Grid energy storage

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

As of 2020^[update], the largest form of grid energy storage is dammed hydroelectricity, with both conventional hydroelectric generation as well as pumped-storage hydroelectricity.<ref>"Grid Energy Storage Technology Cost and Performance Assessment" (PDF). US Department of Energy. Retrieved 23 December 2021.</ref><ref>"Energy Storage Cost and Performance Database". US Department of Energy. Retrieved 23 December 2021.</ref>

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

Two alternatives to grid storage are the use of peaking power plants to fill in supply gaps and demand response to shift load to other times.

Benefits

Any electrical power grid must match electricity production to consumption, both of which vary drastically over time. Any combination of energy storage and demand response has these advantages:

fuel-based power plants (i.e. coal, oil, gas, nuclear) can be more efficiently and easily operated at constant production levels
electricity generated by intermittent sources can be stored and used later, whereas it would otherwise have to be transmitted for sale elsewhere, or shut down
peak generating or transmission capacity can be reduced by the total potential of all storage plus deferrable loads (see demand side management), saving the expense of this capacity
more stable pricing – the cost of the storage or demand management is included in pricing so there is less variation in power rates charged to customers, or alternatively (if rates are kept stable by law) less loss to the utility from expensive on-peak wholesale power rates when peak demand must be met by imported wholesale power
emergency preparedness – vital needs can be met reliably even with no transmission or generation going on while non-essential needs are deferred

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

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

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

The French consulting firm Yole Développement estimates the "stationary storage" market could be a $13.5 billion opportunity by 2023, compared with less than $1 billion in 2015.<ref>Smit, Debra (24 August 2015). "Jay Whitacre and the edible battery". Ozy. Archived from the original on 8 June 2016. Retrieved 15 June 2016.</ref>

Demand side management and grid storage

A sense of units and scale for electrical energy production and consumption

The demand side can also store electricity from the grid, for example charging a battery electric vehicle stores energy for a vehicle and storage heaters, district heating storage or ice storage provide thermal storage for buildings.<ref>"Grid Energy Storage" (PDF). U.S. Department of Energy. December 2013. p. 28. Archived (PDF) from the original on 28 February 2017. Retrieved 13 February 2017.</ref> At present this storage serves only to shift consumption to the off-peak time of day, no electricity is returned to the grid.

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

Regional impacts from the unpredictable operation of wind power has created a new need for interactive demand response, where the utility communicates with the demand. Historically this was only done in cooperation with large industrial consumers, but now may be expanded to entire grids.<ref>Doug Hurley; Paul Peterson; Melissa Whited (May 2013). "Demand Response as a Power System Resource" (PDF). RAP Energy Solutions, Synapse Energy Economics. p. 13. Archived (PDF) from the original on 30 April 2017. Retrieved 13 February 2017.</ref> For instance, a few large-scale projects in Europe link variations in wind power to change industrial food freezer loads, causing small variations in temperature. If communicated on a grid-wide scale, small changes to heating/cooling temperatures would instantly change consumption across the grid.

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

Energy storage for grid applications

Energy storage assets are a valuable asset for the electrical grid.<ref>Lai, Chun Sing; Locatelli, Giorgio; Pimm, Andrew; Wu, Xiaomei; Lai, Loi Lei (September 2020). "A review on long-term electrical power system modeling with energy storage". Journal of Cleaner Production. 280: 124298. doi:10.1016/j.jclepro.2020.124298. hdl:11311/1204822.</ref> They can provide benefits and services such as load management, power quality and uninterruptible power supply to increase the efficiency and supply security. This becomes more and more important in regard to the energy transition and the need for a more efficient and sustainable energy system.

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

Several studies have developed interest and investigated the suitability or selection of the optimal energy storage for certain applications. Literature surveys comprise the available information of the state-of-the-art and compare the storage's uses based on current existing projects.<ref>Palizban, Omid; Kauhaniemi, Kimmo (May 2016). "Energy storage systems in modern grids—Matrix of technologies and applications". Journal of Energy Storage. 6: 248–259. doi:10.1016/j.est.2016.02.001.</ref><ref>Luo, Xing; Wang, Jihong; Dooner, Mark; Clarke, Jonathan (1 January 2015). "Overview of current development in electrical energy storage technologies and the application potential in power system operation". Applied Energy. 137: 511–536. doi:10.1016/j.apenergy.2014.09.081.</ref> Other studies take a step further in evaluating energy storage with each other and rank their fitness based on multiple-criteria decision analysis.<ref>Daim, Tugrul U.; Li, Xin; Kim, Jisun; Simms, Scott (June 2012). "Evaluation of energy storage technologies for integration with renewable electricity: Quantifying expert opinions". Environmental Innovation and Societal Transitions. 3: 29–49. doi:10.1016/j.eist.2012.04.003.</ref><ref>Pham, Cong-Toan; Månsson, Daniel (November 2015). "Suitability analysis of Fuzzy Logic as an evaluation method for the selection of energy storage technologies in Smart Grid applications". 2015 International Symposium on Smart Electric Distribution Systems and Technologies (EDST). Vol. 2015 International Symposium on Smart Electric Distribution Systems and Technologies (EDST). pp. 452–457. doi:10.1109/SEDST.2015.7315251. ISBN 978-1-4799-7736-9. S2CID 42921444.</ref> Another paper proposed an evaluation scheme through the investigation and modelling of storage as equivalent circuits.<ref>Pham, Cong-Toan; Månsson, Daniel (October 2017). "On the physical system modelling of energy storages as equivalent circuits with parameter description for variable load demand (Part I)". Journal of Energy Storage. 13: 73–84. doi:10.1016/j.est.2017.05.015.</ref><ref>Pham, Cong-Toan; Månsson, Daniel (August 2018). "Optimal energy storage sizing using equivalent circuit modelling for prosumer applications (Part II)". Journal of Energy Storage. 18: 1–15. doi:10.1016/j.est.2018.04.015. S2CID 64857425.</ref> An indexing approach has also been suggested in a few studies, but is still in the novel stages.<ref>Raza, Syed Shabbar; Janajreh, Isam; Ghenai, Chaouki (December 2014). "Sustainability index approach as a selection criteria for energy storage system of an intermittent renewable energy source". Applied Energy. 136: 909–920. doi:10.1016/j.est.2018.04.015. S2CID 64857425.</ref> In order to gain increased economic potential of grid connected energy storage systems, it is of interest to consider a portfolio with several services for one or more applications for an energy storage system. By doing so, several revenue streams can be achieved by a single storage and thereby also increasing the degree of utilization.<ref>Moreno, Rodrigo; Moreira, Roberto; Strbac, Goran (January 2015). "A MILP model for optimising multi-service portfolios of distributed energy storage" (PDF). Applied Energy. 137: 554–566. doi:10.1016/j.apenergy.2014.08.080. hdl:10044/1/39706.</ref> To mention two examples, a combination of frequency response and reserve services is examined in,<ref>Lee, Rachel; Homan, Samuel; Mac Dowell, Niall; Brown, Solomon (15 February 2019). "A closed-loop analysis of grid scale battery systems providing frequency response and reserve services in a variable inertia grid" (PDF). Applied Energy. 236: 961–972. doi:10.1016/j.apenergy.2018.12.044. S2CID 116444177.</ref> meanwhile load peak shaving together with power smoothing is considered in.<ref>Reihani, Ehsan; Motalleb, Mahdi; Ghorbani, Reza; Saad Saoud, Lyes (February 2016). "Load peak shaving and power smoothing of a distribution grid with high renewable energy penetration". Renewable Energy. 86: 1372–1379. doi:10.1016/j.renene.2015.09.050.</ref>

