Energy storage

From Wikipedia, the free encyclopedia
Jump to: navigation, search
The Llyn Stwlan dam of the Ffestiniog Pumped Storage Scheme in Wales. The lower power station has four water turbines which can generate a total of 360 MW of electricity for several hours, an example of artificial energy storage and conversion.

Energy storage is accomplished by devices or physical media that store energy to perform useful processes at a later time. A device that stores energy is sometimes called an accumulator.

Many forms of energy produce useful work, heating or cooling to meet societal needs. These energy forms include chemical energy, gravitational potential energy, electrical potential, electricity, temperature differences, latent heat, and kinetic energy. Energy storage involves converting energy from forms that are difficult to store (electricity, kinetic energy, etc.) to more conveniently or economically storable forms. Some technologies provide only short-term energy storage, and others can be very long-term such as power to gas using hydrogen or methane 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 rechargeable 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[edit]

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[edit]

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.[1] A traditional way of storing energy on a large scale is through the use of pumped-storage hydroelectricity. Some areas of the world such as Norway, 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 into their reservoirs. The facilities then release the water which passes through turbine generators and converts the stored potential energy back to electricity when electrical demand peaks.[2] In another example, pumped-storage in hydroelectricity in Norway has an instantaneous capacity of 25–30 GW that can be expanded to 60 GW—enough to be the battery of Europe—with efforts underway in 2014 to expand its power transfer links with Germany.[1][3]

Another 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 and frequency regulation capabilities.[2] As of 2013 some of the newer battery chemistries have shown promise of being competitive with alternate energy storage methods.[4] (See Rechargeable battery below).

Other possible large-scale methods of commercial energy storage include: flywheel, compressed air energy storage, hydrogen storage, thermal energy storage, and power to gas. Smaller scale commercial application-specific storage methods include flywheels, capacitors and supercapacitors.

Short-term thermal storage, as heat or cold[edit]

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.[5] 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. In 2009, thermal storage 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.[6]

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[edit]

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.[7][8] STES projects often have paybacks in the four-to-six year range.[9]

Energy storage in chemical fuels[edit]

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 the production of greenhouse gases.

Advanced systems[edit]

A Flybrid Kinetic Energy Recovery System flywheel. Built for use on Formula 1 racing cars, it is employed to recover and reuse kinetic energy captured during braking.

Several advanced technologies have been investigated and are undergoing commercial development, including flywheels, which can store kinetic energy, and compressed air storage that can be pumped into underground caverns and abandoned mines to store potential energy.[2][10]

Another advanced method used at the Solar Project in the United States and the Solar Tres Power Tower in Spain uses molten salt to store thermal energy captured from solar power and then convert it and dispatch it as electrical power when needed. The system pumps molten salt through a tower or other special conduits that are intensely heated by the sun's rays. Insulated tanks 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 also 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.[11][12]

Grid energy storage[edit]

Main article: 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.

Renewable energy storage[edit]

The 150 MW Andasol solar power station is a commercial parabolic trough solar thermal power plant, located in Spain. The Andasol plant uses tanks of molten salt to store solar energy so that it can continue generating electricity even when the sun isn't shining.[13]

Many renewable energy sources (most notably solar and wind) produce intermittent power.[2] Wherever intermittent power sources reach high levels of grid penetration, energy storage becomes one option to provide reliable energy supplies. Individual energy storage projects augment electrical grids by capturing excess electrical energy during periods of low demand and storing it in other forms until needed on an electrical grid. The energy is later converted back to its electrical form and returned to the grid as needed.

Common forms of renewable energy storage include hydroelectric dams including pumped-storage hydroelectricity, which has long maintained the largest total capacity of stored energy worldwide, as well as rechargeable battery systems, thermal energy storage including molten salts which can efficiently store and release very large quantities of heat energy,[6] and compressed air energy storage. Less common, specialized forms of storage include flywheel energy storage systems, the use of cryogenic stored energy, and even superconducting magnetic coils.

Other options include recourse to peaking power plants that utilize a power-to-gas methane creation and storage process (where excess electricity is converted to hydrogen via electrolysis, combined with CO2 (low to neutral CO2 system) to produce methane (synthetic natural gas via the sabatier process) with stockage in the natural gas network)[14][15] and smart grids[16] with advanced energy demand management. The latter involves bringing "prices to devices",[16] 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.

Another energy storage method is the consumption of surplus or low-cost energy (typically during night time) for conversion into resources such as hot water, cool water or ice, which is then used for heating or cooling at other times when electricity is in higher demand and at greater cost per kilowatt hour (KWh).[6] Such thermal energy storage is often employed at end-user sites such as large buildings, and also as part of district heating, thus 'shifting' energy consumption to other times for better balancing of supply and demand.

