Many wave energy converter (WEC) developers are in the process of scaling up previous point absorber concepts to 100kW+ devices. As sizes increase, serious challenges in power take off (PTO) design are being encountered. Hydraulic transmissions are favoured as gearing up to give high speed rotary motion is easily achieved, and power density is high. They are also highly controllable, for example allowing take-off cylinders to be locked or free-running as conditions require via simple valve control.
Theoretical concepts for extracting the maximum power from waves already exist, using reactive or latching control for example, but these do not account for the real engineering limitations or losses in practical PTO systems. The concepts assume that the PTO can generate any given force/motion relationship with equal efficiency.
This paper presents the results of a simulation study on the design and control of a hydraulic PTO. The control strategies for maximum power extraction are compared with and without consideration of the characteristics (losses) in the PTO. It is shown that efficiency can be surprisingly low. It is also shown that using components (motor and pump in this case) sized for smaller wave conditions may be preferable as their efficiency is better.