In a Catalan laboratory, Spanish researchers have turned an old engineering nuisance into a fresh source of clean energy, using a bare tube, no blades, and almost no moving parts under water.
A cylinder that rocks instead of a turbine that spins
The heart of the concept looks almost disappointingly basic: a cylinder suspended in flowing water, held on an arm like a kind of underwater pendulum. No propeller. No blades. No housing. Just a tube willing to be pushed around by the current.
When water hits a cylindrical object, it does not flow around it smoothly. Instead, tiny rotating eddies appear in its wake. These eddies, called vortices, form alternately on each side of the cylinder. They pull and push rhythmically, nudging the tube first one way, then the other.
This “vortex-induced” rocking, once feared by engineers for damaging bridges and pipelines, is now being turned into electricity.
At the Universitat Rovira i Virgili in Catalonia, a team led by fluid-structure specialists has built a device that captures this oscillation. The cylinder is connected to an axis. As the tube swings, the axis moves, and that motion is carried up to dry land, where a mechanical system and a generator convert it into electrical power.
Only the cylinder stays underwater. Everything delicate and expensive remains safely above the surface, on a floating platform or on the riverbank.
From engineering nightmare to renewable resource
The physics behind the idea are well-known in industry. Engineers talk about “vortex shedding”: when a fluid flows past a cylindrical structure, alternating vortices create fluctuating forces. Bridges, chimneys, offshore rigs and pipelines can start to vibrate strongly under these forces.
Usually, this is bad news. Persistent vibration tires materials, opens cracks and shortens the life of infrastructure. Engineers spend money on dampers, reinforcements and redesigns to stop it.
The Spanish team flipped the logic. If these vibrations are powerful enough to damage steel, they reasoned, then they can also perform useful work.
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What used to be a structural threat is treated as a free mechanical input, already provided by the river or the tide.
Instead of fighting the movement, the system encourages it, within controlled limits. The axis connected to the cylinder feeds its motion to an energy conversion system outside the water. In their experiments, the researchers used an electromagnetic brake to imitate different electrical loads and study how the cylinder responded.
Why walk away from underwater turbines
Most marine energy projects today rely on underwater turbines that look like wind turbines anchored to the seabed. Their performance is not bad: they can convert around 25–35% of the kinetic energy in the water that passes through their swept area into electricity.
The problems begin when these machines meet real oceans. Saltwater attacks bearings, seals and shafts. Sand erodes surfaces. Algae and shellfish grow on blades, changing their shape and trimming their output. Every maintenance mission needs divers, specialist vessels and careful planning around tides and weather.
Those visits are expensive. For small or remote projects, they can make the business case collapse.
The rocking cylinder aims at a different trade-off. It accepts a lower energy conversion rate in exchange for radical simplicity.
- No fast-spinning blades under water
- No submerged gearboxes or complex seals
- Minimal underwater mechanics, mostly just a sturdy tube and supports
- Critical components placed in air, where technicians can reach them
This shift could matter most in locations where long-term reliability and cheap maintenance count more than peak performance on paper.
What the lab results actually show
The team tested its prototype in a hydraulic channel at the university’s fluid-structure interaction laboratory. Controlled flows were sent past the suspended cylinder while sensors measured its angle of oscillation and frequency.
An electromagnetic brake on the axis acted as an artificial electrical load. By adjusting the brake, the researchers could simulate how a real generator would pull against the motion and see how much mechanical power could be extracted without killing the oscillation.
The measurements point to a power coefficient of around 15%. In plain terms, the device captures about 15% of the kinetic energy carried by the water that flows through the cross-sectional area swept by the cylinder.
The 15% figure trails classic turbines, but the entire system is lighter, simpler and far easier to service.
In the niche of vibration-based energy harvesters, 15% is a solid result. The most appealing aspect for investors and operators, though, lies in the economics of installation and upkeep rather than in a headline efficiency number.
A compact system for difficult locations
The Catalan cylinder is not designed to compete head-on with massive tidal farms in prime locations. Its sweet spot lies elsewhere: places that are hard to access, unpredictable or simply not worth the trouble for heavy underwater machinery.
Potential use cases
- Secondary tidal channels with strong but irregular currents
- Free-flowing rivers without dams, where building major civil works is unrealistic
- Estuaries and harbour areas that need small but steady local power
- Remote coastal communities where diesel is costly and supply chains fragile
Each individual unit is relatively compact. Several cylinders can be arranged in series along a current, or in a grid pattern, to build up to higher power levels. The image is closer to a patch of underwater reeds than to a classic turbine farm: multiple thin elements, each waving in the flow.
The basic principle is not restricted to water either. Air flowing across a cylinder can trigger the same alternating vortices. With appropriate tuning and structural design, similar devices could be used in windy areas, blurring the boundary between hydro and wind generation.
Key figures at a glance
| Parameter | Rocking-cylinder system | Typical marine turbine |
|---|---|---|
| Energy capture (power coefficient) | ~15% | ~25–35% |
| Main moving part under water | Single cylinder, oscillating | Multiple blades, rotating |
| Location of delicate components | Above water | Mostly submerged |
| Maintenance needs | Low, easy access | High, diver and vessel support |
| Target sites | Remote, harsh, low-maintenance | Prime tidal and current resources |
What “vortex-induced vibrations” really means
The technical term used in the scientific paper behind this work is “vortex-induced vibrations” (VIV). The idea sounds abstract, but the effect is familiar. When you see a flag fluttering or hear a cable humming in the wind, vortices are at play.
As fluid flows past an obstacle, rotating pockets of water or air peel off one side, then the other, at a regular frequency. If that frequency matches the natural swinging frequency of the object, the motion builds up. For bridges and towers, that can be dangerous. For this device, resonance is the whole point.
The invention tunes the cylinder and its support so that the current “plays” it like an instrument, coaxing a sustained oscillation.
One key challenge is to keep the vibration large enough to generate useful power without letting it grow so strong that it stresses the structure. That balance depends on cylinder size, mass, mounting stiffness and flow speed. The lab tests give early answers, but real rivers and inlets will add layers of complexity.
Opportunities, risks and future scenarios
There are clear advantages for small-scale, distributed energy. A network of rocking cylinders in a tidal creek could power navigation lights, environmental sensors, fish farms or small desalination units. None of these needs megawatts; they need dependable watts with minimal human intervention.
In developing regions, river-based versions could back up solar installations. When clouds roll in or night falls, flowing water would keep lights and communication equipment running. Because installation would avoid big dams or deep foundations, ecological disruption could stay limited.
There are still questions. Long-term durability in gritty, debris-filled rivers must be tested. Floating tree branches or trash might strike the cylinders. Designers will need to consider protective frames or self-cleaning layouts. Noise and vibration impacts on fish and marine mammals also require study, though the lack of spinning blades may prove helpful.
The idea could also combine with other technologies. A floating platform could carry solar panels on top and vortex-harvesting cylinders below, sharing moorings and power electronics. In stormy conditions, when waves and clouds reduce solar output, currents often intensify, letting the underwater system pick up some of the slack.
For readers curious about the scientific side, the research appears in the journal Journal of Fluids and Structures under the title “Energy harvesting from vortex-induced vibrations using a pendulum”. The work places this Spanish invention within a broader effort to turn every kind of motion in nature, even those once seen as threats, into quiet, local, and dependable power sources.