March 10, 2010
Environment, Siting, Technology

Assessing the impacts of siting in heavily wooded areas

Brady, Oisin; and Adams, Jim

Because of the way trees interact with the wind, forestry presents many unique technical challenges that are not adequately addressed by conventional approaches to wind farm site design. Poorly sited turbines can underperform by as much as 50%, according to internal research. …

The effects of trees on the lower atmospheric boundary layer can be summarized as follows: Trees take the momentum out of the wind, introduce turbulence, and cause flow displacement.

Starting with momentum, the swaying of the trees and the myriad movements of the leaves convert otherwise useful energy into heat and turbulence. When trees are in full leaf, the conversion is increased even further as a result of friction drag. Because friction drag is proportional to the square of the velocity, the more wind, the higher the wind speed losses.

The second effect is that trees induce turbulence, again through converting kinetic energy into movement of the trees, which then generates small-scale turbulence in the flow. This explains why, downwind of a large forest, measured turbulence intensity often increases, rather than decreases, with wind speed.

Trees have another effect on turbulence. Turbulence follows what is called the turbulence cascade, whereby large eddies break down into smaller eddies until they get so small they are dissipated by viscosity. Eddies, which describe flow behavior, refer to the swirling of a fluid.

When trees are present, large eddies get absorbed and converted directly into smaller ones. This creates more of a headache for those attempting to model these effects, because it is not just a matter of injecting turbulence, but a function of wind speed. Turbulence is also absorbed by the trees.

Yet another effect is consequential. Trees reduce the critical slopes that provoke flow separation. As a guideline, the critical slope reduces by around half a degree per meter of tree height.

With 20-meter trees, the critical slope (the slope angle beyond which flow detaches from a hill and starts to recirculate) reduces from around 16 degrees to 11 degrees. This means that a wind turbine located behind what seems like a non-complex hill may be subject to severe turbulent wakes if the hill is forested.

Linear flow models work up to a certain slope, which is about 16 degrees. Slopes exceeding 16 degrees can lead to errors. However, the slope at which errors begin to become significant is reduced by forest. In this case, with 20-meter trees, the critical slope is reduced from 16 degrees to 6 degrees. The outcome is that energy yield calculations performed with linear models may have much higher uncertainty than is normally assumed with a given a set of topographic conditions.

Another issue becomes significant when trees are in full leaf. The wind speed within the forest quickly approaches zero as you move away from the forest sides and the top. The forest displaces the flow, effectively creating a virtual floor – the point above the ground at which the wind speed is zero. A correction, commonly called ground plane displacement, is now often applied in wind analysis.

When designing a wind farm in a forest, the same process applies as with any other site: place turbines in the windiest places that are compatible with. the wind turbines’ design conditions. Achieving this requires layers of information that cannot be neglected on a complex, forested site. This information includes mean wind speed at hub height, turbulence intensities at every possible turbine position, vertical shear across the rotor, and vertical inflow angles.

These variables differ significantly over a complex forested site, even within short distances. Single-point measurements alone are, therefore, unlikely to be sufficient to characterize the wind flow across a forested site.

Wind turbines are designed for and certified according to design load conditions described in the International Electrotechnical Commission (IEC) 61400-1 standard. The guidelines describe a limited set of standard wind conditions and, based on the test specification in the IEC 61400-12 standard, turbines are generally tested on flat sites.

At one operational site, several turbines that were installed in a forest were subject to shear that was so high the turbines never reached rated power. Even when the wind was 20 meters per second at hub height, the turbines were producing one-third of rated power. On another site, one wind turbine within an array was subject to severe vibrations, leading to significant operational issues. The problem was traced to a separation bubble provoked by the forest on an upwind slope, which caused the shear across the rotor to vary rapidly with time. …

Furthermore, when it comes to accounting for trees, it is necessary to consider whether forests are deciduous or evergreen. It is not reasonable to assume mean conditions when considering a deciduous broadleaf forest, because winter and summer conditions have a fundamentally different effect on the flow. …

A flat forested site may require substantial felling to achieve any wrthwhile improvement in performance. Conversely, a ridgeline site may only need very limited felling to significantly improve performance. …

—from North American Wind Power, March 2010

Oisin Brady is managing director and Jim Adams is president of U.S. operations at Natural Power.

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