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New Wave Energy Converter Design Inspired by Wind Energy

February 8, 2018

Three metal wave energy converters in open water with large slits that are partially open, much like window blinds.

Conceptual rendering of the variable-geometry, fixed-bottom oscillating surge wave energy converter developed as part of an NREL Laboratory Directed Research and Development project. Illustration courtesy of Joshua Bauer, NREL

Our planet’s energetic seas offer a significant opportunity for researchers. Wave energy converters (WECs) capture electricity from ocean surface waves, but the new industry requires fresh ideas to reduce the cost of energy.

National Renewable Energy Laboratory (NREL) researcher Nathan Tom and the wave energy research team took inspiration from the pitch control of wind turbine blades to develop a new design for WECs. When wind speed increases, turbine control systems adjust the pitch of their blades to account for the added load, which also ensures that the rotor speed does not exceed safe operating levels. NREL research fellow Bob Thresher, who was heavily involved with the wind industry from its inception, acted as the project adviser.

The high loads and harsh environment of open water—what researchers call “slamming events”—are inevitable hurdles for WEC developers. Designs have progressed over the past 20 years, but no one has yet provided a good solution for these slamming events.

“Currently there’s nothing like pitch control for wave energy,” says Tom. “We’re pushing for a next-generation design in wave energy converters with new freedom and control.”

The variable-geometry WEC can alter its shape to change how it interacts with waves. To develop the unique design, the team brainstormed on Venetian blinds, conceptualizing a series of horizontal flaps that span across the device. When opened, these flaps reduce the pressure on the WEC when the waves are particularly energetic—without sacrificing power output.

WECs with these flaps would use less steel, which reduces capital costs and lowers the levelized cost of energy. These devices would also be more resilient. Extreme loading, like breaking waves slamming against a WEC, can “cause greater component fatigue, damage your device, or even destroy your device,” Tom says.

WECs that do not incorporate variable-geometry flaps may experience unmanageable and damaging forces. Imagine holding your hand flat and vertical against a strong current—and then spreading your fingers to let water flow through. Those openings reduce the peak loading, leading to smaller extreme loads and less probability of damage or system failure.

With a fixed-body WEC—that is, one that does not use adjustable geometry to reduce system loading—large sea states may mean “you have to turn your device off, sink it, or pull the device out of the water,” Tom says.

Initially, the system would be controlled by a computer. Load and motion sensors would help the system decide when to open and close the flaps. Wave forecasting and radar could also be useful, for instance, if the system’s controller can see that a large wave will hit in 10 seconds, feedforward controls could reduce loading on the WEC structure and smooth power generation.

There are numerous WEC developers working on their own designs. Like all early-stage technologies, progress comes down to “cost-effectiveness and making sure your device survives,” says Tom.

Getting NREL’s WEC solution to market may be a challenge. Although the team has done small-scale tank validations at the University of Maine, there is lots more to do. Laboratory and open-ocean assessments—as well as a full levelized cost of energy analysis—will be necessary before the variable-geometry WEC concept is adopted by industry.

Still, the team’s design breakthrough could be a boon to a nascent industry.