energiewende | url = http://publications.iass-potsdam.de/pubman/item/escidoc:896902 | journal = Renewable and Sustainable Energy Reviews | volume = 45 | pages = 207–218 | doi = 10.1016/j.rser.2015.01.049 | bibcode = 2015RSERv..45..207V }}{{Dead link|date=March 2024 |bot=InternetArchiveBot |fix-attempted=yes }}

{{Main|Biofuel}}

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

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

Boron, silicon, and zinc have been proposed as energy storage solutions.

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

{{Main|capacitor}}

File:Mylar-film oil-filled low-inductance capacitor 6.5 MFD @ 5000 VDC.jpg.]]

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

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

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

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

{{Main|Superconducting magnetic energy storage}}

Superconducting magnetic energy storage (SMES) systems store energy in a magnetic field created by the flow of direct current in a superconducting coil that has been cooled to a temperature below its superconducting critical temperature. A typical SMES system includes a superconducting coil, power conditioning system and refrigerator. Once the superconducting coil is charged, the current does not decay and the magnetic energy can be stored indefinitely.

The stored energy can be released to the network by discharging the coil. The associated inverter/rectifier accounts for about 2–3% energy loss in each direction. SMES loses the least amount of electricity in the energy storage process compared to other methods of storing energy. SMES systems offer round-trip efficiency greater than 95%.

Due to the energy requirements of refrigeration and the cost of superconducting wire, SMES is used for short duration storage such as improving power quality. It also has applications in grid balancing.

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

{{Main|Home energy storage}}

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

Tesla Motors produces two models of the Tesla Powerwall. One is a 10 kWh weekly cycle version for backup applications and the other is a 7 kWh version for daily cycle applications.{{cite news |first=Matthew |last=Debord |url=http://www.businessinsider.com/here-comes-teslas-missing-piece-battery-announcement-2015-4 |title=Elon Musk's big announcement: it's called 'Tesla Energy' |work=Business Insider |date=May 1, 2015 |access-date=June 11, 2015 |archive-date=May 5, 2015 |archive-url=https://web.archive.org/web/20150505021205/http://www.businessinsider.com/here-comes-teslas-missing-piece-battery-announcement-2015-4 |url-status=live }} In 2016, a limited version of the Tesla Powerpack 2 cost $398(US)/kWh to store electricity worth 12.5 cents/kWh (US average grid price) making a positive return on investment doubtful unless electricity prices are higher than 30 cents/kWh.{{cite news|url= https://electrek.co/2016/11/14/tesla-powerpack-2-price/|title= Tesla slashes price of the Powerpack system by another 10% with new generation|date= May 15, 2017|work= Electrek|access-date= November 14, 2016|archive-date= November 14, 2016|archive-url= https://web.archive.org/web/20161114235216/https://electrek.co/2016/11/14/tesla-powerpack-2-price/|url-status= live|last1= Lambert|first1= Fred}}

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

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

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

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

{{Main|Grid energy storage|Battery storage power station}}

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

File:Andasol 3.jpg in Spain is a parabolic trough solar thermal power plant that stores energy in tanks of molten salt so that it can continue generating electricity when the sun is not shining.]]

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

Many renewable energy sources (notably solar and wind) produce variable power. Storage systems can level out the imbalances between supply and demand that this causes. Electricity must be used as it is generated or converted immediately into storable forms.

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

Some forms of storage that produce electricity include pumped-storage hydroelectric dams, rechargeable batteries, thermal storage including molten salts which can efficiently store and release very large quantities of heat energy, and compressed air energy storage, flywheels, cryogenic systems and superconducting magnetic coils.

Surplus power can also be converted into methane (Sabatier process) with stockage in the natural gas network.

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

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

Since the early 21st century batteries have been applied to utility scale load-leveling and frequency regulation capabilities.

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

{{Main|Ice storage air conditioning}}

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

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

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

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

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

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

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

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

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

Capacitors are widely used in electronic circuits for blocking direct current while allowing alternating current to pass. In analog filter networks, they smooth the output of power supplies. In resonant circuits they tune radios to particular frequencies. In electric power transmission systems they stabilize voltage and power flow.

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

The United States Department of Energy International Energy Storage Database (IESDB), is a free-access database of energy storage projects and policies funded by the United States Department of Energy Office of Electricity and Sandia National Labs.

