This article presents an overview of the Vortex Induced Vibration for Aquatic Clean Energy (VIVACE) converter. Unlike typical hydroelectric power stations, the VIVACE converter does not need to impound water behind a dam to create a head. Water is allowed to flow freely over the cylinders. Also in contrast to many hydrokinetic technologies that have been proposed in the last decade, the converter does not use a turbine, making it safer for aquatic life that swim past or through it. Another advantage of the VIVACE technology is its scalability. Not only can the technology be deployed in few watt-, kilowatt-, megawatt-, or even gigawatt-scale installations, but also the science and engineering is basically the same at every scale. The VIVACE converter is a clean and simple machine that promises to provide cheap and plentiful electricity. There is certainly work that needs to be done to optimize the system, but there is no reason to doubt that the converter can operate as expected.
Water flowing over a cylinder is illuminated to show the vortices that are created.
THE EXPERIMENTAL SET-UP WAS SOLID, BUT IT DIDN’T LOOK EARTH-SHATTERING. My graduate students had built four matched cylinders, each 36 inches long and 3.5 inches in diameter. All four were attached to support structures and immersed in a flow tank in the Marine Renewable Energy Laboratory at the University of Michigan in Ann Arbor.
When the impeller was turned on and water flowed through the tank, however, something amazing happened. The cylinders started to move up and down through the water, and then they began to move in sequence, almost as if they were part of a four-piston reciprocating engine. The motion, first up, then down, was so forceful that it was evident that the cylinders were tapping into some large supply of energy.
All this in water flowing at just 2.6 knots.
The experiment, which was conducted this winter, was the latest in a series at the University of Michigan to help prove the value of a novel, environmentally compatible, and potentially important energy harvesting technology. The cylinders in the tank are being moved by vortices in the water that form and then shed. We don’t have data yet for this experiment but according to our calculations, a matrix of such columns in a steadily flowing ocean current or a river could generate hundreds, even thousands of watts per cubic meter depending on the flow velocity.
In an area far smaller than a typical wind farm, an array of these cylinders could produce 1 gigawatt of power, enough to light a medium-size city.
ENGINEERS HAVE BEEN AWARE OF VORTEX-INDUCED VIBRATIONS FOR CENTURIES. In fact, Leonardo da Vinci first observed VIVs some five hundred years ago, in the form of “Aeolian Tones,” the sound made as wind passes over a wire of the correct diameter and tension, and in the vortices that swirl behind the piers of a bridge. At the latter half of the 19th century and the beginning of the 20th, researchers such as John Strutt (Lord Rayleigh) and Theodore von Kármán discovered that strings in a wind move perpendicular to the flow of air, and that vortices trail behind a bluff body in a regular pattern.
The interaction between the fluid and the structure occurs because of nonlinear resonance of cylinders or spheres through vortex shedding lock-in. That is, the period at which vortices are formed and shed becomes synchronized with the side-to-side motion of the bluff body, and thus the motions become amplified over time. As a reflection of this, the phenomenon can be characterized as vortex induced vibration, wake synchronization, vortex shedding lock-in, or nonlinear resonance.
Vortex induced vibrations of circular cylinders occur over a very large range of Reynolds numbers—between 40 and 150, between 400 and 300,000, and for Re greater than 500,000. Experiments have demonstrated that the formation of VIVs can be affected by such factors as the condition of the cylinder surface, vorticity in the ambient flow, and fluid conditions affecting water viscosity such as salinity and temperature.
VIVs aren’t just an idle curiosity. These vibrations can cause tremendous damage to many kinds of structures. Risers that connect oil production platforms with wells on the seafloor can be shaken loose or broken by VIVs. Even tall brick smokestacks or cooling towers are subject to the force of these vibrations.
Perhaps the most famous and dramatic example of the power of VIVs came in 1940. A newly completed bridge across the Tacoma Narrows in Washington State had developed a reputation for oscillating in a twisting mode in the presence of stiff winds at the site. Eventually, after a morning of persistent motions of about a meter, something gave way and the bridge shook itself apart. Research conducted by von Kármán showed that the Tacoma Narrows bridge collapse was due to VIV. (Recently, the possibility that the bridge collapsed due to flutter instability was raised.)
