Chapter 11

Tidal Energy

Tides are periodic rises and falls of large bodies of water. Gravity is one major force that creates tides. In 1687, Sir Isaac Newton explained that ocean tides result from the gravitational attraction of the sun and moon on the oceans of the earth. Spring tides are especially strong tides that occur when the earth, the sun, and the moon are in a line. The gravitational forces of the moon and the sun both contribute to the tides. Spring tides occur during the full moon and the new moon. Neap tides are especially weak tides. They occur when the gravitational forces of the moon and the sun are perpendicular to one another with respect to the earth. Neap tides occur during quarter moons. Tidal energy is a form of hydropower that converts the energy of the tides into electricity or other useful forms of power. The tide is created by the gravitational effect of the sun and the moon on the earth causing cyclical movement of the seas. Therefore, tidal energy is an entirely predictable form of renewable energy. Until recently, the common plant for tidal power facilities involved erecting a tidal dam, or barrage, with a sluice across a narrow bay or estuary. As the tide flows in or out, creating uneven water levels on either side of the barrage, the sluice is opened and water flows through low-head hydro turbines to generate electricity. For a tidal barrage to be feasible, the difference between high and low tides must be at least 5 m.

11.1 GENERAL

Energy naturally present in ocean water bodies or in their movement can be used for the generation of electricity. This is achieved broadly in the following ways:

1. Tidal energy: During the rising period of tides, water is stored in a water reservoir constructed behind dams on shore. The potential energy of stored water body is used to generate electrical energy similar to that in a conventional hydropower plant. For the tidal energy method to work effectively, the tidal difference (difference in the height of the high and low tides) should be at least 4m. We discuss tidal energy in this chapter.
2. Wave energy: Using the kinetic (dynamic) energy of the ocean, waves is utilized to rotate an underwater power turbine and generate electricity thereon as an underwater wind farm. This will be discussed in Chapter 12.
3. Ocean thermal energy: Chapter 13 focusses on the temperature difference between warm ocean surface water and deep sea cold water is used to generate electricity. This is similar to geothermal power generation where heat trapped in the earth surface is converted into electrical energy.

11.2 TIDAL ENERGY RESOURCE

Tides are the waves caused due to the gravitational pull of the moon and also the sun (although its pull is very low). The rise of seawater is called high tide and fall in seawater is called low tide and this process of rising and receding of water waves happen twice a day and cause enormous movement of water.

Thus, enormous rising and falling movement of water is called tidal energy, which is a large source of energy and can be harnessed in many coastal areas of the world. Tidal dams are built near shores for this purpose in which water flows during high tide and water flows out of dam during low tides. Thus, the head created results in turning the turbine coupled to electrical generator.

Tidal energy has been developed on a commercial scale among the various forms of energy contained in the oceans. When the moon, the earth, and the sun are positioned close to a straight line, the highest tides called spring tides occur. When the earth, moon, and sun are at right angles to each other (moon quadrature), the lowest tides called neap tides occur.

The water mass moved by the moon’s gravitational pull when moon is very close to ocean and results in dramatic rises of the water level (tide cycle). The tide starts receding as the moon continues its travel further over the land, away from the ocean, reducing its gravitational influence on the ocean waters (ebb cycle).

11.3 TIDAL ENERGY AVAILABILITY

Gravitational forces between the moon, the sun, and the earth cause the rhythmic rise and fall of ocean waters throughout the world. Those result in tide waves. The moon exerts more than twice as great a force on the tides as the sun due to its much closer position to the earth. As a result, the tide closely follows the moon during its rotation around the earth, creating diurnal tide and ebb cycles at any particular ocean surface. The amplitude or height of the tide wave is very small in the open ocean where it measures several centimetres in the centre of the wave distributed over hundreds of kilometres. However, the tide can increase dramatically when it reaches continental shelves, bringing huge masses of water into narrow bays, and river estuaries along a coastline. For instance, the tides in the Bay of Fundy in Canada are the greatest in the world, with amplitude between 16 and 17 m near shore. High tides close to these figures can be observed at many other sites worldwide, such as the Bristol Channel in England, the Kimberly coast of Australia, and the Okhotsk Sea of Russia. Table 11.1 gives ranges of amplitude for some locations with large tides. Tidal energy projects are extremely site specific. The quality of the topography of the basin also needs to facilitate civil construction of the power plant. It is a clean mechanism and does not involve the use of fossil fuels. However, environmental concerns exist mainly to do with high silt formation at the shore (due to preventing tides from reaching the shore and washing away silt) and disruption to marine life near the tidal basin. Wave energy projects have lesser ecological impact than tidal wave energy projects.

