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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 battery stores readily convertible chemical energy to operate a mobile phone, and a hydroelectric dam stores energy in a reservoir as gravitational potential energy. Ice storage tanks store ice (thermal energy in the form of latent heat) at night to meet peak demand for cooling. Fossil fuels such as coal and gasoline store ancient energy derived from sunlight by organisms that later died, became buried and over time were then converted into these fuels. Even food (which is made by the same process as fossil fuels) is a form of energy stored in chemical form.
Energy storage as a natural process is as old as the universe itself - the energy present at the initial formation of the universe has been stored in stars such as the Sun, and is now being used by humans directly (e.g. through solar heating), or indirectly (e.g. by growing crops or conversion into electricity in solar cells).
As a purposeful activity, energy storage has existed since pre-history, though it was often not explicitly recognized as such. An example of deliberate mechanical energy storage is the use of logs or boulders as defensive measures in ancient forts—the logs or boulders were collected at the top of a hill or wall, and the energy thus stored used to attack invaders who came within range.
A more recent application is the control of waterways to drive water mills for processing grain or powering machinery. Complex systems of reservoirs and dams were constructed to store and release water (and the potential energy it contained) when required.
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.
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. Until recently electrical energy has not been converted and stored on a major scale, however new efforts to that effect began in the 21st century.
An 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 capabilities; some of which, as of 2013, showed promise of being competitive with alternative methods. A similar possible solution to deal with the intermittency issue of solar and wind energy is found in the capacitor.
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 to the reservoirs, then letting the water pass through turbine generators to retrieve the energy when electrical demands peak.
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 nighttime.
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 4-6 year range.
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 production of greenhouse gases.
Electrochemical devices called fuel cells were invented about the same time as the battery in the 19th Century. However, for many reasons, fuel cells were not well-developed until the advent of manned spaceflight (such as the Gemini Program in the U.S.) when lightweight, non-thermal (and therefore efficient) sources of electricity were required in spacecraft. Fuel cell development has increased in recent years due to an attempt to increase conversion efficiency of chemical energy stored in hydrocarbon or hydrogen fuels into electricity.
Several other technologies have also been investigated, such as flywheels, which can store kinetic energy, and compressed air storage that can be pumped into underground caverns and abandoned mines.
Another method used at the Solar Project and the Solar Tres Power Tower uses molten salt to store solar power and then dispatch that power as needed. The system pumps molten salt through a tower heated by the sun's rays. Insulated containers 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 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.
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.
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. 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. 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. and the hydrogen storage cycle of production for the stationary fuel cell applications like MicroCHP at 93 % 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 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 gas is a technology which converts electrical power to a gas fuel. There are 2 methods, the first is to use the electricity for water splitting and inject the resulting hydrogen into the natural gas grid. The second less efficient method is used to convert carbon dioxide and water to methane, (see natural gas) using electrolysis and the Sabatier reaction. The excess power or off peak power generated by wind generators or solar arrays is then used for load balancing in the energy grid. Using the existing natural gas system for hydrogen Fuel cell maker Hydrogenics and natural gas distributor Enbridge have teamed up to develop such a power to gas system in Canada.
Pipeline storage of hydrogen where a natural gas network is used for the storage of hydrogen. Before switching to natural gas, the German gas networks were operated using towngas, which for the most part consisted of hydrogen. The storage capacity of the German natural gas network 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%). The use of the existing natural gas pipelines for hydrogen was studied by NaturalHy
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. 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 is the simplest hydrocarbon with the molecular formula CH4. Methane could be produced from electricity of renewable energies using power to gas technologies. Methane can be stored more easily than hydrogen and the transportation, storage and combustion infrastructure are mature (pipelines, gasometers, power plants).
As hydrogen and oxygen are produced in the electrolysis of water,
Methane would be stored and used to produce electricity later. Produced water would be recycled back to the electrolysis stage, reducing the need for new pure water. In the electrolysis stage oxygen would also be stored for methane combustion in a pure oxygen environment in an adjacent power plant, eliminating e.g. nitrogen oxides. In the combustion of methane, carbon dioxide and water are produced.
Produced carbon dioxide 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 be a resource having economic value as a component of an energy storage vector, not a cost as in CCS (Carbon capture and storage).
A mass of 1 kg, elevated to a height of 1,000 m 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. Admittedly, this is a bit of an unfair comparison, but it makes it easy to see how it is possible to store more energy in 1 m3 of cheap rock or sand than 1 m3 of lead–acid battery, even if the battery is also moved to a higher elevation, not just charged.
Compressed air energy storage (CAES) 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 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. The paper was Submitted to the Conoco Philips Energy Prize April 2008. This paper precedes ALL others and in that paper Walker defined the CO2 energy storage cycle. Several projects sponsored by the DOE are now underway to develop the body of technology pioneered by Terry L. Walker in 2008.
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:
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. Other options include recourse to peaking power plants, methane storage (excess renewable electricity to hydrogen via electrolysis, combine with CO2 (low to neutral CO2 system) to produce methane (synthetic natural gas 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.
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%.
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.
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 U.S. DOE, Office of Electricity and Sandia National Labs.
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