Imaichi Pumped Storage Power Station

The Imaichi Pumped Storage Power Station (今市発電所) is a large pumped-storage hydroelectric power station in Tochigi Prefecture, Japan. With a total installed capacity of 1050 MW, it is one of the largest pumped-storage power stations in Japan. The facility is run by the Tokyo Electric Power Company (TEPCO). The power plant started operation in July 1988 with a capacity of 350 MW (one unit operational). The other two units entered operation in December 1991. The plant is one of the many large scale pure pumped-storage plants built in Japan since the 1970s to compensate for the increased penetration of base-load nuclear power and peak load from cooling and air-conditioning.

Like most pumped-storage facilities, the power station uses two reservoirs, releasing and pumping as the demand rises and falls. The upper reservoir is contained by the Imaichi Dam, a concrete gravity dam, at an altitude of 512 m. The reservoir is fed by the Togawa river. The lower reservoir is contained by the Kuriyama Dam, a rock-fill embankment dam at an altitude of 1054 m. The reservoirs can store 9100000 m3 of water. Of that storage volume, 6200000 m3 can be used for power generation. This is enough for about 7 hours of operation at full generation capacity, giving a total energy storage capacity of about 7.35 GWh.

The power plant is housed in a large cavern 400 m underground and includes three 350 MW Francis reversible pump-turbines. The cavern dimensions are 33.5 m width, 51 m height, and 160 m length. The power station work on a daily or weekly storage cycle. The plant is connected to the high voltage transmission system with a 500 kV power line. These power lines constituted the world's first practical applications of 500 kV XLPE (cross-linked polyethylene) cables, together with the power connection at the Shimogo Pumped Storage Power Station.