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.
All forms of energy are either potential energy (e.g. chemical, gravitational, electrical energy, temperature differential, latent heat, etc.) or kinetic energy (e.g. momentum). Some technologies provide only short-term energy storage, and others can be very long-term such as power to gas using hydrogen 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 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. A very traditional way doing this on a large scale is by the use of pumped-storage hydroelectricity. Some areas of the world such as Washington and Oregon in the United States, and Wales in the United Kingdom, have used geographic features to store large quantities of water in elevated reservoirs, using excess electricity at times of low demand to pump water up into their reservoirs. The facilities then release the water to pass through turbine generators and convert the stored potential energy back to electricity when electrical demand peaks. In another example, pumped-storage hydroelectricity in Norway has an instantaneous capacity of 25–30 GW, which could be expanded to 60 GW —enough to be the battery of Europe.
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. As of 2013 some of the newer battery chemistries have shown promise of being competitive with alternate energy storage methods.
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 super capacitors.
Grid energy storage
Grid energy storage (also called large-scale energy storage) refers to the methods used to store electricity on a large scale within an electrical power grid. Electrical energy is stored during times when production (from power plants) exceeds consumption and the stores are used at times when consumption exceeds production. In this way, electricity production need not be drastically scaled up and down to meet momentary consumption – instead, production is maintained at a more constant level. This has the advantage that fuel-based power plants (i.e. coal, oil, gas) can be more efficiently and easily operated at constant production levels.
As of March 2012, pumped-storage hydroelectricity (PSH) is the largest-capacity form of grid energy storage available; the Electric Power Research Institute (EPRI) reports that PSH accounts for more than 99% of bulk storage capacity worldwide, around 127,000 MW. PSH energy efficiency varies in practice between 70% to 75%.
In particular, the use of grid-connected intermittent energy sources such as photovoltaics and wind turbines can benefit from grid energy storage. Energy derived from photovoltaic and wind sources is variable by nature – the amount of electrical energy produced varies with time, day of the week, season, and random factors such as the weather. In an electrical power grid without energy storage, energy sources that rely on energy stored within fuels (coal, oil, gas) must be scaled up and down to match the rise and fall of energy production from intermittent energy sources (see load following power plant).
Thus, grid energy storage is one method that the operator of an electrical power grid can use to adapt energy production to energy consumption, both of which can vary over time. This is done to increase efficiency and lower the cost of energy production, or to facilitate the use of intermittent energy sources.
An alternate approach to achieve the same effect as grid energy storage is to use a smart grid communication infrastructure to enable Demand response (DR). The core effect of both of these technologies is to shift energy usage and production on the grid from one time to another.
A report released in December 2013 by the United States Department of Energy further describes the potential benefits of energy storage technologies to the electric grid: “Modernizing the electric system will help the nation meet the challenge of handling projected energy needs—including addressing climate change by integrating more energy from renewable sources and enhancing efficiency from non-renewable energy processes. Advances to the electric grid must maintain a robust and resilient electricity delivery system, and energy storage can play a significant role in meeting these challenges by improving the operating capabilities of the grid, lowering cost and ensuring high reliability, as well as deferring and reducing infrastructure investments. Finally, energy storage can be instrumental for emergency preparedness because of its ability to provide backup power as well as grid stabilization services.” The report was written by a core group of developers representing Office of Electricity Delivery and Energy Reliability, ARPA-E, Office of Science, Office of Energy Efficiency and Renewable Energy, Sandia National Laboratories, and Pacific Northwest National Laboratory; all of whom are engaged in the development of grid energy storage
Renewable energy storage
Many renewable energy sources (most notably solar and wind) produce intermittent power. Wherever intermittent power sources reach high levels of grid penetration, energy storage becomes one option to provide reliable energy supplies. 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 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, 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) and smart grids with advanced energy demand management. The latter involves bringing “prices to devices”, i.e. making electrical equipment and appliances able to adjust their operation to seek the lowest spot price of electricity. On a grid with a high penetration of renewables, low spot prices would correspond to times of high availability of wind and/or sunshine.
