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Pumped storage plants usually use reversible turbine/generator assemblies, which can act both as a pump and as a turbine generator (usually Francis turbine designs). [4] Variable speed operation further optimizes the round trip efficiency in pumped hydro storage plants.
Hydropower technology and attitude began to shift in the second half of the 20th century. While countries had largely abandoned their small hydropower systems by the 1930s, the smaller hydropower plants began to make a comeback in the 1970s, boosted by government subsidies and a push for more independent energy producers. [56]
Hydroelectric power has been a popular method of energy dating back to the late 19th century. The main advantage of using hydropower is that it is a clean form of energy, otherwise known as "green" energy. Since the process of using waterpower does not require burning fossil fuels, it is more environmentally friendly.
Since small hydro projects usually have minimal reservoirs and civil construction work, they are seen as having a relatively low environmental impact compared to large hydro. This decreased environmental impact depends strongly on the balance between stream flow and power production.
Although a dam-like structure is required, no area is enclosed, and therefore most of the benefits of 'damless hydro' are retained, while providing for vast amounts of power generation. Low-head hydro is not to be confused with "free flow" or "stream" technologies, which work solely with the kinetic energy and the velocity of the water.
Pumped storage plants function similarly to a storage battery; they absorb excess power generated by such plants in off-peak hours, such as nighttime, using it to pump water into a reservoir. Later, when demand exceeds the base load, the flow of water from the reservoir generates additional electrical power to meet peak load demands.
This was an efficient machine (~80%) that sent water through a runner with blades curved in one dimension. The stationary outlet also had curved guides. In 1844, Uriah A. Boyden developed an outward flow turbine that improved on the performance of the Fourneyron turbine. Its runner shape was similar to that of a Francis turbine.
There are also small and somewhat-mobile forms of a run-of-the-river power plants. One example is the so-called electricity buoy, a small floating hydroelectric power plant. Like most buoys, it is anchored to the ground, in this case in a river. The energy within the moving water propels a power generator and thereby creates electricity.