# 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).

Roughly 35%–50% of the price of wave energy can be attributed to the structural costs of WEC devices, as they must be designed to withstand large wave loads. Reducing the structural cost of the device and increasing energy capture are two paths toward reducing wave energy's levelized cost of energy. ECs traditionally consist of a fixed-geometry hull and possess a singular means of operational control: the generator, or power take-off (PTO). The PTO is commonly the only means of maximizing power and limiting peak loads. However, NREL researchers are rethinking WEC designs to guide the way to the next-generation, cost-competitive systems of tomorrow.

Employing variable geometry, the WEC shape can be changed so that in more energetic sea states the structural loads are controlled, allowing for extended power production.

Introducing variable geometry into WEC designs would add a second control knob to the system, similar to blade pitch control in a wind turbine.

This secondary means of control would allow for operational adjustment in a variety of sea states, allowing the device to be fine-tuned to shed load in large-to-extreme sea states or increase power absorption in low-to-moderate sea states.

Because of the additional control they offer, variable geometry WECs can be optimized for greater energy capture, improved efficiency of operation, and ultimately, a more cost-competitive wave energy.

## Capabilities

### 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 is adaptable to most developer needs.

### Hydrodynamic Design

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.

### Controls Engineering

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.

### Structural Modeling

NREL has 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 principal 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.

## Publications

Annual Performance of the Second-Generation Variable-Geometry Oscillating Surge Wave
Energy Converter, *Renewable Energy* (2020). In Press, Journal Pre-Proof.

*Numerical Model Development of a Variable-Geometry Attenuator Wave Energy Converter*, 39th International Conference on Ocean, Offshore and Arctic Engineering (2020)

*Submerged Pressure Differential Plate Wave Energy Converter with Variable Geometry*, European Wave and Tidal Energy Conference (2019)

Balancing Power Absorption Against Structural Loads with Viscous Drag and Power-Take-Off
Efficiency, *IEEE Journal of Oceanic Engineering* (2017)

Pseudo-Spectral Control of a Novel Oscillating Surge Wave Energy Converter in Regular
Waves for Power Optimization Including Load Reduction, *Ocean Engineering* (2017)