The 'concrete'-based approach (gravity storage) is pursued, for example by a firm named Heindl Energy https://heindl-energy.com/ https://heindl-energy.com/ The

The 'concrete'-based approach (gravity storage) is pursued, for example by a firm named Heindl Energy https://heindl-energy.com/ https://heindl-energy.com/ There are offerings and deployed energy-storage systems based upon flywheels https://en.wikipedia.org/wiki/Flywheel_energy_storage https://en.wikipedia.org/wiki/Flywheel_energy_storage