All forms of energy are either potential energy (e.g. Chemical, gravitational, electrical energy, temperature differential, latent heat, etc.) or kinetic energy (e.g. momentum). Some technologies provide only short-term energy storage, and others can be very long-term such as power to gas using hydrogen and the storage of heat or cold between opposing seasons in deep aquifers or bedrock. A wind-up clock stores potential energy (in this case mechanical, in the spring tension), a battery stores readily convertible chemical energy to operate a mobile phone, and a hydroelectric dam stores energy in a reservoir as gravitational potential energy. Ice storage tanks store ice (thermal energy in the form of latent heat) at night to meet peak demand for cooling. Fossil fuels such as coal and gasoline store ancient energy derived from sunlight by organisms that later died, became buried and over time were then converted into these fuels. Even food (which is made by the same process as fossil fuels) is a form of energy stored in chemical form.
Energy storage as a natural process is as old as the universe itself - the energy present at the initial formation of the universe has been stored in stars such as the Sun, and is now being used by humans directly (e.g. through solar heating), or indirectly (e.g. by growing crops or conversion into electricity in solar cells).
As a purposeful activity, energy storage has existed since pre-history, though it was often not explicitly recognized as such. An example of deliberate mechanical energy storage is the use of logs or boulders as defensive measures in ancient forts—the logs or boulders were collected at the top of a hill or wall, and the energy thus stored used to attack invaders who came within range.
A more recent application is 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.
Modern era developments 
Storing energy allows humans to balance the supply and demand of energy. Energy storage systems in commercial use today can be broadly categorized as mechanical, electrical, chemical, biological and thermal.
Storage for electricity 
Energy storage became a dominant factor in economic development with the widespread introduction of electricity Unlike other common energy storage in prior use such as wood or coal, electricity must be used as it is being generated, or converted immediately into another form of energy such as potential, kinetic or chemical. Until recently electrical energy has not been converted and stored on a major scale, however new efforts to that effect began in the 21st century.
An early solution to the problem of storing energy for electrical purposes was the development of the battery as an electrochemical storage device. Batteries have previously been of limited use in electric power systems due to their relatively small capacity and high cost. However, since about the middle of the first decade of the 21st century, newer battery technologies have been developed that can now provide significant utility scale load-leveling capabilities. A similar possible solution to deal with the intermittency issue of solar and wind energy is found in the capacitor.
Some areas of the world such as Washington and Oregon in the United States, and Wales in the United Kingdom, have used geographic features to store large quantities of water in elevated reservoirs, using excess electricity at times of low demand to pump water up to the reservoirs, then letting the water pass through turbine generators to retrieve the energy when electrical demands peak.
Short-term thermal storage, as heat or cold 
In the 1980s, a number of manufacturers carefully researched thermal energy storage (TES) to meet the growing demand for air conditioning during peak hours. Today, several companies manufacture TES systems. The most popular form of thermal energy storage for cooling is ice storage, since it can store more energy in less space than water storage and it is also less costly than energy recovered via fuel cells or flywheels. Thermal storage has cost-effectively shifted gigawatts of power away from daytime peak usage periods, and in 2009 was used in over 3,300 buildings in over 35 countries. It works by creating ice at night when electricity is usually less costly, and then using the ice to cool the air in buildings during the hotter daytime periods.
Latent heat can also be stored in technical phase change materials (PCMs), besides ice. These can for example be encapsulated in wall and ceiling panels, to moderate room temperatures between daytime and nighttime.
Interseasonal thermal storage, as heat or cold 
Another class of thermal storage that has been developed since the 1970s that is now frequently employed is seasonal thermal energy storage (STES). It allows heat or cold to be used even months after it was collected from waste energy or natural sources, even in an opposing season. The thermal storage may be accomplished in contained aquifers, clusters of boreholes in geological substrates as diverse as sand or crystalline bedrock, in lined pits filled with gravel and water, or water-filled mines. An example is Alberta, Canada's Drake Landing Solar Community, for which 97% of the year-round heat is provided by solar-thermal collectors on the garage roofs, with a borehole thermal energy store (BTES) being the enabling technology. STES projects often have paybacks in the 4-6 year range.
