Tubercle Technology and wind turbines were made for each other. Tubercles allow turbines to overcome the three major limitations of wind power:
- poor reliability when winds fall or fail
- noise – especially tip chatter caused by tip stalling
- poor performance in unsteady or turbulent air
To really appreciate the advantages, it’s necessary to examine wind power with a critical eye.
WIND POWER’S DEFINING COMPROMISE: THE LOW WIND / STALL LIMITATION
Wind is an unsteady energy resource at best: Winds routinely fail or fall, disrupting power generation. Decades of practical study and application have made it clear that the performance and efficiency of wind turbines is determined by an unforgiving range of compromises which must be mixed in an artful manner in order to derive optimal (or, more often “acceptable”) performance.
The unavoidable reality is that winds routinely change speeds, change angles, swirl and display turbulence. This instability forces turbine designers to play it safe: They back off on performance in favor of more stable operation. The management of such compromises is particularly complex with respect to lift and power relative to drag and stall angle and their combined impact on power generation. Improving performance is inherently difficult. Getting more power from comparatively infrequent higher speed winds has not been a fruitful option.
- Some state of the art turbines shut down completely above the so-called “cut-out speed“ in order to avoid damage to the generator and even the rotor blades themselves.
- Stronger, heavier generators and blades which could operate in higher winds produce lower performance in the more frequent, moderate winds. The trade-off imposes upward limits on wind speeds.
- “Optimized” drives are designed to produce what is called the “rated power” or “nameplate power” at a designated wind speed but above that speed, they routinely waste some of the extra power available at faster wind speeds so they won’t exceed the peak or “rated” power in order to avoid wear and tear on the generator and blades.
That means that every modern turbine can produce power in high speed winds. The trouble is, high speed winds are comparatively rare.
Even more important, that also means that the best hope of increasing the average power output over all conditions, (best measured as the so-called “annualized” power) lies with improvement in low speed operation.
There have been important advances. Multi-generator turbine configurations reduce the load on the drive train in low speed winds by reducing the number of generators under power, which in turn increases efficiency and lowers the minimum, (or “cut-in”) operating speed.
Ultimately, however, the most significant way to improve low speed wind performance is to increase the pitch or operating angle of the rotor blade. Variable pitch turbines balance power production to a degree by increasing pitch as wind speeds fall, but they’re limited by the inherent aerodynamic limits to the stable pitch angles possible with conventional airfoils.
- All airfoils stall (lose lift) at some angle.
- The stall angle is in part a function of the wind speed itself and the direction of the air flow.
- The unsteadiness of the air forces designers to reduce pitch angles to avoid stalling. (In early attempts to increase lift, sudden stalling by one blade in a rotor while the other continued to lift literally tore some the turbines apart.)
- Attempting to pitch blades more steeply induces stalling and drag — both of which effectively act as a brake on power generation.
In the real world, coventional rotor designs must compromise.
- “Stall-regulated” turbines use airfoils which use the stall at different parts of the blade to limit rotation rates.
- “Pitch-regulated” turbines actively vary the pitch of their blades in their efforts to increase lift as winds fall and manage excess power as they rise. (Some large turbines even vary the blade’s pitch between the top and bottom of the rotation).
- The many instabilities of the wind impose further compromises: to operate within a margin of safety, modern turbines limit the operating pitch to angles well below the maximum stall angle of their airfoils. (In one extreme case of a “state of the art” turbine, the tested stall angle of the airfoil is almost 20 degrees but the safety margin imposes a pitch limit of less than 9 degrees.)
It all adds up to poor performance in low wind, which produces inevitable instability of supply which in turn means that there has to be a backup source of power, (usually called a “spinning reserve”), to match demand when the wind falls. Electrical ultilities use a rule of thumb: For every 200 megawatts of wind you need at least 100 megawatts of coal.