Massive batteries? Compressing or liquefying air? Moving gravel uphill on ski lifts? Tom Heap looks at some of the big ideas proposed for storing energy using science or the landscape and explores which may become a reality if we're to keep the lights on.

Huge investment is being made in renewable energy but as solar and wind fluctuate and are intermittent often energy goes to waste because the points at which they generate isn't when the demand occurs. So why not use that energy and store it in another form to be used when it's required? Many companies are proposing ideas to do that - from extending traditional pumped hydro to compressing or liquefying air, electrolysing water or shifting heavy materials up mountains. Or will a revolution in batteries - making them cheaper and from different materials - help the cause?

Tom Heap takes a look at some of the bold ideas to see how far they'll go to keeping the lights switched on, what they'll cost financially and aesthetically and if there's any sign of committing to any of them at all.