To estimate the power of an artificial tide-pool, imagine that it’s filled

rapidly at high tide, and emptied rapidly at low tide. Power is generated

in both directions, on the ebb and on the flood. (This is called two-way

generation or double-effect generation.) The change in potential energy

of the water, each six hours, is *mgh*, where *h* is the change in height of

the centre of mass of the water, which is half the range. (The range is the

difference in height between low and high tide; figure G.1.) The mass per

unit area covered by tide-pool is *ρ* × (2*h*), where *ρ* is the density of water

(1000 kg/m*3*). So the power per unit area generated by a tide-pool is

2ρhgh |
, |

6 hours |

assuming perfectly efficient generators. Plugging in *h* = 2 m (i.e., range

4 m), we find the power per unit area of tide-pool is 3.6 W/m*2*. Allowing

for an efficiency of 90% for conversion of this power to electricity, we get

power per unit area of tide-pool ≅ 3 W/m*2*.

So to generate 1 GW of power (on average), we need a tide-pool with an

area of about 300 km*2*. A circular pool with diameter 20 km would do the

trick. (For comparison, the area of the Severn estuary behind the proposed

barrage is about 550 km*2*, and the area of the Wash is more than 400 km*2*.

If a tide-pool produces electricity in one direction only, the power per

unit area is halved. The average power density of the tidal barrage at

La Rance, where the mean tidal range is 10.9 m, has been 2.7 W/m*2* for

decades (p87).

The tides around Britain are genuine tidal waves. (Tsunamis, which are

called “tidal waves,” have nothing to do with tides: they are caused by

underwater landslides and earthquakes.) The location of the high tide (the

crest of the tidal wave) moves much faster than the tidal flow – 100 miles

per hour, say, while the water itself moves at just 1 mile per hour.

The energy we can extract from tides, using tidal pools or tide farms,

can never be more than the energy of these tidal waves from the Atlantic.

We can estimate the total power of these great Atlantic tidal waves in the

same way that we estimate the power of ordinary wind-generated waves.

The next section describes a standard model for the power arriving in