INTRODUCTION

Like most other renewable energy areas, tidal energy does not have anything new as far as the idea of producing power from the oceans is concerned. Every one knows that the tides are generated by the rotation of the earth within the gravitational fields of the moon and the sun. It is the relative motions of these two bodies that cause the surface of the oceans to rise and than fall in a periodic manner. It is the predictability of these rises and falls that is important to work out and plan the energy output from tidal power systems. Now that the world is going back to nature as far as the energy needs are concerned, it is pertinent to mention here that if less than 0.1%of the renewable energy available within the oceans could be converted into electricity, it would satisfy the existing global demand several times over. The vastness of this resource, if realized, can bring benign energy systems into a less polluted human development effort. Innovations and improvements in technology have been and will continue to expand this process in the next few decades.

BASIC PRINCIPLES

Tides are generated by gravitational forces of the sun and the moon on the earth's waters. The moon roughly exerts twice the tide raising force of the sun due to the proximity of the moon. Very simply put, the gravitational forces of the moon and the sun create "bulges" in the earth's oceans (one closest to the moon and the other farthest). These "bulges" result in the two tides (high water to low water sequences) a day. This is the dominant tidal pattern in most of the oceans of the world. The actual patterns of tides are however fairly complex since other parameters in system also exert forces though of a comparatively minor consequence. Again, there are differences in tide heights between the two daily tides. This pattern repeats on a 14 day cycle as the moon rotates around the sun. One cycle of high water to low water takes an average of 12.4 hours. Following are the interacting cycles of tides.

1. A half day cycle.
2. A 14 days cycle of Spring (maximum height) tide and Neap (minimum height) tide respectively.
3. A half year cycle in March and September on account of the inclination of the moon's orbit to that of the earth and
4. 18.5 year and approximately 1600 year cycles covering other complex gravitational interacting forces.

There are more than 100 harmonic constituents ( cyclic components ) of the tidal movements with small magnitude each with a different cycle time and all these constituents combine so that tides only completely repeat themselves every 18.6 years. The range of spring tide is commonly twice that of a neap tide. The amplitudes of tides are increased substantially towards the coast, particularly in estuaries. At times the tidal range can be further increased by reflection of the tidal wave by the coastline geography or resonance. The amount of energy obtainable from a tidal energy scheme therefore varies with the actual location and time. While in open ocean, the maximum amplitude of the tides is about one meter and output changes as the tide ebbs and floods each day. This change can vary by a factor of about four over a spring-neap cycle. But all the same, it is important to note that tidal energy is, highly predictable in both amount and time and planners can therefore depend on the same.

THE TECHNOLOGY AND PROJECTS

The technology for the exploitation of tidal powers requires conversion of tidal energy into electrical energy and is very similar to the technology used in traditional hydroelectric power generation systems. Just as you need a large dam for a big hydroelectric plant, a barrage is required to be built to have a large reservoir where the water from tides can be collected. A short and shallow dam like structure will be needed so that water in large basin area can be collected in this reservoir. The barrage constructed across an estuary is equipped with a number of sluice valves and gates. Banks of low head axial turbines are installed under the bottom of the dam area from where the water will flow and generate electricity through generators coupled to the turbines. Though, the generation of electricity from tides is very similar to hydropower generation process, the generator is so designed that water can flow in both the directions for generating power - while coming in and going out. Tidal energy technology is, in fact, already a matured technology with a 240 MW commercial unit operating successfully since 1966 at La Rance in France. A few other countries have also installed or are installing tidal power units with different capacities. Notably amongst them are:

• Kislo Guskaya Tidal Power Station (TPP) in Russia
• Lumkara Plant (300 MW) in Russia
• Bay of Fundy TPP in Canada
• Jianagxia TPS in China

Several projects coming up elsewhere include:

• Sihwa TPP in South Korea
• New York's East River TPP
• Sunderban TPP in India
• Doctors' Creek in Derby, Western Australia ( 48 MW)
• Severn Estuary TPP in UK

The Severn Estuary UK plant is the biggest ever project being planned (for a 8640 MW capacity). A list of prospective sites for tidal energy projects is given in APPENDIX - A to this paper.

