pump water out of the low lagoon, making its level even lower than low
water. The energy required to pump down the level of the low lagoon is
then repaid with interest at high tide, when power is generated by letting
water into the low lagoon. Similarly, extra water can be pumped into
the high lagoon at high tide, using energy generated by the low lagoon.
Whatever state the tide is in, one lagoon or the other would be able to
generate power. Such a pair of tidal lagoons could also work as a pumped
storage facility, storing excess energy from the electricity grid.

The average power per unit area of tidal lagoons in British waters could
be 4.5W/m2, so if tidal lagoons with a total area of 800 km2 were created
(as indicated in figure 14.9), the power generated would be 1.5 kWh/d per

Beauties of tide

Totting everything up, the barrage, the lagoons, and the tidal stream farms
could deliver something like 11 kWh/d per person (figure 14.10).

Tide power has never been used on an industrial scale in Britain, so it’s
hard to know what economic and technical challenges will be raised as we
build and maintain tide-turbines – corrosion, silt accumulation, entanglement
with flotsam? But here are seven reasons for being excited about tidal
power in the British Isles. 1. Tidal power is completely predictable; unlike
wind and sun, tidal power is a renewable on which one could depend; it
works day and night all year round; using tidal lagoons, energy can be
stored so that power can be delivered on demand. 2. Successive high and
low tides take about 12 hours to progress around the British Isles, so the
strongest currents off Anglesey, Islay, Orkney and Dover occur at different
times from each other; thus, together, a collection of tide farms could
produce a more constant contribution to the electrical grid than one tide
farm, albeit a contribution that wanders up and down with the phase of
the moon. 3. Tidal power will last for millions of years. 4. It doesn’t require
high-cost hardware, in contrast to solar photovoltaic power. 5. Moreover,
because the power density of a typical tidal flow is greater than the power
density of a typical wind, a 1 MW tide turbine is smaller in size than a
1 MW wind turbine; perhaps tide turbines could therefore be cheaper than
wind turbines. 6. Life below the waves is peaceful; there is no such thing
as a freak tidal storm; so, unlike wind turbines, which require costly engineering
to withstand rare windstorms, underwater tide turbines will not
require big safety factors in their design. 7. Humans mostly live on the
land, and they can’t see under the sea, so objections to the visual impact
of tide turbines should be less strong than the objections to wind turbines.

Figure 14.9. Two tidal lagoons, each with an area of 400 km2, one off Blackpool, and one in the Wash. The Severn estuary is also highlighted for comparison.