Using Furling Control on a Constant Speed Turbine

[RogerS]

Hi,

   After a couple of years of scrounging for parts, reading this forum and talking to the power company, I have about everything I need to start building a Breezy 5.5 inspired grid tied turbine.  The Breezy is a stall regulated, 4 bladed machine using an asynchronous generator (overdriven gearmotor).  I want to use a 10HP induction motor with a 20.75 ft turbine turning about 122 rpm giving a tip speed of about 90 mph.

   Most of these types of machines (Enertech of the 1980"s, Endurance & Aerosmart5 of today) use stall regulation.  Since stall regulation seems a little tricky to achieve, can a furling tail be incorporated into this design to add another element of control.  Since the turbine will only increase a couple of rpm as the wind increases, I suspect that it may not catch proportionally as much of the wind as the axial flux machins usually discussed here.  I think this might not make it furl as soon as you might think but I don't know.  I would sure like to hear any and all opions of this forum.

                                         Thanks for any help,

                                         RogerS

[Flux]

You have 2 issues to contend with.

If the generator is large in relation to the prop then it will stall regulate in all normal winds. If you use appropriate blade profiles I expect it will stay stalled in all winds. If the generator is not large enough to hold it stalled in the highest winds then furling should certainly help.

The big snag is that if you loose load ( grid or machine failure) then it will run away very seriously. I suspect that any furling may give you some protection as the run away blades will extract more energy than the stalled ones for high wind conditions but it is not a situation that I would be prepared to risk.

For a large machine I would infinitely prefer to have it yaw slowly ( servo or fan tail) and I would try very hard to avoid a tail and the associated gyro forces.

Normally large stall regulated machines have tip flaps or some highly reliable brakes to stop it if it sheds load and I think this is very necessary.

Flux

[RogerS]

Thanks for the input.

  I want to use a spring set air released industrial brake on the high speed shaft.  I will use 2 normally open air selonoids.  If I loose power the valves should open and allow the brake to engage.  I hope this will be a less violent stop than the electric released brakes I've seen used.

  I don't know what the fan tail that you mention is.

                                            RogerS

[Janne]

I think the fan tail is a tail hinged to the nacelle, and hold center by springs. This way, the tail is unable to turn the nacelle too fast.

Another choise is to add some sort of dampener to the yaw bearing. In our big mill an air compressor attached to the yaw ring gear, with output choked down has proven to be adequate.

The brake is a good idea, i personally wouldn't try anything much larger than 4m rotor without a mechanical brake.

Another choise for grid failure runaway protection could be a tripping contactor, that would couple capasitors and resistors to the generator in case of grid failure. That way the generator wouldn't became completely free running.

[Flux]

The fantail is a high solidity rotor like a wind pump fixed at right angles to the main prop axis and geared by a high ratio gearbox. It rotates until at right angles to the wind and at which point the main blades are pointing directly at the wind.

Braking on the high speed shaft is far less demanding on the brake. As long as the transmission can't fail it should be fine. Not so clever with belts.

Flux

[trailb4u]

I'm also interested in "Using Furling Control on a Constant Speed Turbine".

I live in a area which is rated between a 2 and 3 (fair to marginal).  

I have the breezy 5.5 book as well as the Homebrew Wind Power book.

I have not built any wind turbines yet, so the extent of my knowledge

comes from what I have read.  I do have some ideas I'd like to run by

the contributors of this forum.

I am not interested in building a turbine to charge batteries, so the

breezy design appeals to me as I am already hooked up to the grid.

In order to increase the amount of energy harvested from the wind,

I would like to upgrade the breezy's turbine blades to extract more

energy from lower winds.  The simplest way to achieve this as I see

it would be to replace the 18 foot 4 bladed rotor with a larger 3 bladed

rotor.  If the size of the rotor were increased to a 27 foot diameter 3 blade

rotor, the 14.55 to 1 gear ratio of the gear motor currently used in the

breezy plans would still be the correct ratio needed.  (If you were to put

a 3 bladed rotor of the same diameter on the breezy, then you

would need a gear box in the range of about 10 to 1 ratio since the

3 bladed rotor would need to run at a faster rpm to efficiently extract the

same amount of enery from the wind.)

So by installing the larger 3 bladed rotor on the breezy, the unit would

reach its full generating potential of 5.5 kw at @ 18 mph instead of the

current 23 mph. The turbine would begin generating at a lower wind speed also,

starting to run continually at @ 14 to 15 mph instead of 18 mph.  These figures

come from using the formulas in the book "Homebrew wind power", so actual

results are prone to vary for the worse or for the better.

A consequence of the above proposed modification is that the larger rotor

could easily overheat the gearmotor/generator in higher winds. It would be

neccesary to furl the tail or use some other standard method to prevent this

and allow the turbine to continue operating at max potenial.

I also have some ideas as to increasing the diameter of the rotor from 18 to

27 feet by making a modified blade hub which would allow the use of the

same blades currently used in the breezy design.  One of the attractions of

the breezy design is that the blades are fairy easy to make and it would be

great if they could still be used in the proposed mod.

If the same blades (3 blades) are used but mounted on a much larger hub of 9 foot

dia, with perhaps no blades in the oversize hub area, then the swept area of

27. feet ( 572 sq ft) minus the vacant area of the 9 foot dia hub ( 63.5 sq ft) =  
    
508. 5 sq ft.  This compares with the swept area of 254.34 sq ft for an 18 foot dia
     
     

rotor, doubling the sq feet swept.    

Like I said earlier, I'm a novice in this field.  Do my ideas have merit, or

am I way off the mark?

