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TIDAL POWER AS ELECTRIC SOURCE.
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Technology, cause of surges, mills, caissons, barrages, turbines, costs, advantages.... More...
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Paper Abstract:
Technology, cause of surges, mills, caissons, barrages, turbines, costs, advantages.

Paper Introduction:
Tidal Power As A Source Of Electricity Tidal energy has been used by humans for hundreds of years. In the modern era, the power is utilized for the production of electricity. The technologies involved are both proven and effective. However, with the exception of a tidal power plant at La Rance, France, the energy resource remains largely undeveloped. This may be in part because of the relatively high cost of the electricity produced. Only in appropriate circumstances will tidal power ever utilize its potential as both a dependable and enduring power source. Tidal surges are caused by complex interactions between the gravitational fields of the sun and the moon. These phenomena occur in cycles of varying length. For the most part, the earth's oceans bulge along an axis which is approximately in line

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However, due to the mature state of thesetechnologies, future advances are unlikely to decrease the cost of tidally-generated power by more than 1 to 2 % (Cavanagh et al., 1993, p. The resourcealso tends to be long-lived. K. 344). Finally, electricity output at a barrage may sometimes be increasedby pumping. Its development is onlypractical though in areas with large-tides and good construction sites. Tide of optimism ebbs over underwaterwindmill. (1991). In addition, duringone fifteen-year span, the turbines attained 91% of their design output(Sanders, 1991, p. Obviously, this machinery requires minimalmaintenance. This method does not generally producegreater power yields, however, because power generation during flood tidemay restrict refilling of the basin. However, with theexception of a tidal power plant at La Rance, France, the energy resourceremains largely undeveloped. Regardless, the method does have theone advantage of continuous energy production. Unfortunately though, from the standpoint ofcost, tidally-generated electric power remains uncompetitive. Over the years, the La Rance planthas established an impressive performance record. Oneof these is caused by interaction between the gravitational fields of thesun and the moon. The electricity is generatedas water flows into the tidal basin. Most approaches to tidal power utilization involve the hydraulic headcreated between high and low tides. Typical problems associated with various ofthe sites include financial constraints, geographic remoteness, andeconomic viability (Cavanagh et al., 1993, p. Modern systems generally employ abarrage, or dam. (199 ). In themodern era, the power is utilized for the production of electricity. Renewable energy resources. The energy output for any given location will varywith the tidal range and geography of the enclosed basin. In T. However, their one drawback is that they tend to be ratherlarge: diameters can be as great as 9 meters. Thetechnologies involved are both proven and effective. Although the last three decades have seen significant development oftidal-energy technologies, most have been demonstration plants. Webb, J. 515). There are a variety of different ways to operate a tidal barrage."Ebb generation" is perhaps the most common and the simplest. Pumping can increase the energygained from ebb generation systems by 5 to 15% (Cavanagh et al., 1993, p.52 ). The result is a half-day tidal cycle of 12 hours and 25 minutesbetween successive high waters. These caninclude the future cost of oil, coal, and uranium, as well as the cost ofmoney and the market for electric power. Smaller ratios are more desirable. These improvements have largely resulted from advances inmachinery, materials, construction methods, barrage configuration, andoperation procedures. The earliest mills were extremelysimple. Rim-generatorturbines have a stator which is outside the water flow. Tidal energy is extremelypredictable with regard to both timing and output. During a flood tide, water would enter a storage pond through asluice. Twidell, J., & Weir, T. Worldwide, somewhere between 5 and 1, terawatt-hours ofelectricity could potentially be produced from tidal power. & F. 273). Perhaps, atsome future date, circumstances will favor the development of this energyresource. 513-547).Covelo, CA: Island Press. Tidal power plants generally employ axial-flow turbines. This may be in part because of the relativelyhigh cost of the electricity produced. (1986). As tidal power technologies have matured, research has tended toemphasize cost reductions, performance improvement, and the enhancement ofeconomic gain. In fact, the overall cost of electricity generated from tidal powertends to be high. As the earthrotates, its surface moves with respect to this bulge (Sanders, 1991, p.273). Since tidal methods involvecontinuously changing hydraulic heads, the angles of either the turbinedistributor, the turbine blades, or both, must be regulated to achievemaximum efficiency. M. Reddy, R. 52 ). 273). This ratio is defined as the "lengthof the dam in meters to the annual energy production" (Sanders, 1991, p.275). All tides, however, may be affected by localinfluences. Tidal energy offers numerous advantages over other forms of electricpower generation. Regardless, engineers in the United Kingdom aredeveloping a 1 kilowatt, two blade tidemill turbine with blades 4 metersacross. 519). Moreover the economics of tidal energy depend greatly onthe particular site. More recently, the technologies required to produce electricity fromtidal power have become highly refined. 