TIDAL ENERGY

ABSTRACT

Twice each day, thanks to a gravitational pull on earth from our rotating moon, the world's oceans produce powerful water currents and rising and falling tides. Humans have studied and exploited the tremendous power of the tides for millennia, including harnessing tidal power in 10^th century dams to turn millwheels for grinding flour. Forty years ago, the first tidal dams were constructed to convert tidal power into electricity. One of the first such tidal dams was constructed on Canada’s Bay of Fundy, where tides rise by as much as 12 meters (45 feet). Now, new energy technologies (NOT dams) that generate electricity from tidal currents could help produce as much electricity as the largest hydroelectric dams or nuclear and fossil fuel generating stations, without producing greenhouse gases or harming the environment. This paper focuses on need of renewable energy sources, tidal power superiority over other types of renewable energy sources. Paper also gives brief information of construction, basic components& types of tidal power plants.Information regarding turbines used in tidal plants are also given. Advantages& disadvantages of tidal power plant is also discussed. The paper also includes case study of La-Rance in France is given for more information regarding this important power source.

                                                                             INTRODUCTION

Creating power using water flow is not a new idea. A Frenchman known only as Monsieur Girard filed the first-ever patent for a wave energy device on July 12, 1799. He thought that if someone used the “motion and successive inequality of waves, which after having been elevated like mountains fall away in the following instant. . One has conceived the idea of the most overfull machine which has ever existed” (Ross 1991). This simple yet inventive idea has changed drastically since its introduction. Today, hydroelectric power, or energy produced by water, is used in various forms ranging from dams to tidal generation.

The sources for 90% of the electric energy generated today are non- renewable (Edinger 2000). Renewable sources of energy are necessary because the Earth will eventually run out of the resources to create non-renewable energy. There are three types of renewable energy sources: solar, wind, and waterpower. Both solar and wind power are drastically affected by weather variations, while tidal power varies little when the weather changes. Seawater is 832 times as dense as air; therefore the kinetic energy available from a 5-knot ocean current is equivalent to a wind velocity of 270 km/m (Blue Energy Canada 2000). Thus, tidal power generation may be the most viable of the three types of renewable sources of energy.

Tides, the daily rise and fall of ocean levels relative to coastlines, are a result of the gravitational force of the moon, the gravitational force of the sun, and the revolution of the Earth. The tides produce the electricity for tidal power by flowing in and out of turbines. A hydrostatic head or adequate water height difference on either side of the turbine is all that is necessary to run the turbine, and the turbines turn an electric generator that produces electricity. The simple idea of utilizing hydrostatic head to power turbines will be the crux of our article.

Chart illustrating the comparative energy advantage of Blue Energy’s Vertical-axis tidal current turbine system over other renewable energy options. (Courtesy, Blue Energy Canada)

BASIC PRINCIPLES OF TIDES

Gravitational Effects and the Centrifugal Force:-The interaction of the Moon and the Earth results in the oceans bulging out towards the Moon, whilst on the opposite side the gravitational effect is partly shielded by the Earth resulting in a slightly smaller interaction and the oceans on that side bulge out away from the Moon, due to centrifugal forces. This is known as the Lunar Tide. This is complicated by the gravitational interaction of the Sun which results in the same effect of bulging towards and away from the Sun on facing and opposing sides of the Earth. This is known as the Solar Tide. As the Sun and Moon are not in fixed positions in the celestial sphere, but change position with respect to each other, their influence on the tidal range (difference between low and high tide) is also affected. For example, when the Moon and the Sun are in the same plane as the Earth, the tidal range is the superposition of the range due to the lunar and solar tides. This results in the maximum tidal range (spring tides).Alternatively when they are at right angles to each other; lower tidal differences are experienced resulting in neap tides. Tidal basics

1. Most locations have two tidal cycles per day: 12 hours, 25 minutes

2. Essentially caused by interaction of moon, earth, and sun centrifugal forces

3. Diurnal tides are generated because the maxima and minima in each daily rotation are unequal in amplitude

                                                              Fig.1 Generation of Tides’

  TYPE OF TIDAL POWER PLANTS

                                                                                                                                        

• First-generation, barrage-style tidal power plants

The oldest technology to harness tidal power for the generation of electricity involves building a dam, known as a barrage, across a bay or estuary that has large differences in elevation between high and low tides. Water retained behind a dam at high tide generates a power head sufficient to generate electricity as the tide ebbs and water released from within the dam turns conventional turbines. Though the American and Canadian governments considered constructing ocean dams to harness the power of the Atlantic tides in the 1930s, the first commercial scale tidal generating barrage rated at 240 MW was built in La Rance.

