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Forest Wind Power

Thomas Arnold | December 17, 2012

Reliable planning based on robust wind resource analysis.

To reach the targets set for renewable energy, an increasing number of German states are establishing the framework conditions needed for harnessing wind power in forested sites. From an economic perspective, forested sites are becoming increasingly attractive thanks to their potential for hub heights of more than 140m and larger rotor diameters. However, these sites are often located on minor mountain ranges, presenting new challenges for the systems applied to calculate the wind profile. Wind farm operators, investors, and forest owners mainly use reliable wind measurements to analyze available wind resources and make investment decisions based on a reliable set of data.

In 2011, more than 22,000 onshore wind turbines were already generating installed capacity of 29,000MW. To expand onshore wind power even further, identification and approval of additional areas as potential wind-power sites is required. North-Rhine Westphalia, for example, passed its new wind-power decree in 2011, permitting the use of forested sites following an appropriate case-by-case review. According to the state government of North-Rhine Westphalia, this step was necessary to increase the share of wind power in electricity generation from its present level of 3% to the minimum of 15% planned by 2020. The German League for Nature and the Environment (Deutscher Naturschutzring, DNR) estimates that other German states such as Brandenburg, Hesse, Rhineland Palatinate, Baden-Wuerttemberg, and Bavaria, can only achieve the targets defined by energy policy if they also use forested sites for harnessing wind power.

The wind turbines of the new generation, with hub heights of 140m and rotor diameters between 100m and 120m, supply the required technological features. They can deliver energy yields previously only possible in coastal regions. To judge the profitability of a site, resource analysis requires wind measurements for solid data from the vicinity that are not available in the majority of cases. Project assessment and licensing of a wind site require wind measurements with a measurement mast as specified in the FGW standard. The directive recommends that measurement extend to at least two-thirds of the hub height of the planned wind turbine. Since a 140m high wind measurement mast would be very costly, masts 100m tall that fulfill the two-thirds criterion of the FGW standard are generally used.

LIDAR Measurements
To determine wind speeds at heights of more than 100m, TÜV SÜD also resorts to LIDAR measurements. The laser-based optical method is useful to measure wind direction and wind speeds in heights of up to 200m and more from ground level. LIDAR is a method for remote measurement of atmospheric parameters that is similar to radar, but uses laser beams instead of radio waves. The combination of wind measurements with LIDAR systems at ground level and measurements performed by multiple anemometers on measurement masts provides a complete profile of wind conditions at heights of over 100m and even beyond the rotor diameter.

LIDAR measurements are not only convenient to assess the suitability of a site; they can find use in optimizing wind turbine operation. Scientists at Stuttgart University worked with colleagues of the National Renewable Energy Laboratory (NREL) in Boulder, CO, and succeeded in controlling a wind turbine via a LIDAR system installed on the nacelle for the first time. Through this innovation, adjustments for the prevailing wind conditions can occur for the rotor speed and other parameters before the wind field reaches the turbine. Previous control methods had the disadvantage that in the case of wind fluctuations, the rpms of the rotor only changed after it exposure to local load. If, by contrast, the wind field data comes from the LIDAR measurement before the wind field arrives, appropriate control of the turbine can begin at an early stage. This can reduce wind-induced loads and increase energy yield.

Simulation of Wind Profile
To assess wind resources and wind profiles, TÜV SÜD's experts rely on established computation models such as WAsP for 2D and WindSim for 3D analysis. The simulation model chosen depends on the ruggedness index (RIX), a measure of the steepness of the terrain. In the case of low RIX values, such as those measured in lowland areas, use of both 2D and 3D simulation can calculate the wind profile. However, 2D analysis is too inaccurate for complex topographies with RIX values of 10 and higher, i.e. RIX values typical of minor mountain ranges. In this case, only 3D simulations deliver sound results, although they are also more time-consuming and more expensive.

Another critical issue for the selection of the simulation model, apart from the steepness of the examined terrain, is the distance of the site under assessment from the edge of the forest. The more cost-effective 2D simulations can be applied near the edge of the forest, where trees have relatively little impact on the wind profile. To compensate for the impact of the trees, the simulation uses a virtually reduced hub height. However, if the site for assessment is located within a forest and more than 1km away from the forest edge, the 3D model should ensure sufficiently accurate simulation results.

Specific aspects of TÜV SÜD's services are the exact categorization of the RIX values and of the distance of the site under assessment from the edge of the forest. In this area of conflict between profitability and forecast accuracy, the choice of the most suitable method of assessment depends on this categorization. Assessment also takes into account wind directions and their frequencies at a specific site. The impact of the trees on a planned turbine can vary depending on wind direction. Wind resource assessment differentiated by the respective wind direction can be determined for the report.

Impact of Turbulence

Compared to open lowlands or plains, wind-power sites in forests have exposure to additional turbulence caused by the treetops, which affect the flow profiles. Turbulence is particularly frequent up to altitudes of three times taller than trees in 90m to 100m, reaching rotor blades up to 140m high, weakening the structure and cause material fatigue, which in turn may affect turbine stability.

In addition, the wind turbines themselves induce turbulence on the lee side, when the wind blows through the rotor and receives additional rotational energy. If the wind turbines of a wind farm stand too close to each other, these wind swirls may cause turbulence that impact the other wind turbines on the lee side. As a rule of thumb, a distance of five times the rotor diameter between wind turbines installed at lowland sites is recommended. In forests, this distance must be significantly larger, as the treetops intensify the turbulence, meaning turbulence reports must be prepared for all forest wind farms.

To reduce the impact of turbulence on the rotor blades, for example, scientists are working intensively on the development of increasingly high-performance materials. Their developments have included new composite materials comprising carbon nanotubes (CNT), where rotor blades can be produced weighing 10% to 30% less while offering more stability than conventional epoxy resin systems, improving rotor blade fatigue characteristics by 50% to 200%.

Pollution Report Required
All wind turbines exceeding a height of 50m need approval and licensing under the Air-Pollution Control Act. Analysis of noise emissions, shadow flicker, and ice shedding is necessary for the approval and licensing procedure. While noise emissions and shadow flicker are mostly of secondary importance in the less densely populated forest areas, where trees partly shield both sound waves and effects of shadow flicker, ice on rotors can reduce energy yield and endanger people. Given this, even though no injuries caused by ice shedding have occurred yet in Germany, the experts must calculate the radius around the turbine that may be a danger zone for this hazard and the probability of ice falling from the turbine in winter hitting a person. Rotor heating may be a solution, as it can prevent the formation of ice altogether or at least accelerate thawing considerably. In addition, a rotor heating system switches off the rotor as soon as it detects an unbalanced state caused, by the formation of ice, reducing the loads acting on the turbine and preventing ice shedding by rotating rotors.

Furthermore, the potential wind farm’s location is important for planning or approving a wind turbine. Considered less ecologically valuable than environments in or near conservation areas are intensively used commercial forests far away from dwellings. The areas that may be eligible for the construction of wind power stations differ from state to state in Germany and require assessment on a case-by-case basis, taking into account the existing flora and fauna. Potential bat populations in forests pose a special challenge. At these sites, wind turbines must meet increasingly stringent requirements. For example, it may become necessary to equip the turbines with a bat protection system. That shuts down the wind turbine at times of increased bat activity and when wind speed is low, under 6m/s.

In addition to technical and legal framework conditions, the designers should take into account that wind turbines at forested sites also represent a challenge in terms of public acceptance. Involving the general public or regional partners right from the planning stage may help to defuse possible problems before they turn into conflicts.


TÜV SÜD Industrie Service GmbH,
Wind Cert Services

Peabody, MA

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