The device consists of a cylindrical tube suspended from an axis so that it behaves like a pendulum when exposed to flow. As water passes around the cylinder, it sheds vortices that cause the tube to oscillate, and those oscillations are transmitted through the shaft to power takeoff components located above the waterline. Only the cylinder needs to remain underwater, while the shaft, transmissions and eventual generator can sit in air, simplifying access and maintenance. Huera designed and tested the system in a water channel at the Fluid Structure Interaction Laboratory at URV.
Conventional approaches to harnessing ocean current energy rely mainly on axial flow or cross flow turbines, underwater analogues of wind turbines. In theory these turbines can exceed 50 percent power conversion efficiency, but in practice they typically capture only about 25 to 35 percent of the kinetic energy passing through the swept area. They also require complex underwater structures with multiple moving parts exposed to corrosion and biofouling, and they demand regular, costly maintenance. Commercial scale tidal turbine farms have yet to move beyond prototype and pilot deployments.
The pendulum based system takes a different route by eliminating rotating blades in favor of a vibrating cylinder. In laboratory tests, a scaled cylinder mounted on air bearings in a controlled water channel allowed researchers to measure the oscillation angle and apply an electromagnetic brake to the shaft. This setup made it possible to quantify the mechanical power available as the cylinder responded to vortex induced vibrations. The experiments produced power coefficients of around 15 percent, in line with previous cylinder vibration based energy harvesters.
According to Huera, this level of efficiency is about half of what a well designed turbine can achieve, but the tradeoff is a much simpler and more compact structure. In his words, "at the end of the day, it's just a tube hanging from an axle." All complex machinery, including generators, transmissions and control systems, can be located on a floating platform or other support at the surface. Underwater, only a robust structural cylinder is required, reducing installation challenges and exposure to harsh conditions.
This simplicity could make the technology attractive in settings where conventional turbines are difficult to install, operate or maintain. The concept targets tidal currents in the first instance, where water flows continuously and predictably through constricted channels or coastal sites. The same principle could also extend to rivers with adequate flow velocity and suitable cross sections, without the need for dams, weirs or diversion channels that alter ecosystems. In addition, the basic idea of extracting energy from flow induced oscillations might be adapted to moving air, opening possibilities for wind applications.
The research joins a broader effort to understand and exploit flow induced vibrations, which engineers have long regarded as a hazard rather than a resource. Large offshore structures such as pipelines linking oil platforms to the seabed can experience vortex induced vibrations that cause fatigue and threaten structural integrity. Huera has previously worked on systems to suppress these unwanted motions and holds a European patent aimed at mitigating the associated risks. The new study turns the same physical phenomenon into a potential source of renewable energy.
The article focuses on the hydrodynamic behavior of the pendulum system in a water channel and on quantifying the mechanical power available at the shaft. It does not present a full scale generator design or a cost analysis. Huera notes that the team has so far described the system theoretically and validated it through laboratory experiments, but has not yet built large prototypes or conducted detailed economic assessments.
Future work will concentrate on optimizing how power is extracted from the vibrating cylinder and on broadening the range of conditions under which the device can operate efficiently. Strategies include adjusting the electromagnetic brake torque as a function of shaft position or hydrodynamic loading and refining control schemes to keep the system tuned to prevailing flow speeds. Researchers also plan to study the interactions among multiple devices placed in arrays so they can evaluate how to maximize energy yield per unit area in real marine or river environments.
Research Report:Energy harvesting from vortex-induced vibrations using a pendulum
Related Links
School of Engineering at Universitat Rovira i Virgili
Water News - Science, Technology and Politics
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