Number of blades

thehardway · March 2011

My site is situated in a mountain gap. I experience average summer day thermals of 5-10 mph starting at about 10:00 AM and dying off at about 6:30. Just about every afternoon in the late spring and early summer, a thunderstorm storm moves through with 30-60 mph winds and lasts for about 2 hours.

During the winter and early spring I experience winds in the 20-30mph category on a consistent basis.

It seems the majority of wind turbine producers have manufactured their turbines and blades to operate on sites with near steady laminar winds at optimal speeds of 15-30mph. Anything lower and they fail to produce and anything higher and they furl or brake to avoid damage. Most of these turbines are three blade designs. These would be pointless at my site as I don't have optimal winds.

I realize that 3 blade designs are the most efficient when noise, drag, and optimal wind speeds are all factored together but in my situation they are impractical as they would only produce in the late winter and early spring at my site.

I have seen articles lately claiming that more blades will produce in lower winds, self-protect in higher winds due to the increased mass,wave drag and vortices, and generally produce better under heavy loads or turbulent wind conditions.
I have seen this principle ring true in other applications such as boat propellers (4 and 5 blade designs provide more thrust at low speed, with heavy loads and while turning) and Airplane props including the 8 blade scimitar design (Airbus A400M)

With this in mind, using 6 or more blades on a turbine seem to make sense for my site because even thought they are less efficient at optimal winds they would produce over a wider range of conditions.

Please keep in mind I am not disputing efficiency of 3 blade designs for optimal wind sites.

Has anyone here used 5+ blade turbines in a site with light wind and high wind application?
What are the results?

Am I crazy?

BB. · March 2011

Re: Number of blades

Blade design also depends on the alternator and charging system design too...

I don't have any answers for you, but here are some other links/forums that have more Wind and DIY Wind than our forum:

Wind Power Links
www.otherpower.com (good forum for DIY Wind Power)
Hugh Piggott - Scoraig Wind Electric site for tons of info (from mike90045)
www.greenpowertalk.org (added from "russ"--Like here but more wind/less solar)

-Bill

waynefromnscanada · March 2011

Re: Number of blades

One thing is sure, and that's that more blades will start the turbine turning at a much lower wind speed. But keep in mind that just because a turbine is turning, doesn't mean it's producing any usable power. On the other hand, it's nice to look at a turbine that APPEARS to be doing something useful.

MisterB · March 2011

Re: Number of blades

I have been studying this myself. I have a 3 blade turbine, I used to have a 2 blade turbine and I have been looking at the 5 blade version of the 3 blade turbine I have. Like all things in small wind, there is a lot of boastful salesmanship around the question and a lack of real facts. Be wary of anything coming from a dealer who stands to make more money by selling you more blades. I have found some information that is unbiased and even has some physics/math explanations that make sense but very little. The general consensus is that more blades will start the turbine at a lower wind speed but unless the turbine alternator is wound to produce power at a low rpm, this will be useless and there is a penalty to pay at higher wind speeds. I can't see any reason to go beyond 5 or 6 blades because the high wind speed losses will go up with more blades.

thehardway wrote: »

My site is situated in a mountain gap. I experience average summer day thermals of 5-10 mph starting at about 10:00 AM and dying off at about 6:30. Just about every afternoon in the late spring and early summer, a thunderstorm storm moves through with 30-60 mph winds and lasts for about 2 hours.

During the winter and early spring I experience winds in the 20-30mph category on a consistent basis.

The summer thunderstorm winds are turbulent and gusty and more likely to do damage than produce useful power. I plan to have my turbine braked, grounded and electrically isolated from the rest of the electrical system during the summer monsoon season. The power from my PV arrays is more than enough in the summer. The wind turbine is more useful during the rest of the year.

Peter_V · March 2011

Re: Number of blades

From what I've read, for a given wind speed more blades produce more torque but lower RPMS.

So more blades will help the turbine overcome cogging in the generator, which means it will be able to turn at a lower wind speed. However the lower RPMs means that it won't produce as much power at ANY wind speed.

As Wayne said during low winds it will "look" like it's doing something, but it won't produce power.

More blades also reduces efficiency.

The fact is that there simply isn't much power in slow wind so it's kind of worthless to try getting it.

Have you done an actual wind survey of your area? Hook up an anenometer and actually measured the wind speeds over a long period of time (ideally at least a year)?
If not you'll likely be disappointed in the actual production. People almost always think it's windier where they live than it actually is.
With a real site survey and detailed records, you could use the power vs wind speed charts for your intended turbine and get a fairly good estimate of actual power production....assuming of course that the manufacturer isn't lying about how well their turbine works. That's apparently a pretty common trend.

russ · March 2011

Re: Number of blades

Below 5 m/s (10 to 12 mph) wind speed there is not a lot of power in the wind to begin with - nothing to go after. The links provided show this well.

