ScottishPower executives yesterday admitted they are exploring sites for new hydro-electric schemes because of the unreliability of wind power.

The news came as Alex Salmond opened Europe's biggest onshore wind farm, at Whitelees, in Lanarkshire, which has 140 turbines capable of producing 322 megawatts of electricity - enough to power 180,000 homes.

The First Minister said that the SNP government had approved plans to increase its capacity by 130MW, enough to power a further 70,000 homes.

However, experts concede that yesterday's conditions - bright and windy - do not always prevail, and that the only proven way of storing wind-produced energy is to link it to a pump-storage hydro scheme.

This uses surplus electricity to pump water to a high-level reservoir where it can be released at times of peak electricity demand downhill into a hydro-electric power station.

Ignacio Galán, chairman of Iberdrola, ScottishPower's Spanish owner said that Iberdrola, which has several wind farms in Spain, was extending one pump-storage plant near Valencia and building a new one in northern Spain. The company has one pump-storage plant in Scotland, the 400MW Cruachan power station on Loch Awe, Argyll, which ScottishPower recently spent £10million to refurbish. "If we are allowed we will build more than this one," he added.

A senior company source later said that ScottishPower has been looking for possible sites. But, he added: "A lot of people, mainly because of environmental concerns, think that there are very few sites."

The problem is that pump-storage schemes require a large body of low-level water, some of which can be pumped up to a large reservoir at least 350ft higher than the power station. This puts most of the potential sites in the Highlands where concerns to protect scenery and wildlife habitats are also at their greatest.

The main environmental impact of the Cruachan scheme is the high-level dam and reservoir as the power station is buried half a mile inside a mountain with a tunnel entrance and popular visitor centre on the lochside.

More such schemes may, however, become vital if Mr Salmond's vision of a Scotland reliably powered by renewable energy is to be realised. "Our potential for electricity generation from renewables is up to 60,000MW - more than ten times our peak demand," he said.

"Harnessing all these opportunities has the potential to create more than 16,000 jobs in Scotland over the next decade," the First Minister said, adding that the construction of Whitelees employed more than 500 people and put £300 million into the economy. Company executives say that many problems need to be overcome. Offshore wind is a big part of Mr Salmond's vision and is generally believed to be the next big area for renewables expansion. Westminster has announced that the offshore subsidy is to be double the £40 per megawatthour given to onshore wind.

Keith Anderson, ScottishPower's director of renewables, said because of the high cost of developing offshore wind, many questions still had to be answered by the government, including when and how such farms would be connected to the national grid, before the company had the certainty it needed to proceed.