Tidal Stream Energy
Tidal streams are water movements caused by forces of gravity between the earth, the moon and the sun, and to a much lesser extent other planets. It is a purely astronomical phenomenon that is observable because the Earth rotates and has oceans.
Tidal current energy converters capture the kinetic energy of the tidal streams by driving a generator that converts the energy into electricity.
The Resource Potential
Tidal stream energy is as of yet a largely untapped resource with no commercial size application, although some are very close. According to a DTI report, the world's accessible tidal stream resources are around 90GW, representing 3% of the total tidal stream energy. That's less than the globally installed wind capacity in 2010. The best sites can be found in Korea, UK and North America. In fact, in the UK alone there is potential for 5 - 16GW that could account for up to 15% of the UK's electricity.
Tidal speeds around the UK
How much power is in tidal streams?
Similar to wind power, the power in tidal streams, depends on the density of water (ρ = 1,023kg/m³), a cross section of the flow and the cubed velocity of the flow. Good tidal stream sites have flow speeds of up to 4-5 m/s, five times the flow speed or hundred times the power per m² than non-tidal rivers like the Amazon!
Tides are greatly influenced by size and location of land masses as well as the shape and depth of the ocean floors. 90% of tidal stream energy is found in deep waters (> 40m), but not on ground level. Some of those "good" sites are near shores.
Tidal Stream Characteristics
Where tidal streams occur, they are highly predictable, driven by patterns caused by the movements of the earth and moon.
Sun - Earth: Solar Tides
Within 24h, gravitational forces between the sun and Earth cause two high tides and two low tides. Maximum currents, and therefore maximum power, are reached at mid tide, every 6 hours (1). As the peak speeds when flowing in are higher than when flowing out, we can observe a 12-hour pattern (2). Plus, the two daily tidal bulges differ, accounting for a 24h pattern (3).The seasonal tilt of the Earth (4) is responsible for maximum peaks around the equinoxes and minimal during solistices.
Finally, because of the elliptic shape of the Earth's orbit around the Sun, peaks observed during June solistice (when Earth is furthest away from the Sun) are less than during December solistice (5).
Moon - Earth: Lunar Tides
Similar to solar tides, though following the lunar cycle of 24h 50' 24" (6,7,8). Due to the differences in mass and distance of Sun and Moon, the power of lunar tides outweighs the solar tides by around 7:3 with the dominant pattern following the 6h 12' 24" cycle.The Moon- Earth- Sun- Constellation
Neap tides occur every 14 days and 18 hours when the Moon's orbit is at 90° to that of the sun, thus cancelling out some of the resulting tidal stream power (9). Maximum peak flows can be observed during spring tides when all three are aligned. During every other spring tide (i.e. every 29 days) Moon and Sun are on the same side of the Earth with combined gravitational pull causing another increase in the maximum peak flow (10).- Horizontal Axis: The most advanced tidal devices are based on the same prinicple as most wind turbines. Most prominent example is Marine Current Turbines
- Vertical Axis: Due to their geometry, these devices can cover a wider area in shallow waters. Their lower cut-in speed (in comparison to horizontal axis) is less of a problem in tidal because there is little energy lost.
- Oscillating Device: The aerofoile-shaped blades of these devices are parallel to the water surface, and perpendicular to the stream. They osciallate up and down - that movement then drives the generator. Most prominent example is Pulse Tidal.
- Ground Mounted: Some devices are mounted directly on the seabed. With depths of hundred or more meters, this means that long pylons are not required. Disadvantage: Maximum speeds are rarely encountered close to the seabed.
- Pylon: This is the "wind turbine under water" principle, where one or more devices are attached to one pylon. The device may be completely or paritally submerged
- Floating: Floating devices are attached by steel cable or similar to the seabed. Obvious advantage: Less material required.
- Suspended: River devices may be suspended from regular bridges over the river.
- Float:Mechanisms which allow the device to be floated
- Raise above sea level: With help of some mechanism.
- Crane: from a vessle.
Axial flow rotors can be stopped or limited to the rated power by pitching the blades to neutral position, dissipating the energy without increasing the thrust. Other mechanisms include hydraulic brakes.
- Synchronous generator: Does not require a gearbox, but is more expensive. Works particularly well with vertical axis machines where the rotor frequency is low.
- Asynchronous generators - mostly with gearbox. Disadvantage: mechanical parts under water
All tidal devices need to operate bi-directionally.
Horizontal axis machines need to have a mechanism to reverse the rotor pitch to work efficiently in both flow directions. Vertical axis machines do not require a special meachnism.
Technology Pipeline
The technology stack is largely divided into two camps: Horizontal and Vertical. With no commercial installation as of yet, there are still many technologies developed with no clear winner yet. One of the moste advanced technologies is Seagen, developed by Marine Current Turbines (now part of Siemens).
Where in the World?
See map below for the global tidal hotspots. Click on locations for details on existing installations or projects.
