Dutch clean technology start-up Ocean Grazer has developed a unique energy storage system for large-scale offshore renewable energy generation. The modular Ocean Battery pumped hydro system is engineered to be located on the seabed near offshore wind turbines or floating solar power plants to store the excess energy produced.

A concrete reservoir buried in the seabed stores up to 20 million liters (5.3 million gal) of fresh water at low pressure. A system of pumps and turbines connects this reservoir to a flexible bladder on the seafloor. Excess electricity from renewable energy sources is 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 for delivery to the grid.

When charged (left), the bladder is full of water and the concrete reservoir is empty. When the battery is discharged (right), the bladder is empty and the reservoir full. Source: Ocean Grazer

The Ocean Battery system has an efficiency of 70% to 80% and is expected to operate for an unlimited number of cycles over a service life of more than 20 years. The scalable concrete reservoir has a capacity of 10 MWh, enabling power system developers to add units to increase the overall capacity. Extra units of the pump and turbine machinery can also be added to boost power output if more energy is needed quickly.