8 Principles of Generation of Tidal Energy

Mr. Prasad Dudhgaonkar and Prof. Purnima Jalihal

1 Introduction

The ocean is a complex entity involving a vast number of processes with various causal factors. The phenomena which have interested oceanographers from times immemorial, are waves, currents, tides,winds. Waves are generally caused on the ocean surface due to winds. They generally keep varying in eight and frequency by location and the driving forces. On the other hand, the periodic rise and fall of water levels that occur daily are the tides. Currents are due to water velocity, which could be due to tidal streams or in general due to water particle velocity. The factors which lead to these phenomena are many and include heating and cooling, evaporation and precipitation, which actually are driven by the sun. Properties like sea surface temperature and air pressure at the sea surface leading to pressure gradients all influence oceanic behaviour. The two oceanic phenomena of interest dealt with in this chapter are tides and currents.

1.2  Causes of tides & ocean currents

It is well known that Newton’s law of universal gravitation states that every object that has mass in the universe is attracted to every other object. Further the gravitational force between these objects is directly proportional to the product of these masses and is inversely proportional to the square of the distance between the two masses. Since the square of the distance is involved, even a small increase in the distance between two objects significantly decreases the gravitational force between them. In short, the greater the mass of the objects and closer they are together, greater is their gravitational attraction. Thus gravitational forces for points on earth caused by the moon vary depending on their distances from the moon. Also the direction of the gravitational attraction between most earth particles and the center of the moon is at an angle relative to a line connecting the center of the earth and the moon. Due to this angle, the gravitational force at different particles is different.

The centripetal force is the force required to keep planets in their orbits and is provided by the gravitational force between them and the sun. This force connects an orbiting body to its parent pulling the object inward toward the parent. The gravitational force between the earth and moon provides centripetal force to hold the moon in its orbit around the earth. The gravitational force between particles on earth and moon is different from the centripetal force to keep the particles in a circular orbit. The difference between these two forces at different locations leads to some resultant forces which are small. If these forces have a significant horizontal component that is tangential to the earth’s surface, it produces tidal bulges on the earth, which are known as tide generating forces. These are inversely proportional to the cube of the distance between each point on earth and the center of the moon or sun. The sun also causes tidal bulges one oriented towards the sun and the other away. However these bulges are much smaller than the ones caused by the moon because the sun is 390 times farther from earth than the moon.

Thus the moon exerts over 2 times the gravitational pull of the sun as far as tides are concerned. The lunar bulges are two in number similar to the solar ones – one towards the moon and the other away from it. Due to the above, tides depend on the lunar day and hence high tides occur every 12 hour and 25 minutes. The time between two high tides is called the tidal period and these would be less high as we go north or south away from the equator. The vertical difference between high and low tides is called the tidal range. When the sun and moon are aligned either with the moon between earth and sun or with moon on the side opposite the sun, the tide generating forces from both sun and moon add up and leads to a high tidal range i.e. high high tide and low low tide called the spring tide. When the low tidal range occur, it is called neap tide.

Tides result in two phenomena

(a) Large volumes of rising and falling of water leading to varying heads

(b) The receding and rising water moves in and out with a certain speed lending to fairly high water velocities at certain locations.

Alternating or reversing currents occur when water moves in and out of restricted passages near the coast or in geological formation in the open sea like near islands.

Other than velocities caused due to tidal stream as discussed above, oceans also have currents which are location specific. Ocean currents are masses of ocean water that flow from one place to another. Ocean currents are either wind driven or density driven. Wind driven currents move surface water horizontally and are called surface currents. Density driven circulation moves water vertically down and the spreading of denser water leads to deep water currents. Many factors influence wind driven currents like the distribution of continents, gravity, friction and the Coriolis effect. The famous Ekman spiral describes the speed and direction of flow of water at various depths as a balance between frictional effects in the ocean and the Coriolis effect.These currents are caused as discussed above and also due to the water velocity as tides flow in and out in tidal streams.

3. Turbines in tidal energy conversion

In the previous section, we saw the causes of tides and how currents and tidal streams have water velocities. These speeds can be used to harness energy. The convertors depend on whether we use the head created due to large water volumes or velocities of tidal streams or ocean currents.

Tidal energy convertors are classified mainly as barrage based tidal energy convertors and tidal stream based tidal energy convertors.

Barrage based tidal energy convertors are similar to conventional hydroelectric power stations. They are more suitable for energy generation where tidal difference is high. A barrage is an artificial wall built across the tidal stream and connects to land mass on both ends. The tide creates head difference on the two sides of the wall. The barrage is tall enough to avoid spilling of water over its top. The barrage houses turbines and the water from high level side is guided over the turbine blades to the low level side through specially designed conduits. This water imparts momentum to the turbine blades thus driving the turbine wheel. The water flow through the turbines reverses with reversing of tide from low to high and from high to low. Accordingly, direction of rotation of the turbines also reverses. The shaft of this turbine wheel drives an electric generator through a gearbox. An electric generator produces electric power which then is supplied to the grid via power cables.

