THE Ben Cruachan hydro-electric scheme is to receive an (pounds) 18.5m upgrade, the biggest refurbishment since it was opened by the Queen in 1965.

Nicknamed the Hollow Mountain, Cruachan became a Scottish icon, a spectacular feat of engineering reflecting the spirit of the post-war years when cheap electricity from the glens was set to fuel an industrial boom in the lowlands.

Cruachan was a flagship project of a hydro power revolution that would guarantee jobs and prosperity far into the future. The 4000-strong hydro workforce wrote its own legends.

The latest investment by ScottishPower comes as a direct result of the renewable energy revolution sweeping the UK.

Hydro power is playing a major part in Scotland's drive to meet climate change targets for reducing carbon dioxide emissions, and green electricity now attracts premium prices.

Brian Wilson, the energy minister, said: ''This is exactly the kind of investment I wanted to see when we brought hydro power into the renewables obligation.

''It was the post-war Labour government, led by Tom Johnston [chairman of the North of Scotland Hydro-Electric Board 1946-1959], which led the drive for hydro, bringing power to remote communities and jobs to the Highlands. I am very pleased to reinforce that great vision.''

With the British electricity, trading, and transmission arrangements likely to be extended to Scotland shortly, Cruachan's turbines are be uprated from 100MW to 120MW each.

Ewan McMillan, Cruachan's manager, said: ''The upgrade makes Cruachan one of the most flexible power plants in the world.

''In a commodity market where electricity is traded on a half-hourly basis, the flexibility will be valuable.''

A thermal power plant like Longannet, which also belongs to ScottishPower, takes around 90 minutes to get from 0 to 100MW. Cruachan can generate that level in 30 seconds.

The Cruachan project, which cost (pounds) 24.5m at the time, was created by the North of Scotland Hydro-Electric Board but was linked to the South of Scotland Electricity Board's Hunterston nuclear power

station in Ayrshire.

Nuclear power stations produce baseload electricity 24 hours a day, regardless of demand. So Cruachan uses the night-time surplus energy, purchased cheaply, to pump water back up the mountain to a reservoir at the base of the great Cruachan corrie.

Electricity was literally stored in the water to be generated when demand peaked. Cruachan made Hunterston viable.

Despite talk in parliament of the need to direct new industry into the Highlands and an outcry for more substantial support for tourism, the Cruachan scheme was opposed in the House of Lords by Lord Chorley, a Middlesex peer who was a member of the executive of the National Trust for England and Wales and a former president of the British Mountaineering Council.

The Glasgow Herald of May 11, 1959, recorded that Lord Chorley was ''interested in the preservation of the countryside and opposed to industrial exploitation if it spoils rural amenities''.

The Sloy scheme, he said, had made a ''dreadful mess of the bonnie banks of Loch Lomond''.

In the event, the Cruachan turbine house was constructed in a vast cavern hollowed out a mile inside the heart of the 3698ft mountain which towers over mid-Lorn. Lord Chorley's objections received short shrift.

The major visible sign of a power station with four 100MW generators which reverse and become pumps is the reservoir in the mountain's summit corrie, which was once used for summer grazings.