Most wind generators face the windβand there’s a good cause for that. For the reason that Eighties, wind turbine builders have been utilizing what is known as the “Danish idea” for his or her designsβthree blades, positioned upwind (i.e., going through the wind), which can be engineered to maintain going through the wind to maximise power manufacturing.
This spring, a bunch of Nationwide Renewable Vitality Laboratory (NREL) researchers, in partnership with the Technical College of Denmark (DTU), sought to problem the Danish idea by answering the query: What occurs once we flip wind turbine blades round?
“There was for many years this ongoing dialogue within the analysis neighborhood about whether or not generators ought to all be upwind,” mentioned NREL wind power researcher (and challenge lead) Pietro Bortolotti. “The generators we began with within the Eighties, when this commonplace was set, had been very totally different than what we deploy immediately. They had been a lot smaller and had chunkier blades, chunkier towers.”
The objective of a latest experiment was to supply laborious knowledge about whether or not the upwind paradigm remains to be legitimate.
On the flip facet
To finish the experiment, NREL and DTU researchers actually flipped the rotor of a 1.5-megawatt analysis wind turbine that lives on NREL’s Flatirons Campus in Coloradoβtogether with the wind vane and nacelle (which homes the gearbox) atop the tower. They then rewired the generator to spin the opposite means. “Plus, one million different issues we needed to do to ensure we weren’t going to interrupt something,” Bortolotti mentioned.
After technicians spent days and nights atop aerial platforms to make the transition, attaching strain belts and putting in microphones at varied distances from the wind turbine, the workforce was in a position to begin capturing knowledge.
“We performed this experiment for 2 causes: to review the techno-economic feasibility of downwind generators and to make the most of this very new instrumentation from DTU that may measure the efficiency of strain distributions on a turbine blade,” Bortolotti mentioned.
Bortolotti was supported by quite a few different wind power researchers and technicians, together with Jason Roadman, Mark Iverson, Chris Ivanov, Jon Keller, and Derek Slaughter, who labored collectively to make the bodily transition and monitor the outcomes.
A techno-economic evaluation to reply decades-old questions
The experiment is likely one of the closing tasks being undertaken by the Huge Adaptive Rotor (BAR) challenge, funded by the U.S. Division of Vitality’s Wind Vitality Applied sciences Workplace. The objective of BAR is to assist the event of land-based wind turbine applied sciences and determine methods to cut back prices, particularly associated to creating lighter-weight blades with elevated flexibility.
“We checked out an entire spectrum of attainable candidates,” Bortolotti mentioned. “And we needed to analyze whether or not going downwind was a chance to additional improve flexibility and reduce prices.”
At first look, a number of facets of the downwind state of affairs appear promising. First, the blades in a downwind turbine are naturally pushed away from the tower by the wind, so there is a chance to design lighter and extra versatile blades that don’t should be stiff sufficient to keep away from the tower. And the lighter the blade, the cheaper they’re to make.
Second, the lean of the wind turbine’s rotor redirects the turbine’s wake towards the bottom, so it is much less prone to intervene with different generators in a farm. And a few research present that downwind rotors improve the ability efficiency of huge wind farms.
However there’s one main situation that comes together with the downwind state of affairs. The truth that, when a blade passes behind the tower, it’s, for a break up second, sheltered from the wind. This adjustments the strain on the blade, producing an oscillation, or fluctuation, that causes fatigue on the blade itself and generates an audible “thump.” And that thump occurs each time one of many blades passes behind the towerβin different phrases, typically.
Over the course of 11 hours of knowledge assortment, the workforce did certainly seize the thump, at audible-enough ranges that might impression communities inside earshot.
Blades underneath strain
The oscillation didn’t simply generate a noticeable sound. It additionally generated strain on the wind turbineβstrain that, through the experiment, was measured through three particular belts that researchers from DTU affixed to one of many blades and the turbine tower.
“The belts are very new instrumentation that DTU has been engaged on,” mentioned Bortolotti, who added that the belt gadgets may help researchers decide the efficiency of the rotor, whether or not it’s upwind or downwind. “We used this experiment to assist DTU push forward this know-how, as a result of it is a very precious and distinctive machine that we hope to be utilizing once more sooner or later for upwind rotors.”
Throughout the experiment, the belts measured the strain distribution alongside the rotor rotation, giving the workforce a exact take a look at the impact of the behind-the-tower oscillation on the blade. These measurements will assist present key insights into the elevated fatigue loading skilled by downwind rotors.
One different key good thing about utilizing the belts within the research was the information they produced. These knowledge can present real-world validation of aeroelastic numerical fashions the workforce developed with NREL’s OpenFAST device.
“At NREL, we develop lots of instruments for the numerical prediction of hundreds, efficiency of wind generators, wind farms, and so forth,” mentioned Bortolotti, who added that gadgets just like the strain belts are crucial for validating these numerical instruments.
The workforce plans to make use of the strain belts in future experiments on extra typical rotors. “Due to the BAR challenge, we at the moment are assured that the belts are a viable method to generate precious experimental datasets that can assist us enhance our understanding of wind generators,” Bortolotti mentioned.
Drawing conclusions
Whereas the formal outcomes of the research won’t be revealed till later in 2024, the preliminary outcomes are sufficient to conclude that the potential advantages of downwind operations don’t outweigh the drawbacks.
Bortolotti factors out that the workforce knew that whereas the noise and blade fatigue considerations had been current with downwind land-based generators, the impression was unclear.
“The analysis neighborhood needed to depend on datasets from the Eighties and anecdotal proof, which in science should not ample,” Bortolotti mentioned. “We are able to now confidently say that the subsequent technology of land-based wind generators will likely be bigger and extra versatile, however the Danish idea will proceed to be the dominant know-how.”
In the end, the experiment was a significant accomplishmentβand never only for the truth that it collected very important knowledge on the efficiency of a utility-scale downwind turbine whereas validating modeling and simulation instruments.
“We did one thing that no person thought that we might do, which was to spin a fairly large wind turbine outfitted with a big set of instrumentation that recorded a broad spectrum of aeroacoustics, hundreds, and strain knowledge,” Bortolotti mentioned. “This 1.5-megawatt turbine is small in comparison with trendy installations, however nonetheless, it is a large beast, and spinning it downwind safely with out breaking a single bolt was an enormous achievement.”
