Affordable Flywheel Energy Storage System Developed with High-Tension Vertical Filament Winding

High-tension, vertical filament winding enables affordable flywheel energy storage system

French startup Energiestro is developing a solar energy flywheel-based storage system designed to reduce costs by utilizing glass fiber composites and prestressed concrete. Founded in 2001 by Anne and André Gennesseaux, Energiestro aims to commercialize affordable energy storage solutions for the renewable energy sector. The team decided to pivot from traditional batteries, which often rely on rare metals and are expensive to recycle, to flywheel energy storage systems. These systems store energy by rotating a cylindrical rotor at high speeds.

Early flywheel designs were all-metal, but modern versions have shifted to carbon fiber composites to take advantage of their lighter weight and higher tensile strength. Although carbon fiber composites are effective, Energiestro sought a more cost-effective solution for underground energy storage compatible with home solar panels. “There’s a very high demand for storing solar energy at very low costs,” explains CTO and COO André Gennesseaux.

The team discovered that concrete alone was insufficient due to its inability to resist external forces effectively, leading to inefficiencies and potential cracking. They decided to enhance the concrete’s strength by prestressing it and overwrapping it with glass fiber composites to achieve the necessary compression without significantly increasing weight.

The manufacturing of the 1-meter-diameter, 1.4-meter-tall cylindrical flywheel posed challenges, particularly in finding a suitable filament winding system capable of meeting the high tension requirements of up to 1,000 MPa necessary to compress the concrete. Energiestro collaborated with Autonational Composites to create a customized vertical winding system, as traditional horizontal winders were impractical for the heavy concrete flywheel.

This vertical winder design allowed for precise tension management, ensuring that the glass fiber did not break while being applied. The team developed a patented system with ten rollers that gradually increased tension along the fiber strands, followed by a spray nozzle that applied liquid epoxy. The final product consists of about 50 layers of glass fiber composite encased in a concrete canister.

Currently, the flywheel system can store up to 10 kilowatt-hours of energy, sufficient to power a home through the night after drawing energy from solar panels during the day. Energiestro plans to scale up their system to handle up to 50 kilowatt-hours of storage. The VOSS (Volant de Stockage Solaire or Solar Storage Flywheel) product is designed to be mostly buried underground, with an access hatch for maintenance, and is expected to have a lifespan of over 30 years.

As for costs, Gennesseaux claims that the concrete’s cost per kilowatt-hour of energy stored is approximately €20 with the VOSS system, significantly lower than the cost of carbon fiber flywheels or lithium battery systems. Energiestro currently has two beta test systems in operation and plans to install ten more this year, with ambitions to build a larger factory for mass production by 2027. They aim to partner with local businesses to sell the VOSS systems for residential and commercial solar installations, ultimately targeting large-scale solar plants that may require around 10,000 systems each.

“This development has already been 10 years in the making. No one had made a flywheel with concrete and glass fiber like this before, which led to the development of the winding technology to build it,” says Gennesseaux. Now, with production underway and plans for scaling, Energiestro is poised to make significant strides in the renewable energy storage market.