day. Here in Britain we don’t directly see these Atlantic crests and troughs
– we are set back from the Atlantic proper, separated from it by a few
hundred miles of paddling pool called the continental shelf. Each time
one of the crests whooshes by in the Atlantic proper, it sends a crest up
our paddling pool. Similarly each Atlantic trough sends a trough up the
paddling pool. Consecutive crests and troughs are separated by six hours.
Or to be more precise, by six and a quarter hours, since the time between
moon-rises is about 25, not 24 hours.

The speed at which the crests and troughs travel varies with the depth
of the paddling pool. The shallower the paddling pool gets, the slower the
crests and troughs travel and the larger they get. Out in the ocean, the
tides are just a foot or two in height. Arriving in European estuaries, the
tidal range is often as big as four metres. In the northern hemisphere, the
Coriolis force (a force, associated with the rotation of the earth, that acts
only on moving objects) makes all tidal crests and troughs tend to hug the
right-hand bank as they go. For example, the tides in the English channel
are bigger on the French side. Similarly, the crests and troughs entering
the North Sea around the Orkneys hug the British side, travelling down
to the Thames Estuary then turning left at the Netherlands to pay their
respects to Denmark.

Tidal energy is sometimes called lunar energy, since it’s mainly thanks
to the moon that the water sloshes around so. Much of the tidal energy,
however, is really coming from the rotational energy of the spinning earth.
The earth is very gradually slowing down.

So, how can we put tidal energy to use, and how much power could
we extract?

Rough estimates of tidal power

When you think of tidal power, you might think of an artificial pool next
to the sea, with a water-wheel that is turned as the pool fills or empties
(figures 14.2 and 14.3). Chapter G shows how to estimate the power avail-
able from such tide-pools. Assuming a range of 4 m, a typical range in
many European estuaries, the maximum power of an artificial tide-pool
that’s filled rapidly at high tide and emptied rapidly at low tide, generating
power from both flow directions, is about 3 W/m2. This is the same as
the power per unit area of an offshore wind farm. And we already know
how big offshore wind farms need to be to make a difference. They need
to be country-sized
. So similarly, to make tide-pools capable of producing
power comparable to Britain’s total consumption, we’d need the total area
of the tide-pools to be similar to the area of Britain.

Amazingly, Britain is already supplied with a natural tide-pool of just
the required dimensions. This tide-pool is known as the North Sea (figure
14.5). If we simply insert generators in appropriate spots, significant
power can be extracted. The generators might look like underwater wind

Figure 14.2. Woodbridge tide-pool and tide-mill. Photos kindly provided by Ted Evans.
Figure 14.3. An artificial tide-pool. The pool was filled at high tide, and now it’s low tide. We let the water out through the electricity generator to turn the water’s potential energy into electricity.
2 m 1 W/m2
4 m 3 W/m2
6 m 7 W/m2
8 m 13 W/m2
Table 14.4. Power density (power per unit area) of tide-pools, assuming generation from both the rising and the falling tide.