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You then take electricity from some renewable source (geothermal, solar, wind, whatever) and use that to electrolyse water.
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The basic idea, as is done here, of using sunlight through solar PV to electrolyse water, store the hydrogen then, when the power is required, reverse the process.
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To use that excess electricity (obviously, only excess when the wind is blowing just right, or the Sun shining) to electrolyse water by passing it through a fuel cell and then store the resultant hydrogen.
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