As developers look to expand the opportunities available onshore, new generations of turbines with higher towers and longer blades are opening up previously unobtainable sites to development. Here some of the key issues are explored, along with the challenges of an environment that can offer a host of attractive locations.

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

Estimates by the European Wind Energy Association (EWEA) anticipate installed onshore wind capacity in Europe will increase from the current 83 GW to 190 GW by 2020. As well as producing the lion's share of renewable energy in Europe, onshore wind 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 opportunities for planners, investors and operators. The German government aims to increase electricity from renewable sources to 25%-30% by 2020. The most recent amendment of Germany's Renewable Energy Sources Act (EEG) in 2009 further increased feed-in tariffs for wind.

Germany's Renewable Energy Sources Act is based on a clean energy cash-back scheme, guaranteeing wind-farm operators fixed feed-in tariffs for the generated power over a period of 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 to be one of the most successful legal acts of its kind at an international level and has been used by over 40 countries 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, installation and maintenance is far less technically challenging for onshore turbines than for offshore projects, which, in turn, cuts investment costs.

Suitable wind-energy regions in commercial forests are generally far from residential areas, which therefore minimises 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 the rural areas of 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 with 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 the wind farm.

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-wide layer of air directly above the tree tops is massively influenced by the trees acting as obstacles to the wind. This zone is characterised by considerable turbulence and low wind speeds, and is therefore unsuitable for profitable exploitation of wind energy.

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 hub heights of over 100 metres, 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 5.8-6.7 metres/second at a height of 120 metres, even in Bavaria and Baden-Wuerttemberg.

Locating wind farms in forest areas only became economically feasible through recent progress in turbine technology

At higher altitudes, wind speeds tend to be both greater and more consistent. Taller towers enable longer rotor blades to be used 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.

Ideal Assessment Tools

The assessment of a forest site calls for interdisciplinary know-how. In this case, information and experience from a host 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 need to be taken into consideration.

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 the width of the tree tops. German forests 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/hectare. The tops of the trees are 4-20 metres wide and the trees have a mean age of 45-65 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 trees.

Comprehensive wind measurements deliver indispensable data based on one or several measurement stations.

Using a quasi-3D model, it is possible to predict annual mean energy generation and the wind speeds for various project configurations. These data form the basis of 'bankable wind reports' and detailed analyses of wind potential, and can also be used for determining the most suitable type of wind turbine for a specific site and the ideal hub height and distances.

Reliable Reports Cut Risk

Reliable wind reports are of particular significance for successful wind-farm operation. They form the basis of the profitability analysis, reflecting the feed-in tariffs for the generated energy and thus the projected income. Incorrect assessments may affect the profitability of the wind farm and jeopardise the funding of the entire project.

Energy yield predictions in Germany are frequently based on the 'IWET index', a value determined and published by IWET (Ingenieurwerkstatt Energietechnik). This wind index is based on the actual energy yields of a large number of wind turbines across Germany. Current data for 25 regions is published monthly and the index is subject to continuous adjustment.

For investors and operators, this has the consequence that 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. Given this, exact measurements are imperative for energy yield prediction.

Thorough reviews frequently reveal that expert reports clearly overrate energy yields compared to the available empirical data on energy yield, and thus upgrade sites' apparent yield. Overrating energy yield can generally be attributed to a conflict of interest. Given this, some companies have established internal classification systems for the reports available to them. These classification systems categorise such reports according to their 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. For example, the largest forest wind farm currently operating in Bavaria is Fasanerie. The project's five 2 MW Enercon E-82 machines, each with a rotor diameter of 82 metres, were connected to the grid in early 2011. The turbines have hub heights of 138 metres. Each machine requires roughly one hectare of forestry area. On average, an area of between 1350 m² 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 to prevent negative impacts on the regional fauna. According to the Bavarian state forest enterprise in Regensburg, the small clearings caused by forest wind farms reduce the density of the forest area and have a positive ecological impact on the forest ecosystem. TUV SUD has proved that energy yields are not enhanced by further deforestation beyond the areas required for the turbines. Further deforestation leads to higher wind speeds at ground level, but not at hub height.

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

Greater Investment Security

For profitable and reliable wind-farm operation, all stakeholders need to protect themselves against potential risks by conducting a comprehensive due diligence review. Following the successful selection of a site, safe and reliable wind-farm construction must be ensured in compliance with the laws and regulations on pollution control, nature and landscape conservation.

In this context, both operators and investors benefit from third-party wind-farm certification. Wind-farm due diligence assesses the economic framework conditions and calculates wind-farm operation costs 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 decentralised electricity supply. As in the planning of the Fasanerie wind farm, local administrations, residents, land owners and authorities should be involved 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.