Forms

Air

Compressed air

One grid energy storage method is to use off-peak or renewably generated electricity to compress air, which is usually stored in an old mine or some other kind of geological feature. When electricity demand is high, the compressed air is heated with a small amount of natural gas and then goes through turboexpanders to generate electricity.<ref>Pendick, Daniel (2007), "Storing energy from the wind in compressed-air reservoirs", New Scientist, 195 (2623): 44–47, doi:10.1016/S0262-4079(07)62476-2</ref>

Compressed air storage is typically around 60–90% efficient.<ref>"LightSail Gets $5.5M From Total, Thiel, Khosla, Gates for Compressed Air Energy Storage". CleanTechnica. 21 February 2013.</ref>

Liquid air

Another electricity storage method is to compress and cool air, turning it into liquid air,<ref>Kevin Bullis (20 May 2013). "The Resurgence of Liquid Air for Energy Storage". MIT Technology Review. Retrieved 7 June 2013.</ref> which can be stored, and expanded when needed, turning a turbine, generating electricity, with a storage efficiency of up to 70%.<ref>"British company offers efficient energy storage using 'liquid air'". ExtremeTech. Archived from the original on 14 December 2012.</ref>

A commercial liquid-air energy storage plant is under construction in the North of England, <ref name="BBC22Oct2019">"How liquid air could help keep the lights on". BBC News. 22 October 2019. Retrieved 23 October 2019.</ref> <ref name="HV21Oct2019">"Highview Power to Develop Multiple Cryogenic Energy Storage Facilities in the UK and to Build Europe's Largest Storage System". Highview power. Retrieved 23 October 2019.</ref> <ref name="BBC-20201107">Roger, Harrabin (6 November 2020). "UK energy plant to use liquid air". BBC News. Retrieved 7 November 2020.</ref> <ref name="HP-20201107">"Highview Power Breaks Ground on 250MWh CRYOBattery Long Duration Energy Storage Facility". Company News and Announcements. Highview Power. Retrieved 7 November 2020.</ref> with commercial operation planned for 2022.<ref name="EIT-202009"> Junior Isles (September 2020). "Really cool storage" (PDF). The Energy Industry Times. 13 (5): 15. ISSN 1757-7365. Retrieved 7 November 2020.</ref> The energy storage capacity of 250MWh of the plant will be nearly twice the capacity of the world's largest existing lithium-ion battery, the Hornsdale Power Reserve in South Australia.<ref name="Economist-20201130">"Powering the future: Electrical energy can be captured as liquid air". The Economist. 30 November 2019. Retrieved 8 November 2020.</ref>

Compressed carbon dioxide

Gaseous carbon dioxide can be compressed to store energy at grid scale. The gas is well suited to this role because, unlike air, it liquifies at ambient temperatures. Liquid CO₂ can be stored indefinitely in high-pressure cylinders, for use when needed. <ref name="Verge-20221012">Justine Calma (12 October 2022). "Meet the CO₂ battery cozying up with a wind energy giant". The Verge. Retrieved 16 April 2023. t faces stiff competition, but the CO₂ battery has some unique strengths that could accelerate the transition to clean energy</ref> <ref name="Maccarini">Mr. Simone Maccarini (2021). "The Carbon Dioxide for energy storage applications" (PDF). DoE. THERMOCHEMICAL POWER GROUP, UNIVERSITY OF GENOA (ITALY). Retrieved 16 April 2023.</ref>

The main proponent of the technology is start-up company Energy Dome, which in 2022 built a 2.5MW/4MWh demonstrator plant in Sardinia. The company claim a round trip efficiency of 75% and a projected cost of EUR220/kWh of storage capacity, which is half that of Li-ion batteries. <ref name="ESN-20230412">Andy Colthorpe (12 April 2023). "Energy Dome: Turning a greenhouse gas into a cheaper form of energy storage than lithium-ion batteries". Editors blog. Energy Storage News. Retrieved 16 April 2023.</ref> <ref>"Energy Dome: This new battery uses CO₂ to store wind and solar power". YouTube.</ref> <ref name="Ansaldo">"CO₂ battery licensed by Energy Dome". Power generation: news and insights. Ansaldo Energia. Retrieved 16 April 2023.</ref>

Batteries

A 900 watt direct current light plant using 16 separate lead acid battery cells (32 volts) from 1917.<ref name="Hawkins1917">Hawkins, Nehemiah (1917). Hawkins Electrical Guide ...: Questions, Answers & Illustrations; a Progressive Course of Study for Engineers, Electricians, Students and Those Desiring to Acquire a Working Knowledge of Electricity and Its Applications; a Practical Treatise. T. Audel & Company. pp. 989–.</ref>

