Load / No load furling question

[Dave B]

I want to protect my 18' from overspeed mechanically with the common side furling if possible. I will be heating water direct no batteries. I want to limit the blade rpm to maybe 250 rpm max. Here is the scenario : Lets say under full load in a 20 mph wind it's starting to furl at 200 rpm. If it helps lets say a carved TSR 7 blade profile and at 200 rpm we have 2kw output. Now, bang ! complete loss of the load or stator failure (open circuit). Obviously if still at 20 mph wind speed the rpm will accelerate like a rocket. 2 questions : If the wind remains at 20 mph is there any guess as to where the rpm will climb to ? In other words, is the TSR of a blade figured for loaded or unloaded or some where in between ? And, if now the wind is still at 20 mph and no load, because the blades are spinning faster would this mean that it would most likely want to furl more or less from it's previous condition of under load and just starting to furl at 20 mph wind speed ? I will have the typical electrical load brake and or overspeed circuit also but I want to have near fail safe protection for total loss of load, stator and or line failure. Suggestions, thoughts or ideas are sure appreciated.  Dave B.  

[scottsAI]

Hello Dave B,

I have studied this, I can sort of answer your question:-)

Like many things it depends.

A properly fully loaded blade's RPM doubles from full load to No load.

Next is the depends part. If the blade is operating in stall, then the loaded speed to No load speed can change more than 2x, 3-4x...

Electrical brakes can be overloaded. Not my idea of protection.

Have fun,

Scott.

[Dave B]

Hi Scott,

 Thank you for your reply, this gives me some idea of what's going on. Like you say the electrical brake idea is not the best and is basically the concept of my heating water. This will help control the speed but if there is an open for any reason ..... off she goes. What are your thoughts on how the furling relates to the rpm / wind speed ? It seems to me that if under load and it starts to furl at say 20 mph then if unloaded and say the rpm doubles at 20 mph then ... the blades appear to be more of a solid and therefore the push would be greater and it would want to furl further toward the high stop. Maybe that thinking is all wrong, I hope not because if it does work that way it will help my idea of the high speed protection. Thanks again for your reply, anyone else care to comment ?  Dave B.

[scottsAI]

Hello Soapman,

Offset blades and the tail are balanced as the blades furl.

Furled blades power = (Wind power) * (cos of angle rotated) from dead on.

the blades appear to be more of a solid and therefore the push would be greater and it would want to furl further toward the high stop.

Yes, the exact number I do not know. Expect it to be less than 2x.

Concern I have with furling for protection. With each doubling of the wind speed the power ups by 8x, even furled blades will see Power * cos of angle, high winds demand almost 90 deg to get protection, with 70 mph winds watch out.

Around here storms winds change direction fast... If the winds rotates even 30 deg the blades see half power! Until the tail can redirect, can your blades handle the power and the turbine the rotation?

Have fun,

Scott.

[harrie]

Hi Dave, I guess I have more or less made up my mind to just plan shut the turbine down in high winds, or even if they are forcasted. Way to much money and work to see it all destroyed.

It would be nice if something could be designed that would be foolproof, but short of rotor turn out with flyweights, Im at a loss to what could be done.

I know it makes it tough to have to shut it down when your not at home to monitor it, but that is the only way you can be assured to come home to a turbine that is still in one peice.

I am hoping that someone on this site will someday come up with something that will take care of this problem, but intill than, I will continue to play it safe.

Good luck, Harrie

[Flux]

Dave

I thought we exhausted this one previously.

Scott is about right, typical blades will double speed from optimum load to runaway.

As normally loaded by stall the speed increase will be about double that. You should be loading to best tsr for direct heating so think of a doubling in speed.

Simple theory says that the furling point is dependent on thrust. That is dependent on the fraction of 1/2 x rho x A xV^2 that you extract as power. If you loose load you extract no fraction of that as useful energy and the thing will go out of furl until the speed rises to the point that the blade losses cause it to extract enough energy to bring the thrust back to furling point.

This will require a fair increase in wind speed and a large increase in prop speed.

