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Current wind turbine technology is not efficient in forest sites

Source:Lappeenranta University of Technology

A new study by the Lappeenranta University of Technology (LUT) have found that forests decrease the actual power produced by wind turbines. According to the study, this can be remedied with an active pitch control.

According to the study, wind turbines produce 20 percent less actual power in forest sites. This is because the wind conditions above the forest are very different from the wind conditions without the presence of forest. More specifically, the turbulence intensity and wind shear are very high in and/or above forest in comparison to flat wind park sites without forest.

The existed wind turbines are designed in according to the latest electronic safety standards set by the International Electrotechnical Commission (IEC). These standards give the maximum values for the wind properties such as wind shear and turbulence intensity. However, these maximums are much smaller than it is found above forests numerically and also by field measurements. The frequency of the turbulent fluctuations oncoming to blades is also different. This may also affect the wind-turbine fatigue and lead to shortened wind-turbine life.

Additionally, the blade angle of attack, the angle between the oncoming wind and the wind-turbine blade, above the forest is different from an optimal angle of attack. The study found that a non-optimal angle of attack is responsible for the power loss in forest sites. The research suggests that an active pitch control in used to reduce the loss of actual power in the forest. Use of the pitch control system will allow to change the turbine blade positions so that an optimal angle of attack is reached for any specific wind condition and existed airfoil types. The power spent on the pitching is assumed to be very small in comparison to the increase in power production due to the pitch control.

“Over 70 percent of Finland is covered with forests and there are already wind turbines in them. But given the large area covered in forests, it is very likely that even more wind turbines will be installed in forest. But as the study showed, wind behaviour in forests is rather complicated. Therefore, before the instalment of a wind farm in and above forests, the investing company has to study the forest influences on the wind turbine wakes, fatigue and power production,” explains the researcher of the study Oxana Agafonova.

For years energy consumption in Finland has not been fully covered by domestic production. Approximately 20 percent of the total energy consumption in Finland has been imported. Despite the suitable wind conditions, the wind energy production in Finland is very small – around 3 percent of the total demand. At the end of 2016, the total installed wind power capacity was equal to 1533 megawatts. However, the wind power production amounted to just over three terawatthours. This is a small amount considering the total wind power capacity, which then should mean about 13,5 terawatthours per year if all turbines perform optimally all the time 24 hours per day 365 days per year.

Source:Lappeenranta University of Technology

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