Energy storage in chemical fuels 
Chemical fuels have become the dominant form of energy storage, both in electrical generation and energy transportation. Chemical fuels in common use are processed coal, gasoline, diesel fuel, natural gas, liquefied petroleum gas (LPG), propane, butane, ethanol and biodiesel. All of these materials are readily converted to mechanical energy and then to electrical energy using heat engines (via turbines or other internal combustion engines, or boilers or other external combustion engines) used for electrical power generation. Heat-engine-powered generators are nearly universal, ranging from small engines producing only a few kilowatts to utility-scale generators with ratings up to 800 megawatts. A key disadvantage to hydrocarbon fuels are their significant emissions of greenhouse gases that contribute to global warming, as well as other significant pollutants emitted by the dirtier fuel sources such as coal and gasoline.
Liquid hydrocarbon fuels are the most commonly used forms of energy storage for use in transportation, but because the byproducts of the reaction that utilizes these liquid fuels' energy (combustion) produce greenhouse gases other energy carriers like hydrogen can be used to avoid production of greenhouse gases.
Advanced systems 
Electrochemical devices called fuel cells were invented about the same time as the battery in the 19th Century. However, for many reasons, fuel cells were not well-developed until the advent of manned spaceflight (such as the Gemini Program in the U.S.) when lightweight, non-thermal (and therefore efficient) sources of electricity were required in spacecraft. Fuel cell development has increased in recent years due to an attempt to increase conversion efficiency of chemical energy stored in hydrocarbon or hydrogen fuels into electricity.
Several other technologies have also been investigated, such as flywheels, which can store kinetic energy, and compressed air storage that can be pumped into underground caverns and abandoned mines.
Another method used at the Solar Project and the Solar Tres Power Tower uses molten salt to store solar power and then dispatch that power as needed. The system pumps molten salt through a tower heated by the sun's rays. Insulated containers store the hot salt solution, and when needed water is then used to create steam that is fed to turbines to generate electricity.
Research is being conducted on harnessing the quantum effects of nanoscale capacitors to create digital quantum batteries. Although this technology is still in the experimental stage, it theoretically has the potential to provide dramatic increases in energy storage capacity.
Grid energy storage 
Grid energy storage (or large-scale energy storage) lets energy producers send excess electricity over the electricity transmission grid to temporary electricity storage sites that subsequently become energy suppliers when electricity demand is greater. Grid energy storage is particularly important in matching supply and demand over a 24 hour period of time.
A proposed variant of grid energy storage is called vehicle-to-grid energy storage system, where modern electric vehicles that are plugged into the energy grid can release the stored electrical energy in their batteries back into the grid when needed.
Storage methods 
Hydrogen is also being developed as an electrical power storage medium. Hydrogen is not a primary energy source, but a portable energy storage method, because it must first be manufactured by other energy sources in order to be used. However, as a storage medium, it may be a significant factor in using renewable energies. See hydrogen storage.
Underground hydrogen storage is the practice of hydrogen storage in underground caverns, salt domes and depleted oil and gas fields. Large quantities of gaseous hydrogen are stored in underground caverns for many years without any difficulties. The storage of large quantities of hydrogen underground can function as grid energy storage which is essential for the hydrogen economy. By using a turboexpander, the electricity needs for compressed storage at 200 bars amounts to 2.1% of the energy content.
With intermittent renewables such as solar and wind, the output may be fed directly into an electricity grid. At penetrations below 20% of the grid demand, this does not severely change the economics; but beyond about 20% of the total demand, external storage will become important. If these sources are used for electricity to make hydrogen, then they can be utilized fully whenever they are available, opportunistically. Broadly speaking, it does not matter when they cut in or out, the hydrogen is simply stored and used as required. A community based pilot program using wind turbines and hydrogen generators is being undertaken from 2007 for five years in the remote community of Ramea, Newfoundland and Labrador. A similar project has been going on since 2004 on Utsira, a small Norwegian island municipality.
