In this article we will discuss about:- 1. Introduction to Tidal Power 2. Advantages and Disadvantages of Tidal Power 3. Generation Methods.
Introduction to Tidal Power:
Tidal power also represents a permanent source of energy. It can be predicted fairly accurately and is free from all types of pollutions. The rise and fall of tides nearly twice a day are associated with the rotation of earth every 24 hours in relation to both Sun and Moon. In large portion of the world tidal ranges are very low (say 1 metre or so) and, therefore, not suitable for electric power generation.
However at some places the various coastal complexities give good tidal patterns owing to resonant effects of local geography. The tide amplitude may be considerable 18 to 21 m have been noted near the Magellan Strait and the shores of America respectively. There is an enormous amount of energy in waves and in tides but it is difficult to harness and control energy.
The use of tides for electric power generation is practical in a few favourably situated sites where the geography of an inlet or bay favours the construction of a large scale hydroelectric power plant. For this a dam would have to be built across the mouth of the bay. It will have large gates in it and also low head water turbines installed in it. For storing water in the storage basin the gates are opened at the time of high tide and after storing water the gates are closed.
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After the tide has receded, there is a working water head between the basin water and open sea and the water is allowed to flow back to the ocean through water turbines installed in the dam. Thus the tidal energy stored at height can be utilised to drive a turbine coupled to an electric generator and thus generate electricity. Because of availability of low head bulb type units are employed for such projects. The principle of operation is illustrated in Fig. 7.47.
Though the arrangement explained above is simple and cheap but it is not useful due to variations in the output (the electric power generation is not continuous). By using reversible water turbine the turbine can run continuously both during high tide and low tide. The principle of operation is illustrated in Fig. 7.48. This arrangement is costlier but provides greater flexibility and continuous power output.
The above source for generation of electricity has been vigorously investigated in some countries like France, Germany, UK, Canada and USA. Probably the best known and the most recent tidal scheme, which utilises the tidal energy for generation of electrical energy has been constructed in France at Ranee Estuary where a power station of 240 MW capacity has been in operation since 1967.
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This plant has 24 units of bulb type turbines, each of 10 MW capacity. The annual output of this plant is around 540 million kWh. The units are designed for two-way operation—from storage basin to sea and from sea to the storage basin. The units operate as pumps accelerating the tidal effect at small water level difference between the storage basin and the sea and they operate as turbine when one level is considerably above the other.
The generators operate as motors driving the turbines as pumps during the pumping operation. The plant gates are used to accelerate filling and emptying of the storage basin at small differential water levels. This can be done with or without the operation of pumps. Operation cycle is shown in Fig. 7.49. Fully automatic operation (for starting, stopping and regulating generator sets and opening/closing valves to control power output) was introduced in 1982.
Tidal fences can also harness the energy in the tides. A tidal fence has vertical axis turbines mounted within a fence structure, known as a caisson that completely blocks a channel, forcing all the water through it. Unlike barrage stations, tidal fences can be employed in unconfined basins, such as in a channel between the main land and a nearly offshore island or between two islands.
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As a result, tidal fences have much less impact on the environment, because they do not need flooding the basin. They are also comparatively cheaper to install. Tidal fences have the advantage of being able to generate electrical power once each individual module is installed. Tidal fences are not free of environmental and economic impacts, however, since the caisson can disrupt the movement of large marine animals and shipping. A 55 MW tidal fence is planned for the San Bemadino Strait in the Philippines.
Tidal turbines are basically wind turbines that can be installed wherever there is strong tidal flow, as well as in river estuaries. Since water is about 800 times as dense as air, tidal turbines will have to be much sturdier than wind turbines. They will be heavier and costlier to build, but will be also to capture more energy.
Advantages and Disadvantages of Tidal Power:
The advantages of tidal power are:
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1. It is free from the problems of uprooting the people and disturbing the ecology balance.
2. It is everlasting and is not influenced by the changing mood of the nature such as failure of the monsoon.
3. No extra submerging of land is involved.
4. Tidal power generation is free from pollution.
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5. The biggest advantage of tidal power plant is that it is inexhaustible.
Disadvantages of tidal power are:
(i) The major drawback of tidal power plants is their uneven operation. Variations in the tidal energy available through the lunar day and lunar month different from their solar counterparts prevent the tidal power from being regularly used in power systems during the periods of peak demand.
