What RPM is the wind turbine blade most efficient?

[MartyM]

I am looking for an efficiency vs RPM graph for the Hugh Piggot style wind turbine blades?  I like a number of things about the Hugh Piggot design and purchased his plans for the axial flux design.  Most of us in North America are grid connected to expensive but reliable electricity.  The Hugh Piggot design is basically best suited for off grid electricty production.  I would like to use the blade design and furling Mechanism but change the generator to an over driven 3 phase generator for easier and quicker construction.  The over driven three phase motor is variable torque and constant RPM.  I can change the gearing and achieve maximum blade efficiency at any given blade RPM. What RPM is the blade the most efficient?

[Geo K]

[IntegEner]

The question seems to be about making the turbine a fixed speed turbine using an induction generator, using more standard terminology. This would mean that the blade TSR varies with the wind speed, rendering the above graph to be of not much use. With wind energy it is often true that the books have been the result of some hard work and you must follow them to the letter all the way from beginning to end!!!

Anthony C.

www.integener.com

[BigBreaker]

Constant RPM means that your blades will need to function over a wide "Tip speed ratio" or TSR.  TSR is the speed of the blade tips divided by the speed of the wind.  As you can imagine the tip speed is directly and linearly related to the RPM of turbine as a whole.

Efficiency for a triple blade turbine peaks around a TSR of 4 and then drops off pretty rapidly.  For a synchronous (60hz) turbine like you are describing you'll need to think about the number of poles in the generator and how that impacts the shaft RPM.  60 hz is "60 RPS" or 3600RPM.  3600 / (# of poles) is the shaft RPM and that shaft RPM better be lower than the RPM of the turbine at a typical wind speed.  Use a TSR of 4, your turbine diameter and some unit conversion to figure out what makes sense.  Chances are you will need a lot of gearing and / or a lot of poles.

Remember that you will produce no power below a certain wind speed and much above that speed your turbine efficiency will drop off so fast that your power output will essentially cap out.  Fixed RPM is the old "Dutch" model is it isn't used anymore in preference to variable RPM combined with power electronics.

[kitno455]

are you saying you want to hold the prop and gen together to a fixed speed? or are you saying that you want to let the prop run at whatever speed it wants, and keep the gen at a fixed speed?

the former will require a fine-grained load controller that can bring coils in the gen online one at a time to match the changes in shaft rpms. the latter will require that, plus a wide-range CVT.

both doable, but failure prone.

allan

[Flux]

You don't say what blade size you intend to use, but with an induction generator you can forget low winds.

If you cut in about 300 rpm you will track fairly well from 12 to 15 mph up to about 30 mph. If you are in a good wind area up to 400 rpm would give more power but even less in lower winds.

Flux

[Geo K]

Actually there is a blade design program by a little known and neglected engineer

named Joe Supercool from Australia called "turbin9" that does run under XP despite

the disclaimer that it won't.

Just keep punching in different RPM and rerunning the program.

This is the link.

http://www.supercoolprops.com/articles/wind_turbines.php

[MartyM]

Hello,

Thanks for the help everyone, this is my first time on the discussion board.

What does the vertical axis say? I can,t read it.

[MartyM]

Anthony C., You have described the situation.  Turbine to gear box to induction generator.  I am a mechanical engineer.  I figure I would need a turbine about 20-24 ft. in diameter to meet my needs.

[MartyM]

I would want to purchase a generator that has many poles as possible.  I can get a 10 HP 3 phase motor 875 rpm.

[MartyM]

I'm saying the generator is at a fixed speed.  The load would be my electric meter.  I want to turn it backwards.

[Flux]

When you said Hugh Piggott design I thought you meant the 8ft machine.

If you want blades 25 ft in diameter forget my previous speeds.

Choose your tsr and find the speed at 12 mph from that.

Flux

[Nando]

>>>I'm saying the generator is at a fixed speed.

The load would be my electric meter.

