Costs

There is little experience with CAES so any cost estimate must be considered tentative. However it would appear to be an economically attractive option for energy storage. Installation costs of around $400/kW have been mooted for the USA.

Large-scale batteries

The traditional way of providing electricity storage has been the battery. This is an electrochemical device which stores energy in a chemical form so that it can be released as required.

A battery comprises a series of individual cells, each of which is capable of providing a defined current at a fixed voltage. Cells are joined both in series in and parallel to provide the required voltage and current rating required for a particular application.

Each cell contains two electrodes, an anode and a cathode. These are immersed in an electrolyte.4 At its simplest, the electrodes are made of materials which will react together spontaneously but the electrolyte in which they are immersed will allow the passage of only one of the components required to complete the reaction.5 An electrical connection must be made between the two electrodes to allow the passage of electrons from one electrode to the other in order to complete the reaction. This is the source of electrical power.

There are two different types of traditional batteries: the primary cell and the secondary cell. A primary cell can only be discharged once, after which it must be discarded. A secondary cell can be discharged and recharged many times. Only the second type is of any use for energy storage systems.

Secondary cells can further be divided into shallow discharge and deep discharge cells. A shallow discharge cell is only partially discharged before being recharged again; an automotive battery would typify this type of cell. A deep discharge cell is normally completely discharged before recharge. This is the type which is most attractive for large-scale electricity storage.

Traditional electrochemical storage systems boast a best case conversion efficiency of 90% but a more typical figure would be 70%. Most batteries also suffer from leakage of power. Left for too long, the cell discharges itself. This means that battery systems can only be used for relatively short-term storage.

An additional problem with batteries is their tendency to age. After a certain number of cycles, the cell stops holding its charge effectively, or the amount of charge it can hold declines. Much development work has been aimed at extending the lifetime of electrochemical cells but this remains a problem.

To their advantage, batteries can respond to a demand for power almost instantaneously. This property can be used to good effect to improve the stability of an electricity network. It is also valuable in both distributed generation and for back-up power applications.

Traditional batteries are completely self-contained. However there is another type called a flow battery in which chemical reagents involved in the generation of electricity are held is tanks separated from the actual electrochemical cell. In this type of device the reagent is pumped through the cell as needed. Such cells suffer less from energy leakage. Several types are being developed for utility electricity storage.

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