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.
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).
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.
Storage methods
Different kinds of energy permits various 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. 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.[33] A dam which impounds a reservoir can store and release a corresponding amount of energy, by raising and lowering its reservoir. - Pumped-storage hydroelectricityWorldwide, 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. PSH reported energy efficiency varies in practice between 70% and 80%, with some claiming 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 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 storageCompressed 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. 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.
- Flywheel energy storageFlywheel 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. - Gravitational potential energy storageA 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. 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.
Thermal storage
Thermal storage is the temporary storage or removal of heat for later use. An example of thermal storage is the storage of solar heat energy during the day to be used at a later time for heating at night. In the HVAC/R field, this type of application using thermal storage for heating is less common than using thermal storage for cooling. An example of the storage of "cold" heat removal for later use is ice made during the cooler night time hours for use during the hot daylight hours. This ice storage is produced when electrical utility rates are lower. This is often referred to as "off-peak" cooling.
Electrochemical
Chemistry also offers facility to transform and store energy.
- Rechargeable batteryA 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).
- Flow batteryA 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.
- SupercapacitorsSupercapacitors, also called electric double-layer capacitors (EDLC) or ultracapacitors, are generic terms for a family of electrochemical capacitors. 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.
- UltraBatteryThe 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. 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.
Other chemical
- HydrogenHydrogen 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.
- Underground hydrogen storageUnderground 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 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.
- Power to gasPower 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. - BiofuelsVarious biofuels such as biodiesel, straight vegetable oil, alcohol fuels, or biomass can be used to replace hydrocarbon fuels. Various chemical processes can convert the carbon and hydrogen in coal, natural gas, plant and animal biomass, and organic wastes into short hydrocarbons suitable as replacements for existing hydrocarbon fuels. Examples are Fischer–Tropsch diesel, methanol, dimethyl ether, or syngas. This diesel source was used extensively in World War II in Germany, with limited access to crude oil supplies. Today South Africa produces most of the country's diesel from coal for similar reasons. A long term oil price above US$35/bbl may make such synthetic liquid fuels economical on a large scale (see coal). Some of the energy in the original source is lost in the conversion process. Historically, coal itself has been used directly for transportation purposes in vehicles and boats using steam engines. Additionally, compressed natural gas is also used as fuel, for instance for buses with some mass transit agencies.
- MethaneMethane 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 (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.
- Aluminium, boron, silicon, and zincAluminium, Boron, silicon, lithium, and zinc have been proposed as energy storage solutions.
Electrical methods
- CapacitorA 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. 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.
- Electromagnetic storageSuperconducting 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.
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