Table 11.1 Highest Tides (Tide Ranges) of the Global Ocean

Source: NOAA Federal

In terms of reliability, tidal energy projects are believed to be more predictable than those harnessing solar or wind energy, since occurrences of tides are fully predictable. Table 11.2 provides glimpses of few potential sites for tidal power generation.

Table 11.2 A Few Potential Sites for Tidal Power Generation

Source: NOAA Federal

11.4 TIDAL POWER GENERATION IN INDIA

Long coastline with the estuaries and gulfs in India has a strong tidal range and height to move turbines for electrical power generation. Important site location and estimated power potential of a few Indian tidal energy plant is given in Table 11.3.

Table 11.3 Indian Tidal Energy Plant

Many organizations and government agencies are busy in the construction of tidal power plants on all those location and harnessing tidal energy at full capacity. There is an ample prospect for tidal power development in India. It has been investigated that Gulf of Cambay may prove the biggest tidal energy reservoir for India. Extensive exploration on the western coast in Gulf of Kutch (at Mandva), Gulf of Combay (at Hazira), Maharashtra (at Janjira and Dharmata) and also in Hoogali, Chhatarpur, and Puri on Eastern coast may be worth attempting.

Nevertheless, the possibility of developing tidal power scheme in India may be examined in the following all aspects:

1. Economic aspects of tidal power schemes when compared to the conventional schemes.
2. Problems associated with the construction and operation of plant.
3. Problems related to the hydraulic balance of the system in order to minimize the fluctuation in the power output.
4. Environmental effects of the schemes.

11.5 LEADING COUNTRY IN TIDAL POWER PLANT INSTALLATION

Worldwide installed capacity of few countries are approximately shown in Table 11.4.

Table 11.4 Installed Tidal Power Capacities of Few Countries

11.6 ENERGY AVAILABILITY IN TIDES

Potential energy and kinetic energy are the two energy components of energy of the tide waves. The potential energy is the work done in lifting the mass of water above the ocean surface.

This energy can be calculated as

(11.1)

where E = the energy; ρ = acceleration of gravity; ρ = the seawater density (which equals its mass per unit volume); A = sea area under consideration; z = vertical coordinate of the ocean surface; and h = the tide amplitude.

Taking an average of the product of (ρg) = 10.15 kN/m³ for seawater, energy for a tidal cycle per m² of ocean surface can be approximated as:

(11.2)

or (11.3)

Since extracting all the available stream power could be environmentally damaging, it is necessary to use a factor that expresses the usable power percentage with apparently no damaging consequences. It is called α (significant extraction factor) that may vary from 0.2 to 0.6.

The kinetic energy (KE) of the water mass (m) is its capacity to do work by virtue of its velocity (V). It is defined by

(11.4)

The total energy of tide waves equals the sum of its potential and kinetic energy components.

Estimation and understanding of the potential energy availability of the tides are key for designing conventional tidal power plants using water dams for creating artificial upstream water heads. Such power plants exploit the potential energy of vertical rise and fall of the water.

The kinetic energy of the tide has to be known for designing other types of tidal power plants (like Soating), which harness energy from tidal currents or horizontal water. They do not involve the installation of water dams.

11.6.1 Calculation of Tidal Power

Potential tidal power can be reckoned based on a mathematical calculation. Let us assume that the surface area of the reservoir as stable between the full stored water level and the emptied floor, the energy produced by the ebbing water could be expressed as

(11.5)

Here, d(w) = energy unit; ρ = density of seawater (about [1.02–1.04] × 10³ kg/m³); g = acceleration of gravity (9.8 m/s²); A = surface area of the reservoir (m²) assumed as a constant from high tide to low tide; h = instant water level height (m); v = volume of reservoirs (m³).

Therefore, its power could be written as

(11.6)

Here, P = potential power (W); T = tidal period (s); and H = tidal ranges (m).

Let us assume ρ = 1.04 ×10³ kg/m³ and T = 6 h.

