Electricity normally has to be used as soon as it has been generated. This is why grid control and electricity dispatching systems are important; they have to balance the demand for electricity with the supply. Once one fails to match the other, problems occur. It would seem obvious, given this situation, that some reservoir of saved electricity would be a major boon to grid operation. Yet storing electricity has proved difficult to master.
Storing electricity in its dynamic form, amperes and volts, is almost impossible. The nearest one can get is a superconducting magnetic energy storage ring which will store a circulating DC current indefinitely provided it is kept cold. A capacitor storage system stores electricity in the form of electric charge. All other types of energy storage convert the electricity into some other form of energy. This means that the energy must then be converted back into electricity when it is needed.
A rechargeable battery may appear to store electricity but in fact it stores the energy in chemical form. A pumped storage hydropower plant stores potential energy; a flywheel stores kinetic energy while a compressed air energy storage (CAES) plant stores energy in the form of compressed air, another type of potential energy. Alternatively one might use electrolysis to turn electricity into hydrogen, yet another chemical form of energy.
All these, and one or two others, represent viable ways of storing electricity. Several are commercially available, others in the development stage. And each has its advantages and disadvantages.
For large-scale utility energy storage there are three possible technologies to chose between, pumped storage hydropower, CAES and, at the low end of the capacity range, large batteries. Batteries can also be used for small- to medium-sized distributed energy storage facilities,1 along with flywheels and capacitor storage systems. Superconducting magnetic energy storage is being used for small storage facilities and would be suitable for large facilities but is prodigiously expensive.
Some of these systems can deliver power extremely rapidly. A capacitor can provide power almost instantaneously, as can a superconducting energy storage system. Flywheels are very fast too, and batteries should respond in tens of milliseconds. A CAES plant probably takes 2-3 min to provide full power. Response times of pumped storage hydropower plants can vary between around 10 s and 15 min.
The length of time the energy must be stored will also affect the technology choice. For very long-term storage of days or weeks, a mechanical storage system is the best and pumped storage hydropower is the most effective provided water loss is managed carefully. For daily cycling of energy, both pumped storage and CAES are suitable while batteries can be used to store energy for periods of hours. Capacitors, flywheels and superconducting magnetic energy storage are generally suited to short-term energy storage, though flywheels can be used for more extended energy storage too.
Table 10.1 Round trip energy efficiencies for storage technologies
Superconducting energy storage 90
Flow batteries 90
Pumped storage hydropower 75-80
Another important consideration is the efficiency of the energy conversion process. An energy storage system utilises two complementary processes, storing the electricity and then retrieving it. Each will involve some loss. The round trip efficiency is the percentage of the electricity sent for storage which actually reappears as electricity again. Typical figures for different types of system are shown in Table 10.1.
Electronic storage systems such as capacitors can be very efficient, as can batteries. However the efficiencies of both will fall with time due to energy leakage. Flow batteries, where the chemical reactants are separated, perform better in this respect and will maintain their round trip efficiency better over time. Mechanical storage systems such as flywheels, CAES and pumped storage hydropower are relatively less efficient. However the latter two, in particular, can store their energy for long periods if necessary without significant loss.
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