Wind Turbines - Wind Energy Systems for Electric Power Generation - page 25

2.4.4 Annual Energy Yield

The annual energy yield depends on the wind speed distribution at the system lo-
cation in nacelle height and on the machine design parameters. A characteristic
parameter is the ratio of the average annual wind speed and the designed rated wind
speed. Under certain assumptions (Rayleigh wind speed distribution, design power
coefficient c p = 0 , 46), Fig. 2.19 shows curves of the relative annual full-load en-
ergy yield [Gas07]. Considering practical design ratios rated/average wind speed of
1 , 5 ... 2, it is obvious that characteristic relative full-load equivalent values are not
larger than 20 ... 35%.
A reference energy yield is defined in the German EEG [BRD04]. A reference lo-
cation is a place with a Rayleigh-distributed wind speed of 5,5 m/s average in 30 m
hub height, with a logarithmic elevation profile and 0,1 m roughness length. The

Fig. 2.19 Full-load equivalent annual energy yield over ratio rated/average wind speed

Fig. 2.20 Energy yield kWh / m 2 per month as recorded in Germany and Austria