Battery storage was used in the early days of direct current electric power. Where AC grid power was not readily available, isolated lighting plants run by wind turbines or internal combustion engines provided lighting and power to small motors. The battery system could be used to run the load without starting the engine or when the wind was calm. A bank of lead–acid batteries in glass jars both supplied power to illuminate lamps, as well as to start an engine to recharge the batteries. Battery storage technology is typically around 80% to more than 90% efficient for newer lithium-ion devices.<ref>Eric Wesoff (2 April 2013). "Aquion Energy's Disruptive Battery Tech Picks Up $35M in VC". greentechmedia.com. Archived from the original on 6 August 2013.</ref><ref>Zachary Shahan (9 May 2015). "Tesla Powerwall & Powerpacks Per-kWh Lifetime Prices vs Aquion Energy, Eos Energy, & Imergy". CleanTechnica. Retrieved 19 March 2018.</ref>

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

Contrary to electric vehicle applications, batteries for stationary storage do not suffer from mass or volume constraints. However, due to the large amounts of energy and power implied, the cost per power or energy unit is crucial. The relevant metrics to assess the interest of a technology for grid-scale storage is the $/Wh (or $/W) rather than the Wh/kg (or W/kg). The electrochemical grid storage was made possible thanks to the development of the electric vehicle, that induced a fast decrease in the production costs of batteries below $300/kWh. By optimizing the production chain, major industrials aimed to reach $150/kWh by the end of 2020, but actually achieved $140/kWh. The rate of decline in battery prices has consistently outpaced most estimates, reaching $132/kWh in 2021.<ref>"Report: EV battery costs hit another low in 2021, but they might rise in 2022". Green Car Reports. Retrieved 8 September 2022.</ref> These batteries rely on a lithium-ion technology, which is suited for mobile applications (high cost, high density). Technologies optimized for the grid should focus on low cost per kWh. Lithium iron phosphate batteries are increasingly being used in both vehicles and grid storage because of their low cost, scale and acceptable energy density for many applications.<ref>Alamalhodaei, Aria (28 July 2021). "What Tesla's bet on iron-based batteries means for manufacturers". TechCrunch. Retrieved 8 September 2022.</ref>

Grid-oriented battery technologies

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

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

Molten-salt/liquid-metal batteries

These batteries are composed of two molten metal alloys separated by an electrolyte. They are simple to manufacture but require a temperature of several hundred degree Celsius to keep the alloys in a liquid state. This technology includes ZEBRA, sodium-sulfur batteries and liquid metal.<ref>David L. Chandler, MIT News Office (19 November 2009). "Liquid battery big enough for the electric grid?". MIT News. Archived from the original on 13 February 2010.</ref> Sodium sulphur batteries are being used for grid storage in Japan and in the United States.<ref>"Appalachian Power Dedicates Mega Battery; New Technology Provides Extra Power, Reliability" (Press release). Appalachian Power. 20 July 2006. Archived from the original on 22 October 2006.</ref> The electrolyte is composed of solid beta alumina. The liquid metal battery, developed by the group of Pr. Donald Sadoway, uses molten alloys of magnesium and antimony separated by an electrically insulating molten salt. It is being brought to market by MIT spinoff company Ambri, which is currently contracted to install a first 250MWh system for TerraScale data centre company near Reno, Nevada.<ref>Andy Colthorpe (26 November 2020). "Ambri's liquid metal battery to be used at desert data centre in Nevada". Energy Storage News.</ref><ref>Eric Wesoff (24 May 2012). "Sadoway's MIT Liquid Metal Battery Startup Adds $15M and Khosla Ventures as Investor". greentechmedia.com. Archived from the original on 25 September 2012.</ref>

Flow batteries

In rechargeable flow batteries, that store energy in liquids, such solutions of transition metal ions in water at room temperature. Flow batteries have the advantages of low capital cost for charge-discharge duration over 2-4 h, and of long durability (many years). Flow batteries are inferior to lithium-ion batteries in terms of energy efficiency.<ref>Flow batteries from 1879 to 2022 and beyond. 2023. Journal of The Electrochemical Society. 170/030505. Y.V. Tolmachev. doi: 10.1149/1945-7111/acb8de.</ref> Flow batteries are currently deployed for storing energy from intermittent renewable sources, such as wind and solar.<ref>""Renewable. Rechargeable. Remarkable.", Feature Article, September 2005". Archived from the original on 15 January 2009.</ref>

Vanadium redox batteries is most technologically and commercially advanced type of flow battery.<ref>"Grid-Scale storage with vanadium redox flow batteries". REDT Energy Storage. Archived from the original on 15 May 2014.</ref><ref>1. An Overview of the Design and Optimized Operation of Vanadium Redox Flow Batteries for Durations in the Range of 4–24 Hours. 2023. Batteries. 9/4. V.V. Viswanathan, A.J. Crawford, E.C. Thomsen, N. Shamim, G. Li, Q. Huang, et al. doi: 10.3390/batteries9040221.</ref> Currently there are dozens of Vanadium Redox Flow batteries installed at different sites including; Huxley Hill wind farm (Australia), Tomari Wind Hills at Hokkaidō (Japan), as well as in non-wind farm applications. A 12 MW·h flow battery was to be installed at the Sorne Hill wind farm (Ireland).<ref>"Wind farm with battery storage in Ireland". Leonardo Energy. Archived from the original on 2 November 2007.</ref>^{[needs update]} These storage systems are designed to smooth out transient wind fluctuations.

Examples

In Puerto Rico a system^{[further explanation needed]} with a capacity of 20 megawatts for 15 minutes (5 megawatt hour) stabilizes the frequency of electric power produced on the island. A 27 megawatt 15-minute (6.75 megawatt hour) nickel-cadmium battery bank was installed at Fairbanks Alaska in 2003 to stabilize voltage at the end of a long transmission line.<ref>Gyuk I, Kulkarni P, Sayer JH, et al. (2005). "The United States of storage". IEEE Power and Energy Magazine. 3 (2): 31–9. doi:10.1109/MPAE.2005.1405868. S2CID 34193246.</ref>

In 2014, the Tehachapi Energy Storage Project was commissioned by Southern California Edison.<ref>International, Edison. "SCE Unveils Largest Battery Energy Storage Project in North America". Edison International. Retrieved 10 May 2020.</ref>

In 2016, a zinc-ion battery was proposed for use in grid storage applications.<ref>"A cheap, long-lasting, sustainable battery for grid energy storage | KurzweilAI". www.kurzweilai.net. 16 September 2016. Archived from the original on 28 December 2016. Retrieved 2 February 2017.</ref>

In 2017, the California Public Utilities Commission installed 396 refrigerator-sized stacks of Tesla batteries at the Mira Loma substation in Ontario, California. The stacks are deployed in two modules of 10 MW each (20 MW in total), each capable of running for 4 hours, thus adding up to 80 MWh of storage. The array is capable of powering 15,000 homes for over four hours.<ref>MICU, ALEXANDRU (30 January 2017). "Rows of Tesla batteries will keep Southern California's lights on during the night". ZME Science. Archived from the original on 1 February 2017. Retrieved 2 February 2017.</ref>

BYD proposes to use conventional consumer battery technologies such as lithium iron phosphate (LiFePO4) battery, connecting many batteries in parallel.