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.[17] In Braestrup, Denmark, the community's solar district heating system also utilizes STES, at a storage temperature of 65°C (149°F). 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°C (176°F) 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%.[18]

In 2011, the Bonneville Power Administration in Northwestern United States created an experimental program to absorb excess wind and hydro power generated at night or during stormy periods that are accompanied by high winds. Under computerized central control, home appliances in the region are commanded to absorb surplus energy at such times by heating ceramic bricks in special space heaters to hundreds of degrees, and by also boosting the temperature of modified hot water heater tanks. After being fully charged the highly insulated home appliances then provide home heating and hot water at later times as needed. The experimental system was created as a result of a severe 2010 storm that overproduced renewable energy in the U.S. Northwest to the extent that all conventional power sources were completely shut down, or in the case of a nuclear powerplant, reduced to its lowest possible operating level, leaving a large swath of the region running almost completely on renewable energy.[19][20]

Storage methods[edit]

Mechanical storage[edit]

A mass of 1 kg, elevated to a height of 1,000 metres stores 9.8 kJ of gravitational energy, which is equivalent to 1 kg mass accelerated to 140 m/s. The same amount of energy is required to raise the temperature of 1 kg of water by 2.34 °C.

Energy can be stored in water pumped to a higher elevation using pumped storage methods and also by moving solid matter to higher locations. Several companies such as Energy Cache and Advanced Rail Energy Storage (ARES) are working on this.[21][22] Other commercial mechanical methods include compressing air and the spinning of large flywheels which converts electric energy into kinetic energy, and then back again when electrical demand peaks.


For regions that have hydroelectric dams with reservoirs, these may 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 its generators when demand is high. The net effect is similar to pumped storage, but without the pumping loss. Depending on the reservoir capacity the plant can provide daily, weekly, or seasonal load following.

While a hydroelectric dam does not directly store excess energy from other generating units, it behaves equivalently by shutting down and storing its "fuel" during periods of excess electricity from other sources. Functioning as a virtual grid storage unit in this way, the dam is one of the most efficient forms of energy storage, because it is only changing the timing of the electricity that it would normally generate. Hydroelectric turbines have a very fast start-up time in the order of a few minutes.[23] A dam which impounds a reservoir can store and release a corresponding amount of energy, by raising and lowering its reservoir.

Pumped-storage hydroelectricity[edit]

The Sir Adam Beck Generating Complex at Niagara Falls, Canada, which includes a large pumped storage hydroelectricity reservoir to provide an extra 174 MW of electricity during periods of peak demand.

Worldwide, pumped-storage hydroelectricity is the largest-capacity form of grid energy storage available, and, as of March 2012, the Electric Power Research Institute (EPRI) reports that PSH accounts for more than 99% of bulk storage capacity worldwide, representing around 127,000 MW.[24] PSH reported energy efficiency varies in practice between 70% and 80%,[24][25][26][27] with some claiming up to 87%.[28]

At times of low electrical demand, excess generation capacity is used to pump water from a lower source into a higher reservoir. When there is higher demand, water is released back into a lower reservoir (or waterway or body of water) through a turbine, generating electricity. Reversible turbine-generator assemblies act as both a pump and turbine (usually a Francis turbine design). Nearly all facilities use the height difference between two natural bodies of water or artificial reservoirs. Pure pumped-storage plants just shift the water between reservoirs, while the "pump-back" approach is a combination of pumped storage and conventional hydroelectric plants that use natural stream-flow.

Compressed air energy storage[edit]

A compressed air locomotive used inside a mine between 1928 and 1961.

Compressed air energy storage (CAES) is a way to store energy generated at one time for use at another time using compressed air. At utility scale, energy generated during periods of low energy demand (off-peak) can be released to meet higher demand (peak load) periods.[29] Small scale systems have long been used in such applications as propulsion of mine locomotives. Large scale applications must conserve the heat energy associated with compressing air; dissipating heat lowers the energy efficiency of the storage system.

The 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, using older CAES technology, 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.[30] Other applications are possible. Walker Architects published the first CO2 gas CAES application, proposing the use of sequestered CO2 for Energy Storage.

Compression of air creates heat; the air is warmer after compression. Expansion requires heat. If no extra heat is added, the air will be much colder after expansion. If the heat generated during compression can be stored and used during expansion, the efficiency of the storage improves considerably.[10] There are three ways in which a CAES system can deal with the heat. Air storage can be adiabatic, diabatic, or isothermal. Several companies have also done design work for vehicles using compressed air power.[31][32]

Flywheel energy storage[edit]

The main components of a typical flywheel.

Flywheel energy storage (FES) works by accelerating a rotor (flywheel) to a very high speed and maintaining the energy in the system as rotational energy with the least friction losses possible. When energy is extracted from the system, the flywheel's rotational speed is reduced as a consequence of the principle of conservation of energy; adding energy to the system correspondingly results in an increase in the speed of the flywheel.

Most FES systems use electricity to accelerate and decelerate the flywheel, but devices that directly use mechanical energy are being developed.[33]

Advanced FES systems have rotors made of high strength carbon-fiber composites, suspended by magnetic bearings, and spinning at speeds from 20,000 to over 50,000 rpm in a vacuum enclosure.[34] Such flywheels can come up to speed in a matter of minutes – reaching their energy capacity much more quickly than some other forms of storage. A typical system consists of a rotor suspended by bearings inside a vacuum chamber to reduce friction, connected to a combination electric motor and electric generator.

Compared with other ways to store electricity, FES systems have long lifetimes (lasting decades with little or no maintenance;[34] full-cycle lifetimes quoted for flywheels range from in excess of 105, up to 107, cycles of use),[35] high energy density (100–130 W·h/kg, or 360–500 kJ/kg),[35][36] and large maximum power output.

