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Modern wind turbines with hub heights of more than 100 metres are opening up the potential of forest sites by reaching low-turbulence layers of air high above the tops of trees Source: TÜV SÜD Industrie Service

Through its support for a major forest wind farm project in Bavaria, TÜV SÜD is cutting through the hurdles posed by an environment that, although long seen as inhospitable to wind generation, could offer a host of attractive locations for Germany’s highly developed wind sector. 

Peter Herbert Meier, TÜV SÜD Industrie Service, Germany

With more than 27 GW of installed wind capacity, Germany is Europe’s leading wind power producer and has plans to increase its capacity further. Commercial forests in particular offer new potential locations – but what are the challenges faced by operators, investors and local administration bodies?

The European Wind Energy Association (EWEA) anticipates that installed onshore wind capacity in Europe will increase from its present level of 83 GW to 190 GW by 2020. Onshore wind turbines will thus produce the lion’s share of renewable energy in Europe by 2020. At the same time, onshore wind energy will continue to be the most cost-effective of all renewable energy sources.

In Germany, the development of land areas for potential new sites is increasingly focusing on inland Southern Germany. Within the scope of option contracts, experts are currently assessing about 100 locations on land owned by the Bavarian state forest enterprise, Bayerische Staatsforsten, for their suitability as wind farm sites. Areas of monoculture forestry, especially, offer the opportunity for profitable and environmentally compatible wind farms.

Overall, the development of onshore wind power continues undiminished across Germany, offering major market opportunities for planners, investors and operators. The German government aims to increase the percentage of electricity from renewable sources to between 25 and 30 per cent by 2020. The most recent amendment of Germany’s Renewable Energy Sources Act (EEG) in 2009 further increased feed-in tariffs for wind energy, rendering planning and investments even more reliable and secure.

Germany’s Renewable Energy Sources Act (EEG) is based on a clean energy cash-back scheme, guaranteeing wind-farm operators fixed feed-in tariffs for the generated power over 20 years. The act places grid operators under the obligation of giving a purchasing preference to electricity produced from renewable sources. The EEG has proved one of the most successful legal acts of its kind at an international level and more than 40 countries have taken it as a role model for developing their own tools promoting renewable energy. 

Forests as wind farm sites 

Firstly, inland sites offer the possibility of injecting power at many different locations, generally permitting direct use of the existing grid infrastructure. Unlike offshore wind farms in the Baltic and the North Sea, they do not require massive expansion of existing grid capacities or the addition of new ones. Secondly, installing and maintaining is far less technically challenging for onshore wind turbines than for offshore projects, which, in turn, cuts investment costs.

Suitable wind energy regions in commercial forests are generally far from residential areas, thus minimizing adverse effects on residents from noise emissions and shadow flicker. The forest also reduces the visual impact of the wind farm. Impacts on residents and the environment can be further reduced by strategically selecting wind farm sites along existing infrastructure routes, including national roads, railway lines, overhead power lines and motorways.

Wind farms may be of considerable economic significance for the region, particularly in rural areas in southern Germany. During construction, for example, wind farm operators generally commission regional planning offices, experts, electrical engineers and contractors for road and foundation building. Operating wind farms also creates jobs at regional service providers, suppliers and servicing companies. In addition, local administrations, municipal utilities, energy co-operatives and residents may have the possibility of investing in wind farms. 

A fresh breeze for inland areas 

Locating wind farms in forest areas was only made economically feasible in recent years by rapid progress in wind turbine technology. Turbine hub heights and rotor diameters are particularly crucial for forest areas, because the wind profile in a 15- to 40-metre broad layer of air directly above the tree tops is massively influenced by the trees acting as obstacles to the wind. This zone is characterized by considerable turbulence and low wind speeds, and is therefore unsuitable for profitable exploitation of wind.

Above this zone, at heights of between 30 and 60 metres, the influence of the trees becomes increasingly negligible. Wind speeds rise while turbulence decreases. Modern wind turbines with hubs over 100 metres in height, which only became the standard some years ago, now extend into these high-wind, low-turbulence layers of air high above the tops of the trees. Mean wind speeds reach up to between 5.8 and 6.7 metres per second at a height of 120 metres, even in Bavaria and Baden-Wuerttemberg.

At higher altitudes, wind speeds tend to be both higher and more consistent. Higher towers enable the use of longer rotor blades that further increase energy yield. As a general rule of thumb, doubling the rotor diameter quadruples the rated capacity. Thanks to this advance in turbine technology, even wind turbines in low mountain ranges can now achieve yields that had only been feasible in coastal and high-mountain areas a few years ago.

TÜV SÜD Industrie Service has successfully supported a host of wind energy projects at many sites across the world and has assisted operators, investors and local administrations in planning, installation and operation. 

Ideal assessment tools 

The assessment of a forest site calls for interdisciplinary know-how. In this case, the results and experience from a range of disciplines including plant engineering, measuring systems, landscape and nature conservation, logistics and pollution control need to be combined and integrated in a reliable manner. In addition, the interests of operators, investors, nature conservationists, residents, forest owners and forest authorities must be taken into consideration.