1 ). Johansson, A. Howes & A.Fainberg (ed's), The energy sourcebook: a guide to technology, resources,and policy. N. Hence, the best basins areconnected to the sea through narrow channels and contain a lot of water.It also helps if the basin is in resonance with the tidal period. This serves to increase both its water level and itshydraulic head. Sanders, M. 518). Single-basin barrages produce one pulse of electricity every 12 hoursand 25 minutes. A third method for barrage electricity production is referred to as"two-way generation." With this technique, tidal power is utilized duringboth the ebb and the flood tides. Begun in the 196 s, the La Rance plant wasbuilt "dry" behind a temporary "cofferdam." It went into operation in 1966. Regardless though, when tidal electric power is a fractionof total system capacity, output can usually be introduced directly into anelectrical distribution network. Moreover, with two-way generation techniques reversepumping mechanisms and double regulation must be employed (Cavanagh et al.,1993, p. These phenomena occur incycles of varying length. Tidal Power As A Source Of Electricity Tidal energy has been used by humans for hundreds of years. However, this method is not onlyrelatively inefficient, it may also have negative environmental impacts(Cavanagh et al., 1993, p. New Scientist, 138, p. This structure can be built out of concrete or steelcaissons (Laughton, 199 , p. Running a power plant's turbines in reverse will pump waterinto the tidal basin. 52 ). New York, NY: American Institute of Physics, pp. Tidal surges are caused by complex interactions between thegravitational fields of the sun and the moon. After a sufficient hydraulic head has developed, the water can bereleased through the barrage's turbines (Cavanagh et al., 1993, p. One full-scale, 24 megawatt (MWe) station has been constructed though at La Rance,in Brittany, northern France. Such factors as tunnelling, reflection, and resonance candramatically alter tidal range at any given point along a coastline(Cavanagh et al., 1993, p. Onemeasure used to determine the suitability of an area for tidal electricityproduction is the Gibrat site ratio. Only in appropriate circumstanceswill tidal power ever utilize its potential as both a dependable andenduring power source. 525). Burnham(ed's), Renewable energy: sources for fuels and electricity (pp. Ocean energysystems. References Cavanagh, J. Spring tide ranges tend to be about twicethose of neap tide. 257-297. Although such pumping does require energy input, a netenergy gain is acquired because the head generated at low tide is greaterthan that pumped against toward high tide. The two most commonly used generators in tidal flow systems are theconventional bulb turbine and the rim-generator turbine. NewYork, NY: E. The mainproblem though is the low-energy density of these streams. Overall, the bulbturbines have been operational about 95% of the time. Different caissons mayincorporate either sluiceways or turbines: the sluice gates may allow forwater flow between the ocean and the tidal basin during flood tide; theturbines generate electricity (Sanders, 1991, p. Renewable energy resources. Areas wherecommercial projects are being considered include, among others, thefollowing: (1) the Bay of Fundy, Canada; (2) the White Sea and the Sea ofOkhotsk, Russia; (3) Garolim, Korea; (4) the Gulf of Kutch, India; (4) theColorado estuary, Mexico; (5) the southeast coast of China; and (5) theSecure Bay region, Australia. It has been estimated though that even atthe best sites, the cost of this electricity would still be above presentcompetitive prices (Webb, 1993, p. N. Then, at low tide, this water would be used to turn a water wheel(Cavanagh et al., 1993, p. For example, tidal power is nonpolluting. The rotor ofrimgenerator turbines is attached to the periphery of the turbine runner. This wouldaccount for about 3% to 7% of all the tidal energy dissipated. 527). In recent years, there has been some innovation in power plantdesign. B. For the most part, the earth's oceans bulgealong an axis which is approximately in line with the moon. Main structures may last up to 12 years, and associated machinerycould endure for 3 to 4 . 525-526). Along the shores of Europe, records indicate that tide mills havebeen in operation since before AD 11 . As the tide and current shifts, the turbine would be able toswing freely on its moorings. 27). In bulb turbines,the rotor is positioned in a plane perpendicular to the tidal flow, whilethe generator is located in a pod behind the turbine runner. New York, NY: Elsevier Applied Science. Spon Ltd. 278). The ratio for the tidal powerplant at La Rance, France, for example, is .36. Williams, & L. While tidal electricity is beinggenerated, fossil fuels might be saved by decreasing the output of any fuel-powered generating plants which additionally contribute to the network. Each bulb units ofthese turbines has a rated capacity of 1 MWe; their rotor diameter israther large, though, at 5.6 meters. The energyavailable from a 4-knot tidal stream is one to two orders of magnitudelower than that available from a turbine of similar diameter containedwithin a barrage. Only then will tidal power technologies realize their fullpotential. In western Europe, at interestrates of around 5%, it has been estimated that a kilowatt hour of tidalelectricity might be produced for less than 18 cents (Cavanagh et al.,1993, p. E.; Clarke, J. H., & Price, R. Hence, the power generation of these systems may notnecessarily correspond to times of peak demand for electricity (Sanders,1991, p. However, other cycles exist as well. In R. Energy from the oceans. Yet another barrage technique is called "flood generation." Thissimply consists of ebb generation in reverse. Caissons may be prefabricated onshore,floated out to the site, and then sunk into place. Such a device would be suspended from a buoy and weighed down bytwo anchors. (1993). Thestraight flow of water through these turbines increases their hydraulicefficiency. H. Laughton, M. However, the magnitudeof this investment may be partly compensated for by the longevity of theseplants. (1993, April 24). 1 .----------------------- 1 These new turbines extract energy from tidal streams. In addition,although maintenance and operating costs are low, tidal power plantconstruction often requires large sums of capital. The ultimate economic viability of anyparticular tidal power plant though, depends on many factors. The plant employs horizontal shaft bulb turbines. Report number22). A. As in theearly tidal mills, water passes through sluices to fill the tidal basin.Then, the sluice gates are closed and the water is held until the tiderecedes. It occurs over a 14 day period and results in tidalmaximums and minimums known as the spring and the neap tides, respectively(Twidell & Weir, 1986, p.

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