Sloy is the largest conventional hydroelectric plant in Britain and is situated on the west bank of Loch Lomond. It uses water from Loch Sloy, which is nearby and higher up. Its water flows along tunnels cut through Ben Vorlich, to power the station below.
Initially, the labour force used to build the scheme was mostly German prisoners of war, but when they were repatriated men came from all over Scotland and from Cornwall to work here. The peak workforce was more than 2,200 men, and 37,600 tonnes of rock were excavated from the site of the generating station.
The new dam (NN289111) at the south east end of Loch Sloy raised its water level by 47m and doubled its length. The catchment area for the loch is some 8,300 hectares. The mass concrete buttress dam is 354m long and 49m high, and impounds 34 million cubic metres if water. There is a spillway of reinforced concrete slabs between three of the buttresses, and a roadway along the top. The dam is founded on faulted and irregular mica-schist rock, with a grout curtain 6-24m deep along its line to seal the fissures.
When the loch is full, there is a 277m operating head of water and the maximum flowrate is 62 cubic metres per second. Water flows into a screened bellmouth intake at the top of a 61m deep shaft near the centre of the dam. From there, it enters the main tunnel through a control gate.
The 3 km long main tunnel through Ben Vorlich (940m) required the excavation of 183,000 tonnes of rock. The first part of the tunnel is 213m long and 4m in diameter with a reinforced concrete lining. This merges into a 2.5km long horseshoe shaped section 4.7m wide.
A surge chamber 27.4m long, 11.6m wide and 8.5m high is situated 83m above the tunnel, reached by a concrete lined vertical shaft 7.9m in diameter with expansion chambers top and bottom. The shaft and chambers were excavated top down through the mountain, necessitating the removal of some 2,400 tonnes of rock.
The tunnel divides into two 152m from the outlet, each 3m in diameter lined with concrete-faced welded steel plate. At the outlet the two tunnels become four 2.1m diameter pipelines, which enter the valve house where four disc valves can close the pipes in an emergency.
The four pipelines are each 457m long and rest on saddle plates supported by intermediate piers, carried on four main anchor blocks. The pipes are made from 7.3m long sections of welded curved steel plates, weighing 15 tonnes, and convey water from the valve house to the turbine hall. The steel framed turbine hall is 58m long and 16m wide, with lightly reinforced concrete walls faced with Rubislaw and Corennie granite.
There are four vertical shaft 32,500kW Francis turbines, each coupled to an 11kV alternator, giving a total capacity of 130MW. The alternators are connected to four 11kV to 132kV step-up transformers. Auxiliary power, if required, is provided by one 450kW Pelton bucket wheel coupled to an alternator. The spiral turbine casings were each cast in one piece of steel weighing 43 tonnes. Each turbine weighs around 200 tonnes and has 24 vanes.
The 132kV transmission lines run from the station to Windyhills near Glasgow. They are carried 58km on a double row of steel towers — 416 towers in all with an average height of 26m. The first electricity was supplied to Arrochar by the diesel station at Loch Sloy in April 1948. However, the first hydroelectricity from the turbo-alternators was produced in March 1950.
A new double span reinforced concrete bridge with rubble masonry parapets carries the road across Inveruglas Bay, replacing the original, which had to be rebuilt after constructing the tail race.
Sloy was opened by HM Queen Elizabeth (later the Queen Mother) on 18th October 1950. It was owned and operated originally by the North of Scotland Hydro-Electric Board, but was later transferred to Scottish and Southern Energy.
The whole scheme was refurbished at a cost of £113m, and was reopened by Scottish Environment Minister Sarah Boyack on 6th December 1999. The work included new turbines, generators, cabling, pipework and internal decoration. Sloy now has an installed capacity of 152.5MW and can be fully operational within five minutes from shut down.
In 2009, Scottish and Southern Energy applied to build a new pumping station to convert Sloy to a pumped-storage hydroelectric scheme.
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