5.2 Tidal Stream Energy Conversion Principles

Tidal stream energy conversion harnesses the power of moving water in tidal currents. This renewable energy source uses turbines and other devices to transform the kinetic energy of predictable tidal flows into electricity.

Various technologies exist for tidal stream energy, including horizontal and vertical axis turbines, oscillating hydrofoils, and Venturi effect devices. Each type has unique advantages and is suited to different tidal conditions and locations.

Tidal Current Turbines

Harnessing Tidal Currents for Energy

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• Tidal current turbines convert the kinetic energy of moving water in tidal currents into electrical energy
• Tidal currents are caused by the gravitational pull of the moon and sun on the Earth's oceans, creating predictable and reliable flow patterns
• Tidal current turbines are typically installed in areas with strong tidal currents, such as narrow channels or straits between islands or coastal areas (Pentland Firth, Scotland)
• The velocity of tidal currents varies throughout the tidal cycle, with peak velocities occurring during spring tides and lower velocities during neap tides

Types of Tidal Current Turbines

• Horizontal axis turbines are the most common type of tidal current turbine and resemble wind turbines with their blades rotating around a horizontal axis
  • Horizontal axis turbines are typically mounted on the seabed or a floating platform and face the direction of the tidal current
  • Examples of horizontal axis tidal turbines include the SeaGen turbine in Strangford Lough, Northern Ireland and the Andritz Hydro Hammerfest turbine in Norway
• Vertical axis turbines have their blades rotating around a vertical axis and can operate in any direction of tidal current
  • Vertical axis turbines are less common than horizontal axis turbines but offer some advantages, such as the ability to operate in shallower waters and easier maintenance access
  • Examples of vertical axis tidal turbines include the Gorlov Helical Turbine and the Darrieus turbine (Uldolmok Tidal Power Station, South Korea)

Alternative Tidal Stream Devices

Oscillating Hydrofoils

• Oscillating hydrofoils are a type of tidal stream device that uses the lift force generated by a hydrofoil in a tidal current to drive a hydraulic system and generate electricity
  • The hydrofoil is mounted on an arm that oscillates up and down as the tidal current flows past, creating a pumping action in the hydraulic system
  • The hydraulic system drives a generator to produce electricity
• Oscillating hydrofoils have the advantage of being able to operate in slower tidal currents than tidal current turbines and can be designed to have a lower environmental impact (Stingray tidal energy device, UK)

Venturi Effect Devices

• Venturi effect devices use the Venturi principle to accelerate the tidal current through a constricted channel, creating a pressure difference that drives a turbine
  • The tidal current enters a wide inlet and is funneled through a narrow channel, increasing its velocity and creating a low-pressure zone
  • The pressure difference drives a turbine, which is connected to a generator to produce electricity
• Venturi effect devices have the advantage of being able to operate in slower tidal currents than tidal current turbines and can be designed to have a lower visual impact (Hales tidal venturi, UK)

Tidal Stream Energy Metrics

Power Density

• Power density is a measure of the amount of power that can be extracted from a given volume of tidal current
  • Power density is calculated as the power available per unit area of the tidal current and is typically expressed in watts per square meter (W/m²)
  • The power density of a tidal current depends on its velocity, with higher velocities resulting in higher power densities
• Tidal stream sites with high power densities are more attractive for development as they can generate more electricity from a smaller area, reducing the overall cost of the project (Pentland Firth, Scotland has a power density of up to 8 kW/m²)

Cut-in Speed

• Cut-in speed is the minimum velocity of the tidal current at which a tidal stream device starts to generate electricity
  • Tidal stream devices are designed to have a specific cut-in speed based on the characteristics of the tidal current at the site and the efficiency of the device
  • Tidal current turbines typically have a cut-in speed of around 0.5 to 1 meter per second, while oscillating hydrofoils and Venturi effect devices can operate at lower velocities
• The cut-in speed of a tidal stream device affects its capacity factor, which is the ratio of its actual energy output to its maximum possible output over a given period (SeaGen tidal turbine has a cut-in speed of 0.7 m/s)