^ Renewable wave energy

Article by Daniel Sweet

When Scott Hunter arrived in Sydney two years into his engineering program, he knew he would eventually end up working on the water—but he didn’t know in what capacity. “I’ve always been into sailing, and at the beginning of my studies, I wanted to design yachts, travel the world, and chase after the coveted sailing trophy, the America’s Cup. But then in my final year, I was working a lot with computational fluid dynamics (CFD), and one of my supervisors by pure chance knew of a wave energy company that was looking for someone with CFD experience. I went along for an interview, and I was offered a job. I thought: this will be something different, maybe a six-month distraction before I go back to boats. 17 years later, and I’m still involved with wave energy.”

Wave Swell Energy

“More specifically,” Mr. Hunter continued, “I am chief technology officer for Wave Swell Energy. Wave Swell’s approach is based on an improvement to the oscillating water column (OWC), a fundamental design in wave energy. An OWC can be thought of as an artificial blowhole. The OWC sits in the water a short distance from the shore. Inside the column is a large chamber into which waves flow in and out. Traditionally, turbines connected to the top of the column are spun as air is forced into and out of the chamber by the movement of the waves, and the energy generated by the turbine is stored for reuse in the grid. But the movement of air in two directions in typical OWCs requires the turbines to spin bi-directionally, which is not efficient. What we found is that we can extract more energy from the ocean by venting the air to the atmosphere when the water enters the column. Then, we extract the energy generated by the air that is sucked into the column as the wave recedes. The obvious question is: by doing it this way, aren’t you missing out on half the cycle’s energy? But interestingly, when you vent air to the atmosphere as the wave enters the chamber, the wave can rise in the column a lot higher than it otherwise would. And that upstroke produces more energy than a complete cycle from a more traditional OWC.”

Venting with valves

Mr. Hunter went on to mention that Wave Swell’s technology relies on valves for the critical venting function in their new approach to OWCs. “With our design, we quickly realized that we would need a reliable way to release the air inside the chamber as the water level rises. Because we don’t want any back pressure pushing on the rising water and restricting it from coming up freely, we also realized that we would need a passive valve instead of a controlled option. After all, the motion of the wave is erratic and based on minute-to-minute water conditions. The waves themselves are usually slow, taking between 12-14 seconds to rise and fall. But the period of that motion changes, and a valve that reacts to a signal, like pressure, would require fast actuators. In turn, that would lead to complexity, which adds cost, which adds potential failure, which leads to downtime. In other words: we wanted a passive valve!”

In-house design
After realizing they would need a valve for their project, the team at Wave Swell set off to find the right one. Because of their unique requirements, this proved difficult. “For our project, we needed the valve to open as soon as there was pressure in the chamber, and we also needed a high flow rate. Based on our research in the literature and talking to other companies, we came to realize that these are characteristics most valves do not possess and that we would have to make something in-house to meet our needs.”
“To begin, we did a lot of experimentation with the Australian Maritime College based in Tasmania. We determined the size of the valve based on model experiments at a scale of 1:25. This gave us an idea of the pressure and flow characteristics. Then we had to determine how to make the valves at full scale and what materials to use. We knew they’d have to be lightweight to open with very little back pressure. First we looked at composites like fiber glass and carbon. But when we tested the composite valve made of fiber glass, it was so heavy that when the pressure dropped the valve would slam shut and make a lot of noise. We thought we would try to dampen that, either with a spring system or seal, but this proved unnecessarily complicated.” 

Mr. Hunter smiled as he recalled the breakthrough: “then it came to us: why not use rubber? Its simple, its durable, and its noncorrosive for the marine environment. We replaced the composite hatch with a rubber flap, and it works very well, right from the first day! It might not be the best solution for years to come, but if we must change the valve out, it would be relatively cheap and easy to accomplish. After implementing an aluminum frame to keep the rubber from deforming as the pressure builds in the chamber, we knew we were ready for serious testing. That’s where we are right now. Going forward into the future, once we get to the full commercial scale and we are working with other partners, the Wave Energy valve solution may not stay rubber. Time will tell. But that’s the great thing about wave energy. Unlike more stable renewables like wind, where the designs haven’t changed in some time, wave energy has always attracted weird and wonderful solutions to engineering challenges.”

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