The largest grid storage batteries in the United States include the 31.5 MW battery at Grand Ridge Power plant in Illinois and the 31.5 MW battery at Beech Ridge, West Virginia.<ref>Invenergy's Grand Ridge energy storage facility wins 2015 Best Renewable Project Award Archived 10 January 2016 at the Wayback Machine, Solar Server, 12 December 2015</ref> Two batteries under construction in 2015 include the 400 MWh (100 MW for 4 hours) Southern California Edison project and the 52 MWh project on Kauai, Hawaii to entirely time shift a 13MW solar farm's output to the evening.<ref>5 battery energy storage projects to watch in 2016 Archived 29 January 2017 at the Wayback Machine, Utility Dive, Krysti Shallenberger, 30 November 2015</ref> Two batteries are in Fairbanks, Alaska (40 MW for 7 minutes using Ni-Cd cells),<ref>Conway, E. (2 September 2008) "World's biggest battery switched on in Alaska" Telegraph.co.uk</ref> and in Notrees, Texas (36 MW for 40 minutes using lead–acid batteries).<ref>"Duke Energy Notrees Wind Storage Demonstration Project". DOE Global Energy Storage Database. Archived from the original on 26 October 2014. Retrieved 13 October 2014.</ref><ref>Lie, Øyvind (12 October 2014). "Her er verdens kraftigste batterier" [Here are the world's most powerful batteries]. Tu.no (in dansk). Teknisk Ukeblad. Archived from the original on 14 October 2014. Retrieved 13 October 2014.</ref> A 13 MWh battery made of used batteries from Daimler's Smart electric drive cars is being constructed in Lünen, Germany, with an expected second life of 10 years.<ref>Media, BioAge. "Green Car Congress: Daimler and partners deploying world's largest 2nd-life EV battery storage unit for grid support". Archived from the original on 7 November 2015.</ref>

In 2015, a 221 MW battery storage was installed in the US, with total capacity expected to reach 1.7 GW in 2020.<ref>"US energy storage market grew 243% in 2015, largest year on record". 4 March 2016. Archived from the original on 5 March 2016.</ref>

The UK had a 50 MW lithium-ion grid-battery installed in Hertfordshire in 2018.<ref>Madelyn Newton (10 July 2018). "UK's 'largest' grid battery storage facility completed in Hertfordshire".</ref> In February 2021, construction began on a 50 MW battery storage development in Burwell, Cambridgeshire and a 40 MW site in Barnsley, South Yorkshire.<ref>Weetch, Bella (21 February 2021). "SMS begins construction of British battery storage projects". Energy Global. Retrieved 1 July 2021.</ref>

In November 2017 Tesla installed a 100 MW, 129 MWh battery system in South Australia.<ref>Megan Geuss (1 December 2017). "Tesla beats deadline, switches on gigantic Australian battery array". Retrieved 29 September 2018.</ref> The Australian Energy Market Operator stated that this "is both rapid and precise, compared to the service typically provided by a conventional synchronous generation unit".<ref>Megan Geuss (11 April 2018). "Australian Energy Market Operator likes its new Tesla battery quite a bit". Retrieved 29 September 2018.</ref><ref>"Initial operation of the Hornsdale Power Reserve Battery Energy Storage Syetem" (PDF). Australian Energy Market Operator. April 2018. Retrieved 29 September 2018.</ref>

Technology comparison
Technology	Moving Parts	Ambient Temperature	Flammable	Toxic Materials	In production	Rare metals
Vanadium flow<ref>Martin Lamonica (20 March 2013). "Flow batteries could back up grid of the future". New Scientist. 217 (2909): 22. Bibcode:2013NewSc.217...22L. doi:10.1016/S0262-4079(13)60735-6. Archived from the original on 6 May 2015.</ref>	Yes	Yes	No	Yes	Yes	No
Liquid Metal	No	No	Yes	No	No	No
Sodium-Ion	No	Yes	Yes	No	No	No
Lead–Acid<ref>"Gridtential Goes After Energy Storage With Improved Lead–Acid Batteries". greentechmedia.com. 2013. Archived from the original on 20 March 2013.</ref>	No	Yes	No	Yes	Yes	No
Sodium–sulfur batteries	No	No	Yes	No	Yes	No
Ni–Cd	No	Yes	No	Yes	Yes	Yes
Al-ion	No	Yes	No	No	No	No
Li-ion	No	Yes	Yes	No	Yes	No
Fe-air	Yes	No	Yes	No	No	No

Electric vehicles

Companies are researching the possible use of electric vehicles to meet peak demand. A parked and plugged-in electric vehicle could sell the electricity from the battery during peak loads and charge either during night (at home) or during off-peak.<ref>"BBC News – New electric car scheme for California". BBC. 19 February 2010. Archived from the original on 20 February 2010.</ref>

