The latest generation of wind turbines enable profitable production of wind energy at sites that 10-15 years ago would have been out of the question. Large rotors and tall towers that raise hub heights enable yields to be achieved that were previously only possible in coastal regions. As a result, developing wind projects in areas such as forests has become a more viable and attractive proposition.

Indeed, a typical 2MW wind turbine creates a value to the local economy of €2.2mn on average throughout its 20-year service life, according to a recent German study carried out by the Institute for Ecological Economy Research (IÖW) and the Centre for Renewable Energy at Freiburg University (ZEE). However, reliable wind energy yield predictions are imperative to ensure the electricity eventually generated matches expectations. To supply reliable yield predictions, measurement and simulation techniques that take into account both flow-related turbulence caused by the trees and the frequently complex topography of minor mountain ranges are critical.

Reliable assessment of the profitability of a site generally requires wind resource measurements. This applies in particular to cases where no comparable data is available from the immediate vicinity. The FGW, a German public association for the renewable energy sector, requires long-term measurements with a cup vane anemometer on a measurement mast.

The hub heights of the latest generation wind turbines reach up to 140 metres. The FGW guideline recommends the mast extends to at least two-thirds of the hub height. This results in a minimum height of 100 metres. As banks press for bankable data, the demand for higher wind masts is rising. In forested areas where trees are high, causing impacting more on air flows, measurements near the hub height have become even more common.

Reaching greater heights

At heights of 40-250 metres, wind mast data can be complemented by means of the LIDAR technique. The on-ground equipment sends laser pulses into the atmosphere, analysing their backscatter with the help of a detector. The measured changes in frequency and signal runtime can then be used to determine wind speed and direction.

Up to heights of 100 metres, the data determined under the LIDAR system will be compared against the wind data measured by the anemometer. Based on this calibration, the wind profile can then be extrapolated up to the upper rotor tips of the planned wind turbine and the expected energy yield calculated.

Simulation of the wind conditions that takes topography into account is another step in the analysis of energy yield. Basically, the following two solutions are feasible:

• WAsP for two-dimensional (2D) analysis; and
• CFD-applications for three-dimensional (3D) analysis.

While the cost-effective 2D simulation model supplies sufficiently accurate results in lowland areas, 3D simulation procedures are frequently used for the high-wind, high-yield sites on minor mountain ranges. After all, the steeper and more rugged the terrain, the less reliable the 2D forecasts.

For quantitative statements on the steepness of the terrain, TÜV SÜD relies on the RIX analysis. Values on a scale from 0 to 15 indicate the percentage of an area around an object that has steepness in excess of a defined value.

At a RIX of 0 to 5, the 2D analysis supplies sound predictions. Certain defined rules of correction and alignment parameters must be taken into account for values between 6 and 10, to avoid incorrect estimates of wind speed and thus energy yield.

The somewhat more complex and slightly more expensive 3D technique should be used at a RIX of 10 and higher. It enables improved yield predictions and reliable cost-benefit analysis even in complex terrains.

Deep in the forest

A wind farm's distance from the edge of the forest also plays a role in the choice of simulation model used to calculate energy yield predictions. The critical factor here is the ‘penetration depth’.

At distances of one kilometre and more from the edge of the forest, 2D simulation becomes too inaccurate because the trees increasingly influence the wind profile. At these sites only 3D analysis will be able to supply realistic simulation results.

Specific aspects of TÜV SÜD's services include exact categorisation of the RIX values and the penetration depth of the site under investigation. Based on the results obtained, the company will then decide which method of analysis supplies sufficiently accurate results at acceptable costs. Since wind directions and the frequencies occurring at the sites are also determined, the wind energy yield potential can be differentiated by wind direction.

Turbulent times ahead

In forested sites (see case study here), the turbulence induced by trees must be taken into account up to heights of 90 metres (roughly two to three times the height of the trees). The forces caused by turbulence may adversely affect wind turbine stability.

Having turbulence assessed in an expert report to find out whether it may be critical for planning purposes and for the wind turbines themselves is recommended.

Turbulence can affect the rotor blades of even modern turbines with hub heights of 140 metres and rotor diameters of 100 metres. To make matters worse, the wind turbines themselves also generate turbulence at their lee sides (the sides sheltered from wind) when the wind blows through the rotors, giving them additional rotational energy. Given this, on the lee sides of wind turbines sufficiently large clearance between the turbines must be ensured to avoid losses in profit.

Before a permit for a wind turbine or wind farm can be issued, an expert report analyzing the potential impacts of a project must be commissioned. This addresses the issues of noise emission, shadow flicker and ice shedding, amongst other factors. As forest wind farms are generally located at larger distances from residential areas, noise emissions and shadow flicker are mostly negligible. However, experts must make sure that people on nearby forest or field tracks or hiking trails will not be in danger of being hit by ice throw. In addition, ice forming on the turbine may reduce energy yield. Special de-icing systems are available to prevent this.

The regulations defining precisely which forested areas can be used for wind turbines differ from one German state to the other, and likewise in other countries. Therefore, projects must be assessed on a case-by-case basis, taking regulations of environmental law and aspects of nature conservation into account.

It should be noted that acceptance by society and the general public may pose a challenge to some forest wind project proposals. The best approach is to involve the general public from an early stage to smooth out any possible conflicts of interest.

On the upside, the trend of citizens investing in wind energy projects is becoming more widespread as the positive impacts on both the overall value chain and municipalities gain recognition. In North Friesland, for example, around 90% of wind farms have been organised by cooperatives.

Interested in wind farm development? Read our guide - Wind Power: from conception to reality

About: Thomas Arnold is from Wind Cert Services at TÜV SÜD Industrie Service GmbH, Regensburg, which offers comprehensive and customised knowledge services and assessment throughout all stages of wind farm development, including wind measurement and wind reports, yield forecasts, site assessment and wind farm and type certification.