Second Order Sliding Mode Control of Oscillating Water Column Wave Energy Converters for Power Improvement

• Published in: IEEE transactions on sustainable energy Vol. 12; no. 2; pp. 1151 - 1160
• Main Authors: Gaebele, Daniel T., Magana, Mario E., Brekken, Ted K. A., Henriques, Joao C. C., Carrelhas, Ana A. D., Gato, Luis M. C.
• Format: Journal Article
• Language: English
• Published: Piscataway IEEE 01.04.2021; The Institute of Electrical and Electronics Engineers, Inc. (IEEE)
• Subjects: Air flow; Algorithms; Angular velocity; biradial turbine; Compressibility; Control algorithms; Control valves; Converters; Energy; Energy conversion; Energy output; Feedforward control; Kinetic energy; Nonlinear dynamics; Nonlinearity; Ocean waves; oscillating water column; Renewable energy; Sea states; Sliding mode control; State space models; Torque; Turbines; Water circulation; Water column; Wave energy; Wave energy conversion; Wave power; Wind turbines
• DOI: 10.1109/TSTE.2020.3035501
• ISSN: 1949-3029

Ocean wave energy is a renewable energy which remains too costly for large-scale electricity generation. The oscillating water column (OWC) wave energy converter (WEC) is a promising device-type with a rectifying air turbine and generator which convert alternating airflow induced by the water motion into kinetic energy then into electric energy. Applying control at each stage of energy conversion could increase the electric energy output of the device. As researchers overcome the modeling challenges of OWC, such as the nonlinearities due to air compressibility and power take-off (PTO) dynamics, we can integrate specific control algorithms to test their ability to improve the efficiency of the OWC. Herein, we present a state-space model of an array of OWC WECs restricted to heave motion with nonlinear PTO dynamics. We apply second-order sliding mode control (SMC) which commands a smooth torque signal to a direct-drive generator to maintain a reference turbine angular velocity. Because the algorithm can yield high turbine torques, we investigate a simple feed-forward relation for the control of a valve to limit the turbine airflow and discard mechanical power. We find that implementing the SMC algorithm and valve control can improve electric energy conversion most effectively in less energetic sea states.