Could this turbine end the reign of the three-bladed machine?

First tests of Vestas’ eye-catching four-rotored concept show higher output than same-rated conventional turbine

Multi-rotor wind turbines could one day be a common sight – offshore as well as on land – following a breakthrough testing programme by Danish OEM Vestas of an experimental four-headed design.

Erected in 2016 at the DTU Wind Energy’s Risø campus west of Copenhagen, the eye-catching prototype, made up of a quartet of V29-225kW turbines, has defied the sceptics by running with a 1.5% higher annual energy production (AEP) than a single-rotor 900kW machine.

The power boost comes from an unlikely source: wind wake. During the trials at the test site, just off the Roskilde fjord in the centre of Denmark, the mixing of the wake created by the four individual turbines was found to drive accelerated flow and so higher production – and as the same force also causes faster ‘wake recovery’, made for more stable levels of generation too.

“From the first simulations we did we could see the [wake] deficit behind the rotors in the multi-rotor design when compared to a single [one-rotor] turbine with the same swept area, so we had a good idea of the qualitative aspect of this,” said Erik Miranda, director for mechanical, loads and control technologies at Vestas.

“But several simulations with various CFD [computational fluid dynamics] tools and test results have given us a huge volume of quantitative data proving the faster [wake] recovery of the multi-rotored concept.”

The original thinking behind the multi-headed design – which features two independent yaw systems at hub heights of 29-metres and 59.5-metres, and bristles with a host of measurement sensors as well as a Lidar system mounted atop the support structure to monitor wind fields – was to enlarge the swept area that is generating energy without increasing the weight of the turbine correspondingly.

While the four rotors on the R&D unit in total cover roughly the same swept area as a single-rotor turbine of the same height and power output, the weight of the unit could be reduced roughly “by half”.

“The wind speeds on the site are quite low and have a low-turbulence area opening toward the fjord. Overall we are seeing this higher AEP,” said Miranda.

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Also, because of the smaller components making up the turbine, the multi-rotor concept would make it possible to have higher volume manufacturing and easier transport and installation at site, when compared to a larger model.

“This was one of our earliest motivations: to see if we could do something else more than just scaling up the rotor size [to get more power from a single turbine],” said Miranda. “Because we are running into more and more challenges [in finding economic wind sites].”

Using “known, proven technology” in a multi-rotor machine he noted, is “another advantage of the design”.

Along with extensive wake, power and noise measurement studies, the test programme at Risø of the four-rotor concept explored potentially show-stopping load increases on the rotor blades, with indications there was less than 3% in flap root bending.

Because offshore wind tends to be less turbulent than onshore, the four-rotor concept could produce more power at sea, said Miranda. “In general where you have low turbulence, the wake recovery effect will be stronger and you will produce more power than single rotor turbines.

“But there are many other aspects to consider before concluding if multi-rotor is the right way forward offshore as well as onshore.”

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