Compared to wind and solar energy, wave
energy has remained relatively expensive and hard to capture, but
engineers from Sandia National Laboratories are working to change that
by drawing inspiration from other industries.
Sandia's 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, making electricity produced from wave energy
less expensive. The team applied classical control theory and robotics
and aerospace engineering design principles to improve the converter's
efficiency.
During a multiyear project funded by the Department of Energy's Water
Power Technologies Office, engineers from Sandia's Water Power program
are using a combination of 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 a harsh ocean environment.
"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 U.S. grid at a competitive price," Sandia
engineer Ryan Coe said. "By getting more energy out of the same device,
we can reduce the cost of energy from that device."
Advanced control of wave energy converters yields increased energy absorption
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
multiple turbines.
"These devices can be in open ocean and deep water, maybe 50 to 100
miles off the coast," Coe said. "An array of wave energy converters,
maybe 100 devices, connected to an underwater transmission line would
send the wave energy back to shore for consumption on the grid."
To capture energy from the ocean's waves, a wave energy 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 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.
"A control algorithm is a set of rules you write that prompts an
action or multiple actions based on incoming measurements," Sandia
engineer Giorgio Bacelli said. "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."
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. The team developed multiple control algorithms for the buoy to
follow and then tested which control system would get the best results.
"Controls is a pretty big field," Sandia engineer Dave Patterson
said. "You can operate anything from planes to cars to walking robots.
Different controls will work better for different machines, so a large
part of this project is figuring out which control algorithm works and
how to design your system to best take advantage of those controls."
Bacelli said 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.
"Resonance also stresses the entire structure of the device, and to
expand the longevity of the device, we need to balance the amount of
stress it undergoes," Bacelli said. "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."
Theory becomes reality in the Navy's world-class wave tank
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 at the Carderock Division in
Bethesda, Maryland, in August to test the new control methods in an
ocean-like environment. The wave tank facility is 360 feet long and 240
feet wide and has a wave maker that can generate precisely measured
waves to simulate various ocean environments for hours at a time. Sandia
used the wave 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 their device.
"The accuracy of the wave they can generate and the repeatability is
outstanding," Bacelli said. "The ability to recreate the same condition
each time allowed us to conduct very meaningful experiments."
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.
"This year, the device can move forward, backward, up and down, and
roll in order to resonate at the frequency of the incoming waves,"
Bacelli said. "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." In fact, the tests showed theory did match reality in the
wave tank. 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.
The team is analyzing the testing data and considering further
options to refine the control systems to maximize energy transfer.
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