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This system is economical as it flattens out the variations in the load on the power grid, permitting thermal power stations such as coal-fired plants and nuclear power plants that provide base-load electricity to continue operating at their most efficient capacity, while reducing the need to build special power plants which run only at peak demand times using more costly generation methods.
As well as energy management, pumped storage systems are important components in controlling electrical network frequency and in provision of reserve generation. Thermal plants are much less able to respond to sudden changes in electrical demand, which cause frequency and voltage instability. Pumped storage plants, in common with other hydroelectric plants, can respond to these changes within seconds.
The first use of pumped storage was in the 1890s in Italy and Switzerland. In the 1930s reversible hydroelectric turbines became available. These turbines could operate as both turbine-generators and in reverse as electric motor driven pumps. The latest in large-scale engineering technology are variable speed machines for greater efficiency. These machines generate in synchronisation with the network frequency, but operate asynchronouslyAsynchrony is the state of not being synchronized. Contrast with synchronous or plesiochronous systems. Examples: Asynchronous communication Asynchronous Transfer Mode Asynchronous serial interfaces Packet switched systems such as Ethernet or Internet Pro (independent of the network frequency) as motor-pumps.
A new concept is wind-pumped water storage where vagaries in wind power can be leveled by using the wind power to fill a reservoir and generating grid power from the reservoir turbines.
In 2000 the United States had 19500 MWeMWe and MWt are units for measuring the output of a power plant. MWe means megawatts of electrical output and MWt means megawatts of thermal output''. For example, a nuclear power plant might use a fission reactor to generate heat (thermal output) which c capacity of pumped storage. This produced a net -5500 MWe of power because they consume more power filling their reservoirs than they generate by emptying them. Still the technique is considered a worthwhile addition to the electrical grid as the most cost effective means to date for storage of mass amounts of electrical power.
In 1999 the EU had 32 GW capacity of pumped storage out of a total of 188 GW of hydropower and representing 5.5% of total electrical capacity in the EU.