Wind index data in these countries is based on the productivity each month of a number of reference wind turbines over a wide geographic area. The index establishes a statistically "normal" period of yearly wind energy content, expressed as 100%. Monthly wind energy content in the summer months in Denmark can be as low as 50% of a statistically average month, but as high as 180% of an average month in winter.
The indices enable wind turbine owners to establish whether variations in energy productivity are due to deficiencies in wind turbine performance or to wind strengths below expected levels. The procedure is analogous to monitoring the efficiency of steam turbine plant -- the operator must demonstrate that the energy content of the fuel (coal, gas, or wind) is not deficient.
The European continental wind indices are also used, unusually, as the basis for predicting energy yield of new wind plant, without which financing terms cannot be established-a cause for controversy (main story).
The reasons for the evolution of wind indices in northern Europe and nowhere else seem to be mainly historical. Without a measure of whether the wind had blown more or less than expected over a given period, owners were unable to claim their machine was not living up to its performance guarantee. Furthermore, local wind measurements were often not available for widely dispersed areas in which single wind turbines were being installed. The use of a wind index circumnavigated the requirement for site specific wind measurements.
Carsten Albrecht, chair of an expert committee on wind assessment, also argues that winds in northwest Europe are particularly volatile, which is possibly another reason why only Scandinavia, Germany and the Netherlands have developed indices. Year-by-year pressure driven climate variations are typical of northwest Europe, he says. Such variations are found to a lesser degree in the UK and hardly at all in southern Europe where winds are temperature-driven and more stable. Others argue there is a good correlation across all the north European indices, including the new British index (Windpower Monthly, November 2005).
In most other countries, a reference level of energy production for a particular site is established by correlating site wind speed measurements taken over periods of up to several years with long term wind speed data from the nearest meteorological station with reliable records.