Italian company Energy Dome has opened the first of its remarkable grid-level energy storage plants. These "CO2 batteries" can store renewable energy over long periods and release it quickly, at less than half the cost of big lithium batteries.

Large-scale energy storage is going to be required on an epic scale all round the world, as green energy begins to take over the world's power supply. Renewable energy is often generated at times and places where it's not needed, and a variety of grid-level storage technologies are jockeying for various energy market niches, each with their own strengths and weaknesses.