Energy storage as a natural process is as old as the universe itself - the energy present at the initial creation 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). Energy storage systems in commercial use today can be broadly categorized as mechanical, electrical, chemical, biological, thermal and nuclear.
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.
Energy storage became a dominant factor in economic development with the widespread introduction of electricity and refined chemical fuels, such as gasoline, kerosene and natural gas in the late 1800s. Unlike other common energy storage used in prior use, such as wood or coal, electricity must be used as it is generated and cannot be stored on anything other than a minor scale. Electricity is transmitted in a closed circuit, and for essentially any practical purpose cannot be stored as electrical energy. This meant that changes in demand could not be accommodated without either cutting supplies (eg, via brownouts or blackouts) or arranging for a storage technique.
An early solution to the problem of storing energy for electrical purposes was the development of the battery, an electrochemical storage device. It has been of limited use in electric power systems due to small capacity and high cost. A similar possible solution with the same type of problems is the capacitor.
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, biodiesel and hydrogen. All of these chemicals are readily converted to mechanical energy and then to electrical energy using heat engines (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.
Electrochemical devices called fuel cells were invented about the same time as the battery. However, for many reasons, fuel cells were not well developed until the advent of manned spaceflight (the Gemini Program) when lightweight, non-thermal (ie, efficient) sources of electricity were required in spacecraft. Fuel cell development has increased in recent years to an attempt to increase conversion efficiency of chemical energy stored in hydrocarbon or hydrogen fuels into electricity.
At this time, liquid hydrocarbon fuels are the dominant forms of energy storage for use in transportation. However, these produce greenhouse gases when used to power cars, trucks, trains, ships and aircraft. Carbon-free energy carriers, such as hydrogen, or carbon-neutral energy carriers, such as some forms of ethanol or biodiesel, are being sought in response to concerns about the possible consequences of greenhouse gas emissions.
Some areas of the world (Washington and Oregon in the USA, and Wales in the United Kingdom are examples) 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 fall through turbine generators to retrieve the energy when demand peaks.
Several other technologies have also been investigated, such as flywheels or compressed air storage in underground caverns, but to date no widely available solution to the challenge of mass energy storage has been deployed commercially.
Grid energy storage lets energy producers send excess electricity over the electricity transmission grid to temporary electricity storage sites that become energy producers when electricity demand is greater. Grid energy storage is particularly important in matching supply and demand over a 24 hour period of time.
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. Hydrogen may be used in conventional internal combustion engines, or in fuel cells which convert chemical energy directly to electricity without flames, similar to the way the human body burns fuel. The hydrogen production requires either reforming natural gas with steam, or, for a possibly renewable and more ecologic source, the electrolysis of water into hydrogen and oxygen. The former process has carbon dioxide as a by-product. With high pressure electrolysis, the greenhouse burden depends on the source of the power.
Energy losses are involved in the hydrogen storage cycle of 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 with biohydrogen, liquification or compression, and conversion to electricity.
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 developed 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.
Nuclear advocates note that using nuclear power to manufacture hydrogen would help solve plant inefficiencies. Here the plant would be run continuously at full capacity, with perhaps all the output being supplied to the grid in peak periods, and any not needed to meet demand being used to make hydrogen at other times. This would mean far better efficiency for the nuclear power plants. High temperature (950-1,000°C) gas cooled nuclear generation IV reactors have the potential to separate hydrogen from water by thermochemical means using nuclear heat as in the sulfur-iodine cycle.
The efficiency for hydrogen storage is typically 50 to 60% overall, which is lower than pumped storage systems or batteries. About 50 kWh (180 MJ) is required to produce a kilogram of hydrogen by electrolysis, 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 U.S., this means hydrogen costs $1.50 a kilogram for the electricity, equivalent to $1.50 a US 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 cavern hydrogen storage is the practice of hydrogen storage in underground caverns. Large quantities of gaseous hydrogen are stored in underground caverns by ICI for many years without any difficulties. The storage of large quantities of hydrogen underground can function as grid energy storage which is essential for the hydrogen economy.
Compressed air energy storage 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 existence in McIntosh, Alabama since 1991 and has run successfully.
Several companies have done preliminary design work for vehicles using compressed air power.
Non-intermittent renewable energy sources include hydroelectric power, geothermal power, solar thermal, tidal power, Energy tower, ocean thermal energy conversion, high altitude airborne wind turbines, biofuel, and solar power satellites. Solar photovoltaics, although technically intermittent, produce electricity largely during peak periods (ie, daylight), and hence do reduce the need for peak power generation, though somewhat unreliably in most areas since weather conditions interfere with terrestrially mounted solar cells.
On the demand side, demand response programs, which send market pricing signals to consumers (or their equipment), can be a very effective way of managing variations in electricity production. For example, electrically powered hydrogen production can be set to increase when electricity is being produced beyond current demand (and prices will be lowest), and conversely, hot water heaters can be automatically set to a lower temperature when demand is high and pricing is also high.