The primary advantages of the modern tidal power system are:

1. High energy density of ocean currents
2. Allowing the turbine to optimally capture the flowing water ( on both the sides of water movements)
3. Regularity of predicted power production from year to year
4. Permits the simultaneous use of the dam for a road and/or a railway line.
5. Low term operation lifetime of plant
6. Protects vulnerable coastline from strong waves and floods
7. Provides a non-polluting and inexhaustible supply of energy
8. The technology does not rely on fuel to produce electricity
9. Does not emit greenhouse gases
10. The system is easy to operate & maintain
11. It is non-polluting and almost silent when running
12. Providing employment to a large number of people

Some of the main drawbacks are:

1. High capital cost
2. Construction time is several years for large projects
3. Limited number of potential sites ( i.e. site-specific)
4. Power generation is intermittent

It may be mentioned here that sustained increases in fossil fuel cost (and may be non availability at times due to political reasons) will ultimately make the tidal power a cheaper proposition in times to come. This is already evident from the campaign toward the establishments of renewable energy projects under operation and being planned all over the world. In just three days time, for example, I have seen not less than ten newspaper reports from different parts of the word urging the government / society to take up projects for tidal generation in their own localities. The rate at which the initiative is being taken, I am convinced that there is indeed a very bright future in tidal energy development on a global scale.

Apart from the tidal barrage technology, there are a few other allied marine power generation projects now being considered for the commercial exploitation in various locations. Some of these are:

1. Tidal currents technology
2. Wave energy technology (sea based)
3. Oscillating water column generation technology
4. Ocean currents technology
5. Tidal fence technology
6. Low-head river power technology

It is true that for sorting out technical and commercial difficulties in all these new technologies, there are quite a few difficulties right now. In the next few years, however, countries will begin to see wave power connected to national supplies. The market is very big from now on to take up these technologies so that ultimately a cleaner, quieter and economically viable power generation system gets developed on a global scale.

OPERATIONAL ASPECTS

The basin is filled during the tide through the sluice and freewheeling turbines in case of the older, traditional designs until high tides. The sluice & turbine gates are then closed. They are kept closed till the sea level falls to create sufficient head to generate turbines. This will continue till the head is again low. The sluices are now opened, turbines disconnected and the basin filled again. The cycle thus repeats itself as a system. The latest design in generating, however, has now two way generation. Generation occurs both as the tide ebbs and floods. This mode is only comparable to ebb generation at spring tides and in general is not very efficient.

The turbines designed to operate in both directions are less efficient but the very fact that the generation takes place during both the directions is quite important. Turbines can be powered in reverse by excess energy in the grid to increase the water level in the basin (The pumping process). This energy is returned during the generation process. Tidal power schemes do not produce energy all the 24 hours a day. A conventional design would produce 6 to 12 hours in a day. Since the tidal cycle is based on the period of revolution of the Moon, while the demand for electricity is based on period of revolution of the Sun, the energy production cycle will not always be in phase with the demand cycle. This problem is equally relevant in solar and wind energy systems because there again the generation in intermittent. The idea of integrating all types of renewable energy systems in the form of a global grid, can greatly improve the average power availability from such a 'Renewable Energy Grid'.

ENVIRONMENTAL ASPECTS

Since a tidal station is operating in France since 1966, it may be easy to have some information about the environmental impact and the ecosystem from this station. The research carried out in 1995 ( by the National Museum of Natural History in France) showed that the Rance estuary has a rich and varied aquatic life. It says that although the construction of the dam modified the currents in the estuary, and consequently, the geographical distribution of the sediments, the studies seem to point to a natural evolution. It thus appears that seemingly there is hardly any adverse impact on the environment due to the setting up of a tidal station. As far as the migration and damage of fishes are concerned, it seems there is no clear indication from studies on existing hydroelectric stations of the number of fish which might be affected. The changes to fish population are also uncertain. Generic R&D has focused on the suitability of acoustic deterrence method which may not damage the fish from entering the turbines thereby eliminating any damage to it.

Many other factors concerning ecosystems are specific-to-type and may have to be studied once the tidal power station starts operating.

