Researchers use wake steering to optimise wind farm

A study conducted by researchers from Stanford University has found that the positioning of wind turbines is critical to optimising the amount of power generated by wind farms.

Although solitary wind turbines produce the most power when pointing directly into the wind, the situation changes when several turbines are installed close together on a wind farm. When tightly packed turbines face the wind, wakes created by one turbine can reduce the energy output of neighbouring ones.

A recent study has revealed that wake steering — pointing turbines slightly away from oncoming wind — can reduce interference and improve the quantity and quality of power from wind farms.

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

“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.”

Turbine wakes can reduce the efficiency of downwind generators by more than 40%. Previous research has used computer simulations to show that misaligning turbines from the prevailing winds could raise production of downstream turbines. However, showing this on a real wind farm has been hindered by the challenge of finding a wind farm willing to halt normal operations to allow the best angles for the turbines to be determined.

The Stanford team developed a faster way to calculate the optimal misalignment angles for turbines, describing their findings in a study published in Proceedings of the National Academy of Sciences.

Then, the researchers tested their calculations on a wind farm in Alberta, Canada, collaborating with operator TransAlta Renewables. By realigning the wind turbines, the overall power output of the farm increased by up to 47% in low wind speeds — depending on the angle of the turbines — and by 7–13% in average wind speeds. Wake steering also reduced the ebbs and flows of power that are normally a challenge with wind power.

“Through wake steering, the front turbine produced less power as we expected,” said mechanical engineering PhD student Michael Howland, lead author on the study. “But we found that because of decreased wake effects, the downstream turbines generated significantly more power.”

Variability

Variable output by wind farms makes managing the grid difficult for two reasons...

One is the need for backup power supplies, like natural gas-fired power plants and large, expensive batteries. In the Stanford study, the power improvement at low wind speeds was particularly high because turbines typically stop spinning below a minimum speed, cutting production entirely and forcing grid managers to rely on backup power. In slow winds, wake steering reduced the amount of time that speeds dropped below this minimum. Notably, the biggest gains were at night, when wind energy is typically most valuable as a complement to solar power.

The other is the need to match the amount of electricity supplied and used in a region to keep the grid reliable. Air turbulence from wakes can make wind farm production erratic minute by minute — a time period too short to fire up a gas generator. This makes matching supply and demand more challenging for system operators in the very short term. They have tools to do so, but the tools can be expensive. In the study, wake steering reduced the very short-term variability of power production by up to 72%.

Additionally, reducing variability can help wind farm owners lower their operating costs. Turbulence in wakes can strain turbine blades and raise repair costs. Although the experiment did not last long enough to prove that wake steering reduces turbine fatigue, the researchers suggested this would happen.

“The first question that a lot of operators ask us is how this will affect the long-term structural health of their turbines,” Dabiri said. “We’re working on pinpointing the exact effects, but so far we have seen that you can actually decrease mechanical fatigue through wake steering.”

Modelling and long-term viability

To calculate the best angles of misalignment for this study, the researchers developed a new model based on historical data from the wind farm.

“Designing wind farms is typically a very data and computationally intensive task,” said Sanjiva Lele, professor of aeronautics and astronautics, and of mechanical engineering.

“Instead, we established simplified mathematical representations that not only worked but also reduced the computational load by at least two orders of magnitude.”

This faster computation could help wind farm operators use wake steering widely.

“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.”

Although the researchers were unable to measure a change in annual power production because of the limited 10-day duration of this field test, the next step, Dabiri said, is to run field tests for an entire year.

“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 optimise aerodynamics, power production and even land use for wind farms everywhere,” he said.

Image credit: ©stock.adobe.com/au/jorisvo