One startup’s quest to store electricity in the ocean

As a child, Manuele Aufiero often explored a unique reservoir in northern Italy with his family, one that would fill and drain based on electricity demand. This reservoir employed a technology known as pumped-storage hydropower, which has been in use for over a century. Globally, these systems store approximately 8,500 gigawatt-hours of electricity, according to the International Energy Agency, proving crucial as renewable energy sources like wind and solar gain traction. Despite the advantages of pumped hydro, Aufiero recognized the limitations posed by geographical constraints. "I’m in love with pumped hydro," he shared, but lamented that current solutions are insufficient to keep pace with the growing demand for renewable energy. To address this challenge, he co-founded Sizable Energy, aiming to innovate by adapting pumped hydro technology for ocean use. Recently, Sizable secured $8 million in funding, spearheaded by Playground Global with contributions from several venture firms, enabling the startup to advance its groundbreaking vision. The proposed energy system resembles an hourglass, consisting of two flexible reservoirs—one floating on the surface and another anchored to the seabed. These reservoirs are interconnected by a plastic tube and turbines. During periods of low energy costs, the system pumps highly saline water from the seabed reservoir to the floating one. When energy demand spikes, the system releases the saline water, which, being denser than the surrounding seawater, descends back down, spinning the turbines to generate electricity. Aufiero explains, "From an energy balance perspective, we are lifting blocks of salt. Instead of using cranes, we dissolve and pump it, which simplifies the process." By transitioning pumped hydro to the ocean, Sizable aims to streamline production, as traditional land-based systems require bespoke designs for each site. The startup has already conducted tests of a smaller model in wave tanks and along the coast of Reggio Calabria, Italy. It is now preparing a pilot project for the floating components before launching a full demonstration plant. By 2026, Sizable plans to roll out multiple commercial projects across various global locations. At optimal capacity, each turbine could generate between 6 to 7 megawatts of electricity, with one turbine for every 100 meters of pipeline. The deeper the site, the greater the storage potential, and multiple reservoirs can be installed at each commercial location. Sizable is targeting an energy storage cost of €20 per kilowatt-hour (approximately $23), significantly lower than traditional grid-scale batteries. The technology is particularly compatible with offshore wind projects, as sharing electrical connections can reduce expenses. Aufiero emphasizes that Sizable's reservoirs could connect to any grid situated near waters that are at least 500 meters deep. "We believe that long-duration energy storage is essential not just for integrating renewables but also for enhancing grid resilience," he asserted. "Traditional pumped hydro or batteries alone cannot meet this need; we require innovative solutions."