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Can tidal lagoons supply base-load power?

By Dave Elliott

The Swansea Lagoon didn’t feature in the Budget announcement, despite Labour’s John McDonnell saying “get on with it”, but with a decision still expected soon it does represent an intriguing idea, especially if it can be extended.  Charles Hendry’s review of Tidal Lagoons threw up some interesting possibilities and issues, including the idea that multiple projects could offer more nearly continuous output, as well as identifying some conflicts over what should be done. He noted that there were “some views that were mutually exclusive. Whilst some, especially in the financial and environmental communities, argue that a smaller tidal lagoon in Swansea Bay needs to be operational before a commitment can be made to larger projects on the most competitive terms; others, in the supply chain, academia and those pushing for faster action on climate change, argue that the cumulative economic and industrial benefits of a programme of tidal lagoons would inevitably be lost by such a delay”.

Hendry backed the latter view and proposed an initial trial go-ahead for the small 320 MW Swansea project – see my earlier post. But the aim was to see if further projects, including possibly cheaper, larger variants, might be worth pursuing and he also looked at some of the wider options that could open up.

So, while Hendry opted for, initially, a relatively low-key approach, with just one small project, he did look at more ambitious ideas, for example at whether “a number of tidal lagoons operated together as a portfolio could come close to providing a consistent level of generation throughout the day”. He notes that “the principle on which this claim rests is that high and low tides come at different times of the day at different points around the UK’s coastline. If tidal lagoons were placed to take advantage of that phenomenon, with generation increasing at one tidal lagoon as it decreased at another in line with the different tidal cycles, the output from the tidal lagoons taken together would be less ‘peaky’, and therefore more consistent”.

He concluded While I believe that such a smoother output profile could have advantages, advice from our technical consultants is that operating tidal lagoons in a portfolio like this would still leave periods of the day when there was no output, particularly at neap tides. This is, in part, a matter of geography: the areas of the coastline that offer suitable locations for tidal lagoons do not offer a perfectly smooth output profile as their tidal cycles do not align (for example, there is a significant gap of tidal lagoon potential in the coast of mid-Wales). Moreover, a programme of tidal lagoons that could deliver the goal of providing constant, or as near as possible to constant, power would be an absolutely huge undertaking, requiring tidal lagoons around much of the country. It is my belief that this is too ambitious a goal to be set at this time, before even one has been built, and could only be considered properly when more progress has been made on building a number of tidal lagoons”.

Energy Matters (EM) came to a similar conclusion on the inability to balance output, at low neap tides especially, though oddly it ignored Hendry’s coverage of this. EM also looked at some earlier studies of multiple dispersed tidal generation options carried out by Liverpool University, which had focused mainly on large barrages (which EM said were “not materially different” from lagoons in energy terms), but had also looked at adding tidal stream turbine projects. The latter arguably would not only have less environmental impact than big estuary-wide barrages, but also offer more siting flexibility: lagoons and barrages are much more geographically defined. So there might be fewer site gaps around the coast.

The 5 barrages chosen, on the Dee, Mersey, Morecambe Bay, Solway Firth and Severn, produced big bursts of ebb generation power with gaps at low tides. The four smaller tidal stream projects chosen, on the Mersey, Lynmouth, the Skerries and west Wales, produced less power, but their net output was better phased. EM, however, focused on another case the Liverpool team had looked at – with the five large barrages and just one very large (6GW!) tidal farm off the Isle of Man, operated in ebb-only generation mode. Its output did fit in the gaps nicely in tide terms, but it still did not help maintain a constant net level of output. So it was no good for baseload, said EM. But why would you expect one project to achieve this? Though even with multiple and more widely dispersed projects, and operating two-way, on the ebb and the flow, output would still be low at neap tide periods for all projects at all sites. You’d need a lot of storage. Assuming baseload is really what you need: there are many ways to balance grids without it.

One of these is pumped storage using large reservoirs. If the containment walls were suitably raised, tidal barrages and lagoons could actually be used for this – an extra head of water being pumped up for release when generation was needed using surplus off-peak grid power at times of low power demand. That could include the surplus output from other tidal projects. Hendry said it has also been suggested that the issue of dispatchability could be addressed by using the tidal lagoons themselves in a similar way to pumped-hydro facilities, where water can be stored during times of excess power supply over demand, and then almost instantaneously released to provide the power needed in times of high demand. The evidence indicates that this would only make a small improvement to the dispatchability of tidal lagoons, as the tides only offer a small window in which to shift dispatch effectively.” So he “was not persuaded by the economic and commercial cases for operating a tidal lagoon in this way”.

 There are other views: a segmented double basin system, although more expensive, might allow for more flexibility, extending the period of generation.

However, for the present, the focus is on the simple Swansea project. The idea has certainly got some attention. And also some backing. Even from UKIP! In my next post I’ll look at the economics – could this and other tidal schemes in time rival wind energy, as some suggest?

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