In many ways, tidal turbines used for energy from tidal streams are analogous to wind turbines, in terms of the general shape, mounting and fixing technology, and power take-off system designs. The one critical difference between wind turbines and wave energy turbines is the size. For instance, tidal turbines generating 1 MW of power have sizes that are only about a third the size of a wind turbine with a similar generating capacity.
Tidal turbines are arrayed underwater in rows, as in some wind farms. Ideal locations for tidal turbine farms are close to shore in water depths of 20–30 meters (65.5–98.5 feet). The turbines function best where coastal currents run at between 3.6 and 4.9 knots (4 and 5.5 miles per hour). In currents of that speed, a 15-meter (49.2-feet) diameter tidal turbine can generate as much energy as a 60-meter (197-feet) diameter wind turbine.
The majority of tidal energy companies are developing horizontal axis turbines that are similar to wind turbines. Other types of tidal turbines being experimented are:
- Reciprocating Tidal Stream Devices - These have hydrofoils which move back and forth in a plane normal to the tidal stream, instead of rotating blades. One design uses hydraulic pistons to feed a hydraulic circuit, which turns a hydraulic motor and generator to produce power.
- Venturi Effect Tidal Stream Devices - In these, the tidal flow is directed through a duct, which concentrates the flow and produces a pressure difference. This causes a secondary fluid to flow through a turbine and generate electricity.