Harnessing tidal power | The Daily Star
12:00 AM, July 13, 2010 / LAST MODIFIED: 12:00 AM, July 13, 2010

Harnessing tidal power

High tide (left) and Low tide

BANGLADESH faces acute power shortages, as everyone can see the hourly switching-off of the lights and fans in the simmering summer. What is the way out ? One important way is to look for green and renewable sources the solar energy, hydroelectricity, geothermal energies and the tidal energy.
Tidal energy is an energy that could be obtained from the changing sea levels. It is a direct result of tide shifting from low to high. It is a form of green energy and is also considered to be inexhaustible because tides always rise and fall due to the gravitational tug-of-war between the Earth, the Moon and the Sun. Tidal power does however have great potential for future power and electricity generation because of the massive size of the oceans and the initiatives for cleaner energy sources. Tidal power can be classified into three types depending how one extracts the energy: Tidal Barrage, Tidal Fence, Tidal Turbine. Tidal energy technology is in the practice 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 and Lumkara Plant (300 MW) in Russia, Bay of Fundy in Canada, Jianagxia in China. The Severn Estuary UK plant is the biggest ever project being planned (for a 8640 MW capacity).
Except for the fact that it involves massive capital outlays, tidal power can result in extremely low costs per kilowatt-hour once it is built. The technology is very simple and has a potential life of more than 40 years. It is a highly efficient (coal/oil efficiency = 30%, tidal power efficiency = 80%) source of power. This potential of the tidal power has not been realized yet, due to two major problems-high capital costs and environmental concerns. Bangladesh has a long coastal area with 2~5 meter tidal head/height rise and fall. Some of the coastal area is protected against flooding by embankment and sluice gates. These barrages, necessary for controlled flow through turbines (to tap tidal power), are also needed for flood control. This avoids the problem of high capital cost as the engineering is either already there or is needed for cyclone and tidal surge protection. Bangladesh has some large tidal sites and many channels of low tidal range in a large number of deltaic islands, where barrages and sluice gates already exist. Analysis of the given sites indicates that Sandwip has a very good prospect for tidal energy. The 5 meter tides experienced at Sandwip results in poor accessibility, with the island constantly surrounded by mud flats, except during high tides. Bangladesh may harness energy from coastal tidal resources by applying two technologies: Low head tidal movements (2~5 m head) & Medium head tidal movements (> 5 m head). Low head tidal movements are Coastal Bangladesh, particularly Khulna, Barisal, Bagerhat, Satkhira and Cox's Bazar regions are, geographically extensively deltaic with levees and sluice gates. These areas are protected by embankments, which had been constructed during 1960s for protection from natural disasters like flooding and tidal surges. Therefore, the infrastructure needed for barrages and sluice gates is already present in these regions. These barrages and sluice gates may be used for electricity generation by applying simple technology. The most favorable locations for tidal power application of this type are on the eastern side of the delta region (e.g. Sandwip). The height tidal ranges occur at the following tidal measurement stations: BIWTA Gauge No 32 : Satalkhal - Sandwip Mean Spring Range = 5.73 meters (as of February 2010). It is hoped that the project will provide an integrated approach to island development where the tidal power outcome is part of a bigger concept involving aquaculture and water management.
This is a powerful motivation for developing tidal power as it promises sustainable energy for Bangladesh. The tides in Sandwip demonstrate roughly a 5 hour 'in' and 7 hour 'out' cycle. Traditional tidal technology would generate large quantities of energy during approximately 6 hours of this cycle. Generation occurs when the water is flowing both in and out, with the change in rotation occurring during a flowing tide. The inconvenience of the loss of electricity experienced when the turbines are in a stalled state will be diminished through the grid connection of a series of power generators around the island. During the night the excess power produced can be directed into battery charging. These batteries can be provided for households living off the grid. The installation of 75 KW turbines, generating 80% of full capacity for 23 hours per day, equates to the production of approximately 1,380 KW-hr per day. It is proposed that the manufacture and installation costs for one site will be in the order of 35 lakh Taka.
In a society with increasing energy demand and decreasing supplies, we must look to the future and develop our best potential renewable and green resources. Tidal power fits the bill as a natural source of energy with many long-term socio-economic benefits.

The authors are graduating students of International Islamic University Chittagong and are researching on tidal energies.

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