Energy storage

Further information: Wind power grid integration and List of energy storage projects

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 the capture of energy produced at one time for use at a later time. A device that stores energy is sometimes called an accumulator. Energy comes in multiple forms including radiation, chemical, gravitational potential, electrical potential, electricity, elevated temperature, latent heat and kinetic. Energy storage involves converting energy from forms that are difficult to store to more conveniently or economically storable forms.

Some technologies provide short-term energy storage, while others can endure for much longer.

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. 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. Food (which is made by the same process as fossil fuels) is a form of energy stored in chemical form.

Ice storage tanks store ice (thermal energy in the form of latent heat) frozen by otherwise wasted energy at night to meet peak daytime demand for cooling. The energy isn't stored directly, but the effect on daytime consumption is equivalent.

History

Prehistory

The energy present at the initial formation of the universe is stored in stars such as the Sun, and is 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 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 was used to attack invaders who came within range.

Methods

Mechanical storage

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

Hydroelectricity

Main article: Hydroelectricity

Hydroelectric dams with reservoirs can be operated to provide peak generation at times of peak demand. Water is stored in the reservoir during periods of low demand and released when demand is high. The net effect is similar to pumped storage, but without the pumping loss.

While a hydroelectric dam does not directly store energy from other generating units, it behaves equivalently by lowering output in periods of excess electricity from other sources. In this mode, dams are one of the most efficient forms of energy storage, because only the timing of its generation changes. Hydroelectric turbines have a start-up time on the order of a few minutes.

Pumped-storage

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.

Main article: Pumped-storage hydroelectricity

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

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

Compressed air

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

Main article: Compressed air energy storage

Compressed air energy storage (CAES) uses surplus energy to compress air for subsequent electricity generation v. Small scale systems have long been used in such applications as propulsion of mine locomotives. The compressed air is stored in an underground reservoir.

Compression of air creates heat; the air is warmer after compression. Expansion requires heat. If no extra heat is added, the air will be much colder after expansion. If the heat generated during compression can be stored and used during expansion, efficiency improves considerably. A CAES system can deal with the heat in three ways. Air storage can be adiabatic, diabatic, or isothermal. Another approach uses compressed air to power vehicles.

Flywheel energy storage

The main components of a typical flywheel.

Main article: Flywheel energy storage

Flywheel energy storage (FES) works by accelerating a rotor (flywheel) to a very high speed, holding energy as rotational energy. When energy is extracted, the flywheel's rotational speed declines as a consequence of conservation of energy; adding energy 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 under consideration.

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. Such flywheels can reach maximum speed ("charge") in a matter of minutes. The flywheel system is connected to a combination electric motor/generator.

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.

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

Gravitational potential energy storage with solid masses

Changing the altitude of solid masses can store or release energy via an elevating system driven by an electric motor/generator.

Companies such as Energy Cache and Advanced Rail Energy Storage (ARES) are working on this. ARES uses rails to move concrete weights up and down. Stratosolar proposes to use winches supported by buoyant platforms at an altitude of 20 kilometers, to raise./lower solid masses. Sink Float Solutions proposes to use winches supported by an ocean barge for taking advantage of a 4 km (13,000 ft) elevation difference between the surface and the seabed. ARES estimated a capital cost for the storage capacity of around 60% of pump storage hydroelectricity, Stratosolar $100/kWh and Sink Float Solutions $25/kWh (4000 m depth) and $50/kWh (with 2000 m depth).

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

ARES claimed advantages including indefinite storage with no energy losses, low costs when earth/rocks are used and conservation of water resources.

Thermal storage

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

Main articles: Thermal energy storage and Seasonal thermal energy storage

Thermal storage is the temporary storage or removal of heat. TES 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.

An example is Alberta, Canada's Drake Landing Solar Community, for which 97% of the year-round heat is provided by solar-thermal collectors on the garage roofs, with a borehole thermal energy store (BTES) being the enabling technology. STES projects often have paybacks in the four-to-six year range. 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 to raise the temperature to 80 °C (176 °F) for distribution. When surplus wind generated electricity is not available, a gas-fired boiler is used. Twenty percent of Braestrup's heat is solar.

