The Earth’s rotation and the alignment of the moon and the sun affect the waters of the ocean. The combined gravitational forces of the moon and the sun make the ocean levels on either side of the planet swell up or fall, causing the tides and tidal movements. As the Earth rotates, these tides seem to go ‘in’ or ‘out’, depending on the pull of the moon.
Tides flow in and out of rivers and estuaries in a periodic pattern. When tides swell up, the waters from the middle of the ocean are forced out and move towards the shoreline. When the tides recede, the waters fall back. This process generates massive amounts of energy in each cycle.
Since they are generated due to the Moon’s and the Sun’s cyclic influences, they form an inexhaustible source of energy. Tidal energy is also more predictable than wind or solar forms of energy.
Though tapping tidal power is sometimes considered expensive and limited, recent technical advances are brightening the prospects of making them more compatible and competitive. One of the most common methods to tap tidal energy is constructing a tidal barrage.
A tidal barrage looks pretty much like a dam, containing caissons, ship locks, sluices and turbines, built across river-mouths or bays to tap the energy from tides making inward and outward movements.
Unlike the conventional dam that has water on one of the two sides, tidal barrages allow water to flow in during high tide and then release it back out during low tides. This is done by controlling the sluice gates that open and close according to the prevailing tide. Turbines, placed near the sluice gates, capture the energy from the water. Revolutions of these turbines help to convert tidal energy into electricity.
There are basically two ways of generating electricity using tidal barrages.
The sluice gates are opened and ocean water is allowed to flow into the basin side of the barrage. At high tides, the sluice gates are closed. The turbines help to pump up the water to increase its level inside the basin. The water is, thus, held back. At low tides, the sluice gates are opened up, allowing the water to flow out into the ocean. When this happens, the turbine rotation converts the energy into electrical energy.
In this method, water allowed to flow in from the ocean is not held back. Hence, the turbines are used to produce electricity during both high and low tides. However, the energy produced this way would be less efficient, since the level of difference between the ocean side and the basin side will not be as much. It also reduces quickly as the water flows out.
Apart from these, the two-basin scheme and the tidal lagoon scheme are also possible implementations. In the two-basin scheme, two basins are constructed; one to be filled at high tide, and the other emptied at low tide. Turbines are placed in the middle of the basins. This method can produce electricity almost continuously. Tidal lagoon schemes are independent pool enclosures built on high-level estuaries that trap the water and release it later, to generate power.
As is the case with the construction of a dam, tidal barrages pose some threats to the flora and fauna prevailing in the area. Barrage construction can result in loss of habitat for many species of plants and animals.
The regular flow of ocean water in and out of tidal barrages affects its salinity and hydrology, which causes some disturbances within the delicate marine ecosystem.
High-speed turbine rotation could be fatal to fish that flow through them, resulting in the death of a number of marine creatures.
Tidal fences block the flow of water in a particular direction, which is detrimental to fishes that are accustomed to those routes during migration.
If, however, tidal barrages are constructed in an environment-friendly manner (and this is something that is becoming more possible every day as we better understand and mitigate the issues), tidal power can be a very good source of renewable energy for mankind.
We could, of course, all do our bit to reduce the amount of energy we need for our lifestyles and our homes by making them super efficient, installing things like radiator reflectors, insulation, draft excluders and double glazing which would mean that less electricity would need to be generated in the first place.