Pitch Versus Stall: The Numbers are In

By Paul Gipe


The following article appeared in an edited form in Renewable Energy World‘s July—August 2003 (Vol. 6 No.4) issue.

As turbines grow beyond the 2-MW threshold, it appears that all manufacturers will use pitch control to some extent for power control. Even the stalwarts of stall, Bonus and NEG-Micon, are using pitch control on their big turbines. The reasons are many, but mostly because variable pitch can be used to actively manage the massive rotors on multimegawatt machines where any mishap can spell disaster.

One claim by pitch control advocates, that variable-pitch rotors are more efficient at winning energy from the wind, has been hard to verify.

However, at the beginning of the new millennium longtime wind veteran Helge Petersen examined the question for the Danish Energy Agency.

Petersen evaluated the theoretical performance of a broad range of turbines from their measured power curves. He included turbines from the 500-kW class and larger, including both Vestas’ V66 and V80, Enercon’s E66, Enron’s 1.5s, and Bonus 1.3 and 2-MW models.

Like most Danes, Petersen doesn’t waste time with capacity factors, plant factors, or full-load hours. He goes to the heart of the matter and using the measured power curve calculates annual energy production under various wind conditions. From that he then derives the annual specific yield in kWh/m2/year and the specific capacity in kWh/kW, measures more appropriate to wind turbines.

Petersen found that there was a “small yet significant” performance advantage to pitch control overall. But more importantly, he found that when turbines are compared to each other on the basis of specific rotor loading in kW/m2 that there was hardly any difference between technologies when stall-regulated turbines operate at dual speeds–as most do. Specific power loading is the ratio of the generator’s rated power to the rotor’s swept area.

Rotor loadings among the turbines evaluated varied from a low of 0.332 kW/m2 for a NEG-Micon NM600/48 to 0.585 kW/m2 for a Vestas V66, 2-MW turbine.

For example, in 7 m/s wind regime, says Petersen, a stall-regulated turbine operating at dual speeds with a rotor loading of 0.35 kW/m2 will produce 1,020 kWh/m2 of rotor area per year. A pitch-regulated turbine operating under the same conditions will produce 1,045 kWh/m2 per year, a 2.5% increase.

Those who track the industry’s performance–in the field and on the test stand–will find Petersen’s report a useful compilation of power curves and energy production estimates for 32 different turbines, many of which are still available on the international market.

Evaluation of Wind Turbine Performance, 2000, by Helge Petersen for the Danish Energy Agency, ENS-51171/00-0016.

Helge Petersen Consult
Klostermarksvej 17
DK-2700 Broenshøj
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