The Tide As A Power Station

AAmong renewable resources the tide alone can be printed in a timetable years in advance. Its rhythm is set by the orbits of the moon and the sun rather than by the weather, so a machine driven by it can be scheduled as no wind farm can. Engineers nevertheless divide the field into two industries that share little beyond the adjective. Tidal range schemes, meaning barrages and artificial lagoons, trap water behind a wall as the sea rises and release it through low-head turbines as it falls; tidal stream devices sit anchored on the seabed in a narrow channel, turned by the horizontal flow itself as a wind turbine is turned by moving air. Their costs, their coastal effects and their opponents differ. Reporting the two under a single heading, as several national energy reviews still do, has obscured more than it has explained.

BGiulia Kuznetsov, a systems engineer at the Fastnet Marine Energy Unit, has spent a decade asking what predictability is actually worth to an electricity network. Between 2015 and 2022 her group logged half-hourly output from fourteen stream turbines in the Ardmore Narrows and arrived at a capacity factor of 38 per cent, respectable, she says, and beside the point. What an operator buys is firm power, the quantity that can be promised for every hour of the year, and on that measure a single channel performs badly: every machine in it goes slack together, four times a day, at an hour that is known but cannot be moved. Her remedy is geographical. Because high water reaches different coasts at different times, output from channels in unlike tidal phases can be summed into something far steadier. Pairing Ardmore with a site 180 kilometres to the north-east lifted the guaranteed minimum from 3 per cent of installed capacity to 21 per cent. The gain, she stresses, is confined to pairs whose phase difference exceeds three hours; below that threshold the two channels merely fail at the same moment.

CThe economic argument has settled more slowly. Tidal stream electricity was contracted at about 280 pounds per megawatt-hour in 2016 and at 150 pounds by 2023, a fall Kuznetsov attributes less to any breakthrough in the machines than to the ordinary effects of building one design repeatedly: crews learn, and a fault that once required a barge now requires a diver. Offshore wind, she observes, was given nearly two decades of guaranteed prices before it reached the scale at which its own costs collapsed, whereas the national allocation for tidal stream was ring-fenced at 53 megawatts in the 2023 auction round and has never run long enough to test the same effect. Critics answer that a technology promised for fifty years has had support enough. Kuznetsov's reply is that the promise was made on behalf of barrages, and that modern seabed rotors have been generating for barely fifteen years, a distinction she concedes means little to a treasury official reading the same word on two applications.

DCallum Hearne, a coastal geomorphologist at the Marram Institute, approaches the subject from the mud. A tidal range scheme works by holding the sea back, and holding the sea back changes where sediment comes to rest. His survey of the Cawsand estuary, where a barrage was completed in 2009, recorded a 34 per cent reduction in exposed intertidal flat within eleven years, together with a coarsening of what remained as the finer particles were retained upstream. Counts of wading birds feeding on the estuary in winter fell across the same period. Hearne is careful about the reach of the claim, since the mechanism requires impoundment: a rotor turning in an open channel, he has said repeatedly, raises a different and much smaller set of questions. What troubles him is the arithmetic of national plans, in which a hectare of mudflat enters the calculation, if at all, as an obstacle to be compensated for somewhere else.

EAt a review commissioned in 2024, the disagreement proved narrower than their published positions suggested. Kuznetsov accepted that her firm-power figures assume transmission lines between distant coasts that nobody has yet built, and that until those lines exist the diversity she describes lives on paper rather than on the network. Hearne accepted that six years of monitoring at Ardmore had found no change in seabed sediment that could be separated from ordinary storm variation, and that his objection should not be carried across to sites of that kind. Each nevertheless drew a different lesson from the same uncertainty. For Hearne, an estuary once altered cannot be returned within the period a licence covers, so the burden of proof belongs with the developer; for Kuznetsov, a decade spent proving safety at one site is a decade of emissions produced elsewhere. The review recorded both remarks and recommended neither.

FWhat has changed since 2020 is the value of the thing tidal energy sells. Cheap batteries have made a few hours of flexibility ordinary, so a resource whose merit is that it can be foreseen must now show that it arrives when the rest of the system is short, and not merely that it arrives on schedule. The early figures are mixed: over the study period the Ardmore fleet was generating during 12 per cent of the winter evening peaks in its region, against 5 per cent for solar and 44 per cent for the local wind fleet. The unresolved question is whether the phase diversity that works between two channels still works once several dozen sites are added, since a coastline offers only so many distinct tidal phases. No published model settles it, and both researchers agree that the industry's habit of quoting the energy present in the water, rather than the electricity a network can use, has cost it credibility it can ill afford.