The disadvantages and problems of tidal energy

The problems with tidal energy are not physics problems. Every one of them is about
cost, access or geography, which is worth knowing because it tells you which of them can be
solved by scale and which cannot.

It costs more than the alternatives

Tidal stream electricity is currently more expensive per unit than offshore wind, and the gap
is not small. Most of that is not the machine: it is the sea around it. Vessels are expensive,
installation windows are short, and every intervention has to wait for weather and tide.

Very few places will work

You need either a large tidal range or a strong reliable current, and those occur where
geography funnels water: narrow channels, island gaps, deeply indented estuaries. That is a short
list worldwide. Solar works everywhere the sun rises. Tidal does not.

The best sites are often the most sensitive

The same constrictions that concentrate flow also concentrate marine life, and act as routes
for migrating fish and marine mammals. Tidal stream rotors turn slowly, which helps, but the
question is legitimate and site specific.

A tidal barrage is a heavier intervention: it changes the tidal regime of an entire estuary
permanently, affecting sediment, salinity and the intertidal habitat that wading birds depend on.
See the three types of tidal energy for how much that varies by approach.

Maintenance is genuinely hard

A wind turbine can be climbed. A tidal turbine has to be recovered to the surface or dived on
during slack water, a window that can be under an hour and arrives on the tide’s schedule rather
than the crew’s. Everything about a good design is bent around reducing how often that is needed.

Output stops four times a day

Predictable is not the same as continuous. Generation falls to nothing around each slack
water, so tidal power needs a grid or storage around it. The mitigation is real though: sites on
different coasts peak at different times, so a distributed fleet smooths out.

Which of these get better

Cost and maintenance are volume problems and they fall with deployment, exactly as they did
for offshore wind. The site limit does not improve. That tension is the subject of
why tidal energy is not widely used.