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The original renewable energy is due for some renewing.

And just some of the potential of hydro power is included in a plan set out today by Scottish and Southern Energy for getting a lot more out of its Sloy plant next to Loch Lomond.

SSE is still known in parts of Scotland as the Hydro Board, and the 'Scottish Hydro-Electric' brand is still the main one for customers north of the Border.

More than 50 years since large tracts of the Highlands were changed forever by the building of dams and installation of giant turbines, the company - long since privatised - is back for more.

The Sloy project, priced at £30m, would see a pumped storage facility built in to the existing scheme.

This means that water could be pumped uphill and stored in Loch Sloy, available when there are peaks in demand.

Of all the options open to generators, there is nothing as effective as hydro for meeting sudden surges in demand.

Open a sluice gate in SSE's new, state-of-the-engineering-art plant at Glendoe, above Loch Ness, and with a drop of 600 metres through its new tunnel, its power is coursing through the National Grid within 30 seconds.

The other attraction in the renewables sector is to provide base load - the energy you can rely on when, for instance, the wind and the waves drop.

Much of the debate is around building a new generation of nuclear plants to meet that need - and nuclear is one of a wide range of joint ventures signed by SSE in the past year to hedge its options across the energy spectrum.

However, a combination of renewables could go some way to avoiding that need. Wind turbines can be used during the night, when demand is low, to pump water uphill.

At present, there are only two such schemes.

SSE chief executive, Ian Marchant, told me this morning - as he was talking through his annual results - that the intention is to move beyond the "splash and dash" approach.

That means they need hydro power that doesn't just fill the gaps, best produced by relatively small amounts of water falling long distances.

Instead, they're looking for much bigger lochs, and the distance from top to bottom of the hydro scheme matters less.

As Mr Marchant put it: "We want pump storage that can run for days and days when there's no wind".

An industry/government study published last September calculated there are another 600 Megawatts of "financially viable" hydro power available in Scotland, much of it from small-scale, run-of-the-river projects.

That's nearly half as much again as is currently installed. And it's twice as much as the new Glendoe project, which is reckoned to provide for 100,000 homes during the short times it's called upon.

SSE is looking at two such minor sites, and it has taken a large stake in a small company, Green Highland Renewables, which specialises in developing more.

Its bid for a Sloy upgrade is on a more significant scale.

It would take the oldest scheme developed by the Hydro Board, opened in 1950 and refurbished in the past ten years, and it would sharply increase the average year's yield.

At present, Sloy generates around 120 Gigawatt hours, and with the enhancement, that could rise by 100 GWh.

Today's SSE annual figures for 2008 has come with confirmation of another application for a giant wind farm on Shetland, lodged yesterday in a joint venture with the islands council.

The company has acquired a power station in Wales, with a view to building a replacement on site.

But its return to its hydro roots are perhaps the most intriguing option for SSE, particularly if it gives that longer-term base load pump storage potential.

Ian Marchant says his team of geographers and engineers think they might have identified such a site.

They're not saying where yet, but expect more on this in the next three or four months.