Inside a Spanish breakwater 16 turbines keep spinning the same way no matter which direction the sea drives the air through them, and that quirk is what turns ocean swell into grid power
On a breakwater on the Basque coast, the Bay of Biscay throws ocean swell against the concrete.
Behind the wall sit sixteen sealed chambers, each open to the sea below the waterline.
With every wave the trapped air is shoved upward, then sucked back down, a violent draft that reverses several times a minute through a narrow throat.
Other bidirectional concepts, among them an underwater lung that pushes air through a turbine, chase the same bidirectional payoff while dodging that stall.
In that throat spins a single rotor, and it does not reverse with the air. It holds one direction, wave after wave, feeding a generator wired to the grid.
Any conventional turbine dropped into that slot would stall or run backward the instant the flow reversed.
So how does a rotor keep turning one way while the air through it keeps switching direction?
Why the rotor never reverses
The trick is the blade shape. The rotor uses symmetrical airfoils, the same curve on both faces, mounted edge on to the airflow.
That symmetry is the whole secret.
When air rushes up through the blades, each meets the stream at an angle of attack and generates lift with a small forward push that turns the rotor.
When the air reverses, the blade meets the flow at the mirror angle, and the lift tips forward in the same rotational direction again. Push or pull, the torque never changes sign.
The payoff is what the design leaves out. Because the rotor ignores flow direction, the plant needs nothing to rectify the air, none of the delicate check valves that would otherwise slam shut and open on every wave.
A column that breathes
The chamber feeding that rotor is an oscillating water column, and it is deliberately crude: a hollow concrete box, open to the sea below the surface and sealed but for a small vent at the top.
The water inside rises and falls with each wave.
That surface acts as a piston, compressing the air on the upstroke and pulling a partial vacuum on the downstroke, so the column breathes in and out through the throat. The harder the sea heaves, the harder it drives the air.
The intent was to sidestep mechanical complexity. Earlier wave machines drowned in moving parts and valves that seawater wrecked, so a turbine indifferent to flow direction was the piece that made the breathing column practical.
What Mutriku actually delivers
The idea became hardware on the Basque coast. Professor Alan Wells set out the rotor at Queen’s University Belfast in the late 1970s, and in 2011 Mutriku built sixteen of them into a 330 foot section of its harbor breakwater.
The numbers are modest.
Each turbine is rated at 18.5 kW, for a combined 296 kW, and it was the first breakwater anywhere to run a whole bank of wave turbines at once. By the end of 2023 it had passed 3 GWh of lifetime output, enough in a good year for about 100 homes.
It was not the first of its kind on the grid. That belongs to a 500 kW unit built into the shore of Scotland’s Islay in 2000, the first commercial wave power station to feed a national network, and it ran on the same bidirectional rotor.
The efficiency it trades away
Simplicity has a price, and here it is efficiency. A symmetrical blade is a compromise, converting the pulsing airflow to shaft power less effectively than a purpose shaped rotor in steady one way flow.
The rotor also has a temper.
Driven too hard at a wave’s peak, the blades exceed their angle of attack and stall, losing lift exactly when the energy is highest, and the rotor is a poor self starter, often spun up by its own generator before it bites.
Other bidirectional concepts, among them an underwater lung that pushes air through a turbine, chase the same bidirectional payoff while dodging that stall.
Measured end to end, the conversion stays low. At Mutriku only about 30 percent of the pneumatic energy in the breathing column reaches the wires, a figure that leaves a great deal of swell energy on the table, as reviews of the rotor design lay out.
Where the idea goes next
The response is not to abandon the rotor but to sharpen it. Mutriku is lining up replacement turbines meant to roughly double the conversion toward 50 percent, a retrofit budgeted at several million euros.
Designers are also hedging the concept.
Variable pitch blades, paired rotors and impulse turbines all keep the advantage of ignoring flow direction while cutting stall losses, and some drop the spinning airfoil altogether, as a blade free design proposes for wind.
Each chases the same goal, a machine that survives the sea while wasting less of the incoming swell.
What keeps the breathing column alive is durability. With one moving assembly, no gearbox in the water and nothing that must open and close on every wave, it rides out storms that wreck more elegant machines, which is why grid operators keep funding it, as project records for the Basque plant document.
The strangest turbine in the water may also be the hardest to break.
Carlos is an engineer with strong expertise in technical and industrial topics. He previously worked at international companies such as Siemens and is multilingual.
