Hydrokinetic technology targets low-knot water currents that are off-limits to conventional devices.
In 1940, the Tacoma Narrows Bridge in Washington State collapsed due to the effects of Vortex Induced Vibration (VIV), where vortices are formed and shed on the downstream side of bluff bodies or rounded objects in a fluid current.
The vortex shedding alternated from one side of the bridge to the other, creating a pressure imbalance that resulted in an oscillatory lift, leading to the total destruction of the bridge.
Now, University of Michigan professor Michael Bernitsas has invented a novel hydrokinetic power-generating system that takes advantage of the same VIV phenomenon, allowing it to harness the hydrokinetic energy of river and ocean currents moving as slow as 2 to 3 knots previously off limits to conventional turbines, which only work in rivers and oceans with water currents greater than 4 knots.
Having demonstrated that the concept works in the laboratory, the Vortex Induced Vibration for Aquatic Clean Energy (VIVACE) converter is currently being commercialised by University of Michigan spin-out Vortex Hydro Energy. Bernitsas is heading up further development efforts as its chief technology officer.
Unlike most existing tidal-energy systems, the VIVACE converter does not use turbines, propellers, or dams. Instead, the system converts the horizontal hydrokinetic energy of water currents into mechanical energy through the movement of several horizontally placed cylinders, supported in a stream of water by vertical struts and springs.
As the water passes over the cylinders, its flow is slowed though contact with their surfaces, creating boundary layers. These then separate from the cylinders and vortices are shed from them, changing the pressure distribution along the surfaces of the cylinders and causing them to move vertically. The vertical motion of the supporting struts can then be converted to electricity through rotary power generators.
Although Bernitsas cites VIV as one of the means by which the system generates power partly due to the modifications he has made to the geometric symmetry of the cylinders and their surface roughness the system also takes advantage of other hydrodynamic phenomena, including several forms of galloping, and buffeting.
“Water turbines need 5 to 7 knots to be financially viable – we’ve made our system work at 1 knot”
MICHAEL BERNITSAS, VORTEX HYDRO
The galloping effect is created due to the aerodynamic instability of the cylinders’ cross-section in the water flow. These oscillations occur above a certain critical velocity and are also in the direction perpendicular to the direction of the flow of the water.
Even though, in galloping mode, vortices shed from both sides of the cylinders, they are no longer the hydrodynamic mechanism driving the cylinders. Instead, the movement of the two shear layers of water on either side of the cylinders creates a pressure difference that causes them to oscillate.
When several cylinders are in close proximity, a proximity galloping effect is created in the forward-most cylinder and an interference galloping effect is produced in the one behind it. The wake from the upstream cylinder can also drive the rear cylinder in the system, due to another hydrodynamic effect called wake galloping.
When the motion of the water becomes vigorous, the oscillating shear layers shed and separate from the cylinders, causing buffeting an effect where an unsteady flow of water beats on the structure of the cylinder, causing an even more pronounced response.
The power output from the system is dependent on the length and diameter of the cylinders. But, despite the fact that the various flow-induced effects vary as a result of the distance that the cylinders in the system are separated, the power produced by the system is relatively insensitive to that distance. This is an advantage, as the system can be optimised to create as much disturbance in the tidal flow as is necessary, taking any environmental considerations into account.
Bernitsas explained that his system differs dramatically from many other tidal devices. An oscillating buoy, for example, only provides power efficiently when its frequency of excitation is close to its natural frequency of oscillation. This system, however, is inherently non-linear and can effectively provide power over a range of water speeds, due to the complex varying hydrodynamic interactions that take place between the cylinders and the flowing water.
Because of that, the system has a range through which it synchronises itself to the water speed its natural frequency plus or minus 50-60 per cent. This means that Bernitsas’ team has been able to design a prototype system to work at 2.6 knots, and between 2 and 4 knots.
Bernitsas chose to configure his system to work at this speed as it provided a means to compare its effectiveness with that of a wind turbine the water speed is the equivalent to an air speed of 12m/sec. At that speed, all four cylinders oscillate at their maximum stroke and the effect is like watching a four-cylinder reciprocating engine where the fuel and the only connections between the two cylinders is water. The power density achieved is about 15,000 times that of a wind farm.
’While water turbines need an average of 5 to 7 knots to be financially viable and hardly deliver any energy when the speed is slower than 4 knots, we have made our system work at 1 knot and higher,’ said Bernitsas.
Bernitsas has already demonstrated the effectiveness of a prototype small-scale four-cylinder converter in the Low Turbulence Free Surface Water Channel at his Marine Renewable Energy Laboratory at the University of Michigan. He has also developed a larger device with 2.7m-long, 25cm-diameter cylinders that he is testing this year in the 32-mile-long Detroit River.
This could be used for small systems that produce many hundreds of watts and large ones that create thousands
As VIVACE is scaleable, it could be used to produce small-scale systems that produce hundreds of watts and larger systems that generate thousands.
Bernitsas estimates that the energy produced by such tidal systems would cost about $0.055/kWh, which would certainly make them competitive with other forms of renewable energy in use today.