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As the world's energy crisis progresses, renewable energy sources like tidal power are being explored. Tides, the rise and fall of sea levels, are created by changes in the gravitational forces of the sun and moon as well as the earth's rotation. Tidal Energy is generated by converting gravitational potential energy of high tides into hydroelectricity. Most ocean environments experience two high tides and two low tides  everyday. There are three main ways to harness tidal energy, each with their own pros and cons. The first method of tidal energy generation is the tidal stream generator, the second is the tidal barrage and lastly there is dynamic tidal power. Tidal energy is not yet widely used but many people believe that it has great potential for future energy generation  . This paper will examine the environmental effects of the different methods.
The concept behind the tidal stream generator method is very similar to that of an underwater windmill; as the tide flows in and out of a bay it spins an underwater turbine which generates power. Tides move slowly, but because water is much denser than air, it can move much slower while providing the same power  .
The environmental benefits of this technology are numerous. Tides are always either flowing in or out allowing tidal stream generators to supply constant, consistent and emission free power. This is a benefit over other the other methods of tidal generation that can only generate power when the tide moves in a specific direction. Also, tidal stream generators are much, much cheaper than other methods of tidal generation allowing them to be easily implemented and tested. Although very few studies have been done, it would seem fish avoid the turbines used in tidal stream generators resulting in almost no aquatic fatalities  . This means tidal stream generators have no known impacts on the surrounding ecosystem.
There are not many environmental disadvantages to tidal stream generators. The turbines may cause some minor disruption to aquatic animals, but this is yet to be proven. The only environmental damaging aspect of this technology would be the work involved with maintaining and installing the underwater turbines. However, as a relatively unknown source of power, there is much of time for improvements.
The tidal barrage is a large dam-like structure built at the mouth of a bay, stream, or estuary which experiences a large tidal range  . It allows water to flow into the basin through the sluice gates  during high-tide and during low tide releases the water through turbines. Turbines trap the energy from the movement of water in and out of the basin.
This method of collecting energy has many environmental benefits. Firstly, because tides come twice a day, a tidal barrage will supply predictable, consistent and emission free power. This method is able to collect more energy than tidal stream generators because it essentially converts the potential gravitational energy of the tidal range in an entire region into electricity. Although barrages are expensive to build, they can be conveniently built under existing bridges. This reduces cost and some impact on the natural surroundings.
There are, however, many environmental concerns with this kind of operation. Barrages have a great effect on the surrounding ecosystem. One of the main problems with the construction of a tidal barrage is it can halt the flow of saltwater into and out of estuaries  . This can negatively affect the resident marine life  . Also, migrating fish are only able to pass through the sluice gates as the basin fills with water. When the gates close, the fish will swim through the turbines. Currently even the safest designed turbines kill fifteen percent of the fish that pass through. This can have a signification effect on the fish population. Finally, tidal barrages block sediment movement  . This can causes an accumulation of sediment around and in the barrage which could impair its functionality. Also, by blocking sediment movement it decreases the turbidity  in the basin allowing for an increase in phytoplankton which effects the food chain of the ecosystem.
Lastly there is dynamic tidal power, a promising new idea with great potential. A large barrier, perpendicular to the coast, is built out from the shore and an adjoining barrier is built along the far end of the first barrier making a ââ‚¬Å“Tââ‚¬Â shape. This barrier blocks the acceleration of the tides parallel to the coast  creating a difference in the water levels on each side of the barrier. A series of turbines installed in the dam can convert the potential energy of the higher water into electricity. The main difference between this source of tidal generation and the others is that to be economically viable it must be over 30 km in length. This is because the power output is proportional to the square of the dam length. The idea was proposed by Dutch coastal engineers Kees Hulsbergen and Rob Steijn. It has only been tested in computer simulations.
Dynamic tidal power like the other types of tidal generation is emission free and resource independent. The environmental benefits of this method over the others boil down to the huge amount of energy that can potentially be generated in coastal regions. Even though it must be built on such a large scale, simulations project that it could potentially generate and 8GW of installed capacity which is thirty percent more power than the Bruce Nuclear Generating Station  . If two are built around 200km apart an almost constant stream of power can be generated. Also beneficial is that the dams donââ‚¬â„¢t require a naturally high tidal range like both tidal barrages and stream generators. So countries like China, Korea, and the United Kingdom could potentially generate huge amounts of power. It is estimated China alone could generate over 100 GW of power.
The environmental risks of dynamic tidal power is mostly in the construction of such a large structure. The building of something so massive wastes a lot of fossil fuels. Also, a barrier of that size could disrupt migration patterns of aquatic animals. Although as the barrier does not seal anything off sea creatures can always swim around.
Each method has its own benefits and detriments. Tidal Stream power although not harming to the environment returns the least power, tidal barrages have the capability for large scale power generation they can hurt the ecosystem they are uses in, and dynamic tidal power which can be used to generate large amounts of power in many coastal locations is very very costly to implement and has never been tested. One method does not replace the other as they all are operate in different conditions. Development of this new technology is important and must be continued.