La Rance: learning from the world’s oldest tidal project 

Tidal plants use a barrage to build an artificial reservoir, which helps to establish the differential in water levels needed to drive the turbines.

The Rance river estuary was chosen due to its large tidal range, with the site actually boasting the highest range in the country with an average of 8m between low and high tide. In the spring and neap range, this figure can jump to as high as 13.5m.

During the first two years of construction, two dams were built to block the Rance, ensuring the estuary was completely drained and enabling the reservoir’s establishment across the site. In total, construction took five years, with an initial building cost of $100m

The project highlights the high financial investment needed to develop tidal power - the main reason for opponents to claim the energy source is less worthy of exploration than the cheaper alternatives of wind, solar or nuclear.  

Yet the plant also shows the long-term rewards tidal installations can bring. While it took around 20 years to pay for itself, the project has now recovered all of its costs through savings made from its energy generation - and the tidal energy produced costs less than nuclear or solar power.

Since its construction, the plant has produced approximately 27,600GWh of electricity, equivalent to around £3.3bn at today’s prices.

“Rance being the first barrage of its kind, I think it proves the arguments for tidal,” says professor Phil Hart, director of energy and power at Cranfield University. “I’m not sure how the lifetime economics have worked out overall but seeing as most energy projects have a life of 25-40 years and Rance is still going strong after 50 years plus with no signs of slowing down, it is difficult to think that it’s not paid for itself a few times over.”

“Tidal barrages' painful expense is the very high cost of initially building the structure,” Hart adds. “However, once you have the civil works done the rest is pretty cost effective. The turbines are or can be very typical water flow turbines, such as those you see in many other applications. They have to be modified to work with sea water instead of fresh water (seawater being much more corrosive), but that’s just a materials and servicing issue.”

A huge benefit of tidal power is that it is predictable. Reliant on the movements of the tides, engineers have access to daily, bi-weekly, biannual and even annual cycles on which they can base predictions of energy production. In addition, inexhaustible energy can be generated both day and night with minimal influence from weather conditions. As such, some say it has great potential to be a main replacement for fossil fuel sources. 

“Tidal barrage energy is pretty straightforward, incredibly predictable and environmentally carbon zero (or almost so),” says Hart. “All of these are really good and unusual things for energy production. How you would do it now is very similar to the way that Rance does it, though of course technology gets more efficient with time, and tidal barrage is no exception.”

“At Rance you can predict and rely on the energy produced, and know how much you are going to get when, hour by hour, day in day out, with 100% accuracy (neglecting any faults in the turbines). That’s a real strength when it comes to renewables, as most other forms are at the whim of nature.”

The design and operating principles are very well understood, having been developed over many decades, so challenges are fairly controllable. And when it comes to lifespan, Hart says, tidal is leading the way.

“Barrages just trounce free standing turbines,” he says. “The structure is essentially life unlimited, because you’re constricting the flow and having high speed water around the turbine inflow/outflows, the design needs to be carefully done and maintenance and repairs are needed, but that’s a cost which is pretty trivial over a lifetime of 100 years plus.”

Thomas Adcock, associate professor in the Department of Engineering Science at Oxford University, says there has been a "major environmental impact" on the Rance estuary as a result of the project, adding: “this would make it very difficult to get permission to do such a barrage again.”

Indeed, the barrage has been found to cause a progressive silting in the Rance’s surrounding ecosystem. Numbers of sand eels and plaice in the area have diminished, although the river has seen the return of sea bass and cuttlefish. EDF has previously said it will attempt to mitigate this ecological impact, but general consensus seems to be that more research into the environmental impacts of tidal plants is needed/. 

Researchers from Cardiff and Liverpool University have previously published reports on how tidal power can negatively impact marine life by altering sedimentation patterns, as well as oxygen and nutrient levels in the water. Similarly, a report from IRENA says costs of fixtures to the seabed and maintenance and installation costs need to be brought down, while more extensive research on the materials and methodologies of tidal power is needed before the technology can come into mainstream use.