Startup Sizable Energy Aims to Bring Pumped Hydro Storage to the Ocean

2025-10-22 0 By

Sizable Energy, a startup founded by Manuele Aufiero, is developing a novel approach to pumped-storage hydropower, moving the technology from land-based reservoirs to the ocean. Traditional pumped hydro, a well-established method of storing electricity, utilizes reservoirs that drain and refill using pumps. Globally, these facilities currently store 8,500 gigawatt-hours of electricity, but suitable locations are limited.

Sizable’s design employs two sealed, flexible reservoirs – one floating at the ocean surface and the other resting on the seabed – connected by a plastic tube and turbines. When electricity is inexpensive, the turbines pump highly saline water from the lower reservoir to the upper. When grid demand rises, a valve is opened, allowing the denser, salt-rich water to flow downwards, spinning the turbines and generating electricity.

The startup recently secured $8 million in funding led by Playground Global, with participation from EDEN/IAG, Exa Ventures, Satgana, Unruly Capital, and Verve Ventures. This offshore deployment offers scalability advantages compared to land-based pumped hydro, enabling mass production of the technology due to standardized components and deployment across diverse locations.

Sizable has tested prototype reservoirs and plans to deploy a pilot of the floating components ahead of a full demonstration plant by 2026. Full-scale turbines are projected to generate approximately 6 to 7 megawatts of electricity each, with multiple reservoirs planned for commercial sites. The company anticipates delivering energy storage at a cost of €20 per kilowatt-hour (approximately $23), significantly lower than current grid-scale battery costs. The technology is particularly suited for pairing with offshore wind projects, minimizing infrastructure costs by sharing connection points.