Storage capacity is the amount of energy extracted from an energy storage device or system; usually measured in joules or kilowatt-hours and their multiples, it may be given in number of hours of electricity production at power plant nameplate capacity; when storage is of primary type (i.e., thermal or pumped-water), output is sourced only with the power plant embedded storage system.{{cite web

| url = http://www.nrel.gov/csp/troughnet/pdfs/nava_andasol_storage_system.pdf

| title = Thermal Storage Concept for a 50 MW Trough Power Plant in Spain

| last1 = Herrman

| first1 = Ulf

| last2 = Nava

| first2 = Paul

| date = February 13, 2016

| website = nrel.gov

| publisher = NREL

| archive-url = https://web.archive.org/web/20160402190815/http://www.nrel.gov/csp/troughnet/pdfs/nava_andasol_storage_system.pdf

| archive-date = April 2, 2016

| url-status = dead

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|url = http://www.iea-eces.org/files/a6.5.2_2014-11-06_defintion_of_storage_capacity_fraunhofer_umsicht.pdf

|title = Electric Storage Devices – "Definition" of Storage Capacity, Power, Efficiency

|last = Doetsch

|first = Christian

|date = November 6, 2014

|website = iea-eces.org

|archive-url = https://web.archive.org/web/20170213095950/http://www.iea-eces.org/files/a6.5.2_2014-11-06_defintion_of_storage_capacity_fraunhofer_umsicht.pdf

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

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

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

Therefore, traditional techniques based on deterministic Discounted Cash Flow (DCF) for the investment appraisal are not fully adequate to evaluate these risks and uncertainties and the investor's flexibility to deal with them. Hence, the literature recommends to assess the value of risks and uncertainties through the Real Option Analysis (ROA), which is a valuable method in uncertain contexts.

The economic valuation of large-scale applications (including pumped hydro storage and compressed air) considers benefits including: curtailment avoidance, grid congestion avoidance, price arbitrage and carbon-free energy delivery. In one technical assessment by the Carnegie Mellon Electricity Industry Centre, economic goals could be met using batteries if their capital cost was $30 to $50 per kilowatt-hour.

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

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

In 2013, the German government allocated €200M (approximately US$270M) for research, and another €50M to subsidize battery storage in residential rooftop solar panels, according to a representative of the German Energy Storage Association.

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

From 2023 onwards, a new project by the German Research Foundation focuses on molecular photoswitches to store solar thermal energy. The spokesperson of these so-called molecular solar thermal (MOST) systems is Prof. Dr. Hermann A. Wegner.

In 2014, research and test centers opened to evaluate energy storage technologies. Among them was the Advanced Systems Test Laboratory at the University of Wisconsin at Madison in Wisconsin State, which partnered with battery manufacturer Johnson Controls. The laboratory was created as part of the university's newly opened Wisconsin Energy Institute. Their goals include the evaluation of state-of-the-art and next generation electric vehicle batteries, including their use as grid supplements.

The State of New York unveiled its New York Battery and Energy Storage Technology (NY-BEST) Test and Commercialization Center at Eastman Business Park in Rochester, New York, at a cost of $23 million for its almost 1,700 m² laboratory. The center includes the Center for Future Energy Systems, a collaboration between Cornell University of Ithaca, New York and the Rensselaer Polytechnic Institute in Troy, New York. NY-BEST tests, validates and independently certifies diverse forms of energy storage intended for commercial use.

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

In grid models with high VRE share, the excessive cost of storage tends to dominate the costs of the whole grid – for example, in California alone 80% share of VRE would require 9.6 TWh of storage but 100% would require 36.3 TWh. According to another study, supplying 80% of US demand from VRE would require a smart grid covering the whole country or battery storage capable to supply the whole system for 12 hours, both at cost estimated at $2.5 trillion.

In the United Kingdom, some 14 industry and government agencies allied with seven British universities in May 2014 to create the SUPERGEN Energy Storage Hub in order to assist in the coordination of energy storage technology research and development.

{{Portal|Energy|Renewable energy}}

{{Div col}}

• Efficient energy use
• Energy storage as a service (ESaaS)
• Grid energy storage
• Hybrid power
• List of energy storage power plants
• Outline of energy
• Power-to-X
• Power transmission
• Renewable energy
• Thermal battery
• Variable renewable energy

{{div col end}}

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Gies, Erica. [https://www.nytimes.com/2012/10/02/business/energy-environment/a-storage-solution-is-in-the-air.html?pagewanted=all Global Clean Energy: A Storage Solution Is in the Air] {{Webarchive|url=https://web.archive.org/web/20190508092207/https://www.nytimes.com/2012/10/02/business/energy-environment/a-storage-solution-is-in-the-air.html?pagewanted=all |date=May 8, 2019 }}, International Herald Tribune online website, October 1, 2012, and in print on October 2, 2012, in The International Herald Tribune. Retrieved from NYTimes.com website, March 19, 2013.