I want to turn it backwards

~~~~~~~~~~~~~~~~~~~~~~~

With the present technology, what you want is not possible.

A wind mill has a "variable applied power source" therefore the generator can not have a "fixed speed", it is variable .

The STANDARD SOLUTION is to convert the wind mill generated power into a DC voltage and from there to have a SPECIALLY DESIGNED DC/AC inverter for GRID tied service.

WHAT YOU WANT IS DESIRED BY THE GREAT MANY -- but the in-expensive idea elude all of those interested individuals.

There is a technique that is somewhat difficult to do and requires a lot of wind mill manipulation to keep generating GRID tied energy.

A wind mill/generator package where the generator operates at a RPM around the GRID frequency values and with a microprocessor the generator is loaded or unloaded with either a ballast load or a brake and when the RPM is in sync with the GRID the connection is made, and this may occur for a few seconds and then disconnected when the phasing starts to fall, allowing the wind mill to lock again for a few seconds.

This varies a lot depending on the wind regime and wind mill/generator package.

I wrote the highlights of this system that has been analyzed many years ago and let go, though it seems that there is one company trying to sell one with 4 large blades CAVEAT EMPTOR .

Nando

[MartyM]

The generator can't run any faster then it's rated RPM plus slip.  Remember fixed rpm and variable torque. The load is infinitly variable because it's the load behind the electric meter.  I sent Praire Turbines e-mail concerning the bogas information on there web site.  All I got back was BS and changing the subject.  But lets think about their turbine with an open mind.

[MartyM]

Lets assume 875 rpm generator. 24 ft dia blades.  The cut in speed would be what ever turns the meter back the most during the year.  Maybe just generate at higher wind speeds and forget the slower wind speeds.  Oversize the generator so it doesn't burn out.

[MartyM]

The average wind speed is low (10 MPH) hence the need for the 24 ft dia.

[kitno455]

so the cut-in speed is ~950 rpms. bigger props turn slower, so you will need a gearbox to get to cut-in in low winds. now what happens when the windspeed increases? do the slip rpms continue to increase, and hence, iron losses in the rotor? can the prop run away?

allan

[Nando]

To explain in depth, it would take a lot of my time plus the need to include a lot of mathematical data.

If the wind is much higher/lowwer even though the "generator" is limited to RPM + slip the generated PF will go well away from PF 1.0.

For this system to work, as demonstrated by the many thousands small, medium and large wind mills, there is a need of doing impedance transfer between the wind mill - mechanical energy and the GRID impedance that the generator stops producing the "perfect transfer impedance and energy" around a well defined wind-mill-generator matching Power to wind speed ratio -- which is a narrow band.

For this reason alone, the industry has to do a double or triple power transfer, - Wind Mill mechanical energy - to generator with variable output energy and this energy converted to the GRID energy requirements with variable applied power at fixed GRID frequency.

It is best, to spend a lot of time, investigating why an industry does certain ways with certain systems, find its faults and good procedures, also to examine the patents available to see how the principles are evolving for a possible definition of a new well analyzed principle -- if possible.

Just a general question : what are your technical capabilities in this subject or any other subject ?.

Nando

[MartyM]

Hello Nando and others,

I am a mechanical engineer and of course I don't know as much about wind energy as most of your members.  I can design and personally build just about anything.  I can weld and operate machine tools, I have a CNC mill and can do 3D contours on turbine blades if necessary.  Oh and most importantly complete a project.  I am a humble person and only say this because you ask.  I do appreciate the time you spent with me.  Questions that may seem dumb to you help me learn.  After all thats why I ask the question to begin with.    

[DanB]

"With the present technology, what you want is not possible."

Why do you say that Nando?  It may not be very workable in low winds and not too efficient in high winds but I've certainly seen very simple grid tie systems that worked this way and it is certainly possible.  To me flux's comments make sense - I can see why one might need to pick a high cutin speed and forget about high wind performance and I can see how overall the system might not be ideal, but it does seem like a fairly simple/workable way to go.  I have no experience with it -but I have seen it done.

[MartyM]

Does anyone know of a specific blade design that will function over a wide "Tip speed ratio" or TSR

[Geo K]

Marty  the vertical axis says (Solidity Percent).

A water pumpper mill has more solidity than a two blade electric generator.

Solidity and TSR both influence RPM.

That chart comes from the chapter 4 doc. That you can download from the link below.

The first 4 chapters are on wind power and the rest are on water.

http://www.ee.unb.ca/powereng/courses/EE6693/

Geo K

[MartyM]

I agree, thanks for the support.  Your statement, "It may not be very workable in low winds and not too efficient in high winds" captures the problem at hand.  I think we shouldn't focus on total efficency but strike a balance between efficency and simplicity of the "system".  I agree the Hugh turbine is efficient and simple.  I can tell most of you are partial to this style of turbine.  I am shooting for a 5-7 KW machine and the axial flux design is not proven in this size range (your getting close).  My problem comes with the expensive grid tie electronics and total system cost.    

