Development of Feedforward Control Strategies for Wave Energy Conversion Technologies
The future of wave energy will depend on developing a new generation of wave energy converters (WECs) that maximize energy extraction and mitigate critical loads while reducing costs. Today’s WECs are relatively inefficient compared to their theoretical upper limit and lack the ability to concurrently maximize power capture and minimize structural loads. The majority of existing WECs consist of fixed geometrical bodies relying predominantly on control of the power take-off system to meet design objectives. This low control authority in existing devices limits the achievable load mitigation in moderate-to-extreme sea states.
Engineers at the National Renewable Energy Laboratory (NREL) have optimized an existing advanced WEC concept by adding controllable surfaces for load mitigation and enhanced capabilities for device tuning. Advanced feed-forward controls coordinate controller performance of this multi-actuated system. This project is positively impacting the WEC industry through technology advancements that dramatically reduce the levelized cost of energy (LCOE).
It was found that of the existing WEC categories an oscillating surge wave energy converter was the best candidate for incorporating structural components that were controllable.
Detailed technical analyses using linear frequency domain methods were used to select the device geometry, dimensions, flap pitch angle, flap cross section, and the number of activated flaps. Figure 1 depicts a sample configuration, showing overall device layout with four flaps having elliptical cross sections chosen to reduce viscous losses in order to improve power. Each flap can be rotated about its centerline to orient the section at different angles to the waves, with the flaps fully closed at 0°, and open at 90° to control the wave forces exerted on the converter. The entire wave energy converter panel assembly with control flaps shown in Figure 1 is hinged at the bottom to allow the entire system to rotate with the wave motion to produce power.
The panel assembly is connected to one or more power-take-off (PTO) systems that allow the device to extract power in heave, surge, and pitch for a floating converter, or in surge for a fixed bottom converter. As a wave propagates past the device the hydrodynamic forces, which occur because of the dynamic pressure variation over the device, force the device to oscillate in its available degrees of freedom. A PTO system is connected to the device providing a force that resists the device motion, thereby extracting power contained within the propagating wave. The device supporting structure is secured to the seafloor through mooring lines and anchors, or bottom fixed foundations. The oscillating converter panel assembly is held in place by the PTO system connections and hinge bearing at the bottom.
One example of an actuated geometry design resembles a large "Venetian blind" (see Figure 1). The device as a rigid body extracts energy from its translational and rotational motion. Each Venetian blind element of the flap can be independently opened and closed. This provides a mechanism to control hydrodynamic forces (i.e. hydrodynamic coefficients) and additionally the natural frequency in surge, heave, and pitch. The flaps would ideally be actuated on a time-scale less than one typical wave period. This property provides several benefits:
· The device can be tuned for optimal performance as each wave passes by actuating the blinds in the correct way
· Opening the Venetian blinds reduces the hydrodynamic loads on the device, allowing it to operate in harsher sea conditions
· Blinds can be actuated to insure PTO forces remain within PTO specifications
· This concept allows for "feed-forward" controls that tune PTO and device parameters for maximum power extraction and or load minimization
· Articulated geometry allows for advanced control structures to be implemented
· Optimized power under any given sea condition
· Minimized fatigue
· Adds an additional degree of freedom when considering wave-to-wave control capabilities
Applications and Industries
· Wave energy
· Feed-forward control
· Wave energy conversion
· Actuated geometry
|Technology ID||Development Stage||Availability||Published||Last Updated|
|NREL ROI 15-58||Proposed||Available||12/29/2015||12/29/2015|