Another method of converting the low-speed oscillating
motion of the primary WEC interface is to
employ a hydraulic system. Waves apply large forces
at slow speeds and hydraulic systems are well suited
to absorbing energy in these situations [51]. The use of
hydraulics operating at a pressure of 400 bar is a distinct
advantage of some types of WEC where size and
weight are an issue [42], and the force created by these
pressures are considerably greater than those from the
best electrical machines.
Figure 9 is a circuit diagram for a basic hydraulic
PTO system for a WEC. The rod of the hydraulic cylinder
is forced up and down by a floating buoy, which
forces fluid through check valves, rectifying the flow,
to a hydraulic motor. In this case, the generator could
be a constant speed device, and the hydraulic motor
has variable capacity, to drive the generator at closeto constant speed despite a variable flowrate. The
control of the motor capacity could be based on measured
or predicted sea states around the WEC, or fluid
flow measurements within the system. Additionally, a
throttling valve could also be used to control the flow to
the motor. Accumulators are included in the circuit to
provide energy storage and to maintain constant flow
to the hydraulic motor. In addition, the low-pressure
accumulator provides a small boost pressure to reduce
the risk of cavitation on the low-pressure side.
If the incident waves are close to sinusoidal, then
the flow from one port of the actuator is represented
in Fig. 10(a). Rectification through the check valves
results in the flow represented in Fig. 10(b). The accumulator
would then smooth this, with the variable
capacity motor driving the generator.
The hydraulic circuit employed in the Pelamis WEC
(see section 2.4.1 for further details) essentially follows
the design shown in Fig. 9. The accumulators provide
a decoupling between the hydraulic cylinders and the
motors, providing enough energy storage to supply the
pumps with a relatively constant energy supply. Active
valves are employed to rectify the flow, and are also
used to vary the reaction torque. In reference [51], it is
noted that losses in the primary transmission can be
kept below 20 per cent over a wide range of operating
conditions.