How schools of fish inspired scientists to harness underwater power
Earth’s water is a natural medium for collecting energy, taking in about 97% of what we receive from the sun. After reflection and radiation, water stores over 2 million TWh (terawatt hours) per year. The world’s annual energy consumption is about 150,000 TWh. Clearly, we could benefit from using water for power.
Traditionally, the majority of the hydroelectric power we collect is from stopping water with dams and releasing it through turbines at high speed. But this process can have negative impacts on marine life, flora, and people. Sometimes as many as 40,000 to 50,000 people have been relocated so that a hydroelectric dam can be built. The natural seasonal movements of fish up and down a river may be blocked by dams, and flora may lose natural habitat. In the last few decades, tidal dams have been built, but they have similar impacts on the environment.
Traditionally, the majority of the hydroelectric power we collect is from stopping water with dams and releasing it through turbines at high speed. But this process can have negative impacts on marine life, flora, and people.
But now, devices are being developed to collect marine hydrokinetic energy using the motion of water in waves or currents. The challenges are many: The environment is harsh, the power-to-volume density is low, the structures are heavy and expensive, and several failures have discouraged private investors. Moreover, some devices may present a potential threat to people and the environment, as they may have large moving parts like propellers. Or, they may simply be visible and face the “not in my back yard” response.
So, what is The Next Idea?
What if we could build a device that mimics some aspects of nature in converting flow motion to moving a something, like a cylinder? If we did, would it be efficient? In nature, we see that fish swim efficiently individually or in schools. What if we could learn from this efficiency? And, would the cost of a device like this be competitive with fossil fuels, which are millions of times more dense in power-to-volume ratio than renewable energy technologies?
My background is in offshore mechanics. That involves studying the dynamics of flexible structures in severe ocean environments. Pipelines, cables, legs of offshore oil/gas platforms are long cylindrical structures, subject to destructive Flow-Structure Interaction. A good example of FSI on a land structure can be seen in the unfortunate Tacoma Narrows bridge incident.
Several FSI phenomena occur in nature, but I won’t burden you with their complexities. The bottom line is that they have an important common characteristic with fish dynamics. They all move with alternating lift in a steady flow, not with steady lift, like wings and sails.
I used to study those movements with the goal of suppressing them so that underwater structures could survive longer. Then, about 10 years ago, it dawned on me that we should “play along” with nature and do the opposite – enhance rather than suppress those destructive phenomena to convert more hydrokinetic energy to mechanical energy.
How do we do this? By placing cylinders on springs – a very simple device. We call this the VIVACE hydro energy device. VIVACE stands for Vortex Induced Vibrations for Aquatic Clean Energy.
Will it hurt fish? A 10-year-long study by the Department of Energy, Massachusetts Institute of Technology, and Harvard University, shows that fish thrive (and even spawn more) in the alternating wake of such cylinders. So, that is hopeful for the environment.
Then the question becomes, how many cylinders can you deploy close to each other to get more energy? We are fortunate at the University of Michigan to have developed a lab dedicated to understanding the complex hydrodynamics around these issues. We have been studying this for about a decade, and we recently made another breakthrough. We initially thought that it would be best to place such devices as far apart as possible to make each one work efficiently by avoiding interference. But when we tried that, it didn’t work well.
So, we looked again at the dynamics of fish schools, and we tried the opposite: We brought the cylinders very close together and we managed to get two of them last year and three this year to work together into synergistic FSI. The results were astounding. With two cylinders in synergy we got between 2.6 and 7.5 times the energy we could harness with one isolated oscillator. With three cylinders, preliminary results show that we can get up to 15 times the energy of one.
Another reason this development is significant is because of the types of currents we see around the world. The vast majority are slower than three knots. Typical rivers are slower than 2 knots. Traditional turbines require a minimum of four knots to operate efficiently. We have made VIVACE work in the lab with flows as slow as 0.75 knots. So, the VIVACE technology potentially gives us access to energy that is available no matter the current speed.
The DOE, with matching dollars from the state of Michigan, funded our latest deployment in the St. Clair River for three months in the summer of 2016. We were very fortunate to have great support from the Port Huron community, starting with Dunn Paper, who gave us access not only to the river but also to their facility with Internet and utility access. They are real fans of renewable energy with genuine interest in our technology and progress.
We also had strong support from the business development office of Port Huron and the Binational Public Advisory Council, and First Nation. After asking many questions about the technology, local business people became believers and followed our progress. We have received invitations to consider other locations for deployment as well. When people see a technology that mimics and respects the environment and is unobtrusive they really want to see it succeed.
VIVACE has great flexibility. It could be used near populated areas, or places like the Caribbean where there are few other energy resources. It respects the environment by mimicking it and is unobtrusive to people as it remains submerged at all times. With the recent breakthrough of synergistic FSI, we have achieved high power density – about 60,000 times that of wind farms at equivalent speeds. In addition, this technology is highly scalable and adjustable to locations. We can build small devices to power electronics, larger ones to power houses, or large ones near shore to connect to grids.
We know we need clean renewable energy. Developing a technology that is also environmentally compatible would give us a candidate for a sustainable energy portfolio. We owe it to ourselves and to our planet to give renewables a chance to become cost competitive. That effort is happening right now, right here in Michigan.
Michael Bernitsas is a professor of Naval Architecture and Marine Engineering at the University of Michigan. At the university, he's also a professor of Mechanical Engineering.
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