The company has completed a two-year feasibility study to see whether it is possible to expand the 440MW power station, and found that the capacity could be increased by 400-600MW with an investment of £300-400 million.
It said extending the plant would take eight to 10 years in total, with two to three years spent obtaining consent and six to seven years taken up by construction and commissioning.
A new cavern would need to be excavated within the Ben Cruachan mountain, which would require a peak workforce of 800 people. Scottish Power said new dams could also be constructed to increase the capacity of the upper reservoir.
Generation director at Scottish Power Hugh Finlay said: “A new generation of pumped storage hydro would be a major asset for electricity systems worldwide as more renewable electricity continues to come on line.
“As well as being able to further support peak demand, expanded pumped storage would also be able to effectively store greater levels of electricity at times when renewable energy output is high but demand is low.
“Pumped storage hydro is the most cost effective and well-developed large-scale electricity storage technology in existence. We will now take forward our Cruachan findings with government and regulators.”
Also known as “The Hollow Mountain”, Cruachan was opened by the Queen in 1965 after six years of construction undertaken by a workforce of 4,000. Consisting of two reservoirs at different heights connected by water tunnels and turbines, it effectively acts as a battery.
In periods of low demand surplus power is stored by using the plant to pump water from the lower reservoir to the upper one. In period of high demand the power is released by allowing the water to flow back down via the turbines. The plant can start producing energy for the grid within just two minutes – or 30 seconds if the turbines are already primed.
Scottish Power has called on the government to help de-risk the project. Speaking to the Financial Times, Scottish Power’s regulation director Rupert Steele said a “promising approach” would be some form of cap and floor agreement, similar to the arrangements being put in place to support interconnectors. Under Ofgem’s cap and floor regime interconnectors are guaranteed a minimum income but are also limited in the amount they can earn.
Last week Scottish Power owner Iberdrola said it was planning to invest €8.3 billion in the UK by 2020.