Tidal

Tidal power, also called tidal energy, is a form of hydropower that converts the energy of tides into useful forms of power - mainly electricity. Tidal power generates electricity by capturing the energy contained in moving water caused by tides.

Although not yet widely used, tidal power has potential for future electricity generation. Tides are more predictable than wind energy and solar power. Among sources of renewable energy, tidal power has traditionally suffered from relatively high cost and limited availability of sites with sufficiently high tidal ranges or flow velocities, thus constricting its total availability. However, many recent technological developments and improvements, both in design (e.g. dynamic tidal power, tidal lagoons) and turbine technology (e.g. new axial turbines, cross flow turbines), indicate that the total availability of tidal power may be much higher than previously assumed, and that economic and environmental costs may be brought down to competitive levels.

Capturing and Using Tidal Power

Tidal energy can be captured in two ways:

Tidal Barrage

Tidal power can be harnessed using a barrage (dam) built across an estuary that captures the potential energy generated by the change in height (or head) between high and low tides. As the tide goes in and out, the water flows through tunnels in the dam. The ebb and flow are used to either turn a water turbine or compress air through a pipe that then turns a turbine, which generates electricity.

Tidal Fences and Turbines

Tidal fences and turbines can also be used to capture tidal power. Tidal fences are turbines that operate like giant turnstiles, while tidal turbines are similar to wind turbines. In both cases, electricity is generated when the turbines are turned by the tidal currents that occur in coastal waters. Ocean currents generate relatively more energy than wind (air currents) because ocean water has a higher density than air and therefore applies greater force on the turbines.

Benefits

• Tides are completely predictable and therefore produce reliable power.
• Tidal current turbines should have low environmental impacts.

Challenges

• Compared to river dams, tidal barrages are very expensive and can affect a wide area upstream and downstream. Tidal barrages reduce the time mud flats are exposed and covered and therefore affect bird and other wildlife and fisheries.
• The relatively low head of water above the turbines restricts the capacity of individual generators to about 25—50 megawatts.
• All components and machinery must be able to withstand salt water and moving silt and sand.

Global Status and Potential

Only a few tidal energy sites are in operation around the world. Larger sites include the White Sea in Russia and the Rance River in France (the largest site in the world). Smaller tidal power plant have been built in Canada, such as the site at Annapolis Royal in Nova Scotia, and several in Norway. Together they have a total capacity of less than 250 MW.

However, the potential for tidal energy is immense; potential global tidal power exceeds 450 terawatts, most of it in Asia and North America.