Energy storage

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.

Prehistory

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.^[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

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

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

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

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

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

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

Mechanical storage

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.

Hydroelectricity

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

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

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

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

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.^[37][38][39] 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.^[40] 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

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.^[41] 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.^[42]

Ice storage air conditioning

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)

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.

Electrochemical

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.^[43] 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

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 electrical 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.

Supercapacitors

Supercapacitors, also called electric double-layer capacitors (EDLC) or ultracapacitors, are generic terms for a family of electrochemical capacitors.^[44] 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:^[45][46][47]

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,^[48] up to 10,000 times that of electrolytic capacitors, but deliver or accept less than half as much power per unit time (power density).^[44]

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

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.^[49] 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.^[50] In hybrid-electric vehicle tests, millions of cycles have been achieved.^[51] The UltraBattery is also highly tolerant to the effects of sulfation compared with traditional lead-acid cells.^[52] 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.^[53]

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

Hydrogen

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 , external storage will become important.^[54] 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.^[55] 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.^[56] and the hydrogen storage cycle of production for the stationary fuel cell applications like MicroCHP at 93 %^[57] 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.

Underground hydrogen storage

Underground hydrogen storage is the practice of hydrogen storage in underground caverns, salt domes and depleted oil and gas fields.^[58][59] Large quantities of gaseous hydrogen have been stored in underground caverns by Imperial Chemical Industries (ICI) for many years without any difficulties.^[60] The European project Hyunder indicated in 2013 that for the storage of wind and solar energy, an additional 85 caverns are required as it can't be covered by PHES and CAES systems.^[61]

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.^[62]

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.^[63]

Biofuels

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.^[64] 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

Methane is the simplest hydrocarbon with the molecular formula CH₄. Methane can be produced from electricity using power to gas technologies.^[65] 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

Aluminium,^[66] Boron,^[67] silicon,^[68] lithium, and zinc^[69] have been proposed as energy storage solutions.

Electrical methods

Capacitor

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.^[70] 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.^[71]

Electromagnetic storage

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.^[72]

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%.^[73]

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.^[72]

Broad listing

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

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.^[74]

Research

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.^[75]

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][76][77] 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.^[78] 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.^[78]

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.^[79]

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.^[80][81]

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.^[82][83]

See also

References

Further reading

General:

• Barringer, Felicity. Green Blog: In California, a Grid Storage Mandate, The New York Times, October 1, 2010.
• Baxter, Richard. Energy Storage: A Nontechnical Guide, September 10, 2005. ISBN 1-59370-027-X
• Belli, Brita. ‘Battery University’ Aims to Train a Work Force for Next-Generation Energy Storage, The New York Times, April 8, 2013. Discusses a professional development program at San Jose State University.
• Browne, Malcome W. New Hunt for Ideal Energy Storage System, The New York Times, January 6, 1988. Discusses superconducting magnetic energy storage.

• Cardwell, Diane. Battery Seen as Way to Cut Heat-Related Power Losses, July 16, 2013 online and July 17, 2013 in print on July 17, 2013, on page B1 in the New York City edition of The New York Times, p. B1. Discusses Eos Energy Systems' Zinc–air batteries.
• Cardwell, Diane. SolarCity to Use Batteries From Tesla for Energy Storage, December 4, 2013 on line, and December 5, 2013 in the New York City edition of The New York Times, p. B-2. Discusses SolarCity, DemandLogic and Tesla Motors.

• Galbraith, Kate. In Presidio, a Grasp at the Holy Grail of Energy Storage, The New York Times, November 6, 2010.
• Galbraith, Kate. Filling the Gaps in the Flow of Renewable Energy, The New York Times, October 22, 2013.
• 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.
• Gies, Erica. I.H.T. Special Report: Global Clean Energy: A Storage Solution Is in the Air, The International Herald Tribune, October 1, 2012. Retrieved from NYTimes.com website. Discusses SustainX, LightSail Energy, and both adiabatic and isothermal compressed air energy storage.

• Kumagai, Jean. How Much Energy Storage Do You Need to Back Up the London Array?, IEEE Spectrum, July 15, 2014.
• LaMonica, Martin. A Big Bet on How to Store Energy, Cheaply, June 24, 2014, Smithsonian magazine website. Retrieved from Smithsonian.com July 20, 2014. Discusses CAES, SustainX, LightSail Energy, and others.
• Malewitz, Jim. Project Tests New Storage for Energy, The New York Times website, June 21, 2014, and in print on June 22, 2014, on p. A27A of the National edition.
• 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 Groupe S.A.
• Wald, Matthew L. Seeking to Start a Silicon Valley for Battery Science, The New York Times, November 30, 2012.
• Wald, Matthew L. Arizona Utility Tries Storing Solar Energy for Use in the Dark, The New York Times, October 18, 2013, p. B1 (New York edition); also published online at NYTimes.com on October 17, 2013. Retrieved October 18, 2013. Discusses BrightSource Energy, Electric Power Research Institute, Ivanpah Solar Power Facility project, and the Solana Generating Station project.
• Wald, Matthew L. Catching Rays in California, and Storing Them, The New York Times, December 23, 2013 online, and in print on December 24, 2013, p. B3 of the New York edition. Discusses the California Public Utilities Commission's role in mandating the use of energy storage within that state.
• Wald, Matthew L. From Harvard, a Cheaper Storage Battery, The New York Times, January 8, 2014. Discusses research into flow-batteries utilizing carbon-based molecules called quinones.

• 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.

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 ac.els-cdn.com 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 ieeexplore.ieee.org May 13, 2014. Synopsis: A discussion of the important aspects of energy storage including emerging battery technologies and the importance of storage systems in key application areas, including electronic devices, transportation, and the utility grid. (PDF)

External links

Wikimedia Commons has media related to Energy storage.

• U.S. Dept of Energy - Energy Storage Systems Government research center on energy storage technology.
• U.S. Dept of Energy - International Energy Storage Database The DOE International Energy Storage Database provides free, up-to-date information on grid-connected energy storage projects and relevant state and federal policies.
• US Energy 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).

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