Tidal stream based energy convertors are turbines which harness kinetic energy in the tidal stream which is a fast-flowing body of seawater created by tides. These types can also be used in any oceanic location where naturally occurring ocean currents are large enough. These turbines, also termed as hydrokinetic turbines, operate in a similar way to wind energy turbines. These turbines are to be placed inside water either on seabed or to be suspended from a floater thus intercepting the water flow. These turbines drive electric generators coupled to them through a gearbox for generating electricity. This electricity is transmitted via under water power cables to the nearest shore where it can be consumed or can be transmitted further via power grid.

4. Potential to harness tidal energy in India

Barrage based tidal energy harnessing becomes feasible for tidal ranges greater than five metres (approximately 16 feet). In India the Gulf of Khambat and the Gulf of Kutch in Gujarat on the west coast have the maximum tidal range of 11 m and 8 m with average tidal range of 6.77 m and 5.23 m respectively. That apart, the locations mentioned above and a few locations in Andaman have high tidal currents where tidal stream based hydrokinetic turbines can harness tidal energy. Deeper regions in Gulf of Khambat and Gulf of Kutch are suitable for tidal stream based hydrokinetic turbines for harnessing tidal energy; whereas shallower regions of these gulfs are suitable for tidal barrage based energy conversion. Other potential regions along Indian coastline are delta regions of Hooghly River and Mahanadi River, and creeks nearby Mumbai coast offer potential for the development of small to medium capacity barrage based tidal power plants. According to a study, with tidal barrage technology, Gulf of Khambat and Gulf of Kutch has an estimated potential of 9000 MW and 2000 MW respectively. Also a few places in Sunderbans in West Bengal like DurgaDuani Creek have a potential of 100 MW. Total estimated tidal energy potential in India is 11.5 GW according to this study. [2] However a reassessment of tidal energy potential in India needs to be carried out and locations with high potential need to be identified in order to systematically realize tidal energy harvesting at large scale.

5. Components of Tidal Power plantThe components of a tidal power plant are Dam or dyke: The function is to form a barrier between the sea and the reservoir.Sluiceways are provided in the dam so that the water can enter into basin during high tides

Powerhouse: A powerhouse consists of turbines, electric generators and auxillary equipments. The water with high potential energy is made to run through turbines to run generators for power generations

6. Types of tidal turbines

Several types for turbines are considered suitable for harnessing tidal energy. Their main classification is based on the form of energy to be harnessed namely (a) potential energy by barrage based tidal energy convertor, and (b) kinetic energy by tidal stream based hydrokinetic turbines.

6.1   Turbines for barrage based tidal energy convertors

As explained earlier, the barrage based tidal energy convertors are similar to conventional hydroelectric power plants in construction. These convertors employ axial flow turbines named bulb turbines or tubular turbines to operate at low heads and high flow rates offered by tidal barrages. The flow over turbine reverses during tide cycle. The blades of these turbines are designed to suit this flow reversal. Rotor blades of the bulb turbines look very similar to a ship’s propeller. By virtue of the blade construction, these turbines spin in opposite direction as the flow reverses. The name bulb or tubular turbines is due to a large bulb at the center of the water tube that houses the turbine. The bulb holds the generator, wicket gate and runner. Wicket gate is essentially a set of guide vanes that control the flow of water over the turbine blades.

Generally these turbines are large in size and weight which is also an important factor as its cost further adds to the capital investment. One of the bulb turbine units in La Rance Tidal Energy Plant has a tip diameter of 5.35 metres and it weighs 470 tonnes. It has four blades and 24 guide vanes. Its operating head range is from 3 metre to 11 metre with peak output of 10 MW.

6.2  Tidal stream turbines

Basic classification of tidal stream turbines, which are also termed as hydrokinetic turbines, is done according to the flow direction with respect to the axis of rotation of the turbine blades. Thus two types emerge on this basis namely axial flow tidal stream turbines and cross flow tidal stream turbines.

6.2.1  Axial flow turbines

The axial flow turbines have their axis of rotation oriented along the incoming water flow. It may be noted here that the bulb turbines explained above is also an axial flow turbine. These turbines have two or more radial blades. Construction of axial flow tidal stream turbines is similar to modern wind turbines seen today.

Axial flow turbines have be oriented continuously in order to maximize energy capture by aligning their rotational axis with the direction of water flow. However the direction of rotation of turbines of this class changes as the tidal stream reverses the flow of water with change in tide. Also all turbines of this type experience huge drag which has to be countered appropriately by thrust bearings. The mast holding these turbines has to be designed to withstand the overturning drag moment.

6.2.2 Cross flow turbines

In cross flow turbines, the rotational axis of the rotor is perpendicular to the incoming water stream and the water ‘crosses’ the axis as it flows through the turbine. Blades of these turbines undergo a complete rotation around their own axis (tumble) as they rotate around the turbine axis. The relative motion between each of these blades and the incoming water generates a hydrodynamic force on the blade which is responsible for generating a moment around the turbine axis. Sum of moments exerted by water on all blades generates a torque at the turbine shaft which in turn is used to generate electricity by coupling the shaft to an electric generator through a gearbox. The torque transmitted to the shaft varies within a single rotation as the blades change position with rotation. This variation is also called as torque ripple. Designers have to ensure that this torque ripple is kept to a minimum for an optimum design with long life.

