Image: United States Government
-
The Best Science Writing Online 2012
Showcasing more than fifty of the most provocative, original, and significant online essays from 2011, The Best Science Writing Online 2012 will change the way...
Read More »
The differences between wind turbine and ceiling fan blades arise from the contrasting design criteria: the wind turbine is intended to capture high-velocity wind to generate electricity efficiently; the ceiling fan needs to move air at low velocity with inexpensive components.
To keep drivetrain costs low, a wind turbine must capture the energy in fast-moving air and rotate at relatively high speed—within limits, so as to avoid excessive noise generation. (Slow rotation would increase the torque and require heavier and more expensive drivetrain components.) Such high-efficiency energy conversion dictates the use of lift-type turbine blades, similar to airplane wings, of twisted and tapered airfoil shapes. The blade design creates a pressure difference in wind—high pressure on one side and low pressure on the other—that causes the blades to turn. A combination of structural and economic considerations drives the use of three slender blades on most wind turbines—using one or two blades means more complex structural dynamics, and more blades means greater expense for the blades and the blade attachments to the turbine.
The ceiling fan, on the other hand, is built to keep the occupants of a room comfortable by moving air gently. Its engineers work to minimize noise while the fan rotates at low speed (for safety reasons) and to keep the construction costs, and therefore the purchase price, low. Energy efficiency is not a primary concern, because operation is inexpensive—a typical ceiling fan running 24 hours a day consumes about 60 kilowatt-hours a month, for an average electricity cost of about six dollars. For this reason, most ceiling fans incorporate blades that are comparatively inefficient drag devices; rotating the pitched blades pushes air vertically out of the way. Wide, flat blades are inexpensive to build and work well as drag devices. More blades are better, up to a point, and the usual layout of four or five blades is the result of balancing trade-offs between efficiency and expense.
A 2001 article in Mechanical Engineering chronicled the quest of a man named Danny Parker to create a more efficient ceiling fan. Parker’s initial blade prototype looked a lot like a wind turbine blade, but the end result (because of manufacturing, safety and operating concerns) was a hybrid between a standard ceiling fan blade and a wind turbine blade.
Note: This question was submitted by J. Lester, Stroudsburg, Pa., and was printed in the February 2009 issue of Scientific American.
See what we're tweeting about
12 Comments
Add CommentYou described the 3 blades wrong. The wind was not at high velocity, but the rotation of the blade was (faster cutting the slow wind) - high speed low torque. Yes cost is the determining factor. But the "arc / sec" of the blade I think allows for a lower wind speed. For example look at the rpm in different wind speeds. we are probably talking about the same thing here. And I just understood your english a little off.
Reply | Report Abuse | Link to this"similar to airplane wings, of twisted and tapered airfoil shapes. The blade design creates a pressure difference in windhigh pressure on one side and low pressure on the otherthat causes the blades to turn."
Reply | Report Abuse | Link to thisIt's a common but widely believed myth that airplane wings generate lift primarily from Bernoulli type pressure differences, I imagine the same myth is being perpetuated here.
So what is the primary mechanism generating lift in airplane wings--angle of attack with nearly symmetrical upper and lower curve of the airfoil?
Reply | Report Abuse | Link to this"wind turbines have three narrow blades."
Reply | Report Abuse | Link to thisAs is known to everone that three piont is stable enough to make up a plate when blades are turn.So we can cost the minimum and get the maximal energy .
If you look at the Lift Equation (how a wing or turbine blade generates lift) and Betz's Law (Gives the maximum energy that can be derved from a given swept area - called the "Betz Limit") you will see that indeed the more blades the better. Three blades will give you no where near the Betz Limit. the problems with more blades are noise and mechanical complexity in the hub to feather all the blades in high wind.
Reply | Report Abuse | Link to thisAbout Bernoulli: The pressure on the "top" of the wing is definitely lower than the pressure on the "bottom" due to speed differential - not a myth. But the air on the bottom still strikes the underside of the wing, pushes it up into the partial vacuum and then bounces down. So the wing causes air to move down while developing lift. MS
Related question: What should the turbines in the Gulf Stream ocean current look like?
Reply | Report Abuse | Link to thisShould the design resemble a wind turbine or should it look like a boat propeller or some other design? Water is denser than air. The current is relatively slow at perhaps 5 mph.
Gulf Stream Turbine project at Florida Atlantic University (FAU)
http://www.scientificamerican.com/article.cfm?id=gulf-stream-renewable-energy
I have a question, if the cost is not considered, how many blades could make the efficiency highest.
Reply | Report Abuse | Link to thisThese blades spin so fast and yet they are so simple
Reply | Report Abuse | Link to thiswww.windynation.com/shop/index.php?act=viewProd&productId=12
I built a wind generator using these blades and I can get enough electricity run all of my outdoor lighting. Not bad at all Ithink.
I have read that the more blades the merrier.
Reply | Report Abuse | Link to thisI am doing research on a new turbine, and have found the same. As of now, the most blades I have tested are 18. This provided a slight improvement over 9 blades. There is most likely a practical limit. Several narrower blades are better than a few wide ones. You cannot plan to have the exit smaller than the entry to the blades, as this will restrict the flow of air.
Hi folks, http://www.youtube.com/user/spiralairfoil#p/u/4/gO3WK89BB5Q At some point in the near future we intend to come up with a ceiling fan that very much looks like this earth Spiral, keep in mind that on the ceiling fan the top will be on the bottom and the overall shape will be much flater, using the Spiralairfoil concept.
Reply | Report Abuse | Link to thisDan Parker
I disagree with this article a little. I use this five blade design on my re-wound car alternator turbine:
Reply | Report Abuse | Link to thishttp://www.windynation.com/shop/index.php?act=viewProd&productId=26
and I have found that I can get more power out of my turbine using five blades as opposed to three. I have actually measured this with a power meter. I am pretty sure it is because my turbine needs lots of torque to get started. Especially when the wind is in the 6-10 mph range.
Since the start of winged flight, we have seen the development of many different types of engine propellers and many different wings. Call me disagreeable, but I will not be satisfied that the three bladed wind turbine design is the most efficient one possible until I have seen other types tested and rejected.
Reply | Report Abuse | Link to this