Gravitational potential energy storage with solid masses[edit]

Several solutions exist to store potential energy without using pumped storage hydro electricity. By rising or descending solid masses, it is possible to store or release electricity with an electric motor and a generator. ARES system made possible to do it by using rails to move concrete weights from a low elevation point to a high elevation point.[37] Stratosolar proposes to use winches supported by buoyant platforms at an altitude of 20 kilometers, to rise or lower solid masses in order to store or release electricity on the grid.[38] Sink Float Solutions proposes to use winches supported by a barge for taking advantage of 4 kilometers (13,000 feet) elevation difference between the surface and the seabed.[39] On their respective websites, ARES assumes a capital cost for the storage capacity of around 60% of pump storage hydroelectricity,[40] Stratosolar $100/kWh[41] and Sink Float Solutions $25/kWh (with 4000 meters depth) and $50/kWh (with 2000 meters depth).[39]

A newer concept called potential energy storage or gravity energy storage system, has generated some proposals, at least one of which was under active development in 2013 in the U.S. state of Nevada in association with the California independent system operator.[42][43][44] Whereas pumped hydro storage is a form of potential energy storage that uses water, the newer schemes are predicated on the movement of solid masses (such as hopper rail cars filled with plain earth driven by electric locomotives) from lower to higher elevations. The masses can then be stored there at a higher elevation with no loss of efficiency until power is required to be returned to the grid, at which time the masses are returned to their lower elevation storage site, generating electricity on their way down.[22]

Advantages of one such system, called Advanced Rail Energy Storage (ARES), include the indefinite storage of potential energy with no efficiency losses over time, the low bulk filler material costs when earth or rocks are used, the non-usage of water resources in areas where water is scarce, plus, since water is unused in that scheme there is no efficiency lost due to evaporation on hot days, one of several efficiency issues encountered with most pumped hydro storage reservoirs.[45] As of 2014 ARES has started initial planning on a commercial-scale project in Nevada near its California border, partnered with Valley Electric Association Inc.[22]

Thermal storage[edit]

District heating accumulation tower from Theiss near Krems an der Donau in Lower Austria with a thermal capacity of 2 GWh

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.[6] This ice storage is produced when electrical utility rates are lower.[46] 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.[47]

Ice storage air conditioning[edit]

Air conditioning based on stored ice for thermal energy storage has become an accepted commercial technology in the 21st century. This is practical because of water's large heat of fusion: the melting of one metric ton of ice (approximately one cubic metre in size) can capture 334 megajoules (MJ) (317,000 BTU) of thermal energy.

Replacing existing air conditioning systems with ice storage based air conditioning offers a cost-effective energy storage method, enabling surplus wind energy and other such intermittent energy sources to be stored for use in chilling air at a later time, possibly months later. The most widely used form of this technology can be found in campus-wide air conditioning or chilled water systems of large buildings. Air conditioning systems, especially in commercial buildings, are the biggest contributors to peak electrical loads seen on hot summer days in various countries. In this application, a standard chiller runs at night to produce an ice pile. Water then circulates through the pile during the day to produce chilled water that would normally be the chiller's daytime output.

A partial storage system minimizes capital investment by running the chillers nearly 24 hours a day. At night, they produce ice for storage and during the day they chill water for the air conditioning system. Water circulating through the melting ice augments their production. Such a system usually runs in ice-making mode for 16 to 18 hours a day and in ice-melting mode for six hours a day. Capital expenditures are minimized because the chillers can be just 40 - 50% of the size needed for a conventional design. Ice storage sufficient to store half a day's rejected heat is usually adequate.

A full storage system minimizes the cost of energy to run that system by entirely shutting off the chillers during peak load hours. The capital cost is higher, as such a system requires somewhat larger chillers than those from a partial storage system, and a larger ice storage system.

Latent heat thermal energy storage (LHTES)[edit]

Latent heat thermal energy storage systems works with materials with high latent heat (heat of fusion) capacity. These kind of materials are known as phase change materials (PCMs). The main advantage of these materials is that the latent heat storage capacity of them is much more than sensible heat. Therefore, in a specific range of temperature by phase changing from solid to liquid a large magnitude of thermal energy could be stored in the material and then, in the other time the stored heat could be released and used again.


Rechargeable battery[edit]

A rechargeable battery bank used as an uninterruptible power supply in a data center
Main article: Rechargeable battery

A rechargeable battery, also called a storage battery or accumulator, is a type of electrical battery. It comprises one or more electrochemical cells, and is a type of energy accumulator. It is known as a 'secondary cell' because its electrochemical reactions are electrically reversible. Rechargeable batteries come in many different shapes and sizes, ranging from button cells to megawatt systems connected to stabilize an electrical distribution network. Several different combinations of chemicals are commonly used, including: lead–acid, nickel cadmium (NiCd), nickel metal hydride (NiMH), lithium-ion (Li-ion), and lithium ion polymer (Li-ion polymer).