Because of the disruptive power of VIVs, engineers have long been interested in trying to spoil vortex shedding and suppress VIV to prevent damage to equipment and structures. Anything circular cylindrical and flexible, from tiny fishnet filaments to 120-foot diameter SPAR platforms used in offshore petroleum production, gets into VIV. The list includes such everyday objects as car antennas, flagpoles, traffic-light posts, buildings, and cooling towers. There are hundreds of patents on VIV suppression devices. Typically, they are helical strakes, large extrusions, fairings, or even weathervaning fairings.
GIVEN ALL THAT EFFORT TO ELIMINATE VORTEX INDUCED VIBRATIONS, IT'S AMAZING THAT ALMOST NO ONE HAS TRIED TO TAP THEIR POWER.
Back in 2004, I had a very good doctoral student, Kamal Raghavan, who wanted to work with me in the area of marine riser mechanics and VIV. I had moved on to other areas of offshore mechanics research more than ten years before and could not see my interests and expertise fitting in that area again. I also had a visiting faculty from Australia with whom we introduced some renewable energy material in the curriculum. I could not see fundamental research of interest to me in that area either.
One day, however, it just dawned on me that I could utilize VIV in generating renewable energy. First, Kamal and I looked into the literature for papers and patents; there was nothing meaningful. Then we looked for data in high-damping, high-Reynolds number VIV; there was nothing there, either. All of a sudden there was a double revelation: virgin territory in both fundamental research and product development. We did some testing and analysis for four to five months and then filed for a patent to cover flow induced motions (VIV, galloping, and so on) for energy harnessing. The patent was granted in February 2009. We filed for three more relevant patents since then. I also started a company, Vortex Hydro Energy, with great help from the Office of Technology Transfer of the University of Michigan in 2005.
We call this hydrokinetic technology the Vortex Induced Vibration for Aquatic Clean Energy converter, or VIVACE for short. As we envision it, a typical converter would consist of a three-dimensional array of long cylinders, each attached to a power take-off system which could be electrical or hydraulic depending on the size of the application. As water flows over the cylinders, the formation and shedding of vortices make the cylinders oscillate thus converting some of this horizontal hydrokinetic energy from the flow to mechanical energy. The attached PTO captures this energy and uses it to turn an electric generator.
Such a design has many advantages over conventional hydroelectric and hydrokinetic applications. Unlike typical hydroelectric power stations, the VIVACE converter doesn’t need to impound water behind a dam to create a head. Water is allowed to flow freely over the cylinders. And in contrast to many hydrokinetic technologies that have been proposed in the last decade, the converter does not use a turbine, making it safer for aquatic life that swim past or through it.
In addition, several proposals have been made and prototypes built for capturing energy not from the flow of water but from the waves on the surface (vertical hydrokinetic energy). The energy of waves that can be captured by devices, such as point absorbers (buoys), line absorbers (Pelamis), or surface absorbers (oscillating water columns) has small power density, particularly when the required spacing between such devices and the footprint volume are considered. Low power density is the Achilles heel of renewable energy technologies. VIVACE is a real three-dimensional absorber and has achieved remarkable power density in the lab. Equally important is that ocean currents and river flows are much more predictable and reliable than ocean waves, wind, or solar energy.
Another advantage of the VIVACE technology is its scalability. Not only can the technology be deployed in few watt-, kilowatt-, megawatt-, or even gigawatt-scale installations, but the science and engineering is basically the same at every scale.
For instance, the most basic unit of the VIVACE converter is a simple rigid cylinder mounted on springs and connected to a power take-off system via a transmission mechanism. As water flows past the cylinder, the primary response mode is transverse to the flow. In-line oscillations of smaller magnitude are also observed in VIV and these strengthen transverse oscillations. As the flow velocity increases, lock-in for a low mass ratio system (typical in water but not in wind) is reached when the vortex formation frequency is even as far as 60 percent from the oscillator's natural frequency. Vortex synchronization occurs over a broad range of the reduced velocities as well. The practical implication of it is that a VIVACE converter designed for a 3-knot flow can operate efficiently without any changes between 2 and 4 knots.
