Sail versus Turbine?

[Frank S]

I know it is the general accepted norm for a wind generator to have a 2 or 3 bladed turbine and in some cases 4 or 5 blades and i know only so much kinetic energy can be extracted from a given area of air mass according to Betz's law. try to extract a higher percentage you ultimately  disrupt the flow by creating turbulent eddy's which in turn reduces the overall efficiency of the turbine. We can only borrow the linear flow of or the air mass, and convert it to rotary motion via the cord area and cross sectional profile of the blades. How much of this energy we are able to borrow depends largely on the cross sectional profile, including the angle of attack on the windward or up stream side versus the angle & profile of the downstream or lea side. a nose cone on the front of the turbine helps to divert this kinetic energy around the nacelle of the machine
 Am I wrong with this line of thinking?
 Now a sail or water windmill fan extracts this same energy and uses it as torque through gear reduction in most cases converting it in most cases to reciprocating motion serving as a means of lifting a column of liquid.
 Sails make high amounts of torque at very low RPMs and low wind speeds I would venture to say at the expense of  a much lower efficiency at higher RPMS
 is my line of thinking on par with the rest of the world or not?
 I know I have personally seen windmills with 20 sails actually being used to pump oil out of the ground in East Texas on wells about 5 or 600 ft deep. And the Iron-man windmills have been reported to lift as much as 500 tons of water a short distance per hour. in good winds.
 So ultimately is it better to have fried chicken or boiled eggs
  or you could do this I guess

[fabricator]

Not sure there is a question in there, but nose cones are really not much more than eye candy, there is virtually no work done at the root of the blade.

[oztules]

It's horses for courses Frank.

If the pumper had a centrifical pump on it with no gears.....useless
The pumper with a fixed displacement pump..... no problem (with or without gears)
The pumper with a direct drive alternator.... not possible until neo's, and then huge.

So for power generation, we need shaft speed, and so rely almost exclusively or lift blades.

Faster the mill, smaller the alt.


...............oztules

[Frank S]

Not sure there is a question in there, but nose cones are really not much more than eye candy, there is virtually no work done at the root of the blade.

yes exactly that is due to the shortness of the moment arm no real torque can be realized near the center, the further out from center the better for achieving the transference of linear kinetic energy into rotary motion creating torsional inertia or tangential force.
That being said blunt frontal impact without a nose cone creates a column of blocked air this air has to go somewhere so it spills sideways in the form of turbulence robing energy from the turbine blades the larger the hub Dia is or the nacelle of the unit the more pronounced the blockages.
  My thinking is that back in the day of the sail fan they were more interested in actual work than speeds of 4 to 700 RPM which would pull the leathers right off the end of the pump  even with the reduction gearing of the aermotor not to mention cause the sail fan to self-destruct from centrifugal forces, of course with a linear action pump they could lift water even at 1 RPM a huge advantage over trying to produce electricity at low RPM They also didn't have to concern themselves as much with frictional coefficients as is necessary  with lift blade turbines. which as I gather it, are actually trying to out run the wind much like trying to make a prop driven airplane fly 400 MPH against a head wind.

[finnsawyer]

I think you're missing some things.  Anything you put up in the air stream will deflect the air to some extent.  Consider the extreme case of a solid object like a garbage can lid.  All the flow must be deflected and must flow around the edges of the lid. In order for that to happen and the total air flow to remain constant the air flow past the lid must speed up (air can not pile up behind the lid without limit).  This speed up in air speed past the lid results in a lower pressure behind the lid.  It is this drop in pressure which allows one to harvest energy from the air flow.  One may view the pressure term in Bernoulli's equation in this context as an energy potential, so pressure differences translate into energy differences and some of the energy can be captured.  Any object, including a windmill. experiences a pressure dome in front of it that deflects some of the air flow around the object.  So, a nose cone would have at best marginal effect since much of the air would be delected before it encounters the cone.  

[Frank S]

Have any of you guys ever seen a pitch compensated  wind mill sail fan?

this is a 22 ft diameter sail obviously furling such a monster would be like attaching a sparrow's tail on a teradactal so the solution was to control the speed by reducing the total frontal surface as in this

[fabricator]

That thing will be a torque monster, but will be useless for a direct drive machine, it would do fairly well for a geared machine.

