By, N Venkata Srinath, MS Power Systems. Tidal and Wave By, N Venkata Srinath, MS Power Systems.

Introduction Tide is a periodic rise and fall of the water level of sea which are carried by the action of the sun and moon on the water of the earth. Here, the large scale up and down movement of sea water represents an unlimited source of energy. The main feature of the tidal cycle is the difference in water surface elevation at the high tide and at the low tide. If this differential head could be utilized in operating a hydraulic turbine, the tidal energy could be converted into electrical energy by means of an attached generator.

Tide Tides are produced mainly by the gravitational attraction of the Moon and the Sun. Earth is able to hold onto everything except the water. Since the water is always moving, the Earth cannot hold onto it, and the moon is able to pull at it.  About 70% of the tide producing force is due to the moon and 30% to the sun. The moon is the major factor in the tide formation.

Components of Tidal Power Plants There are three main components of a tidal power plant i.e. The power house. The dam or barrage. Sluice-ways. The turbines, electric generators and other equipments are the main equipments of power house. The function of dam is to form a barrier between the sea and the basin or between one basin and the other in case of multiple basin. The sluice ways are used either to fill the basin during the high tide or empty the basin during the low tide. These are gate controlled devices.

Classification of Tidal power systems Single basin arrangement Single ebb cycle system Single tide cycle system Double cycle system Double basin arrangement

Single ebb cycle system When the flood tide comes in, the sluice gates are opened to permit sea-water to enter the basin or reservoir, while the turbine sets are shut. The reservoir thus starts filling while its level rises, till the maximum tide level is reached. At the beginning of the ebb tide the sluice gates are closed. Then the generation of power takes place when the sea is ebbing and the water from the basin flow over the turbine into lower level sea water.

Single tide cycle system In single tide cycle systems, the generation is affected when the sea is at flood tide. The water is admitted into the basin over the turbines. As the flood tide period is over and the sea level starts falling again, the generation is stopped. The basin is drained into the sea through the sluice ways.

Flood operation scheme need larger size plant, operating for shorter period and hence less efficient as compared to ebb tide operation. The ebb operation plant will be of smaller size, but will operate over a large period. The main disadvantage in both the ebb-cycle as well as the tide cycle systems is the intermittent nature in their operation. A system can be geared to generate power, both during the ebb and the flood tides with the help of single basin only. This system is known as the double cycle system.

Double cycle system The power generation is affected during the ebb as well as in flood tides. The direction of flow through the turbines during the ebb and flood tides alternates, but the machine acts as a turbine for either direction of flow. In this method, the generation of power is accomplished both during emptying and filling cycles. Both filling and emptying process take place during short periods of time, the filling when the ocean is at high tide while the water in the basin is at low tide level, the emptying when the ocean is at low tide and the basin at high-tide level. The flow of water in both directions is used to drive a number of reversible water turbines, each driving an electrical generator.

Double Basin Arrangement Though the double cycle system has only short duration interruptions in the turbine operation, yet a continuous generation of power is still not possible. These problems are solved to some extant in the two-basin scheme. Double Basin Arrangement It requires two separate but adjacent basins. In one basin called ‘upper basin’, the water level is maintained above that in the other, the ‘lower basin’. Because there is always a head between upper and lower basins, electricity can be generated continuously.

Power in single basin tidal system Pav (watts) = W/time = 0.5gρAR2/time . Where A = basin surface area R = Range(head/height available) Power in a double cycle system P(HP) = (ρQh/75)*η Q = average discharge h= head available.

Advantages The biggest advantage of the tidal power is inexhaustible. It is completely independent of the rain or continuous dry of any number of years. Tidal power generation is free from pollution, as it does not use any fuel and also does not produce any unhealthy waste like gases, ash, atomic residue. These power plants do not demand large area of valuable land because they are on the sea shore.

Limitations The variability in output caused by the variations in the tidal range. The tidal ranges is highly variable and thus the turbine have to work on a wide range of head variations. This affects the efficiency of the plant. Sea water is corrosive. Construction in sea or near too sea is found difficult. Cost is not favorable compared to the other sources of energy.

