Tidal Energy

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Tides are the vertical rise and fall of the sea level as a result of gravitational attraction between the earth, the moon and, to a lesser degree the sun. The gravitational attraction of the moon causes the water facing toward the moon to bulge. On the other side of the earth, the moon pulls the earth from the water, and we have a second bulge and another high tide. At the same time, low tides occur at the right angle to the moon’s pull. Other factors that affect the force, shape, and height of the tides are the time of the year (position of the sun), size, depth, and shape of the ocean basin.

The highest highs and the lowest lows are called the spring tides. Spring tides (they come from the Saxon word springen meaning the swelling of water and have nothing to do with the spring season) occur a few days after the full and the new moons, when sun, moon, and earth are aligned. Spring tides take place about twice a month. When the sun-earth line is at the right angle to the earth-moon line (i.e during half moons), the gravitational effects of the sun and the moon tend to cancel each other, tides are lower than average range and we have the neap tides (neap means scanty or lacking). Since with respect to the moon, the earth rotates about its axis once every 24 hours 50 minutes, we expect an interval of 12 hours and 25 minutes between two high or two low tides; each tidal day is therefore 1.035 times a solar day.

Accompanying the vertical rise and fall of water, there are also horizontal or lateral movements. As the tides channel between islands or into bays and estuaries tidal currents occur. During the high tide, the tidal current known as the flood tide flows towards the shore. About six hours later, the current reverses and flows away from shore. This is the ebb tide.

Using tides for power generation is not a new concept. Tidal mills operated off the western coasts of Europe for many centuries before being replaced by cheaper methods of producing energy. These mills took advantage of the natural rise and fall of coastal tides by allowing water to fill up a pond during flood tides and then emptying that water over a watermill during the ebb tide.( 1 ) Modern versions of tide mills are similarly designed. A barrier is built across an estuary and is allowed to be filled by water during the flood tide. The barrier is equipped with gates that are closed as the basin fills with water. As the tide recedes into the open sea during its ebb, the released water drives a number of turbines which in turn drive generators and produce electricity. Upon emptying, the basin gates are closed to allow the water in the sea to rise. This creates a head against the basin during the flood which can be emptied for a second time, turning the turbines in the opposite direction and generating power (Figure 4-6).

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Figure 4-6: Operation of a tidal plant (a) During period of high-tides water fills the basin. (b) At low tides the gate is opened and water is allowed to empty into the open sea while turning a turbine. (c) Once all the potential energy is exhausted gates are closed again, allowing water rise in the open sea. (d) The flow is from the open sea to the basin and turbine runs in the opposite direction. (e) The gate is closed until water rises again and repeats the operation.

The largest tidal power station that was ever constructed, a 240-MW plant built in 1966, is still operating today in La Rance estuary in Brittany, France (Figure 4-7). The annual generation is some 640 million kWh. Two other plants are operating commercially. The first, with an electrical generating capacity of 18-MW, is the Annapolis Royal Station in Nova Scotia off the Bay of Fundy in Canada; the second is a small 1.8 MW tidal plant in Kislaya Bay near Murmansk in the Russian Arctic.( 2 ) Currently, there are no tidal power plants in the United States and none are planned.

The potential energy of tidal power has been shown to increase with the basin area and the square of the average range between the low and high tides.(3,4) To be practical, tidal ranges of at least 5 meters (16.4 feet) are recommended. Oceans have tidal ranges between 5 to 10 feet; in narrow passages and estuaries ranges as high as 50 feet are common. Unfortunately, there are only about 40 sites on the planet that meet this requirement.(a)

Tides are clean, entirely predictable and renewable sources of energy. The major drawbacks are the large distances of most suitable sites from the population centers and the high capital cost of building the barrage across the estuaries. In addition, tidal power can be used only a few hours a day when the tide is moving in or out of the barrage.

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Figure 4-7: Tidal power plant at the Bay of Rance estuary in France.

Contents

References

(1) Gies, F. J., Cathedral, Forge, and Waterwheel: Technology and Invention in the Middle Ages, Harper Perennial, New York, 1994.

(2) Charlier, R. H., “Sustainable co-generation from the tides: A review,” Renewable & Sustainable Energy Reviews, 7, 187-213, 2003.

(3) Bernstein, L. B., “Central tidal-power stations in contemporary energy production,” State Publishing House, Moscow, 1961.

(4) Frau, J. P., “Tidal energy: promising projects: La Rance, a successful industrial-scale experiment,” IEEE Transactions on Energy Conversion, Volume: 8, Issue: 3, September 1993. pp. 552-558.

Additional Comments

(a) See the Department of Energy website at http://www.eere.energy.gov/consumerinfo/factsheets/nb1.html.

Further Reading

External Links