• Second-generation, tidal current power production

Engineers have recently created two new kinds of devices to harness the energy of tidal currents (AKA ‘tidal streams’) and generate renewable, pollution-free

Electricity. These new devices may be distinguished as Vertical-axis and Horizontal axis models, determined by the orientation of a sub sea, rotating shaft that turns a gearbox linked to a turbine with the help of large, slow-moving rotor blades. Both models can be considered a kind of underwater windmill. While horizontal-axis turbine prototypes are now being tested in northern Europe (the UK and Norway) a vertical-axis turbine has already been successfully tested in Canada. Tidal current

Energy systems have been endorsed by leading environmental organizations, including Greenpeace, the Sierra Club of British Columbia and the David Suzuki Foundation as having “the lightest of environmental footprints,” compared to other large-scale energy system.

TYPE 1ST: BARRAGE TYPE TIDAL

POWER PLANT CONSTRUCTIONAL DETAILS BASIC COMPONENTS:

The four main components of a tidal power generation plant will be subsequently discussed. These components (as shown in Figure 2) are a tidal basin, a tidal barrage, sluice gates, and the tidal turbines themselves.

The first component of a tidal power generation plant is a tidal basin, or estuary. Finding a proper site containing an estuary is essential for the successful operation of a tidal power generation plant. One must note that the estuary will not be man-made; rather, the tidal basin will be a geographical feature that is not easily

Figure 2: Ebb generating system with a bulb turbine

replicated. A suitable estuary is typically a large body of water that is almost entirely surrounded by land with a small opening to the sea. The amount of power that a tidal power generation plant can produce is proportional to the size of the estuary (Taylor 1982).

The second component of a tidal power generation plant is the tidal barrage. This barrage looks like a wall that cuts off the estuary from the remainder of the sea. The bottom of the barrage sits on the sea floor, and the top of the barrage sits above the highest level that seawater can reach at high tide (Edinger 2000). The tidal barrage serves the purpose of cutting off seawater from water in the estuary so that water can be channeled through the wall in a beneficial manner for tidal power to be created.

 The third component of tidal power generating plants is sluice gates. Basically defined, sluice gates are areas of the barrage where water can freely flow in and out of the estuary. These gates are not always open: rather, they are controlled by the power plant operators such that water flows in and out of the estuary in a favorable method to the tidal turbines. Sluice gates do not have a uniform location on the tidal barrage.

The fourth major component of tidal power generation plants is the tidal turbines themselves. These turbines are located within the tidal barrage, and sit near the bottom of the sea floor. The turbines are designed in the same manner as a steam turbine. The turbines lie between sluice gates located on both the estuary and seaside of the tidal barrage. When these gates are opened, water rushes through the turbines, spinning the blades and creating electricity.

SINGLE EFFECT &DOUBLE EFFECT POWER GENERATION

There are two unique designs for tidal power generation plants. The first is single effect, which is also referred to as ebb generating flow. The second, more complex, design is termed double effect and will be discussed after single effect is understood. Single effect tidal power generation plants create power from water flowing through turbines in only one direction (Ross 1995). In the same way that steam turbines cannot operate if steam flows through in the opposite direction, single effect turbines cannot function unless water is run through them in a uniform direction. The tidal cycle of single effect operation is discussed below. Assume that water in the estuary is low and high tide conditions exist outside of the estuary

Figure 3: The tidal cycle for single effect turbines (Newsome 2002).

 When the water level in the sea is sufficiently high, sluice gates located away from the tidal turbines are opened and water rushes into the estuary, eventually filling the tidal basin to the level of the sea. When the water level inside the estuary reaches the water level of the sea, the sluice gates are closed and the high water sits inside of the estuary. While the water level inside of the estuary stays constant, the water level in the sea goes down and low tide conditions are ultimately reached. When the sea water level is suitably low, sluice gates located in front of and behind the turbines are opened. By opening these sluice gates, water is forced to flow through the turbine, spinning the blades and creating electricity. The sluice gates are closed when the estuary water level reaches the low tide water level of the sea. The water level in the sea rises back to high tide, and the cycle starts over again (Banal   1981).

