Electricity-generating tidal lagoons located in the Severn Estuary could provide an economically attractive and environmentally acceptable way of supplying up to 7% of England and Wales’s electricity consumption with low-cost, low-carbon electricity.
There are a large range of potential environmental and economic benefits and disbenefits associated with siting lagoons or the proposed Severn Barrage in the Estuary. However, initial comparisons strongly suggest that lagoons could be significantly less extensive and environmentally damaging and more cost effective and powerful than the Barrage. Lagoons would not directly impound the ecologically highly valuable inter-tidal areas of the Estuary.
Indeed, lagoons may offer potentially significant wildlife habitat. Yet, lagoons would generate twice as much power per square mile impounded than the Barrage and could extract about 25 - 40% more energy from two thirds of the impounded area.
Considering the wider environmental and economic issues, the sourcing of large volumes of aggregates for the lagoons would be critical because this could result in substantial adverse impacts. Yet, every tonne of aggregate used in lagoon construction would enable the generation of about three times more electricity than a tonne of coal burnt in a power station, and there would be no greenhouse gas or acid gas emissions.
Lagoons would not impede shipping but the Barrage could provide a novel transport link.
Both technologies would generate significant quantities of low-carbon electricity close to
large populations. However, the Unit generation cost, output timings, storage capability and smaller capital costs of lagoons are likely to be far more attractive to private investors and consumers.
Several large lagoons could be built over time to impound as much as 115 square miles of
the Estuary (roughly an area 11 miles by 11 miles). Even then the lagoons would cover 70
square miles less than the 185 square miles the Barrage would impound. Yet this area of
lagoons would capture about 26 - 41% more of the Estuary’s tidal energy than the Barrage (2.75 GW average from lagoons, 1.95-2.15 GW average from Barrage). This is because the lagoons can generate electricity on both the ebb and flood tides while the Barrage is limited to generating mostly on the ebb tide to reduce silting. Both types of scheme would enhance their output by innovative pumping techniques. Overall, lagoons would generate just over twice as much electricity per square mile impounded than the Barrage.

Technically speaking, the capacity factor of a multi-pool lagoon is about 61%, compared to about 26% for the Barrage. For comparison, the nominal capacity factors for other electricity generation technologies are 30% for onshore windfarms, 35% for offshore windfarms, and 33% for marine current turbines. The capacity factor of the Wylfa nuclear power station has been about 56% to date, and the now closed Trawsfynydd station was 60%.

Both technologies would produce electricity close to major demand centres in south Wales
and the south west of England. This geographical proximity is highly valuable because of
grid security and transmission losses. The major electricity flows along the UK grid are from power stations in the north to demand centres in the south. Also, by around 2010 it is likely that the two remaining nuclear power stations in the region, namely Oldbury (440 MW capacity) and Hinkley Point B (1,325 MW capacity) will be near to closure or closed. Hinkley Point A (475 MW capacity) has recently closed.
The combined power of all these three nuclear stations when operating at maximum output would have been 2,240 MW but they would probably have averaged about 1600 MW due to maintenance and fault outages (assuming a 70% capacity factor). So, the average output of the nuclear stations would be roughly 75% of the average Barrage output, and only 60% of the average output of lagoons covering an area of 115 square miles. In population terms the lagoons could supply over 3.5 million people at current demand levels. Both schemes would require Grid strengthening. However, the maximum output of the lagoons would be 4.5 GW so requiring less expensive reinforcement than the 8.64 GW Barrage.
Because tides are predictable, the electrical output of both lagoons and Barrage would be
predictable. However, both technologies might also provide significant and cost-effective,
demand-responsive and energy storage capabilities. This would depend on pumping losses and the market value of electricity as traded under the New Electricity Trading Arrangements (NETA). Such capabilities may well reduce the need for ‘spinning reserve’ as a back-up for the existing unpredictable power generating technologies and facilitate the integration of the more intermittent renewable energy technologies particularly wind and solar energy.

Both Barrage and lagoon schemes, given normal maintenance, could last well over one
hundred years with little change in comparison with the 50 years of a nuclear power station. The low-head turbines which would be used by either
scheme are a well proven, long-lasting technology and aggregates have an unlimited life
span. Indeed, the lagoon aggregates, which are for the most part loose, could be reclaimed for further uses as part of any decommissioning process at some point in the future.
The high tidal range of the sensitive Severn Estuary may yet be harnessed for electricity
generation, possibly sooner rather than later, and not in a way previously envisaged. On the basis of this preliminary analysis and comparison tidal lagoons could provide a major source of safe, clean, regionally generated renewable electricity. Lagoons also appear to offer numerous significant economic and environmental advantages over a Severn Barrage. Yet only a few years ago the Royal Commission on Environmental Pollution included the Severn Barrage in three out of four of its 2050 energy scenarios to reduce UK greenhouse gas emissions. For the reasons stated in the Summary Friends of the Earth Cymru continues to call on the Welsh Assembly Government to investigate the potential of tidal lagoons, to help fund specific lagoon studies and to support acceptable proposals.

The appearance of tidal lagoons off the Welsh coast could be a ‘world first’ for Wales as
would befit a country with a world-leading sustainable development remit. Observable from space, large lagoons would be a spectacular symbol of a species harnessing natural
planetary forces for the protection of the Earth’s biosphere and all the diverse species it
supports.

Friends of the Earth Cymru recommends that the National Assembly for Wales and all other relevant agencies provide the necessary support to assist in detailed studies to identify if the construction of tidal lagoon schemes in the Severn Estuary, and elsewhere, is viable and sustainable.

Written by Neil Crumpton
Campaigner and Energy Spokesperson
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