This formula can be simplified as (11.7)

11.6.2 Tidal Stream Generator

It is often referred to as a tidal energy converter (TEC). It is a machine that extracts kinetic energy from moving masses of water in particular tides. A TEC device extracts energy from a tidal flow much in the same way that a windmill extracts energy from the wind. The following equation can be used to calculate the power output of either devices,

(11.8)

where P = power (W); ρ = density of seawater (kg/m³); A = capture area (m²); η_T = combined efficiency from water to electric wire; and V = flow speed (m/s).

The abovementioned power calculation is valid, provided the capture area of the tidal energy conversion device(s) is small in comparison to the cross-sectional area of the channel. The significant difference between wind and tidal power calculations is the typical range of flow speeds and the density of the fluid.

11.7 TIDAL POWER BASIN

The basin system is the most practical method of harnessing tidal energy. It is created by enclosing a portion of sea behind erected dams. The dam includes a sluice that is opened to allow the tide to flow into the basin during tide rise periods and the sluice is then closed. When the sea level drops, traditional hydropower technologies (water is allowed to run through hydro turbines) are used to generate electricity from the elevated water in the basin. From Equation 11.7, we can observe that the tidal power varies as the square of the head and since the head varies with the tidal range, the power available at different sites shows very wide variation. In order to overcome this wide variation in availability of tidal power, various tidal basin systems have, therefore, been developed. They are discussed in the following sections.

11.7.1 Single-basin System

This is the simplest way of power generation and the simplest scheme for developing tidal power is the single-basin arrangement as shown in Figure 11.1. Single water reservoir is closed off by constructing dam or barrage. Sluice (gate), large enough to admit the water during tide so that the loss of head is small, is provided in the dam.

Figure 11.1 Single-basin system

The single-basin system has two configurations, namely:

1. One-way single-basin system: The basin is filled by seawater passing through the sluice gate during the high tide period. When the water level in the basin is higher than the sea level at low tide period, then power is generated by emptying the basin water through turbine generators. This type of systems can allow power generation only for about 5 h and is followed by the refilling of the basin. Power is generated till the level of falling tides coincides with the level of the next rising tide.
2. Two-way single basin: This system allows power generation from the water moving from the sea to the basin, and then, at low tide, moving back to the sea. This process requires bigger and more expensive turbine.

Single-basin system has the drawbacks of intermittent power supply and harnessing of only about 50% of available tidal energy.

11.7.2 Two-basin Systems

An improvement over the single-basin system is the two-basin system. In this system, a constant and continuous output is maintained by suitable adjustment of the turbine valves to suit the head under which these turbines are operating.

A two-basin system regulates power output of an individual tide, but it cannot take care of the great difference in outputs between spring and neap tides. Therefore, this system provides a partial solution to the problem of getting a steady output of power from a tidal scheme.

This disadvantage can be overcome by the joint operation of tidal power and pumped storage plant. During the period, when the tidal power plant is producing more energy than required, the pumped storage plant utilizes the surplus power for pumping water to the upper reservoir. When the output of the tidal power plant is low, the pumped storage plant generates electric power and feeds it to the system. This arrangement, even though technically feasible, is much more expensive, as it calls for high installed capacity for meeting a particular load.

This basic principle of joint operation of tidal power with steam plant is also possible when it is connected to a grid. In this case, whenever tidal power is available, the output of the steam plant will be reduced by that extent that leads to saving in fuel and reduced wear and tear of steam plant. This operation requires the capacity of steam power plant to be equal to that of tidal power plant and makes the overall cost of power obtained from such a combined scheme very high. In the system shown in Figure 11.2, the two basins close to each other, operate alternatively. One basin generates power when the tide is rising (basin getting filled up) and the other basin generates power while the tide is falling (basin getting emptied). The two basins may have a common power house or may have separate power house for each basin. In both the cases, the power can be generated continuously. The system could be thought of as a combination of two single-basin systems, in which one is generating power during tiding cycle, and the other is generating power during emptying.

Figure 11.2 Two-basin system

11.7.3 Co-operating Two-basin Systems

This scheme consists of two basins at different elevation connected through the turbine. The sluices in the high- and low-level basin communicate with seawater directly, as shown in Figure 11.3. The high-level basin sluices are called the inlet sluices and the low level asoutlet sluices.

