Tidal power? No thanks

1/4/2010 New Scientist THE vastness of the ocean has always created the illusion of infinite resources, whether for food or waste disposal. Yet despite its huge size, the ocean is vulnerable to exploitation.The ocean also seems like an attractive source of vast amounts of sustainable energy, including tidal power. Just last month, the UK’s Crown Estate announced four tidal energy schemes off the north coast of Scotland with the aim of generating 600 megawatts of electricity. Scotland’s first minister, Alex Salmond, described the waters as the “Saudi Arabia of marine power”.

Again, this is an illusion. In practice, only relatively small amounts of energy are available from tides, and extracting it will have devastating effects on the ocean ecosystem.

Tides created by the moon and sun generate about 3.5 terawatts of power in total. This may sound like a huge amount, but is in fact only about 20 per cent of global energy demand. The amount of this energy that can be used is necessarily lower: to make tidal power viable, the speed of the current has to be at least 1.2 metres per second. This rules out the vast majority of tidal energy because it is found in the open ocean where tidal currents are too weak to be useful, generally less than 0.1 metres per second.

Viable speeds are only found in the shallow seas around the perimeter of oceans. In fact, there are only about 20 suitable sites in the world, including the north of Scotland and the Severn estuary in the UK. In the Netherlands a test plant is proposed for the Wadden Sea, a UNESCO World Heritage site.

Unfortunately, these sites are all in extraordinarily rich and ecologically fragile straits and estuaries that are critically important spawning grounds for marine life. Strong tides are what make these waters so productive: their turbulence stirs up nutrients vital for life.

In total, less than 100 gigawatts of power could be generated by the suitable sites, and it is debatable whether even this can ever be extracted efficiently. Tidal currents vary greatly over time and maximum power-generating currents are only a minor part of a tidal cycle. Even small decreases in current speed have large impacts on electricity generation.

Recent evidence also questions the efficiency of electricity generation once tidal barrages and turbines are in place (Renewable Energy, vol 33, p 2485). Obstructing 25 per cent of the area through which the tide flows alters currents so substantially that the potential power is no longer extracted efficiently. Thus permanently exploitable tidal power is reduced to a few tens of gigawatts.

On top of that, turbines kill up to 80 per cent of fish passing through them, and changes in current affect nutrient supply, thereby altering the ecology of estuarine life.

Tides are indispensable for life in shallow seas. Without them, ocean life would come to a halt. Extraction of their energy may seem attractive, but in reality there is very little tidal energy to be had - and what there is comes at high ecological cost. We should save the tides.

Hans van Haren is an oceanographer at the Royal Netherlands Institute for Sea Research in Den Burg
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