24 C O N T E X T 1 0 9 : M A Y 2 0 0 9 turbine generators to the upper reservoir ready for use again at peak load. The scheme spent a long time in the development phase, with McColl determined to optimise the design of the scheme, ensuring that every drop of available water was used to its full potential. The four 100 MW Francis turbines that were eventually installed in the plant were also world firsts.They required a significant amount of testing off-site. The civil engineering requirements were designed by James Williamson & Company of Glasgow. The main construction contractors were Edmund Nuttall of Camberley and WilliamTawse of Aberdeen. The design of the scheme was unique in setting the turbine hall within the mountain. The station is accessed by a tunnel some seven metres wide and four metres tall. This opens out on to a massive turbine hall, 90 metres long and 36 metres high. Over 220,000 cubic metres of rock and soil had to be removed to create the chamber. Further chambers were created to house the transformer halls and the control room, although the starting and stopping of the scheme is controlled remotely from Cathcart near Glasgow. The transformer rooms step up the voltage from 16 kilovolts to the grid transmission standard of 275 kilovolts. The power is transmitted to the grid via a cable shaft with six 275-kilovolt, oil-filled cables. All the chambers were carefully sited following a detailed geological assessment of the site. McColl was concerned about choosing a section that balanced geological and cost considerations. The other major constructional component of the scheme was the Cruachan dam. The large buttress dam relies upon the action of gravity on the sheer weight of concrete used in the 316 metre long dam, which holds back the water in the Cruachan reservoir. The catchment receives water from an area of around 23 square kilometres, with a complex network of 19 tunnels diverting water from surrounding streams into the catchment. One of the most important factors in the development of the Cruachan scheme was that the bulk of the infrastructure would be invisible and accommodated within the mountain itself. The dam was the one feature which could not be hidden. The design was subject to significant environmental restrictions. In order to provide a single clean sweeping line, all the operational equipment is located within the dam wall itself, leaving the exterior unbroken by control towers. Water is let into and out of the reservoir through two inlets, both with flow control gates. From the reservoir, two high-pressure tunnels, some 400 metres long and at an angle of 55 degrees to the horizontal were sunk to meet steel-lined penstocks with a diameter of 2.7 metres terminating in special flow valves. Water then passes through these valves on to the turbine runners that spin the turbine. The turbine is linked to the generator by a shaft, which transfers the motion to the generators above.These then use the rotational motion to generate electricity. The valves at the end of the steel penstocks are vital to the success of the scheme. One of the main strengths of a pumped storage system is that it can move quickly to generation (in the case of Cruachan, in under two minutes). This makes pumped storage facilities ideal for satisfying peak load on the National Grid where extra power is required at short notice. To create such a fast response time, the turbines can spin in reserve with the penstock valves closed. Full output can be achieved in a matter of seconds, with the penstock valves moved to open and water rushing through to spin the turbines. Once the water has passed through the turbines, it enters a deep surge chamber, designed to protect the system from sudden influxes of additional water. The tailrace of the scheme is a chamber seven metres in diameter and over 900 metres long. At the end of the tail race the water leaves the scheme into Loch Awe. When the system is put into reverse, water runs the opposite way through the system. A small pony motor spins the turbines to a synchronous speed. Water is drawn back up to the reservoir, each turbine drawing a total of 110 megawatts from the grid to pump water back up hill. Hydroelectric power is a vital technology in the creation of renewable energy. It is important to the functioning of the National Grid and, ultimately, to the provision of power to homes and businesses around the country. The flexibility of the Cruachan scheme was one of its major advantages. The short start-up time of two minutes is ideal for dealing with periods of peak demand. The station can generate for 22 hours before the upper reservoir is exhausted, although the station is required to maintain a 12-hour emergency supply, referred to as ‘black start’. The Ben Cruachan scheme is characteristic of the dynamism of the development of Scotland’s resources for hydroelectricity between 1943 and the late 1960s. With the development of Ben Cruachan, the hydro sector in Scotland achieved a world first and Sir Edward McColl underlined his status as one of the most prominent hydroelectric engineers of his generation. The Cruachan scheme is also an excellent example of the balance achieved in the development of hydro schemes between the necessary engineering requirements and the highest quality technical and aesthetic design to ensure that developments had as small an impact on the landscape as possible. Ben Cruachan has been recognised by Docomomo as one of the 60 key post-war monuments of Scottish architecture. The high quality design of the buildings associated with the scheme is an overt display of pride in a piece of civil engineering of international importance. The real legacy of the attention to detail and success of the scheme is its continued operation. The pioneering technology, which provides clean and renewable energy, is set to work hand-in-hand with pioneering developments of the 21st century, including the development of electric vehicles and the supply of carbon neutral homes. David Fleetwood is working as an assistant inspector of historic buildings at Historic Scotland on a thematic survey of hydroelectric power. He is director of Landlines Heritage, a specialist photographic survey company for historic buildings.
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