The demand for electricity varies daily and seasonally and therefore some centralized power stations may only be required for short periods or to operate at limited capacity. In general, a fully interconnected electricity network will use low-cost, very large power stations for the base load and more expensive units for peak loads. The fossil fuels used for either base load or peak units are easily stored and trans­ported. Most renewable energy sources with the exception of biomass cannot be stored and transported easily, unless they are converted into electricity or other energy carriers. In addition, some renewable power systems only supply electricity intermittently, for example wind, wave and photovoltaics. In order to incorporate these intermittent electricity sources and to deal with peaks and troughs in electricity

demand some form of storing either electricity or energy which can be converted back into electricity is required. The advantages of storage would be the following:

• Bulk storage of energy would allow the decoupling of production from supply.

• Allows the incorporation of smaller power stations into the network.

• Improves power quality and reliability.

• Reduces transmission losses as transmission distances reduced.

• Cost reduction, as smaller, more efficient power stations can be constructed.

• Allows the use of intermittent renewable power sources.

• Decreased environmental impact associated with renewable sources.

• Strategic advantages of generating energy from indigenous energy sources, avoid­ing imports.

At present two large-scale energy storage systems are in operation: pumped hydro storage and compressed air energy storage (Dell and Rand, 2001).

In the case of pumped hydro storage, excess electricity at times of low demand is used to pump water into a lake or reservoir some distance above a hydroelectric power station. When a peak in electricity demand occurs conventional power stations are too slow to respond but the stored water can be released and the hydroelectric plant comes online rapidly. In the UK, there is such a system in Wales. The second large-scale energy storage system is to compress air in large reservoirs when electricity is in excess and release this to drive electricity-producing turbines. Such systems have been operat­ing for some time in Germany and the USA (van der Linden, 2006).

On the small scale a number of systems are under development including the following:

• Flywheels.

• Hydrogen production.

• Batteries.

• Thermal storage.

• Superconducting magnetic coils.

Flywheels have also been used to store energy and using new technology small high- density systems have been constructed and megawatt modules can be installed.

On the island of Utsira, Norway, electricity is provided by wind turbines as there is no link to a mainland power station. Wind power is intermittent so that any excess electricity generated when the wind blows is used to electrolyse water, producing hydrogen. The hydrogen is stored and burnt to produce electricity when the wind is insufficient to run the turbines. The feasibility of a wind-photovoltaic system using compressed hydrogen has also been tested in Australia, where the costs of the hydro­gen storage was the most critical factor (Shakya et al., 2005).

Five types of batteries can be used to store electricity. The lead-acid battery was developed a long time ago and is used widely in the automotive industry. These batter­ies have also been used for small wind and solar installations but they require periodic maintenance and are poor at low and high temperatures. Alkaline batteries, nickel-iron and nickel-cadmium, were also developed a long time ago, around 1900. The best is the nickel-cadmium which performs better than the lead-acid at high and low temperatures. It is however more expensive but the nickel-metal-hydride has been developed. This battery, although more expensive, holds more charge and has seen widespread use in mobile phones and laptop computers. It has also been used in

electric and hybrid vehicles. The third type of battery is the flow batteries, sometimes known as ‘regenerative fuel cells’ (rated to 12 MW). The cells are charged, converting electricity into chemical energy. The two compartments of the cell are separated by an ion-exchange membrane and the electrolyte in the compartments is circulated in a closed-loop system. The last two types of battery are the high temperature battery and the rechargeable lithium battery. The high temperature battery uses molten sodium at 300-400°C, and both these types have problems for large-scale use, although lithium ion batteries are widely used in portable electronic devices.

The thermal storage of energy from electricity using hot water or solid material is used to heat buildings in the form of night storage radiators where off-peak electri­city is used. The heat cannot efficiently be converted back to electricity so this is not suitable for energy storage. However, phase-change materials have been used to store solar energy (Kenisarin and Mahkamov, 2007).

In systems where there is fluctuating power, superconductive magnetic energy storage can be used, and though the system is expensive, it can respond in millisec­onds. Energy is stored in a magnetic field formed by a DC current in super-cooled superconductive coils.