•

Very low hydraulic head from the seawater in the reservoir

•

Large variation in head from 4 m to less than 1 m

•

Debris and seaweed in the reservoir

•

Difficulties performing maintenance at sea requires high reliability.

Fortunately there is lots of experience in hydrogenation with turbines operating with hydraulic heads as low as one meter. The Kaplan turbine is widely used and operates reliably in salt water tidal plants. The picture shows a Kaplan turbine rotor. Unfortunately, the Kaplan turbine’s efficiency drops considerably at low hydraulic head unless it is equipped with variable pitch blades on both the runner and the guide. This makes the turbine more complex and less reliable mechanically. Alternately a fixed-pitch turbine can maintain efficiency by changing speeds at different hydraulic heads. This, however, requires a variable speed/frequency generator, which in turn requires an inverter to change the variable frequency power to 60 Hz to feed into the grid. So what appears to be a simple design of the hydrodynamic system, may require a complex PTO.


Kaplan turbine rotor
Photo: Ser. Ge. M. A. srl

The Oscillating Water Column device also tends to be rather simple in design. However, the intermediate energy it produces (compressed air) is not unidirectional, but reverses pressure with the frequency of the water waves. An ordinary air turbine would reverse directions under this reversing pressure and air flow. The Wells turbine, shown in the picture, however, is specially designed to maintain the same direction of rotation with reversing air flow. The blades on the turbine rotor are not angled to the direction of rotation as in other turbines. Air passing the rotor in either direction will transfer energy to the rotor blades. The turbine is not self-starting, but must be connected to a generator/motor to operate. When there is no air flowing past the turbine rotor, the generator/motor acts as a motor and drives the rotor. As soon as air passes the rotor in either director the generator/motor begins generating electricity.


Wells Turbine
Photo: Wave Energy Research Team. Limerick University

The mechanical motion of the Archimedes Wave Swing is typical of the output of a number of hydrodynamic systems. A device that is designed to make use of this motion directly is the linear generator shown in the diagram. The stator that contains the armature winding is fixed, while the translator containing permanent magnets moves. The motion of the translator creates a varying current in the armature which is connected to the load. The linear generator is completely analogous to a permanent magnet rotary generator.


Linear Generator

Another form of intermediate energy produced by a hydrodynamic system is a high pressure fluid. The pressurized fluids may be sea water as in the AquaBouy, or a hydraulic fluid or oil as in the Pelamis. Sea water is easier to handle in marine surroundings as it is environmentally benign. Sea water can be used in an open system and released after running a turbine generator. It also allows the PTO facilities to be located centrally on shore and fed by high pressure flexible piping from an array off-shore WECs.

In the Pelamis the hydraulic fluid from the high pressure accumulator tanks in each hinged joint is fed to a large hydraulic motor. The hydraulic motors in the joints drive electric generators, and the electricity from all the joints is fed down a single submarine cable to a junction on the sea bed. Several devices can be connected together and linked to shore through a single seabed cable. The Pelamis uses commercially available hydraulic components
.