As the world transitions from fossil fuels to renewables, a fundamental question remains: where do we get our energy when the wind is not blowing and the sun is not shining?

One solution is to store excess energy when the sun is shining and the wind is blowing — then discharge it when necessary. Large lithium ion rechargeable batteries are already being used to store energy to some extent, but “currently, battery technology only has a capacity of covering up to four hours”, notes Carlos Torres Diaz, director of power and gas market research at consultancy Rystad Energy.

While that is likely to improve in the coming years, “there will still be periods where batteries will not be enough, so you need something else to back up the energy system”, Diaz says. “This is when long-term energy storage becomes crucial.”