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

Common rechargeable battery chemistries include:

  • Lead–acid battery: Lead acid batteries still hold the largest market share for all electric storage products today. A single lead-acid cell produces about 2V when charged. In the charged state the metallic lead negative electrode and the lead sulfate positive electrode are immersed in a dilute sulfuric acid (H2SO4) electrolyte. In the discharge process electrons are pushed out of the cell as lead sulfate is formed at the negative electrode while the electrolyte is reduced to water.
  • Nickel–cadmium battery (NiCd): Uses nickel oxide hydroxide and metallic cadmium as electrodes. Cadmium is a toxic element, and was banned for most uses by the European Union in 2004. Nickel–cadmium batteries have been almost completely superseded by nickel–metal hydride (NiMH) batteries.
  • Nickel–metal hydride battery (NiMH): First commercial types were available in 1989.[48] These are now a common consumer and industrial type. The battery has a hydrogen-absorbing alloy for the negative electrode instead of cadmium.
  • Lithium-ion battery: The technology behind the lithium-ion battery has not yet fully reached maturity. However, the batteries are the type of choice in many consumer electronics and have one of the best energy-to-mass ratios and a very slow loss of charge when not in use.
  • Lithium-ion polymer battery: These batteries are light in weight and can be made in any shape desired.
Flow battery[edit]

A flow battery is a type of rechargeable battery where rechargeability is provided by two chemical components dissolved in liquids contained within the system and separated by a membrane. Ion exchange (providing flow of electric current) occurs through the membrane while both liquids circulate in their own respective space. Cell voltage is chemically determined by the Nernst equation and ranges, in practical applications, from 1.0 to 2.2 Volts.

A flow battery is technically akin both to a fuel cell and an electrochemical accumulator cell (electrochemical reversibility). While it has certain appealing to large market niches (such as independent scaling of power and energy (runtime), very long durability compared to conventional batteries with solid electroactive materials) current implementations have rather low areal powers which translate into the cost of power being too high for stationary energy storage. Commercial applications of most flow batteries are appealing only for long half cycle duration stationary energy storage (such as back up grid power for emergency), since increasing a system's overall energy capacity (measured in MWh) basically requires only an increase in the size of its liquid chemical storage reservoirs.


One of a fleet of electric capabuses powered by supercapacitors, at a quick-charge station-bus stop, in service during Expo 2010 Shanghai China. Charging rails can be seen suspended over the bus.
Main article: Supercapacitor

Supercapacitors, also called electric double-layer capacitors (EDLC) or ultracapacitors, are generic terms for a family of electrochemical capacitors.[49] Supercapacitors don't have conventional solid dielectrics. The capacitance value of an electrochemical capacitor is determined by two storage principles, which both contribute indivisibly to the total capacitance:[50][51][52]

Supercapacitors bridge the gap between conventional capacitors and rechargeable batteries. They store the most energy per unit volume or mass (energy density) among capacitors. They support up to 10,000 farads/1.2 volt,[53] up to 10,000 times that of electrolytic capacitors, but deliver or accept less than half as much power per unit time (power density).[49]

By contrast, while supercapacitors have energy densities that are approximately 10% of conventional batteries, their power density is generally 10 to 100 times greater. This results in much shorter charge/discharge cycles than batteries. Additionally, they will tolerate many more charge and discharge cycles than batteries.

Supercapacitors support a broad spectrum of applications, including:

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


Main article: UltraBattery

The UltraBattery is a hybrid lead-acid cell and carbon-based ultracapacitor (or supercapacitor) invented by Australia’s national research body, the Commonwealth Scientific and Industrial Research Organisation (CSIRO). The lead-acid cell and ultracapacitor share the sulfuric acid electrolyte and both are packaged into the same physical cell.[54] The UltraBattery can be manufactured with similar physical and electrical characteristics to conventional lead-acid batteries making it possible to cost-effectively replace many existing lead-acid installations with UltraBattery technology.

The key difference between conventional lead-acid batteries and UltraBattery technology is that the UltraBattery performs like an ultracapacitor when necessary and like a lead-acid cell at other times meaning it can work across a very wide range of applications. The constant cycling and fast charging and discharging necessary for applications such as renewable smoothing, grid regulation, electric and hybrid-electric vehicles can have deleterious effects on chemical batteries but are well handled by the ultracapacitive qualities of UltraBattery technology.

The UltraBattery will tolerate high charge and discharge levels and very large numbers of cycles during its lifetime, outperforming previous lead-acid cells by more than an order of magnitude.[55] In hybrid-electric vehicle tests, millions of cycles have been achieved.[56] The UltraBattery is also highly tolerant to the effects of sulfation compared with traditional lead-acid cells.[57] This means it can operate continuously in partial state of charge whereas traditional lead-acid batteries are generally held at full charge between discharge events. It is generally electrically inefficient to fully charge a lead-acid battery so by decreasing time spent in the top region of charge the UltraBattery achieves high efficiencies, typically between 85 to 95% DC-DC.[58]

The technology has been installed in Australia and the U.S.A. on the megawatt scale performing frequency regulation and renewable smoothing applications.

Other chemical[edit]


Main article: Hydrogen economy

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.

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[citation needed], external storage will become important.[59] 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.[60] A similar project has been going on since 2004 on Utsira, a small Norwegian island municipality.