Galbraith, Kate. [https://www.nytimes.com/2013/10/03/business/energy-environment/Filling-the-Gaps-in-the-Flow-of-Renewable-Energy.html Filling the Gaps in the Flow of Renewable Energy] {{Webarchive|url=https://web.archive.org/web/20170410212050/http://www.nytimes.com/2013/10/03/business/energy-environment/Filling-the-Gaps-in-the-Flow-of-Renewable-Energy.html |date=April 10, 2017 }}, The New York Times, October 22, 2013.

Wald, Matthew L. [https://www.nytimes.com/2014/04/22/business/energy-environment/ice-or-molten-salt-not-batteries-to-store-energy.html Ice or Molten Salt, Not Batteries, to Store Energy] {{Webarchive|url=https://web.archive.org/web/20201112021117/https://www.nytimes.com/2014/04/22/business/energy-environment/ice-or-molten-salt-not-batteries-to-store-energy.html |date=November 12, 2020 }}, The New York Times website, April 21, 2014, and in print on April 22, 2014, p. F7 of the New York edition. Retrieved May 29, 2014.

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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=April 30, 2014 }}, ScientificAmerican.com website, March 25, 2014. Retrieved March 28, 2014.

Schmid, Jürgen. [http://www.uni-kassel.de/upress/online/frei/978-3-89958-798-2.volltext.frei.pdf Renewable Energies and Energy Efficiency: Bioenergy and renewable power methane in integrated 100% renewable energy system] {{Webarchive|url=https://web.archive.org/web/20111202051707/http://www.uni-kassel.de/upress/online/frei/978-3-89958-798-2.volltext.frei.pdf |date=December 2, 2011 }} (thesis), Universität Kassel/Kassel University Press, September 23, 2009.

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

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

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

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

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

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Journals and papers

• Chen, Haisheng; Thang Ngoc Cong; Wei Yang; Chunqing Tan; Yongliang Li; Yulong Ding. [http://ac.els-cdn.com/S100200710800381X/1-s2.0-S100200710800381X-main.pdf?_tid=a5208c44-dab8-11e3-8ec6-00000aacb362&acdnat=1399997413_3fc617bbbeca49f3fd02e976897508df Progress in electrical energy storage system: A critical review], Progress in Natural Science, accepted July 2, 2008, published in Vol. 19, 2009, pp. 291–312, doi: 10.1016/j.pnsc.2008.07.014. Sourced from the National Natural Science Foundation of China and the Chinese Academy of Sciences. Published by Elsevier and Science in China Press. Synopsis: a review of electrical energy storage technologies for stationary applications. Retrieved from ac.els-cdn.com on May 13, 2014. (PDF)
• Corum, Lyn. [https://web.archive.org/web/20130531074306/http://www.distributedenergy.com/DE/Articles/9027.aspx The New Core Technology: Energy storage is part of the smart grid evolution], The Journal of Energy Efficiency and Reliability, December 31, 2009. Discusses: Anaheim Public Utilities Department, lithium ion energy storage, iCel Systems, Beacon Power, Electric Power Research Institute (EPRI), ICEL, Self Generation Incentive Program, ICE Energy, vanadium redox flow, lithium Ion, regenerative fuel cell, ZBB, VRB, lead acid, CAES, and Thermal Energy Storage. (PDF)
• {{cite journal | last1 = de Oliveira e Silva | first1 = G. | last2 = Hendrick | first2 = P. | year = 2016 | title = Lead-acid batteries coupled with photovoltaics for increased electricity self-sufficiency in households | journal = Applied Energy | volume = 178 | pages = 856–867 | doi = 10.1016/j.apenergy.2016.06.003 | bibcode = 2016ApEn..178..856D }}
• {{cite journal|last1=Sahoo|first1=Subrat|last2=Timmann|first2=Pascal|title=Energy Storage Technologies for Modern Power Systems: A Detailed Analysis of Functionalities, Potentials, and Impacts|journal=IEEE Access|volume=11|date=2023|issn=2169-3536|doi=10.1109/ACCESS.2023.3274504|doi-access=free|pages=49689–49729|bibcode=2023IEEEA..1149689S |url=https://ieeexplore.ieee.org/ielx7/6287639/10005208/10121760.pdf|access-date=2024-12-14}}
• Whittingham, M. Stanley. [https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=06184265 History, Evolution, and Future Status of Energy Storage], Proceedings of the IEEE, manuscript accepted February 20, 2012, date of publication April 16, 2012; date of current version May 10, 2012, published in Proceedings of the IEEE, Vol. 100, May 13, 2012, 0018–9219, pp. 1518–1534, doi: 10.1109/JPROC.2012.219017. Retrieved from ieeexplore.ieee.org May 13, 2014. Synopsis: A discussion of the important aspects of energy storage including emerging battery technologies and the importance of storage systems in key application areas, including electronic devices, transportation, and the utility grid. (PDF)

Books

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

{{Commons category|Energy storage}}

{{Wikiversity}}

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

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