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That looks very cool. Mechanical energy storage. Any idea what the return efficiency of such a device is? And guesses about its cost?

No idea. But interestingly enough, it looks like someone built a grid scale version in PA with funds from the 2008 Recovery Act. There's a final tech report here https://www.smartgrid.gov/project/hazle_spindle_20mws_flywhe... in the Related Docs