Tidal energy is one of the oldest forms of energy used by humans.  Tide mills, in use on the Spanish, French and British coasts, date back to 787 A.D.
Tidal power generates electricity by capturing the energy contained in moving water caused by tides.
It can be captured in two ways:

Tidal Barrage:
Tidal power can be harnessed using a barrage (dam) built across an estuary that captures the potential energy generated by the change in height (or head) between high and low tides. As the tide goes in and out, the water flows through tunnels in the dam. The ebb and flow are used to either turn a water turbine or compress air through a pipe that then turns a turbine, which generates electricity.  Barrages allow tidal waters to fill an estuary via sluices and to empty through turbines.

Tidal Fences and Turbines:
Tidal fences and turbines can also be used to capture tidal power. Tidal fences are turbines that operate like giant turnstiles, while tidal turbines are similar to wind turbines. In both cases, electricity is generated when the turbines are turned by the tidal currents that occur in coastal waters. Ocean currents generate relatively more energy than wind (air currents) because ocean water has a higher density than air and therefore applies greater force on the turbines.

Benefits: Tides are completely predictable and therefore produce reliable power.  Tidal power is non-polluting, reliable and predictable. Tidal current turbines should have low environmental impacts.

Challenges: Compared to river dams, tidal barrages are very expensive and can affect a wide area upstream and downstream. Tidal barrages can affect bird and other wildlife and fisheries.  Most modern tidal concepts employ a dam approach with hydraulic turbines. A drawback of tidal power is its low capacity factor, and it misses peak demand times because of 12.5 hr cycle of the tides.

Wave power:
Waves are caused by the wind blowing over the surface of the ocean. In many areas of the world, the wind blows with enough consistency and force to provide continuous waves. There is tremendous energy in the ocean waves.
Wave power devices
extract energy directly from the surface motion of ocean waves or from pressure fluctuations below the surface.
While all wave energy technologies are intended to be installed at or near the water's surface, they differ in their orientation to the waves with which they are interacting and in the manner in which they convert the energy of the waves into other energy forms, usually electricity.

pelamis wave power

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tidal power
tidal map

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Wave Hot-Spots