Foyers hydropower schemes

There are two hydropower schemes at Foyers, Highland, Scotland, which is located on the south-eastern shore of Loch Ness about half-way along its length. There is a conventional 5 MW hydropower scheme taking water from the River Foyers, and a 300 MW pumped-storage hydro-electric scheme using Loch Ness as the lower reservoir and Loch Mhòr for the upper.

The first hydropower scheme was built in 1895 by the British Aluminium Company to power an aluminium smelting plant on the shore of Loch Ness. This had a rated power of 3.75 MW, and took water from a dam on the River Foyers above the Falls of Foyers, significantly reducing the flow over the falls. The Aluminium smelter closed in 1967, and the hydropower scheme was then taken over by the North of Scotland Hydro Electric Board (now SSE). The scheme was upgraded to 5 MW in 1968, has a gross head of 108 m (354 ft), and is referred to as Foyers falls or Foyers 5 MW.

The Hydro Board promoted plans for the pumped storage scheme in 1968, with work commencing the following year. It has two reversible 150 MW turbines, which can pump water uphill at times of low demand for electricity, then generating when demand is high. The pumped storage scheme has a gross head between the lochs of 179 m (587 ft) and was completed in 1974.

The scheme was designed with sufficient storage to operate on a weekly cycle, with additional hydropower generation from the inflow to the reservoir. It therefore operates as a hybrid scheme, with about 25% of the planned output coming from inflow to Loch Mhòr. As part of the new development, some flow of the River Fechlin was diverted via a tunnel into the River E upstream of Loch Mhòr, increasing the inflow into the reservoir, and thus the amount of power which could be produced.