If you have the 5 m/s wind you don't need the additional blades.

Seems to be running in circles to me.

oil pan 4 · March 2011

Re: Number of blades

I am about to try more blades, I will let you know how it turns out.
I have the blades and the hub, I just need to install them on the PMA.

MisterB · March 2011

Re: Number of blades

I'm seeing small but still measurable power out of my turbine at low wind speeds. Less than 10mph gives me a trickle of a few ma to 1 amp. 10-12mph gives something like 2-4 amps @ 12 volts. Enough to power this laptop and my long range wireless internet system at least and my batteries are trickle charged at night which brings the batteries up faster when the sun starts hitting the panels. If 5 blades would increase this even a little, it would actually increase the total output of the turbine a lot because these breezes happen a lot while higher winds are rarer but usually happen when they're really uselful--when there is serious weather happening and the output from my solar arrays is anywhere from 5-50% of what it would be on a sunny day.

And I've still yet to see any real physics or math on 5 blades vs 3 or 2 to either prove or disprove anything.

russ · March 2011

Re: Number of blades

Of course the thing puts out the milliamps at low wind speeds - not enough to bother with but there.

Take a look at the sites Bill posted and you can see - quite easily.

One big thing - no reputable wind turbine manufacturer uses the 5 blades - should be a good indication.

thehardway · March 2011

Re: Number of blades

russ wrote: »

One big thing - no reputable wind turbine manufacturer uses the 5 blades - should be a good indication.

Russ,

To draw the conclusion that 5 blades is not a good design because no reputable mass producers use it is horrible logic at best. By that logic I could say the converse is true as well. There are a lot of non-reputable companies using the three blade design so it must be a bad design.

Reputable companies typically gear up for production in quantity and for maximum profit, that is what determines whether or not they survive. The bulk of wind turbines should be sold to people in areas with steady, laminar winds, this is the low hanging fruit, hence they make a 3 blade version which is optimized for a broadest and most lucrative market.

Most outboard motors come stock with a 3 blade prop. Most airplanes come standard equipped with either a 2 or 3 blade prop. That does not mean they are the best design, and certainly not best for specialized application. They would actually perform poorly and inefficiently in specialized services. They are popular merely because that is what the bulk of the market demands. It is more costly and difficult to produce multi-blade designs which makes them less competitive in the mass market.

According to the links you all recommended, the one on blade theory proposes the most efficient rotor design is the "windflower" design which uses an infinite number of narrow blades.
http://www.scoraigwind.com/wpNotes/bladeDesign.pdf
It is true that this becomes less efficient as tip speed increases and that is where 3 blades become more efficient, however, a 5 or 6 blade turbine which is spinning in 60 mph winds cannot be as inefficient as a 3 blade turbine that is braked or furled.

It is also true that I could produce more power with 1 hour of 60 mph winds than could be produced in 12 hours of steady 15mph wind or more power in 12 hours of trickle charging than in one hour of 12mph winds.

Understanding that an alternator must be optimized for lower speeds I am contemplating a design which utilizes a two stage alternator.

http://www.youtube.com/watch?v=tr0bBPUqJCc

The low wind speed coil would operate until tip speed ore electrical output reached a specific level, at that point a centrifugal clutch would engage a second alternator which is optimized for higher speed rotation and the load would be transferred when its current exceeded that of the low speed alternator.

This would add minimal amounts of weight to the rotor structure at low speed and add weight at high speed for additional dampening and protection.

In addition, a trickle at low wind speed is better than zero.

I doubt this would be a commercially viable design but I would also guess there are more potential sites for this type of variable speed wind turbine than there are sites with perfect steady winds.

I am very tired of hearing there is no power in low wind speeds. It is there, it just hasn't been the low hanging and profitable fruit.

Talk to competitive sailors and they will tell you there is of power in low wind speeds, you just have to know which sails to put up and how to trim them.

Blades are nothing more than sails that go in circles. I believe there is a special blade for every wind just as there is a special sail for each wind condition.

I will be doing an anemometer wind study as soon as I am consistently on site to take readings.

Mister B and oil pan please keep your multi-blade experience posted.

john p · March 2011

Re: Number of blades

Russ can you define "a reputable manufacturer"?
I changed over my AIRX 3 blade set of 4ft dia for a 6 blade set of 5ft dia and the improvement was dramatic.
(1) much less blade noise
(2) would generate power in winds that the 3 blades would not even rotate
(3) it achieved 40a in very strong winds against about 30a for the 3 blade set

I recommend the 5 blade set to anyone. but if you do use them you have to add an extension tail to control the generator as the orig tail is way too small

here is how I worked out the tail size as posted on another thread
To design a suitable tail for your genenerator, just use the following formula.
measure half the length of one blade multiply that by the cosine angle of the support pipe then when you have that figure multiply that by 3.1274. that should give you the vector angle of the tip of the blade.. use that figure to then obtain the weighted adjusted height index. which is done by subtracting the first calculated cosine angle figure..