Energy losses are involved in the hydrogen storage cycle of hydrogen production for vehicle applications with electrolysis of water, liquification or compression, and conversion back to electricity. and the hydrogen storage cycle of production for the stationary fuel cell applications like MicroCHP at 93 % with biohydrogen or biological hydrogen production, and conversion to electricity.
About 50 kW·h (180 MJ) of solar energy is required to produce a kilogram of hydrogen, so the cost of the electricity clearly is crucial, even for hydrogen uses other than storage for electrical generation. At $0.03/kWh, common off-peak high-voltage line rate in the United States, this means hydrogen costs $1.50 a kilogram for the electricity, equivalent to $1.50 a U.S. gallon for gasoline if used in a fuel cell vehicle. Other costs would include the electrolyzer plant, hydrogen compressors or liquefaction, storage and transportation, which will be significant.
Various biofuels such as biodiesel, straight vegetable oil, alcohol fuels, or biomass can be used to replace hydrocarbon 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, or syngas. This diesel source was used extensively in World War II in Germany, with limited access to crude oil supplies. Today 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 synthetic liquid fuels economical on a large scale (see coal). Some of the energy in the original source is lost in the conversion process. Historically, coal itself has been used directly for transportation purposes in vehicles and boats using steam engines. Additionally, compressed natural gas is also used as fuel, for instance for buses with some mass transit agencies.
Methane (SNG Synthetic Natural Gas) 
Methane is the simplest hydrocarbon with the molecular formula CH4. Methane could be produced from electricity of renewable energies using power to gas technologies. Methane can be stored more easily than hydrogen and the transportation, storage and combustion infrastructure are mature (pipelines, gasometers, power plants).
As hydrogen and oxygen are produced in the electrolysis of water,
- 2H2O → 2H2 + O2
- CO2 + 4H2 → CH4 + 2H2O
Methane would be stored and used to produce electricity later. Produced water would be recycled back to the electrolysis stage, reducing the need for new pure water. In the electrolysis stage oxygen would also be stored for methane combustion in a pure oxygen environment in an adjacent power plant, eliminating e.g. nitrogen oxides. In the combustion of methane, carbon dioxide and water are produced.
- CH4 + 2O2 → CO2 + 2H2O
Produced carbon dioxide would be recycled back to boost the Sabatier process and water would be recycled back to the electrolysis stage. The carbon dioxide produced by methane combustion would be turned back to methane, thus producing no greenhouse gases. Methane production, storage and adjacent combustion would recycle all the reaction products, creating a low carbon cycle.
The CO2 would be a resource having economic value as a component of an energy storage vector, not a cost as in CCS (Carbon capture and storage).
Aluminium, boron, silicon, and zinc 
Mechanical storage 
A mass of 1 kg, elevated to a height of 1,000 m stores 9.8 kJ of gravitational energy, which is equivalent to 1 kg mass accelerated to 140 m/s. To store the same mass of water, if increased in temperature by 2.34 Celsius, requires the same amount of energy. Admittedly, this is a bit of an unfair comparison, but it makes it easy to see how it is possible to store more energy in 1 m3 of cheap rock or sand than 1 m3 of lead–acid battery, even if the battery is also moved to a higher elevation, not just charged.
Compressed air energy storage (CAES) technology stores low cost off-peak energy, in the form of compressed air in an underground reservoir. The air is then released during peak load hours and heated with the exhaust heat of a standard combustion turbine. This heated air is converted to energy through expansion turbines to produce electricity. A CAES plant has been in operation in McIntosh, Alabama since 1991 and has run successfully. Other applications are possible. Walker Architects published the first CO2 gas CAES application, proposing the use of sequestered CO2 for Energy Storage on October 24, 2008. In that paper Walker defined the CO2 energy storage cycle. Several projects sponsored by the DOE are underway to develop the body of technology pioneered by Terry L. Walker in 2008.
