The current proposals for the barrage will generate power in one direction
only. This reduces the power delivered by another 50%. The engineers’
reports on the proposed Severn barrage say that, generating on the ebb
alone, it would contribute 0.8 kWh/d per person on average. The barrage
would also provide protection from flooding valued at about £120M per

Tidal lagoons

Tidal lagoons are created by building walls in the sea; they can then be
used like artificial estuaries. The required conditions for building lagoons
are that the water must be shallow and the tidal range must be large.
Economies of scale apply: big tidal lagoons make cheaper electricity than
small ones. The two main locations for large tidal lagoons in Britain are
the Wash on the east coast, and the waters off Blackpool on the west coast
(figure 14.9). Smaller facilities could be built in north Wales, Lincolnshire,
southwest Wales, and east Sussex.

If two lagoons are built in one location, a neat trick can be used to
boost the power delivered and to enable the lagoons to deliver power on
demand at any time, independent of the state of the tide. One lagoon can
be designated the “high” lagoon, and the other the “low” lagoon. At low
tide, some power generated by the emptying high lagoon can be used to

Figure 14.8. The Severn barrage proposals (bottom left), and Strangford Lough, Northern Ireland (top left), shown on the same scale as the barrage at La Rance (bottom right). The map shows two proposed locations for a Severn barrage. A barrage at Weston-super-Mare would deliver an average power of 2 GW (0.8 kWh/d per person). The outer alternative would deliver twice as much.
There is a big tidal resource in Northern Ireland at Strangford Lough. Strangford Lough’s area is 150 km2; the tidal range in the Irish Sea outside is 4.5 m at springs and 1.5 m at neaps – sadly not as big as the range at La Rance or the Severn. The raw power of the natural tide-pool at Strangford Lough is roughly 150 MW, which, shared between the 1.7 million people of Northern Ireland, comes to 2 kWh/d per person. Strangford Lough is the location of the first grid-connected tidal stream generator in the UK.