NWTC Researchers Develop Wave Energy Conversion Technology
Robert Thresher may be considered the wizard of wind at the National Renewable Energy Laboratory (NREL), having worked in the field since 1973. At the laboratory since 1984, Thresher’s credited with the buildup of what is now the National Wind Technology Center and the startup of the Energy Department's Water Energy Program. His longtime efforts at turning wind and water into renewable energy has now yielded a record of invention (ROI) titled “Wave Energy Conversion Devices with Actuated Geometry.”
Just as wind turbines generate power, electricity can be generated by capturing the energy of waves. But therein is the problem.
“It’s not like a wind farm where you can jump in your pickup and be there in a few minutes,” Thresher said.
Wave devices typically look like buoys that move up and down in the water. The simplest type is a device that, as the waves hit, pulls on a line attached to a generator on the ocean floor. That pulling motion creates the power, and that works fine except when a storm rolls in and the waves grow too large.
The pitch of blades on a wind turbine can be adjusted to reduce the amount of power generated. “There’s no need to build a giant generator to accommodate the highest wind speed possible,” Thresher said, “because those winds don’t come along that often.”
Something similar was needed to handle swells caused by powerful storm surges hitting a wave device. Thresher said the idea behind the ROI came from a “kind of group think” during a meeting with researchers who had expertise in everything from ocean engineering to control systems. “We thought to add something like a pitch control on a wind turbine.”
Some existing wave devices have something that resembles a barn door with a hinge at the bottom. The waves move it back and forth. But control of a system like that is tricky, because a large wave slapping against the door can overpower the generator.
Thresher and his colleagues came up with the notion that the door should be more like flaps on a venetian blind, able to close fully when waves are optimal and to open to allow the excess energy to flow through.
“The flaps across it can open and close depending on wave conditions,” Thresher said. “As a result, in very small or moderate waves, the flaps can be closed to get maximum exposure. As the wave height increases, the flaps can be opened to reduce the driving force and let a lot of the energy pass by. This ability can accommodate a wide set of conditions, reducing the chances of overpowering the generator.”
Control over when to open or close the flaps is key. Thresher’s team included Michael Lawson, Yi-Hsiang Yu, and Nathan Tom, all of whom are among the principal developers at NREL of the Wave Energy Simulator, or WEC-Sim. Funded by the Energy Department's Wind and Water Power Technologies Office, the software was developed through collaboration with NREL and Sandia National Laboratories. Using WEC-Sim will help analyze how well the idea behind the ROI will work.
Better control of the wave loads hitting these devices could allow them to become a more economical source of renewable energy, said Nathan Tom, a postdoctoral researcher and the lead researcher on the laboratory directed research and development wave energy project.
“Wave devices are really heavy,” he said. “We have a lot of steel. That is where we see the greatest savings coming from. We believe the only way we’ll cut the cost of energy in half is to have a system where the loads are more controlled than they are now.”
According to Thresher, getting the wave device into the water will take some time, with a working prototype taking 4 or 5 years. “It’s truly in its infancy,” he said.