Tidal stream turbines are renewable energy machines that turn the motion of ocean tides into electricity. They are often called underwater windmills because moving water pushes their blades much like wind pushes a wind turbine. Tides are driven mainly by the Moon's gravity, so their timing is highly predictable.
This makes tidal power useful for planning electricity generation from a clean source.
Key Facts
- Power in a moving fluid scales as P = 1/2 rho A v^3, where rho is fluid density, A is swept area, and v is current speed.
- Seawater is about 800 times denser than air, so slow tidal currents can carry large amounts of energy.
- A turbine cannot convert all flow energy into electricity because some water must keep moving past the rotor.
- Swept area for a circular rotor is A = pi r^2, so longer blades greatly increase available power.
- Generator output depends on turbine efficiency: P_electric = eta P_available.
- Tidal stream turbines work best in narrow channels, around headlands, and other places where tides create fast currents.
Vocabulary
- Tidal stream turbine
- A machine placed in moving tidal water that uses rotating blades to drive a generator and produce electricity.
- Swept area
- The circular area covered by the turbine blades as they rotate through the water.
- Current speed
- The speed at which water flows past the turbine, usually measured in meters per second.
- Generator
- A device that converts mechanical rotation into electrical energy using electromagnetic induction.
- Turbulence
- Irregular swirling motion in a fluid that can reduce efficiency and increase forces on turbine parts.
Common Mistakes to Avoid
- Treating tidal turbines as wave machines is wrong because they use horizontal tidal currents, not the up and down motion of waves.
- Forgetting the v^3 dependence is wrong because doubling current speed increases available power by a factor of eight, not by a factor of two.
- Using blade length instead of swept area in power calculations is wrong because the flow energy depends on A = pi r^2.
- Assuming all water energy becomes electricity is wrong because real turbines have efficiency losses and the water must continue flowing downstream.
Practice Questions
- 1 A tidal turbine has blade radius 8 m and sits in seawater with density 1025 kg/m^3. If the current speed is 2.0 m/s, calculate the available fluid power using P = 1/2 rho A v^3.
- 2 A turbine receives 820 kW of available fluid power and has an overall efficiency of 35 percent. What electrical power does it produce in kW?
- 3 A site has predictable tides but frequent slow currents, while another site has less convenient access but faster currents in a narrow channel. Explain which site is likely better for a tidal stream turbine and why.