As the most viable option for clean energy, improvements in wind power technology are unlocking new areas for wind generation and driving interest towards onshore locations.
In the search for new sites, the European wind industry is increasingly targeting woodlands, with new generations of turbines opening up previously unattainable sites to new development.
Unlike offshore, the installation and maintenance of onshore wind farms is far less technically challenging.
The energy yield can be fed into existing infrastructure, which makes the expansion or construction of new power grids redundant. In the low-turbulence and high-wind areas high above the top of forests, winds can speed up to 6.7m a second, which comes close to offshore wind speeds of 6m-8m a second.
Results from the first successful operating forest wind farms have shown the turbines have no negative impact on the forest environment and could therefore develop into a cost-effective and sustainable alternative to offshore wind farms.
Germany, as the leading hub of this European high-tech industry, has currently more than 27GW of installed wind capacity and plans to further increase its capacity; in the wake of the Fukushima nuclear disaster in Japan in March 2011, the government has decided to give renewable energy greater priority.
Plans to expand onshore wind farms to forests have also been made in the UK, where wind energy overtook hydropower to become the largest renewable generation source.
Moving wind farms to forests
"Renewable energy, not nuclear, is going to be the growth area in the future," said EU energy commissioner Günther Oettinger after the events in Japan in March this year. Experts predict that onshore wind energy, as the forerunner of European renewable power production, will continue to be the most cost-effective of all renewable energy sources.
Estimates by the European Wind Energy Association (EWEA) expect the installed onshore wind capacity in Europe will increase from 83GW to 190GW by 2020.
With a new generation of modern turbines with longer rotor blades and higher towers, forest wind farms could soon play a major role in the European renewable energy sector.
According to technical service corporation TÜV SÜD Industrie Service head of wind services Peter Herbert Meier, the rapid progress in wind turbine technology in recent years has established wind farms in forest areas and made them economically viable. With the help of assessment tools such as interdisciplinary know-how, including plant-engineering, measuring systems, landscape, nature conservation, logistics and pollution control, combined with feasibility studies and modern laser instruments "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."
What is particularly crucial for forested areas are increased turbine heights and rotor diameters to reach the best possible energy outcome of the wind farms.
Directly above the tops of the trees a 15m-40m wide layer will bring no energy yield, as the trees act 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," wrote Peter Herbert Meier in a report for Renewable Energy World in June 2011.
Above this zone, at around 30m-60m of height, the trees loose their influence on the wind, turbulences decrease and wind speeds rise. Modern state-of-the-art turbines, equipped with hubs more than 100m high, reach into these 'low-turbulence and high-wind' areas, where the winds can reach speeds of 5.8m - 6.7m a second.
The higher the altitude, the greater and more consistent are the wind speeds. Taller and usually hollow steel towers with long fibreglass rotor blades are widely used in Germany to further increase the energy outcome. According to Meier, results from the first commercially used forest wind farms show that doubling the rotor diameter has quadrupled the rated capacity.
But while in Germany, forest wind farms are already an established alternative source of wind energy, the proposed projects in the UK still attract criticism from local communities and activist groups.
The proposals, submitted by one of the country's leading renewable energy companies, RWE npower renewables, for different locations in Wales, are widely seen as 'controversial'.
In February 2011, the company opened a public consultation process on the suggested Brechfa Forest East Wind Farm in south-west Wales, which soon will accommodate 12 turbines within an area of 270ha and accommodate a capacity of between 24MW and 36MW.
On 3 August, the company presented a second project, which is currently undergoing public scrutiny.
According to the company, 32 turbines at Clocaenog Forest in North Wales could soon power up to 42,100 homes in the area.
Both projects however have stirred up criticism and disapproval among locals and activist groups. In north Wales, Denbigshire councillor Paul Marfleet said in a letter to the Foresty Commission Wales: "It appears that they (RWE npower renewables) intend to develop the site from the north to the south covering a considerable area of this forest. The development plan will necessitate considerable construction traffic and earth moving equipment."
In a 56-day long formal public consultation process the public is asked to give their opinion on the proposed wind farm project. Earlier in December 2010, RWE npower renewables' proposal to install 15 45MW turbines at Raera Forest, near the west coast of Wales, was turned down by the planning committee, who said the impact of the proposal on visuals and the landscape in the area would have been too great.
However, the reduced visibility in midst of forests and the usually long distance to housing areas have increased the acceptance of forest wind farms - at least in Germany. Moreover, the country's Federal Wind Energy Association claims results from the first wind turbine projects in forested areas have shown they have no negative impact on the surrounding forest and its fauna.
Construction and transport disturbances only take place in the start-up phase and make no difference to standard forest work. To further minimise changes in the ecosystem of the forest, experts suggest keeping the clearings small, to plan the project in cooperation with local forest authorities and even to schedule the forestation of comparable spaces to the cleared areas.
What is more, unlike offshore wind farms, onshore projects do not involve massive expansion of existing grid capacities or the addition of new ones as they can feed power into many different locations of the existing infrastructure.
Also, the installation and maintenance of turbines in forests is technically far less challenging than for offshore turbines, which not only has a beneficial impact on the environment but also on the investment costs.
Case study: Bavaria, South Germany
The largest forest wind farm currently being operated in Germany is the Fasanerie wind farm in Bavaria, in the south of the country.
The project, which was connected to the grid in early 2011, consists of five 2MW Enercon E-82 machines, each with a rotor diameter of 82m.
The turbines have a hub height of 138m of which each requires an area of 1,350m² - 2,400m², used for the foundation, the crane and turbine assembly. Together, they are expected to produce about 22.5 million KW hours of electricity every year, enough to supply about 7,500 households in the area. According to TÜV SÜD, whose experts supported the project process on questions such as energy yield, noise emission and shadow flicker, prior consultation with forestry companies helped to prevent negative impacts on the regional fauna.
The Bavarian state forest enterprise in Regensburg has even claimed the small clearings caused by the wind farms have reduced the density of the woods and therefore had a "positive ecological impact on the forest ecosystem."
Previously conducted studies have shown energy yields cannot be improved by further deforestation beyond the areas required for the turbines. Therefore, the operation of the wind farm will have no further influence on the forest and its surroundings. In case of a complete dismantling, both turbines and foundations will be removed and the ground refilled, giving the opportunity to reforest the area without causing further harm to the environment.