[harrie]

Hi Marty, I am always glad to hear new ideas, and better yet see them work. Your efforts are never a complete loss even if they dont work, cause it may lead to something that would. keep us informed.

[Flux]

This is nothing new. Until a few years ago all large scale wind turbines used induction ( asynchronus) generators and the thing has supplied much of Denmark's power for Years. Some used two generators with different slip speeds to make things a bit more flexible but I have seen it quoted somewhere that the annual energy capture for a fixed speed machine is only about 12% lower than a variable speed one.

The idea is perfectly possible, don't choose too many poles for your alternator, you need a gearbox so you might as well choose 4 or 6 poles which work better as generators than the high pole numbers ( don't consider above 8 pole).

Keep it simple and make sure the generator is big enough to hold it in stall in a storm, if it pulls out you need emergency brakes pretty fast. You need some back up scheme to stop it because it will loose load if you come off line. If you have power factor correction capacitors and you loose line it will self excite and you will have serious voltages flying around.

Flux

[Nando]

DanB:

The problems are synchronization and keeping it for variable winds, and the PF that changes with such wind changes.

There is a group now selling, or trying to sell a wind mill with 4 large blades with a HOIST Induction Motor that ( it has a gear multiplier) and a microprocessor for RPM detection and electric phase detection to GRID connect that 3 phase motor to produce GRID tied energy, in addition it has a brake ( I don't know the type) to slow down the RPM.

The microprocessor makes the connection/disconnection to the GRID on the fly in spurs, also if too fast RPM braking it for re-synchronizing.

I do not know the power in to GRID connection ratio, but it should be low -- considering that to solve this problem the large Wind MILLS industry has implemented two generators with variable SLIP and set for different wind velocities ranges -- the efficiency, as noted by FLUX, runs around 12 % as much.

In the 1970's we were experimenting with a walking or variable rotor wind magnetic field.

This walking magnetic field ROTOR had in principle, let's say 48 poles and the stator just, let's say 3 ( 3 phase), to make the output frequency stable with variable wind velocity, the magnetic rotor field was rotated electronically at a faster or slower rate than the mechanical rotation, to maintain the Generated AC power in phase with a REFERENCE that could be the GRID if tied to it or to a stable 50 or 60 Hertz reference.

We also experimented with walking stators for the same purpose.

Also, there were private experimentation using Pitch controlled wind mills with electric means to maintain the RPM fixed within certain wind velocities, I believe that patents may not have been issued, I have not kept track of such development because I changed my working environment.

There were other types, one still required to keep my mouth shut about it.

It makes sense that FLUX says and some may have done some building but the systems may not have "grown" to high levels of production for some reason.

We have here, a mechanical engineer, that may find a solution, seeked for many decades.

There is a possible solution, I do not know the efficiency, it is the use of a hydraulic system between the wind mill and the generator with capabilities of fast VARIABLE RPM ratio conversion -- may be possible !!

Nando

[MartyM]

To Flux - I would like to keep the gear ratio of turbine to generator as low as possible.  I say this because the turbine will start easier at lower wind speed.  Also a gear ratio above 12:1 will require 2 pairs of gears and the exrta pair of gears cause a lower power transfer efficency.  I would think the generator should run at the slowest possible RPM?  This means more poles are better (maybe)?  How many poles would my example 875 RPM motor have.

[MartyM]

I didn't think the 4 bladed turbine you speak of was syncronized with the microprocessor.  I thought they used two phases for their power generation and the third phase was energised from the grid to maintain syncronization.  I thought the microprocessor was only for cut-in speed.  I could be all wrong on this one.  Does anyone know for sure?  

[oztules]

It may be useful in looking at the drawbacks of some of the tricks being touted thus far.

It would seem that Flux has correctly ruled out low wind usage if you require gearbox/hydraulicpump/or other torqueconverter systems.

It seems to be the case that low wind speeds would be the norm in your area (10mph av). It may be time to bite the bullet and do the unthinkable, and just charge batteries... a proven use of low to medium windspeeds, and use the batts as tempory storage for your low wind capture energy.... every watt here is a bonus, you would not have captured it for your use before, and yet now it may provide the bulk of your disposable energy income.

Yes you can see whats coming next, just run a 3hp dc motor to direct drive the induction generator. Here you will lose maybe 15% (a decent dc motor is capable of 85% i believe.. ie 2600w in for 2200 w output.

This will give you an energy storage for the low winds, and the gennie will direct drive (with batteries as filter) when decent winds arrive.

