Tidal power has an advantage no other renewable source can claim: its output can be calculated years ahead. Commercial plants nevertheless remain rare, and the reasons are structural rather than technical. The first is arithmetic. The energy a barrage can extract rises with the surface area of the basin it encloses and with the square of the tidal range, so a coast where the water rises four meters offers not half of what an eight-meter site would give but a quarter. Length of shoreline is therefore no guide to where a plant can pay for itself, and the fifty largest tidal ranges on Earth fall within six regions. Those regions narrow again once the estuaries themselves are weighed. A barrage alters how sediment settles behind it and how much intertidal ground is uncovered twice a day, and the estuaries with the most workable range tend to be the ones carrying the most ecological value. The site that is best by one measure is the costliest by the other. Construction is expensive in its own right, and the strongest ranges lie mostly far from the cities that would draw on them. Forecasting the output, though, is not among the difficulties. A site measured for a single month can be predicted across the thirty-year life of a plant, because the driver is astronomical rather than meteorological. The peaks nonetheless arrive about fifty minutes later each day, since the moon returns overhead on a cycle slightly longer than the solar day. Demand keeps to that solar day, and across a month the two cycles slide past one another and back again.
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