Energy losses involved in the hydrogen storage cycle come from the electrolysis of water, liquification or compression of the hydrogen, and conversion to electricity.[61]

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.[citation needed]

Underground hydrogen storage[edit]

Underground hydrogen storage is the practice of hydrogen storage in underground caverns, salt domes and depleted oil and gas fields.[62][63] Large quantities of gaseous hydrogen have been stored in underground caverns by Imperial Chemical Industries (ICI) for many years without any difficulties.[64] The European project Hyunder indicated in 2013 that for the storage of wind and solar energy with underground hydrogen a total of 85 caverns would be required. The energy storage required is well beyond what PHES and CAES systems can provide.[65]

Power to gas[edit]

Main article: Power to gas

Power to gas is a technology which converts electrical power to a gaseous fuel. There are currently three methods in use; all use electricity to split water into hydrogen and oxygen by means of electrolysis.

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

The excess power or off–peak power generated by wind turbines or solar arrays can then be used for load balancing in an energy grid. Using the existing natural gas system for hydrogen, fuel cell manufacturer Hydrogenics and natural gas distributor Enbridge have teamed up to develop such a power to gas system in Canada.[66]

Hydrogen can be stored in natural gas pipeline networks. Before switching to natural gas, German gas networks were operated using towngas, which for the most part consisted of hydrogen. The storage capacity of the German natural gas network, which also contains many man-made caverns (artificial caves produced by mining), 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%) (besides High-voltage direct current). The use of the existing natural gas pipelines for hydrogen was studied by NaturalHy.[67]


Main article: biofuel

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.[68] 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.


Methane is the simplest hydrocarbon with the molecular formula CH4. Methane can be produced from electricity using power to gas technologies.[69] Methane is more easily stored than hydrogen and the transportation, storage and combustion infrastructure (pipelines, gasometers, power plants) are mature.

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

In the combustion of methane, carbon dioxide (CO2) and water are produced. The carbon dioxide created 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 therefore be a resource having economic value as a component of an energy storage vector, not a cost as in Carbon capture and storage.

Aluminium, boron, silicon, and zinc[edit]

Aluminium,[70] Boron,[71] silicon,[72] lithium, and zinc[73] have been proposed as energy storage solutions.

Electrical methods[edit]


Main article: capacitor
This mylar-film, oil-filled capacitor has very low inductance and low resistance, to provide the high-power (70 megawatts) and the very high speed (1.2 microsecond) discharges needed to operate a dye laser.

A capacitor (originally known as a 'condenser') is a passive two-terminal electrical component used to store energy electrostatically in an electric field. The forms of practical capacitors vary widely, but all contain at least two electrical conductors (plates) separated by a dielectric (i.e., insulator). A capacitor can store electric energy when disconnected from its charging circuit, so it can be used like a temporary battery, or like other types of rechargeable energy storage system.[74] Capacitors are also commonly used in electronic devices to maintain power supply while batteries are being changed. (This prevents loss of information in volatile memory.) Conventional capacitors provide less than 360 joules per kilogram of energy density, whereas a conventional alkaline battery has a density of 590 kJ/kg.

Unlike a resistor, a capacitor does not dissipate energy. Instead, a capacitor stores energy in the form of an electrostatic field between its plates. When there is a potential difference across the conductors (e.g., when a capacitor is attached across a battery), an electric field develops across the dielectric, causing positive charge (+Q) to collect on one plate and negative charge (-Q) to collect on the other plate. If a battery has been attached to a capacitor for a sufficient amount of time, no current can flow through the capacitor. However, if an accelerating or alternating voltage is applied across the leads of the capacitor, a displacement current can flow.

The capacitance is greater when there is a narrower separation between conductors and when the conductors have a larger surface area. In practice, the dielectric between the plates passes a small amount of leakage current and also has an electric field strength limit, known as the breakdown voltage. The conductors and leads introduce an undesired inductance and resistance.

Capacitors are widely used in electronic circuits for blocking direct current while allowing alternating current to pass. In analog filter networks, they smooth the output of power supplies. In resonant circuits they tune radios to particular frequencies. In electric power transmission systems they stabilize voltage and power flow.[75]

Electromagnetic storage[edit]

Superconducting Magnetic Energy Storage (SMES) systems store energy in a 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.[76]

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%.[77]

Due to the energy requirements of refrigeration and the high cost of superconducting wire, 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 baseload power at night and meeting peak loads during the day.[76]

Broad listing[edit]

The following list includes natural and other non-commercial types of energy storage. in addition to those designed for use in industry and commerce:

Energy Storage Use Cases[edit]

The United States Department of Energy International Energy Storage Database (IESDB), is a free-access database of energy storage projects and policies funded by the United States Department of Energy Office of Electricity and Sandia National Labs.[78]

Energy Storage On Investment[edit]

A metric for calculating the energy efficiency of storage systems is Energy Storage On energy Invested (ESOI) which is the useful energy used to make the storage system divided into the lifetime energy storage. For lithium ion batteries this is around 10, and for lead acid batteries it is about 2. Other forms of storage such as pumped hydroelectric storage can be expected to have high ESOI, such as 210.[79]


Research in energy storage is being coordinated by several governments.

