Compared to wind and solar energy, wave energy is relatively difficult and expensive to capture, but engineers from Sandia National Laboratories are working to change that by drawing inspiration from other industries.
A Sandia engineering team has designed, modeled, and tested controls that double the amount of power a wave energy converter can absorb from ocean waves, making electricity generated by wave energy less expensive. The team applied classical control theory, robotics, and aerospace design principles to improve the converter’s efficiency.
During a multiyear project funded by the Department of Energy’s Water Power Technologies Office, Sandia engineers have been using modeling and experimental testing to refine how a 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 harsh ocean environments.
“We are working on methods and technologies private companies can use to create wave-energy devices that let them sell power to the U.S. grid at competitive prices,” says Sandia engineer Ryan Coe.
Sandia’s 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. Commercial wave energy-converters can be large and are generally part of a group of devices, like a wind farm with several turbines.
“These devices can be in deep water and 50 to 100 miles off the coast,” Coe explains. “An array of maybe 100 converters connected to an underwater transmission line would send the wave energy as electricity to shore for consumption on the grid.”
To capture energy from the ocean’s waves, a converter moves and bobs in the water, absorbing power from waves when they generate forces on the buoy. Sandia’s previous testing focused on studying and modeling how converters moved in an ocean-like environment to create a numerical model of its device.
Using the model it developed and validated last fall, the team wrote and applied multiple control algorithms to see if the converter could capture more energy.
“A control algorithm is a set of rules that prompts an action or multiple actions based on incoming measurements,” says Sandia engineer Giorgio Bacelli. “The converter’s sensors measure position, velocity, and pressure on the hull of the 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, and this increases the amount of power that can be absorbed.”
The control system uses a feedback loop that responds to the behavior of the device by taking measurements 1,000 times per second and continuously refines the buoy’s movement in response to the variety of waves. The team developed several control algorithms for the buoy to follow and tested them to see which got the best results.
Although the primary objective of the control algorithm is to increase energy transfer between the wave and buoy, the amount of stress being applied to the device also must be considered.
“Resonance stresses the entire structure of the buoy, and to increase its longevity, we need to limit the amount of stress it undergoes,” Bacelli notes. “Designing and using controls helps find the best trade-off between loads and stress applied to the buoy while maximizing the power absorbed, and we’ve seen that our controls do that.”
Results from numerical modeling with the control algorithms showed a large potential, so the team took the converter to the U.S. Navy’s Maneuvering and Sea Keeping facility in Bethesda, Md. in August to test the new controls in an ocean-like environment. The wave tank measures 360 ft long by 240 ft wide and has a wave maker that generates precisely measured waves to simulate various ocean environments for hours at a time. Sandia used the tank to simulate a full-size ocean environment off the coast of Oregon, but scaled down to 1/20th the size of typical ocean waves to match its device.
“The accuracy of the wave they generate and the repeatability is outstanding,” Bacelli says. “The ability to recreate the same conditions each time let us conduct very meaningful experiments.”
The team ran a baseline test to see how the converter performed with simple controls directing its movements and actions. Then it ran a series of tests to study how its various algorithms affected the buoy’s energy-absorbing ability.
“Currently, the buoy can move forward, backward, up and down, and roll to resonate at the incoming waves’ frequency,” says Bacelli. “All degrees of freedom were actuated, meaning there are motors in the device for each direction it can move. During testing, it was 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.” In fact, tests showed theory matched reality in the wave tank. The control algorithms more than doubled the amount of energy the wave energy converters could absorb without a control system.
The team is analyzing the testing data and considering further options to refine the control systems to maximize energy transfer.
