The message
from Andy Champ contains these words:

The output of a tidal system is roughly sinusoidal, peaking at half tide.

Add two systems, 90 degrees out of phase, and you have an approximation
to a continuous supply.

I can't find anything at the moment that predicts the period during
which tidal energy can usefully be extracted but the current front
runner for the Severn Barrage seems to be designed only to generate on
the flow tides. That would leave each flow tide on its own if there were
just 2 systems 90 degrees out of phase.

I presume output could be controlled so that once the barrage starts to
generate it gives even output as much as half the time from low tide to
high tide might be lost waiting for the head to build. One reference
suggested output is proportional to the square of the tidal range* which
suggests they will be going for shorter generating times rather than
longer.

*Not clear to me why that should be the case (I thought mgh would rule)
but no doubt someone will soon be along to point out I have managed to
overlook something really obvious.

--
Roger Chapman