Entracque Power Plant

The Entracque Power Plant, also known as The Upper Gesso Plant, is a pumped-storage hydroelectric power station located in Valle Gesso just south of Entracque, Italy. The power station contains pump-generators for two co-located but hydraulically separated power schemes; the Chiotas-Piastra Plant and Rovina-Piastra Plant. Both plants use separate upper reservoirs but use Lago della Piastra as their common lower reservoir. To produce power, water is released from the upper reservoirs to the power station located at the lower reservoir. The pump-generators re-fill the reservoirs and the process repeats as needed. The Chiotas' upper reservoir, Lago del Chiotas, is located much higher in the valley and larger than Rovina's Lago della Rovina which affords it the ability to produce more electricity. The installed capacity of Chiotas is 1184 MW with a hydraulic head (water drop in elevation) of 1048 m while Rovina has an installed capacity of 133.67 MW and a head of 598 m. Construction on the plant began in 1962 and operations started in 1982. It is owned and operated by Enel.

Design and operation
The Entracque Power Plant is supported by a scheme that also primarily consists of three reservoirs. The lower reservoir used by both the Chiotas and Rovina is Lago della Piastra. The reservoir is located at the base of the valley and was formed by the construction of an 88 m tall gravity dam. Its water level has a normal operating elevation of 956 m and 9000000 m3 of its total 12000000 m3 storage capacity can be pumped up to the upper reservoirs. The pumping is carried out by the power station's nine Francis pump turbine-generators, eight belong to Chiotas and one to Rovina. When either of the upper reservoirs needs to be filled, water is pumped from Piastra to the Chiotas or Rovina upper reservoirs through a series of penstocks and tunnels. This usually occurs during periods of low energy demand, such as at night, when electricity is cheap. The Rovina's upper reservoir is at an elevation of 1535 m and is formed by an embankment dam. It has an active capacity of 1200000 m3 and depth of 10 m. Its catchment area is 77.2 km2. The upper Chiotas reservoir was formed with the construction of a 130 m tall arch-gravity dam which lies at an elevation of 1978 m. The dam's thickness ranges from 37.5 m at the base to 5 m at its crest. Its crest is 230 m long and the dam has a structural volume of 360000 m3. The Chiotas reservoir is also supported by a 30 m tall and 70 m long saddle dam, called Colle Laura, directly to its east. The saddle dam is also equipped with a spillway that has a discharge capacity of 240 m3/s. The storage capacity of the reservoir is 30200000 m3 while 27300000 m3 can be used for power generation. The catchment area for Chiotas is 11.6 km2 and the reservoir reaches a maximum depth of 118 m.

When power generation is required, water is released from either upper reservoir back down to the power station and its generators. The power station is located underground and consists of transformer, valve gallery and generator hall caverns. From the Chiotas reservoir, water is sent back through a 7.4 km long tunnel and when near the power plant, it splits into a 1.7 km system of penstocks which feed each of its eight 148 MW pump-generators. Water from the Rovina is processed by a single 133.67 MW generator. After the water is used for power generation, it is discharged into a 500 m long tail-race tunnel and into Lago Della Piastra. Both the tail-race and power plant intake are protected from water hammer by surge tanks. The drop in elevation between the upper reservoirs and the power plant afford the Chiotas a maximum hydraulic head of 1048 m and the Rovina 598 m.