The only thing to worry about then is the turn on/turn off of the dc/induction generator set. Looks pretty simple to me. a very basic voltage detect on the battery pack would start induction motor from the grid, and then turn on the dc drive to over run the ac motor.when the voltage drops below cut off point, all stops,and batteries resume their charging on winds too small to usefully convert in real time.

This intermittant use of the gen set will extend the brush life to a large extent, and my experience of this kind of dc motor is that used 2 hours/day,it will last decades.  (my experience on over a hundered floor scrubbing machines.. Ohio 36vdc 2hp pm motors  some ran 25 years with no brush maintenance 2hrs/day)

So by introducing an inefficiency of a motor, instead of a torqueconverter or hydraulic, or gearbox even, we get to use the lower wind speeds, get maximum wind capture, and solve the highwind problems that Flux eluded to....batteries act os load plus the dc genset.

This then uses current technology, is solid as a brick s---house, and has no obvious weaknesses.....that I can see.

Yes we are introducing inefficiencies, but we are also getting power from incidental winds and winds which we would have otherwise ignored.

ok time to kick me around I guess

..........oztules

[Nando]

I lost the URL for the site, so I can not go there a read again.

If you have the info send it to me, please and will try again.

Nando

[Flux]

Marty

875 rpm is slip speed of an eight pole machine at 60 Hz ( synchronus speed 900 rpm).

As a generator you will run about 930 rpm.

Flux

[Nando]

I have found the site, by a mistake I made.

This is what they say.

A 3 phase motor and one phase loading so a 7.5 KVA may be capable of producing around 5 KVA

IT IS CLEAR that NO phasing is done to insure that the PF is good, even though the GRID does Field energy.

The Wind mill has a torque limiter between mill and generator for protection when the motor brake is energized.

I found this thread that may be interesting to you or others

http://www.fieldlines.com/story/2005/10/24/212159/94

GOOD LUCK TO ANYONE USING THIS SYSTEM -- CAVEAT EMPTOR

Nando

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http://www.prairieturbines.com/theory.htm

Theory of Operation: In basic terms, our turbine generator is an electric motor, in our case a 3-phase induction motor, with a designed rotational speed of 1725 rpm. Induction motors are also known as "squirrel cage motors" and are used as furnace blowers, pump motors, washing machines, etc. Current flow in stationary windings induces a magnetic field into the rotor. This magnetic field causes the rotor to chase the fields created by the stator windings. There is no physical connection between the rotor and stator windings. There are no brushes or slip rings. When our turbine rotor blades cause the induction motor to rotate faster then the designed speed the motor begins to generate power back into the leads. Even though the motor is turning faster then the nameplate rpm it remains locked in frequency to the line power. As the torque applied to the motor shaft is increased the output current is also increased. More wind, more power.

Even though we are connected to incoming 1-phase power we use a 3-phase motor. Large 3-phase motors are much easier to find and less expensive to purchase (the unloaded 3rd phase is used to provide voltage regulation). The motor we have chosen is commonly used to drive overhead cranes in an industrial environment. It has a 15-1 low friction gearbox on the front and an electric brake on the rear. The nameplate specifies that it is "inverter duty" which to us means heavy-duty. The motor will probably be the largest single expense when gathering components but it is heart of the wind turbine.

The generator speed must be monitored electronically. We have designed a micro- controller based circuit and written the program that runs in it. It utilizes a hall-effect device attached to the rear of the motor to determine the speed at which the generator is turning. When power is applied to the generator controlling circuits the micro-controller releases the motor brake and looks at pulses coming down from the hall-device. As the wind turbine speeds up and the micro-controller determines that the generator is running at the correct speed it energizes the main relay connecting the generator windings to the power lines. If the wind speed decreases the main relay is de-energized and the generator is allowed to turn until the wind speed increases enough to allow the generator to begin producing power again. As a safety feature should the micro-controller determine that the generator is turning faster then our programmed upper speed limit the main relay is de-energized and the motor brake is applied. The controller will hold the circuits in this state until it is manually reset.

The turbine rotor blades have the task of applying torque to the shaft of the generator. The contoured shape of the blade surface (similar to an aircraft wing) forces the wind to travel farther over the top of the blade then the bottom creating a difference in air pressure between the upper and lower surfaces of the blade. It is this difference in pressure known as lift that causes the blades to rotate around center. There is a balance between the motor we've chosen and the size, pitch, and number of blades in the rotor.

[vawtman]

Marty I think once the motor would start to generate the blades would drop out and the process would have to start over again.Not sure how the grid effects the gen.

 Has anyone here tried this and ifso what where the results?

 Also permits to gridtie can be expensive.At least in Wi.

With a expensive variable pitch monster turbine than yes.