Applications for Stall and Pitch Control in Wind Turbines

Prior to the mid-1990s — stall controlled wind turbines were the norm for regulating power on the rotor of a wind turbine. Then the turbines got bigger, with much more output capability, and stall control gave way to a blade pitch control actuators.

Today, stall and pitch regulated wind turbines still both have their uses, and this post will discuss the difference between the two, and why some turbines are better suited for blade pitch control than others.

What makes pitch control the choice for utility-scale wind turbines? Why are a growing number of distributed-scale wind turbine manufacturers turning to blade pitch control? Keep reading to learn how both pitch and stall controlled turbines have a place in the wind energy industry.

Stall and Pitch Control in Wind Turbines: What’s the Difference?

What is pitch control in wind turbines? And what is stall control? 

Stall controlled, or stall regulated wind turbines come in two flavors: active and passive. The most simple designs are passive stall control. 

This means that the shape of the wind turbine blades themselves, and the behaviors of the entire nacelle in high winds are all designed to become less aerodynamically efficient as wind speeds increase. As a consequence, wind speeds above a certain point will begin to slow the wind turbine.

Needless to say, the aerodynamic design of passive stall controlled wind turbines need to make some compromises to both speed up and slow down the wind turbine. Compare this to holding the accelerator and the brake in your car at the same time – it’s not very good for gas mileage.

This brings us to active stall control, the second category of stall controlled wind turbines. 

Active stall control means that the turbine will actively change the shape of the blade to begin slowing the rotor when wind speeds reach a certain level. Tip brakes, or speed brakes can deploy mechanically on the blades when the turbine reaches a certain speed. Just like speed brakes on the wing of a plane, these mechanisms spring out and add drag to the blades, which cause rotor power and speed to decrease.

Pitch control, on the other hand, uses a system that instructs each individual blade to rotate from power to feathered position. It’s an active, mechanical process.

Which Control System Will Maximize Efficiency?

The short answer: pitch control.

One hundred percent of the kinetic energy contained in the wind that passes through the rotor of a wind turbine cannot be converted into electricity. There are losses almost every step of the way. Among these are:

• The Betz Limit
• Aerodynamic efficiency losses
• Power electronics efficiency losses

Active pitch regulation gives the wind turbine a chance to configure itself perfectly for any rated wind speed. The blades are shaped to speed the rotor with no consideration for slowing or stopping the wind turbine. For this reason pitch controlled wind turbines are the most efficient, compared to their stall controlled counterparts. All gas, no brakes.

Active stall control is next down on the efficiency list. This is because, like pitch control, the active stall controlled turbine can make changes to the shape of the blades “on the fly” by deploying tip brakes or spoilers when wind speeds get too high.

Passive stall control provides the least aerodynamic efficiency of these categories because it is always trying to both speed and slow its rotor. At certain wind speeds, the slowing effects start to “win” and the turbine will gently furl, yaw out of the wind or progressively unload the forces on the rotor by some other means.

Stall controlled wind turbine technology doesn’t require the electronic components that modern day pitch actuators have and are consequently lower in cost.

Which System Is Easier to Install, Repair, & Replace?

We’ve sung the praises of pitch control, but stall control isn’t without its positives. 

Stall control turbines are less complex and smaller than pitch control turbines, so they’re easier to install, repair, and replace.

That being said, pitch control still takes the win in this debate. Modern advanced pitch systems from Windurance are completely maintenance free.

So if there are so many features that pitch has over stall control, then why choose stall control?

Which Control System Is the Right One?

It all depends on the size of the turbine.

Multi-megawatt turbines need pitch control because stall controlled shutdown methods are not effective. Good luck finding a disc brake large enough to stop a massive wind turbine, or dealing with the huge forces necessary to yaw a 90 meter rotor out of the wind 

On the opposite end of the spectrum, 10-20 kW wind turbines generally use stall control because a complex blade pitch actuator system is relatively expensive, and these turbines don’t generate enough energy to justify the cost of a pitch control system. 

But, in the sweet spot area between the 10-20 kW turbines and the multi-megawatt, utility-scale installations are new candidates for blade pitch control that are rapidly emerging as feasible thanks to the groundbreaking work by Windurance to lower the cost of blade pitch actuator systems.

Generally speaking, turbines that are 50 kW to 300 kW can justify the cost of blade pitch control. Windurance is committed to continuing to decrease the cost for blade pitch actuators, and applying this technology to smaller and smaller machines in a cost effective manner. 

At the end of the day, the Levelized Cost of Energy (LCOE) that the turbine generates needs to be competitive. This alone will dictate if the cost of a blade pitch actuator system, and increased generation capacity (or savings elsewhere in the machine) can be justified by the turbine OEM.

Experts in Pitch Control in Wind Turbines

Choosing the right control system — whether pitch or stall — is critical to the reliability, performance, and service life of any wind turbine. Making the right choice can lead to lower costs and a higher ROI.

Partnering with an expert in pitch control systems can help you make that choice. Click below to learn more about how pitch systems might improve your wind turbine performance.