The tidal cycle of double effect turbines (see Figure 4) is shown below. The cycle begins as the single effect cycle does, with the water level in the estuary low nd the water level in the sea at high tide conditions. Sluice gates in front of and behind the turbines are opened so that water rushes through the turbines, creating electricity. When the water level inside the estuary gets to the same level as the sea water level, the sluice gates are closed. The water in the estuary stays high, and the water in the sea will finally reach low tide conditions. When the water level in the sea is low enough, the same sluice gates in front of and behind the turbine are reopened and water flows out of the estuary through the turbines (Banal 1981).

Figure 4: The tidal cycle for double effect turbines.

Turbines that generate electricity when water flows over the blades in two directions are the largest innovation in tidal power technology. The blades are designed such that they spin in the same direction regardless of the direction that water flows over them. Allowing the blades to spin due to multi-directional flow allows double effect turbines to have a greater power output than comparable single effect turbines. Intuition tells people that double effect turbines should create about twice as much power as single effect turbines. Double effect turbines do produce more power than comparable single effect turbines: however, double effect turbines do not produce twice the amount of power that single effect turbines create (Ross 1991).

 TURBINES USED IN TIDAL POWER STATION

Tidal Turbines:

Several different turbine configurations are possible. For example, the La Rance tidal plant near St Malo on the Brittany coast in France uses a bulb turbine (figure 5). In systems with a bulb turbine, water flows around the turbine, making access for maintenance difficult, as the water must be prevented from flowing past the turbine. Rim turbines (figure 6), such as the Straflo turbine used at Annapolis Royal in Nova Scotia, reduce these problems as the generator is mounted in the barrage, at right angles to the turbine blades. Unfortunately, it is difficult to regulate the performance of these turbines and it is unsuitable for use in pumping. Tubular turbines have been proposed for use in the Severn tidal project in the United Kingdom.

In this configuration, the blades are connected to a long shaft and orientated at an angle so that the generator is sitting on top of the barrage.

Fig.5 Bulb Turbine (Copyright Boyle, 1996)

Fig.6 Rim Turbine (Copyright Boyle, 1996)

Details of Bulb turbine:

The Bulb turbine is a reaction turbine of Kaplan type which is used for the lowest heads. It is characterized by having the essential turbine components as well as the generator inside a bulb, from which the name is developed. A main difference from the Kaplan turbine is fore over that the water flows with a mixed axial-radial direction into the guide vane cascade and not through a scroll casing. The guide vane spindles are inclined (normally 60o) in relation to the turbine shaft. Contrary to other turbine types this results in a conical guide vane cascade. The Bulb turbine runner is of the same design as for the Kaplan turbine, and it may also have different numbers of blades depending on the head and water.

                                                 Fig7. Constructional Details of Bulb Turbine

Basic components of bulb turbine

- Stay cone

- Runner chamber

- Draft tube cone- stay cone

- Runner chamber

- Draft tube cone

- Generator hatch

- Stay shield

- Rotating parts

- Turbine bearing

- Shaft seal box

- Guide vane mechanism

 

The power available from the turbine at any particular instant is given by

 

Where,

Cd = Discharge Coefficient

A = Cross sectional area (m2)

G = gravity = 9.81

r = density (kg/m3)

The discharge coefficient accounts for the restrictive effect of the flow passage within the barrage on the passing water.

The equation above illustrates how important the difference between the water levels of the sea and the basin, (Z₁-Z₂), is when calculating the power produced

 

TYPE 2ND: TIDAL CURRENT POWER GENERATION

Advantages of tidal current power generation Like the ocean dam models of France, Canada and Russia, vertical and horizontal axis tidal current energy generators are fueled by the renewable and free forces of the tides, and produce no pollution or greenhouse gas emissions. As an improvement on ocean dam models, however, the new models offer many additional advantages:- because the new tidal current models do not require the construction of a dam, they are considered much less costly, they are considered much more environmentally-friendly., further cost-reductions are realized from not having to dredge a catchments area.- tidal current generators are also considered more efficient because they can produce electricity while tides are ebbing (going out) and surging (coming in),whereas barrage-style structures only generate electricity while the tide is ebbing. Vertical-axis tidal generators may be stacked and joined together in series to span a passage of water such as a fiord and offer a transportation corridor (bridge), essentially providing two infrastructure services for the price of one. Vertical-axis tidal generators may be joined together in series to create a ‘tidal fence’ capable of generating electricity.

