WAVE POWER Q


CAPTURING OCEAN ENERGY


Robotic principles


are helping wave energy converters


in the US to absorb even more ocean wave energy


Words: Kristen Meub A


team of marine technologists at the Sandia National Laboratories in the US has designed and tested a new control system which, they claim, doubles the amount of power a wave energy


converter can absorb from ocean waves. With funding from the US Department of


Energy’s Water Power Technologies Offi ce, the team applied classical control theory and robotics to improve the converter’s effi ciency. “We’re working to create technologies that companies can use to create wave energy devices which will allow them to sell power to the US grid at a competitive price,” explains Sandia engineer Ryan Coe. “By getting more energy out of the same device, we will be able to reduce the cost of energy from that device.”


Energy absorption Sandia’s wave energy converter is a one-ton ocean buoy with motors, sensors and an onboard computer which has been built at a scaled-down size for a testing environment. Commercial versions are much larger and are generally part of an array of devices, like a wind farm with multiple turbines. “They can be out in the open ocean and


deep water up to 100 miles off the coast,” explains Coe. “An array can consist of 100 devices, connected to an underwater transmission line sending the wave energy back to shore for consumption on the grid.” To capture energy, a wave energy converter


moves and bobs in the water, absorbing power from waves when they generate forces on the buoy. The team modelled how the devices moved in an ocean-like environment in order to create a numerical model of their device. Then, using this model, they wrote and


applied multiple control algorithms to see if the converter could capture even more energy. “A control algorithm is a set of rules you


write that prompts an action or multiple actions based on incoming measurements,” says Sandia engineer Giorgio Bacelli. “The sensors on the device measure position, velocity and pressure on the hull of the buoy and then generate a force or torque in the motor. This action modifi es the dynamic response


of the buoy so that it resonates at the frequency of the incoming waves – which maximizes the amount of power that can be absorbed.” The control system uses a feedback loop to


respond to the behaviour of the device by taking measurements 1,000 times per second to continuously refi ne the movement of the buoy in response to the variety of waves. The team developed multiple control algorithms for the buoy to follow and then tested which control system would get the best results. While the primary objective of the control


algorithm is to maximize energy transfer between the wave and the buoy, the amount of stress being applied to the device must also be considered. “Resonance also stresses the structure, so,


to expand its longevity, we need to balance the amount of stress it undergoes,” says Bacelli.


“Designing and using a control system


helps fi nd the best trade-off between the loads and stress applied to the buoy while maximizing the power absorbed.”


Test tank Results from numerical modelling with the control algorithms showed great potential, so the team took the converter to a US Navy test tank in Maryland to test the new control methods in an ocean-like environment. The wave tank measures 360ft by 240ft and can generate precisely measured waves to simulate various ocean environments for hours at a time. Sandia used the wave tank to simulate an ocean environment, but scaled down 20 times to match their device. “The accuracy and repeatability of the


waves they generate is outstanding,” says Bacelli. “The ability to recreate the same condition each time allowed us to conduct very meaningful experiments.” The team then ran a baseline test to see


how the converter performed with a simple control system directing its movements and actions. Then they ran a series of tests to study how the various control algorithms they had designed affected the ability of the device to absorb energy. “Now it can move forward, backward, up


and down, and roll in order to resonate at the frequency of the incoming waves,” says Bacelli. “All degrees of freedom have been actuated so that there are motors in the device for each direction it can move.”


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