FOREST WIND FARMS
COMPLEX TOPOGRAPHY CALLS FOR 3D SIMULATIONS Basically, there are two calculation models for determining wind potential and wind profiles: WAsP (based on Risø’s European Wind Atlas model) for 2D analysis, and WindSim for 3D analysis. The choice of model to be applied depends on the steepness and complexity of the terrain. One measure for these two factors is the RIX, the ruggedness index. In lowland areas where the ruggedness index is low, the more cost-effective 2D modelling method can indeed be used for wind profile calculation.
However, the steeper and the more complex the terrain, the higher the inaccuracies of 2D simulation. In areas with ruggedness indices of 10 and higher, such as those typical of minor mountain ranges, only 3D modelling supplies sufficiently accurate results. In this case, the higher costs should be accepted to obtain a reliable basis for financial project calculation.
THE DISTANCE FROM THE EDGE OF THE FOREST IS OF CRITICAL IMPORTANCE Which simulation model supplies the best results also depends on how far the planned wind turbine will be positioned from the edge of the forest. If the wind turbine is located near the edge of the forest, the trees will have only minor impact on the wind profile. To save costs, 2D simulation, which is more economical, is generally used in this case. The impact of the trees can be compensated for by using a ‘virtually’ reduced hub height in the simulation. However, this procedure also has its limits.
At sites that are located more than one kilometre from the edge of the forest, the results become too inaccurate. In this case, the 3D simulation procedure, which delivers more accurate simulation results, should be used.
METHOD CONSIDERATION Given this, future forest wind farm operators should consider which method of analysis will supply sufficiently accurate results at reasonable costs. The exact categorisation of the site’s ruggedness index and its distance from the forest edge are of critical importance to answer this question. Planners should also consider how often the wind blows from each direction, as the influence of the trees may be small when the wind blows from the south, but tree tops may considerably influence wind potential in case of winds blowing from a northern or western direction.
INCREASED TURBULENCE OVER TREE TOPS
Compared to open lowlands or plains, forest sites involve more turbulence – this can occur at heights of 90 to 100 metres, i.e. three times the height of the trees.
Assuming hub heights of 140 metres and rotor diameters of 100 metres, this turbulence thus reaches the rotor blades and may cause weakening of the structure and the material, and thus ultimately affects wind turbine stability.
If plans provide for several wind turbines at a specific site, the planners further must take into account that the wind turbines themselves also generate turbulence at their lee side, i.e. the side sheltered from the wind. Wind flowing through the rotor blades receives additional rotational energy.
If a turbine is erected too close to the lee side of another turbine, wind swirls may adversely affect turbine operation. As tree tops further increase turbulence, turbines in forest areas must be installed at greater distances from each other than turbines on open lowland sites. Regarding this aspect, an expert report should assess whether the turbulence to be expected is critical for planning and for the wind turbines.
AIR-POLLUTION CONTROL REQUIRED BY LAW However, wind-resource and cost-benefit analyses are not the only relevant criteria when planning forest wind farms. To obtain a permit, the future operators must also submit an assessment of noise emissions, shadow flicker and ice shedding.
While noise emission and shadow flicker are of secondary importance for forest wind farms, which are generally located at larger distances from residential areas, planners must make sure that no persons on nearby paths or roads may be endangered by ice shedding. In addition, ice on rotors reduces the energy yield of the turbine. To prevent ice on rotors or accelerate its thawing, wind turbines may be equipped with a de-icing system. If ice causes rotor imbalance, the rotor is switched off to minimise the stress acting on the system and prevent ice shedding.
INVOLVE THE PUBLIC AT AN EARLY STAGE In Germany, the regulations governing possible wind farm sites vary from state to state. To cater to this aspect, individual assessments must be carried out and also take into account the flora and fauna at the respective site.
If the economic, technical and legal requirements for the construction of a wind turbine have been fulfilled, the planners should address acceptance by the general public.
Integrating regional partners and the general public at an early stage has proved helpful to smooth possible disputes between various interest groups even before commencement of the permission process.
Thomas Arnold, TÜV SÜD Industrie Service
www.tuev-sued.de/windenergie
www.windenergynetwork.co.uk
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