Foyers can operate as a conventional hydro - electric power station, but is also used as a pumped storage system.
The site has been used for 100 years to generate hydro - electricity. The British Aluminium Company first built a 5MW power station in 1896 to provide the energy required to smelt aluminium ore mined locally.
The power station sits on the eastern shore of Loch Ness and is connected by an underground tunnel to Loch Mohr, 180m above Loch Ness and 3.5km distant.
The diagram below shows the arrangement of the power station, the two lochs and the connecting tunnel.
Foyers Pumped Storage Scheme, Loch Ness
When generating electricity, water runs from Loch Mhor to the power station through a tunnel adapted from the tunnel first cut 100 years ago. Water pressure is low in this 3km long, D-shaped, concrete-lined tunnel. The tunnel floor is now almost 7m wide. Water flow is controlled by valves at the tunnel inlet at Loch Mohr.
A surge chamber is connected to the end of the low pressure tunnel. By filling and emptying, this absorbs sudden changes in pressure as water flowrate through the power station is adjusted while generating electricity.
Water from the low pressure tunnel fills a 112m deep, 7.3m diameter, concrete-lined vertical shaft, generating very high pressure at its base. The water then divides between two steel-lined tunnels which connect to the 3m diameter turbine inlet valves. This last section of tunnels taper - reduce in diameter- to produce a further increase in water pressure.
The very high pressure forces water through the two turbines and out into a short, concrete-lined tunnel leading to Loch Ness. A screened forebay, which can be seen in the photograph of the power station, stops the water from discharging directly into the Loch. It also prevents obstructions from floating into the mouth of the tunnel. When the power station is pumping water from Loch Ness up to Loch Mohr, the screens stop any man-made or natural debris from being drawn into the tunnel and therefore prevent damage to the turbines.
When pumping water, the power station draws electricity from the National Grid. It does this when demand for electricity from the national grid is low - particularly at night. Using the generators as electric motors, the turbines are driven in reverse and act as pumps to drive water back to Loch Mohr through the tunnel system.
The two turbines generate 150MW each. Water flows through them at a rate of 200 m3 (200 tonnes) per second when generating electricity and 163 m3 (163 tonnes) when they are pumping. On average, 400 GW h of electrical energy is produced by the station each year. 300 GW h are generated by pumped-storage capacity, and 100 GW h are produced by rainwater flowing off the catchment area into Loch Mohr.
The power station turbines and generators can switch on from a standstill to generating maximum power output in under 2 minutes. Reversing the turbines to pump water back to Loch Mohr takes about 3 minutes.
To ensure that this system does not have any serious effect on the level of water in Loch Ness, and the River Ness, which connects the Loch to the sea, a water flow control system was built at the outlet of the Loch.