Tidal turbines

Tidal energy is produced by the surge of ocean waters during the rise and fall of tides.

Tidal energy or power, is a form of hydropower that converts the energy of tides into electricy or other useful power.

Tidal stream generator
Tidal stream generators make use of the kinetic energy of moving water to power turbines, in a similar way to wind turbines that use wind to power turbines. Land constrictions such as straits or inlets can create high velocities at specific sites, which can be captured with the use of turbines. These turbines can be horizontal, vertical, open, or ducted and are typically placed near the bottom of the water column.

Tidal barrage
Tidal barrages make use of the potential energy in the difference in height (or hydraulic head) between high and low tides. When using tidal barrages to generate power, the potential energy from a tide is seized through strategic placement of specialized dams. When the sea level rises and the tide begins to come in, the temporary increase in tidal power is channeled into a large basin behind the dam, holding a large amount of potential energy. With the receding tide, this energy is then converted into mechanical energy as the water is released through large turbines that create electrical power through the use of generators. Barrages are essentially dams across the full width of a tidal estuary

Formula for tidal energy
The energy available from a kinetic system is:

C_P = the turbine power coefficient
P = the power generated (W)
ρ = the density of the water (if seawater =1027 kg/m³)
A = the sweep area of the turbine (in m²)
V = the velocity of the flow

A ducted turbine are capable of 3 to 4 times the power of the same turbine rotor in open flow.