The Physics of Windmill Design

This video is about how physics dictates the design of modern windmills - why they are so big, have so few blades, and have such skinny blades.

REFERENCES
H. Glauert: Aerodynamic Theory, 1935 Division L (Airplane Propellers), Chapter XI: Windmills and Fans

Wind power extraction fundamentals

Betz's Law

Tip Speed Ratio

Aerodynamics of Wind Turbines Book

Penn State Wind Turbine Aerodynamics Lesson

Wind Power Physics youtube video

Why do Wind Turbines Have Three Blades?

Wind Power Fundamentals

Wind Power Explained

Drag Coefficient

Reynolds Number and Drag

Reynolds Number

Viscosity of Air


MinutePhysics is on twitter - @minutephysics

Minute Physics provides an energetic and entertaining view of old and new problems in physics -- all in a minute!

Created by Henry Reich

[Squantle]

I’m scared for that bass player, forever trapped in the recording studio playing the same riff.

[JohnSmith-kjod]

I'm not gonna lie, This hasn't ever occured to me and don't think it would ever have without this video

[jorndielen]

Fun fact: now a days a lot of wind mills come with serrations. Which are tinny triangle like shapes put in rows on the back side of the blade to reduce the noise the blade creates from cutting true the wind.
The idea is based of the wings of an owl.
I work as a techician on these things so further questions are welcome :)

[EEVblog]

In theory and simulations there are designs that use ducting around the blades and other mechanisms that claim to have beat Betz's Law. But in practice none of them have worked. Many wind turbine startups have tried and wasted a lot of investor dollars. Commercial wind turbines usually cap out at around 80% of Betz's law due to the hubs and other design factors.

[flupoop]

I have a master's in energy engineering but the fact about the rotation speed was new to me. You never run out of learning. Thanks.

[physicsfun]

I love a video that answers questions I only know I had as I watch the video- noticing lately smaller turbines along the highway being replaced by larger ones.

[theboulder]

I read it as "Why don't windmills have snails" at first and I was granted a glimpse into a much brighter timeline

[ImKibitz]

Never thought about this, cool video!

[filippoorologio]

watched these guys since middle school, and they haven't changed much.

[isaach]

Not to mention the shear force being applied to the tower. If you replaced the blades with sails, The tower would have to be massive to support the horizontal force being applied to the nacelle.🌬

[yeyo]

Teacher: there are no stupid questions
Student: why don’t windmills have sails?
Teacher : bill please don’t ask stupid questions

[saali]

“What kind of music do you like?”
“I’m a huge metal fan”

[MotoCat]

I'd love to see a follow up to this comparing the 3 bladed direction windmill to those smaller profile helical ones which work in all directions

[Canadian_Ry]

Isn't it supposed to be a 'wind turbine' as opposed to a wind mill?
I suppose 'rotating bladed wind energy capture device' isn't very relatable.

[MordecaiV]

I appreciate that you have to vastly simplify to get these across in a short time, but another design element that affects the design is the designed wind speed range. Lower cut-in and lower operating wind favors a higher blade count. This is why 'old west' homestead water pump windmills are multi-bladed with a 'solidity' of around 60-80%. They are optimized not for peak power, but low speed wind operation and better torque. Source: Graduate Level Wind power design course at the Technion in Haifa, Israel.

[TheScienceBiome]

Its always hype when Minutephysics uploads

[PaulPaulPaulson]

Beware: After watching this video, you might get spam mails with subject "enlarge your windmill"

[JAzzWoods-ikvv]

Bass riff is in 5/4 and this is the first time I’ve seen someone talk about in the comments. Interesting

[schmidt]

Last semester I had a month-long freshman engineering project where we investigated all kinds of aspects of windmill design. My team struggled to understand some of the results of our measurements, and this video was a major epiphany moment for me. For instance, we found that power generation didn't necessarily scale with blade count - a medium number of blades generated the most power, and also allowed for a faster windspeed through them. We figured this had something to do with turbulence, but this video really put it all together for me. I wish I had seen this last semester!

[ChadEichhorn]

As homework in my Computational Fluid Dynamics class, we had to determine what percentage of the wind's energy was extracted by a provided modern windmill design as we altered the number of blades. Everyone thought their results were wrong - 3 bladed windmills were notably less efficient than 4 bladed ones, which were a tiny bit less efficient than 5 bladed ones (efficiency started falling past 5 blades). If our results were correct, why did we only use 3 bladed windmills in practice?

The answer, our professor explained, was cost-benefit analysis. Sure you could get more energy out of a 4 or 5 bladed windmill, but the blades cost a lot of money. Someone determined that for the lifetime of a windmill, the cost of the 4th blade wasn't recovered by the additional energy gained, and so we only see 3 bladed ones.