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Turning to the tide

Consultants Halcrow and Mott MacDonald have joined forces to promote tidal energy around the world. But their biggest challenge is UK-based - the Severn barrage. Ed Owen reports.

Tidal barrage schemes such as the proposed Severn Barrage have been on the drawing board for decades, yet few have been built. Problems include long lead in times, environmental concerns and very high upfront costs.

But with the global drive for more renewable energy sources, tidal barrages are worthy of serious consideration, and Mott MacDonald energy director Simon Harrison is quick to point out the advantages.

“A tidal barrage is one of the few ways that can provide high energy capacity with no emissions. Provided you can overcome the energy demand issues that can vary with tidal generation patterns, you have serious value there,” he says.

Potentially huge opportunities

Halcrow director of maritime Ben Hamer says the opportunities are potentially huge and both companies are looking to benefit. “We are both committed to sustainable energy, as are our companies. We also have a good fit with our skills sets. And tidal power is a substantial market which makes it an appealing proposition.”
Harrison adds: “If all the possible UK schemes were approved - admittedly unlikely - then the collected worth would be £100bn,” he says.

“It would only take one large scheme somewhere in the world to go ahead to demonstrate the prize is worth fighting for. And with one large scheme on the go, people will copy it, so it is something you need to position for and something we are putting to the market.”

Hamer and Harrison are leading a formal collaboration between Halcrow and Mott MacDonald to promote the growing opportunities for the sustainable development of global tidal range energy projects. They are working together to identify, study and deliver schemes around the world.

As a team they have been exploring issues surrounding the development of the Severn Barrage, and they have won a feasiblility study to evaluate options for harnessing tidal energy on the Solway Firth.

“If all the possible UK schemes were approved - admittedly unlikely - then the collected worth would be £100bn”

Ben Hamer, Halcrow

The Severn Barrage is the largest project the two companies have been engaged in so far. The technical challenge of building the massive 8GW, £23bn Cardiff-Weston option is not a primary concern, according to Harrison.

“The 240MW Rance tidal barrage in France that was completed in the 1960s has run trouble-free since it was built. Technically, Severn would not be as big a step as building deep offshore wind farms,” he says.

One significant barrier to constructing tidal barrages is the development of a smart grid to allow electricity demand to be managed to match output.

Harrison says the long lead-in time for building a Severn Barrage is actually a benefit. “If the consenting time for the Severn Barrage is about 10 years, this takes us to 2020.

“Looking at the proposed rate of renewables deployment in the UK to 2020, you are not going to be able to accommodate that extent of variable power generation on the existing grid network and will need a smart grid.

Ofgem has a £500M fund to develop this for distribution operators,” he says.

Smart grid

A smart grid would be able to divert power generated by the Severn Barrage at off-peak times to charge electric vehicles and power heat pumps for commercial and domestic space heating.

Harrison says: “Some analysts think you could get to a tipping point when people start to see substantial cost savings from using electric cars and suddenly nobody buys petrol any more.

“If you are not clear by that stage where you are going with smart grids, and which low carbon sources of electricity will provide the power for them, then you have a problem.”

Working together, Halcrow and Mott MacDonald have come up with some new options for the Severn Barrage.
Hamer says they began by looking to optimise the design. “We modified the number of sluices, introduced new turbines etc. We had a view to value engineer this, but we found it was pretty well optimised already, which was not what we expected, but an interesting outcome all the same.”

What they did deduce was a very strong argument for an ebb and flood system - two-way water flow electricity generation - rather than an ebb-only - one-way flow generation.

“With ebb and flood, you maintain much more inter-tidal habitat and maintain much more of the natural water level. With ebb only you would increase water levels upstream, causing potential problems there,” says Harrison.

Hamer says the ebb-only model would involve a four-hour generation window every 12 hours with large generation surges that rapidly tail-off. Such power surges would be problematic to manage.

“With ebb and flood you can better manage the release, and better manage the production, smoothing output to almost 70% of the time.”


The only drawback is that less power is produced overall. “People have fixated on the numbers, and high output equals good, but the ebb and flood model would fit very well into creating capacity off peak, for example for charging electric cars,” says Harrison.

Costings would also change - with 10%-15% less power produced overall, costs per kWh inevitably rise compared to the ebb-only model.

“With an ebb and flood design, you do produce less power overall, but it is of more value to the marketplace,” says Harrison.

“Decarbonisation will mean nuclear, carbon capture and storage and renewables look like solid options”

Simon Harrison, Mott MacDonald

Hamer adds: “You also get 65% less habitat loss than with an ebb-only scheme. So when you come to planning application, it is much more attractive.”

Work on the Severn study continues, despite many thinking the Cardiff-Weston barrage option is simply too large to be feasible.

“I think it is worthy of further consideration. It is certainly not dead as a concept,” says Hamer, “and it is something we are continuing to look at as a project. If you were looking for a significant quantum of renewable energy in a short timeframe, you would opt for the barrage.”

Consultant Parsons Brinckerhoff has completed its feasibility study into the Severn Barrage for the Department for Energy and Climate Change (DECC), and the report is now with ministers for consideration and publication in the near future.

The Mott MacDonald/Halcrow work was fed into the study, and NCE understands that ebb and flood designs were looked at.

“That is the largest collaborative effort we have undertaken at the moment, but there are other smaller projects on the way - in the North West for example there are so many small projects it calls for a regional perspective,” says Hamer.

The two companies have also earned fees for their Solway Firth study.

“For joint work we seek parity, half and half,” says Harrison. “But that will not compromise the fact that we will always put the most able people forward for each job from the combined pool of staff. Then we split the tasks up between these key technical people.”

Their agreement is a signed memorandum of understanding, not a joint venture. It leaves scope to extend the working relationship.

“We have the option of looking at other renewable projects if we feel there is the need to do so, but we have not had this tested,” says Harrison.

Hamer adds: “People are now working as a co-ordinated team. Solway is the first project where we earned fees together, but we have nine work streams to look at different aspects of the technology and its integration. The Solway has been one example, and this will act as a blueprint for further fee-earning jobs to come.”

Specific tidal projects come from a long list of well known schemes throughout the world, and a second list of much less well known schemes the two companies are keeping secret.

Opportunities abroad

They are looking at opportunities in India and Russia, and Hamer mentions further opportunities in the north-eastern US. “They have offshore opportunities and because it is so densely populated, they are looking at offshore wind and tidal range.”

But underlying much of the work done so far by the two companies is a desire to keep decisions future-proofed as far as possible.

Harrison explains: “This world is going in a certain direction - decarbonisation is a pretty sure bet. This makes nuclear, carbon capture and storage and renewables look like solid options.

“Greater electric transport looks likely, but there are other options. Replacing gas in space heating could involve air and ground source heat pumps, but might instead use biogas in the existing network, or community energy schemes. For this uncertainty, flexibility becomes key.”

Hamer agrees: “The most important thing is building flexibility into the design of the project, and not to go great guns into one type of operation.”

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