A consideration in the transition to renewables is the matter how to store excess energy to use when demand is higher than supply. Perhaps the most obvious idea is batteries. Unfortunately they require rare minerals mined under harsh conditions, stirring social turmoil, especially in countries such as DRC. Pumped hydro works well, but needs mountains and valleys; tricky in certain flat countries, such as Denmark and BeNeL….iterally that’s all! But still, a system proposed by researchers from Oxford (UK), Melbourne (Australia) and Dublin (Ireland) works in any sea. An the idea floats.
The concept is vast, floating flywheels as grid-scale energy stores, capable of spinning up when electricity is cheap and slowing down to feed it back when prices rise. Imagine an enormous concrete ring — nearly two kilometres wide — rotating gently on the sea surface, near offshore wind farms. When there’s a power surplus, the grid supplies energy to spin it faster. When demand spikes, the rotation slows, converting that motion back into electricity.
What’s striking is not only the concept’s ambition but the researchers’ finding that its operation could be remarkably simple. Their modelling shows that the system’s most efficient behaviour reduces to an elegant rule: charge when prices fall below a critical threshold, discharge when they rise above it. In effect, a “switching point” defines the flywheel’s entire strategy — no complex forecasting required.
This simplicity matters because electricity markets are volatile. A device that can make money, or at least stay in the black, by following a single rule is a rare thing. When simulated using real data from Germany’s wholesale market, the model kept the flywheel spinning at nearly constant speed while automatically buying low and selling high.
The researchers also explored how wind-driven sails mounted on the structure could provide a steady torque — offsetting drag from seawater and even generating profit when electricity prices are flat. In calm seas but breezy conditions, the flywheel could effectively act as a hybrid between a wind turbine and an energy battery.
For northern regions like Canada, Scandinavia or the UK, such systems could offer valuable resilience. Offshore locations already hosting wind farms could co-locate floating flywheels, storing gigawatt-hours of power without needing new land or invasive infrastructure. And unlike chemical batteries, a concrete flywheel can last decades with little maintenance or environmental risk.
In short, this is energy storage stripped back to physics: mass, motion, and momentum. While the concept remains at an early stage, it points to a future where parts of the ocean double as vast, slow-turning reserves of clean power — ready to keep the lights on long after the wind drops.
Source
Optimal operation of floating flywheel energy storage under fluctuating electricity prices, SSRN, 2025-10-24
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