The aspect ratio of the cylinder (that is, length to diameter) is a crucial design parameter. Both the aspect ratio and the VIVs have a major impact on the correlation length of the flow along the cylinder. Obviously, higher correlation length induces higher overall forces on the oscillating cylinder; in theory, the correlation length becomes infinite when a cylinder undergoes VIV. In our marine hydrodynamics laboratory, we’ve tested models that have had aspect ratios of 7.2, 10.29, 12.0, and 14.4. But we have found that the optimal aspect length is around 20. Cylinders that are more slender may start to flex, and that would divert hydrokinetic energy from harnessable oscillatory energy to useless and potentially damaging vibratory energy.In addition, the basic geometry of the cylinder can be modified to ensure that VIVs develop over the likely range of Reynolds numbers the converter would experience. For instance, we can change the diameter of the cylinder, since the Reynolds number changes in linear proportion with the diameter. Or the surface of the cylinder can be textured; this alters the Reynolds number at which VIVs develop and can be very effective when the flow conditions are well known
And here is where a researcher's dream comes true. By mimicking fish kinematics we are answering vital questions, such as how to harness more hydrokinetic energy and how to make the technology fit even better into the environment. Every probing test that we have conducted in the lab has been so successful that we need to be especially disciplined to complete and publish our results for one test before we jump to the obvious next step.
Let me give you one example. Our first step was to start with VIV of smooth circular cylinders in the high-lift regime of fully turbulent shear layers. We were very successful achieving amplitudes of oscillation of nearly two diameters, for a total stroke of four diameters—nearly double a typical VIV test in water. With passive turbulence control and passive fish tails, we achieved all important breakthroughs we needed. We have, for instance, increased the range of synchronization. We have triggered back-to-back VIV and galloping. We have maintained more than two diameters of amplitude in the flow transition region from laminar to turbulent flow (for a Reynolds number between 300,000 and 500,000) where VIV would be otherwise suppressed. We reached three diameters of amplitude in galloping. We changed the distance between cylinders from the original design of eight diameters center-to-center to less than two, which we demonstrated in the 4-cylinder VIVACE converter.
These breakthroughs have made VIVACE a truly three-dimensional energy absorber as cylinders can be deployed in all three dimensions underwater. This flexibility reduces the gradient of energy absorption, making it possible to harness more energy while enabling us to design a technology that can be adapted to the specific environmental and marine life requirements of the location of deployment. Scaling the device now does not mean necessarily making it out of bigger cylinders to make it more financially viable, as with turbines and open propellers. A large VIVACE converter could be made of a great many small cylinders or fewer and bigger cylinders as determined in a trade off between environmental constraints versus complexity and maintenance constraints. (Smaller cylinders in the same space will provide higher power density but will introduce more complexity.) You can imagine the analogy of power density in the propulsion of many small fish or a big whale in the same fluid volume.
Keep in mind that a bluff body with a tail curving to collect a vortex and stretching to shed it is the natural way of moving in dense media. From tiny sperm to small fish to big whales, that is the natural shape in the marine environment. Alternating vortex shedding generates oscillatory lift as opposed to steady lift from lifting surfaces such as airplane and bird wings or sails and wind turbine blades.
WHAT MAKES THE CONCEPT EXCITING IS JUST HOW MUCH POWER CAN BE TAPPED FOR A GIVEN AMOUNT OF INFRASTRUCTURE. Wind power makes a very good comparison: both wind turbines and the VIVACE systems are machines that capture power from a fluid to generate electricity. In the case of wind turbines, the flow of air across the blades creates lift that turns the rotor. Wind machines are becoming increasingly large; one giant turbine may have a nameplate capacity of 5 megawatts. The world's largest wind farm, the Horse Hollow Wind Energy Center in Texas, generates 735.5 MW at peak power with 291 GE Energy 1.5 MW wind turbines and 130 Siemens 2.3 MW wind turbines spread over an area of 190 square kilometers. If you factor in the height of the turbines, the farm occupies a volume of almost 22 cubic kilometers.
But wind power starts with a marked disadvantage: compared to flowing water, wind has a low power density. And that shows up when comparing a large wind farm to the VIVACE converter. For instance, the rated wind speed for the wind turbines at the Horse Hollow wind farm is 12 meters per second. Because water is 830 times denser than air, the comparable water flow velocity is 1.3 meters a second. According to our experiments, a VIVACE converter working in water flowing at that speed has a power density of 185 watts per cubic meter. On a per volume basis, factoring lack of availability due to maintenance and low wind speed, the power density of the VIVACE converter is 14,600 times greater than that of the wind farm.
