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PowerPedia:Energy storage

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Energy storage (or power storage) is the storing of some form of energy that can be drawn upon at a later time to perform some useful operation.



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). As a purposeful activity, energy storage has certainly existed since pre-history, though it was often not recognized as such. An example 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 only became a major concern, however, with the introduction of electricity. Unlike the other common power sources at the time, such as natural gas, electricity had to be used as it was generated. 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 was the battery, but this is of limited use both due to its small capacity and relatively high cost. A similar solution with the same type of problems is the capacitor.

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, but to date no widely available, affordable solution to the challenge of mass energy storage has been found.

Details of storage

Grid energy storage

Grid energy storage is the use of various energy storage techniques to complement electric power generation plants on the transmission grid. By using the grid connection, intermittent sources of power need not be installed with their own energy storage facilities. For example, a solar panel installed on a home may produce more energy than is required during the day, which can be exported to the grid; at night, the needs of the home can be met from the grid, as if the solar energy had been stored locally.

Demand for electricity from the world's various grids varies over the course of the day and from season to season. For the most part, variation in electric demand is met by varying the amount of electrical energy supplied from primary sources, usually hydroelectric power plants and natural gas-fired turbines. Increasingly, however, operators are storing lower-cost energy produced at night, then releasing it to the grid during the peak periods of the day when it is more valuable.

Energy storage is economical when the marginal cost of electricity varies more than the energy losses of storing and retrieving it. For instance, 1.2 gigawatt-hours might be stored at night in a pumped-storage reservoir, at a cost of 1.5 cents/kilowatt-hour. The next day, 1 gigawatt-hour might be recovered, and 200 megawatt-hours lost, and sold at 4.0 cents/kilowatt-hour, for a profit of $22,000. If this profit can be realized on most days, and if the storage facility cost less than perhaps $100M, the operator makes a profit.

The marginal cost of electricity varies because of the varying economics of different kinds of generators. At one extreme, water from a dam can go through a turbine for little more than the cost of releasing it down a spillway, so the marginal cost of generation is nearly zero. Coal-fired and nuclear power plants are also low marginal cost generators, as they have high capital and maintenance costs but low fuel costs. At the other extreme, most peaking generators burn natural gas, which is expensive. Operators prefer cheaper electricity, so they run the low-marginal-cost generators most of the time, and run the more expensive ones only when necessary. This is called "economic dispatch".

Renewable supplies with prediction errors and variable production, like wind and solar power, tend to increase the net variation in electric load. Because they are not dispatchable and must run when available, power from these supplies is generally sold to grid operators for less than power available on demand. As renewable supplies become increasingly popular, this difference in price opens an increasingly large economic opportunity for grid energy storage.

Reactive electrical demand

The easiest way to deal with varying electrical loads is to decrease the variation. For decades, utilities have sold off-peak power to large consumers at lower rates, to encourage these users to shift their loads to off-peak hours, in the same way that phone companies do with individual customers. Usually, these time-dependent prices are negotiated ahead of time. In an attempt to save more money, some utilities are experimenting with selling electricity to large users at minute-by-minute spot prices, which allow those users to detect demand peaks as they happen, and shift demand to save both the user and the utility money.

Pumped water storage

In many places, pumped storage is used to even out the daily generating load, by pumping water to a high storage reservoir during off-peak hours and weekends, using the excess base-load capacity from coal or nuclear sources. During peak hours, this water can be used for hydroelectric generation, often as a high value rapid-response reserve to cover transient peaks in demand. There is over 90 GW of pumped storage in operation, which is about 3% of global generation capacity. Pumped storage recovers about 80% of the energy consumed. The chief problem with pumped storage is that it usually requires two nearby reservoirs at considerably different heights, for which there are few suitable locations, and often requires considerable capital expenditure.

Compressed air storage

Another grid energy storage method is to use off-peak electricity to compress air in compressed air energy storage, which is usually stored in an old mine or some other kind of geological feature. When electricity demand is high, the compressed air is burned with natural gas to run a turbine and generate electricity.

Thermal energy storage

Off-peak electricity can be used to make ice from water in thermal Energy Storage. The ice can be stored until the next day, when it is used to cool either the air in a large building (thereby shifting that demand off-peak) or the intake air of a combustion gas turbine generator (thereby increasing the on-peak generation capacity).

Battery storage

Battery storage was used in the very early days of electric power networks, but is no longer common. Many "off-the-grid" domestic systems rely on battery storage, but means of storing large amounts of electricity as such in giant batteries or by other means have not yet been put to general use. Batteries are generally expensive, have maintenance problems, and have limited lifespans. One possible technology for large-scale storage are large-scale flow batteries. Sodium-sulfur batteries could also be inexpensive to implement on a large scale and have been used for grid storage in Japan. Vanadium redox batteries and other types of flow batteries are also beginning to be used for energy storage including the averaging of generation from wind turbines.

If battery electric vehicles were in wide use with modern high cycle batteries [1] [2], such mobile energy sinks could be utilized for their energy storage capabilities. Vehicle to Grid technology could be employed, turning each vehicle with its 20 to 50 kWh battery pack into a load-balancing device or emergency power source. This represents 2 to 5 days per vehicle of average household requirements of 10 kWh per day, assuming annual consumption of 3650 kWh. This quantity of energy is equivalent to between 40 and 300 miles of range in such vehicles consuming 0.5 to 0.16 kWh per mile. These figures can be achieved even in home-made electric vehicle conversions.

