One big challenge of the switch to renewable energy is unreliability. Critics often ask what happens when the sun doesn’t shine, and the wind doesn’t blow? But there is one renewable source which is consistent and never stops. For millions of years, the moon has been pulling all the water in the earth’s oceans up and down with absolute consistency for 22+ hours a day. That is a gigantic amount of energy.
And the UK is one of the places where the effects are greatest. There is only one place on earth where the tidal range is higher than the south west of England, where it rises and falls by 10 metres every day. Meanwhile, tides pull vast quantities of water between the Atlantic and North Sea through narrow gaps between Scotland, Orkney and Shetland.
Tidal power
Tidal power works by channelling water through turbines. The most established way uses reservoirs, which fill as the tide rises and empty as it falls. Turbines are driven by the flow. When this works in both directions, the energy supply is almost constant. This is the principle of the tide mill, which was a feature of coastal communities from early medieval times. Until the nineteenth century, East Anglia had many, grinding grain and fulling cloth, and Woodbridge still has the last tide mill in working order in England.
But now tidal energy is being considered on a much larger scale, for electricity generation. The principle has been used successfully in Brittany, where the Rance estuary was dammed in 1966 to generate electricity, and is still producing 240 megawatts of electricity. It is also the principle proposed for the abandoned Severn Barrage, which would have captured the power of the huge tidal rise in the Severn estuary, but at high financial and environmental cost. And recently such a barrage has been proposed across the Wash, where it would also provide new harbour facilities, and a direct road link from Norfolk to Lincolnshire.
Tidal flow
But this is not the only way of harnessing tidal energy. A number of experimental ‘tidal flow’ projects have been placing turbines in the sea in channels where the tide naturally sets up strong currents. The most promising of these are round the north of Scotland, where water can flow at 8 mph.
Until recently these were experimental projects, designed to test different approaches, materials and engineering solutions. By comparison with wind and solar, the technological challenges of creating large moving machines able to operate reliably in very hostile environment are much greater.
There have been three significant projects. The MayGen project operates four turbines mounted on the sea bed in the Pentland Firth, and now generates enough power for 6,000 homes. Orbital Marine takes a different approach. To address the problems of assembling and maintaining sea bed devices, they mount their turbines on a floating platform, anchored in a very fast tidal stream off Orkney. A third firm, Nova Innovation, has trialled six devices in Shetland’s Bluemell Sound, and is planning a new 16-device array near the Orkney islands.
These very different projects have demonstrated that tidal flow can work in practice, and firms are now moving into commercial production. In the last three rounds of negotiation of renewable energy contracts, the government has approved projects aiming to generate 122 megawatts of tidal energy. This is very small, by comparison with wind and solar, but an important step for a new technology. At the same time the EU is funding Nova Innovation’s new Orkney project.
What about the environment?
Just as windfarms pose a threat to birds, so putting large, fast-moving machinery underwater inevitably poses a risk to marine life, including fish, seals and whales. One oceanographer has suggested that large scale underwater turbine systems could disrupt the natural flows of nutrients in the sea, leading to unknown impacts on marine life. However, she also recognises that these would be small, compared to the costs of climate change.
Tidal flow in the energy mix
The future of renewable energy supply will not be with a single technology. Rather it will require a mix of sources, adapted to particular niches and circumstances. Most commentators agree that tidal flow can make a modest but significant contribution to the total, alongside much larger wind and solar sources.
Planned developments already expect to generate 130 megawatts in the UK by 2029, and there is a theoretical potential of 11 gigawatts, which would be 11% of the UK’s current consumption. This would be a significant contribution to the energy mix, especially in providing reliable baseload generation when the wind doesn’t blow, and the sun isn’t shining.
Richard Parkinson, CEO of Inyanga, is optimistic,
I think the sweet spot would be to put in 30 megawatt-plus projects, because I think that’s the point where the big investors will start taking an interest in tidal. In the next five years, I think we’ll be going through a lot of these demonstration projects. As long as these can get delivered and get the investment, I think tidal will be in a very strong place.
Unlike wind and solar, where the UK is well behind international competitors, this is a field where we have a technological lead, and one of the world’s largest potential tidal resources.
More on this subject can be found here.
