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'Energy storage' is the storing of some form of energy that can be drawn upon at a later time to perform some useful operation. All forms of energy are either potential energy or kinetic energy. A wind up clock stores potential energy in the spring, a battery stores electrical energy to keep a clock chip in a computer running even when the computer is turned off, and a hydroelectric dam stores power in a reservoir. Even food is a form of energy storage.

Contents

History

Grid energy storage

Storage methods

Hydrogen

Biofuels

Synthetic hydrocarbon fuel

Boron, silicon, and zinc

Mechanical storage

Intermittent power

See also

External links

History

Energy storage as a natural process is billions of years old - the energy produced in the initial creation of the Universe has been stored in stars such as our Sun, and is now being used by humans directly (e.g. through solar cells) or indirectly (e.g. by growing crops). Energy storage systems in commercial use today can be broadly categorized into the following forms: mechanical, electrical, chemical, biological, thermal and nuclear.
As a purposeful activity, energy storage has certainly existed since pre-history, though it was often not recognized as such. An example of mechanical storage would be the use of logs or boulders as defensive measures in ancient forts - the logs or boulders would be collected at the top of a hill, and the energy thus stored would be released as a defense against invaders.
A more recent application was the control of waterways to power water mills for processing grain or powering machinery. Often 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 major factor in economic development, however, with the widespread introduction of electricity and refined chemical fuels, such as gasoline, kerosine and natural gas in the late 1800s. Unlike the other common energy carriers used at the time, such as wood or coal, electricity had to be used as it was generated. Electricity must be transmitted in a closed circuit and for practical purposes cannot be stored as electrical energy. This meant that changes in demand were difficult to cater for without either cutting supplies at times, or having expensive excess capacity.
An early solution to the problem of storing electricity was the development of the battery, an electrochemical conversion device of limited use in electric power systems due to its small capacity and relatively high cost. A similar solution with the same type of problems is the capacitor.
Refined chemical fuels have become the dominant form of energy storage, both in the electrical generating sector and the transportation sector. Refined chemical fuels in common use are processed coal, gasoline, diesel fuel, natural gas, liquefied petroleum gas (LPG or propane), ethanol, biodiesel and hydrogen. All of these chemical energy carriers are readily converted to mechanical energy and then to electrical energy with heat engines that are used to power electical generators. Heat engine powered generators are ubiquitous and range in size from small automobile alternators that produce a few kilowatts to utility-scale generators with ratings up to 800 megawatts.
Electrochemical devices called fuel cells were also invented at the same time as the battery. However, for many reasons, fuel cells were not developed until the advent of manned spaceflight (the Gemini Program) when lightweight, efficient sources of electricity were required to keep astronauts alive in a very demanding environment. Many types of fuel cells are now being commercialized to allow the efficient conversion of chemical energy stored in refined hydrocarbon or hydrogen fuels directly into electricity.
At this time, liquid hydrocarbon fuels are the dominant forms of energy storage for the transportation sector. Unfortunately liquid hydrocarbon fuels emit greenhouse gases when combusted in heat engines 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 aggressively being sought in response to concerns about greenhouse gas emissions from the production, distribution and use of energy.
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 reservoirs at the top of hills, using excess electricity at times of low demand to pump water into the reservoirs, then letting the water fall through generators to retrieve the energy when demand peaks.
A number of other technologies have 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 commercialized.

Grid energy storage

Main articles: Grid energy storage


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.

Storage methods

★ Electrochemical

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★ Batteries

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★ Flow batteries

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★ Fuel cells

★ Electrical

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★ Capacitor

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★ Supercapacitor

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★ Superconducting magnetic energy storage (SMES)

★ Mechanical

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★ Compressed air energy storage (CAES)

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★ Flywheel energy storage

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★ Hydraulic accumulator

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★ Hydroelectric energy storage

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★ Spring

★ Thermal

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★ Molten salt [1]

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★ Cryogenic liquid air or nitrogen