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The Fasanerie wind farm features five Enercon E-82 turbines, each with a capacity of 2 MW Source: TÜV SÜD

A reliable assessment of the potential site first necessitates a survey of the forest stand structure with a focus on diversity, average stand height, tree density and width of the tree tops. The German forest consists mainly of spruce, pine, beech, oak and Douglas fir with average heights of between 15 and 30 metres and a density of between 400 and 1000 trees per hectare. The tops of the trees are between 4 and 20 metres wide and the trees have a mean age of between 45 and 65 years. Wind-relevant parameters can be determined efficiently with the help of modern laser instruments. A factor that must be taken into account is variance, in particular with respect to the densities, stand heights and distances between the trees.

Comprehensive wind measurements deliver indispensable data for preparing reliable expert opinions. The data is determined in wind measurement campaigns based on one or several measurement stations. In this context, the experts at TÜV SÜD can make use of experience and reference data from their supervision and support of over 100 wind measurement campaigns across the world.

Using a quasi-3D model, the experts can predict mean energy generation per year and the wind speeds for various project configurations. These data form the basis of ‘bankable wind reports’ and detailed analyses of wind potential on the one hand, and, on the other, can also be used for determining the most suitable type of wind turbine for a specific site and the ideal hub height and distances. 

Case study: Fasanerie wind farm 

The experts at TÜV SÜD Industrie Service supported the largest forest wind farm currently operating in Bavaria, providing expert reports on aspects including energy yield, noise emission and shadow flicker. The wind farm is on land owned by the Bavarian state forest enterprise, situated in one of the high-wind regions of Bavaria close to a motorway. The five wind turbines of the Enercon E-82 type, each with a capacity of 2 MW and a rotor diameter of 82 metres, were connected to the grid in early 2011. The turbines have hub heights of 138 metres and deliver electricity to around 7500 households, saving about 17 500 tonnes of CO2 per year.

Reliable wind reports are especially significant for successful wind farm operation. They form the basis of the profitability analysis for a wind farm, reflecting the feed-in tariffs for the generated energy and thus the projected income. Professional and impartial assessments by experts are recommended to ensure an objective evaluation. Inaccurate estimates may affect the profitability of the wind farm and jeopardize the funding of the entire project.

Energy yield predictions in Germany are often based on the ‘IWET Index’, a value determined and published by IWET (Ingenieurwerkstatt Energietechnik). This index is based on actual energy yields of many wind turbines across Germany, with data for 25 regions published on a monthly basis. The index is subject to continuous adjustment and has been revised and reduced several times over recent years. For investors and operators, this means a predicted energy yield is probably incorrect – i.e. too high – if an obsolete index is used in the calculation. Numerous studies in which the energy yield of an existing wind farm was re-calculated have shown that the current index also tends to significantly overrate the energy yield. Exact measurements are therefore imperative for energy yield prediction.

Engineers at TÜV SÜD review the wind reports and other project documentation impartially and independently. Thorough reviews frequently reveal that expert reports clearly exaggerate energy yields compared to the available empirical data, and thus upgrade sites’ apparent potential.

Overestimates of energy yield can generally be attributed to a conflict of interest. Given this, some companies have, according to their own information, established internal classification systems for the expert reports available to them. These classification systems categorize expert reports according to the experts’ trustworthiness. 

Meeting concerns over sustainability 

Even though most commercial forests are monocultures and total forest area is increasing annually across Germany, the extent of deforestation required must always be given consideration within the scope of forest wind farms. The five turbines of the Fasanerie wind farm require roughly one hectare of forestry area. On average, an area of between 1350 and 2400 m2 is required per turbine. This total area comprises the area needed for the foundation, the crane and turbine assembly. Generally, the existing network of forestry roads can be used for the development of the wind farm site.

Prior consultation with forestry companies helps protect regional fauna. According to the Bavarian state forest enterprise in Regensburg, small clearings caused by wind farms reduce the density of the forest and have a positive ecological impact on the forest ecosystem. TÜV SÜD experts have found that further deforestation beyond the areas required for the turbines does not raise energy yields. Additional deforestation leads to higher wind speeds at ground level, but not at hub height.

The required deforestation may be compensated for by afforestation in other areas. Furthermore, bank guarantees for the cost of the final dismantling of the wind turbines ensure that the original condition will be restored at the end of the roughly 20-year service life of the wind turbines. In the case of complete dismantling, both the turbines and the foundations will be removed and the ground then refilled with soil. At a forest wind farm such as the Fasanerie wind farm, the area can then be reforested. 

Greater investment security 

For profitable and reliable wind-farm operation, all stakeholders need to protect themselves against risks by conducting a comprehensive due diligence review. Following selection of a site, safe and reliable wind farm construction must be ensured in compliance with laws and regulations on pollution control and on nature and landscape conservation. Both operators and investors benefit from third-party wind-farm certification. Wind farm due diligence assesses the economic framework conditions and calculates the costs operation and maximum yields. In addition, it reviews pollution forecasts, permits, contracts and approvals.

The expansion of onshore capacities in commercial forests offers major opportunities and may contribute significantly and cost-effectively to strengthening decentralized electricity supply. As in the planning of the Fasanerie wind farm, local administrations, residents, land owners and authorities should be involved in the planning from an early stage.

The project’s success has shown that thorough co-ordination and consideration of the interests of all stakeholders places wind farm operations on a profitable and environmentally sound footing. The experts at TÜV SÜD Wind Cert Services support projects across the world, from site assessment to construction and commissioning. 

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