With the perpetual motion of its waves and tides, the Earth's ocean represents a highly predictable, theoretically limitless source of kinetic energy.
At a power density of 10 – 50 kilowatts per meter, in fact, wave energy is a more efficient, more concentrated form of energy than all other renewable energy resources, including solar and wind. Efficiently harnessed, waves along the U.S. coastline alone have the potential for generating 64% more electricity than is currently generated through all sources across the country.
For his senior design project, Stevens Institute of Technology engineering major Tyler Brunquell '22 helped design and develop an Ocean Wave-Powered Autonomous Robot to take advantage of just such untapped potential.
Given his major concentration in naval engineering and minor in green engineering, Brunquell said the project afforded him the opportunity to combine these two areas while exploring ways of developing sustainable renewable energy technologies.
Brunquell's areas of responsibility on the project were two-fold: to design an optimized hull for the buoy section of the vessel and to develop a half-scale model to test its power take-off (PTO) system's electricity-generating capabilities.
The primary use case for which the ocean-powered vessel was designed, Brunquell said, was for the monitoring and maintenance of offshore farms, such as wind farms and marine aquaculture.
Equipped with sensors, cameras, and wireless communication systems, such a self-sustaining robot could monitor and respond to changes in facility or environmental conditions, while generating enough electricity to propel and maneuver itself and to provide electricity to the farm's operations and other manned or unmanned surface and underwater vehicles.
Functioning autonomously, the vessel would also provide a cost savings through its reduced need for personnel.
"Instead of sending an individual to monitor the offshore farm, you could send out this device, and it would be able to check on how everything was working," Brunquell said.
Designed as a catamaran, the vessel is composed of a double-hulled buoy that floats on the ocean's surface and a heave plate submerged below it, which provides stability. A PTO system generates electricity through the difference in relative motion between the two.
To optimize the buoy's hull design, Brunquell developed a design matrix comparing different types of hulls and how well they performed according to each of three primary parameters: stability, heave motion, and maneuverability. Heave motion is the motion of a vessel as it moves vertically.
“The way I judged how well a hull type would work would be running it through multiple programs that allowed me to see the relative heave response amplitude operator, which is how much motion compared to the wave amplitude the boat will actually respond to," he said.
This suite of conceptual ship design programs can simulate different operating conditions, such as wave height, wave period, ship heading, and rudder direction, Brunquell said. "This allowed me to compare how my theoretical designs matched up in maneuverability, heave response and thrust power."
Given the vessel's primary use case and expected operating conditions — with waves occurring on average every five seconds, with the highest third among them averaging three feet — the need for stability remained foremost in Brunquell's mind.
"[Stability] was weighed a lot more in my design matrix because when you're operating with different wave heights and with different vertical center of gravity values, you can get some unstable characteristics of the model," he said. "I wanted to make sure, no matter what range of vertical center of gravity and weight that they have on the vessel, I was able to see if the design was decent enough to operate in the wave conditions that [the team is] hoping for."
Brunquell developed his 3D design models and conducted stability testing using CAD modeling software Rhinoceros 3D with a naval architecture design plug-in called Orca 3D.
"I looked at how well the vessel is able to right itself at different degrees of roll," he said. "I used values from zero to 120 degrees — meaning that this vessel was basically upside down — to see how well it's able to right itself back."
Then Brunquell shifted his focus from the vessel's hull design to its energy capabilities, developing and testing a half-scale model to test the validity of its electricity-generating PTO system.
Although a PTO system can capture energy in a variety of ways, Brunquell said, "for this system specifically it works through the relative heave motion of a surface vehicle as it goes up and down over the waves."
Riding along the surface of the waves, the vertical motion of the buoy is greater than the vertical motion of the heave plate that is submerged beneath the waterline. The relative motion represented by the difference in kinetic energy between the two objects is how wave energy ultimately is converted into electricity.
"That relative motion between the two is connected through a string," Brunquell explained. "That string is wrapped around a spool a few times to create some friction, and as the distance between the submerged body and surface body changes, it's able to spin the spool, which spins a motor and creates electricity."
"If you have a larger wave, you get much larger values and more speed in the motor, which creates more electricity," he said.