Latent heat thermal energy storage (LHTES)

Latent heat thermal energy storage systems works with materials with high latent heat (heat of fusion) capacity, known as phase change materials (PCMs). The main advantage of these materials is that their latent heat storage capacity is much more than sensible heat. In a specific temperature range, phase changes from solid to liquid absorbs a large amount of thermal energy for later use.

Electrochemical

Rechargeable battery

A rechargeable battery bank used as an uninterruptible power supply in a data center

Main article: Rechargeable battery

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

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

Common rechargeable battery chemistries include:

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

Flow battery

Main articles: Flow battery and Vanadium redox battery

A flow battery operates by passing a solution over a membrane where ions are exchanged to charge/discharge the cell. Cell voltage is chemically determined by the Nernst equation and ranges, in practical applications, from 1.0 to 2.2 V. Its storage capacity is a function of the volume of the tanks holding the solution.

A flow battery is technically akin both to a fuel cell and an electrochemical accumulator cell. Commercial applications are for long half-cycle storage such as backup grid power.

Supercapacitor

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 that do not have conventional solid dielectrics. capacitance is determined by two storage principles, double-layer capacitance and pseudocapacitance.

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

While supercapacitors have energy densities that are approximately 10% of batteries, their power density is generally 10 to 100 times greater. This results in much shorter charge/discharge cycles. 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

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 unit. The UltraBattery can be manufactured with similar physical and electrical characteristics to conventional lead-acid batteries making it possible to cost-effectively replace many lead-acid applications.

The UltraBattery tolerates high charge and discharge levels and endures large numbers of cycles, outperforming previous lead-acid cells by more than an order of magnitude. In hybrid-electric vehicle tests, millions of cycles have been achieved. The UltraBattery is also highly tolerant to the effects of sulfation compared with traditional lead-acid cells. 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.

The UltraBattery can work across a wide range of applications. The constant cycling and fast charging and discharging necessary for applications such as grid regulation and leveling and electric vehicles can damage chemical batteries, but are well handled by the ultracapacitive qualities of UltraBattery technology. The technology has been installed in Australia and the US on the megawatt scale, performing frequency regulation and renewable smoothing applications.

Other chemical

Hydrogen

Main article: Hydrogen storage

Atomic hydrogen is a form of stored energy. It can produce electricity via a hydrogen fuel cell.

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

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

About 50 kW·h (180 MJ) of solar energy is required to produce a kilogram of hydrogen, so the cost of the electricity is crucial. At $0.03/kWh, a common off-peak high-voltage line rate in the United States, hydrogen costs $1.50 a kilogram for the electricity, equivalent to $1.50/gallon for gasoline. Other costs include the electrolyzer plant, hydrogen compressors or liquefaction, storage and transportation.

Underground hydrogen storage

Underground hydrogen storage is the practice of hydrogen storage in underground caverns, salt domes and depleted oil and gas fields. Large quantities of gaseous hydrogen have been stored in underground caverns by Imperial Chemical Industries (ICI) for many years without any difficulties. The European Hyunder project indicated in 2013 that storage of wind and solar energy using underground hydrogen would require 85 caverns. This amount of storage is well beyond what PHES and CAES systems can provide.

Power to gas

Main article: Power to gas

Power to gas is a technology which converts electricity into a gaseous fuel. The three commercial methods use electricity to reduce water into hydrogen and oxygen by means of electrolysis.

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

Using the existing natural gas system for hydrogen, fuel cell manufacturer Hydrogenics and natural gas distributor Enbridge teamed up to develop a power to gas system in Canada.

Hydrogen can be stored in natural gas pipeline networks. Before switching to natural gas, German gas networks were operated using towngas, which was mostly hydrogen. The German network's storage capacity, which includes many man-made mining caverns, is more than 200,000 GW·h, enough for several months of energy requirement. By comparison, the capacity of all German pumped storage power plants amounts to 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%). The use of the natural gas pipelines for hydrogen was studied by NaturalHy.

