PART 4 - CONTROL SYSTEMS
Continuing our review of the output from the European Commission's workshop; an aspect that can be overlooked in the early development of a device concept is the control system. However, how a wave device or tidal turbine is controlled can have a huge impact upon the peak loads experienced by the device and the annual energy yield.
Advances in control systems are one of the factors in the increasing size of offshore wind turbines and in many cases the control principles are very applicable to tidal turbines. Traditional control systems attempt to follow an idealised torque speed curve with no regard for turbine loading or whether that is in fact the most efficient state to operate in under the current conditions.
Prototype devices and simulation models can be used to correlate controller parameters to structural loading. Then load limits can be set in the controller and the control system can be allowed to maximise power capture within loading limits. Indeed this logic has been introduced on tidal turbines to date and developments of this are likely to include fatigue limiting controllers, farm controllers, AI and controllers with an internal numerical model of the turbine for active load estimation in place of strain gauges.
As we mentioned for tidal turbines the ability to robustly test controllers within numerical simulations is a highly useful tool; however, to date there is no reliable, realistic universal numerical modelling tool for wave energy devices. Yes, tidal flow modelling is not perfect, but it has been shown to be accurate, on the other hand numerical models of WECs tend to need to be tailored to the specific device via tank testing and validation.
Another significant challenge facing wave energy converters is that optimal performance is achieved when short term predictions of waves can be achieved, but measuring or predicting the incoming waves can be very costly and complex.
The control system also has an important role to play in ensuring the device takes appropriate safety actions in extreme wave conditions and acts to broaden the operating bandwidth of the device.
The priority for WEC control systems is to get systems tested as thoroughly as possible at scale and within a numerical model. Before being implemented at sea to best understand performance and change in performance over time with factors such as biofouling. Once a baseline concept is defined there is some transferable learning from tidal energy converters with control strategies that load limit and learn the device's response to the environment.