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★ Seasonal thermal store

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★ Solar pond

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★ Hot bricks

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★ Steam accumulator

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★ Fireless locomotive

Hydrogen

There is a widely held misconception that hydrogen is an alternative energy source. Hydrogen is a chemical energy carrier, just like gasoline, ethanol or natural gas. The unique characteristic of hydrogen is that it is the only carbon-free or zero-emission chemical energy carrier. Hydrogen is a widely used industrial chemical that can be produced from any primary energy source. Most of the world's production is by the thermal reformation of natural gas (methane) into hydrogen that is used immediately to refine petroleum into gasoline, diesel fuel and other petrochemicals. The carbon dioxide produced by the reforming process is either captured and processed into liquid carbon dioxide or vented to the atmosphere.
Hydrogen can be used to fuel all types of internal and external combustion heat engines, including ubiqitous, highly reliable and inexpensive reciprocating engines and turbines. In addition to being a zero-emission fuel, hydrogen fueled heat engines can be optimized by the manufacturer to operate at much higher thermal efficiencies than heat engines powered with traditional hydrocarbon fuels. Although engineers have demonstrated the superior performance and environmental benefits of hydrogen fueled piston engines, engine manufacturers do not mass produce hydrogen engines for consumer markets.
Pure hydrogen can also be used to power electrochemical engines, such as the much publicized proton exchange membrane (PEM) fuel cell. Hydrogen powered fuel cells hold the promise of being even more efficient in electrical output than hydrogen fueled heat engines and much more efficient and cleaner than hydrocarbon fuel heat engines. Massive investments have been made by several companies to develop reliable, inexpensive PEM fuel cells. However, these devices are not mass produced and the limited quantities available for purchase are hand made and much more expensive than conventional heat engines.
Just like gasoline, diesel fuel, ethanol or biodiesel, there are no uncombined hydrogen reserves on Earth that could be tapped directly to provide energy. Just like any other hydrocarbon fuel, hydrogen must be manufactured from a primary energy source, like natural gas, crude oil, coal, biomass, solar energy or uranium. Uncombined hydrogen can be produced from any and all of these primary energy sources. Hydrogen is the only chemical energy carrier that can be produced from any primary energy source, including renewable energy sources, and converted into useful work without producing pollution. Because hydrogen is produced and distributed in such huge quantities, the technology needed to build infrastructure to serve wholesale and retail energy markets is proven, reliable and commercially available.
The environmental and efficiency benefits of hydrogen fuels are not yet fully recognized by energy markets. hydrogen is the only chemical energy carrier that has the potential to be produced, distributed and used without discharging carbon to the atmosphere. When the cost of greenhouse gas pollution is fully incorporated into the market price of traditional hydrocarbon fuels, then hydrogen fuels will begin to play an important role in development of highly efficient, distributed energy systems that supply clean, efficient power for our homes, businesses and vehicles.

Biofuels

Various biofuels such as biodiesel, straight vegetable oil, alcohol fuels, or biomass can be broken down into transportation fuel. 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 transportation fuels. Examples of such fuels are Fischer-Tropsch diesel, methanol, dimethyl ether, or syngas. Such diesel was used extensively in World War II by the Germans, who had limited access to crude oil supplies. Today South Africa produces most of country's diesel from coal.^[1] A long term oil price above 35 USD may make such liquid fuels economical on a large scale (See coal). Some of the energy in the original source will be lost in the conversion process. Historically coal itself has been used directly for transportation purposes in vehicles and boats using steam engines. Compressed natural gas can itself be used as a transportation fuel.

Synthetic hydrocarbon fuel

Carbon dioxide in the atmosphere can be converted into hydrocarbon fuel with the help of energy from another source. The energy can come from sunlight using future artificial photosynthesis technology.^[2][3] Another alternative for the energy is electricity or heat from solar energy or nuclear power.^[4][5] Compared to hydrogen, many hydrocarbons fuels have the advantage of reusing existing engine technology and existing fuel distribution infrastructure. Manufacturing synthetic hydrocarbon fuel reduces the amount of carbon dioxide in the atmosphere until the fuel is burned, when the same amount of carbon dioxide returns to the atmosphere.

Boron, silicon, and zinc

Boron,^[6] silicon,^[7] and zinc^[8] have been proposed as energy storage solutions.

Mechanical storage

Energy can be stored as water pumped to a higher elevation, compressed air, and spinning flywheels, but mechanical methods of storing energy have limited capacity or efficiency.
Several companies are proposing vehicles using compressed air for power.^[9][10]

Intermittent power

Many renewable energy systems produce intermittent power. Other generators on the grid can be throttled to match varying production from renewable sources, but most of this throttling capacity is already committed to handling variations in load. Further development of intermittent renewable power will require some combination of grid energy storage, demand response, and spot pricing. Intermittent energy sources may be limited to at most 20-30% of the electricity produced for the grid without such measures. If electricity distribution loss and costs are managed, then intermittent power production from many different sources would increase the overall reliability of the grid.
Renewables that are not intermittent include hydroelectric power, geothermal power, tidal power, Energy tower, ocean thermal energy conversion, high altitude airborne wind turbines, biofuel, and solar power satellites. Solar photovoltaics, although technically intermittent, produces electricity during peak periods, and hence does reduce the need for peak power plants. Demand response programs, which send market pricing signals to consumers, can be a very effective way of managing variations in electricity production; for example, hydrogen production can increase when excess electricity is being produced, and conversely, hot water heaters can be automatically set to a lower temperature when production is lower.

See also

★ Grid energy storage

★ List of energy topics

★ Energy

★ Energy Density

★ Distributed generation

★ Power Transmission

External links

★ U.S. Dept of Energy - Energy Storage Systems Government research center on energy storage technology.

★ Electricity Storage Association Good comparison of technologies.