Tides are the daily swells and sags of ocean waters relative to coastlines due to the gravitational pull of the moon and the sun. Although the moon is much smaller in mass than the sun, it exerts a larger gravitational force because of its relative proximity to earth.
This force of attraction causes the oceans to rise along a perpendicular axis between the moon and earth. Because of the earth’s rotation, the rise of water moves opposite to the direction of the earth’s rotation, creating the rhythmic rise and sag of coastal water.
These tidal waves are slow in frequency (about one cycle every 12 h), but contain tremendous amounts of KE, which is probably one of the major untapped energy resources of earth.
All coastal areas experience two high tides and two low tides over a period of slightly more than 24 hours. For those tidal differences to be harnessed into electricity, the difference between high and low tides must be more than 16 feet (or at least 5 meters). However, the number of sites on earth with tidal ranges of this magnitude is limited.
How does Tidal Energy Work
The technology required to convert tidal energy into electricity is very similar to the technology used in wind energy. The common designs are the free-flow system (also called tidal stream or tidal mill) and the dam system (also known as barrage or basin system).
Tidal Stream
The tidal energy turbine has its blades immersed in oceans or rivers at the path of strong tidal currents. The current rotates the blades that are attached to an electrical generator mounted above the water level.
Tidal mills produce much more power than wind turbines because the density of the water is 800–900 times the density of air. The power of the tidal current Ptidal
Where A = the sweep area of the turbine blades (m2 ); v = the velocity of water (m/s); δ = the water density (1000 kg/m3 ).
Because the water density is high (about 1025 kg/m3 ), the tidal current has much higher power density than wind.
When tidal turbines are placed in areas with strong currents, they can produce large amounts of power. Tidal turbines look like wind turbines. They are arrayed underwater in rows as in some wind farms.
The turbines function best where coastal currents run between 3.6 and 4.9 knots (4 and 5.5 mph). In currents of that speed, a 49.2-foot (15-meter) diameter tidal turbine can generate as much energy as a 197-foot (60-meter) diameter wind turbine. Ideal locations for tidal turbine farms are close to shore in water 65.5–98.5 feet (20–30 meters) deep.
Barrage System
A barrage or dam is typically used to convert tidal energy into electricity by forcing water through turbines, which rotate a generator. Gates and turbines are installed along the dam.
When the tides produce an adequate difference in the level of water on the opposite sides of the dam, the gates are opened. The water then flows through the turbines. The turbines turn an electric generator to produce electricity.
The barrage energy system, which is also known as a dam-type tidal system, is shown in Figure 1. It is most suited for inlets where a channel connects an enclosed lagoon to the open sea. At the mouth of the channel, a dam is constructed to regulate the flow of the tidal water in either direction. A turbine is installed inside a conduit connecting the two sides of the dam.
At high tides, the water moves from the sea to the lagoon through the turbine as shown in Figure 1a. The turbine and its generator convert the KE of the water into electrical energy. When the tide is low, the water stored in the lagoon at high tides goes back to the sea and turns the turbine in the appropriate direction, thereby producing electricity.
If Hhigh is the head of the high water side of the dam and Hlow is the head of the low water side of the dam, as shown in Figure 1b, the average of the difference in heads ∆H between the waters on the two sides of the dam is given by
The difference in hydraulic heads of the tide determines the amount of energy that can be captured. We can compute the PE of a body of water with a higher head than the rest of the ocean.
PE = mg∆H (Equation 3)
Where m = the mass of water moving from the high head side to the low head side; g = the acceleration of gravity. Therefore, the PE of water is directly proportional to the difference in head.
The barrage of the tidal energy should be located in areas with high amplitude of tides. Tidal fences look like giant turnstiles. They can reach across channels between small islands or across straits between the mainland and an island. The turnstiles spin via tidal currents typical of coastal waters.
Some of these currents run at 5.6–9 miles/ hour and generate as much energy as winds of much higher velocity. Because seawater has a much higher density than air, ocean currents carry significantly more energy than air currents (wind).
Environmental and Economic Challenges
Tidal power plants that dam estuaries can impede sea life migration, and silt buildups behind such facilities can affect local ecosystems. Tidal fences may also disturb sea life migration. But new types of tidal turbines can be designed to avoid any migratory paths and can be less environmentally damaging.
It does not cost much to operate tidal power plants, but their construction costs are high, which lengthens payback periods. As a result, the cost per kilowatt-hour of tidal power is not competitive with conventional fossil fuel power.
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