Environmental Engineering Reference
In-Depth Information
held behind it at high tide. Even so, it is claimed that, given that construction is expected to
be easier and quicker than for barrages, costs for power from lagoons can be competitive
(Atkins 2004).
In its review of tidal option, the SDC was however concerned that there '
was a lack of
available evidence on the costs and environmental impacts, mainly due to the absence of any
practical experience'
. But it
' called on Government to support the development of one or
more demonstration project, which would help provide real-life data on their economic and
environmental viability.
' (SDC 2007)
Subsequently, the UK government announced that it would allow lagoons, and barrages,
of up to 1GW capacity, to be eligible for support under the Renewable Obligation Certificate
trading system, with two Renewable Obligation Certificates being offered for each MWh
delivered. This is double the level of support compared with more developed renewables like
on-land wind farms. The justification for this extra cross-subsidy was that, although barrages
and lagoons were capital intensive, they would both continue to operate for decades after the
initial costs had been paid back, so providing some extra support was reasonable. As yet no
projects have come forward in response, but it seems likely that some will.
A government decision of the much larger Severn Barrage is still awaited. In 2008, the
government set up a feasibility study to look into the financial and environmental
implications of larger tidal barrages, and also tidal lagoons. It is expected to report in 2010,
in conjunction with a public consultation exercise.
Tidal Turbines
While plans for new Tidal Barrages are still being considered, and Tidal Lagoons are still
at the conceptual stage, there are many tidal current turbine projects underway or planned in
the UK and elsewhere (NATTA 2007). This is not surprising since they are modular and can
be installed individually, and are much less invasive then either lagoons or barrages. Most
can also easily operate in two-way mode, e.g. by swivelling the rotors around when the tidal
direction changes. In which case the system can operate on four tidal movements every 24
hour cycle, instead of just two, as with simple ebb barrages or lagoons.
The UK has led the way in this field. Following work by IT Power on a 15kW pontoon
supported prototype in the 1990's, in 2003 its offshoot, Marine Current Turbines Ltd (MCT),
installed a 300kW prototype Seaflow machine, essentially a propeller-like device, off the
North Devon coast. MCT are installing a larger two rotor 1.2 MW rated commercial version ,
'SeaGen', in Strangford Narrows, Northern Ireland. After that the aim is to move up scale to
tidal farms with seven machines. The company is looking at possible sites off Wales and N
Devon and also possibly Guernsey.
There are many other designs under test. Propeller type designs have dominated so far,
with in the case of the MCT devices, the rotors mounted on piles driven into the sea bed.
However some designers have tried to get enhancement of the tidal flow using annular ducts
around the rotor, as with Lunar energy's device, which sits on the sea bed. There are plans for
an eight machine tidal farm off the Welsh coast using this system.
There have also been some novel designs, like the 'Open Centre Turbine' developed by
Dublin based Open Hydro. This has blades running in a circular cam ring, which also acts as
the generator, thus avoiding gearing. A 250 kW prototype has been tested at the European
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