The earliest pumped storage power plants used separate turbines and pumps but this made the projects costly to build. The development of reversible pump turbines made the economics of the pumped storage plant look much more attractive.

Most reversible pump turbines used in storage plants are Francis turbines. The Francis design is well suited to both generation and pumping and can pump water to a considerable height (see Chapter 8 for more detail about hydropower turbines).

There is one drawback with the Francis design for this application; the turbine blade angle is fixed. A fixed blade does not provide the best efficiency for both pumping and generating power. An alternative design called the Deriaz turbine, similar to the Francis turbine in design but with movable blades, has been used in several pumped storage projects to try and achieve greater efficiency.

Propeller-like Kaplan turbines (see Chapter 8) can also be used as pumps, though not to transfer water to a reservoir of any great height. The La Rance tidal power plant in France, for example, uses such turbines to pump water across its tidal barrage in order to increase efficiency of operation of this station.

The best efficiency that a hydraulic turbine can provide for generating power is around 95%. Pumps are less efficient, operating at best at around 90%. This means that the best efficiency that can be expected from a pumped storage power plant through a storage and regeneration cycle is around 86%. In practice the efficiency is normally between 75% and 80% as shown in Table 10.1.

Francis and Deriaz turbines can be built today to operate at heads of up to 700 m in a single stage. Beyond that it will usually be necessary to use a combination of a pump and a Pelton turbine. Several plants in Switzerland employ this configuration.

Pumped storage hydropower plants can be brought on-line extremely quickly. The 1800-MW power station at Dinorwig in Wales, for example, can be run up from zero output to 1800-MW output in around 10 s. This ability makes pumped storage extremely attractive as a system reserve to be brought into service if a major base-load unit breaks down.

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