In its heyday in the 19th century the Port of Swansea on the south coast of Wales was one of the most important in the British imperial sphere. Over half of the world’s copper was exported from its bustling waterways and docks, which provided a critical link between the resource rich Welsh valleys and demand throughout the empire and beyond.

Today, of course, things are rather different. Whilst still a working port, parts of the old docks are being turned into a new waterfront development, and Swansea Bay in the 21st century is known as much for its water-sports as its metal exports. 

And now, with plans for the world’s first tidal lagoon progressing fast, the city is set to become a global exemplar for harnessing the tides to generate power. 

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Last week, the environmental assessment for the project was completed with unprecedented levels of public support. According to the developers, when completed in 2017 at a cost of £650m, the 10km long horseshoe shaped breakwater wall will impound 11km2 of water and provide over 100,000 homes with electricity. The impounded area could be used for water-sports and recreation.

So what are tidal lagoons, and could they really play a serious role providing the world with significant amounts of low-carbon, predictable power? 

What is a tidal lagoon?

According to Dr Simon Boxall of the University of Southampton’s National Oceanography Centre, tidal power is nothing new. Since the 7th century in Europe, mills have been built across coastal inlets to trap water in ponds at high tide, releasing it later to turn wheels and grind grain into flour. 

The same principal lies at the heart of tidal lagoons. Similar in appearance to harbours or marinas, large lagoon structures would impound sea water and separate it from the natural ebb and flow of the tides. When the water level between the inside and outside of the lagoon is large enough, sluice gates would open, allowing water to rush through and turn turbines to produce power.

The fundamental technology is established and proven, and the concept has been promoted by environmental groups like Friends of the Earth as a way of generating power from the tides whilst minimising damage to habitats and wildlife.

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What are the advantages of tidal lagoons? 

More than anything, it is the fact that tidal energy is both low carbon and completely reliable that is its greatest asset. The Swansea Bay project would operate on both ebb and flow tides to provide power at four periods each day. According to David MacKay, chief scientific advisor to the UK Department for Energy, in the future lagoons could even be divided into separate pools to generate power on demand, much like pumped storage hydro plants do today. This would be extremely valuable in any power system with high levels of intermittent renewables. 

There are of course other methods of generating power from the tides, such as the barrage built at La Rance in France, or the proposed 6GW barrage across the UK’s Severn Estuary (a true mega project). However it is the disadvantages and controversy of these barrages that give lagoons such comparative appeal.

For example, whilst both approaches involve trapping water with breakwater walls, a barrage cuts straight across an estuary or river, whereas lagoons are ‘curved’ harbour-type which do not. Thus lagoons need not impede shipping - a big plus in many people’s eyes.

In addition, large barrages such as the one proposed for the Severn Estuary would likely have a very significant adverse impact on bird breeding grounds (due to loss of intertidal habitats) and fish populations (blocked from their migration routes and vulnerable to turbine blades). 

Tidal lagoons would likely be less environmentally damaging. This is both because estuaries and rivers would remain open to fish movement, and because if built ‘offshore’ - i.e. away from the coast - lagoons could avoid the loss of inter-tidal habitats. 

Even with ‘land-connected’ lagoons like the one proposed at Swansea Bay, it is suggested that the impact would be substantially less than barrages. Mark Shorrock, head of the Swansea project, told me that “our modelling suggests a large number of smaller land-connected lagoons will have a net-positive environmental benefit by creating localised slower or faster water movements but leaving the intertidal area exposed and mud flats intact”

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How much energy could tidal lagoons provide?

A recent study by the Crown Estate - the body that owns and manages UK territorial waters - estimated the total theoretical tidal lagoon resource in the UK to be 14GW of capacity producing 25TWh energy each year - about 7% of current total UK electrical demand.

But the UK’s tidal resource is one of the best in the world. So whilst lagoons may be able to provide significant power in some other high tide countries like Canada and France, they are never likely to match solar or wind in terms of deployed global capacity.

In addition, a very specific set of geographical conditions is needed for tidal lagoons. The Crown Estate suggests the tidal range needs to be large (above 4m), the water shallow in order to minimise construction costs (no more than 25m below mean sea level), yet with the turbines themselves located in or adjacent to deeper water. 

I not aware of any surveys of such sites other than in the UK, and until these are undertaken the global potential of tidal lagoons will remain highly speculative.

Is there a catch?

Assuming enough sites with favourable resource and geography are found, a number of important questions still need answering before we can really know whether tidal lagoons can be viable and more beneficial that other forms of power generation.  

  • Environmental impact

Will tidal lagoons deployed at scale really prove to be environmentally acceptable? Here a cautionary note was sounded in a 2010 UK Government assessment, which concluded that some large tidal lagoon options in the Severn Estuary would still lead to the loss of 1/5 of the estuary’s intertidal habitat and lead to significant declines in 13 bird species. 

  • Cost

Will they be affordable? The Swansea Bay developers say their project will deliver energy at a cost of 14p/kWh - comparable to the current cost of offshore wind. This is a good start for a first-of-a-kind project, but bringing future costs down enough to be competitive with other technologies may be tricky if deployment potential is limited. There could also be difficult trade-offs between cost and environmental performance.

  • Public acceptance

Will tidal lagoons benefit from strong public support or will they generate opposition like wind farms? Whilst the Swansea Bay project has got off to a good start, it is far from guaranteed this will continue or extend more widely. After all, large lagoons will have a visual impact on landscapes people hold dear to their hearts.

One step at a time

There is reason for cautious optimism about the potential of tidal lagoons, but the challenges ahead should not be underestimated. Experience tells us that navigating the complex issues that inevitably arise as a new renewable technology is developed is not easy. 

In any case, perhaps for now we should resist the temptation to get ahead of ourselves. As things stand there is not one operational tidal lagoon anywhere in the world, and focussing on changing this fact seems like challenge enough. Over to you then, Swansea.