Argument: Tidal energy production varies with tides; requires stand-by power
Lewis Page. "Tidal power project fails to start onschedule". 23rd Aug. 2007 - The tidal lads, though, are being a bit naughty here in suggesting that they wouldn't need standby generators like wind, sun etc. Even though their SeaGen submarine windmill works with the tide going both ways, it won't produce a steady, reliable flow of power. Every seaman knows the "rule of twelfths" which says that in the hour after high tide one-twelfth of the total water will flow back out; the next hour two-twelfths; then three; then three; then two; then the final twelfth, until low tide is reached approximately six hours later.
That's just a rule of thumb, but it's a fairly good one. Tidal turbines are going to vary from zero output at slack water to maximum when the flood and ebb are flowing at full speed; and the peak flow will vary between spring and neap tides too. The tidal turbine will operate on average at well under 50 per cent of what it must be built to take, and will dip to nothing four times every day.
In other words, it will need full standby from somewhere, just like wind, wave and solar; and it will have to be inefficiently overengineered, just like them and unlike hydrocarbon or nuclear plants, which hatefully run at 90 per cent of max cap or better year after year.