Innovative design of fiberglass wound prestressed concrete flywheel energy storage system?
The global energy transition is accelerating and driving a surge in energy storage demand. Lithium batteries are limited by resource scarcity and environmental risks, and physical energy storage technology has become an important supplement. Flywheel energy storage achieves kinetic energy-electric energy conversion through high-speed rotating flywheels. The core challenge is to balance material performance and economy. Although traditional carbon fiber flywheels have high energy storage density (>4000MPa tensile strength), the cost is as high as 200 euros/kWh, which is difficult to meet the needs of household and industrial and commercial scenarios.
French Energiestro company proposed an innovative solution with concrete as the matrix and fiberglass as the reinforcement material. It breaks through the material limitations through prestressed winding technology and provides a new path for low-cost flywheel energy storage. The company's VOSS ("Solar Energy Storage Flywheel") products, including prestressed concrete/glass fiber composite flywheels, are designed to be buried in concrete tanks in residential or corporate courtyards equipped with solar panels, and have access ports on the ground for maintenance. The system is designed to have a service life of 30 years.
In terms of material selection, early flywheel energy storage systems used an all-metal design, and later a newer and more efficient carbon fiber composite version appeared. This version uses carbon fiber's lighter weight and higher tensile strength than steel to make it rotate faster, thereby storing more energy at the same mass. However, carbon fiber composite flywheels are expensive, so Energiestro has developed an alternative solution based on fiberglass composites and concrete to create a lower-cost and more durable flywheel.
This solution uses a combination of fiberglass + concrete, with a flywheel tensile strength of about 1000~1500MPa and a cost of only 20 euros/kWh, which is one-tenth the cost of a carbon fiber flywheel, and has a significant cost-effectiveness advantage. To achieve efficient prestressing, Energiestro and Autonational Composites developed a vertical winding system and tension control technology: in view of the weight limit of the 3-ton concrete flywheel, a vertical winding layout is adopted, combined with crane-assisted transportation to solve the problem that traditional horizontal equipment cannot bear the load; 10 sets of independent roller systems are used to gradually increase the fiber tension to avoid fiberglass fracture due to instantaneous high stress, while matching the fiber modulus change (length extension with increasing tension) to ensure winding uniformity; epoxy resin is sprayed in real time during the winding process, and after curing, a dense composite material layer is formed with a thickness of about 2cm (50 layers), taking into account both lightweight and structural strength.
In terms of energy storage density, the energy storage density of the fiberglass-concrete flywheel reaches 8Wh/kg, which is 300% higher than the full concrete solution, but still 32% of the carbon fiber flywheel (25Wh/kg). And in a vacuum environment, the overall efficiency of the system is 85%, which is the same as the carbon fiber flywheel. The price of glass fiber is about 1/10 of that of carbon fiber, and the cost of the concrete matrix is negligible; the customized winding system realizes automated production, the manufacturing cycle of a single flywheel is controllable, and the marginal cost is further reduced after scale-up; from the perspective of the product's full life cycle cost, the 30-year design life is far longer than that of lithium batteries (usually 5-10 years), and basically no maintenance or replacement is required. The unit electricity cost in the 30-year operating cycle is 0.05 euros/kWh, which is lower than that of lithium batteries (0.12 euros/kWh). Energiestro's VOSS system is positioned in household and industrial and commercial energy storage scenarios. The buried design reduces space occupation and is directly integrated with solar panels. Its target customers have a clear demand for "low cost and long life", and the cost-effectiveness of fiberglass composite materials perfectly fits this market gap.
The team finally successfully manufactured a prestressed concrete/fiberglass composite flywheel that can currently store up to 10 kWh of solar panel electricity when operating in an energy storage system. Energiestro plans to further develop larger-scale systems to increase the energy storage capacity to 50 kWh.
As of early 2025, Energiestro has installed two beta test systems for commercial and industrial solar panel systems, and plans to install 10 more this year. The systems are all produced at the company's pilot plant in France. "We are now raising funds to build a larger factory so that we can achieve large-scale production," said founder Gennesseaux. The company hopes to start commercial production in 2027.
