From the archive: Swansea’s tidal lagoon – how does it work and where are the costs?

Tidal Lagoon’s Swansea Bay project is at once “innovative” and “tried and tested”, depending on whether it is looking for subsidy or investment. Janet Wood took a look at the project

The tidal lagoon planned for Swansea Bay is a project that combines many very familiar elements. What’s new is the combination, the type of site, and some developments in turbine technology. The company says: “Each component part is proven. The configuration and pulling them together has never been done before.” That degree of familiarity makes it an attractive investment: “In terms of technology risks it’s in a different ball game. For institutional investors it’s entirely different from other renewable energy.”

“For institutional investors it’s entirely different from other renewable energy”

The tidal barrage is a familiar technology – one has been in operation for 50 years at La Rance in France. Swansea Bay uses the same principle. But instead of filling a natural space, it has to create its own: six miles of sea wall will enclose a horseshoe-shaped area that will be filled and emptied by the tides twice a day.

The sea wall is one area where the technology is familiar. Two options went into planning and the preferred one was very similar to a typical sea wall, with sediment and different grades of rock infill. Seabed areas have already been identified for dredging for the sediment infill.

The turbines too are based on familiar technology, but with a new application. They are “variable speed” turbines, which means the generator can generate at any frequency which is then corrected through power electronics to synchronise with the grid frequency of 50Hz. It means the project has a wide operating load and can “chase hydraulic efficiency”. The company says variable-speed technology has not been used at large scale in this type of hydro plant before.

With 16 bidirectional turbines housed in the powerhouse, the project will have four 3.5-hour ­generating periods every 24 hours, predictable decades in advance. The turbines will each have a 20MW ­capacity, totalling 320MW.

The company will use the pumping ability of its turbines to control the water level difference. That will  maximise the power produced on each tide, and ensure the lagoon is emptied at low tide for ­environmental reasons, to match the natural flow.

The company explains that when there is high tide outside the lagoon and the level inside is almost the same, it will pump into the lagoon. “We pump for half an hour and then close the [turbine’s] wicket gates. Then we wait for the tide to turn. It means [the water level inside the lagoon is slightly higher so] we ­generate at slightly higher head.”
That is not unique – the plant at La Rance can also pump, but its efficiency is very low. Designing this in from the start, Swansea Bay expects to make annual energy gains of 10%.

In all there are four generation cycles, either side of the two high tides, and up to four pump periods (importing power). The company says fine tuning when to pump will be part of the lessons learned from the project: “The devil’s in the detail on deciding when to pump. We don’t necessarily have to pump on every tide.” The company says it may want to avoid importing power during triad periods, for example, so there will be some tides when it will not pump.

Can costs come down?
Power from Swansea Bay will be expensive – the company initially suggested a Contract for ­Difference (CfD) at £168/MWh for 35 years. But a deal is now under bilateral negotiation with the Department of Energy and Climate Change (Decc) and that price may well come down. It compares to £140/MWh for ­offshore wind and £92/MWh for Hinkley Point C – all ­index-linked.
The Institution of Civil Engineers suggests the maximum payment for the project is clear in advance, as it is unlikely to overproduce, unlike offshore windfarms that have had higher load factors than expected.

In fact, there may be some increase – annual generation was 495GWh in planning, but guaranteed annual production is 540GWh per year, and the company thinks realistic estimates are higher. More uncertain is the power price against which the strike price is paid; a falling reference price requires bigger ‘top up’, and it’s not clear what the situation will be in the event wholesale power prices are negative.

“For 90 turbines, the supply chain has to expand. The pilot project builds a readiness to step up in the supply chain”

The company has a buyer for 10% of the power – Good Energy, which is an early-stage investor – but the remainder has still to be placed.

Follow-on projects are intended to be much cheaper, although capital costs could be up to seven times the £1 billion investment in Swansea Bay. That’s partly because of technology, and experience from Swansea that will reduce the construction risk. But mainly it is because the follow-on projects would be much bigger.
The company is looking at five potential full-scale projects. Three are in Wales – Cardiff, Newport and Colwyn Bay – and the others are in west Cumbria (north of Workington) and Bridgwater (Somerset). Cardiff is likely to be the next project.

At that site, the sea wall would be twice as long as the one at Swansea and it would enclose six times the area of water – 12 times the volume. That means 12 times the volume of water going through the turbines – and low-head hydro is all about the volume through the turbine. But Cardiff also has a better tidal range, so the project would have more head (adding more power). The larger lagoon would have more turbines – 90, compared with 16 at Swansea. That puts minimum capacity potentially at 1,800MW and maximum at 2,800MW.
Economies of scale is an immediate cost reduction. There will also be “a degree of learning” in the technology, the company said. But it also requires investment from the supply chain.

