Sizable Energy is developing an offshore gravity-based energy storage system that uses seawater depths and high-density brine to store and return renewable electricity to the grid. The technology applies the principle of pumped hydro storage at sea, using flexible reservoirs on the seabed and at the surface instead of conventional water reservoirs at different elevations.

When surplus electricity is available, pumps move brine from a flexible reservoir on the seabed to a floating reservoir at the surface. When electricity is needed, the brine flows back down through a turbine to generate power. The company describes the system as a closed-loop design that uses commonly available materials, requires no lithium and consumes no land.

Sizable Energy co-founder Simone Biondi said that the company had completed its demonstration and concept-validation phase and moved into project development over the past year. It completed offshore tests in Reggio Calabria with the University of Reggio Calabria, following earlier tank testing at the Dutch marine research institute Marin. The tests validated key components under real conditions and demonstrated that deployment operations could be completed without specialized vessels.

The company has also received the European Commission’s STEP Seal and secured additional funding for an integrated demonstrator with a data center in Naples, scheduled for the first half of 2027. Pre-front-end engineering design (pre-FEED) and front-end engineering design (FEED) activities are underway for an initial project in Calabria and a second in Sicily.

Biondi said Sizable Energy has built an international pipeline comprising more than 10 GWh of potential projects in the Mediterranean and Middle East, with advanced discussions also taking place in the Caribbean.

Ocean storage

Sizable Energy’s platform is designed to integrate with floating solar and offshore wind. Floating solar panels could share infrastructure, moorings and grid connection cables with the storage reservoirs. The company’s Power Shell concept also combines offshore storage, floating solar and data centers on a single platform.

Biondi said long-duration storage could shift solar generation from periods of high daytime production to evening and nighttime demand, while also helping cover longer periods of low renewable output. He identified data centers and other continuous loads as potential applications, particularly where renewable electricity supply is required around the clock.

The system separates power and energy capacity. Power output depends on water depth and the number of machines installed, while storage capacity depends on reservoir volume and depth. This allows the two parameters to be sized independently for different applications.

The company does not view lithium-ion battery energy storage systems (BESS) as direct competitors, but as complementary technologies. Biondi said its system could provide grid response, frequency services, reserves, balancing and physical or synthetic inertia, depending on its configuration.

He said the technology is particularly suited to utility-scale projects measured in gigawatt-hours (GWh), because storage costs scale with reservoir volume rather than the number of battery cells. Sizable Energy estimates a service life of more than 30 years, without cycle-related degradation, compared with the 10- to 15-year lifespan it attributes to conventional batteries.

Site requirements

The technology requires offshore sites with water depths of at least 500 meters and reasonable proximity to the coast and grid. Biondi said it is intended for utility-scale applications rather than smaller projects, where conventional battery systems remain more suitable.

He also acknowledged that the technology is at a first-of-a-kind stage. Although it uses components and materials already deployed in the offshore industry, integrating them into a new system architecture carries technical risks that must be addressed through field demonstrations.

In Italy, Biondi identified Terna’s Macse mechanism, which provides long-term contracts for storage capacity, as a potential framework for grid-connected projects. Sizable Energy is also exploring direct renewable power supply agreements with large consumers, including data centers, to secure long-term revenue.

Floating offshore wind is another potential application. Sizable Energy says its storage system could share export cables with offshore wind farms, allowing the connection to be sized closer to average rather than peak generation. Excess electricity could then be stored offshore and exported when cable capacity becomes available, reducing connection costs and curtailment.