details. Now let’s estimate the power that could be delivered by three
specific solutions: tide farms, barrages, and offshore tidal lagoons.
One way to extract tidal energy would be to build tide farms, just like wind
farms. The first such underwater windmill, or “tidalstream” generator, to
be connected to the grid was a “300 kW” turbine, installed in 2003 near the
northerly city of Hammerfest, Norway. Detailed power production results
have not been published, and noone has yet built a tide farm with more
than one turbine, so we’re going to have to rely on physics and guesswork
to predict how much power tide farms could produce. Assuming that the
rules for laying out a sensible tide farm are similar to those for wind farms,
and that the efficiency of the tide turbines will be like that of the best wind
turbines, table 14.7 shows the power of a tide farm for a few tidal currents.
Given that tidal currents of 2 to 3 knots are common, there are many
places around the British Isles where the power per unit area of tide farm
would be 6 W/m^{2} or more. This power per unit area can be compared to
our estimates for wind farms (2–3 W/m^{2}) and for photovoltaic solar farms
(5–10 W/m^{2}).
Tide power is not to be sneezed at! How would it add up, if we assume
that there are no economic obstacles to the exploitation of tidal power at
all the hot spots around the UK? Chapter G lists the flow speeds in the
best areas around the UK, and estimates that 9 kWh/d per person could
be extracted.
Tidal barrages are a proven technology. The famous barrage at La Rance
in France, where the tidal range is a whopping 8 metres on average, has
produced an average power of 60 MW since 1966. The tidal range in the
Severn Estuary is also unusually large. At Cardiff the range is 11.3 m at
spring tides, and 5.8 m at neaps. If a barrage were put across the mouth of
the Severn Estuary (from WestonsuperMare to Cardiff), it would make a
500 km^{2} tidepool (figure 14.8). Notice how much bigger this pool is than
the estuary at La Rance. What power could this tidepool deliver, if we let
the water in and out at the ideal times, generating on both the flood and
the ebb? According to the theoretical numbers from table 14.4, when the
range is 11.3 m, the average power contributed by the barrage (at 30 W/m^{2})
would be at most 14.5 GW, or 5.8 kWh/d per person. When the range is
5.8 m, the average power contributed by the barrage (at 8 W/m^{2}) would be
at most 3.9 GW, or 1.6 kWh/d per person. These numbers assume that
the water is let in in a single pulse at the peak of high tide, and let out in a
single pulse at low tide. In practice, the inflow and outflow would be
spread over a few hours, which would reduce the power delivered a little.
speed  power density (W/m^{2}) 


(m/s)  (knots)  
0.5  1  1 
1  2  8 
2  4  60 
3  6  200 
4  8  500 
5  10  1000 