A glance at hydroelectricity evolution

The ecological transition that many governments are implementing, notably the EU, will stand on the phase-out of fossil fuels and the consequent expansion of renewable sources, which will develop as a technological revolution.
\nHowever, hydropower is a major renewable source already at a mature stage and widely exploited. It early uses appeared quite soon in early isolated plants when mechanical generator became available, but in limited cases were flowing water was easily accessible. Plants and lines had limited extension at that time and thermal engines (both steam and internal combustion) allowed a more flexible adoption. 
\nThe advent of alternating current, which allowed the use of stepping-up and stepping-down transformer and thus power transmission at high voltage over long distances opened the way to the exploitation of large water resources in remote areas. An early hydroelectric power plant of this kind was put into service at Willamette Falls, Oregon, in 1889, to supply Portland through a 4-kV 125 Hz 22-km line. The soundness of the technology was proven in the 1890s. A major step ahead occurred in 1891, on occasion of the International Electrotechnical Exhibition at Frankfort, where the first three-phase power line, rated 240 kW at 15 kV and 40 Hz, extending over 175 km between the Lauffen waterfalls, were a hydrogenator was installed, and Frankfurt, was presented. Shortly after, some hydropower stations powering long lines were opened, e.g.: 
\n1892: Aniene–Rome, Italy (1.2 MW, 5 kV, 42 Hz single-phase, 28-km)
\n1893: Lake Hellsjön–Grängesberg, Sweden (300 kW, 9.5 kV three-phase, 14 km)
\n1893: Mill Creek n.1 hydroelectric plant, California US (250 kW, 2.4 kV 50 Hz three-phase,12 km)
\nIn 1895, the Niagara hydropower station was started, with three (increased to 10 by 1898) two-phase alternators each rated 3.7 MW 2.2 kV 25 Hz. Step-up transformers with Scott connection fed the three-phase 11 kV line powering Buffalo 35 km apart. Two similar systems appeared in the Alps, Europe, in 1898, namely the Paderno d’Adda three-phase hydropower station, rated 9 MW at 13.5 kV 42 Hz that fed Milan, 30 km apart; and the Rheinfelden three-phase hydroelectric power plant, rated 12.5 MW at 50 Hz (Germany-Switzerland). Following these achievements, countries and regions rich with water resources exploited them increasingly in the early decades of the 20th century, while extending their power lines and network which eventually were interconnected into national grids.
\nIn the first decade of the century, also pumped hydro power station appeared, with early notable installations in Germany and Switzerland, in 1908. US followed starting in 1929 and major developments occurred in different countries after World War Two. 
\nThe growth of hydropower was massive in countries rich with water resources, notably Switzerland, Italy, Canada, Sweden, Norway, Soviet Union, and, more recently, Brazil and China, as long as more water resources were exploitable. By 1920, 40% of the electricity produced in the United States was hydroelectric and in the mid 20th century, 96% of the installed power in Canada, 94% in Switzerland, 90% in Italy, and 80% in Sweden 80%, came from hydroelectricity.
\nHowever, when the water resources were saturated, the growing demand was satisfied with thermoelectricity fed with fossil fuels. For the sake of example, hydroelectric energy share in Norway was 96.2% of the total production and 117.9% of the domestic demand in 2016; conversely, in Italy hydroelectric energy production remained substantially constant in the last 60 years, counting 44,257 GWh in 2016, but hydroelectric energy share had dropped to 15.3% of the total domestic electricity production, although flanked by 22% of other renewables. On the other hand, pumped hydro is today by far the largest-capacity form of grid electric energy storage worldwide, accounting for 181 GW of power capacity and 1.6 TWh of storage capacity, which correspond to 95% of the global figures for energy storage.

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