As useful as renewable energy sources are, they need to be backed up by storage systems that hold energy for times when the Sun isn’t shining or the wind isn’t blowing. Ocean Battery is a new design for an energy storage system that functions a bit like a hydroelectric dam at the bottom of the sea.

Developed by Dutch startup Ocean Grazer, the Ocean Battery is designed to be installed on the seafloor near offshore renewable energy generators, like wind turbines, floating solar farms, tidal and wave energy systems. It is made up of three components that together function on a principle similar to that of a hydro dam.

Buried in the seabed is a concrete reservoir that holds up to 20 million liters (5.3 million gal) of fresh water, stored at low pressure. A system of pumps and turbines connects this reservoir to a flexible bladder on the seafloor. Excess electricity from the renewable sources can be used to pump water from the reservoir into the bladder. When the energy is needed, the bladder releases and, driven by the pressure of the seawater above it, squeezes its water back down to the reservoir, spinning turbines on the way to generate electricity that’s fed out into the grid.