Plug-in hybrid or electric cars could be used<ref name="RSC">Eberle, Ulrich; von Helmolt, Rittmar (14 May 2010). "Sustainable transportation based on electric vehicle concepts: a brief overview". Royal Society of Chemistry. Archived from the original on 21 October 2013. Retrieved 8 June 2010.</ref><ref>"Charge a battery in just six minutes". Archived from the original on 15 October 2008.</ref><ref>"Toshiba : Press Releases 29 March 2005". toshiba.co.jp. Archived from the original on 30 December 2016.</ref> for their energy storage capabilities. Vehicle-to-grid technology can be employed, turning each vehicle with its 20 to 50 kWh battery pack into a distributed load-balancing device or emergency power source. This represents two to five days per vehicle of average household requirements of 10 kWh per day, assuming annual consumption of 3,650 kWh. This quantity of energy is equivalent to between 60 and 480 kilometres (40 and 300 mi) of range in such vehicles consuming 0.1 to 0.3 kilowatt-hours per kilometre (0.16 to 0.5 kWh/mi). These figures can be achieved even in home-made electric vehicle conversions. Some electric utilities plan to use old plug-in vehicle batteries (sometimes resulting in a giant battery) to store electricity<ref name="woody">Woody, Todd. "PG&E's Battery Power Plans Could Jump Start Electric Car Market." Archived 8 February 2008 at the Wayback Machine (Blog). Green Wombat, 2007-06-12. Retrieved on 2007-08-19</ref><ref>Planet Ark Environmental Foundation. "E.on UK Plans Giant Battery to Store Wind Power". Positive Environment News. Archived from the original on 18 September 2007.</ref> However, a large disadvantage of using vehicle to grid energy storage would be if each storage cycle stressed the battery with one complete charge-discharge cycle.<ref name="RSC" /> However, one major study showed that used intelligently, vehicle-to-grid storage actually improved the batteries longevity.<ref>"V2G found to improve the lifetime of electric vehicle batteries". Clean Energy News. Archived from the original on 28 March 2018. Retrieved 5 May 2018.</ref> Conventional (cobalt-based) lithium-ion batteries break down with the number of cycles – newer li-ion batteries do not break down significantly with each cycle, and so have much longer lives. One approach is to reuse unreliable vehicle batteries in dedicated grid storage<ref>Kelly-Detwiler, Peter (18 March 2014). "The Afterlife For Electric Vehicle Batteries: A Future Source of Energy Storage?". Forbes.</ref> as they are expected to be good in this role for ten years.<ref>Garthwaite, Josie (12 November 2012). "Second Life for Old Electric-Car Batteries: Guardians of the Electric Grid". National Geographic. Archived from the original on 18 November 2012.</ref> If such storage is done on a large scale it becomes much easier to guarantee replacement of a vehicle battery degraded in mobile use, as the old battery has value and immediate use.

Flywheel

Mechanical inertia is the basis of this storage method. When the electric power flows into the device, an electric motor accelerates a heavy rotating disc. The motor acts as a generator when the flow of power is reversed, slowing down the disc and producing electricity. Electricity is stored as the kinetic energy of the disc. Friction must be kept to a minimum to prolong the storage time. This is often achieved by placing the flywheel in a vacuum and using magnetic bearings, tending to make the method expensive. Greater flywheel speeds allow greater storage capacity but require strong materials such as steel or composite materials to resist the centrifugal forces. The ranges of power and energy storage technology that make this method economic, however, tends to make flywheels unsuitable for general power system application; they are probably best suited to load-leveling applications on railway power systems and for improving power quality in renewable energy systems such as the 20MW system in Ireland.<ref>"Energy Storage Plant in Europe announced in Midlands". Department of Business, Enterprise and Innovation. 26 March 2015. Archived from the original on 28 November 2016. Retrieved 28 January 2020.</ref><ref>"New energy storage plant could 'revolutionise' renewable sector". The Guardian. Archived from the original on 4 December 2016.</ref>

Applications that use flywheel storage are those that require very high bursts of power for very short durations such as tokamak<ref>"Joint European Torus facility – Flywheel details". Archived from the original on 1 February 2014. Retrieved 18 January 2014.</ref> and laser experiments where a motor generator is spun up to operating speed and is partially slowed down during discharge.

Flywheel storage is also currently used in the form of the Diesel rotary uninterruptible power supply to provide uninterruptible power supply systems (such as those in large datacenters) for ride-through power necessary during transfer<ref>David Hamilton (8 January 2010). "Terremark Installs Space-Saving Flywheel UPS in New Data Center". Web Host Industry Review. Archived from the original on 28 April 2010. Retrieved 16 November 2010.</ref> – that is, the relatively brief amount of time between a loss of power to the mains and the warm-up of an alternate source, such as a diesel generator.

This potential solution has been implemented by EDA<ref>"EDA – Electricidade dos Açores". Archived from the original on 28 November 2007.</ref>^{[better source needed]} in the Azores on the islands of Graciosa and Flores. This system uses an 18 megawatt-second flywheel to improve power quality and thus allow increased renewable energy usage. As the description suggests, these systems are again designed to smooth out transient fluctuations in supply, and could never be used to cope with an outage exceeding a couple of days.

Powercorp in Australia have been developing applications using wind turbines, flywheels and low load diesel (LLD) technology to maximize the wind input to small grids. A system installed in Coral Bay, Western Australia, uses wind turbines coupled with a flywheel based control system and LLDs. The flywheel technology enables the wind turbines to supply up to 95 percent of Coral Bay's energy supply at times, with a total annual wind penetration of 45 percent.<ref>"Coral Bay PowerStore Flywheel Project". DOE Global Energy Storage Database. Archived from the original on 26 August 2017. Retrieved 26 August 2017.,</ref>

Hydrogen

Hydrogen is being developed as an electrical energy storage medium.<ref name="RSC" /><ref name=RSC2>Eberle, Ulrich; Mueller, Bernd; von Helmolt, Rittmar (15 July 2012). "Fuel cell electric vehicles and hydrogen infrastructure: status 2012". Royal Society of Chemistry. Archived from the original on 9 February 2014. Retrieved 8 January 2013.</ref> Hydrogen is produced, then compressed or liquefied, cryogenically stored at −252.882 °C, and then converted back to electrical energy or heat. Hydrogen can be used as a fuel for portable (vehicles) or stationary energy generation. Compared to pumped water storage and batteries, hydrogen has the advantage that it is a high energy density fuel.<ref name="RSC2" /> Green hydrogen, from electrolysis of water, is a more economical means of long-term renewable energy storage in terms of capital expenditures than pumped-storage hydroelectricity or batteries.<ref name="Schrotenboer">Schrotenboer, Albert H.; Veenstra, Arjen A.T.; uit het Broek, Michiel A.J.; Ursavas, Evrim (October 2022). "A Green Hydrogen Energy System: Optimal control strategies for integrated hydrogen storage and power generation with wind energy" (PDF). Renewable and Sustainable Energy Reviews. 168: 112744. doi:10.1016/j.rser.2022.112744. S2CID 250941369.</ref><ref name="Lipták">Lipták, Béla (24 January 2022). "Hydrogen is key to sustainable green energy". Control. Retrieved 12 February 2023.</ref>

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

The AC-to-AC efficiency of hydrogen storage has been shown to be on the order of 20 to 45%, which imposes economic constraints.<ref name="RSC2" /><ref name=anscombe /> The price ratio between purchase and sale of electricity must be at least proportional to the efficiency in order for the system to be economic. Hydrogen fuel cells can respond quickly enough to correct rapid fluctuations in electricity demand or supply and regulate frequency. Whether hydrogen can use natural gas infrastructure depends on the network construction materials, standards in joints, and storage pressure.<ref>"Conversion of the UK gas system to transport hydrogen". Archived from the original on 16 May 2016.</ref>

The equipment necessary for hydrogen energy storage includes an electrolysis plant, hydrogen compressors or liquifiers, and storage tanks.

