Long Term Variation of Wind Potential: Are we Moving into a Low ...

1. Long Term Variation of Wind Potential: Are we Moving into a Low NAO-Cycle Period? Axel Albers Dipl.-Phys. Deutsche WindGuard Consulting GmbH Oldenburger Straße 65, D-26316 Varel a.albers@windguard.de testing- and calibration laboratory with quality management system according EN ISO/IEC 17025:2000

2. State of the Art Energy Yield Prediction How accurate is this assumption? Can we find a better approximation?

3. Relevant Climate Variation Studies by Deutscher Wetterdinst No real long term trend since 1880 Indication of dominant periodicities of geostrophic wind of about 35a (German North Sea coast)Indication of periodicities of NAO in winter half year of about 6-8a, 2a and in summer half year of about 10-20a, 3a, 5a Periodicities not stable enough to be used for prediction purposes

4. WindGuard Production Index WindGuard World Wind Atlas index (WWW-Index) Data Source: World Wind Atlas by Sander& Partner, NCEP/NCAR reanalysis data WWW-Index is adapted to individual wind farm conditions (site conditions, turbine design etc.) Good correlation between wind turbine energy production and WWW-index

5. Comparison WWW-Index to other Long Term Sources Good agreement between WWW-Index and average of 7 met stations at German North Sea Coast Good agreement between WWW-Index and GEO-Index,GEO-Index= production index calculated from DWD-time series of geostrophic wind (based on air pressure measurements)

6. Long Term Variation in Germany Even 5-year mean values show high variation between 80-120%. At low wind sites higher variation than at high wind sites 1990’s in Germany clearly above average Variation in South of Germany different than in North of Germany

7. Relation NAO-Index and Wind Potential in Germany Relation week but existent

8. Periodicities of Wind Potential in Germany Periodicities and phases of harmonics are shifted, if 125-year time series of DWD is considered (GEO-index) Periodicities not stable enough for prediction purpose

9. Deviation of Wind Potential (GEO-Index) of Successive Time Periods

10. Uncertainty of AssumptionWind Potential Past=Wind Potential Future Tendency of standard uncertainty to converge for long lengths of past time periods Recommendation: Past Time Period at least 30a Magnitude of uncertainty is dependent on site and lengths of future period

11. Model for Uncertainty Wind Potential Past Time Period and Future Period are independent Approximation standard deviation of n year averages: Standard uncertainty assumption Wind Potential Past= Wind Potential Future:

12. Variation of Wind Direction Frequency Distribution Variations of wind direction pdf correlate with wind potential variations In tendency shift to opposite wind directions Resulting uncertainty on modelling of wake effects (wind farm efficiency) not yet analysed

13. Situation Germany expected wind farm energy production strongly oriented towards wind potential of the nineteen nineties IWET-production index widely in use, since recently with base period 1989-2002, until 2003 with base period 1989-1999. Wind potential of nineteen nineties was above long term average

14. Conclusions Significant risk linked to long term cycles of wind potential in northern Europe; longest known cycles larger than expected lifetime of wind farm projects In Germany expectations often too high because of orientation towards wind potential of nineteen nineties (above average) Recommendation: base period for definition of 100% wind potential at least 30a Reliable climate prediction for the next 20-50 years would be extremely useful for planning of wind farms.