Some strong potential benefits of applying nonlinear Model Predictive Control to the low-level WEC control were demonstrated:.
The use of models in design and commissioning, for example, will shorten the overall engineering time thus lowering the CAPEX and improving affordability. ISC's role in the project was to investigate the real-time implementation feasibility of the neural network-based short-term prediction methods. These methods will benefit WEC developers as it will be possible for them to obtain the present wave state as an input for their device enabling improved control to deliver higher performance power capture and survivability, leading to an overall lower LCOE.
This is a Wave Energy Scotland contract research project Stage 2 led by Mocean Energy and aims to develop the performance and engineering of Mocean Energy's novel wave energy converter, which is a hinged raft a so-called floating attenuator WEC with a rotational power take-off PTO with design innovation in the geometry of the two hulls.
Our engineers are highly qualified and have many years of experience in: Representing process dynamic behaviour with first-principle models and designing and de-risking controls for a wide variety of applications; Designing, developing, implementing and commissioning real-time control systems for challenging practical applications ISC is an award-winning National Instruments NI Alliance Partner ; Developing advanced model-based optimal control methods for renewables applications, including multi-objective optimal control we have published many peer reviewed papers on these applications; Contract and grant funded research , including in wave energy example below.
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Unit-Operation Nonlinear Modeling for Planning and Scheduling Applica…
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Need help? How do I find a book? Can I borrow this item? Can I get a copy? This technique often incorporates an objective function to minimize the output error over the controller time horizon. MPC is an advanced supervisory control method that is the middle level of a multilayer control hierarchy figure 1 consisting of:.
This hierarchical approach is applied by analyzing the causes of the disturbances in each part of the process and then layering the solutions that deal with the disturbances from the bottom up. Each layer adds complexity, and its use depends upon the nature of the disturbance and the preferred response. Basic control includes single input — single output systems connecting field measurements to field actuators through PID controllers figure 2.
This layer provides the foundation. Without effective basic control, the higher levels will be ineffective. Figure 2. Example basic single input — single output control scheme. One of the main weaknesses of PID is its inability to maintain stable operation when there is significant dead time or lag time between the valve movement and the effect on the set point.
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For example, in a distillation column, the reflux flow is normally adjusted to maintain a stable key column tray temperature. If the tray is well down the tower, an unmeasured feed composition change may upset the tray temperature. The controller responds by adjusting the reflux flow, but there may be dead time of several minutes before the tray temperature responds to the change.
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Many distillation towers have longer lag times and may take from 30 minutes to multiple hours for the full effect of the disturbance and the MV moves to fully play out. These types of loops are extremely difficult to tune in a PID controller. The controller is typically detuned except for heavy derivative action to prevent overshoot. An example of advanced regulatory control ARC is composition control of distillation tower product streams figure 3. The first type of disturbance that typically occurs is caused by changes in ambient conditions, which change the temperature of the column reflux and overhead product.
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This changes the degree of column separation and product qualities. Figure 3. Example multiple input — single output control scheme. The IRC calculates the net internal reflux as a function of the heat of vaporization, heat capacity and flow of the external reflux, the overhead vapor temperature, and the reflux temperature.
The external reflux flow is determined to maintain constant internal reflux and then adjust the set point of the external reflux flow controller. This type of control has improved stability by responding faster to disturbances caused by changes in ambient conditions. There are multiple inputs to the control, but only one set point for the reflux flow controller.
Further stabilization of the overhead product composition is achieved by controlling a key temperature in the upper part of the tower, since temperature at constant pressure is directly related to composition.
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When the tower pressure varies, which often happens through a pressure minimization strategy to save reboiler heat input, then the key temperature should be corrected for pressure variations.