Figure 11.3 Co-operating two-basin systems

The basic operation of the scheme is as follows:

1. The rising tide fills the high-level basin through the sluiceways.
2. When the falling seawater level is equal to the water level in the high-level basin, the sluiceways are closed to prevent the outflowing high-level basin water back to the sea.
3. The water from high-level basin is then allowed to flow through the turbine generators to the low-level basin.
4. When the falling seawater level becomes lower than the rising water level in the low-level basin, the sluiceways are opened to allow water to flow into the sea from the low-level basin. This process continues until the water level in the low-level basin equals to the rising sea level. Then, the sluiceways are closed to prevent the filling of low-level basin from the seawater.
5. When the seawater again rises during the next rising tide equals to low level of high-level basin, sluices of high-level basin is again open for filling of water in high-level basin. Thus, the cycle is repeated.

Figure 11.4 gives another schematic diagram of co-coordinating two-basin tidal power stations. With two basins, one is filled at high tide and the other is emptied at low tide. Turbines are placed between the basins and between the basin and the sea. This two basin systems allow continuous power generation. However, they are very expensive to construct due to the cost of the extra length.

Figure 11.4 Co-ordinating two basin systems

11.8 TURBINES FOR TIDAL POWER

Tidal power plants operate using a rapidly varying head of water, and therefore, their turbines must have high efficiency at varying head.

These are as follows:

1. The Kaplan type of water turbine operates quite favourably under these conditions.
2. The propeller type of turbine is also suitable because the angle of the blades can be altered to obtain maximum efficiency while water is falling.
3. A compact reversible horizontal turbine (bulb-type turbine) has been developed by French Engineer and it acts with equal efficiency both as a pump and as a turbine.

11.8.1 Bulb-type Turbine

The bulb-type turbine shown in Figure 11.5 consists of a steel shell completely enclosing the generator that is coupled to the turbine runner.

Figure 11.5 Bulb-type turbine

The turbine is mounted in a tube within the structure of the barrage, and the whole machine being submerged at all times. When the power demand on the system is low during the rising tides, the unit operates as a pump to transfer water from sea to the basin. When the load on this system is high, the unit will work as a generator, and deliver the stored energy that is a valuable additional input to the system.

Bulb turbines incorporated the generator–motor unit in the flow passage of the water. These turbines are used at the La Rance power station in France. The main drawback is that water flows around the turbine, making maintenance difficult.

11.8.2 Commercial Status of Tidal Stream Devices (as on 2009)

Table 11.5 Status of Tidal Stream Devices

11.9 ADVANTAGES AND DISADVANTAGES OF TIDAL POWER

The following are the advantages of tidal power:

1. About two-third of earth’s surface is covered by water, there is scope to generate tidal energy on large scale.
2. Techniques to predict the rise and fall of tides as they follow cyclic fashion and prediction of energy availability is well established.
3. The energy density of tidal energy is relatively higher than other renewable energy sources.
4. Tidal energy is a clean source of energy and does not require much land or other resources as in harnessing energy from other sources.
5. It is an inexhaustible source of energy.
6. It is an environment friendly energy and does not produce greenhouse effects.
7. Efficiency of tidal power generation is far greater when compared to coal, solar, or wind energy. Its efficiency is around 80%.
8. Despite the fact that capital investment of construction of tidal power is high, running and maintenance costs are relatively low.
9. The life of tidal energy power plant is very long.

The following are the disadvantages of tidal power:

1. Capital investment for construction of tidal power plant is high.
2. Only a very few ideal locations for construction of plant are available and they too are localized to coastal regions.
3. Unpredictable intensity of sea waves can cause damage to power generating units.
4. Aquatic life is influenced adversely and can disrupt the migration of fish.
5. The energy generated is not much as high and low tides occur only twice a day and continuous energy production is not possible.
6. The actual generation is for a short period of time. The tides only happen twice a day so electricity can be produced only for that time, approximately for 12 h and 25 min.
7. This technology is still not cost effective and more technological advancements are required to make it commercially viable

11.10 PROBLEMS FACED IN EXPLOITING TIDAL ENERGY

1. Usually the places where tidal energy is produced are far away from the places where it is consumed. This transmission is expensive and difficult.
2. Intermittent supply: Cost and environmental problems, particularly barrage systems are less attractive than some other forms of renewable energy.
3. Cost: The disadvantages of using tidal and wave energy must be considered before jumping to conclusion that this renewable, clean resource is the answer to all our problems. The main detriment is the cost of those plants.
4. Altering the ecosystem at the bay: Damages such as reduced flushing, winter icing, and erosion can change the vegetation of the area and disrupt the balance. Similar to other ocean energies, tidal energy has several prerequisites that make it only available in a small number of regions.

