Energy storage
Energy storage is the storing of some form of energy that can be drawn upon at a later time to perform some useful operation.  A device that stores energy is sometimes called an accumulator. All forms of energy are either potential energy (eg. chemical, gravitational or electrical energy) or kinetic energy (eg. thermal energy).  A wind up clock stores potential energy (in this case mechanical, in the spring tension), a battery stores readily convertible chemical energy to keep a clock chip in a computer running (electrically) even when the computer is turned off, and a hydroelectric dam stores power in a reservoir as gravitational potential energy. Even food is a form of energy storage, chemical in this case.

# History
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. Unfortunately, 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 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
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
- Chemical
Hydrogen
Biofuels
- Hydrogen
- Biofuels
- Electrochemical
Batteries
Flow batteries
Fuel cells
- Batteries
- Flow batteries
- Fuel cells
- Electrical
Capacitor
Supercapacitor
Superconducting magnetic energy storage (SMES)
- Capacitor
- Supercapacitor
- Superconducting magnetic energy storage (SMES)
- Mechanical
Compressed air energy storage (CAES)
Flywheel energy storage
Hydraulic accumulator
Hydroelectric energy storage
Spring
- Compressed air energy storage (CAES)
- Flywheel energy storage
- Hydraulic accumulator
- Hydroelectric energy storage
- Spring
- Thermal
Molten salt 
Cryogenic liquid air or nitrogen
Seasonal thermal store
Solar pond
Hot bricks
Steam accumulator
Fireless locomotive
- Molten salt 
- Cryogenic liquid air or nitrogen
- Seasonal thermal store
- Solar pond
- Hot bricks
- Steam accumulator
- Fireless locomotive

## Hydrogen
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. 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.
Hydrogen can be used as a fuel for all types of internal and external combustion heat engines and turbines (with adjustments to compensate for the difference between, say, diesel fluid and hydrogen gas). Hydrogen fueled heat engines can be optimized to operate at higher thermal efficiencies than traditional heat engines using traditional hydrocarbon fuels. The increased thermodynamic efficiency, and reduced pollution, would be beneficial, but they are not produced in quantity largely because hydrogen is not industrially available.
Sufficiently purified hydrogen can also be used to power electrochemical engines, such as the proton exchange membrane (PEM) fuel cell. Hydrogen fuel cells can be more efficient than hydrogen fueled heat engines, and thus much more efficient than hydrocarbon fuel heat engines. They are also less polluting. Several companies are attempting to develop reliable, inexpensive PEM fuel cells. However, designs are not sufficiently developed to be routinely mass produced. The limited quantities available for purchase are hand made and much more expensive than conventional heat engines.
Hydrogen production in quantities sufficient to replace existing hydrocarbon fuels is not possible. Such production will require more energy than is currently being used, and require large capital investment in hydrogen production plants. Because of the increased costs, hydrogen is not yet in widespread use. If the cost of  greenhouse gas production is fully included into the market price of hydrocarbon fuels, hydrogen fuels may become more attractive commercially, providing clean, efficient power for our homes, businesses and vehicles.
Disadvantages of hydrogen include a low energy density per volume (even when highly compressed) compared to traditional hydrocarbon fuels, changing such things as the volumes of fuel required for equivalent performance. And, for many hydrogen production methods, there is a significant loss of energy during the conversion. Some production methods, for instance, electrolytic generation from water, are more efficient.

## Biofuels
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 country's diesel from coal for similar reasons. A long term oil price above 35 USD 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. And compressed natural gas is being used in special circumstances  fuel, for instance in busses for some mass transit agencies.

## Synthetic hydrocarbon fuel
Carbon dioxide in the atmosphere has been, experimentally, converted into hydrocarbon fuel with the help of energy from another source. To be useful industrially, the energy will probably have to come from sunlight using, perhaps, future artificial photosynthesis technology. Another alternative for the energy is electricity or heat from solar energy or nuclear power. Compared to hydrogen, many hydrocarbons fuels have the advantage of being immediately usable in existing engine technology and existing fuel distribution infrastructures. 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. If usable on a wide scale, this approach may help in the long term to avoid some of the deleterious effects of greenhouse gas emission.

## Boron, silicon, and zinc
Boron, silicon, and zinc have been proposed as energy storage solutions.

## Mechanical storage
Energy can be stored in water pumped to a higher elevation, in compressed air, or in spinning flywheels, but mechanical methods of storing energy on a large scale are expensive and water pumping systems require considerable capital investment.
Several companies have done preliminary design work for vehicles using compressed air power.

## 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 the existing throttling capacity is already committed to handling load variations. Further development of intermittent renewable power will require some combination of grid energy storage, demand response, and spot pricing. Intermittent energy sources is 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 could increase the overall reliability of the grid.
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.