HOT TOPICS

SEVERN BARRAGE - TAKING THE LONG VIEW

Peter Ackers concludes his series of articles weighing up the benefits and concerns associated with the proposed Severn Barrage

Some environmentalists have stated that the closing off the estuary, thus excluding the tides for many years during the construction, would be unacceptable, and so it would. That is one of the reasons why caissons are the preferred method of construction, both for the turbines and sluice gates. After floating each one in place, the passages through them would be opened so that the tides may continue to have access to the estuary upstream right up to the time that the scheme is commissioned. Only then would the tidal regime change significantly.

Environmental impact

Clearly, environmental impacts would be significant. There would be certain improvements, as well as possible negative effects. The reduction in flood risk would be very significant. But if environmental costs are factored into the equation, then so should the benefits.

Is there a better option for generating pattern?

The optimisation for the Bondi Committee was on the basis of maximising output, and most subsequent studies have taken the same line. However, it would have been possible to optimise on the basis of value, which clearly varies at different times of day.

With the present free market conditions no-one will commit themselves to the price they would pay in ten or 100 years time.  But it should be within the wit of man to make a shot at value, hour by hour. Clearly output at times of peak demand has a greater value that output in the middle of the night when demand is low. (One might reason that the value at any time is proportional to the balance of demand in excess of say a 20 per cent baseload from future nuclear.) It was because of the desire to maximise output that ebb generation was the clear winner. However, the system could be more flexible.  The Rance project in France, for example, has operated for some 35 years and has reversible turbines that can also pump.

Because of the tide times in the Severn, the maximum output (mid spring tides) occurs from 10am to 4pm, and then from 10pm to 4am. The first is of moderate value, the second is of low value because it would be at the time of minimum demand, and therefore rather more difficult to incorporate into the national system. (There is a limit to how far nuclear can be turned down to compensate). It is perhaps just conceivable that using flood, rather than ebb, generation for a few days over the peak of large spring tides could pay off. But it is a very complex matter.

Global warming

There is no doubt that global warming will give rising sea levels for at least the next 100 years as the oceans expand. The need for a tidal barrier in the Severn will therefore increase irrespective of the need for renewable resources. Apparently the Crown Estates require allowance to be made for removing any barrage, and, even taking a minimum working life of 120 years, this would add to the generation cost. I believe this is so illogical as to be beyond comprehension. How could any government body decide to abandon Gloucester to a sudden considerable increase in flood risk when sea levels are higher probably by 0.7 metres? Such fanciful financial burdens should not be allowed to affect any economic assessment.

Indigenous resources

The only indigenous resources we have are renewables and coal, once North Sea oil and gas from the British section are exhausted. Even nuclear involves imported uranium ore. With the increasing world wide demand for oil and gas and the resultant increase in price, who can say how the economics might change during the very long life of a barrage in the Severn? There is also the risk that fuel supplying nations will see their assets as a useful political tool, and restrict supplies. As a nation we should be maximising the use of indigenous sources of energy, and tidal power is one of them. Can we really justify not using one of the largest such resources in the world?

The role of nuclear

The future role of nuclear power is now subject to even closer scrutiny following the Japanese tsunami, which closed down and seriously damaged the coastal power stations at Fukushima, in effect sterilising large areas. Several nations, including Germany, have since reversed policy about future nuclear power generation. All of the UK's nuclear plants are near the coast because of the need for copious cooling water.

The overall management of the Bondi Committee and final reporting were the responsibility of ETSU, the Energy Technology Support Unit located at Harwell. One of their objectives over 25 years ago may well have been to ensure that no other technology would supplant nuclear - but this was, and still is, a mistaken concept. Tidal power and nuclear power have very different characteristics, and investment in them does not preclude the other.

Renewables and nuclear need to be looked at on their own merits in terms of the UK's energy mix and environmental impact. Unfortunately, neither nuclear nor the renewables are flexible energy sources (though if we had a significant amount of hydropower from large dams on perennial rivers, that would be much more flexible). Most renewable resources have to be supplemented by generating capacity that can be switched on and off to meet the significant diurnal variation in demand. The nature of British winters is such that periods of extreme cold tend to be anticyclonic with low wind speeds, so even wind farms would fall far short of optimum performance Sadly, only fossil fuels seem able to provide the required standby flexibility.

Conclusion

These notes have been prepared in the hope that by summarising some of the facts, the present debate will not be based on misinformation, but on what has already been established regarding the likely effect of a barrage in the Severn. There has been plenty of speculation based on inadequate understanding in the past. It should also be borne in mind in any cost comparisons that a tidal power project has a life in excess of 100 years, perhaps three times the length of most other generation systems.

Peter Ackers is a member of CIWEM