Please give me some constructive feedback positive or negative.

Clint

[RogerS]

Hi Clint,  

Being a newbie also, I probably shouldn't be the one to try to answer you, but I see you responded to an older post of mine.  I know that the guys here do not like to try to furl anything too large because of the high gyroscopic forces (I believe that is the reason) that are generated.  The tail takes a real beating trying to turn the the turbine disc that does not want to change direction.  

As far as trying to generate  more power at lower winds by using a larger turbine, I'm sure that more power will be the result,  but since the power increases at the cubed rate I think you are going to get to much power to handle as the wind speeds up.  I think these machines will furl over a  range of several mile per hour, and before you get it furled you will overpower the generator.  Figure the sq feet of the elipse that is formed by the furling blades when it is furled 45 degrees and the power produced when the wind is 3mph higher.  The power doesn't drop as fast as it needs to.

I would like to know how you arrived at the numbers for the Breezy's start up and also the ratio needed for a 3 bladed one.  I have a 12:1 gear reducer that I thought would make a good 3 bladed Breezy.

RogerS

[trailb4u]

Hey RogerS,

The gyroscopic issue is one I have read about, but I haven't been able to find any

definitive information about what size rotor is the limit for furling.  When is big

too big for furling tails? There are some people who have built 20 foot diameter

axial flux turbines and they are using furling tails.  I guess these are still

considered experimental so I don't know how well the system is working for the

larger rotors long term but they seem to be working short term.

I made approximations for the startup wind speed and full power windspeed using a

formula used for determining the "power available in the wind".

The formula being Power in watts = 1/2 x air density x swept area x wind velocity

(cubed).

The gear ratios I listed for the different diameter rotors and number of blades are

based on the assumption that the tip speed ratio of the breezy is ideal.

Looking at the numbers again, I come up with 10.91/1 ratio for a 3 blade rotor.

So I can't say the 12/1 gearbox wouldn't work perfectly on a 3 bladed 18 foot dia

breezy.  Also, looking again it looks like a 24 foot 3 blade rotor would be

ideal for the 14.55/1 ratio gearbox from a blade tip speed perspective, which

makes all of my earlier numbers relating to cut-in speed and windspeed for

max generating power wrong.

In reading the comments of the breezy 5.5 authors, they settled on a 4 blade rotor

For a number of reasons.  One of the reasons was for cut in speed.  They indicated

that the larger surface area associated with the 4 bladed rotor allowed it

to start generating at a lower wind speed than a 3 blade design.  They also

said that they did try using some larger dia 3 bladed rotors on the early

prototype that would overpower the generator in higher winds.  I vauguely

recall a mention of a tower strike due to the longer blades flexing in high

winds.  The four bladed, 18 foot rotor gave them the best match for the

nord 7 1/2 hp motor/generator with the corresponding 14.55/1 ratio with a

lower cut in speed for a diameter that wouldn't overpower the generator at

higher wind speeds.

I'm currently leaning toward a multiple (two) generator design with belt drive

for the generators.  A gearbox would step up the speed most of the way and

the pulley drives would set the final ratio for the corresponding generators.

A brake would of course need to be incorporated onto the highspeed output

shaft coming from the gearbox.  This would also make it easier to

play around with the final drive ratios a little bit to see where

you get the best efficiencies.

Let me know if I've made any more mistakes in my calculations.

Interesting stuff.

Clint

[cawest]

I am currently working on my Breezy 10.0 version 2. Version 1 was with a 28 ft diameter, four bladed rotor. It worked great (22amps @240 volts at 9mph)when it kicked on at 106 rpm. The blades were way to big for the 11kw motor (15hp). Peak power was at 15 mph (hub height). The mechanical break used was not large enough to stop the rotor. When a strong northern blew in, my rotor over spun and destroyed my blades. Version 2 will have a rotor diameter of 22 ft. and a gear ratio of 29:1 to keep the rpm down to 60. 106 RPM is very fast for a large diameter rotor. I am currently trying to devise a variable pitch hub that is activated mechanically instead of centrifically. I hope someone reading this can give me some help.

Keep the rotor diameter reasonbale:

Keep the RPM's down:

Have a good brake, much bigger than you think you'll need:

[RogerS]

Hi ca,  Hope you see this as it is a rather old posting.  What kind/size of brake did you have on it? Did you have a lot of jerking or problems with yawing on that big of a turbine?  Going from 106 rpm @ 28 ft to 60 rpm @ 22 ft is a whole lot of change.  This would make your tip speed down around 45MPH I believe.  That doesn't sound very fast.  

Are you trying to use only mechanical feedback of some kind to control your pitch? I can see where using centrifical force would only work for control of a run-a-way on a constant speed turbine.  I assume you are using turbine torque in some way.  This is an interesting idea that I'm going to have to think about.  Hope you see this...I'll be checking back,

RogerS

[CoolBreeze]

To CAWEST:

Congratulations, according to the following website your Breezy 10k wind turbine has exceeded the Betz limit by as much as a factor of four.

Your 28 foot turbine has a swept area of 14x14x3.14= 615 square feet.

In a 9mph wind at the Betz limit there is 2.15 watts/ square foot.
Therefore the max power of your turbine at 9mph = 615 square feet x 2.15 watts/ square foot= 1322 watts.
You claim 5280 watts.

In a 15mph wind at the Betz limit there is 9.95 watts/ square foot.
Therefore the max power of your turbine at 15mph = 615 square feet x 9.95 watts/ square foot = 6119 watts.
You claim 11000 watts.



http://www.ndsu.nodak.edu/ndsu/klemen/

Someone please correct me if I have miscalculated.