The German Federal government has allocated €200M (approximately US$270M) for advanced research, as well as providing a further €50M to subsidize battery storage for use with residential rooftop solar panels, according to a representative of the German Energy Storage Association.[80]

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.[6][81][82] In one technical assessment by the Carnegie Mellon Electricity Industry Centre, economic goals could be met with batteries if energy storage were achievable at a capital cost of $30 to $50 per kilowatt-hour of storage capacity.[6]

In 2014, several research and test centers opened to evaluate energy storage technologies and efficacy. Among them in the United States was the Advanced Systems Test Laboratory at the University of Wisconsin at Madison in Wisconsin State, which partnered with the multinational conglomerate (and battery manufacturer) Johnson Controls.[83] The laboratory was created as part of the university's newly opened Wisconsin Energy Institute. Their goals include the evaluation of state-of-the-art and next generation electric vehicle batteries, including the use of those batteries when they are connected to the electrical grid in order to supplement it during demand peaks, according to Professor Tom Jahns.[83]

Also in 2014, the State of New York unveiled its New York Battery and Energy Storage Technology (NY-BEST) Test and Commercialization Center at Eastman Business Park in Rochester, New York, at a cost of $23 million for its almost 1,700 m2 laboratory. The center, a consortium, also includes the Center for Future Energy Systems, a collaboration between Cornell University of Ithaca, New York and the Rensselaer Polytechnic Institute in Troy, New York. NY-BEST will conduct testing, validation and independent certification of diverse forms of energy storage intended for commercial use. The center's director stated there were currently 3,000 New Yorkers working in the energy storage industry, expected to eventually grow to 40,000 as the sector matures.[84]

In the United Kingdom, some fourteen industry and government agencies allied themselves with seven British universities in May 2014 to create the SUPERGEN Energy Storage Hub in order to assist in the coordination of energy storage technology research and development.[85][86]

Germany's Siemens AG conglomerate began commissioning a production-research plant to open in 2015 at the Zentrum für Sonnenenergie und Wasserstoff (ZSW, the German Center for Solar Energy and Hydrogen Research in the State of Baden-Württemberg), a collaboration of industry plus universities in Stuttgart, Ulm and Widderstall, staffed by approximately 350 scientists, researchers, engineers, and technicians. The plant will develop new near-production manufacturing materials and processes (NPMM&P) using a computerized Supervisory Control and Data Acquisition (SCADA) system. Its goals will enable the expansion of rechargeable battery production with both increased quality and reduced manufacturing costs.[87][88]

See also[edit]


