mills. Because the density of water is roughly 1000 times that of air, the
power of water flow is 1000 times greater than the power of wind at the
same speed. We’ll come back to tide farms in a moment, but first let’s
discuss how much raw tidal energy rolls around Britain every day.
The tides around Britain are genuine tidal waves – unlike tsunamis, which
are called “tidal waves,” but are nothing to do with tides. Follow a high
tide as it rolls in from the Atlantic. The time of high tide becomes progress-
ively later as we move east up the English channel from the Isles of Scilly
to Portsmouth and on to Dover. The crest of the tidal wave progresses up
the channel at about 70 km/h. (The crest of the wave moves much faster
than the water itself, just as ordinary waves on the sea move faster than
the water.) Similarly, a high tide moves clockwise round Scotland, rolling
down the North Sea from Wick to Berwick and on to Hull at a speed of
about 100 km/h. These two high tides converge on the Thames Estuary.
By coincidence, the Scottish crest arrives about 12 hours later than the crest
that came via Dover, so it arrives in near-synchrony with the next high tide
via Dover, and London receives the normal two high tides per day.
The power we can extract from tides can never be more than the total
power of these tidal waves from the Atlantic. The total power crossing the
lines in figure 14.6 has been measured; on average it amounts to 100 kWh
per day per person. If we imagine extracting 10% of this incident energy,
and if the conversion and transmission processes are 50% efficient, the
average power delivered would be 5 kWh per day per person.
This is a tentative first guess, made without specifying any technical