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

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

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

A community based pilot program using wind turbines and hydrogen generators was started in 2007 in the remote community of Ramea, Newfoundland and Labrador.<ref>Oprisan, Morel (April 2007). "Introduction of Hydrogen Technologies to Ramea Island" (PDF). IEA Wind – KWEA Joint Workshop. Archived from the original (PDF) on 30 July 2016. Retrieved 2 February 2017.</ref> A similar project has been going on since 2004 in Utsira, a small Norwegian island municipality.

Underground hydrogen storage

Underground hydrogen storage is the practice of hydrogen storage in caverns, salt domes and depleted oil and gas fields.<ref name="RSC" /><ref>Olaf Kruck; Fritz Crotogino (14 August 2013). "Benchmarking of selected storage options" (PDF). HyUnder.</ref> Large quantities of gaseous hydrogen have been stored in caverns by Imperial Chemical Industries (ICI) for many years without any difficulties.<ref>Reinhold Wurster; Werner Zittel. "Hydrogen Energy". HyWeb – The LBST Information Portal on Hydrogen and Fuel Cells. Archived from the original on 2 January 2004.</ref> The European project Hyunder<ref>"Why storing large scale intermittent renewable energies with hydrogen?". HyUnder. Archived from the original on 11 November 2013.</ref> indicated in 2013 that for the storage of wind and solar energy an additional 85 caverns are required as it cannot be covered by PHES and CAES systems.<ref>"Storing renewable energy: Is hydrogen a viable solution?" (PDF).</ref>

Power to gas

Power to gas is a technology which converts electrical power to a gas fuel. There are 2 methods, the first is to use the electricity for water splitting and inject the resulting hydrogen into the natural gas grid. The second less efficient method is used to convert carbon dioxide and water to methane, (see natural gas) using electrolysis and the Sabatier reaction. The excess power or off peak power generated by wind generators or solar arrays is then used for load balancing in the energy grid. Using the existing natural gas system for hydrogen, fuel cell maker Hydrogenics and natural gas distributor Enbridge have teamed up to develop such a power to gas system in Canada.<ref name="anscombe">Anscombe, Nadya (4 June 2012). "Energy storage: Could hydrogen be the answer?". Solar Novus Today. Archived from the original on 19 August 2013. Retrieved 3 November 2012.</ref>

Pipeline storage of hydrogen where a natural gas network is used for the storage of hydrogen. Before switching to natural gas, the German gas networks were operated using towngas, which for the most part consisted of hydrogen. The storage capacity of the German natural gas network is more than 200,000 GW·h which is enough for several months of energy requirement. By comparison, the capacity of all German pumped-storage power plants amounts to only about 40 GW·h. The transport of energy through a gas network is done with much less loss (<0.1%) than in a power network (8%)^{[clarification needed]}. The use of the existing natural gas pipelines for hydrogen was studied by NaturalHy<ref>"Preparing for the Hydrogen Economy by Using the Existing Natural Gas System as a Catalyst" (PDF). Naturalhy. October 2009. Archived from the original (PDF) on 18 January 2012.</ref>

The power-to-ammonia concept

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

Just like natural gas, the stored ammonia can be used as a thermal fuel for transportation and electricity generation or used in a fuel cell.<ref>Lan, Rong; Tao, Shanwen (5 May 2018). "Ammonia as a Suitable Fuel for Fuel Cells". Frontiers in Energy Research. 2. doi:10.3389/fenrg.2014.00035.</ref> A standard 60,000 m³ tank of liquid ammonia contains about 211 GWh of energy, equivalent to the annual production of roughly 30 wind turbines. Ammonia can be burned cleanly: water and nitrogen are released, but no CO₂ and little or no nitrogen oxides. Ammonia has multiple uses besides being an energy carrier, it is the basis for the production of many chemicals, the most common use is for fertilizer.<ref>Service, Robert F. (12 July 2018). "Ammonia—a renewable fuel made from sun, air, and water—could power the globe without carbon". Science | AAAS. Retrieved 15 April 2021.</ref> Given this flexibility of usage, and given that the infrastructure for the safe transport, distribution and usage of ammonia is already in place, it makes ammonia a good candidate to be a large-scale, non-carbon, energy carrier of the future.

Hydroelectricity

Pumped water

In 2008, world pumped-storage generating capacity was 104 GW,<ref name=eia>"International Energy Statistics". Archived from the original on 3 October 2011.</ref> while other sources claim 127 GW, which comprises the vast majority of all types of grid electric storage – all other types combined are some hundreds of MW.<ref name="epri">Rastler; et al. (2010). "Electric Energy Storage Technology Options: A White Paper Primer on Applications, Costs, and Benefits". EPRI. Archived from the original ((Free download)) on 17 August 2011. Retrieved 30 September 2011.</ref>

In many places, pumped-storage hydroelectricity is used to even out the daily generating load, by pumping water to a high storage reservoir during off-peak hours and weekends, using the excess base-load capacity from coal or nuclear sources. During peak hours, this water can be used for hydroelectric generation, often as a high value rapid-response reserve to cover transient peaks in demand. Pumped storage recovers about 70% to 85% of the energy consumed, and is currently the most cost effective form of mass power storage.<ref>"Pumped Hydro (PH)". Electricity Storage Association. Archived from the original on 15 March 2013. Retrieved 26 March 2013.</ref> The chief problem with pumped storage is that it usually requires two nearby reservoirs at considerably different heights, and often requires considerable capital expenditure.<ref>"Pumped Hydroelectric Energy Storage". Imperial College London. Archived from the original on 29 October 2007.</ref>

Pumped water systems have high dispatchability, meaning they can come on-line very quickly, typically within 15 seconds,<ref name=Dinorwig>"First Hydro Dinorwig Power Station". Archived from the original on 12 May 2016.</ref> which makes these systems very efficient at soaking up variability in electrical demand from consumers. There is over 90 GW of pumped storage in operation around the world, which is about 3% of instantaneous global generation capacity. Pumped water storage systems, such as the Dinorwig storage system in Britain, hold five or six hours of generating capacity,<ref name=Dinorwig/> and are used to smooth out demand variations.