  1. ^ a b Erik Ingebretsen; Tor Haakon Glimsdal Johansen (July 16, 2013). "The Potential of Pumped Hydro Storage in Norway (abstract)" (PDF). Retrieved February 16, 2014. 
  2. ^ a b c d Wald, Matthew, L. Wind Drives Growing Use of Batteries, The New York Times, July 28, 2010, p. B1.
  3. ^ Norway: Energy storage for Europe (video report), Deutsche Welle, July 7, 2014. Retrieved July 21, 2014.
  4. ^ Diane Cardwell (July 16, 2013). "Battery Seen as Way to Cut Heat-Related Power Losses". The New York Times. Retrieved July 17, 2013. 
  5. ^ Thermal Energy Storage Myths, website.
  6. ^ a b c d e f Wald, Matthew L. Ice or Molten Salt, Not Batteries, to Store Energy, The New York Times website, April 21, 2014, and in print on April 22, 2014, p. F7 of the New York edition. Retrieved May 29, 2014.
  7. ^ Wong, B. (2013). Integrating solar & heat pumps. [1].
  8. ^ Wong, B. (2011). Drake Landing Solar Community.
  9. ^ Hellström, G. (19 May 2008), Large-Scale Applications of Ground-Source Heat Pumps in Sweden, IEA Heat Pump Annex 29 Workshop, Zurich.
  10. ^ a b Gies, Erica. Global Clean Energy: A Storage Solution Is in the Air, International Herald Tribune online website, October 1, 2012, and in print on October 2, 2012, in The International Herald Tribune. Retrieved from website, March 19, 2013.
  11. ^ Talbot, David (December 21, 2009). "A Quantum Leap in Battery Design". Technology Review (MIT). Retrieved June 9, 2011. 
  12. ^ Hubler, Alfred W. (Jan–Feb 2009). "Digital Batteries". Complexity (Wiley Periodicals, Inc) 14 (3): 7–8. doi:10.1002/cplx.20275. 
  13. ^ Edwin Cartlidge (18 November 2011). "Saving for a rainy day". Science (Vol 334). pp. 922–924. 
  14. ^ 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.
  15. ^ Scénario NégaWatt 2011 (France)
  16. ^ a b Weeks, Jennifer (2010-04-28). "U.S. Electrical Grid Undergoes Massive Transition to Connect to Renewables". Scientific American. Retrieved 2010-05-04. 
  17. ^ Canadian Solar Community Sets New World Record for Energy Efficiency and Innovation, Natural Resources Canada, 5 Oct. 2012.
  18. ^ 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.
  19. ^ Wald, Matthew L. Taming Unruly Wind Power, The New York Times, November 4, 2011, and in print on November 5, 2011, p. B1 of the New York edition.
  20. ^ Wald, Matthew, L. Sudden Surplus Calls for Quick Thinking, The New York Times online website, July 7, 2010.
  21. ^ The Technology, website. Retrieved April 19, 2014.
  22. ^ a b c 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, website, March 25, 2014. Retrieved March 28, 2014.
  23. ^ Robert A. Huggins (1 September 2010). Energy Storage. Springer. p. 60. ISBN 978-1-4419-1023-3. 
  24. ^ a b "Energy storage - Packing some power". The Economist. 2011-03-03. Retrieved 2012-03-11. 
  25. ^ Jacob, Thierry.Pumped storage in Switzerland - an outlook beyond 2000 Stucky. Accessed: 13 February 2012.
  26. ^ Levine, Jonah G. Pumped Hydroelectric Energy Storage and Spatial Diversity of Wind Resources as Methods of Improving Utilization of Renewable Energy Sources page 6, University of Colorado, December 2007. Accessed: 12 February 2012.
  27. ^ Yang, Chi-Jen. Pumped Hydroelectric Storage Duke University. Accessed: 12 February 2012.
  28. ^ Energy Storage Hawaiian Electric Company. Accessed: 13 February 2012.
  29. ^ Wild, Matthew, L. Wind Drives Growing Use of Batteries, New York Times, July 28, 2010, pp.B1.
  30. ^ Wald, Matthew L. Using Compressed Air To Store Up Electricity, The New York Times, September 29, 1991. Discusses the McIntosh CAES storage facility.
  31. ^ Diem, William. Experimental car is powered by air: French developer works on making it practical for real-world driving,, March 18, 2004. Retrieved from on March 19, 2013.
  32. ^ Slashdot: Car Powered by Compressed Air, website, 2004.03.18
  33. ^ Torotrak Toroidal variable drive CVT, retrieved June 7, 2007.
  34. ^ a b Castelvecchi, Davide (May 19, 2007). "Spinning into control: High-tech reincarnations of an ancient way of storing energy". Science News 171 (20): 312–313. doi:10.1002/scin.2007.5591712010. 
  35. ^ a b Storage Technology Report, ST6 Flywheel
  36. ^ "Next-gen Of Flywheel Energy Storage". Product Design & Development. Retrieved 2009-05-21. 
  37. ^
  38. ^
  39. ^ a b
  40. ^
  41. ^
  42. ^ Packing Some Power: Energy Technology: Better ways of storing energy are needed if electricity systems are to become cleaner and more efficient, The Economist, March 3, 2012
  43. ^ Downing, Louise. Ski Lifts Help Open $25 Billion Market for Storing Power, Bloomberg News online, September 6, 2012
  44. ^ Kernan, Aedan. Storing Energy on Rail Tracks, website, 30 October 2013
  45. ^ Markham, Derek. Using Trains and Gravity for Energy Storage, website, April 3, 2013
  46. ^ Fire and Ice based storage, website, April 2009.
  47. ^ Air-Conditioning, Heating and Refrigeration Institute, Fundamentals of HVAC/R, Page 1263
  48. ^ Katerina E. Aifantis et al, High Energy Density Lithium Batteries: Materials, Engineering, Applications Wiley-VCH, 2010 ISBN 3-527-32407-0 page 66
  49. ^ a b B. E. Conway (1999). Electrochemical Supercapacitors: Scientific Fundamentals and Technological Applications. Berlin: Springer. ISBN 0306457369. Retrieved May 2, 2013. 
  50. ^ Marin S. Halper, James C. Ellenbogen (March 2006). Supercapacitors: A Brief Overview (PDF) (Technical report). MITRE Nanosystems Group. Retrieved 2014-01-20. 
  51. ^ Elzbieta Frackowiak, Francois Beguin, PERGAMON, Carbon 39 (2001) 937–950, Carbon materials for the electrochemical storage of energy in Capacitors [2]