Indeed, in a 6-knot current, we calculate that the converter would have a power density of 1,980 watts per cubic meter. That is only an order of magnitude less than the power density of a diesel engine and still at a safe distance from the Betz limit. These numbers were measured for a single smooth cylinder in VIV, where spacing of eight diameters center-to-center is used in the denominator in calculating the power density. We are in the process of testing the 4-cylinder machine in two diameter spacing and galloping so we can measure the new power density.
Also, unlike wind turbines, which have to be sited carefully not only to find the most favorable wind conditions but also to avoid unfavorable interactions with neighbors and wildlife, a VIVACE converter can be placed in a wide variety of free-flowing water. The water must be deep enough to enable surface vessels to pass by safely, and to avoid interaction between the wake of the converter and both the surface and the riverbed or seafloor. But even this is not much of a constraint, since the modular nature of the VIVACE converter means that smaller machines could be set up in more restrictive spaces.
To provide a sense of the range over which VIVACE converters can be built, an early paper on the research provided the following scenarios. A small converter with 180 5-meter cylinders operating in 5 meters of water could generate 100 kW in a three-knot current. Such a converter would have a footprint of about 300 square meters and could power a neighborhood. To build a 10 MW converter—enough to power a small town—one would need only to build more cylinders and place them in deeper water.
The technology is such that one can envision a gigawatt-scale VIVACE converter, comparable to a nuclear power plant in output. And while the numbers seem large— more than 14,000 cylinders, each some 40 meters long operating in 60 meters of water—the operations shouldn’t be qualitatively different from the smallest version.
Of course, an energy resource, no matter how abundant, is not useful if it is too expensive to exploit. (That is a problem facing solar power, for example.) Our calculations indicate this should not be a stumbling block for the VIVACE converter. For one, the system is completely mechanical and requires no engineering breakthroughs to build. All scientific breakthroughs needed to have a successful system have been achieved; more are being researched to make the system even more powerful and environmentally compatible. The converters themselves are somewhat simple to erect using established offshore construction techniques. There are no sophisticated material requirements.
Cost estimates that were conducted a few years ago indicate that the per-watt capital costs of a 100 MW VIVACE converter would be about twice that of a new coal-fired power plant. But since the VIVACE converter would require no fuel and should have low variable operation and maintenance costs, in contrast to a fossil fuel plant, the cost per kilowatt of electricity would be within a penny or two of a new coal power plant. And the electricity would be cheaper than what could be generated by wind, solar, or natural gas.We are working toward proving these estimates in a real-world situation. The first phase of a testing campaign would be to build a small-scale converter in a nearby body of water. We have completed three weeks of fullsize testing in the towing tank of the Marine Hydrodynamics Laboratories of the University of Michigan and two more have been scheduled before the deployment target date in August 2010. The St. Clair River north of Port Huron is near the University of Michigan campus and would provide an ideal home for a 5-10 kW pilot facility. We have secured the state permit and we are waiting for the federal permit.
Even though hydrodynamics research has driven the successful steps in the first years, it is economics and manufacturing that will have the final say on bringing this technology to the world. We must continue keeping the VIVACE simple to manufacture, inexpensive, and simple to maintain in the often harsh marine environment. Presently, VIVACE is made out of off-the-shelf components. Most important, nature is very kind to flow induced motions, such as VIV, galloping, and flutter. There is tremendous leeway in cylinder spacing, location of the passive turbulence control, and passive fish tails. The more tests we do and the more we understand the underlying hydrodynamics, the more we develop the feeling that you cannot go wrong with these variables. The practical implication is that manufacturing tolerances are large and this will lead to lower maintenance requirements.
The process has been really gratifying. For instance, we met with 14 fisheries experts from the State of Michigan who were pleased to see an environmentally compatible device be considered for the St. Clair River. Now we are eager to observe the operation of the prototype in the river and to measure its performance and interaction with the environment over a three-month period. The lessons we learn from that experience will be used to design and build a larger test prototype to be installed in a major river or in ocean waters close to shore.
The VIVACE converter is a clean and simple machine that promises to provide cheap and plentiful electricity. There is certainly work that needs to be done to optimize the system, but there is no reason to doubt that the converter can operate as expected. Much the way that power from hydroelectric dams transformed the western United States during the middle part of the 20th Century, VIVACE has the potential to revolutionize the renewable power industry in this century.