Tidal Barrages


Tidal_Barrage_Diagram.jpg
Fig 1: Typical tidal barrage design (Benelghali, et al., 2007).

Figure 1 shows a typical design set up for a tidal barrage. They apply the same principles used by hydroelectric dams with the major difference being that the current flows in both directions as opposed to a river where the current is monodirectional (Benelghali, et al., 2007). Typically, a structure similar to a dam is constructed across the mouth of a bay with an appropriate tidal range, which is typically about 5m (O Rourke, et al., 2010). There are four types of tidal barrages: ebb generating systems, flood generating systems, two-way generation, and double-basin tidal barrages.

Ebb generation (O Rourke, et al., 2010)

In this method, the flood tide is allowed to fill the bay through a series of sluice gates. Once high tide is reached, the sluice gates are closed in order to trap the water within the estuary. As the tide outside the barrage flows out, the water within the bay is held in place until a sufficient head difference is created. At this point the sluice gates are opened, releasing the water through a series of low-head turbines and generating electricity. This generation continues until the pressure head has dropped to a level at which the turbines can no longer operate efficiently.

Flood generation (O Rourke, et al., 2010)

This method of generation is similar to ebb generation except that they harness the energy of the incoming flood tide. After the water has been allowed to flow out of the bay, the sluice gates are closed at ebb tide. As the tide begins to flood back in, the pressure head is built up on the seaward side of the tidal barrage. As with the ebb generation method, the sluice gates are then opened once a sufficient hydrostatic head has been achieved.

Two-way generation (O Rourke, et al., 2010)

Two-way generation harnesses the power of both the incoming flood tide, and outgoing ebb tide. The sluice gates are kept closed until towards the end of the flood cycle. Once a sufficient pressure head is achieved, the gates are opened, allowing water to flood into the bay through the turbines. Once high tide is reached within the bay, the sluice gates are again closed in order to create a hydrostatic head with the ebb tide. Again, once a sufficient head difference is achieved, the gates are opened, allowing the water to turn the turbines and generate electricity. Two-way generation has a distinct advantage over both ebb and flood generation in that the period of non-generation is greatly reduced. This method also reduces the cost of generators due to lower peak power.

Double-basin tidal generation (O Rourke, et al., 2010)

As the name implies, double-basin systems consist of two basins. The main basin of the system operates in a manner similar to that of an ebb generation system. The key difference with a double-basin system is that a portion of the electricity generated is used to power pumps which pump water into a secondary holding basin. This system effectively allows for storage in order to more effectively adjust the rate of electricity generation to better match demand. This is a major advantage of double-basin systems in that it allows the system to deliver more electricity during periods of increased demand. However, inefficiencies associated with their utilization of low-head turbines in addition to far greater construction costs compared to single-basin systems means that it is unlikely that this system will be widely implemented.