5.5.1 Pumped storage
At present the only practicable and economically viable way to store electrical energy in large quantities is to use it to pump water up a mountain, so pumped storage has become increasingly important, with installed capacity worldwide having grown from 78 GW in 2005 to reach 127 GW in 2009 – nearly a seventh of world total hydro capacity (REN21, 2010).
The principle is simple. Electrical energy is converted into gravitational potential energy when the water is pumped from a lower reservoir to an upper one, and the process is reversed when it is released to run back down, driving a turbo-generator on the way (Figure 7). The economic viability of the method depends on two nice technological facts:
- A suitably designed generator can be run ‘backwards’ as an electric motor.
- A suitably designed turbine can also run in either direction, either extracting energy from the water as a turbine or delivering energy to the water as a pump.
The complete reversal is turbo-generator to electric pump, with the machines designed for this dual role, but the cost saving is obviously significant. Turbines and generators are very efficient with nearly 80% of the input electrical energy being retrieved as electrical output when needed.
The value of the system is enhanced by its speed of response. Pumped storage is thus particularly useful as back-up in case of sudden changes in demand, or failure elsewhere in a grid system.
The location must of course be suitable, and a low-level reservoir of at least the capacity of the upper one must be available/constructed. Sites such as Cruachan in Scotland, where the mountains rise from a large loch or lake, are obviously ideal.
The high-level reservoir, behind a large dam, provides an operating head of 365 metres. Running the four 100 MW reversible machines for one hour at full capacity, as electric pumps or turbo-generators, raises or lowers the reservoir level by about a metre, storing or releasing about 400 000 kilowatt-hours of energy (Cruachan, 2011).
We now move on to discuss the rate at which a hydro plant can deliver energy.