The reliability of the economy of a wind farm is based on the accuracy of the assumed inputs for the energy yield prediction. Wind potential assessment and the power performance of the turbines are the most crucial points. The following risks are to be taken into account by evaluating a wind farm project (due diligence):
The site assessment starts with one (or several) wind speed measurements (preferably at hub height). The uncertainties related to wind speed measurements are varying between 3 and 20 (!) % in terms of energy production depending on the quality of the measurement system and the anemometer calibration (see IEA Recommendation 11: Wind Speed Measurement and Use of Cup Anemometry). Not only the mounting, the quality and calibration of the anemometers is important, but also the regular plausibility check of the data.
If the wind speed is not measured at hub height an extrapolation of the wind speed from the measurement height to the hub height has to be performed. The results obtained during the measurement period (usually one year) have to be correlated with the data from a nearby met station (MCP= Measure Correlate Predict) in order to get the long term wind potential.
Then a so called micro siting is performed in order to determine the wind potential at each turbines position within the wind farm area. The micro siting is usually done with a flow model and/or a mobile SODAR system. SODAR is a kind of acoustic radar with which we can measure the wind seed up to a height of 150 m with 5 m resolution. This system is very flexible and can be easily transported from one site within the wind farm to another site of planned turbine positions.
Next the wind farm efficiency has to be determined taking the wake effects of the turbines into account. Finally with the help of a measured power curve the energy yield of the wind farm has to be calculated taking the guaranteed availability and the guaranteed power curve of the turbines into account.
The plausibility check of a guaranteed power curve is a very complex task within the due diligence work. Questions have to be answered like: according to which standard and which quality assurance system the power curve is measured. Is the turbine planned in the project technically identical to the measured power curve (use of stall strips or vortex generators, contamination of blades, changes in the blade angle, noise reduction measures which influence the performance of the turbine) ? The verification method of the power performance has to be defined in the contract in all details in order to reduce the financial risks to an acceptable limit.