How efficient is tidal energy?

Tidal turbines are efficient in the narrow sense, and the more useful measure is how
much of the time they generate. Confusing the two is behind most of the odd numbers quoted about
tidal power.

Conversion efficiency

A turbine in an open flow cannot capture all the energy passing through it: taking all of it
would mean stopping the water, which would stop the flow arriving. The theoretical ceiling for an
open rotor is a well known limit of a little under sixty percent, and it applies to water exactly
as it does to air.

Good tidal rotors operate near the practical share of that ceiling, comparable with a modern
wind turbine.

Capacity factor is the number that matters

Capacity factor is the share of the year a machine actually produces, against producing flat
out continuously. Tidal stream sites score well because the resource arrives on schedule and the
machine generates through most of the flood and most of the ebb.

The comparison with wind is the point: a wind farm may have a higher peak rating and spend far
more of the year waiting for wind. Predictable output at a known time is worth more to a grid
than a higher number that arrives at random.

What reduces it

Generation falls to nothing around slack water four times a day. Output also varies over the
spring and neap cycle of roughly two weeks, as tidal range grows and shrinks with the alignment
of sun and moon. Both are entirely predictable, which is the difference from weather.

System efficiency

Losses continue after the rotor: gearbox or direct drive, generator, subsea cable and the
onshore connection each take a share. Nothing unusual there, and the losses are comparable with
offshore wind at similar distances.