Indian Scenario India is surrounded by sea on three sides. The most attractive locations are the Gulf of Cambay and the Culf of Kachchh on the west coast where the maximum tidal range is 11 m and 8 m with average tidal range of 6.77 m and 5.23 m respectively.  The Ganges Delta in the Sunderbans in West Bengal also has good locations for small scale tidal power development. The maximum tidal range in Sunderbans is approximately 5 m with an average tidal range of 2.97 m. The identified economic tidal power potential in India is of the order of 8000-9000 MW with about 7000 MW in the Gulf of Cambay(10.8m) about 1200 MW in the Gulf of Kachchh (7m) and less than 100 MW in Sundarbans.(4m)

In, 1970, the CEA had identified this tidal project in the Gulf of Kachchh in Gujarat. The investigations were formally launched in 1982.  More than twelve specialized organizations of Govt. of India and Govt. of Gujarat were involved in the field of investigations. The techno-economic feasibility study has been completed in a very scientific and systematic manner and the feasibility report completed in 1988. The proposed tidal power scheme was an installation of 900 MW project biggest in the world, located in the Hansthal Creek, 25 Kms. from Kandla Port in Dist Kachchh of Gujarat State.

The Durgaduani Creek of Sundarbans site is selected for tidal energy project because there the difference between the high tide and the low tide is 4 meters and this is considered to be good for tidal energy projects. The 3.75 mw capacity Durgaduani Creek tidal energy project is a technology demonstration project. The cost of the project is Rs 48 crore, the project is to be completed in 36months, Work on the project started in September.

References Books Websites G.D.Rai ‘Non-Conventional energy sources’ Khanna publishers. John & Tony ‘Renewable energy resources’ Taylor & Francis group. Websites Project monitor <www.projectmonitor.com> Power professionals <www.powertoday.co.in> Gujarat energy development authority <www.geda.org.in>

WAVE Ocean and sea waves are caused indirectly by solar energy like the wind. Wave energy derives from wind energy, which drives in turn from solar energy. As stated earlier, the wind energy is caused by the uneven solar heating. Wave energy is much more concentrated than that solar energy. Devices that convert energy from wave can therefore produce much higher power densities that solar devices.

The power in the ocean waves have been part of the human experience for thousands of years. However, the history of attempts to exploit this power for human purpose has been extensive. It was the recent energy crisis that prompted serious attempts at harnessing the waves for the production of electricity. Power density P/A = 0.5g ρa2.f Where a= amplitude

Wave energy conversion Devices Wave energy conversion by floats High level reservoir wave machine

Wave energy conversion by floats Wave motion is primarily horizontal, but the motion of the water is primarily vertical. Mechanical power is obtained by floats making use of the motion of water, and then electrical power is generated. The very first Wave Power Plant was set up off of Portugal’s coast and the new power plant contains three individual giant rubber tubes to harness wave power. Named as Pelamis Wave Energy Converters (PWEC), the partially submerged power generators are 3.5 m in diameter and are 140 m long. They can each individually produce 750 KW of energy.(September 23, 2008)

High level reservoir wave machine The pressurized water is elevated to a reservoir above the wave generator. The water in the reservoir is made to flow through a turbine coupled to an electric generator, and then back to sea level.

Advantages The wave energy has the advantage over wind or solar that the energy has been naturally more concentrated. High power density. It is a free and renewable energy source. Wave power devices do not use large land masses. These devices are relatively pollution free, because they remove energy from the waves.

Limitations The major disadvantage of wave energy, as compared to wind, is that the energy is available on the ocean. The extraction equipment must operated in a marine environment will all that implies in terms of maintenance, life time, and reliability. The energy must be transported a greater distance to shore. Wave, energy converters must be capable of withstanding very severe peak stresses in storms. Capital investment, repair and replacement cost are high.

Indian scenario The Indian Wave Energy Program started in 1983 at the Indian Institute of Technology, Madras.  Oscillating Water Column (OWC) type of device was most suitable for Indian conditions and a 150 kW pilot plant was actually built and commissioned in October 1991 in the breakwater of the Vizinjham Fisheries Harbor near Trivandrum in Kerala. In 1993 the National Institute of Ocean Technology was established within the IIT-M campus and it took over the wave energy program. The Maharashtra Govt has set up a small wave power plant in 2 coastal villages in Ratnagiri district. A pilot of sorts it will produce between 15 to 20 kW of power

References Books Websites G.D.Rai ‘Non-Conventional energy sources’ Khanna publishers. John & Tony ‘renewable energy resources’ Taylor & Francis group. Websites Ecofriend <www.ecofriend.org> Youtube <www.youtube.com> The indic view (alternate energy and the indian infrastructure and energy sense) indicview.blogspot.com