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Low-wind turbines tested by conditions

Data collated in September from nearly 12,000 wind turbines with a cumulative installed capacity of close to 17GW across Germany and Denmark revealed a continuation of the summer's main trend.

Warm weather and consistently low wind speeds — over Germany in particular — have been suppressing volumes of wind-generated electricity.

The average capacity factor for the German turbines contributing data was just 11%, down marginally on August's average of 12%, and representing the seventh consecutive month of sub-20% output.

Denmark enjoyed slightly better wind resources during the month, with its turbines recording a respectable average capacity factor of 19%, slightly up on its weakest month of the year, June, when the capacity factor was just 17%, but well down on August's impressive average of 27%.

The data suggests that older turbine models, many from manufacturers no longer in business or subsumed within other firms, have struggled in the summer's low winds. Only contemporary long-bladed machines on tall towers have the design specification to make the most from these conditions. That can be seen most clearly in the performance of Vestas, GE and Senvion (middle chart), where their average capacity factors have generally been a few percentage points higher than companies such as HSW, Sudwind and DeWind.

Conversely, both Enercon and Nordex, leaders in low-wind turbine design and development, appear to have performed less impressively, but this is more of a reflection of their concentration in the market most affected by the month's low winds.

Siemens is the clear outlier in terms of overall manufacturer performance, a result of the high number of its offshore turbines that are reporting data.

But the offshore sector was also affected by low wind speeds during September according to the evidence provided by individual turbine types.

Siemens leads on land and offshore

Siemens' 3.6MW offshore workhorse was the only reporting turbine to achieve an average capacity factor above 30% in September, although its 31% rating was well down on August's 36%.

More surprising was that Siemens' 3MW onshore platform was such a clear winner on land, too. The 57 units reporting provided an average capacity factor for the month of 24%. This compares to 16% for the 3MW Vestas, 3.05MW Enercon, and 2.4MW Nordex turbines, and just 13% for GE's 2.75MW unit.

Smaller onshore turbines all reported low capacity factors, ranging from 9% for Gamesa's 2MW machine to 15% for Nordex's 1.5MW model.