For a tidal power plant to produce electricity effectively (about 85% efficiency), it requires a basin or a gulf that has a mean tidal amplitude (the differences between spring and neap tide) of 7 m or more. It is also desirable to have semi-diurnal tides where there are two high and low tides everyday.

A barrage across an estuary is very expensive to build and affects a very wide area—the environment is changed for many miles upstream and downstream. Many birds rely on the tide uncovering the mud flats so that they can feed. There are few suitable sites for tidal barrages.

1. Only provides power for around 10 h each day, when the tide is actually moving in or out.
2. Present designs do not produce a lot of electricity, and barrages across river estuaries can change the flow of water, and consequently, the habitat for birds and other wildlife.
3. Expensive to construct.
4. Power is often generated when there is little demand for electricity.
5. Limited construction locations.
6. Barrages may block outlets to open water. Although locks can be in stalled, this is often a slow and expensive process.
7. Barrages affect fish migration and other wildlife; many fish like salmon swim up to the barrages and are killed by the spinning turbines.
8. Fish ladders may be used to allow passage for the fish, but these are never 100% effective.
9. Barrages may also destroy the habitat of the wildlife living near it.
10. Barrages may affect the tidal level—the change in tidal level may affect navigation, recreation, cause flooding of the shoreline, and affect local marine life.
11. Tidal plants are expensive to build.
12. They can only be built on ocean coastlines; this mean that for communities that are far away from the sea, it is useless.

SUMMARY

• Tidal energy is clean, renewable, and sustainable form of energy resource. It has no impact on climate because it does not produce any greenhouse gases.
• The tidal basin system is a portion of the sea enclosed behind a dam or dams and water is allowed to run through turbines, as the tide subsides.
• The tidal basin system is either single-basin or two-basin or co-operated two-basin system.
• La Rance Barrage is the world’s first tidal power station. The facility is located on the estuary of the Rance River, in Brittany, France. It is opened on the 26th November 1966.
• Large investment and long time is required to build barrage. These are the main causes of slowing down the tidal energy development.
• The biggest and main use of tidal energy is in the generation of electricity.
• Tidal barrages and power plants have several environmental drawbacks including changes to marine and shoreline ecosystems, most notably fish populations.

REVIEW QUESTIONS

1. What are the special problems in the construction of barrage for tidal scheme?
2. A typical tidal power station has 24 generators each of 10 mW operating at maximum head of 3.5 m. It generates for 6 h twice a day. The density of seawater is 1,025 kg/m³, and the efficiency of turbine and generator each is 93%. Assuming that power decreases linearly and that the reservoir has rectangular cross-sectional area, calculate the following:
   1. Basin capacity in m³
   2. Annual energy production
3. Explain the ‘single-basin’ and ‘two-basin’ systems of tidal power harnessing. Further, discuss their advantages and limitations
4. A tidal power plant of single-basin type has a pool area 80 × 104 m². The tidal has a range of 8 m. However, the turbine stops operating when head on it falls below 2 m. Calculate the energy generated in one filling process in kW-h, if the turbine–generator efficiency is 90%.
5. Derive an expression for total energy generated by a modulated single-pool tidal power plant with a tidal range of R meters. The water level in the pool rises during filling process as 0.06 R (t – t₁)’ where t₁ is the instant when power generation starts and t is time in hours. The basin has an area of A km² and water density is 1,025 kg/m³. Power generation stops at t2, when head falls below generation threshold. Assume sinusoidal tidal pattern.
6. Explain the difference between tidal range and head of water in tidal scheme.
7. Derive an expression for tidal energy per tidal cycle for a simple single-pool single effect tidal scheme.
8. State the types of tidal energy conversion schemes.
9. Explain the principle of operation of a simple single-effect tidal power plant and give a graph of sequential operating modes.
10. Explain the use of additional pumping feature in a single-pool single effect tidal scheme.
11. Discuss the relative merits and limitations of tidal power.
12. What are the difficulties in tidal power developments?
13. Write a short note on Sundarban tidal power scheme.