There is a secondary and more awkward factor that is even more difficult to deal with and that is the blade seeking force that tries to keep it into the wind and is reducing the effect of the thrust trying to furl it. In most cases this seeking force seems to increase as you move from stall towards runaway.

I have said it before, furling is meant to provide control under load conditions, it will provide some ultimate protection with loss of load but at very high speed. If you build strong enough you may survive but you will frighten yourself when it happens.

Your case is no worse than loosing the load from a battery charging machine and others just seem happy to live with the problem.

All my machines have had to be so close to habitation that I could never consider building something without a mechanical means of shut down. I prefer to use ways of turning the tail at right angles rather than a brake, but either method can work.

This issue seems a worry to you and you are not going to find a solution in some magic furling system.

Flux

[outback]

Has anyone experimented with tip brakes on this sight? that would cure Daves concerns.

[Nando]

Dave:

You have gotten good info from several members.

An ultimate protection, for furling wind mills, is the use of an electrical setup forcing the furling into a protection arrangement.

A small electronic circuit to detect voltage level (frequency as well) and lack of current which fires a clutch to engage a screw driven by a motor to bring the tail into furling at an angle that produces just certain enough voltage to keep the circuit engaged, current drawn may release the system OFF.

There are several wind mills in the lower part of South America with this type of protection that has worked well for many years.

Nando

[Flux]

I don't know of anyone who has mounted tip brakes on a carved wooden blade. It shouldn't be impossible but the biggest problem I see is to make sure they all operate at once. If all three didn't operate together the vibration would be dreadful.

Flux

[Nando]

Dave:

Trying NOT to be repetitious the solution is a torque Pitch controlled hub.

Which has a natural lower RPM when the load is disconnected protecting the whole wind mill system as set by the wind mill internal frictions, the generator cogging load and maximum allowed blade pitched angle ( if such latter arrangement is added).

In days past, I presented the link to a friend's wind mill that was done, just as a test, using in this case, a pitch controlled 11 KW wind mill flying weights which has survived a 90+ wind period, producing it speak power.

Test data will forthcoming soon.

Nando

[Dave B]

Thanks Nando,

  Yes, there are many ways to limit the speed and for those with the resources and figuring the design from the start it would be easier indeed. I'm working on a couple ideas to keep it as simple as possible and one that can be worked into my current design. All comments are welcome. Thank you for your input,  Dave B.

[scorman]

Maybe I am missing something basic here.

Why aren't you simply "unloading" the turbine by disconnecting the electrical load while spinning in a stiff wind. You can empirically determine the increase in rpm by measuring the output voltage which should be linear (sort of) and note the noise level as well. You can also observe whether it furls more or less compared to applying the load again.

I agree with most of the posters who claim that the rpm will probably double, but remember that as TSR goes from 6 to 12 (rpm doubling), the angle of attack becomes much shallower, the lift is reduced (or goes negative), and the drag goes way up. There is also some mechanical friction which acts as a load and never goes away.

If you or anyone viewing have interest in an Excel calculator, I have posted it on GreenPower:

http://www.greenpowertalk.org/showpost.php?p=1476&postcount=26

you can easily see what happens if you double or triple the TSR by simply plugging in the new value

Stew Corman from sunny Endicott

[Dave B]

Hello Stew,

  I want to automatically protect from overspeed for an "open load" condition and preferrably as a mechanical arrangement. Ideally I want to limit the rpm to 250 max. with maximum out put at around 200 rpm. I will most likely go way above my limit before furling kicks in if I loose the load at full power and approx. 200 rpm. I am thinking a stepped furling arrangement, I am almost looking for a "furled not furled" set up for my application of hot water heating direct. I'll figure something out, comments and ideas are welcome.  Dave B.

[rickysmartz]

Hi DaveB,

That was in 2007. Did you figure this problem out. I have a similar problem but slightly different in that im trying to find a way to protect my 17'er grid tied unit from running away if the grid goes black. I need something that is reliable and maintenance free.

Cheers

Richard