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). 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. 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%.
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 power plant, reduced to its lowest possible operating level, leaving a large swath of the region running almost completely on renewable energy.
Short-term thermal storage, as heat or cold
In the 1980s, a number of manufacturers carefully researched thermal energy storage (TES) to meet the growing demand for air conditioning during peak hours. Today, several companies manufacture TES systems. The most popular form of thermal energy storage for cooling is ice storage, since it can store more energy in less space than water storage and it is also less costly than energy recovered via fuel cells or flywheels. Thermal storage has cost-effectively shifted gigawatts of power away from daytime peak usage periods, and in 2009 was used in over 3,300 buildings in over 35 countries. It works by creating ice at night when electricity is usually less costly, and then using the ice to cool the air in buildings during the hotter daytime periods. Latent heat can also be stored in technical phase change materials (PCMs), besides ice. These can for example be encapsulated in wall and ceiling panels, to moderate room temperatures between daytime and night time.
Interseasonal thermal storage, as heat or cold
Another class of thermal storage that has been developed since the 1970s that is now frequently employed is seasonal thermal energy storage (STES). It allows heat or cold to be used even months after it was collected from waste energy or natural sources, even in an opposing season. The thermal storage may be accomplished in contained aquifers, clusters of boreholes in geological substrates as diverse as sand or crystalline bedrock, in lined pits filled with gravel and water, or water-filled mines. An example is Alberta, Canada’s Drake Landing Solar Community, for which 97% of the year-round heat is provided by solar-thermal collectors on the garage roofs, with a borehole thermal energy store (BTES) being the enabling technology. STES projects often have paybacks in the four-to-six year range.
Energy storage in chemical fuels
Chemical fuels have become the dominant form of energy storage, both in electrical generation and energy transportation. Chemical fuels in common use are processed coal, gasoline, diesel fuel, natural gas, liquefied petroleum gas (LPG), propane, butane, ethanol and biodiesel. All of these materials are readily converted to mechanical energy and then to electrical energy using heat engines (via turbines or other internal combustion engines, or boilers or other external combustion engines) used for electrical power generation. Heat-engine-powered generators are nearly universal, ranging from small engines producing only a few kilowatts to utility-scale generators with ratings up to 800 megawatts. A key disadvantage to hydrocarbon fuels are their significant emissions of greenhouse gases that contribute to global warming, as well as other significant pollutants emitted by the dirtier fuel sources such as coal and gasoline.
Liquid hydrocarbon fuels are the most commonly used forms of energy storage for use in transportation, but because the by-products 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.
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. To store the same mass of water, if increased in temperature by 2.34 Celsius, requires the same amount of energy.
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.
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. 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 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. 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. 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. 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.
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.
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. 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; 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 large maximum power output.
Thermal storage is the temporary storage or removal of heat for later use. An example of thermal storage is the storage of solar heat energy during the day to be used at a later time for heating at night. In the HVAC/R field, this type of application using thermal storage for heating is less common than using thermal storage for cooling. An example of the storage of “cold” heat removal for later use is ice made during the cooler night time hours for use during the hot daylight hours. This ice storage is produced when electrical utility rates are lower. This is often referred to as “off-peak” cooling.
When used for the proper application with the appropriate design, off-peak cooling systems can lower energy costs. The U.S. Green Building Council has developed the Leadership in Energy and Environmental Design (LEED) program to encourage the design of high-performance buildings that will help protect our environment. The increased levels of energy performance by utilizing off-peak cooling may qualify of credits toward LEED Certification.
The advantages of thermal storage are:
Commercial electrical rates are lower at night;
it takes less energy to make ice when the ambient temperature is cool at night. Source energy (energy from the power plant) is saved.
a smaller, less costly system can do the job of a much larger unit by running for more hours.
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.
Another advanced method used at the Solar Project in the United States and the Solar Tres Power Tower in Spain utilizes 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.