The Kannagawa Hydropower Plant (神流川発電所) is an under construction pumped-storage hydroelectric power plant near Minamiaiki in Nagano Prefecture and Ueno in Gunma Prefecture, Japan. The power plant utilizes the Minamiaiki River along with an upper and lower reservoir created by two dams, the upper Minamiaiki Dam and the lower Ueno Dam. The power station in between the two dams will contain six 470 MW pump-generators for a total installed capacity of 2820 MW. Unit 1 commenced commercial operation in 2005 and Unit 2 in 2012. When completed, the plant will have the third-largest (after Fengning Pumped Storage Power Station and Bath County Pumped Storage Station) pumped-storage power capacity in the world.

Construction

In July 1993, the Kannagawa Hydropower Field Survey Office was initiated and in July 1995, the power plant was approved by the Electric Power Development Coordination Council. In May 1997, construction on the project began and by October 2003, the area behind the Ueno Dam was being inundated with water and the next year, the Minamiaiki Dam's reservoir began to fill as well. Both dams were completed and the upper reservoir was filled by 2004. The first generator was commissioned on 22 December 2005 and the second on 7 June 2012. The remaining units 3-6 are scheduled for commissioning by 2032.

Power station

The power station is 1600 ft underground and measures 708 ft long, 108 ft wide, and 169 ft high. It will contain 6 x 470 MW pump generators for a total capacity of 2,820 MW. Water from the upper Minamiaiki Reservoir is transferred through the power house and after producing electricity, it runs to the lower Ueno Reservoir. The pump-generators can then pump water from the lower reservoir back up to the upper reservoir for re-use in hydroelectric power production. The water tunnel connecting the two reservoirs is 3.8 mi long. The power station also has an effective hydraulic head of 653 m and maximum discharge of 510 m3/s.

Dams

The Minamiaiki Dam is located in Nagano Prefecture and is a 136 m high and 444 m long rock-fill dam. It is made of 7300000 m3 of material and withholds a 19170000 m3 reservoir. The Ueno Dam, in Gunma Prefecture, is a 120 m high and 350 m long concrete-gravity dam. It is made of 720000 m3 of material and withholds a 18400000 m3 reservoir.