FOREST WIND FARMS
Since the latest generation of wind turbines have hub heights of 140 metres and more, the wind mast should be at least 100 metres high. Higher anemometer masts are rarely used at present, to limit the costs incurred by their building and installation.
LIDAR INSTEAD OF SODAR MEASUREMENTS Wind speeds at heights above these anemometer masts can basically be determined by two remote sensing techniques – SODAR and LIDAR.
ACHIEVING RENEWABLE ENERGY TARGETS Most German states will only be able to achieve the renewable energy targets defined in energy policies if they include forest areas in their wind power plans. The government of the German state of North- Rhine Westphalia, for example, assumes that it will be unable to expand its share of wind power in total electricity generation from currently three to the targeted 15 per cent without the use of forest wind farms.
CHALLENGE
FOREST WIND FARMS
INNOVATIVE TECHNIQUES FOR WIND RESOURCE ANALYSIS WITH THE HELP OF LASER-BASED SYSTEMS Reliable measurement and simulation of wind conditions are imperative to estimate the yield of wind farms located in topologically complex terrain. Forest areas present wind-farm owners and operators with particular challenges.
What technologies are the most suitable, and what cost-benefit ratio do they provide?
COMPLEX WIND CONDITIONS However, reliable prediction of the yield of forest wind farms requires measurement and simulation techniques that take into account the complex wind conditions caused by trees and uneven terrain – the FGW Technical Guideline requires wind measurement masts.
Unless data measured in the immediate vicinity of the planned wind farm is available, the measurement of wind conditions is indispensable for wind- resource analysis and reliable conclusions regarding the profitability of a potential wind site.
FGW TECHNICAL GUIDELINE The Technical Guideline published by the FGW, the German public association of the renewable energy sector, defines how these measurements must be performed. For the assessment and approval of a project, wind speed and directions must be measured with the help of an anemometer mast. According to the FGW guideline the mast should have a height of at least two-thirds of the hub height of the planned wind turbine.
SODAR The SODAR technique is based on a mobile acoustic system, measuring wind direction and speed with the help of sound waves. However, this technique is only applied in open plains as the trees in forest areas shield both the acoustic waves and their reflections.
In addition, at heights over 100 m excessive scattering of the reflected waves reduces the accuracy of the measurement instruments.
LIDAR Given this, TÜV SÜD relies on the LIDAR technique as one of its measurement methods. LIDAR stands for “LIght Detection and Ranging”. The laser-optical method works similarly to radar but uses laser beams instead of radio waves. The technique permits wind direction and speed to be determined at heights of up to 200 metres and ensures accurate measurements even at these heights. If the results obtained with an anemometer mast are combined with those obtained by LIDAR, complete wind profiles can be determined at heights of over 100 metres and beyond rotor diameters.
RESEARCH Researchers at Stuttgart University have worked with scientists of the National Renewable Energy Laboratory (NREL) on using the LIDAR technique to optimise the operation of existing wind turbines. By knowing the parameters of a wind field before it actually hits the turbine, the owners/operators can take appropriate control measures at an early stage and thus may reduce wind-induced loads and increase energy yield at the same time. The research results are aimed at facilitating the dimensioning and construction of future wind turbines, thus saving materials and costs.
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www.windenergynetwork.co.uk
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