Healthy Economy Marine Studies Initiative

Ripples Upon Ripples

Engineers use sensors and software to refine wave-energy systems

By Rachel Robertson

Out on the ocean, a wave-energy converter heaves, rocks and pitches to the motion of the waves. How the converter interacts with the water to generate power is at the core of creating devices that can be commercially viable. And it has been a missing piece of information for wave-energy simulation tools — until now.

Researchers in the School of Electrical Engineering and Computer Science at Oregon State University recently completed a year-long experimental testing project at the O.H. Hinsdale Wave Laboratory to do just that. The research was part of a $4.5 million effort initiated by the U.S. Department of Energy for Sandia National Laboratories and the National Renewable Energy Laboratory to create a simulation software tool to advance wave energy research.

“Wave energy is a nascent technology — there’s a long way to go before we have utility-scale deployments of wave farms like you see with wind and solar,” said Kelley Ruehl, co-principal investigator from Sandia National Laboratories and an Oregon State alumnus in mechanical engineering. She was co-advised by Professor Bob Paasch (mechanical engineering) and Ted Brekken (electrical and computer engineering).

“Using simulations is an inexpensive way of evaluating the performance of different devices in comparison to one another or in testing out design changes. So having a good simulation tool is a critical step in design optimization,” she said.

Although other simulation tools exist, they are proprietary and not designed specifically for wave energy converters. The open-source code that the team developed is called WEC-Sim (for wave energy converter simulator).

“The intent of having an open-source code project was to have a tool that users can add features to themselves,” Ruehl said.

And so they have. The code was initially released in 2014, and in the first year there were only a handful of users. But Ruehl said over the last year there has been widespread adoption of the code. Programmers have been actively incorporating user modifications.

Last year the project moved to the experimental testing phase, which is when Oregon State entered the picture. “When we were looking at different test facilities, Oregon State, with its expertise in wave energy and the facilities at Hinsdale, was the obvious choice,” said Ruehl, who headed up the testing.

Video of the wave energy converter built for the WEC-Sim project.

The Oregon State team involved with tank testing and data collection of the scale model (1/33 size of a full wave energy converter) included Asher Simmons and Ratanak So, graduate students in electrical and computer engineering. They were advised by Professor Ted Broken. Another collaborator, Bret Bosma, was a post-doc at the time and is now a faculty research associate.

Testing with an actual wave-energy converter was critical. Its structure is more complicated than that of other ocean vessels. Data from boats interacting with water are available, but the information is insufficient because wave-energy converters have two or more rigid bodies that interact in close proximity. In turn, that leads to more complicated interactions with the water.

“If you are standing in a lake, and you throw a rock as far out as you can, by the time the ripples get to your legs they’re hardly anything. But if you drop a rock right next to you, the ripples hit your leg and cause more ripples. It is the interaction causing more ripples that we don’t understand,” Simmons said.

Sensors on the body of the device and in the tank contributed to 62 total data signals, including measurements of force, position and the height of the waves. The system included a new use of a sensor — a pressure mat — located on the flaps of the device. It generates a field of pressure measurements that engineers could use to create a map of the wave impact on the flap.

Because the device was so highly instrumented, Simmons used the opportunity for a side project to determine the most efficient use of sensors. Some of the sensors have price tags that are out of reach for developers of wave-energy devices in the early stages of testing. So his research will help inform researchers about which sensors are critical to collecting good data, where the sensors would be best placed and how to correct the data if they are using non-optimal sensors.

“We really need this,” said Simmons, explaining that developers often don’t have the funds to properly test power take-off until far too late in the process, creating products that are not commercially viable. “My hope is that developers can use this information much earlier to get a better feel for how much power they will be able to extract,” he said.

The data from the testing phase will be publicly available through the WEC-Sim website and it will also be used to validate the WEC-Sim software.

For the researchers at Oregon State, the project was a great opportunity to be part of a collaborative effort to advance wave-energy technology. In addition to the two national labs and Oregon State, a local engineering firm (Andrews-Cooper) and a Spanish company (+D), collaborated on the project.

“I like physical modeling and working on real engineering problems, and wave energy is an extremely challenging one,” Bret Bosma said. “This was a very complicated system with a lot of sensors and a lot of data, and so it was a really nice platform to work on.”