Another example is the 1836 MW Tianhuangping Pumped-Storage Hydro Plant in China, which has a reservoir capacity of eight million cubic meters (2.1 billion U.S. gallons or the volume of water over Niagara Falls in 25 minutes) with a vertical distance of 600 m (1970 feet). The reservoir can provide about 13 GW·h of stored gravitational potential energy (convertible to electricity at about 80% efficiency), or about 2% of China's daily electricity consumption.<ref>"CIA – The World Factbook – China". Archived from the original on 13 August 2008.</ref>

A new concept in pumped-storage is utilizing wind energy or solar power to pump water. Wind turbines or solar cells that direct drive water pumps for an energy storing wind or solar dam can make this a more efficient process but are limited. Such systems can only increase kinetic water volume during windy and daylight periods. A study published in 2013 showed rooftop solar, coupled to existing pumped-storage, could replace the reactors lost at Fukushima with an equivalent capacity factor.<ref>Stoll, B L; Smith, T A; Deinert, M R (1 March 2013). "Potential for rooftop photovoltaics in Tokyo to replace nuclear capacity". Environmental Research Letters. 8 (1): 014042. Bibcode:2013ERL.....8a4042S. doi:10.1088/1748-9326/8/1/014042. ISSN 1748-9326. S2CID 56317922.</ref>

Hydroelectric dams

Fetsui hydroelectric dam in New Taipei, Taiwan

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

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

A dam which impounds a large reservoir can store and release a correspondingly large amount of energy, by controlling river outflow and raising or lowering its reservoir level a few meters. Limitations do apply to dam operation, their releases are commonly subject to government regulated water rights to limit downstream effect on rivers. For example, there are grid situations where baseload thermal plants, nuclear or wind turbines are already producing excess power at night, dams are still required to release enough water to maintain adequate river levels, whether electricity is generated or not. Conversely there's a limit to peak capacity, which if excessive could cause a river to flood for a few hours each day.<ref>"Rethinking our Water Ways - 5.3 Water Use Plans". www.rethinkingwater.ca. Archived from the original on 5 October 2017. Retrieved 5 May 2018.</ref>

Superconducting magnetic energy

Superconducting magnetic energy storage (SMES) systems store energy in the magnetic field created by the flow of direct current in a superconducting coil which has been cryogenically cooled to a temperature below its superconducting critical temperature. A typical SMES system includes three parts: superconducting coil, power conditioning system and cryogenically cooled refrigerator. Once the superconducting coil is charged, the current will not decay and the magnetic energy can be stored indefinitely. The stored energy can be released back to the network by discharging the coil. The power conditioning system uses an inverter/rectifier to transform alternating current (AC) power to direct current or convert DC back to AC power. The inverter/rectifier accounts for about 2–3% energy loss in each direction. SMES loses the least amount of electricity in the energy storage process compared to other methods of storing energy. SMES systems are highly efficient; the round-trip efficiency is greater than 95%. The high cost of superconductors is the primary limitation for commercial use of this energy storage method.

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

There are significant technical challenges yet to be solved for superconducting magnetic energy storage to become practical.

Thermal

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

Molten salt is used to store heat collected by a solar power tower so that it can be used to generate electricity in bad weather or at night.<ref>Advantages of Using Molten Salt Archived 5 June 2011 at the Wayback Machine Tom Mancini, Sandia National Laboratories, Albuquerque, NM Accessed December 2007</ref>

Building heating and cooling systems can be controlled to store thermal energy in either the building's mass or dedicated thermal storage tanks. This thermal storage can provide load-shifting or even more complex ancillary services by increasing power consumption (charging the storage) during off-peak times and lowering power consumption (discharging the storage) during higher-priced peak times.<ref>Lee, Zachary E.; Sun, Qingxuan; Ma, Zhao; Wang, Jiangfeng; MacDonald, Jason S.; Zhang, K. Max (February 2020). "Providing Grid Services With Heat Pumps: A Review". Journal of Engineering for Sustainable Buildings and Cities. 1 (1). doi:10.1115/1.4045819. S2CID 213898377.</ref> For example, off-peak electricity can be used to make ice from water, and the ice can be stored. The stored ice can be used to cool the air in a large building which would have normally used electric AC, thereby shifting the electric load to off-peak hours. On other systems stored ice is used to cool the intake air of a gas turbine generator, thus increasing the on-peak generation capacity and the on-peak efficiency.

A pumped-heat electricity storage system uses a highly reversible heat engine/heat pump to pump heat between two storage vessels, heating one and cooling the other. The UK-based engineering company Isentropic that is developing the system claims a potential electricity-in to electricity-out round-trip efficiency of 72–80%.<ref name=Isentropic>"Isentropic's PHES Technology". Archived from the original on 10 October 2014.</ref>

A Carnot battery is a type of energy storage systems that stores electricity in heat storage and converts the stored heat back to electricity via thermodynamics cycles. This concept has been investigated and developed by many research projects recently.<ref name="DumontFrate2020">Dumont, Olivier; Frate, Guido Francesco; Pillai, Aditya; Lecompte, Steven; De paepe, Michel; Lemort, Vincent (2020). "Carnot battery technology: A state-of-the-art review". Journal of Energy Storage. 32: 101756. doi:10.1016/j.est.2020.101756. ISSN 2352-152X. S2CID 225019981.</ref> One of the advantage of this type of system is that the cost at large-scale and long-duration of thermal storage could be much lower than other storage technologies.