Thermal storage 
Thermal storage is the temporary storage or removal of heat for later use. An example of thermal storage is the storage of solar heat energy during the day to be used at a later time for heating at night. In the HVAC/R field, this type of application using thermal storage for heating is less common than using thermal storage for cooling. An example of the storage of "cold" heat removal for later use is ice made during the cooler night time hours for use during the hot daylight hours. This ice storage is produced when electrical utility rates are lower. This is often referred to as "off-peak" cooling.
When used for the proper application with the appropriate design, 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 high-performance buildings that will help protect our environment. The increased levels of energy performance by utilizing off-peak cooling may qualify of credits toward LEED Certification.
The advantages of thermal storage are:
- Commercial electrical rates are lower at night;
- it takes less energy to make ice when the ambient temperature is cool at night. Source energy (energy from the power plant) is saved.
- a smaller, less costly system can do the job of a much larger unit by running for more hours.
Renewable energy storage 
Many renewable energy sources (most notably solar and wind) produce intermittent power. Wherever intermittent power sources reach high levels of grid penetration, energy storage becomes one option to provide reliable energy supplies. Other options include recourse to peaking power plants, methane storage (excess renewable electricity to hydrogen via electrolysis, combine with CO2 (low to neutral CO2 system) to produce methane (synthetic natural gas sabatier process) with stockage in the natural gas network) and smart grids with advanced energy demand management. The latter involves bringing "prices to devices", i.e. making electrical equipment and appliances able to adjust their operation to seek the lowest spot price of electricity. On a grid with a high penetration of renewables, low spot prices would correspond to times of high availability of wind and/or sunshine.
Seasonal thermal energy storage (STES) stores heat deep in the ground via a cluster of boreholes, the Drake Landing Solar Community in Alberta, Canada has achieved a 97% solar fraction for year 'round heating, with solar collectors on the garage roofs as the heat source. In Braestrup, Denmark, the community's solar district heating system also utilizes STES, at a storage temperature of 65 oC. A heat pump, which is run only when there is surplus wind power available on the national grid, is used when extracting heat from the storage to raise the temperature to 80 oC for distribution. This helps stabilize the national grid, as well as contributing to maximal use of wind power. When surplus wind generated electricity is not available, a gas-fired boiler is used. Presently, 20% of Braestrup's heat is solar, but expansion of the facility is planned to raise the fraction to 50%.
Economic evaluation 
The economic valuation of large-scale applications (including pumped hydro storage and compressed air) must evaluate benefits including: wind curtailment avoidance, grid congestion avoidance, price arbitrage, and carbon free energy delivery.
See also 
- Wild, Matthew, L. Wind Drives Growing Use of Batteries, The New York Times, July 28, 2010, pp. B1.
- Thermal Energy Storage Myths, Calmac.com website.
- Wong, B. (2013). Integrating solar & heat pumps. .
- Wong, B. (2011). Drake Landing Solar Community.
- Hellström, G. (19 May 2008), Large-Scale Applications of Ground-Source Heat Pumps in Sweden, IEA Heat Pump Annex 29 Workshop, Zurich.
- Gies, Erica. Global Clean Energy: A Storage Solution Is in the Air, International Herald Tribune, October 1, 2012. Retrieved from NYTimes.com website, March 19, 2013.
- Talbot, David (December 21, 2009). "A Quantum Leap in Battery Design". Technology Review (MIT). Retrieved June 9, 2011.
- Hubler, Alfred W. (Jan/Feb 2009). "Digital Batteries". Complexity (Wiley Periodicals, Inc) 14 (3): 7–8. doi:10.1002/cplx.20275.
- Hydrogen and Fuel Cell News: 1994 - ECN abstract, HyWeb.com website.[dead link]
- Energy technology analysis, International Energy Agency. p.70
- Oprisan, Morel. Introduction of Hydrogen Technologies to Ramea Island, CANMET Technology Innovation Centre, Natural Resources Canada, April 2007.
- Zyga, Lisa (2006-12-11:15-44). "Why a hydrogen economy doesn't make sense". Physorg.com web site (Physorg.com). Retrieved 2007-11-17.
