TURBINE SPEED REGULATION
The speed of a conventional turbine generator has to be closely regulated so that it is synchronized with the electrical transmission system to which it is attached. 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 con- junction 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 that does not connect into the grid 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 is more costly than a conventional generator with mechanical speed control of the turbine. Variable-speed generators are being used on some hydropower schemes today (see Chapter 8). Capacity is limited but that is unlikely to be a problem with a tidal power plant where unit size is generally small.
SLUICES AND SHIPLOCKS
The sluices in a tidal barrage must be large and efficient enough to allow the tidal basin behind the barrage to fill with water quickly. Unless the water level behind the barrage effectively follows that on the seaward side, the efficiency of the plant is reduced. Where the water is sufficiently deep, efficient sluices can be built using the concept of the Venturi tube. Such a design will transfer water through the barrage extremely efficiently but it must be completely submerged. More conventional sluice gates usually need to be larger than Venturi tubes to provide the same rate of transfer.
Many of the rivers suitable for tidal development carry significant ship- borne trade and water traffic. To enable ships and boats to continue to use a river, ship locks must be included in the barrage. There must also be facilities to allow fish and other forms of marine life to pass the barrage. This is particularly important if the river is one used by migratory fish such as salmon.