When set up in groups, wind turbines in the front rows cast a wind shadow on those behind them, lowering their performance. These effects dissipate fastest under convective conditions, according to Swiss researchers from EPFL, in a recent journal.
Mahdi Abkar and Fernando Porté-Agel, the authors of the study, used a mathematical model to study the turbulence generated by a wind turbine as its rotors slice through the wind that drives them. With turbines often set up to form dense wind farms, understanding the turbulent wakes generated by upwind turbines and how long it takes for them to recover are important in predicting the overall power output of these wind farms, as turbines set up in the wind shadow of others are driven by strong wind. Today, turbines are spaced between five and seven turbine diameters apart – about 500 to 700 meters for large turbines – to maximize the amount of power that can be produced on a given area.
Optimizing production over a 24-hour cycle
“What we were interested in was understanding the effect of thermal stability on the characteristics and dynamics of the turbulent wake behind wind turbines,” says Abkar.
So, why does the turbulence recover so much faster in the convective case? According to Abkar, this is mostly due to the higher levels of turbulence in the incoming wind, which enhances turbulent mixing within, thereby breaking apart the turbulent wake behind the turbines. By contrast, in the stable setting, the turbulence is allowed to linger on for much longer, as turbulent mixing remains low.
Accounting for windfarms in weather models
Unraveling the detailed physics at play within wind farms is not only important to optimize their power output. Wind turbines alter the atmospheric dynamics by absorbing some of the wind and increasing turbulence, which can modify a number of other properties, such as atmospheric humidity and temperature. That is why meteorologists are interested in improving how they account for them in weather models. In a second paper, published in the journal of Renewable and Sustainable Energy, Akbar and Porté-Agel propose a new way to account for wind farms in large-scale atmospheric models that considers the number of turbines in the wind farm, the way that they are set up, and the wind direction.
- Journal Reference: Physics of Fluids, scitation.aip.org/content/aip/… /3/10.1063/1.4913695
- Provided by: Ecole Polytechnique Federale de Lausanne
- Retrieved from: Phys.org (March 2015); http://phys.org/news/2015-03-farms-sun.html
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