Sandia National Laboratories’ engineering team has designed, modeled and tested a control system that doubles the amount of power a wave energy converter can absorb from ocean waves.
The team applied classical control theory and robotics and aerospace engineering design principles to improve the converter’s efficiency in an effort to cut costs of electricity produced from wave energy, Sandia Labs said.
The research is being done as part of the multiyear project funded by the Department of Energy’s Water Power Technologies Office where Sandia Labs’ engineers are exploring how wave energy converter moves and responds in the ocean to capture wave energy while also considering how to improve the resiliency of the device in a harsh ocean environment.
Ryan Coe, engineer at Sandia Labs, said: “We are working to create methodologies and technologies that private companies can harness to create wave energy devices that will enable them to sell power to the US grid at a competitive price. By getting more energy out of the same device, we can reduce the cost of energy from that device.”
Sandia Labs’ wave energy converter is a large 1-ton ocean buoy with motors, sensors and an onboard computer built at a scaled down size for a testing environment.
The previous testing conducted by Sandia Labs researchers focused on studying and modeling how the devices moved in an ocean-like environment to create a numerical model of their device.
Using the model they developed and validated last fall, the team wrote and applied multiple control algorithms to see if the converter could capture more energy.
The control system uses a feedback loop to respond to the behavior of the device by taking measurements 1,000 times per second to continuously refine the movement of the buoy in response to the variety of waves, according to Sandia Labs.
The team developed multiple control algorithms for the buoy to follow and then tested which control system would get the best results.
Sandia Labs engineer Giorgio Bacelli said: “A control algorithm is a set of rules you write that prompts an action or multiple actions based on incoming measurements. The sensors on the device measure position, velocity and pressure on the hull of buoy and then generate a force or torque in the motor. This action modifies 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.”
Bacelli added that 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 also must be considered.
“Designing and using a control system helps find the best trade-off between the loads and stress applied to the buoy while maximizing the power absorbed, and we’ve seen that our systems can do that,” said Bacelli.
After obtaining results from numerical modeling with the control algorithm, the team tested the new control methods in an ocean-like environment at US Navy’s Maneuvering and Sea Keeping (MASK) facility back in August.
Sandia Labs used the wave tank to simulate a full-size ocean environment off the coast of Oregon, but scaled down to 1:20 scale of typical ocean waves to match their device.
The team 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, Sandia Labs informed.
The tests showed theory did match reality in the wave tank as the control algorithms were able to more than double the amount of energy the wave energy converters were able to absorb without a control system, according to Sandia Labs.
Bacelli said: “This year, the device can move forward, backward, up and down, and roll in order to resonate at the frequency of the incoming waves. All degrees of freedom were actuated, meaning there are motors in the device for each direction it can move. During testing we were able to absorb energy in each of these modes, and we were able to simulate the operating conditions of a device at sea much more accurately.”
Sandia Labs added the team is currently analyzing the testing data and considering further options to refine the control systems to maximize energy transfer.