Wind power in German forests: technical case study

Peter Herbert Meier

As hub heights of wind turbines increase, new sites such as forests are becoming more attractive. Important technological and geophysical features must however be taken into account – together with ensuring the accuracy of data – as this case study from Germany shows.

The expansion of wind energy plays a central role in the discussion surrounding energy transition, with much attention still focusing on onshore potential. A study conducted by the Fraunhofer Institute on behalf of the German Wind Energy Association showed that theoretically at least, around 8% of the land area in Germany would be suitable for harnessing wind power.

This study was the first to assess not only the areas identified already, but all geophysically feasible areas where wind speeds are adequate for the newer generation of large wind turbines.

While these percentages will probably not be realised fully in practice, the study calculated that as little as two per cent of the land area would be sufficient for 189GW of installed wind capacity. Thus, says the study, it would be possible for wind to generate enough electricity to meet over half of Germany's annual energy demand of around 600TWh.

In southern Germany in particular, where wind development has been relatively rare to date, an increasing number of new potential areas for harnessing wind power are being identified. In Bavaria alone, experts are currently assessing around 100 sites for their suitability for wind farm development.

In addition, the German state of Baden-Wuerttemberg, in the most south-westerly corner of the country, plans to build around 150 further wind turbines – generating a total capacity of around 350MW by 2020.

Thanks to hub heights of up to 150 metres, rotor diameters of around 80 metres and generators with a power capacity of up to 6MW, forest areas have now also become feasible sites for the effective use of wind energy. In Germany, forests account for around 30% of the land area.

In contrast to coastal areas, the variety in relief structures greatly influences the factor known as ‘near-ground wind field' in inland areas. Given this, the requirements for spatial modelling of wind speeds are higher in these areas. At altitudes of 120 metres and over, air turbulence decreases while wind speeds are high enough for profitable wind-turbine operation.

To ensure data of the highest possible quality, experts preparing the wind potential assessment should cooperate as closely as possible with regional wind-farm operators, wind-turbine manufacturers, associations or authorities. This also applies when it comes to clarifying the requirements of a specific site.

Understanding site requirements

To identify suitable sites, for example, a wind atlas was produced for Baden-Wuerttemberg's State Ministry of Economics. This mapped the entire land area of the state. To determine high-yield sites, the wind potential was assessed at altitudes of 100 and 140 metres, broken down into a 50 × 50-metre grid. High-yield sites are locations offering average wind speeds of over 6.0 metres per second.

A comparison of wind speeds measured at different altitudes showed that an additional 40 metres in height more than doubles the number of high-yield sites.

The wind atlas also identified particularly high-yield areas on the Hohenlohe plateau and in the Black Forest. Open hilltop sites in mountainous South Germany are particularly profitable.

Remote commercial forests presenting decentralised sites and large areas offer profitable (yet also environmentally and socially acceptable harnessing of wind energy) without adversely affecting nearby residential areas by sound emissions or shadow flicker.

Generally, the existing network of forestry roads can be used for site development.

The possible impact of wind turbines on regional fauna of course needs to be discussed with forestry authorities at an early stage and kept to a minimum. Detailed assessments of the sites, taking into account the wind speeds measured at different altitudes and the air turbulence in different zones, is imperative.

Furthermore, trees act as obstacles impeding wind flow, so the wind profile changes depending on the stand height. Any necessary yield analysis must consider the forest growth structure; tree density and tree-crown width; seasonal changes; foliage growth; pest infestation; and the thermal properties of the forest.

Case study: first projects started

In late 2010, the first Wind in the Woods project was developed near Hof in Bavaria. With a hub height of 140 metres, the five wind turbines of the “Fasanerie” feed 22.5 million kWh/year of electricity from renewable sources into the public grid. This is enough to supply around 7,500 households with electricity and save around 17,500 tonnes of CO2 a year.

From an early stage, the project planners proactively involved the municipalities, project funders, site owners, approval authorities and residents in the planning process. The success of the project demonstrates that planning and implementation tools fine-tuned to each other help to make profitability, safety and social and environmental acceptability compatible.

Nevertheless, the expansion of wind power also has its critics. One point of criticism is the deforestation required for wind-turbine construction. However, experience has shown that the minimum area required for the foundation, the crane and the assembly is only 1,350 to 2,400 square metres and this area can be re-forested to some extent after the wind turbine's service life is complete.

Meanwhile, extensive measurements and modelling has proved that deforestation over larger areas only influences wind speeds near ground level. At high hub heights, however, potential energy-related advantages of additional deforestation have not been found. Given this, further deforestation is not required from an economic perspective, when using larger turbines.

Significantly, according to research by the Bavarian State Forest Enterprise, in Regensburg, Bavaria, the clearings made for wind turbine installation reduce the density of the forest area and, in doing so, have a positive ecological impact on the ecosystem of commercial forests, which are frequently characterised by monocultures. The German branch of Friends of the Earth also welcomes the development of potential wind-power sites in forest areas for this reason.

So an increasing number of German states are opting to produce a comprehensive inventory of their potential wind-power areas. As profitable and sustainable wind energy benefits all stakeholders, contributions to this innovative form of energy need not be limited to political and site operator level. Society as a whole can be involved.

Proactive involvement of all interested parties in an open and unbiased dialogue is essential. Third-party expert opinions, extensive risk assessments and the assessment of wind potential are important building blocks on the path to achieving these goals.

About the author: Peter Herbert Meier works for Wind Cert Services, TÜV SÜD Industrie Service GmbH, which assesses potential areas for wind-farm sites by measuring wind speed and turbulence and analysing wind potential and yield.

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