Energy storage plays a vital part in the modern global economy. At a national level, oil and gas are regularly stored by both utilities and governments, while at a smaller scale petrol stations store gasoline and all cars carry a storage tank to provide them with the ability to travel a significant distance between refueling stops. Domestic storage of hot water is also usual in modern homes. Yet when it comes to electrical energy, storage on anything but a small scale in batteries is rare.
Part of the reason for this is that storage of electricity, although it can be achieved in a number of ways, is difficult. In most storage technologies the electricity must be converted into some other form of energy before it can be stored. For example, in a battery it is converted into chemical energy, while in a pumped storage hydropower plant the electrical energy is turned into the potential energy contained within an elevated mass of water. Energy conversion makes the storage process complex and the conversion itself is often inefficient. These and other factors help to make an energy storage system costly.
In spite of such obstacles, large-scale energy storage plants have been built in many countries. In the majority of cases these installations are pumped stor- age hydropower plants, often built to capture and store power from base-load nuclear power plants during off-peak periods. Many of these storage plants were built in the 1970s. More recently there has been renewed interest in technologies such as pumped storage for grid support, particularly in European countries that are installing large capacities of renewable capacity such as wind and solar power. However, the economics of energy storage often make construction difficult to justify in a liberalized electricity market.
While economics may not always favor their construction, energy storage plants offer significant benefits for the generation, distribution, and use of electric power. At the utility level, for example, a large energy storage facility can be used to store electricity generated during off-peak periods (typically over- night), and this energy can be delivered during peak periods of demand when the marginal cost of generating additional power can be several times the off- peak cost. Energy arbitrage of this type is potentially a lucrative source of rev- enue for storage plant operators and is how most pumped storage plants operate.
At a smaller scale, energy storage plants can supply emergency backup in case of power plant failure, as well as other grid support features that help to maintain grid stability. They can also be employed in factories or offices to take over in case of a power failure. Indeed, in a critical facility where an instantaneous response to loss of power is needed, a storage technology may be the only way to ensure complete reliability.
Energy storage also has an important role to play in the efficient use of electricity from renewable energy. Many renewable sources of energy, such as solar, wind, and tidal energy, are intermittent and so are incapable of supplying electrical power continuously. Combining some form of energy storage with a renewable energy source helps remove this uncertainty and increases the value of the electricity generated. It also allows all the renewable energy available to be used. Today, the shedding of excess renewable power when demand does not exist for it, or when the grid cannot cope with it, is becoming common on some grid systems with high renewable capacity.
While there are many types of electrical energy storage systems, pumped storage hydropower plants account for virtually all grid storage capacity avail- able today with perhaps 130 GW of generating capacity in operation, based on estimates by the International Hydropower Association.1 This was effectively the only large-scale energy storage technology available until the late 1970s but in the past 30–40 years new interest has been stimulated and a range of other technologies have been developed. These vary in size so that some are suitable for transmission system–level storage while others are more suited to the distribution grid or even for small micro-grids. They include a range of battery storage systems, compressed-air energy storage, large storage capacitors and flywheels, superconducting magnetic energy storage, and systems designed to generate hydrogen as an energy storage medium. The widespread adoption of electric vehicles that use battery energy storage could potentially offer a major new means of storing grid electricity too.
If deployed widely, these technologies could potentially transform the way the grid-based delivery of electrical energy operates by eliminating the need for expensive peak power plants while at the same time integrating the range of renewable generation technologies now available. This would, in turn, eliminate the vulnerability of electricity production to the vagaries of the global fossil fuel markets, creating more stable economic conditions everywhere. There is no consensus on how much storage capacity would be required to achieve this on a mature national grid but it could be equivalent to around 10–15% of the avail- able generating capacity.
In spite of the apparent advantages offered by energy storage, widespread adoption remains slow. Cost appears to be the main obstacle, although developments are slowly bringing costs down. At the same time the growth of distributed generation is offering new opportunities for small-scale energy storage facilities. This chapter will look at the range of technologies available and where they might fit into the electricity system.