Innovative technologies ready for large-volume production unlock onshore wind potential in Southern Germany. Commercial forests offer particularly attractive new wind-farm sites.

The south knows where the wind blows: forest sites in focus

Peter Herbert Meier and Thomas Arnold | TV SD Industrie Service

A current study carried out by the Fraunhofer Institute reveals that suitable sites for harnessing wind energy can be found not only in the northern part of Germany, but across the entire country. Only two per cent of the land area of Germany would be needed to serve 65 per cent of the country's electricity demand with onshore wind power. However, to reach this goal, Germany would have to identify more priority areas and use wind turbines with hub heights of up to 150 metres.

 

Potential offered by forest sites

Commercial forests offer particularly attractive wind sites. Remote locations involve low costs of connection to the regional grid and take some pressure off the urgency of expanding and extending cross-regional grids. As the wind turbines are not fully visible and are erected at great distances from residential areas, the impacts of possible noise emissions and shadow flicker are minimal. In many instances, the existing network of forestry roads can be used for site development, reducing both costs and the level of impact on the forest.

Forest wind farms only became possible some years ago when turbines with hub heights of over 100 metres were developed. These turbines can now be operated in the high-wind and low-turbulence layers of air high above the tree crowns, thus overcoming the typical problem of low wind speeds at inland sites, which only prevails near ground level. The wind atlas for Baden-Wuerttemberg, a German state located in the south-west of Germany, which TÜV SÜD published recently, confirms this fact. Prepared on behalf of the German state's Ministry of the Economy, the atlas provides a detailed assessment of wind potential for the entire land area of the state. Baden-Wuerttemberg plans to build around 150 further wind turbines, providing a total capacity of around 350 megawatts (MW), by 2020.

 

Profitability and hub height

For the wind atlas, the TÜV SÜD experts measured wind speeds at 100 and 140 metres and broke them down into a 50 x 50-metre grid. A comparison of the air layers at different heights demonstrated that the number of eligible sites can be doubled by increasing hub height by only 40 metres. Higher towers mean longer rotor blades, and doubling the rotor diameter may potentially quadruple the rated capacity. Taken together, these factors now enable wind farms located on inland sites in southern Germany to reach yields which, some years ago, were only possible in the coastal regions of Northern Germany or on exposed high-mountain areas.

However, thorough and comprehensive assessment of forest sites is imperative to exploit the potential offered to the full. Important aspects that must be considered in this context include the forest growth structure in terms of diversity, average stand heights, tree density and tree crown width. The forest in Germany consists mainly of spruce, pine, beech and oak with average stand heights of between 15 and 30 metres. As tree density and heights vary, individual assessment is indispensable for an accurate determination of wind-flow conditions.

Directly above the tree crowns is a 15 to 50-metre thick layer of air characterised by high turbulence and low average wind speeds. Above this zone, the wind profile is more stable, with wind speeds rising exponentially with height. The wind experts model the site-specific wind layers and turbulence zones to develop reliable yield forecasts and assessment of wind potential, taking into consideration the thermal properties of the forest and seasonal changes, storm damage, deforestation, pest infestation and foliage growth.

Third-party due diligence then offers the possibility of determining whether, and if so when, a project is profitable. Wind-farm due diligence assesses the economic framework conditions, calculates the costs of wind-farm operation and the wind-farm layout required for maximum yield. The wind experts also review pollution forecasts, approvals and permits, contracts and supply commitments.

 

Bavaria's first forest wind farm

The Fasanerie wind farm in the Bavarian Vogtland region, which TÜV SÜD supported by providing yield calculations and air-pollution control forecasts, is one example of a successful project. Bavaria's first forest wind farm, which was connected to the grid in late 2010, produces 22.5 million kWh/a of electricity. The project planners proactively included municipalities, local residents, property owners and authorities in the planning process from an early stage. The success of the project proves not least that profitability and sustainability can go hand in hand provided acceptance is high among all stakeholders.

In the case of forest sites, like the one in Bavaria, the required deforestation measures must not be ignored. For example, the five wind turbines of the Fasanerie wind farm with hub heights of 140 metres require roughly one hectare of forestry area. On average, the foundation, crane and assembly require an area of between 1,350 and 2,400 square metres per wind turbine. The required deforestation may be made up for by compensatory afforestation. Bank guarantees for the costs of final dismantling ensure that the original condition will be restored at the end of the roughly 20-year service life of the wind turbines.

In this context, both operators and investors benefit from third-party wind-farm certification. A qualified third-party approach not only provides the required level of certainty for planning and investments, but also helps to exploit the full potential offered by the wind farm.


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