Mechanical battery (gravity battery) demonstration
Not the most convenient energy storage device, but it does have some unique advantages, like high efficiency, reliability and a very long lifespan if designed well.
This device could be "charged" by a wind turbine, essentially storing "free energy" ("free" in the financial sense of the word, not thermodinamically free, of course). After it's been partially or fully charged, it can then be discharged, spinning an electrical generator or some other machine in the process.
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