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
Dr. Sagarkumar Agravat says
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Please go through below link and download the complete publication. Shall look forward for all sort of comments and technical discussions which are in favour of my concept or which disapprove the same. All sort of comments without any bias shall help wind industry.
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Dr. Sagar Agravat
Dr. Sagar Agravat says
Is it possible to dynamically change swept area by modifying rotor dia and chord to match wind and generate more out of it?
Paul Dvorak says
Thanks to all for the good comments.
One point of the post is that just making things bigger is not an immediate solution to capturing more wind power, and that capturing more power from a single wind turbine will take some unconventional thinking. To anyone who says, “That won’t work”, I ask, has it been tried? If a wacky idea produces a working prototype, maybe the idea is not so wacky. OEMs have done a good job wringing incrementally more power from the wind with good blades and better controls. I think the stage is set for a disruptive idea that produces more than an incremental improvement. Keep an eye on left field.
Helmuth Geiser says
New airfoil developments already harness more of the wind power, and even newer developments are on the way to increase efficiency at low wind speeds. Also the development of “intelligent” multistage direct drive PM generators is well on his way.
Stay tuned for news coming to you soon 🙂
This is very interesting discussion. I don’t know the background of Paul Dvorak, but it appears he has done his homework and knows what he is talking about. In general I like what he has written as long as a few points are clarified.
There are a few areas that I don’t agree. He wrote “… but a huge percentage of the energy blowing through a turbine rotor is lost, especially at high speed. …” Well the Betz limit for power coefficient is 59% and some turbines claim to have power coefficient (or Cp) of about 49% to 50% (Vestas is about 48%), that is pretty good. I cannot call this huge percentage lost since capturing half of the wind power, is exactly that half. But as wind speeds get faster, the CP drops to 20% and then to almost zero. But there is a reason for this. First, the pitch of the blades is controlled intentionally to reduce wind loads on turbine blades since they are huge (over 40 m long). Second, the industry has decided to optimize the blades to have maximum Cp in the range of 15 to 20 mph (7 to 9 m/s) because majority of time wind are in this range. In fact only 20 to 25% of the year, one has wind speed in the range of 12 m/s or higher.
The general statement that one has to forget about higher wind speed is simply wrong no matter how one views it. From physics point of view, the air energy density is much higher. Perhaps one cannot have it both ways, keep making the blades larger and largest (using the sweep area or squared of blade size) and high speed at the same time. So if we decide to capitalize on high speed, first we have to figure out how to control and generate high speed wind most of the year, and second how to harvest wind power using smaller blades so we are not limited by wind loads when speeds are high.
None of the proposed solutions will work. Variable gearbox system sounds good, but it will not work. Gearboxes are a major problem for industry to start with. They are the number one or two in failures and because of downtime that is huge cost for utility scale wind power plants. Also, they increase O&M.
The 2nd suggestion we should explore how we can capture more between 3 to 11 m/s is exactly what industry has done. They have optimized the blades at 7 to 9 m/s and have made the blades as big as we can see them now. The current turbines could be as big as 160 m in diameter. Even those in favor of wind can see we have reached the limitation of blades sizes due to manufacturing, transportation, installation, and maintenance.
The last suggestion of “electrostatic generator” will also not be the solution we are looking for. In fact, the issue raised in his write up has to do with blades (or rotor) not generators. The wind industry has not been able to take advantages of state of the art in the generator technologies because the rotor should turn at 10 to 15 RPM. There are a number of high speed and direct drive generators. But at the end of the day, or perhaps we should write at the beginning of the day, one should convert wind power to mechanical rotation, and that is the function of rotors, and NOT generators.
One solution that Dave did not suggest was to be able to control wind itself. That might be the only hope left.
Drew Devitt says
Thanks for the challenge! You know we are good for it here at American Offshore Energy, Remember the piece you and Steven Bushong did on our floating VAWT? We called out that at median wind speed say, 6 meters per second, Aerodynamic HAWT have lower efficiency because of their low solidity, note they are near their cut in speed where they have zero efficiency. Drag type machines using sail fabric are low weight and low cost and would catch more of that most frequently encountered wind.
Here is a link to your November coverage, thanks for that;
Here is a link to a no sign in required white paper on our site that shows efficiency curves for different types of aero and drag turbines related to tip speed. Also a chart that shows the total power in the wind for each wind speed, times the time the wind blow at each of those speeds, this give an intuitively obvious image that help visualize your chart above. Turns out 11m/s is the max power point;
Our floating wind turbines combine well with the energy storage you covered in this link, because the deep water 30 or more miles offshore provides head for pumped storage.
And credit to you for covering electrostatic motors, they play to the low speed of that most frequently encountered wind and generate high voltage direct current (HVDC). This dramatically reduces the balance of plant costs by eliminating AC frequency and phase conditioning. I can’t wait to see your June issue.
John Friedson says
Lower speed solution exists. http://www.sheerwind.com