Wind researchers conduct extensive wind turbine validation study

Most wind turbines face into the wind—and for good reason. Since the 1980s, wind turbine designers have used the so-called “Danish concept” in their designs—three blades that are oriented in the direction of the wind (i.e., face into the wind), and are designed to face into the wind, maximizing energy production.

This spring, a group of researchers from the National Renewable Energy Laboratory (NREL) in collaboration with the Technical University of Denmark (DTU) set out to challenge the Danish concept by answering the question: What happens when we turn the blades of wind turbines?

“There’s been a continuing debate in the scientific community for decades about whether all turbines should be upwind,” says Pietro Bortolotti, an NREL wind scientist (and project manager). “The turbines we started with in the 1980s, when that standard was set, were very different from the ones we’re deploying today. They were much smaller, with thicker blades, thicker towers.”

The goal of a recent experiment was to obtain hard data on whether the upwind wind paradigm is still valid.

On the other hand

To complete the experiment, NREL and DTU scientists literally turned the rotor of a 1.5-megawatt wind turbine located on NREL’s Flatirons campus in Colorado—complete with the wind vane and the nacelle (which houses the gearbox) at the top of the tower. Then they rewired the generator to turn the other way. “There were a million other things we had to do to make sure we didn’t mess anything up,” Bortolotti said.

After technicians spent days and nights atop suspended platforms, attaching pressure belts and installing microphones at various distances from the wind turbine, the team could begin collecting data.

“We did this experiment for two reasons: to investigate the technical and economic feasibility of wind turbines and to take advantage of this new DTU instrumentation, which can measure the pressure distribution performance on a turbine blade,” Bortolotti said.

Bortolotti was assisted by many other wind energy researchers and technicians, including Jason Roadman, Mark Iverson, Chris Ivanov, Jon Keller, and Derek Slaughter, who worked together to conduct the physical transformation and monitor the results.

Technical and economic analysis that will answer questions that have been asked for decades

The experiment is one of the final projects under the Big Adaptive Rotor (BAR) project, funded by the U.S. Department of Energy’s Wind Energy Technology Office. The BAR aims to support the development of onshore wind turbine technology and identify ways to reduce costs, particularly those related to producing lighter, more flexible blades.

“We looked at a whole range of possible candidates,” Bortolotti said. “And we wanted to explore whether traveling downwind was an opportunity to further increase flexibility and reduce costs.”

At first glance, several aspects of the downwind scenario seem promising. First, the blades in a downwind turbine are naturally pushed away from the tower by the wind, so there is an opportunity to design lighter and more flexible blades that do not have to be stiff enough to stay away from the tower. And the lighter the blade, the cheaper it is to make.

Second, the tilt of a wind turbine rotor redirects the turbine’s wake toward the ground, so it’s less likely to collide with other turbines in the farm. And some studies show that downwind rotors increase the energy output of large wind farms.

But there is one major problem that comes with the wind scenario. The fact that as the blade passes behind the tower, it is shielded from the wind for a fraction of a second. This changes the pressure on the blade, causing an oscillation or fluctuation that fatigues the blade itself and generates an audible “thump.” And that thump occurs every time one of the blades passes behind the tower—in other words, often.

Over the course of 11 hours of data collection, the team managed to record the sound of an impact that was loud enough to have had an impact on communities within earshot.

Blades under pressure

The vibrations not only generated noticeable sound. They also generated pressure on the wind turbine—pressure that was measured during the experiment using three special straps that the DTU scientists attached to one of the turbine blades and the tower.

“The belts are very new devices that DTU is working on,” said Bortolotti, who added that the belt devices can help researchers determine the performance of a rotor, whether it is upwind or downwind. “We used this experiment to help DTU develop this technology because it is a very valuable and unique device that we hope to reuse in the future for windward rotors.”

During the experiment, the belts measured the pressure distribution along the rotor’s rotation, giving the team an accurate picture of the effect of oscillations behind the tower on the blade. These measurements will help provide key insights into the increased fatigue loading experienced by downwind rotors.

Another key benefit of using the belts in the study was the data they produced. These data can provide real-world validation of the aeroelastic numerical models the team developed using NREL’s OpenFAST tool.

“At NREL, we develop a lot of tools to numerically predict the loads, the performance of wind turbines, wind farms, and so on,” said Bortolotti, who added that devices such as pressure belts are key to validating these numerical tools.

The team plans to use the pressure belts in future experiments on more conventional rotors. “Thanks to the BAR project, we are now convinced that the belts are a viable way to generate valuable experimental data sets that will help us better understand wind turbines,” Bortolotti said.

Draw conclusions

While formal study results will not be published until 2024, preliminary results suggest that the potential benefits of downwind operations do not outweigh the disadvantages.

Bortolotti stresses that the team knew that while there were issues with noise and blade fatigue for onshore turbines downwind, their impact was not clear.

“The research community had to rely on data sets from the 1980s and anecdotal evidence, which is not enough in science,” Bortolotti said. “We can now confidently say that the next generation of onshore wind turbines will be larger and more flexible, but the Danish concept will continue to be the dominant technology.”

Ultimately, the experiment was a major achievement—and not just because it allowed them to collect key data on the operation of a utility-scale wind turbine while also validating their modeling and simulation tools.

“We did something that no one thought we could do, which was spin up a fairly large wind turbine with a large set of instruments that recorded a wide range of aeroacoustic, load and pressure data,” Bortolotti said. “This 1.5-megawatt turbine is small compared to modern installations, but it’s still a big beast, and to spin it up safely into the wind without breaking a single bolt was a great achievement.”