It happens that Chris Hogg formulated :
The last two paragraphs, referring to a tidal barrage being proposed
in the US, worried me slightly. The last sentence includes the phrase
"The setup would use pumps to replicate natural tides when
necessary...". Sounds suspiciously like perpetual motion to me! OK, so
it won't be, but it's not obvious what it does actually mean.

I would guess, it would pump the water to storage at off peak, then use
the stored water to generate power at peak. Its a well known and cost
effective method.

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Regards,
Harry (M1BYT) (L)
http://www.ukradioamateur.co.uk