Tilting of windmills could lead to performance gains, study finds

In a new paper published in the Proceedings of the National Academy of Sciences, Stanford University researchers have detailed a subtle, but effective technique for boosting the power output from wind farms by “steering” the wake created as the wind passes through a wind farm.

“To meet global targets for renewable energy generation, we need to find ways to generate a lot more energy from existing wind farms,” Stanford professor of civil and environmental engineering John Dabiri said.

“The traditional focus has been on the performance of individual turbines in a wind farm, but we need to instead start thinking about the farm as a whole, and not just as the sum of its parts.”

The researchers found that by tilting the wind turbines at a slight angle to the direction of the wind, it was possible to steer the wake of a wind turbine away from other turbines that are located downwind.

“To increase wind farm power production, we developed a wake steering control scheme. This approach maximizes the power of a wind farm through yaw misalignment that deflects wakes away from downstream turbines,” the paper says.

The wake from wind turbines disrupts of the flow of wind and can result in significantly lower power output from wind turbines located downwind.

The Stanford researchers developed a method of producing faster calculations for optimal “misalignment angles”, allowing for near real-time optimisation of the orientation of wind turbine blades, and tested the technique at an operational wind farm in Alberta, Canada.

The researchers collaborated with wind turbine operator TransAlta Renewables to test the methodology, finding that it was possible to boost output from the wind farm by up to 47 per cent during times of low wind conditions.

In normal wind conditions, the tilting technique was able to boost output by between and 7 and 13 per cent.

The results were highest for particular wind directions and provides the greatest benefit when turbines are aligned downwind of one another.

“Through wake steering, the front turbine produced less power as we expected,” researcher and study lead author Michael Howland said.

“But we found that because of decreased wake effects, the downstream turbines generated significantly more power.”

The researchers were also hopeful that by avoiding placing turbines in situations where they were operating in turbulent winds, the tilting technique could potentially reduce the amount of fatigue experienced by the turbines.

By providing less interrupted wind flows to wind turbines, the researchers were also able to reduce the level of variability of wind farm generation, improving the reliability and consistency of output from the wind. The researchers found that the level of variability in wind farm output could be reduced by as much as 72 per cent.

“The first question that a lot of operators ask us is how this will affect the long-term structural health of their turbines,” author Dabiri added.

“We’re working on pinpointing the exact effects, but so far we have seen that you can actually decrease mechanical fatigue through wake steering.”

While the benefits of the ‘wake steering’ technique are likely to be small for individual wind farms and is dependent on the right wind conditions, the researchers hope that the adoption of the method globally could add up to significant energy gains.

“Our model is essentially plug-and-play because it can use the site-specific data on wind farm performance,” Howland said.

“Different farm locations will be able to use the model and continuously adjust their turbine angles based on wind conditions.”

“If we can get to the point where we can deploy this strategy on a large-scale for long periods of time, we can potentially optimize aerodynamics, power production and even land-use for wind farms everywhere,” Dabiri added.