ECONOMIES OF TIDAL POWER

The major factors in determining the cost effectiveness of a tidal power site are the size (length and height of the barrage required), and the difference in height between high & low tides. These factors can be expressed in a site's "Gibrat" ratio. This ration is the ratio of length of barrage (in meters) to the annual energy production (in KWh). The smaller the Gibrat site ratio, the more desirable the site. Examples of Gibrat Ratio are:

1. La Rance at 0.36
2. Severn at 0.87 and
3. Pasamaquoddy in the Bay of Fundy at 0.92.

While it is true that the fossil fuel costs are going up, the existing economics do not make the tidal plant an economically viable proposition. It is however important to note here that with oil prices going up further and political issues in the Arab world creating problems with the West, the present conditions may make it imperative to make use of the local energy potential to be used to its maximum capacity. This scenario therefore, brings out more of latent renewable energy technologies to come out for fuller exploitation. A number of wind farms coming up on a large scale in the world and a much greater business coming up in solar energy systems (and fuel cells development) on a global scale is now moving the energy business to yet another renewable source - the tidal power. Increasing pollution due to fossil fuel based power plants is also pressurizing the public to take to clean air thro' these developments in renewable power systems. The concept of "avoided costs" is also looming large in the public mind. The fact that making use of benign energy systems like wind, solar and tidal can avoid the fossil fuel generation (and costs) thus reducing the global pollution is picking up at a much faster rate and all these above mentioned developments do have the economic parameters in mind in the ultimate analysis. The large scale initiation of tidal power developments have to be taken in this context for all practical purposes.

CONCLUSION

With increasing energy demands all over the world, we have to look for a future that can supply energy in abundance without any pollution. As part of an initiative, the global task should have two pronged approach (1) to mitigate environmental pollution by gradually reducing fossil fuel based power stations and (2) by initiating clean air power generation systems. Tidal power fits into the system as a natural resource of energy and provides benign power taking into consideration the availability of local resources and thus reducing (if not eliminating) the dependence on monopolies of oil suppliers. Now that wind and solar energy generation systems are coming up on a large scale, this natural resource of tidal power is also getting ready for fuller exploitation.

With policies both in US and Europe planning for a systematic exploitation of renewable energy on a strategic scale, the stage is all set now for the fuller development of tidal power on a world wide scale. In view of the fact that all these renewable energies have long term financial implications and that the fuel itself has no cost of its own, strategically planned funding systems once developed will accelerate the growth of economy ensuring a better life-style and a cleaner environment.

The earlier such a development process gets going globally, the better it is.

REFERENCES

1         World Energy Council

2         Australian Institute of Energy

3         www.blueenergy.com/tidal.htm

4         www.deccanherald.com/deccanherald/jul11.2005/sut194362005710.asp

5         www.marketresearch.com/map/prod/924854.htm/

6         Researchassistance.com

7         www.alternative-energy-news.info/tidal-power/

8         www.planetark.org/dailynewsstory.cfm/newsid/19416/story.htm

9         www.verdantpower.com

10     British Council. Eurobserver.com (Dated 11-3-06)

11     CSICOP on line

12     Dept of Trade and Energy. Tidal Power  www.dti.gov.uk/renewables/rener_1.5.2.htm

13     www.cc.utah.edu/~ptt25660/tides.htm

14     www.thenorthernlights.org

15     Connaught  Telegraph  on-line ( Ireland ) Mar 9,2006

16     Tidal Power  :  By Baker A C  :  ISBN 086241 189 4  ( 1990 )

17     UN Commission on sustainable Development 2006 Conference: Sustainable Energy : Tidal Energy and Low-head River Power.

18     www.electricite-de-france.com/htm/en/decouvertes/voyage/usine/usine.htm/

19     www.darvill.clara.net/altenerg/tidal.htm

20     BWEA  Marine Resource. Tidal Power Potential

21     www.poemsinc.org/FAQ tidal.htm/

22     Energy Fact Sheet. Tidal Enerfy ( Energy Education of Ontario )

23     www.iere.org/documents/tidal.pdf

24     www.aedsb.org/JBS-June 2000-Salequzzaman.doc

25     www.iclei.org/E FACTS/TIDALEN2.GIF

26     Fujita Research Report. Wave and Tidal Power

27     Helifexline.com , Nova Skutia, Darell. Dexter Report Mar 06

28     Cape Gazette , Delaware, USA  03 Mar 06

29     www.vicnews.com/portal-code/list-cgi%paper.~

30     www.marineturbines.com/technical.htm

31     "Prospects of Electricity from Tidal Power in Coastal Regions of Bangladesh" Published from Murdock University , Australia

APPENDIX - A

Prospective Sites for Tidal Energy Projects

• 7 000 MW variant also studied  

Source:  World Energy Council