Marinet comments on Swansea “tidal lagoon” proposal

The Swansea Bay lagoon scheme will be the first of its kind in the UK.

On 9th April we reported on a project to power 107,000 homes, using a tidal lagoon in Swansea Bay, which is seeking £10m of funding from the public.The first of its kind in the world, the scheme intends to generate up to 250MW of renewable power by capturing tidal differences in an artificial lagoon, to drive large turbines.

The company “Tidal Lagoon” set up by entrepreneur Mark Shorrock, who previously developed wind farms in Scotland and solar arrays in Cornwall and Spain, talks of starting construction in 2017.

We offer the following comment:

From the website Tidal Lagoon Swansea Bay and presentations in Swansea, we can’t have a lot of confidence in this proposal, though remain open-minded on it. The newly arrived company appear not to have learned from the previous company’s (Tidal Electric Ltd.) studies regarding key problems of silting up, both inside and outside the lagoon.

Nor do they plan to capitalise on the advantages of tidal lagoon generation for energy storage and grid balancing. Peter Ullman of Tidal Electric, told the parliamentary inquiry ^[1] :

• The tidal lagoon impoundment can double as a pumped storage facility. Off-peak power is used to pump up (or down) the water level in the lagoon thereby increasing the output from generation on the subsequent generation pulse.
• If unpredictable power, such as wind power or wave power, is used for pumping, then the unpredictable (wind or wave) power is transformed into predictable tidal power. This transformation mitigates the need for maintaining balancing capacity and enhances the value of the wind or wave power. In addition, the tidal lagoon’s output is increased.

Lagoon generation has the facility to meet peaks in demand, by switching individual turbines in and out. However, the point can be overstated as most electrical units are generated at low tide when height differences are maximal (thus giving the “fall” necessary to generate electricity), whereas few or zero units are generated for much of the time due to the absence of this height difference. The tidal barrage at La Rance is operated with pumping to store power for an hour or two and contribute to balancing, so there is plenty of experience with the limitations of the technique.

Stuart Anderson developed the ECOSTAR (Energy Capture Obtainable by Storage for Tidally Amplified Release) concept for the North Wales consortium – see Marinet article on the Severn Estuary Debate – which we assessed as:
In principle, one could design for two-way generation with pumping, generating only when the height differences are large. Stuart Anderson calculates that the total efficiency could increase by four times. However, this exaggerates the discontinuities in output. The DTI-sponsored studies have been directed instead to extending the generating cycle and phasing with electricity demand, by adjusting discharge times and using pumping for ~1hr in the cycle.

These discussions led to two conclusions:

• payments for renewable units need to reward the flexible timing capability of lagoons with pumping, unlike the current subsidy per unit delivered at the generator’s convenience, and
• a prototype should be developed and tried out as soon as possible. The north Wales coast off Rhyl would be a good site for trialling any attached or free-standing lagoon.

We doubt that Swansea Bay is good for such a prototype lagoon, because any design would interfere with the back-circulatory currents that are all important for sediment/sand transfer in the Bay and sensitive beaches in the Gower (east side). Studying this problem would take a great deal of time and cost.

Peter Ullman himself accepted ^[2] that the key potential problems are the depositing of sediments where they are not wanted, such as shipping lanes, and the erosion of sand from places such as beaches.

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