This French company uses glass fiber reinforced flywheels for energy storage instead of steel. Quote:
<< Gennesseaux went on to say. “We are around 10 grams of CO2 per kWh, while lithium-ion batteries are around 40g CO2/kWh, at the same level as steel flywheels.”>>
Initially the flywheel will store 10 kWh. It’s buried underground. Steel flywheels cost about 200 Euros per kWh – concrete is less expensive – a few Euros – and uses common raw materials.
https://www.pv-magazine.com/2021/06/21/concrete-flywheel-storage-system-for-residential-pv/