Turning the tide
By Ashhad A. Khan
Saturday, 27 Feb, 2010
Due to the noncompressibility and high density of seawater (approximately 832 times that of air), tidal power holds a unique potential for generating renewable energy in comparison with other sources, especially wind.
Every passing day, millions of gallons of oil and petrol are used, millions of tonnes of coal burned, and vast amounts of carbon dioxide emitted into the atmosphere—followed by more mining and drilling which further eats up our fossil fuels. We know there is a limit to how much we can use these fossil fuels and that if we continue upon our current path of energy production and consumption, they will ultimately run out.
So how do we prolong their availability? After these fossil fuels have been consumed by the never-ending gluttony of energy on which we thrive, what will happen to modern civilisation? Will it continue to function as we know it—in a state entirely dependent on energy, or will there be drastic changes that will alter the fabric of the infrastructure on which we have come to depend?
Tidal energy could play a major role in the overall solution: an alternative form of power generation that harvests the energy of tides and waves and converts them into electrical energy or other useful forms of power. It is generated by the relative motion of water which interacts via gravitational forces due to the Earth-moon system. Because the tides are caused by the tidal forces that are generated due to the gravitational forces, and also partly due to the Earth’s rotation, tidal power is virtually limitless, which is why it is classified as a renewable energy resource. And due to the noncompressibility and high density of seawater (approximately 832 times that of air), it holds a unique potential for generating renewable energy in comparison with other sources, especially wind. Moreover, if the flow rates of the seawater at many coastal locations are factored in, the potential for power generation is further amplified.
This technique has been used for power generation earlier as well, although with mixed results, relating to cost versus revenue—through the building of tidal barrage power generation plants. The barrage is similar to a large dam that is built across a river, bay or estuary. Turbines installed in the walls of the barrage are used to generate electricity as the water flows out of the basin. The feasibility of their construction is however questionable due to the extremely high civil infrastructure cost associated with their construction, and the fact that power is only generated when the tide is ebbing. Barrages also create serious environmental concerns due to their blocking channels and estuarine mouths which disrupts fish migration and causes the build up of sediments within basins. The dredging of which is another costly procedure and these concerns are another reason for their unfeasibility.
However, new engineering research and technological advances in the field have brought about the creation of new kinds of devices to harness power from the tides and generate renewable energy. Vertical-axis and horizontal-axis turbines, very similar in design to windmills, have rotor blades that move due to tidal streams, and in turn rotate the shaft which turns a gearbox linked to a turbine. Some newer models eliminate the need for a gearbox and the rotor blades are linked directly to an electrical generator.
These turbines seem to be the future of tidal power generation. The vertical-axis design has already been successfully tested in Canada, while the horizontal axis is now being tested in the UK and Norway. These turbines have a huge advantage in that there is not a very large capital infrastructure cost required in their construction, and environmentally speaking, they are relatively benign. They are also more energy efficient when compared to barrages, because unlike them, these turbines can generate power both during the surging and ebbing of a tide.
Vertical-axis turbines can also be stacked and joined together in series to create a tidal fence that is capable of producing energy comparable to the largest existing fossil fuel-based, hydroelectric and nuclear energy generation facilities. A road can also be constructed on top of the tidal fence so as to effectively create a bridge over an estuary which would provide two infrastructure facilities within one.
Tidal current energy is very predictable; tidal activity can be predicted many years in advance with exceptional accuracy, which makes tidal power a much more reliable generator of energy than wind or wave and would also allow for the easy integration with electrical grids, as well as the introduction of backup sources well in advance of requirements. It requires no fuel to operate, and produces no greenhouse gases or pollutants. Since they will be entirely submerged, the visual impact argument that is thrown against windmills would not apply here either. The construction of these turbines would also result in larger areas of sheltered water and their structure—especially that of the concrete base that is used to anchor some types of these turbines to the seabed—is also an ideal artificial coral reef which would stimulate the growth of fish populations in the area.
Tidal energy construction so far has not happened at the rate at which it possibly could, and thus the amount of energy being generated currently—approximately 260MW—is a fraction of the potential. Despite the advantages, though, one fact is clear to tidal energy researchers. Tidal energy alone would not be able to supply all our energy needs. There will need to be an amalgamation of multiple sources of renewable energy that will have to be combined together to provide for our future energy needs and solve the global energy crisis due to the impending end of fossil fuels. But tidal energy, no doubt, will be a crucial component of that energy solution.
Turning the tide
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