Flywheel storage

Mechanical inertia is the basis in Flywheel energy storage. A heavy rotating disc is accelerated by an electric motor, which acts as a generator on reversal, slowing down the disc and producing electricity. Electricity is stored as the kinetic energy of the disc. Friction must be kept to a minimum to prolong the storage time. This is often achieved by placing the flywheel in a vacuum and using magnetic bearings, tending to make the method expensive. Larger flywheel speeds allow greater storage capacity but require strong materials such as steel or composite materials to resist the centrifugal forces (or rather, to provide centripetal forces). The use of carbon nanotubes as a flywheel material is being researched. The ranges of power and energy storage technically and economically achievable, however, tend to make flywheels unsuitable for general power system application; they are probably best suited to load-levelling applications on railway power systems.

Superconducting magnetic energy storage

Superconducting magnetic energy storage (SMES) systems store energy in the magnetic field created by the flow of direct current in a superconducting coil which has been cryogenically cooled to a temperature below its superconducting critical temperature. A typical SMES system includes three parts: superconducting coil, power conditioning system and cryogenically cooled refrigerator. Once the superconducting coil is charged, the current will not decay and the magnetic energy can be stored indefinitely. The stored energy can be released back to the network by discharging the coil. The power conditioning system uses an inverter/rectifier to transform alternating current (AC) power to direct current or convert DC back to AC power. The inverter/rectifier accounts for about 2-3% energy loss in each direction. SMES loses the least amount of electricity in the energy storage process compared to other methods of storing energy. SMES systems are highly efficient; the round-trip efficiency is greater than 95%.

Due to the energy requirements of refrigeration, and the limits in the total energy able to be stored, SMES is currently used for short duration energy storage. Therefore, SMES is most commonly devoted to improving power quality. If SMES were to be used for utilities it would be a diurnal storage device, charged from baseload power at night and meeting peak loads during the day. The high cost of superconductors is the primary limitation for commercial use of this energy storage method.

Hydrogen fuel cells

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. [3] It is widely seen as a possible fuel for hydrogen cars as part of a Hydrogen economy, if the problem of energy return on energy invested can be overcome. It 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. Making hydrogen 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, which exacerbates greenhouse gas emissions relative to present technology. With electrolysis, the greenhouse burden depends on the source of the power, and both intermittent renewables and nuclear energy are considered here.

With intermittent renewables such as solar and wind, the output may be fed directly into an electricity grid, with a penalty due to the requirement to operate some extra conventional plant on part load to allow for the fluctuations in power output. At penetrations below 20% of the grid demand, this penalty is usually small and does not severely change the economics; but beyond about 20% of the total demand, the penalty may make the generated power uneconomic. 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.

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 (950-1000°C) gas cooled nuclear reactors have the potential to split hydrogen from water by thermochemical means using nuclear heat (i.e. without using electrolysis).

About 50 kWh (180 MJ) is required to produce a kilogram of hydrogen by electrolysis, so the cost of the electricity clearly is crucial. 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, compressors, liquifaction, storage and transportation, which will be significant.

Load Levelling

The demand for electricity from consumers and industry is constantly changing, broadly within the following categories:

  • Seasonal (during dark winters more electric lighting and heating is required, while in other climates hot weather boosts the requirement for air conditioning)
  • Weekly (most industry closes at the weekend, lowering demand)
  • Daily (such as the peak as everyone arrives home and switches the television on)
  • Hourly (one method for estimating television viewing figures in the United Kingdom is to measure the power spike when advertisements are shown and everyone goes to switch the kettle on)
  • Transient (fluctuations due to individual's actions, differences in power transmission efficiency and other small factors that need to be accounted for)

There are currently three main methods for dealing with changing demand:

  • Electrical devices generally having a working voltage range that they require, commonly 110-120V or 220-240V. Minor variations in load are automatically smoothed by slight variations in the voltage available across the system.
  • Power plants can be run below their normal output, with the facility to increase the amount they generate almost instantaneously. This is termed 'Spinning Reserve'.
  • Additional power plants can be brought online to provide a larger generating capacity. Typically, these would be combustion gas turbines, which can be started in a matter of minutes.

The problem with relying on these last two methods in particular is that they are expensive, because they leave expensive generating equipment unused much of the time, and because plants running below maximum output usually produce at less than their best efficiency. Grid energy storage is used to shift load from peak to off-peak hours. Power plants are able to run closer to their peak efficiency for much of the year.


This is the area of greatest success for current energy storage technologies. Single-use and rechargeable batteries are ubiquitous, and provide power for devices with demands as varied as digital watches and cars. Advances in battery technology have generally been slow, however, with much of the advance in battery life that consumers see being attributable to efficient power management rather than increased storage capacity. This has become an issue as pressure grows for alternatives to the internal combustion engine in cars and other means of transport. These uses require far more energy density (the amount of energy stored in a given volume or weight) than current battery technology can deliver. Liquid hydrocarbon fuel (such as gasoline,ethanol/petrol and diesel) have much higher energy densities.


Virtually all devices that operate on electricity are adversely affected by the sudden removal of their power supply. Solutions such as UPS (uninterruptible power supplies) or backup generators are available, but these are expensive. Efficient methods of power storage would allow for devices to have a built-in backup for power cuts, and also reduce the impact of a failure in a generating station. Examples of this are currently available using fuel cells and flywheels.

Storage methods

Related concepts

External articles and references

Sites on Energy storage
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Images of Energy storage
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Newsgroups with Energy storage
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See also

- PowerPedia main index
- PESWiki home page

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