Sizable Energy, a start-up developing long-duration energy storage technology, has announced that it has raised $8 million to accelerate its path to commercialisation. The round was led by Californian venture capital firm Playground Global, with participation from Exa Ventures, Verve Ventures, Satgana, EDEN/IAG and Unruly Capital , who had led the previous round.
Following successful tests conducted at the Maritime Research Institute Netherlands (MARIN) research centre and with new sea trials underway in Reggio Calabria, in collaboration with the NOEL laboratory, Sizable is now advancing in the technological qualification of its offshore hydroelectric pumping system to provide high-capacity, low-cost energy storage using gravity.
“Without economically sustainable long-term storage systems, the electricity grid risks running into difficulties, regardless of the energy mix,” said Manuele Aufiero, CEO and co-founder of Sizable Energy, in a statement. “Our system is designed to store millions of kWh in an environmentally sustainable and economically competitive way, making the grid more stable and resilient.”
This round of investment accelerates the development and deployment of Sizable Energy’s innovative offshore hydroelectric pumping system, designed to provide a cost-effective and reliable long-duration energy storage (LDES) solution.
According to the International Energy Agency, by 2040 there will be a need for up to 120 terawatt hours of long-duration storage worldwide, ten times more than current capacity. Today, most of the available capacity comes from onshore hydroelectric pumping, which is slow to build and limited by geographical and environmental constraints. Sizable Energy offers a faster and more flexible solution: offshore hydroelectric pumping that is modular and ready for industrial scale.
Sizable Energy’s patented system stores energy by pumping saturated brine (denser than seawater) in a closed-loop system from a lower tank anchored to the seabed to a floating tank on the surface, exploiting the depth of the ocean rather than the difference in height between two tanks in the mountains. When energy needs to be fed into the grid, the brine descends by gravity through the pipe via reversible turbines, whose rotation generates electricity that is fed back into the grid.
Invisible from the coast, Sizable Energy’s solution is expected to be economically competitive in terms of total costs per unit of stored energy, even considering the most optimistic projections for lithium battery cost reductions. In addition, it adopts a modular and scalable design, capable of providing from one to hundreds of gigawatt hours of capacity. Finally, it uses readily available materials that can be manufactured, assembled and installed at depths of 500 metres or more, utilising existing shipbuilding and maritime infrastructure.
“As a society, we have demonstrated that wind and solar can provide the cheapest electrons. Now we urgently need long-duration, economically scalable storage solutions,” says Bruce Leak, general partner at Playground Global and board member at Sizable Energy. – The depth of the ocean is a virtually unlimited resource, and Sizable Energy is harnessing it to offer long-duration storage at a fraction of the cost of batteries. This gravity-based solution is the clear path to energy security.
Sizable Energy has validated the innovative components of its technology both in various laboratories (wave tanks that simulate real-world operating conditions) and through experimental campaigns at sea, verifying its actual resistance to marine conditions. Tests conducted in September at the MARIN research centre in the Netherlands confirmed that the system is capable of operating reliably even in severe sea conditions.
Sizable Energy has also commenced preliminary activities for its upcoming sea trial off the coast of Reggio Calabria. This project will validate all key floating components, demonstrate the assembly and installation process, and pave the way for a multi-MWh demonstration plant in the Mediterranean Sea off the Italian coast.
As this milestone approaches, the company is preparing for the development of commercial projects in 2026 at several global sites, in collaboration with local partners and energy suppliers.
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