Environmental Engineering Reference
In-Depth Information
There is something of a flurry of innovative activity underway in a number of countries
in the tidal current turbine area at present, with forty or more devices of various designs being
tested. But there is also still interest in tidal barrages and lagoons.
This paper provides an overview of the state of play with tidal energy developments
around the world, looking in particular at progress with tidal current turbine projects. It then
explores some of the key design issues and looks at the overall technological and economic
prospects for the future use of tidal energy.
Tidal Barrages
Tidal energy is the result of the gravitational pull of the moon acting on the seas,
modified by the lesser pull of the more distant sun. The regular rise and fall of the tides has
been used for centuries in tidal mills of various designs, but it was only in the last century that
electricity production was attempted, using barrages across river estuaries.
The largest tidal barrage so far built is the 240 Megawatt (MW) La Rance barrage in
Brittany, France, which was commissioned in 1966. A few smaller projects have been
developed e.g. in Canada (20MW) Russia (400 kilowatt) and China (500kW), and there have
been many proposals for barrages around the world (Elliott 2004). Most recently, there have
been proposals for some large projects in S Korea- a 254MW barrage on Sihwa Lake and an
812MW project at Ganghwa Island.
However the main emphasis has been on the Severn estuary in the UK, which has one of
the world's largest tidal ranges. Many detailed studies have indicated that a 11mile long 8.6
Gigawatt (GW) rated barrage could be built between Weston super Mare on the English side
and Lavenock Point on the Welsh side. It would generate around 17 Terawatt-hours (TWh)
per annum, about 4.6% of UK electricity requirements. The cost however would be
significant, around £15 billion ($30 billion). Given that the construction time would also be
relatively long (up to 10 years), while it might be feasible as publicly funded project, it seems
unlikely to be attractive in the current very competitive UK climate of privatised electricity
generation and liberalised energy markets.
It has been estimated that at 2% public sector discount rate, the Barrage would generate at
between 2.27 and 2.31pence/kWh (around 4.5 US cents/kWh), depending on how long it took
to build, whereas at 10% private sector discount rate, the cost rises to 11.18-12.37p/kWh
(around 23 c/kWh) (SDC 2007).
In addition to the high capital costs, there are problems related to the nature of tidal
energy. Since the barrage would only operate during the (roughly) twice daily tidal cycles, its
8.6 GW turbine capacity could only offer the same output, averaged out over a year, as
around 2 GW of conventional plant. In addition, given the shifting lunar phasing, its output
would not always be well matched to the cyclic daily pattern of demand, thus reducing the
value of the electricity it produced.
Two-way operation, on the tidal flow (in) as well as the ebb (out) is possible, and would
extend the available output pattern to some degree, but having to install two-way turbines
adds to costs, and, in the main, barrages are often seen as, in effect, fuel saving hydro damns
filled by the tides. It is possible to use off-peak power from other sources to pump water
behind the barrage to increase the head of water for later power generation. The barrage could
thus be used a pump storage facility, although that would require either separate pumps, or
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