- Home heat and power: Fuel cell or combustion engine, GreenEnergyNews.com website, May 1, 2005, Vol.10 No.6.
- Clean Alternative Fuels: Fischer-Tropsch, Transportation and Air Quality, Transportation and Regional Programs Division, United States Environmental Protection Agency, March 2002.
- Quirin Schiermeier (April 10, 2013). "Renewable power: Germany’s energy gamble: An ambitious plan to slash greenhouse-gas emissions must clear some high technical and economic hurdles.". Nature. Retrieved April 10, 2013.
- White Paper: A Novel Method For Grid Energy Storage Using Aluminum Fuel, Alchemy Research, April 2012.
- Cowan, Graham R.L. Boron: A Better Energy Carrier than Hydrogen?, June 12, 2007]
- Auner, Norbert. Silicon as an intermediary between renewable energy and hydrogen, Frankfurt, Germany: Institute of Inorganic Chemistry, Johann Wolfgang Goethe University Frankfurt, Leibniz-Informationszentrum Wirtschaft, May 5, 2004, No. 11.
- Engineer-Poet. Ergosphere Blog, Zinc: Miracle metal?, June 29, 2005.
- Wald, Matthew L. Using Compressed Air To Store Up Electricity, The New York Times, September 29, 1991. Discusses the McIntosh CAES storage facility.
- Diem, William. Experimental car is powered by air: French developer works on making it practical for real-world driving, Auto.com, March 18, 2004. Retrieved from Archive.org on March 19, 2013.
- Slashdot: Car Powered by Compressed Air], Freep.com website, 2004.03.18
- Fire and Ice based storage, DistributedEnergy.com website, April 2009.
- Air-Conditioning, Heating and Refrigeration Institute, Fundamentals of HVAC/R, Page 1263
- Schmid, Jürgen. Renewable Energies and Energy Efficiency: Bioenergy and renewable power methane in integrated 100% renewable energy system (thesis), Universität Kassel/Kassel University Press, September 23, 2009.
- Scénario NégaWatt 2011 (France)
- Weeks, Jennifer (2010-04-28). "U.S. Electrical Grid Undergoes Massive Transition to Connect to Renewables". Scientific American. Retrieved 2010-05-04.
- Natural Resources Canada, 2012. Canadian Solar Community Sets New World Record for Energy Efficiency and Innovation. 5 Oct. 2012.
- Solar District Heating (SDH). 2012. Braedstrup Solar Park in Denmark Is Now a Reality! Newsletter. 25 Oct. 2012. SDH is a European Union-wide program.
- Rodica Loisel, Arnaud Mercier, Christoph Gatzen, Nick Elms, Hrvoje Petric, "Valuation framework for large scale electricity storage in a case with wind curtailment", Energy Policy 38(11): 7323-7337, 2010, doi:10.1016/j.enpol.2010.08.007.
- Wald, Matthew. Green Blog: The Convoluted Economics of Storing Energy, The New York Times, January 3, 2012.
Further reading 
- Barringer, Felicity. Green Blog: In California, a Grid Storage Mandate, The New York Times, October 1, 2010.
- Browne, Malcome W. New Hunt for Ideal Energy Storage System, The New York Times, January 6, 1988. Discusses superconducting magnetic energy storage.
- Corum, Lyn. 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.
- Galbraith, Kate. In Presidio, a Grasp at the Holy Grail of Energy Storage, The New York Times, November 6, 2010.
- Gies, Erica. The Challenge of Storing Energy on a Large Scale, The New York Times, September 29, 2010.
- Gies, Erica. Making the Consumer an Active Participant in the Grid, The New York Times, November 29, 2010. Discusses distributed generation and the U.S. Federal Energy Regulatory Commission.
- Middlesboro Daily News. The Case For A Fifth Wheel, Middlesboro, Kentucky: Middlesboro Daily News, Jan 15, 1974, p. 4. Discusses research into flywheel powered autos.