(ii) The other factors tending to limit the use of tidal schemes are the heavy initial capital and long construction period. The interest and depreciation component of the generation cost is significant.
(iii) Sea water is corrosive.
(iv) The conversion devices are relatively complicated in construction.
(v) Because of variable tidal range, the plant efficiency is affected.
(vi) Sedimentation and silteration of basins are the serious problems associated with tidal power plant.
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) are suggesting that the total availability of tidal power may be much more than previously assumed, and that economic and environmental costs may be reduced to competitive levels. Such a scheme, if developed, can play a vital role in conservation of conventional resources.
Historically, tide mills have been used, both in Europe and on the Atlantic coast of North America. The earliest occurrences date from the Middle Ages, or even from Roman times. France is currently the only country that has significantly harnessed tidal energy and has the tidal power plant of 240 MW in the world. The Seven Barrage is a proposed tidal power station to be built across the Bristol Channel (Seven Estuary) in UK.
The River Severn has a tidal range of 14 m, making it perfect for tidal power generation. The Severn Barrage would involve the construction of a 16 km long barrage between Lavernock Point (Wales) and Brean Down (England). A total of 214 turbines each of 40 MW would be built into the barrage, making it a colossal of power plant of 8,560 MW of installed capacity with an average generation of 17 GWh/annum.
The drawback of uneven operation of tidal power plants however, can be overcome by operating the tidal power plant in conjunction with pumped-storage plant or run-of river power plant having a storage reservoir. In the tidal power-pumped storage scheme, the surplus energy generated (during the periods, the tidal power plant is generating electrical power more than requirements) by the tidal power plant can be used by the pumped storage plant to pump water into the upper reservoirs. When the tidal power plant output is at minimum, the pumped storage plant is run to deliver power to the system. This arrangement is attractive technically, but is costlier.
In the tidal power run-of river plants, the hydro plant raises its output when that of the tidal plant is declining until it shutdown. When the tidal plant is operating at adequate capacity, water is stored in the reservoir of the hydro plant.
Tidal Power Generation Methods:
There are three methods of tidal power generation as detailed below:
1. Tidal stream systems use kinetic energy of flowing water to drive power turbines as moving air is employed for driving windmills. This method is becoming popular because of the lower cost and lower ecological impact compared to barrages.
2. Barrages use the potential energy in the difference in height (or head) between high and low tides. Barrages are essentially dams across the full width of a tidal estuary, and involve very high civil infrastructure costs, and drawbacks of a worldwide shortage of viable sites, and environmental issues.
3. Dynamic tidal power exploits a combination of potential and kinetic energy by constructing long dams of 30-50 km length from the coast straight out into the sea or ocean, without enclosing an area. Both the obstruction of the tidal flow by the dam—as well as the tidal phase differences introduced by the presence of dam (which is not negligible in length compared to the tidal wavelength)—lead to hydraulic head differences along the dam. Turbines in the dam are used for conversion of power (6-15 GW per dam). In shallow coastal seas featuring strong coast-parallel oscillating tidal currents (common in UK, China and Korea), a significant water level difference (2-3 m) will appear between both sides of the dam.
Modern advances in turbine technology may eventually see large amounts of power generated from the ocean, especially tidal currents using the tidal stream designs but also from the major thermal current systems such as the Gulf Stream, which is covered by the more general term marine current power.
Tidal stream turbines may be arranged in high velocity areas where natural tidal current flows are concentrated such as the west and east coasts of Canada, the Strait of Gibraltar, the Bosporus, and numerous sites in Southeast Asia and Australia. Such flows occur almost anywhere where there are entrances to bap and rivers, or between land masses where water currents are concentrated.
The turbines used in tidal power plants are- axial turbines; vertical and horizontal axis cross flow turbines; oscillating devices using aero foils and venture effect.
Turbine Power:
Various turbine designs have different efficiencies and therefore different power outputs.
The energy available from these kinetic systems may be given as:
Power generated, P (watts) = ½ ξρ Av3 …(7.17)
Where ξ is turbine efficiency, ρ is the density of sea water (1,025 kg/m3), A is the swept area of the turbine in m2 and v is the velocity of flow.
Relative to an open turbine in free stream, depending on the geometry of the shroud shrouded turbines are capable of as much as 3 to 4 times the power of same turbine rotor in open flow.