  52. ^ Yu.M. Volfkovich, A.A. Mikhailin, D.A. Bograchev, V.E. Sosenkin and V.S. Bagotsky, Studies of Supercapacitor Carbon Electrodes with High Pseudocapacitance, A. N. Frumkin Institute of Physical Chemistry and Electrochemistry, Russian Academy of Sciences, Moscow, Russia, Dr. Ujjal Kumar Sur (Ed.), ISBN 978-953-307-830-4, free PDF copy available here.
  53. ^ "Capacitor cells - ELTON". Retrieved 2013-05-29. 
  54. ^ "UltraBattery". Ecoult. Retrieved 18 August 2014. 
  55. ^ "Ultrabattery Test Results for Utility Cycling Applications" (PDF). Ecoult. Retrieved 18 August 2014. 
  56. ^ "Further demonstration of the VRLA-type UltraBattery® under medium-HEV duty and development of the flooded-type UltraBattery® for micro-HEV applications". Journal of Power Sources 195: 1241. 2010. doi:10.1016/j.jpowsour.2009.08.080. 
  57. ^ "UltraBattery Test Results for Utility Cycling Applications". International Seminar on Double Layer Capacitors And Hybrid Energy Storage Devices: 195. 2008. Retrieved 18 August 2014. 
  58. ^ "Development of UltraBattery®. Furukawa Review" (PDF). Furukawa. Retrieved 18 August 2014. 
  59. ^ "Solar Hydrogen Fuel Cell Water Heater (Educational Stand)". Scribd. 
  60. ^ Oprisan, Morel. Introduction of Hydrogen Technologies to Ramea Island, CANMET Technology Innovation Centre, Natural Resources Canada, April 2007.
  61. ^ Zyga, Lisa (2006-12-11:15-44). "Why A Hydrogen Economy Doesn't Make Sense". web site ( Retrieved 2007-11-17.  Check date values in: |date= (help)
  62. ^ Eberle, Ulrich and Rittmar von Helmolt. "Sustainable transportation based on electric vehicle concepts: a brief overview". Energy & Environmental Science, Royal Society of Chemistry, 14 May 2010, accessed 2 August 2011
  63. ^ Benchmarking of selected storage options
  64. ^ 1994 - ECN abstract
  65. ^ Storing renewable energy: Is hydrogen a viable solution?
  66. ^ Anscombe, Nadya (4 June 2012). "Energy storage: Could hydrogen be the answer?". Solar Novus Today. Retrieved 3 November 2012. 
  67. ^ Naturalhy, website Archived January 18, 2012 at the Wayback Machine
  68. ^ Clean Alternative Fuels: Fischer-Tropsch, Transportation and Air Quality, Transportation and Regional Programs Division, United States Environmental Protection Agency, March 2002.
  69. ^ 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. 
  70. ^ White Paper: A Novel Method For Grid Energy Storage Using Aluminum Fuel, Alchemy Research, April 2012.
  71. ^ Cowan, Graham R.L. Boron: A Better Energy Carrier than Hydrogen?, June 12, 2007
  72. ^ 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.
  73. ^ Engineer-Poet. Ergosphere Blog, Zinc: Miracle metal?, June 29, 2005.
  74. ^ Miller, Charles. Illustrated Guide to the National Electrical Code, p. 445 (Cengage Learning 2011).
  75. ^ Bird, John (2010). Electrical and Electronic Principles and Technology. Routledge. pp. 63–76. ISBN 9780080890562. Retrieved 2013-03-17. 
  76. ^ a b Hassenzahl, W.V., "Applied Superconductivity: Superconductivity, An Enabling Technology For 21st Century Power Systems?", IEEE Transactions on Magnetics, pp. 1447–1453, Vol. 11, Iss. 1, March 2001.
  77. ^ Cheung K.Y.C; Cheung S.T.H.; Navin De Silvia; Juvonen; Singh; Woo J.J. Large-Scale Energy Storage Systems, Imperial College London: ISE2, 2002/2003.
  78. ^ DOE Global Energy Storage Database, United States Department of Energy, Office of Electricity and Sandia National Labs.
  79. ^
  80. ^ Galbraith, Kate. Filling the Gaps in the Flow of Renewable Energy, The New York Times, October 22, 2013.
  81. ^ 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.
  82. ^ Wald, Matthew. Green Blog: The Convoluted Economics of Storing Energy, The New York Times, January 3, 2012.
  83. ^ a b Content, Thomas. Johnson Controls, UW Open Energy Storage Systems Test Lab In Madison, Milwaukee, Wisconsin: Milwaukee Journal Sentinel, May 5, 2014.
  84. ^ Loudon, Bennett J. NY-BEST Opens $23M Energy Storage Center, Rochester, New York: Democrat and Chronicle, April 30, 2014.
  85. ^ SUPERGEN hub to set the direction of the UK’s energy storage, website, May 6, 2014. Retrieved May 8, 2014.
  86. ^ New SUPERGEN Hub to set UK's energy storage course, website, May 2, 2014.
  87. ^ Aschenbrenner, Norbert. Test Plant For Automated Battery Production, website, May 06, 2014. Retrieved May 8, 2014.
  88. ^ Produktionsforschung | Prozessentwicklung und Produktionstechnik für große Lithium-Ionen-Zellen, Zentrum für Sonnenenergie- und Wasserstoff-Forschung Baden-Württemberg website, 2011. (German)

Further reading[edit]


Journals and papers:

  • Chen, Haisheng; Thang Ngoc Cong; Wei Yang; Chunqing Tan; Yongliang Li; Yulong Ding. Progress in electrical energy storage system: A critical review, Progress in Natural Science, accepted July 2, 2008, published in Vol. 19, 2009, pp. 291–312, doi: 10.1016/j.pnsc.2008.07.014. Sourced from the National Natural Science Foundation of China and the Chinese Academy of Sciences. Published by Elsevier and Science in China Press. Synopsis: a review of electrical energy storage technologies for stationary applications. Retrieved from on May 13, 2014. (PDF)
  • 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. (PDF)
  • Whittingham, M. Stanley. History, Evolution, and Future Status of Energy Storage, Proceedings of the IEEE, manuscript accepted February 20, 2012, date of publication April 16, 2012; date of current version May 10, 2012, published in Proceedings of the IEEE, Vol. 100, May 13, 2012, 0018-9219, pp. 1518–1534, doi: 10.1109/JPROC.2012.219017. Retrieved from May 13, 2014. Synopsis: A discussion of the important aspects of energy storage including emerging battery technologies and the importance of storage systems in key application areas, including electronic devices, transportation, and the utility grid. (PDF)

External links[edit]