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WEEK 03: GENERATION: ALTERNATIVE SOURCES: WAVE & TIDAL POWER Sections: Introduction | Salter's Ducks | Oscillating Water Column | Tidal Power | Challenges Out at sea, the wind gives some of its energy to the water when it blows the sea into waves. Each wave on the open sea moves a body of water -- all the droplets that make it up -- up and down. Then the next wave comes along and carries the same droplets up and down again. All waves carry energy, and the energy of the ocean waves can be converted to energy. Several experimental projects are now in the stages of development. Hereunder are a few of them as an overview. Sections: Introduction | Salter's Ducks | Oscillating Water Column | Tidal Power | Challenges
Sections: Introduction | Salter's Ducks | Oscillating Water Column | Tidal Power | Challenges
Sections: Introduction | Salter's Ducks | Oscillating Water Column | Tidal Power | Challenges Introduction4 Tidal changes in sea level can be harnessed to generate electricity by building a dam for generating hydroelectric power across a coastal bay or estuary. Sluice gates and specially designed turbine/generators are incorporated into tunnels, called penstocks, in the dam; the turbines are turned by the inflow and outflow of water. The two common operating strategies for a tidal power plant are single-effect/ebb-generation, and double-effect. Schemes5 In single-effect/ebb-generation schemes, sluice gates are opened during flood tide, permitting water to flow into the basin behind the dam without generating power. The sluice gates are closed at high tide, and as the tide ebbs, the sea level falls relative to the level of water in the basin. When the difference in water levels, the "head", equals about half the tidal range, the penstocks are opened, and water is directed back to the sea through the turbine/generators. In double-effect schemes, reversible turbines are used, allowing the plant to generate power at both rising and ebb tides. Since the difference in "head" between the basin and the ocean must be significant, the turbines can generate for only about three to four hours within a full tidal period.
The world's largest tidal power plant is a 240-megawatt (MW), double-effect scheme located on the Rance River estuary along the coast of Brittany in France, where the mean tidal range is 8.5 m (28 ft) and the maximum range is 13.5 m (44 ft). It consists of a barrage blocking the 750 m (2460 ft) wide estuary of the Riven Rance.3 The tidal waters are channelled through 24 tunnets in the barrage(seen in cross-section above). Each tunnel houses a reversible turbine generator that can operate efficiently both on the flood tide (when the water flow is from sea to basin) and on the ebb tide (from basin to sea). At high tide, the sluices are closed, trapping the water in the tidal basin. At high and low tides, the turbines are operated as pumps, in order to increase the water level difference between basin and ocean just before generation begins. The water can then be released to turn the turbines when the tide is low but when demand for power is high. Each of the 24 turbines can generate up to about 10 MW. In Russia6, a 0.4-MW tidal power plant has been operating on the Barents Sea north of Murmansk since 1968. Built at about the same time as La Rance, this plant pioneered the use of prefabricated dam sections: turbo-generating machinery, penstocks, and sluice gates were installed in caissons and sunk onto a prepared foundation. In Canada, a single-effect, ebb-generation plant has been operating at Annapolis Royal, Nova Scotia, to demonstrate a 20-MW Straflo low-head turbine designed specifically for tidal operations. Unlike the Kaplan turbine used at La Rance, where the generator is housed in a steel bulb along the turbine axis, the Straflo generator is mounted on the turbine rim, providing less obstruction to water passage. Eight tidal plants, with a total installed capacity of 6 MW, operate in the People's Republic of China. Sections: Introduction | Salter's Ducks | Oscillating Water Column | Tidal Power | Challenges Future The problems with such machines, discussed above, are strength, corrosion and collision-prone, that is, making them strong enough to withstand constant battering from wind and waves and the corrosive effects of saltwater and avoiding collisions with ships at sea. 1. TechnoQuest 128, Waves and Wavepower Eaglemoss Publications Ltd, U.K, 1989. Picture courtesy of SPL Ltd (Martin Bond) and Jamstec. |
One method uses a fixed platform anchored out at sea with a hinged floating 'duck' attached to it.
The 