- Ricketts, Camille. DOE Charges Up Flywheels, Finalizes $43M Loan to Beacon Power, VentureBeat, The New York Times, August 10, 2010. Discusses Beacon Power and flywheel energy storage.
- Sorkin, Andrew Ross. Storage: Clean Energy’s Killer App, DealBook, The New York Times, September 30, 2010.
- Sorkin, Andrew Ross. Despite Setbacks, Investor Is Bullish on Clean Technology, DealBook, The New York Times, November 29, 2012. Discusses LightSail and Khosla Ventures.
- Wald, Matthew L. New Ways to Store Solar Energy for Nighttime and Cloudy Days, The New York Times, April 15, 2008. Discusses molten salt thermal energy storage, power tower.
- Wald, Matthew L. Green Blog: Storing Solar Power in Salt, The New York Times, March 2, 2009. Discusses molten salt thermal storage, SolarReserve.
- Wald, Matthew L. Green Blog: Harnessing the Sun to Store the Wind, The New York Times, December 28, 2009. Discusses CAES and Southwest Solar Technology.
- Wald, Matthew L. Hold That Megawatt!, The New York Times, January 7, 2011. Discusses AES Energy Storage.
- Wald, Matthew L. Green Blog: Storing Energy as Ice?, The New York Times, January 27, 2010. Discusses Thermal energy storage.
- Wald, Matthew L. Green Blog: ARPA-E Is Poised to Put Products on the Grid, The New York Times, April 14, 2011. Discusses: Advanced Research Projects Agency (ARPA-E), General Compression, CAES.
- Wald, Matthew L. Can Batteries Replace Power Generators?, The New York Times, May 18, 2011. Discusses: AES Energy Storage, Long Island Power Authority.
- Wald, Matthew L. Green Blog: Google’s Green Energy Wish List, The New York Times, June 28, 2011.
- Wald, Matthew L. Solar Power for Darker Times, The New York Times, July 15, 2011. Discusses salt thermal storage, SolarReserve.
- Wald, Matthew L. Batteries at a Wind Farm Help Control Output, The New York Times, October 28, 2011. Discusses: AES Corporation, energy storage at wind farms.
- Wald, Matthew L. Taming Unruly Wind Power, The New York Times, November 4, 2011.
- Wald, Matthew L. Storehouses for Solar Energy Can Step In When the Sun Goes Down, The New York Times, January 2, 2012. Discusses molten salt thermal storage, SolarReserve, Brightsource, solar power towers.
- Wald, Matthew L. Green Blog: Surplus Renewable Energy: An Update, The New York Times, February 8, 2012.
- Wald, Matthew L. Green Blog: Is That Onions You Smell? Or Battery Juice?, The New York Times, May 9, 2012. Discusses vanadium redox battery technology.
- Wald, Matthew L. Green Blog: Cutting the Electric Bill with a Giant Battery, The New York Times, June 27, 2012. Discusses Saft.
- Wald, Matthew L. Seeking to Start a Silicon Valley for Battery Science, The New York Times, November 30, 2012.
- Witkin, Jim. Building Better Batteries for Electric Cars, The New York Times, March 31, 2011, p. F4. Published online March 30, 2011. Discusses batteries and lithium ion battery.
- Witkin, Jim. Green Blog: A Second Life for the Electric Car Battery, The New York Times, April 27, 2011. Describes: ABB; Community Energy Storage for the use of electric vehicle batteries for grid energy storage.
|Wikimedia Commons has media related to: Energy storage|
- U.S. Dept of Energy - Energy Storage Systems Government research center on energy storage technology.
- Electricity Storage Association Good comparison of technologies.
- List of grid scale energy storage systems Currently being used today.
- IEEE Special Issue on Massive Energy Storage
- IEA-ECES - International Energy Agency - Energy Conservation through Energy Conservation programme.
- IEA-SHC - International Energy Agency - Solar Heating and Cooling programme. (Includes storge aspects.)
- SDH - Solar District Heating Platform. (European Union) (Includes storage aspects).
- Reports and White Papers on flywheel-based energy storage systems, Beacon Power