Tidal power, while clean and green, should not be called renew-
able. Extracting power from the tides slows down the earth’s
rotation. We definitely can’t use tidal power long-term.

False. The natural tides already slow down the earth’s rotation. The
natural rotational energy loss is roughly 3 TW (Shepherd, 2003). Thanks to
natural tidal friction, each century, the day gets longer by 2.3 milliseconds.
Many tidal energy extraction systems are just extracting energy that would
have been lost anyway in friction. But even if we doubled the power ex-
tracted from the earth–moon system, tidal energy would still last more
than a billion years.

Notes and further reading

page no.

82The power of an artificial tide-pool. The power per unit area of a tide-pool is
derived in Chapter G, p311.

Britain is already supplied with a natural tide-pool . . . known as the North
. I should not give the impression that the North Sea fills and empties
just like a tide-pool on the English coast. The flows in the North Sea are
more complex because the time taken for a bump in water level to propagate
across the Sea is similar to the time between tides. Nevertheless, there are
whopping tidal currents in and out of the North Sea, and within it too.

83The total incoming power of lunar tidal waves crossing these lines has been
measured to be 100 kWh per day per person
. Source: Cartwright et al. (1980).
For readers who like back-of-envelope models, Chapter G shows how to
estimate this power from first principles.

84La Rance generated 16 TWh over 30 years. That’s an average power of
60 MW. (Its peak power is 240 MW.) The tidal range is up to 13.5 m; the
impounded area is 22 km2; the barrage 750 m long. Average power density:
2.7 W/m2. Source: [6xrm5q].

85The engineers’ reports on the Severn barrage...say 17 TWh/year. (Taylor,
2002b). This (2 GW) corresponds to 5% of current UK total electricity con-
sumption, on average.

86Power per unit area of tidal lagoons could be 4.5 W/m2. MacKay (2007a).

Figure 14.10. Tide.