Tidal current energy, though intermittent, is predictable with exceptional accuracy many years in advance. Present tidal current or tidal stream technologies are capable of exploiting and generating renewable energy in many marine environments that exist worldwide. It is proximal to existing, significant electro transportation infrastructure - is blessed with exceptional opportunities to generate large scale, renewable energy for domestic use and export

            TURBINES USED IN TIDAL CURRENT TYPE POWER PLANT

                                                         Fig .8 Tidal Turbine

• Vertical-axis tidal turbine– Canadian connection

A Canadian company – Blue Energy Canada Inc. – has completed six successful prototypes of its vertical-axis ‘Davis Hydro Turbine, named after its inventor, the late Barry Davis. Barry Davis trained as an aerospace engineer, working on the renowned Canadian Avro ‘Arrow’ project, then on the equally-remarkable ‘Bras D’Or’ hydrofoil project of the Canadian Navy. Barry then decided to apply his knowledge of hydrodynamics in creating a tidal energy generator. Barry received support from the Canadian National Research Council and successfully tested 5 turbine prototypes in the St. Lawrence Seaway and on the eastern seaboard. Blue Energy is presently raising funds for a commercial demonstration project of the Davis Hydro Turbine.

Figure 9: cutaway graphic depicting an array of vertical-axis tidal turbines stacked and joined in series across a marine passage.

              Tidal currents push on vertical mounted hydrofoils that apply a torque force to rotating shafts, which are coupled to generators housed just above the water level. A transportation corridor (bridge, etc.) may be constructed along the top surface providing two-for-one infrastructure service (courtesy, Blue Energy Canada Inc.).

Figure 10: cutaway graphic of a ‘mid-range scale’ (2 x 250 kW) vertical-axis tidal turbine. (Courtesy, Blue Energy Canada Inc.)

Trends in Generation Technologies:-

It has been over 30 years since the world's largest tidal power station was constructed on the Rance Estuary in France. At 240MW, it easily dwarves the 18MW

Station at Annapolis Royal, Canada which was completed in 1984 and smaller, (less than 500 kW) systems in the Bay of Kislaya and Janga Creek completed around the time of the La Rance project. Concerns over the environmental effects of barrage tidal plants since the construction of the La Rance tidal power station have lead to the development of technologies which have less impact on the environment. Two key areas of development have been in tidal fences and tidal turbines (also known as tidal mills) Tidal Fences Tidal fences are composed of individual, vertical axis turbines which are mounted within the fence structure, known as a caisson, and they can be thought of as giant turn styles which completely block a channel, forcing all of the water through them as shown in figure in operation.

                                                    Figure 11: Artists impression of a tidal fence

Unlike barrage tidal power stations, tidal fences can also be used in unconfined basins, such as in the channel between the mainland and a nearby off shore island, or between two islands. As a result, tidal fences have much less impact on the environment, as they do not require flooding of the basin and are significantly cheaper to install. Tidal fences also have the advantage of being able to generate electricity once the initial modules are installed, rather than after complete installation as in the case of barrage technologies. Tidal fences are not free of environmental and social concerns, as a caisson structure is still required, which can disrupt the movement of large marine animals and shipping. A 2.2GWp tidal fence using the Davis Turbine is being planned for the San Bernadino Strait in the Philippines. The project, estimated to cost $US 2.8 Billion and take 6 years to complete. Tidal Turbines Proposed shortly after the oil crisis of the 1970s, tidal  turbine s have only become reality in the last five years, when a 15kW 'proof of concept' turbine was operated on Loch Linnhe. Resembling a wind turbine, tidal turbines offer significant advantages over barrage and fence tidal systems, including reduced environmental effects.

