Few things are more dependable than the tides. As long as the moon is in the sky and gravity rules the earth, the ocean will always run in and run out. So, it’s perhaps not crazy to think of harnessing tidal movements for energy generation, as the United Kingdom is now planning to do in a big way. While solar and wind power are always going to be intermittent, tidal has the great benefit of being predictable.
The British government recently gave the go-ahead to six potential projects, four in Wales and another two in England. The first, on the Welsh coast near Swansea, is already in the planning stages and could begin generating energy early in the next decade. If so, the project would be the first of its kind anywhere in the world.
The idea goes like this. Engineers build a long semi-circular wall round a bay, trapping water inside. At the furthest point, midway around, is a set of turbines that exploit a water height difference between the inside and outside. As the tidal level rises, the gates are closed, so the water is higher outside than in. Then the gates are opened, and the water rushes in, generating power. When the tide goes out, the process is reversed, with water flowing back out to sea. With two tides a day, the lagoon generates four times within 24 hours.
The proposed Swansea causeway is 13.6 miles long, with a turbine section of about 1,800 feet. It could power about 150,000 homes, according to the company behind the project. A second planned scheme, up the coast near Cardiff, would be seven times larger still and generate electricity for more than a million households.
The wall–which is built from gravel taken from the bay bed and placed into “geo-tubes” that sit one on top of another–creates a nice path for walking or cycling, and damage to the bay is projected to be minimal. Environmental groups generally support the idea, though some fishermen on nearby rivers are opposed.
The big concern is cost. At $1.5 billion, the Swansea project is jaw-droppingly expensive. Each megawatt-hour could cost as $250. That’s even more than offshore wind in the UK, which currently costs more than $200 per megawatt hour. On the other hand, the scheme’s backers argue that the first-of-anything is always pricey and that the follow-up projects will be cheaper–perhaps half as much per megawatt. Plus, there are secondary benefits. The causeway creates a walking and cycling path, a tourist attraction and contributes little or nothing to climate change, or local pollution. Those things are all worth something, even if the cost calculations don’t account for it.