Tidal power is generated by the bulk motion of water. Tidal flow arises from the cyclical motion of the moon relative to the earth, and to a lesser extent, the cyclical motion of the sun relative to the earth. As the distance between the earth, sun and moon changes, gravitational forces change as well. This causes periodic changes of water level which cause tidal currents. These currents can then be used to generate power.
Tidal power has two main classifications:
Tidal Stream Systems
These systems derive their energy from the bulk motion of water, similar to the way wind turbines derive their energy from the motion of air (wind). This method of tidal power generation is attracting the most attention.
Since water is much denser than air, there is much higher power density in moving water than in moving air. So for a given rotor speed, power output is much higher in a tidal stream generator than in a wind turbine. This also means that rotor size can be smaller than for wind turbines, because of the higher power density of moving water.
However, for power generation to be practical the tide must be moving at a speed of at least 1 m/s.
The World Offshore Renewable Energy Report 2002-2007, suggests that 3,000 GW of tidal energy is estimated to be available world-wide. However, less than 3% (90 GW) is located in areas suitable for power generation (ref: http://www.bwea.com/marine/resource.html). One region with good potential is The Bay of Fundy.
Tidal stream generation is a relatively new technology undergoing development. The above installation is among those being evaluated for large-scale commercial implementation.
Barrage Tidal Power
This is essentially a type of dam in which power is generated due to the difference in water level from the rise and fall of tides. This is similar to a hydro-electric dam in which water “falls” across a height difference and flows through turbines to generate electricity. In the case of a barrage, the water is falling across a difference in height created by the rise and fall of the tide.
This mode of power generation tends to be less favored than tidal stream generators because of higher construction cost and greater environmental impact; such as mortality of marine life resulting from direct contact with the turbines as it gets pulled through the barrage.
Tidal stream generators may also be used in areas of flow that are not a result of tides, such as the entrance to bays and rivers, or between land masses where water flow is concentrated.
These type of naturally occurring flows are constant year around. They are referred to as marine currents.
The total worldwide power available from marine currents is estimated to be 5,000 GW. This is five times the electrical production capacity of the United States. For instance, the Gulf Stream is an example of a potentially very large source of marine energy. It is estimated that capturing just 1/1000th of the available energy of the Gulf Stream (which moves at around 2 m/s) would supply 35% of the electricity used in Florida (ref: http://ocsenergy.anl.gov/documents/docs/OCS_EIS_WhitePaper_Current.pdf).
Energy storage is necessary due to the intermittent nature of tidal power. Pumped hydro storage facilities, which are a proven technology, can be built on elevated land located near shore, as close as possible to tidal power generators. This allows dispatching of power when needed. These pumped hydro facilities can also be used for storing energy from other renewables such as wind turbines (located offshore or near shore), and wave power generators. Operating together these can provide a reliable level of peak, intermediate and even baseload power to coastal communities.
Links:
http://www.marineturbines.com
http://www.seageneration.co.uk
http://www.cleancurrent.com
http://www.verdantpower.com
http://uekus.com