Power from the Tides

The pull of the moon moves enormous quantities of water twice a day, and unlike wind or sunshine, the timing of that movement can be predicted decades in advance. For engineers searching for renewable power that does not depend on the weather, this reliability is the great attraction of tidal energy. Tidal power is hardly new: mills driven by the tide have ground grain on European coasts for nearly a thousand years. A tide table for the year 2050 could be printed today with confidence. No forecast of cloud cover or wind speed can offer anything like it. Two main designs exist. A tidal barrage is a low dam built across an estuary. Gates let the incoming tide fill the basin behind it, and the water is then released through turbines as the tide falls. The oldest large barrage, built across a river in northern France, has operated since 1966 and still produces electricity at a competitive cost. The alternative is the tidal stream turbine, which resembles a wind turbine fixed to the seabed and turned by fast currents flowing past it, so that no dam is needed. Barrages have a serious drawback. By holding back water they change the movement of silt and the habitat of birds and fish upstream, and the mudflats on which wading birds feed can be permanently altered. For this reason, several proposals for very large barrages have been abandoned after long public inquiries. Stream turbines avoid most of these objections, since the water passes around them, but they face a different problem: the sea itself. Equipment submerged in a fast current must withstand salt corrosion, marine growth and the force of storms, and repairs require calm weather and specialised vessels. Cost remains the largest obstacle. Because few machines have been built, each one is close to a prototype, and prototypes are expensive. Wind power became cheap only after thousands of turbines had been manufactured and the lessons of each had been applied to the next. Supporters argue that tidal stream power is simply at an earlier point on the same curve, while sceptics reply that the sea is a far harsher setting than a hillside, so the comparison flatters tidal energy. Banks and insurers, who dislike risk, are reluctant to finance a technology with such a short record, which keeps the price of borrowing high. Tidal energy is unlikely ever to supply more than a modest fraction of a country's electricity, because only a limited number of coastlines have the necessary tidal range or current speed. Even so, I would argue that it deserves public support out of proportion to its size. A grid dominated by wind and solar needs sources that can be switched on at known times, and the tide offers a schedule that nothing else can. To dismiss it because it cannot meet all demand is to apply a test that no single source of energy would pass.