Blade cross section of cross flow turbines is generally of hydrodynamic shape and its design enables all turbines of this class to rotate in the same direction even when the water flows in the opposite direction. However these turbines are sensitive to the minor changes in tidal stream direction in the same tide phase as they are mounted on the seabed.

Cross flow tidal stream turbines can further be categorized in two types namely horizontal axis turbines and vertical axis turbines.

Rotational axis of a horizontal axis cross flow turbine rotor is oriented perpendicular to the incoming water stream. The number of blades in turbines of this class can be varied. Most commonly two and three bladed designs are considered. It is simple to construct and handle in marine conditions.

In a vertical axis cross flow turbine, the rotational axis of rotor is vertically oriented. It is perpendicular to the flow direction of the incoming water stream akin to its horizontal axis counterpart. Advantage of this orientation is that the turbine performance does not get affected by all possible changes in flow direction of incoming water stream. For all flow directions, the turbine blades rotate in the same direction and it can generate power even when it is not completely immersed.

Various configurations of cross flow turbines have been developed namely: (i) Darrieus turbines with straight blades parallel to axis, (ii) helical turbines (also termed as Gorlov turbines) with blade following a helical path around the axis, and (iii) Savonious rotor as shown in illustration.

In the cross flow turbines, the use of H-Darrieus and Squirrel Cage Darrieus is rather common. However the instances of classical type of Darrieus turbines with curved blades being used to produce hydropower are nearly non-existent. The Gorlov turbines, where the blades are of helical structure, generate lower torque ripple and have better starting characteristics as compared to all derivatives of Darrieus turbines. Savonious rotors are designed to operate on differential drag on blades on either side of the axis exerted by the water stream. Their blades may consist of straight or skewed blades or vanes. They suffer from very high drag and are not suitable for large scale power generation.

There are a few disadvantages associated with cross flow tidal stream turbines. These turbines generate low starting torque and they are affected by torque ripple – repeating changes in shaft torque. These turbines may not be self-starting for low current speeds and therefore some kind of external starting mechanisms need to be adopted. However for high current speeds these turbines can be designed to have good self-starting characteristics.

7. Potential of tidal range

The potential energy of the water layer of small height dh at a height of h from turbine inlet, entrapped water at high tides in a reservoir having surface area A, tidal range of R, dm- mass of water layer of height dh is given by

dE = dmgh

and E= ½ ρAgR2

8. Advantages, limitations in harvesting tidal energy

Today with the severe power deficit and with fossil fuels dwindling, it has become imperative to harness energy from all renewable sources. To this end, tidal energy shows great potential, whether it is due to the tidal range or the velocity in a tidal stream. These factors are sustainable and constant, hence it can prove to be a non exhaustibleform. However long term impacts are unknown today. Environmental activists have alerted regarding fish being caught in barrage based tidal turbines, since water keeps moving in and out carrying large volumes of biological life. However from constant power to grid, these types of tidal power plants have shown to be robust and sustainable. Tidal stream turbines especially in the open ocean have to be designed for waves, winds and other environmental conditions. Long term behaviour of such turbines is yet to be understood. However tidal stream turbine blades rotate slowly and are more ‘open’. This allows small to medium sized fish to easily pass through the rotating blades without any harm. The corrosion of tidal energy converter components due to content exposure to sea water is another issue that needs to be addressed by choosing appropriate corrosion resistant materials. The converters also need to be designed considering biofouling which may adversely affect the performance of the turbines. Also due care needs to be taken in order to avoid lubricating oil leakage from the turbines as it can be detrimental to the environment.

7. Summary

   Harnessing tidal energy is a challenging prospect. While tidal range is easier to address, velocities due to tidal streams are more complex from design perspective. Such energy is renewable and green and needs more focus from long term survival and design perspective. Today the capital costs are high but over a period of time more optimized components can lead to lower costs both capital and from operational and maintenance view point. Indian coasts have few tidal streams and low number of locations, where tidal range is high as discussed in an earlier section. Hence judicious choice of turbine types are required depending upon the location.

8. References

•  Trujillo AP. and Thurman H.V., “Essentials of Oceanography”, Pearson Pnentiu Hall, USA, 2011.
• Study on Tidal and Waves Energy in India: Survey on the potential and preposition of a roadmap, CRISIL-IIT Madras, December 2014.Vincent de Lateu. “La Rance Tidal Power Plant 40. Year operation feedback & lessons learnt”. BHA Annual Conf. Liverpool. 14&15 Oct 2009.
• Khan M.J., Iqbal M.T., Quaicoe J.E. “River current energy conversion systems:Progress,prospects and challenges”. Renewable and Sustainable Energy Reviews, 12. pp. 2177–2193. 2008.