III.

Advances in Turbine Design

The trend in modern hydraulic turbine installations has been toward higher heads and larger units. Depending on the size of the unit, Kaplan turbines are now used with heads up to about 60 m (about 200 ft), and Francis turbines up to 610 m (2000 ft). The world's highest head installation (about 1770 m/about 5800 ft), using a Pelton wheel, is at Reisseck, Austria, and the largest single units are installed in a plant at Itaipu, Brazil, where 18 Francis-type turbines sized at 700 megawatts (Mw) each have a total capacity of 12,600 Mw. The largest installation in North America is at La Grande on James Bay, in eastern Canada, where 22 units rated 333 Mw each have a total capacity of 7326 Mw. The largest installation in the United States is at the Grand Coulee Dam on the Columbia River, which has a total capacity of about 6500 Mw.

Many of the small dam hydroelectric systems built before 1930 were later abandoned because of high maintenance and labor costs. Increases in the cost of fossil fuels have focused renewed attention on these low head installations. With the development of standardized propeller turbines with nearly horizontal shafts, small installations have again become attractive.

Turbines can also be designed to run in reverse as pumps. This is done by inverting the electric generator to operate as a motor. Because electric power cannot be stored economically, the operation of the so-called pump-turbines with electricity generated from nuclear and fossil fuel power plants during off-peak hours enables additional water to be stored in a reservoir. It can then be reused to drive the turbine during peak periods. In recent years, pump-turbine technology has been developed to allow for heads up to about 600 m (about 2000 ft) of water and for unit capacities of more than 400 Mw.