Tidal Stream Generator
Tidal Stream Generators (also called Tidal Energy Converters) are simple machines that extract energy from the movement of water with the tides. The name derives from the fact that the machines are placed in path of the tide, called the tidal stream. The diagram below illustrates one type of tidal stream generator.
Like all electricity generating schemes, tidal stream generation relies on turbines. The turbines can come in many shapes, but some look just like the windmills used on land to capture energy from the wind. In fact, tidal stream generation is more similar to wind energy than it is to hydroelectric power generation. If the turbines can run in both forward and reverse directions, then the system will produce energy as the tide comes in and as it goes out. As it stands, a water flow rate of 2-3 meters per second (which is about average in most places except at neap tide) would translate into four times more energy for every turn of the turbine as compared to wind. This happens because water is 800 times denser than air and thus provides more contact with the blades of the rotor, which translates into more energy being transferred to the turbine.
Turbine design also affects the amount of energy that can be derived from a tidal stream. Different turbines are depicted in the following diagram.
Horizontal axis turbines are what most people are familiar with. These turbines are favored for their rapid spinning and low torques on the drive train. Vertical axis turbines are beneficial from the standpoint that they need not be directed perfectly into the stream of the tide and thus can generate electricity in settings where the direction of the tidal stream varies. As variation is relatively rare underwater (unlike wind direction variation), HAWT-type turbines are most prevalent. Because the blades of HAWT turbines spin slower underwater than they do in the air, they are less of a threat to wildlife. Nevertheless, VAWT turbines are the least likely to cause damage to passing organisms.
Advantages of Tidal Stream Generators
Tidal stream generation is very cheap to implement. This inexpensive implementation occurs in part because each turbine stands on its own without the need for a massive dam as in the tidal barrage system. Such modularity also allows for small applications to be expanded upon once the economic feasibility is settled in a small test project and once environmental impacts are better understood.
One of the key advantages of tidal stream generators is the ability to incorporate them into existing structures like bridges and docks. This not only further reduces the cost, but also reduces the environmental impact.
Finally, the acoustic properties of the blades in water seem to prevent wildlife from straying into their path. This is very different from how blades operate on land where birds, bats, and owls are often killed by them. It seems that the impact of underwater turbines may be much less than that of surface turbines.
Disadvantages of Tidal Stream Generators
Tidal stream generators generally cannot produce as much power as barrage systems. They are, like barrage systems, prone to corrosion from saltwater. Materials science has advanced enough, however, to make corrosion only a minor problem.
Tests on the full impact of tidal stream generators on the environment are currently being tested at the Northwest National Marine Renewable Energy Center in the United States. Most predictions are that tidal stream generators will become an important, though small, part of the power supply in coastal nations.