Physical battery; Gravitational potential energy storage with solid masses

Alternatives include storing energy by moving large solid masses upward against gravity. This can be achieved inside old mine shafts<ref>"How UK's disused mine shafts could be used to store renewable energy". The Guardian. 21 October 2019.</ref> or in specially constructed towers where heavy weights are winched up to store energy and allowed a controlled descent to release it.<ref>Gourley, Perry (31 August 2020). "Edinburgh firm behind incredible gravity energy storage project hails milestone". www.edinburghnews.scotsman.com. Retrieved 1 September 2020.</ref><ref name="quartz">Akshat Rathi (18 August 2018). "Stacking concrete blocks is a surprisingly efficient way to store energy". Quartz.</ref> In rail energy storage, rail cars carrying large weights are moved up or down a section of inclined rail track, storing or releasing energy as a result;<ref name="Scientific American-2014.03.25">Massey, Nathanael and ClimateWire. 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 Archived 30 April 2014 at the Wayback Machine, ScientificAmerican.com website, 25 March 2014. Retrieved 28 March 2014.</ref> In disused oil-well potential energy storage, weights are raised or lowered in a deep, decommissioned oil well.

Economics

The levelized cost of storing electricity depends highly on storage type and purpose; as subsecond-scale frequency regulation, minute/hour-scale peaker plants, or day/week-scale season storage.<ref>"Some energy storage already cost competitive, new valuation study shows". Utility Dive. 24 November 2015. Archived from the original on 18 October 2016. Retrieved 15 October 2016.</ref><ref>"Lazard's Levelized Cost of Storage Analysis" (PDF). Archived (PDF) from the original on 2 February 2017. Retrieved 2 February 2017.</ref><ref>Lai, Chun Sing; McCulloch, Malcolm D. (March 2017). "Levelized cost of electricity for solar photovoltaic and electrical energy storage". Applied Energy. 190: 191–203. doi:10.1016/j.apenergy.2016.12.153. S2CID 113623853.</ref>

Using battery storage is said to have a levelized cost of $120<ref>Chip Register (13 January 2015). "The Battery Revolution: A Technology Disruption, Economics and Grid Level Application Discussion with Eos Energy Storage". Forbes. Archived from the original on 11 November 2016.</ref> to $170<ref>"Eos Energy Storage – Technology and Products". eosenergystorage.com. Archived from the original on 6 February 2014.</ref> per MWh. This compares with open cycle gas turbines which, as of 2020, have a cost of around $151–198 per MWh.<ref>"Levelized Cost of Energy and of Storage".</ref>

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

However, the marginal cost of electricity varies because of the varying operational and fuel costs of different classes of generators.<ref>Lai, Chun Sing; Jia, Youwei; Xu, Zhao; Lai, Loi Lei; Li, Xuecong; Cao, Jun; McCulloch, Malcolm D. (December 2017). "Levelized cost of electricity for photovoltaic/biogas power plant hybrid system with electrical energy storage degradation costs". Energy Conversion and Management. 153: 34–47. doi:10.1016/j.enconman.2017.09.076.</ref> At one extreme, base load power plants such as coal-fired power plants and nuclear power plants are low marginal cost generators, as they have high capital and maintenance costs but low fuel costs. At the other extreme, peaking power plants such as gas turbine natural gas plants burn expensive fuel but are cheaper to build, operate and maintain. To minimize the total operational cost of generating power, base load generators are dispatched most of the time, while peak power generators are dispatched only when necessary, generally when energy demand peaks. This is called "economic dispatch".

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

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

The United States Department of Energy's International Energy Storage Database provides a free list of grid energy storage projects, many of which show funding sources and amounts.<ref>"Projects". DOE Global Energy Storage Database. Archived from the original on 15 November 2014. Retrieved 13 November 2013.</ref>

Load leveling

The demand for electricity from consumers and industry is constantly changing, broadly within the following categories:

Seasonal (during dark winters more electric lighting and heating is required, while in other climates hot weather boosts the requirement for air conditioning)
Weekly (most industry closes at the weekend, lowering demand)
Daily (such as the morning peak as offices open and air conditioners get switched on)
Hourly (one method for estimating television viewing figures in the United Kingdom is to measure the power spikes during advertisement breaks or after programmes when viewers go to switch a kettle on<ref>"BBC News – Christmas Television – The great TV ratings war". BBC. Archived from the original on 12 January 2009.</ref>)
Transient (fluctuations due to individual's actions, differences in power transmission efficiency and other small factors that need to be accounted for)

There are currently three main methods for dealing with changing demand:

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

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

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

Portability

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

There are synthetic pathways for using electricity to reduce carbon dioxide and water to liquid hydrocarbon or alcohol fuels.<ref name="great_lakes_wind_potential">Bradley, David (6 February 2004). "A Great Potential: The Great Lakes as a Regional Renewable Energy Source" (PDF). Archived from the original (PDF) on 25 March 2009. Retrieved 4 October 2008.</ref> These pathways begin with electrolysis of water to generate hydrogen, and then reducing carbon dioxide with excess hydrogen in variations of the reverse water gas shift reaction. Non-fossil sources of carbon dioxide include fermentation plants and sewage treatment plants. Converting electrical energy to carbon-based liquid fuel has potential to provide portable energy storage usable by the large existing stock of motor vehicles and other engine-driven equipment, without the difficulties of dealing with hydrogen or another exotic energy carrier. These synthetic pathways may attract attention in connection with attempts to improve energy security in nations that rely on imported petroleum, but have or can develop large sources of renewable or nuclear electricity, as well as to deal with possible future declines in the amount of petroleum available to import.

Because the transport sector uses the energy from petroleum very inefficiently, replacing petroleum with electricity for mobile energy will not require very large investments over many years.^{[citation needed]}

Reliability

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

References

Saving For a Windless day by Sean Davies in The E&T Magazine Vol 5 Issue 9 from the www.IET.org

External links

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Proposals	EU SuperSmart Grid Smartgrids Technology Platform USA Unified Smart Grid Pickens plan super grid Electranet
Efficient energy use	Smart grid Smart meter Nonintrusive load monitoring Demand response Demand management Dynamic demand Negawatts
Other technologies/concepts	HVDC bulk transmission FACTS Power line communication Phasor measurement unit Load following Load control Peak demand Power-to-X
Policies	Renewable energy policy Feed-in Tariff Net metering Renewable Energy Certificates Renewable energy payments Soft energy path Carbon capture and storage
Related Issues	Energy security Electrification United States energy independence Peak oil Energy crisis Renewable energy commercialization Rural electrification
Categories Energy policy Electricity economics Portals Energy Renewable energy

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