Wind turbines have now reached the pinnacle of how much energy they can pull out of the air. But an enormous amount of energy still lies hidden in wind farms, as they don’t yet work optimally due to extremely turbulent and complex wind conditions.

In March 2012, an energy agreement was adopted in Denmark with historically broad political backing. The framework for the development of renewable energy had been set, and the target for wind energy was that one-half of Danish electricity consumption is to be covered exclusively by wind turbines in 2020.

The same tendency can be seen in many countries around the globe. There will be increasing focus on the climate and sustainability, and this requires efficient energy solutions. The technology behind wind energy and wind turbines dates back more than two thousand years. A wind turbine was used to generate electrical energy for the first time in 1887. Modern wind turbines are close to exhausting the possibilities for further development within the three-winged offshore and onshore turbines as we know them.

Therefore, researchers like Assistant Professor Mahdi Abkar are working hard to improve the output of wind turbines in other ways. Instead of looking at the output of the individual wind turbine, they are taking a broader perspective and looking at entire wind farms.

Maximizing the output
“The increasing focus on renewable energy and sustainability will spawn a need for more and ever larger wind turbines and wind farms. The problem is that just having larger wind turbines and wind farms is not enough - they also need to be more efficient and reliable than they are today,” he says and continues:

“We’re therefore working to develop models that can describe how wind farms can be designed so that they get the best output with the least loads on the individual turbines. It’s not about maximizing the output from the individual turbine; it’s about getting the most out of the whole farm - and that’s not the same thing,” he stresses.

Even though the wind turbine technology is proven, the physics behind wind energy is much more complex and unpredictable. Assistant Professor Abkar is working to develop numerical and analytical models that can predict the complex interaction that takes place between the wind farm and the atmospheric boundary layer; i.e. the layer of air immediately above the Earth’s surface.

In contrast to the upper layers of the atmosphere, the airflow in the atmospheric boundary layer is highly turbulent. The wind is constantly changing direction and speed and is made turbulent by the landscape and surface topography, for example. The sun also plays a major role for the characteristics of the wind, which is why we talk about particular cycles for day and night.

“There’s also a loss of wind energy in the wind farm itself due to the so-called wake effect,” explains Mahdi Abkar and he stresses that this effect is also affected by the cycles of day and night.

Huge benefits in new design
Wake effect occurs in wind farms when the wind hits a turbine’s blades. Energy from the wind is absorbed, but at the same time, the flow and speed of the wind are disturbed in the wake of the turbine. Depending on a large number of factors, this effect can have minor or major significance for the total output of the wind farm, but Mahdi Abkar explains that the effect can result in up to 40 per cent less energy output from the turbines in the wake.

The computer models will take into account these factors and others, so that, firstly, we can build and design an optimal wind farm array with different types of wind turbines, and secondly, so that we can develop a system that can automatically control the turbines individually to minimise the wake effect.

“The well-known three-bladed turbine technology is now fully mature, and the changes we’re still making in blade design, for example, are very small with only very limited effect in relation to energy production. What we lack are improved applied techniques to optimally control the individual wind turbines in wind farms in order to maximize the power output. We need a very accurate model that contains all the physical parameters at stake. There are huge benefits to be gained here, if we can design a wind farm that can take account of all these variables,” says Assistant Professor Abkar.