Surface turbulence intensity as a predictor of extrapolated wind resource to the turbine hub height: method's test at a mountain site

Following testing at the Cabauw (Netherlands) flat and inland site, and at the FINO3 offshore platform in the North Sea (Germany), the alpha-I wind resource extrapolating method was tested at the Boulder (CO, USA) mountain site (1855 m), another substantially different location in terms of surface characteristics, stability conditions, and wind energy pattern. Data from local 82-m M2 met mast between 10 and 80m were used, with extrapolations to 50-m and 80-m turbine hub heights performed based on 10-m and 20- m turbulence intensity observations. Trained over a 2-year period (1997-1998), the method was validated on the year 1999. Slightly better results than those at both Cabauw and FINO3 were achieved in 50-m and 80-m wind speed extrapolations, with bias within 5%, NRMSE=0.17-0.23, and r=0.96-0.98. In predicting the annual energy yield, a bias within 1% was achieved at 50 m, which at worst increased to 6.44% at 80 m. The method was less stability-sensitive than at Cabauw and particularly FINO3. It proved to be reliable even over a mountain site affected by fairly complex terrain, which is noteworthy if considering the power law the method is based upon was actually developed for flat and homogeneous terrain.