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.