A21 Tidal power plants

Tidal energy, i.e. the tidal range, can be exploited according to different methodological approaches.

The easiest method consists of a one-basin-system that is only used in one direction (Fig. A.6, top). A bay is separated from the open sea by means of a dam, but remains connected to the sea by a floodgate and a turbine. The control system of the turbine and the floodgate ensures that water can only enter the bay via the floodgate and can only exit the bay through the turbine. This turbine is connected with an electrical generator providing the electrical energy. A fundamental disadvantage of such types of tidal power plants is that energy can only be generated during comparatively short time periods (Fig. A.6, top). Its advantage is the simple turbine design.

To avoid this disadvantage a cross-flow turbine can be applied for power generation (Fig. A.6, centre). The floodgate, which is also needed for this system design, accelerates the inflow and outflow of water during periods when there is almost no water level difference between the basin and the sea (i.e. high tide and low tide). This design ensures power generation over much longer time periods (Fig. A.6, centre). This is only possible with a higher technical expenditure. But in general the higher possible energy provision overcompensates the higher costs.

Tidal power stations can also be designed as a two-basin-system (Fig. A.6, bottom). Between the two basins another turbine is integrated. This turbine is either built into a dam or into a connecting canal located between the two basins connected to the sea also via turbines. Water inflow and outflow is controlled as water enters one basin at high tide and exits through the other basin at low tide, after having passed through the corresponding turbines. Energy production is thus even more regular and can be performed without any interruptions. However, the power plant design is more demanding and the two basins require more space.

In most cases projects for tidal power plants equipped with such storage reservoirs that hold back the water until the water level outside of the reservoir has reached a lower level are often not put into practice due to high costs and significant environmental effects of this space-consuming design. Currently, there are only very few tidal power plants in operation worldwide. The power station located at the estuary of the river Rance, near St. Malo in France has been operated since 1966. The Canadian 20 MW prototype plant, located at Fundy Bay, is in successful operation since late 1984. Moreover, there is one plant in Koslogubsk in Russia and two plants in China. However, only the first one can be regarded as prototype of large-scale tidal power plant, as for an average tidal range of approximately 8.5 m, 240 MW have been installed.

/Turbine

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Floodgate

Turbine

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Energy provision Sea level /Basin water level

Energy provision Sea level

Basin 1 Basin 2

Turbine

Turbine

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Energy provision Sea level

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Energy provision *Sea level

Energy provision *Sea level

Fig. A.6 Operating principles of tidal power stations (top: one-basin tidal power station using one flow direction; centre: one-basin tidal power station using opposite flow directions; bottom: two-basin tidal power station; according to /A-2/)

On the whole, the technical potential of tidal power stations worldwide is too low to largely contribute to global energy provision. However, local conditions may be favourable.

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