“When you are looking at the supply chain and industrial opportunity, most industry turbine makers can cope with an order for 16 turbines, they don’t have to expand. But for 90 they do. The pilot project builds a readiness to step up in the supply chain,” the company says. “A year ago we guessed the strike price at Swansea at £168/MWh – at Cardiff we would have a strike price of £95/MWh and a lot more power produced.” In addition, the company says that in the first project, the risk is in the construction, not the technology, and that is borne by the private sector.

What Swansea does not offer
Usually, a characteristic of hydro power is its responsiveness and flexibility in operation. Hydro plants with dams can stop generating and store water when power is abundant; pumped storage plants with two reservoirs go further, using excess power to pump water back up the chain to be used to generate later. Although so-called run of river plants cannot store water, and may have limited generating capa­city according to river conditions, they can reduce ­generation on demand.

All these options help balance the grid – and although it sounds unlikely, there are times when local or national conditions mean there is oversupply and it is necessary for generation plant to turn down.

Swansea Bay has little of this type of flexibility. Although it can pump, that ability is used to maximise generation around high or low tide. In theory it could store water between one tide and another – but that would affect the next cycle of generation and, importantly, the tidal habitat enclosed by the sea wall, which has to empty regularly to mimic natural tides.

“Usually, a characteristic of hydro power is its responsiveness and flexibility in operation… Swansea Bay has little of this type of flexibility”

There are some services the plant could offer to help balance and stabilise the grid, if not continuously over 24 hours. The plant has some flexibility to store ­short-term, generating half an hour before or after the optimum time, and of course it could open the turbine vanes and allow water to flow without generating. It could also alter the amount of pumping or change the time slightly.

At present, however, that is not on the project’s radar. The aim is to generate as much as possible whenever possible.
The company says a fleet of lagoons around Great Britain could mean the lagoons will generate 24 hours a day somewhere in the country. But it says that in addition, National Grid is interested in taking advantage of the projects because during pumping periods a lagoon would be a big demand customer – albeit at more or less fixed times.

“We can’t use the lagoons like a pumped storage unit. But more lagoons means more flexibility. At the moment we have fairly simple optimisation – [any other options] are outweighed by the income from CfDs. Once the CfD comes to an end, that may change.

We can’t use the lagoons like pumped storage

“In future we may want to optimise, working with day-ahead pricing, and we’ll need a trading desk.” That will require detailed work on operating ­models, but there are examples to be built on, such as ­“cascades” of hydro plant on a river, as there is in the Rhine.

What’s next?
The first major construction activity at the Swansea Bay site, in year one, would be building a cofferdam so the company can work through the winter constructing the turbine hall. We are not yet at that stage, of course.

The company has proudly announced that it wants to maximise the UK supply chain, and it says there are UK suppliers for nine of 11 major turbine components. The turbine suppliers “have had to rewrite their supply chain to do that”.

Tier 1 contracts have been awarded. The project’s major delivery partnerships announced to date are: China Harbour Engineering Company Limited (CHEC) for marine works; General Electric and Andritz Hydro as turbine suppliers; Laing O’Rourke for turbine housings; and Alun Griffiths for public realm ancillary works. When the company signs fixed-price contracts, it will have more certainty on costs.

Meanwhile, negotiations with Decc over the CfD continue.


This article was first published in the October 2015 issue of New Power.

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1 comment for “From the archive: Swansea’s tidal lagoon – how does it work and where are the costs?

  1. December 18, 2017 at 11:58 PM

    The Swansea Bay Tidal Lagoon proposal’s primitive and inferior single lagoon design is not appropriate even as a “first of a kind” renewable energy generation project.

    The advanced and superior double lagoon design, which I explain here -
    - is much more appropriate for generating electricity.

    The main scientific, engineering and technical flaw with the inferior single lagoon design is that it does not offer valuable “dispatchable” power, on demand, when it is wanted by c ustomers, when they are happy to pay for power.

    Rather, this inferior single lagoon scheme supplies a lot of worthless power only when it is convenient for the tides and “NEVER MIND” with what power the customer demands when and is willing to pay for.

    With the inferior single lagoon design there are times during the day when no power whatsoever can be generated, whenever the level of the water inside the single lagoon is the same as the level of the sea outside the lagoon.

    Scottish Scientist
    Independent Scientific Adviser for Scotland

    * Wind, storage and back-up system designer
    * Double Tidal Lagoon Baseload Scheme
    * Off-Shore Electricity from Wind, Solar and Hydrogen Power
    * World’s biggest-ever pumped-storage hydro-scheme, for Scotland?
    * Modelling of wind and pumped-storage power
    * Scotland Electricity Generation – my plan for 2020
    * South America – GREAT for Renewable Energy

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