Turbines

The turbines in a tidal power station must operate under a variable, low head of water. The highest global tidal reach, in the Bay of Fundy in Canada, is 15.8 m; most plants would have to operate with much lower heads than this.

Such low heads necessitate the use of a propeller turbine, the turbine type best suited for low-head operation. The fact that the head varies appreciably during the tidal cycle means that a fixed-blade turbine will not be operating under its most efficient conditions during the majority of the tidal flow; consequently a variable-blade Kaplan turbine is usually employed.

The most compact design of propeller turbine for low-head applications is the bulb turbine in which the generator attached to the turbine shaft is housed in a watertight pod, or nacelle, directly behind the turbine runner. The La Rance tidal plant employs 24 bulb turbines, each fitted with a Kaplan runner and a 10 MW generator.

Bulb turbines were new when La Rance was built and construction of the plant involved some experimental work; of the 24 turbines, 12 had steel runners and 12 had aluminium bronze runners. Experience has led the operators to prefer the steel variety.

The turbines at La Rance were designed to pump water from the sea into the reservoir behind the barrage at high tide to increase efficiency. This

Turbine generator support and power take-off

Turbine generator support and power take-off

Propeller turbine

Water flows -in

Straflo Turbine

Water > flows out

Nacelle-containing generator

Figure 9.2 Bulb turbine

Propeller turbine

Water flows -in

Water > flows out

Nacelle-containing generator

Figure 9.2 Bulb turbine was found to cause severe strain on parts of the generator and the design had to be modified. Work was carried out between 1975 and 1982. Since then the plant has operated smoothly and with high availability.

An alternative to the bulb turbine is a design called the Straflo turbine. This is unique in that the generator is built into the rim of the turbine runner, allowing the unit it operate in low-head conditions while keeping most of the generator components out of the water. A single large Straflo turbine generator was installed at the Annapolis tidal power plant at Annapolis Royal in the Bay of Fundy, Canada. This 18 MW unit is the only one of similar size that has been built so experience with the design in limited.

Speed regulation

The speed of a conventional turbine generator has to be closely regulated so that it is synchronised with the electrical transmission system to which it is attached. In order to aid frequency regulation under the variable conditions of a tidal power plant, a set of fixed blades called a regulator are often placed in front of the turbine blades to impart a rotary motion to the water. The use of these blades in conjunction with a variable-blade Kaplan turbine provides a considerable measure of control over the runner speed.

In small applications where such tight speed control may not be essential and where costs are critical it may be possible to use one method of control - either a variable-blade turbine or a regulator - rather than both. An isolated unit could operate without regulation.

An alternative option is to use a variable-speed generator. This electronic solution will permit the turbine to run at its optimum speed under all conditions while delivering power at the correct frequency to the grid. This allows some efficiency gains. However the solution tends to be more costly than a conventional generator with mechanical speed control of the turbine.

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