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.

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). 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.

Reasons

Load Levelling

Main article: Grid energy storage

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

Electrochemical
Electrical
- Capacitor
- Superconducting magnetic energy storage (SMES)
Mechanical
Potential (Gravity)
- Hydro-electric
Thermal

External links

U.S. Dept of Energy - Energy Storage Systems Government research center on energy storage technology.
Electricity Storage Association Good comparison of technologies.
Energy Storage: A Nontechnical Guide Good book on energy storage technologies and applications.