From prognosis to reality : about the accuracy of energy yield assessments

The accuracy of pre-finance energy yield assessments (EYA) and the uncertainties connected with them is of special importance for developers, operators and investors of wind farms, as these form the basis of financing and economic decisions. In Germany, it is as well the basis for the determination of the site quality ("Standortgüte") and correction factor according the renewable energy sources act EEG 2017. For verifying the accuracy of rheir energy yield assessments, UL DEWI performed a backcast study for 235 wind farms globally between 1997 and 2015. This investigation covers the "German" method for wind farms located in Germany, for which energy assessments mainly base on reference energy yields of neighboring wind farms, and international projects, mainly in France, which base on on-site wind measurements. Several aspects that affect the comparison are discussed and several dependencies identified. Within specific case studies, possible reasons for deviations are explained.