Turbulence is a permanent state for wind turbines, says wind physicist Joachim Peinke. He uses mathematical methods to describe the small-scale fluctuations of the wind and how these depend on large-scale flows.

“The physics of wind as an energy source still hasn't been adequately researched,” says Prof. Dr. Joachim Peinke, a turbulence researcher at the University of Oldenburg's ForWind Center for Wind Energy Research. Although equations that describe the behaviour of the atmosphere’s air flow exist, even the world's most high-performance computers can't solve them accurately.

The turbulent boundary layer close to the ground is simply too chaotic: wind speeds vary not just over days, months and years, but within minutes and seconds. A gentle breeze can be followed by a sudden squall which then subsides as quickly as it came. Some air vortexes are as large as continents, others as small as a mosquito. Obstacles such as mountains, forests and buildings change air flow in unpredictable ways.

This volatility poses a major challenge for the green energy transition. In 2018 wind was the second most important resource in the German power mix after lignite, providing 20 percent of the country's electricity. But it is far more difficult to plan electricity generation from wind energy than from fossil fuels. "In the quest to make optimum use of wind as an energy source we face various constraints" says Peinke.  

The scientists at ForWind are working to improve their knowledge of the wind in order to make wind-based electricity generation more reliable and plannable. Peinke, a renowned turbulence researcher, is among these scientists. He and his research group Turbulence, Wind Energy and Stochastic (TWIST) are trying to decipher the properties of the wind using complex mathematical methods.

Small-scale turbulences are difficult to predict

As well as large-scale flow patterns and long-term average values, small-scale air turbulences also play an important role for wind power. “Wind structures of just one metre in size determine which local forces act on a wind turbine,” turbulence researcher Peinke explains. An airplane taking off in a heavy storm encounters the turbulent layer of air close to the ground only during take-off and landing, while for the rest of the flight wind currents will generally be calm. “But for wind turbines, turbulence is a permanent state,” says the researcher.

This places tough demands on materials – especially as wind turbines have grown dramatically in recent decades. Whereas back in the 1980s rotor blades were typically just seven to eight metres long, today they are around eighty metres long, surpassing the wingspan of an A380 aircraft. To keep weight down, many components are designed to function at the limits of their capacity. “If rotors were still made out of metal, as they were in the 1980s, they would break under their own weight,” says Peinke.

Manufacturers use turbulence models to calculate the forces a turbine must withstand. But small-scale turbulences in particular are difficult to predict. “There are so many local fluctuations in the wind that surprises are not rare,” says Peinke.

The frequency of extreme events is underestimated

To learn more about the mysteries of turbulence he and his colleagues use complex mathematical methods to study, for example, how best to characterize small-scale fluctuations, or whether properties exist that are common to all turbulent flows. Here they look into questions such as how much small-scale turbulences depend on the structure of the large-scale flow from which they emerge, or whether details such as the duration or stability of a wind gust depend on environmental conditions.

Peinke's research group also carries out experiments in the ForWind Research Center's purpose-built, 30-metre wind tunnel. Inside the tunnel a one-of-a-kind grid with almost a thousand movable, diamond-shaped aluminium blades creates natural wind fields, including large and small turbulences. “The great thing about the wind tunnel is that, unlike in nature, we can repeat certain turbulence patterns to see what impact they have on model systems,” explains Dr. Michael Hölling, a colleague of Peinke who is responsible for the wind tunnel.

Statistics offers another possibility for analysing the properties of flow patterns. Peinke and his research group study physical parameters such as turbulence intensity in air flows and wind speed distribution in order to deduce general rules. In a scientific publication from 2012, the team demonstrated that a special statistical principle applies for wind gusts: extreme fluctuations in wind speed occur far more frequently than the statistics currently used by industry suggest, meaning that in some cases the frequency of extreme events is drastically underestimated.

Fluctuations in the power grid

“An extreme event that – according to the Gaussian statistics applied to date should only occur once every 1,250 years – actually occurs once every hour,” Peinke explains. The scientists were also able to establish that this property of the wind, known as intermittency, is present at all stages of the process of converting wind energy into electricity, from the forces acting on the turbine to the electricity generation process. By comparing measurement data the team established that all these variables fluctuate just as much as wind speeds do. “For example the amount of power generated by a wind turbine can increase or decrease by fifty percent within seconds,” Peinke explains.

