Let’s look at the generation possibilities above and below 11 m/s. First, consider high speed wind.
And this is just a observation and not criticism, but a huge percentage of the energy blowing through a turbine rotor is lost, especially at high speed. For instance, a typical power curve shows that at about 11 m/s wind, a turbine begins producing its nameplate capacity. Above that figure, the controls are such that the turbine’s output is limited to its rating, and justifiably so.
Consider a typical 2 MW turbine. From about 11 to 25 m/s the output is capped at 2 MW. If the rotor were allowed to speed up with wind, the rotor could fly apart (as some youtube videos show) and who knows what internal damage might happen from excess rpm.
To understand the problem and opportunity of high-speed wind, recall that the power in the wind rises exponentially with speed. To be precise, energy in the wind is proportional to the cube of wind speed, like this:
Where P = power, and V = speed.
Knowing that a conventional 2 MW turbine produces 2 MW at 11 m/s wind, we can find the proportionality factor for this equation
P = kV3,
Solving for k
k = P/V3
So for our 2 MW turbine:
k = 2 MW / (11 m/s)3
For our purposes, we can ignore units.
Now we can find the extractable power, Pe, by conventional methods from other wind speeds by calculating
Pe = kV3.
A table is helpful showing the power in the wind
What does this mean?
It means that in 24 m/s wind, when a conventional turbine is close to shutting down, it is capturing just 10% (2 MW/ 20 MW = 1/10 = 10%) of the wind’s energy, meaning 90% just blows through the rotors. The figures are based on an actual turbine, not a theoretical possibility, so the proportionality factor includes the Betz limit, the 59% limit of what is really an upper limit of energy a turbine can capture.
A consolation thought is that winds of 24 m/s and faster occur only about 5% of the time. Most often, about 85%, if memory serves, a turbine is working in wind from 3 to 11 m/s.
A couple ways around the mechanical limitations might be variable-speed gearboxes that would keep rotor speed constant and let the generator speed up. Readers might recall just such a transmission featured on these pages about September of 2011, here: https://www.windpowerengineering.com/design/mechanical/variable-speed-hard-geared-transmission-may-improve-wind-power-efficiency-5-to-10/
Another development might be a generator that is tolerant of greater speed or output. High-temperature superconducting wire might building a small-diameter generator or some other device.
Or the question should be: How can a rotor capture more wind between 1 and 11 m/s, effectively moving the power curve’s upslope to the left, or make it more vertical? Vortex generators do this to some extent, but is there another way? (Hint: Wait for the June issue).
But I’m thinking linearly, modifying what I know and taking it one step further. This situation calls for some out-of-the-box thinking. A possibility, for example, might be the electrostatic generator, news of which appeared here: https://www.windpowerengineering.com/design/electrical/generators/can-new-breed-generator-recharge-wind-industry/. At least two companies are working on such a generator, but commercialization is several years off.
So the challenge to you inventive minds is to find a way to capture those evasive MW available in frequently encountered wind.
Editor’s note: I edited this lightly to include comments from sharp-eyed readers.
Filed Under: News, Turbines