Figure12: Schematic of an axial flow, seabed mounted marine current turbine

Tidal turbines utilize tidal currents which are moving with velocities of between 2 and 3 m/s (4 to 6 knots) to generate between 4 and 13 kW/m2. Fast moving current (>3 m/s) can cause undue stress on  he blades in a similar way that very strong gale force winds can damage traditional wind turbine generators, whilst lower velocities are uneconomic. Little research and development has been until taken until very recently in this area, with only the small 3kW, Australian Tyson turbine, for river systems, available commercially. Funding for a 300kW tidal turbine, manufactured by IT Power Ltd has just been funded by the European Commission and is expected to be installed during the year 2000.

 CONSTRAINTS TO TIDAL POWER GENERATION

There are also some significant environmental disadvantages which make tidal power, particularly barrage systems less attractive than other forms of renewable energy. Tidal Changes The construction of a tidal barrage in an estuary will change the tidal level in the basin. This change is difficult to predict, and can result in a lowering or rising of the tidal level. This change will also have a marked effect on the sedimentation and turbidity of the water within the basin. In addition, navigation and recreation can be affected as a result of a sea depth change due to increased sedimentation within the basin. A rising of the tidal level could result in the flooding of the shoreline, which could have an effect on the local marine food chain.

             Ecological Changes Potentially the largest disadvantage of tidal power is the effect a tidal station has on the plants and animals which live within the estuary. As very few tidal barrages have been built, very little is understood about the full impact of tidal power systems on the local environment. What has been concluded is that the effect due to a tidal barrage is highly dependent upon the local geography and marine ecosystem energy.

CASE STUDY

La Rance Tidal Generation Plant the La Rance Tidal Generation Plant is currently the world’s largest and oldest operational tidal power plant. Some brief historical and background information about the plant will first be provided, followed by the advantages and disadvantages of the plant. Background

                                                      Fig.13 La Rance Tidal Generation Plant

The La Rance tidal power plant was initially designed in 1954, but construction was not complete until 1967. This tidal power plant is so named because it sits on the La Rance River, near St. Malo, on the Brittany coast in France. Although the La Rance plant sits on a river, it has the same mannerisms as any other tidal power plant located in the sea. The La Rance plant is located on the river close enough to the sea for the river water to have tides similar to the sea tides. The enclosed estuary of the La Rance has tidal range of up to 13.5 meters this large tidal range provides a large hydrostatic head, which aids the plant’s power production. The plant has 24 separate horizontal 10 MW turbines. The La Rance plant utilizes double effect instead of single effect turbines. When all turbines are functioning, the turbines provide an overall output of 240 MW of power. This 240 MW is enough power to meet the electricity needs of about 300,000 homes (Banal 1981).

Figure 14: This shows the difference in sea

Levels at high tide on the La Rance River (“The Rance Tidal Power Plant” 2002). Plant Advantages and Disadvantages Many traits have allowed the La Rance plant to successfully generate power over the past years. During the plant’s 30 years of operation it has produced a total of 16 billion kWh and maintained an average reliability of 90%. This reliability statistic means that at any given time, only 10%, or roughly two of the 24 turbines will be inoperative. Turbine efficiency is also an issue. Efficiency is the ratio of actual power output to expected power output (Moran 2000). The operating turbines in the La Rance power plant produce an output of 95% efficiency. While this efficiency statistic seems good, it seems even better when compared to the efficiency of traditional energy sources. Traditional coal burning technology operates at about 35% efficiency (Shaw 1980).

Reliability and efficiency are just a few of the many advantages from the La Rance tidal plant. La Rance’s minimal impact on the environment also illustrates advantages of tidal power. A tidal power station does not result in any chemical or thermal pollution of the natural environment. Expected consequences from the tidal plant stem from the obstacle that a tidal barrage creates. The La Rance plant greatly impacted the environment only at initial construction. Marine flora and fauna suffered as a result of human intervention in the environment. The biological diversity in the basin recovered once the construction phase of the plant ended.

The flooding of the estuary is another environmental problem that can be caused when building a tidal power plant. Although flooding of the estuary did not Happen with the La Rance tidal power generation plant, flooding could be a major Problem elsewhere. The La Rance bas in did not flood because the plant was carefully planned and is always monitored to ensure that water levels in the estuary do not become dangerously high. People do not inhabit the area around the plant, so no homes were lost when the La Rance plant was created (Banal 1981). If people have environmental concerns about tidal power plants, they can look to the La Rance tidal power generation plant and see that it is possible to build an effective plant without destroying the plant’s surrounding environment.

