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ISIS Report 30/03/09
Reef to Replace Barrage to Tap the Tides?
How to get renewable energy from the tides in the Severn without damaging the environment
Peter Bunyard
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The Barrage
The Severn estuary has long attracted the attention
of engineers and various governments, who see the ebb and flow of its tides
– reckoned to be third highest tidal range in the world - as a potential major
contributor to the UK’s use of renewable energy. By building a barrage across
the estuary, engineers estimate that the 7 to 8 metre average tidal range
could provide up to 5 per cent of the UK’s electrical energy, with up to 8
gigawatts (GW) being generated at low tide when the water from the high tide
has gathered behind the dam. According to the Department
for Business Enterprise & Regulatory Reform (BERR), total primary energy
consumption in the UK in 2007 was about 230 million tonnes of oil equivalent [1], with electricity
consuming at least one third. On the basis of the barrage generating a 24
hour average of 2 GWs, and, as 1 Mtoe is 11.630 GWh, it would yield 17 520
GWh/y, about 0.65 percent of UK’s total primary energy and 2 per cent of total
electricity [2]. That is under half the 5 per cent
claimed by the Chartered Institution
of Water and Environmental Management which has stated that the barrage would
pay back its carbon footprint in construction in no more than 6 months [3].
Yet, since the Severn Barrage scheme estimated to
cost £15 000 million was first proposed many decades
back, it has triggered environmental concerns among a wide range of
groups. A 2008 report [4],
commissioned from Frontier Economics by the Royal
Society for the Protection of Birds (RSPB), WWF-UK, the Anglers’ Conservation
Association, the National Trust, as well as fishery interests, such as the
Wye Salmon Fishery Owners’ Group, concluded that the costs of the barrage
could not be justified on economic grounds, let alone on the grounds of the
environmental devastation that its construction and operation would cause.
In its final form, and after a massive undertaking, the Severn
Barrage would not only have to be big enough to extract the potential energy
from the tidal head of 8 metres or more, it would also have to cope with powerful
weather events, such as storm surges, and even sea level rise, now projected
to be faster than estimated a few years ago. Electricity
would be generated by a series of 40 megawatt underwater turbines during
the two extremes of the tide. This would put a considerable strain on the
central grid, which would have to cope with large surges in generation at
times that while predictable are not controllable.
As pointed out in the Frontier Economics report, the barrage
would inevitably lead to the loss of hundreds of square kilometres of mudflats
and salt marsh, home to waders and other coastal birds as well
as to a host of migratory species. Furthermore, on account of the delay in
the natural tidal rhythm from penning in the water and then from the surge
of water over each of the turbines when the gates
are opened, its construction and use would alter drastically the currents
in the estuary, playing havoc with the deposition of silt and having a profound
impact on estuarine life, including fisheries and salmon runs.
The Reef
Cornish hydraulics engineer Rupert Armstrong Evans believes the Cardiff-Weston
Barrage across the Severn Estuary (as currently planned) is massively ill-conceived [5]. Instead, he has proposed
a substantially different concept that he claims would generate as much electricity,
but far more steadily than the big barrage and would have a much reduced environmental
impact. In particular, it would leave most
of the mudflats and salt marsh intact.
Having pioneered electronic control systems that revolutionised the use of
mini-hydro in the 1970s; and installed low-head hydraulic turbines
which he designed for use in different parts of the world, from Cornwall,
Wales, Scotland, to Nepal, India and South America, Rupert
has come up with the innovative idea of a structure that has parallels
with a tidal reef and so is designed to extract the energy from no
more than two metres of tidal head
In his tidal reef concept, Rupert proposed
a semi-floating set of caissons (water-tight structures) to stretch across
the estuary, thereby avoiding the massive high-head structure implied in the
construction of the Severn Barrage. The fundamental difference between
the barrage and reef is that, in the latter, the 1 000 turbines of some 10
metres in diameter would be housed within the floating caissons, themselves
designed to ride over a fixed base reef on the estuary floor. By using a moveable
‘crest gate’ to track the tide level and therefore to maintain a small head
difference, irrespective of the stage of the tide, the turbines would operate
for long periods, and for at least double the generation period compared to
the proposed big barrage. In addition, the reef would be far less vulnerable
under adverse conditions than the barrage, on account of its smaller size
and lower operating ‘head’. In that respect, storm surges would easily
go over the structure rather than battering it, as would be the case for the
barrage (see Fig. 1).
Figure 1. Cross section of the
reef with caisson housing a turbine
On account of the structure being more modest than the barrage,
the saving on rock fill alone would amount to more than 10 million tonnes.
At the same time, the passage of ships would be easier, as a single gate, similar in principal to the Thames
Barrier, would allow the passage of even the largest ships with minimal disruption.
This is only possible because of the small head difference across the structure.
This system would also avoid the need to dredge a new deep-water shipping
channel.
Figure 2. Artist impression of tidal reef with road/rail link
(above) or wind machines superimposed on the underlying structure.
As Rupert points out, migratory fish should have no problems
navigating the slow-moving turbines, and with the low head required any changes
to the estuary flows will be significantly reduced, causing far less impact
on the mud banks and salt marshes than would the barrage. Moreover, the time
taken to construct the reef would be considerably less than for the barrage.
Rupert refers to an excellent precedent in the construction of the Mulberry
Harbour floating dock that was put together for the D-day Normandy landings.
The various pieces of the dock were built in six months, before being successfully
installed under enemy fire.
Rupert has had the backing of WS
Atkins, the international engineering consultancy which, in 2008,
declared that Rupert’s Reef Scheme would not only generate more electricity
but would cost considerably less – by some £2 000 million - while simultaneously avoiding the
worst environmental aspects of the Severn Barrage [6].
In the face of mounting concerns over the ecological damage
that would result from constructing the barrage across the Severn estuary, Rupert’s
reef scheme has met with the approval of the RSPB. Furthermore,
the government has just announced [7] that
it believes the ‘Severn Tidal Power Reef’ project to have merit, and is to
commit financial support towards its future development, thereby postponing,
at least for a year, any decision as to which project it gives the go-ahead.
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