An in-stream tidal turbine, also called a tidal current turbine, works a lot like an underwater windmill. In-stream technology is designed to use the flow of the tides to turn an impellor, just like a windmill uses the flow of air to turn its blades. Each turbine technology deals with this challenge differently, but each uses the rotation of a turbine to turn an electrical generator.
OpenHydro, for example, houses its impellors in a shroud or duct, to accelerate the flow of water over the blades, and improve the efficiency of the units. Marine Current Turbines uses two reversing pitch propellers, just like a conventional wind turbine, and uses the design of their blades to maximize efficiency.
The turbines are designed to operate in the open flow of water. In the Minas Passage, they must operate in a range of speeds from zero to 8 knots, depending on where they are sited and how deep they are positioned. Water speed is fastest at the surface and slowest near the sea floor. Tidal power output is very sensitive to water speed, just as windmills are to wind speed. For example, if the water speed doubles, the turbine will produce eight times more power!
FORCE is designed to accommodate a number of turbines throughout the demonstration site. These turbine “berths” are supported by four 34.5kV subsea power cables (each 2 to 3 km in length) designed to transfer power to the shore and on to the Nova Scotia electricity grid.