Variable-Geometry Wave Energy Conversion and Control
NREL is working to develop next-generation power maximization, load-shedding, cost-reduction, and peak-to-average power control strategies for variable-geometry wave energy converters (WECs).
The novelty of the variable-geometry design is the ability to alter WEC surfaces normal to the principle degree of freedom for energy capture, thus reducing the wave pressure and corresponding loads. In current practice, the power take-off (PTO) is commonly the only control knob used to maximize power and limit peak loads. However, NREL's goal is to create a paradigm shift in the control of WECs by adding an additional control knob to reduce the structural mass and levelized cost of energy of these devices.
Controlling the position and location of the variable-geometry sections allows the frontal surface area of the device to be changed, which increases or decreases the wave-excitation forces/torques as well as the hydrodynamic radiation coefficients of the device. NREL researchers believe that advanced control strategies will include conventional PTO control but also control of variable-geometry components.
Wave Energy Converter Dynamic Modeling
NREL maintains and supports the development of the open-source Wave Energy Converter SIMulator code in collaboration with Sandia National Laboratories. The code has been used to model a range of WEC architectures and has been shown to be adaptable to most developer needs.
NREL is familiar with the boundary element method hydrodynamic solvers WAMIT, NEMOH, and AQWA. Our experience with meshing tools, first- and second-order forces, and post-processing ensures developers are confident in their results.
NREL has significant experience in the development of feedforward control strategies for WEC technologies. We have researched control strategies such as complex conjugate control, model predictive control, and pseudo-spectral control for SISO and MIMO systems.
NREL has recently incorporated flexible body modes into the WEC-Sim modeling tool. This provides an opportunity to run WEC-Sim and identify hot-spot stress concentrations that can assist in WEC structural optimization.
Novel Control Options
A novel control option for WEC design is the use of control surfaces that allow for changing or variable geometries. The novelty of the proposed design is the ability to alter WEC surfaces normal to the principle degree of freedom for energy capture, thereby reducing the wave pressure and corresponding loads. In current practice, the PTO is commonly the only control knob used to maximize power and limit peak loads. However, NREL is suggesting that an additional control knob be added that uses the WEC geometry for advanced load-shedding.
Furthermore, the extreme loads the device must withstand cannot be limited when the WEC geometry is fixed, thereby limiting the effectiveness of the PTO to minimize loads. Without the ability to shed greater hydrodynamic loads, the WEC must be placed in survival mode, and power production will not just be decreased but halted, reducing availability and limiting the number of operational sea states. The reduced availability has negative impacts on technology acceptability as a result of reduced capacity factors and increased intermittency on the grid.
Control of Peak-to-Average Power Ratio and Fatigue Damage Accumulation
One of the primary challenges for WECs is the fluctuating nature of waves. As a result, WEC components must be designed to handle loads (i.e., torques, forces, and powers) that are many times greater than the average load, which requires a much larger PTO capacity than the average power output. The generated peak power from the WEC can be more than one order of magnitude larger than the absorbed average power. A large peak power implies a much higher PTO cost for the WEC system.
In addition, these fluctuations will have important implications for the stability of voltage and frequency to the grid and can be a problem for sensitive equipment. Therefore, it is essential to reduce the peak-to-average power ratio while trying to maximize, or at least maintain, the power output from the WEC by implementing energy storage/relief and advanced load-shedding methods such as WEC variable geometry control.
A significant reduction in the peak-to-average power will reduce the size, weight, and peak power of the entire PTO system. The peak structural loads can be set by the designer, who can balance energy production against structural costs and generator size and cost by designing the variable geometry control parameters for the site wave conditions. Possibly the most important impact of variable geometry load control is the ability to greatly reduce the cyclic fatigue loads on all of the system components.
A wave converter will most probably be designed for a 25-year life span, and over that lifetime will experience about 109 wave fatigue loading cycles (about the same cycle count as a wind turbine), so the ability to reduce peak loads will reduce the root mean square loads and fatigue damage. All of these benefits will be directly reflected in a reduced levelized cost of energy.
Balancing Power Absorption Against Structural Loads with Viscous Drag and Power-Take-Off Efficiency, IEEE Journal of Oceanic Engineering (2017)
Balancing the Power-to-Load Ratio for a Novel Variable Geometry Wave Energy Converter with Nonideal Power Take-Off in Regular Waves, European Wave and Tidal Energy Conference (2017)
Numerical Model Development and Validation for the WECCCOMP Control Competition, 37th International Conference on Ocean, Offshore Mechanics, and Artic Engineering (2018)
Power-to-Load Balancing for Heaving Asymmetric Wave-Energy Converters with Nonideal Power Take-Off, Renewable Energy (2019)