7.2 Tidal Stream Turbine Designs

Tidal stream turbines come in various designs, each with unique features. From horizontal and vertical axis turbines to ducted and open-centre models, engineers have developed diverse solutions to harness tidal energy efficiently.

Control mechanisms like blade pitch and yaw systems optimize turbine performance. Key specs include rotor diameter, rated power, and cut-in speed. These factors determine a turbine's ability to generate electricity from tidal currents effectively.

Turbine Types

Horizontal and Vertical Axis Turbines

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• Horizontal axis turbines have blades that rotate around a horizontal axis parallel to the direction of the tidal current flow (similar to wind turbines)
• Vertical axis turbines have blades that rotate around a vertical axis perpendicular to the direction of the tidal current flow
  • Can capture tidal flow from any direction without the need for a yaw mechanism
  • Examples include Darrieus turbines (egg beater shape) and Savonius turbines (helical shape)

Ducted and Open-Centre Turbines

• Ducted turbines have a shroud or duct surrounding the rotor blades
  • Concentrates and accelerates the tidal flow through the turbine, increasing power output
  • Protects the rotor blades from debris and marine life
• Open-centre turbines have a large hole in the center of the rotor
  • Allows marine life to pass through safely
  • Reduces the risk of cavitation by reducing the pressure drop across the turbine

Cross-Flow and Oscillating Hydrofoil Turbines

• Cross-flow turbines have blades that are arranged across the flow direction, allowing them to capture tidal flow from any direction
  • Example is the Gorlov Helical Turbine which has helical shaped blades wrapped around a cylindrical rotor
• Oscillating hydrofoils have hydrofoil shaped blades that oscillate up and down in the tidal current
  • Blade motion creates lift forces that drive a hydraulic system to generate electricity
  • Allows for a slower blade speed compared to rotary turbines, reducing environmental impact

Turbine Control and Mechanisms

Blade Pitch Control

• Blade pitch control involves adjusting the angle of the rotor blades to optimize power output and protect the turbine in high flow conditions
  • Pitching blades to a neutral angle in high flows reduces loads on the turbine structure
  • Pitching blades to an optimal angle in lower flows maximizes power generation
• Can be achieved through active hydraulic or electric pitch actuators, or passively through blade geometry and material properties

Yaw Mechanism

• Yaw mechanism allows horizontal axis turbines to rotate and align with the direction of the tidal current flow
  • Maximizes power output by ensuring the rotor is always facing the flow
  • Can be achieved through active hydraulic or electric yaw drives, or passively through tail vanes
• Some turbine designs (vertical axis, cross-flow) eliminate the need for a yaw mechanism by being able to capture flow from any direction

Turbine Specifications

Rotor Diameter and Rated Power

• Rotor diameter is the diameter of the swept area of the turbine blades
  • Larger rotor diameters allow for greater power output, but also increase the size and cost of the turbine
  • Typical tidal turbine rotor diameters range from 5-20 meters
• Rated power is the maximum power output of the turbine at optimal flow conditions
  • Determined by the rotor diameter, blade design, and generator capacity
  • Typical tidal turbine rated powers range from 100 kW to 2 MW

Cut-In Speed

• Cut-in speed is the minimum tidal current speed at which the turbine begins to generate power
  • Determined by the blade design, generator efficiency, and power electronics
  • Lower cut-in speeds allow for power generation in a wider range of tidal conditions
  • Typical tidal turbine cut-in speeds range from 0.5-1 m/s