The 24 turbines not only give the La Rance plant its plentiful power source, but also make it possible to build a four-lane road on top of the tidal barrage. This barrage reduced the distance from two neighboring cities from 45 km to 15 km. A bridge averaging 26,000 vehicles per day now connects these cities, once separated by the river, and traffic rises to around 50,000 vehicles per day during the summer.

Research has shown that building a bridge over a tidal power generation plant will  ost little less than the initial construction cost of the La Rance plant (Shaw 1980). If this is true, the La Rance plant serves two purposes for the price of one. Now some of the disadvantages of La Rance will be explored, although the disadvantages are minimal.

The biggest disadvantage of the La Rance was the high initial cost of construction. When built 1967, the plant cost 617 million francs, which is equivalent to 3.7 billion U.S. dollars today (Banal 1981). Although the initial cost is high, the plant has been in operation over 30 years and maintenance costs are minimal. Therefore, the plant has paid for itself over the years. Several different turbine configurations are possible in tidal power plants. For example, the La Rance uses a Bulb turbine .Water surrounds bulb turbines, making access to the turbines difficult. The regular maintenance for the turbines includes work on the turbines for 6 days every 4 years and 4 weeks every 10 years. This sort of maintenance is typical for turbine operations. Double effect bulb turbines are more difficult to maintain as a result of the incoming and outgoing flow producing extra stress on the rotors. This extra stress resulted in non-functional turbines, which led to losses in reliability. As a result, La Rance does not generally run double effect because plant operators found that running the turbines in single effect was more cost effective than running double effect (Clark 1997). Thus, double effects urbane are not necessarily the proper choice for every tidal power generation plant.

 PROBABLE SITES IN INDIA

1) Gulf of cambay: The range is 10.8m.Some of the sites on western banks are Sonari & Bhavnagar creek & sites on eastern bank are Dhodar &Kim river outfalls. His potential estimate is around 15MW. The major problem is high sliy index 5000ppm causing erosion of barrage.

2) Gulf of Kutch: The maximum range is 7.5M. Lara creek & Wank creek near Navlakhi are of attraction. Power potential is greater than Cambay.Slit charge is much smaller than Cambay.

3) Sundarban area in West Bengal: The tide range 4.8m.Power of 40MW can be produced in this area.

                                                                                                                                                          

 ADVANTAGES &DISADVANTAGES OF TIDAL POWER

 PLANT

Advantages:

1) Exploitation will in no case make demand for large area of valuable land, because they are on bays.

2) It is free from any pollution as it does not use any fuel.

3) It is much more suitable than hydropower plant as it is independent of rain.

4) It is independent on season cycle.

5) It has unique capacity to meet the peak power demand effectively when it works in combination    with thermal or hydroelectric.

Disadvantages:

1) Can only be developed if natural sites are available.

2) Transportation cost is more as sites are away from the load center.

3) The navigation is obstructed.

4) The output is varies with lunar cycle.

5) Capital cost is considerably high.

6) Supply is not continuous as it depends on timing of tides.

CONCLUSION

Advances in tidal power technology have occurred in a relatively short amount of time, and engineers have more incentive than ever to improve tidal power Generation. When many engineers began experimenting with the idea of creating electricity from the tides, tidal power was not taken seriously. Currently, the search for renewable energy sources has become serious. More and more people are committed to finding alternatives to the burning of natural resources because people realize that soon enough, other power options must be explored. They see that there   is no reason to delay the switch to another power source. Although another source of energy will be needed in the future, tidal power will not be this source. Tidal power can help ease the strain on other types of power production. The entire United States could be powered by the tides; yet the cost is more than most people would be willing to pay. So long as engineers have the ability to dream up new ideas and constantly improve on them, humanity will have some sort of power source. Tidal power has the potential to generate significant amounts of electricity at certain sites around the world. Tidal power can be a valuable source of renewable energy, although the United States electricity needs could never be met by tidal power alone. The negative impacts of tidal barrages are much smaller than those of other sources of electricity; however this reason alone is not enough to pursue implementation on a global scale. The technology required for tidal power is well developed, and the main barrier to increase the use of tides is that of construction cost. The prospect of natural resources and cost of other forms of energy will ultimately decide the future of tidal power generation.