Gusts of wind and whirlwinds that occur locally without affecting all the turbines in a wind farm or a larger area still have an impact on the power grid as a whole – and in particular on the grid frequency, as a team led by Peinke and his PhD student Hauke Hähne demonstrated in a paper published in 2018 in the science journal Europhysics Letters. The researchers studied the oscillations of the AC voltage in the regional power grid over a period of four-and-a-half months.

“The mains frequency indicates the current ratio of electricity production to consumption, and is thus a stability parameter,” explains Peinke. If several megawatts of wind power are suddenly fed into the grid without consumption rising at the same time, the frequency changes. If it deviates too much from the desired value - 50 hertz, or 50 oscillations per second - within a few seconds a control mechanism intervenes to ensure that the power grid remains stable. Minor fluctuations are automatically compensated for by rotational inertia in large power stations.

Solar power systems switch on and off repeatedly

To find out how gusts of wind and other turbulences affect this interaction, the team made 10,000 measurements per second of the frequency, allowing the researchers to record fluctuations on very small timescales. Their analysis showed that the phenomenon of intermittency is also present in the grid frequency: its fluctuations obey a statistical law similar to that which governs wind speed distribution, with relatively large fluctuations occurring far more frequently than expected.

The researchers also discovered that the more wind energy was fed into the grid within a given period, the more frequently strong fluctuations in frequency occurred. “This suggests that the fluctuations in wind power caused the fluctuations in frequency,” says Peinke.

The documented frequency fluctuations were so small that they didn't pose a threat to the power grid. But in the future, if the proportion of renewable energies in electricity generation increases, the fluctuations could increase. The observed probability distribution can help scientists to correctly identify risks and develop control strategies, Peinke explains. “It's essential to have a precise knowledge of the fluctuations in order to be able to correctly assess the probability of large, potentially critical fluctuations.”

Solar energy is even more disruptive for the power grid than wind energy: “On days when the sky is alternately cloudless and cloudy, solar power systems switch on and off repeatedly within very short periods,” says Peinke. Such episodes are particularly likely to destabilise smaller subnetworks, which are expected to multiply as the energy transition advances.

Developing a wind energy doctor

In 2017, a team of ForWind turbulence research and energy meteorology scientists working in collaboration with electrical engineers from the Clausthal University of Technology published a study in the science journal Solar Energy. It provides a mathematical description of the erratic nature of solar energy.

In their paper the researchers present a method for deriving parameters directly from a measured time series that describe the stochastic – that is random – properties of solar power. Network operators could potentially use these parameters to develop an algorithm to filter out and suppress dramatic fluctuations. Batteries, capacitors and inverters with just a few percent of the installed capacity can be used to stabilise microgrids, including those running on a high proportion of renewable energies, Peinke points out.

Word is now spreading in the wind energy sector that the complex challenges of wind energy can be tackled using statistics and turbulence research. Peinke is currently preparing a project for monitoring the condition of wind turbines using statistical data analysis. “We want to develop a kind of wind turbine doctor,” says Dr. Matthias Wächter, who runs the project. Each wind turbine contains hundreds of sensors that monitor numerous parameters, from the oil level of the generator to the bending moment of the rotors.

Using big data technologies

“Because the turbines wobble constantly, the data is very noisy and so far hardly anyone has been able to do anything with it,” explains Peinke. He and his team now want to work with computer scientists to evaluate and analyse these mountains of data using big data technologies. They aim to extract information from the noise such as whether everything is running smoothly or whether certain components of a system, for instance the motors, are showing signs of fatigue and need replacing.

Particularly in the case of offshore turbines, which are difficult to access, it would be useful to be able to better plan maintenance work so that it can be carried out when the wind is low. “The costs for outages and maintenance work account for a significant proportion of wind energy costs,” stresses Peinke. “To reduce these costs, it will be necessary not only to increase the efficiency of wind turbines, but to understand them better - so that they can operate maintenance-free even in turbulent times.”