To give example mine came to a tail size of 1 sqft of 1/8 al sheet.. held in place by 2 lengths of flat aluminium 2" x3/16.. held with 2 bolts..

MisterB · March 2011

Re: Number of blades

russ wrote: »

Of course the thing puts out the milliamps at low wind speeds - not enough to bother with but there.

I said starts at a few milliamps. The alternator will jump between this and 1 amp at the lowest wind speeds and at a just slightly higher windspeeds puts out 3-5 amps which is a uselful trickle charge at the wattages of my system. My turbine is 3 blade. It is the good quality 3 phase alternator that gives it it's excptional low wind speed perfomance.

The equations and basic physics of power vs windsweep area(rotor diameter) are easily found and understood. Blade count theory is not so easily found or well explained.

I plan on posting more about the performance of my HY400, especially with spring winds coming on and the turbine seeing wind conditions that stress it. I've already seen it surge to 40 amps @14 volts before dumping which equates to 560 watts. One problem is getting real accurate wind speed data. An anemometer that would give me real accurate data that I could mount on the tower at close to the turbine height costs almost as much as the turbine itself. I keep looking at less expensive ones and almost inevitably, they don't give accurate readings or look sturdy enough to last long up on the tower. My basic conlusion about the HY400 so far is that it is the best turbine currently out there in the 500 watt and under range that costs under $1000. It beats the SW Windpower AirXs and any of the Delco PMA based turbines handily in any real performance tests.

And I did check out the links. Greenpowertalk.org is very good. I got some information that I found useful in designing a manual braking circuit across the 3 phase lines which is something I'm working on at the moment.

russ · March 2011

Re: Number of blades

Reputable manufacturers such as - Scirocco, ARE, Jacobs - the ones that have been around and have good reputations.

For the DIY guy that wants to mess with the turbine all the time it makes no difference but most people don't fit into that category.

Agreed they are far more costly.

MisterB · March 2011

Re: Number of blades

I've found a bit more reading on the whole issue of turbine blade design and blade count from an engineering math/physics perspective. This is what an engineering student studying turbine blade design is likely to get:

http://www.dur.ac.uk/g.l.ingram/download/wind_turbine_design.pdf

A lot of very complicated math but in the section on blade design, the blade count is derived from the tip speed ratio and the count is determined from a table which is derived from "practical experience". For electrical power the desired tip speed ratio is results in 1-3 blades.

This article on "Optimal Tip Speed Ratio" has easier math and in general is easier to follow:

https://netfiles.uiuc.edu/mragheb/www/NPRE%20475%20Wind%20Power%20Systems/Optimal%20Rotor%20Tip%20Speed%20Ratio.pdf

And finally here is the previously mentioned article from Scoraigwind:

http://www.scoraigwind.com/wpNotes/bladeDesign.pdf

This is the easiest read and Hugh Piggott has been around awhile. I was reading articles by him about wind turbine design in the 90s.

Digesting all of this, I don't see any real reason to expect more power out of more blades except at low wind speeds. One factor that will increase power is increasing sweep area which is what John P has done as well as increasing the number of blades. The equation for power that can be derived from the wind by a hawt is pretty straightforward: P=AD*D^2*V^3*cP which is the Air Density times the square of the diameter of the rotor times the cube of the wind velocity times the coeffecient of performance which is a measure of the turbines effeciency and is limited to 59%(betz law). If you put bigger blades on a turbine, you are going to get more power at a given wind speed provided that the alternator is capable of it.

One thing that is not taken into account often is that wind speed is constantly fluctuating and when I take measurments from my turbine, I usually have to read for about a minute to see what it is really doing. At low wind speeds, I've seen current jump from almost nothing to 3 or 4 amps in a few seconds. If you start a turbine at a lower wind speed, that turbine will be already spinning when wind speed increases and will capture power that a stalled turbine will use to start up.The heavier rotor will also have more angular momentum which will tend to keep it spinning longer when wind speed drops momentarily. Apart from power, putting more blades on a rotor to decrease noise and reduce rpms at high wind speeds is not necessarily a bad idea. I get the idea that more blades could result in a smoother power output at all wind speeds if done right. I still don't see any reason to go beyond 5 or 6 blades in a hawt unless it's pumping water mechanically. The 7-12 bladed rotors seem to come from companies that don't have very good reputations for engineering and that have an interest in making more money by selling more blades.

Number of blades

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