This could end up taking many forms, from conventional lithium-based "big battery" installations, to flow batteries, silicon phase-change batteries, molten salt batteries, iron-air batteries, gravity batteries, carbon dioxide expansion batteries, and other more unusual ideas like buoyancy batteries.

Each has its own advantages and disadvantages in terms of efficiency, size, location, installation costs, operating costs, input and output power ratings, longevity and how long it can store the energy for. That's good, since different solutions will fill different needs – some backing up the power grid during instantaneous demand spikes, others smoothing out the mismatched daily curves between demand and renewable supply, and others still helping to address seasonal supply drops, like when solar drops off through the winter.

Here's another for the pile, coming out of Finland. Polar Night Energy says it's just opened its first commercial sand battery at the premises of "new energy" company Vatajankoski, a few hours out of Helsinki.

This is a thermal energy storage system, effectively built around a big, insulated steel tank – around 4 metres (13.1 ft) wide and 7 metres (23 ft) high – full of plain old sand. When this sand is heated up, using a simple heat exchanger buried in the middle of it, this device is capable of storing an impressive 8 megawatt-hours of energy, at a nominal power rating of 100 kW, with the sand heated to somewhere around 500-600 degrees Celsius (932-1112 °F).