Energy Storage
Supercapacitors are one method of storing solar energy
Energy storage has always been closely associated with solar installations, including both solar heating and photovoltaic (PV) applications. Today you can find many different energy storage methods in use, all competing by the following criteria.
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.
Portability
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.
Reliability
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
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Electrochemical
- Batteries
- Flow batteries
- Fuel cells
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Electrical
- Capacitor
- Supercapacitor
- Superconducting magnetic energy storage
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Mechanical
- Compressed air energy storage
- Flywheel energy storage
- Hydraulic accumulator
- Hydroelectric energy storage
- Spring
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Thermal
- Molten salt
- Cryogenic liquid air or nitrogen
- Seasonal thermal storeage
- Solar pond
- Hot bricks
- Steam
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