Biofuels

Main article: Biofuel

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

Methane

Main article: Substitute natural gas

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

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

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

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

Aluminium, boron, silicon, and zinc

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

Electrical methods

Capacitor

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

Capacitors store energy in an electrostatic field between their plates. Given a potential difference across the conductors (e.g., when a capacitor is attached across a battery), an electric field develops across the dielectric, causing positive charge (+Q) to collect on one plate and negative charge (-Q) to collect on the other plate. If a battery is attached to a capacitor for a sufficient amount of time, no current can flow through the capacitor. However, if an accelerating or alternating voltage is applied across the leads of the capacitor, a displacement current can flow.

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

Research is assessing the quantum effects of nanoscale capacitors for digital quantum batteries.

Superconducting magnetics

Main article: Superconducting magnetic energy storage

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

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

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

Interseasonal thermal storage

seasonal thermal energy storage (STES) allows heat or cold to be used months after it was collected from waste energy or natural sources. The material can be stored in contained aquifers, clusters of boreholes in geological substrates such as sand or crystalline bedrock, in lined pits filled with gravel and water, or water-filled mines.

Outline

See also: Outline of energy storage

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

• Mechanical
  • Compressed air energy storage (CAES)
  • Fireless locomotive
  • Flywheel energy storage
  • Gravitational potential energy (device)
  • Hydraulic accumulator
  • Liquid nitrogen
  • Pumped-storage hydroelectricity
• Electrical
  • Capacitor
  • Superconducting magnetic energy storage (SMES)
• Biological
  • Glycogen
  • Starch
• Electrochemical
  • Flow battery
  • Rechargeable battery
  • Supercapacitor
  • UltraBattery
• Thermal
  • Brick storage heater
  • Cryogenic liquid air or nitrogen
  • Eutectic system
  • Ice Storage
  • Molten salt
  • Phase Change Material
  • Seasonal thermal energy storage
  • Solar pond
  • Steam accumulator
  • Thermal energy storage (general)
• Chemical
  • Biofuels
  • Hydrated salts
  • Hydrogen
  • Hydrogen peroxide
  • Power to gas
  • Vanadium pentoxide

Applications

Mills

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.

Home energy storage

In 2015, multiple manufacturers announced rechargeable battery systems for storing energy, generally to hold surplus energy from home solar/wind generation.

Tesla Motors announced the first two models of theTesla Powerwall. One is a 10 kWh weekly cycle version for backup applications and the other is a 7 kWh version for daily cycle applications.

Enphase Energy announced an integrated system that allows home users to store, monitor and manage electricity. The system stores 1.2 kWh hours of energy and 275W/500W power output.

Grid electricity

Main article: Grid energy storage

Renewable energy storage

Construction of the Salt Tanks which provide efficient thermal energy storage so that output can be provided after the sun goes down, and output can be scheduled to meet demand requirements. The 280 MW Solana Generating Station is designed to provide six hours of energy storage. This allows the plant to generate about 38 percent of its rated capacity over the course of a year.

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.

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

The first method of large scale storage was pumped-storage hydroelectricity. Areas of the world such as Norway, Washington and Oregon states and Wales have used geographic features to supply elevated reservoirs, using electricity to fill them. When needed, the water passes through generators and converts the potential energy back to electricity. Pumped storage in Norway has an instantaneous capacity of 25–30 GW expandable to 60 GW—enough to be "Europe's battery".

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

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

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

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

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

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

Generation

Chemical fuels remain the dominant form of energy storage for electricity generation. Natural gas is crowding out other forms, such as oil and coal.

Air conditioning

Main article: Ice storage air conditioning

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

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

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

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

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

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

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

Transport

Liquid hydrocarbon fuels are the most commonly used forms of energy storage for use in transportation. Other energy carriers such as hydrogen can be used to avoid producing greenhouse gases.

Electronics

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.

Use cases

Main article: United States Department of Energy International Energy Storage Database

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

Metrics

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 generally have higher ESOI, such as 210.

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

Research

Germany

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.

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

United States

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

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

United Kingdom

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

See also

• Distributed generation
• Energy
• Energy density
• Efficient energy use
• Grid energy storage
• Hybrid renewable energy system
• List of energy topics
• List of energy storage projects
• Power transmission
• Renewable energy

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. 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. 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. 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. 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

• 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).