Wind Turbines - Wind Energy Systems for Electric Power Generation - page 26

reference yield is the calculated energy supplied by a specific type of wind system
with trhe relevant hub height at the reference location, on the basis of a measured
power characteristic over 5 years. Expertise is required that the wind system is capa-
ble of at least 60% of the reference yield, in order to receive the established energy
price from the utility.
The energy yield per month is of course strongly dependent on the time of the
year. For two countries on the Northern Hemisphere, Austria and Germany, reported
values of 2002-2005 of onshore wind parks collected in a database are visualized
by Fig. 2.20 (Source: Betreiber-Datenbasis and Interessengemeinschaft Windkraft
Osterreich). It gives an impression of differences not only between months but also
between several years. Note that in the meantime the distribution of participating
WES has changed to higher unit ratings, together with a better utilization.