[gizmo]

Those farm pump windmills are designed for torque, especially from a standstill. If the pump is a bottom dead center, and the pipe is full of water, then the turbine has to have enought torque to lift the column of water, and there could be many 10's or 100's of kg to lift on each stroke. My old Southern Cross with its 2 meter fan has enough torque to lift a 20kg bag of cement in light winds, and its sheared a 6mm bolt. But a 3 blade 2 meter turbine for electricity generation is a weakling from standstill. Once the RPM's get over 100 or so, the farm windmill is left far behind and the 3 blade turbine races ahead in output power. Different machines, designed for a purpose.

The Betz limit and multi blades. The way I see it is you need to restrict the wind stream to get power form it, and the more you restrict it, the more the wind stream is inclined to go around the turbine instead of through it. So the Betz limit is the best balance, any less and your loosing potential power, any more and the wind is spilling around the sides of the turbine, again loosing potential power.

Glenn

[Frank S]

did I mention anything about using it to power a flux capacitor, axial flux, radial flux. or any other type of generator?
to use a sail as a power source for a generator you would need a very efficient way having a variable gearing system. so a generator could run at optimum RPMs irregardless of sail speed.
 TO get absolute  performance out of a turbine or a sail you tunnel the perimeter preventing spillage of the air column this way you can take full advantage of belts law and extract the maximum kinetic energy about 40% possibly 50% above that the resistance to flow will begin to restrict and reduce power output.
 None of the CVTs that have been developed are without their drawbacks
 
I just asked if any of you had ever seen one of those wind sails up close and personal.   

[gizmo]

Shaft power comparison between multiblade sails and 3 blade high speed turbines, the 3 blade turbine makes much more power. Gearing up the low RPM of the multiblade sail wont make the same power as a un-geared 3 blade of the same diameter.

TO get absolute  performance out of a turbine or a sail you tunnel the perimeter preventing spillage of the air column this way you can take full advantage of belts law and extract the maximum kinetic energy about 40% possibly 50% above that the resistance to flow will begin to restrict and reduce power output.
 

Doesn't work like that unless its a closed system, like a wind tunnel, where the air has no choice but to go through the funnel/turbine. Adding "ducts" or funnels to existing wind turbines gives a tiny gain in power for a lot of extra hardware. Would be easier to add winglets to a existing blades, or just extend the blades out a couple of inches.

I just asked if any of you had ever seen one of those wind sails up close and personal.   
 

Yes.

Glenn

[Frank S]

Shaft power comparison between multiblade sails and 3 blade high speed turbines, the 3 blade turbine makes much more power. Gearing up the low RPM of the multiblade sail wont make the same power as a un-geared 3 blade of the same diameter.
 
 

Yes.

Glenn
[/quote]
I can see that since the sails are usually very low in total mass even though there are a lot of blades they have a small overall cord area lots of down stream resistance and the whole construction is very light weight then add in the gearing drag the power would have to drop .
 the 3 blade turbine may not have any more mass but with much lower down stream resistance the higher RPM helps to serve as a flywheel storing more usable kinetic energy it is only in the extreme low end loaded start up where the sail excels at least that is my take on it 
 I have never been up close to a feathering sail like the picture but I have been around a couple  of the windmills with double sails connected VIA chain drive or belt drive they were used to pump oil from shallow wells

[finnsawyer]

     Here's a little problem for you "experts".  Take a piece of paper and pencil.  Draw in a pressure field that decreases linearly left to right.  Place a slab of length l, width w, and depth (into the paper) d into the pressure field.  Calculate the force on the slab for different orientations of the slab.  So, the question then becomes, "How does the pressure drop across the windmill affect the operation of the multi-bladed water pumper windmill?"  As I mentioned befor. one can, in principle, harvest energy if there is a pressure drop to establish two different pressures in Bernoulli's Equation.  Does this do it?

[fabricator]

It might if a guy could do those equations

[finnsawyer]

     So, fabricator, how do you know the thing will be "a torque monster"? 

[fabricator]

The more blades the more torque the lower the speed of the rotor, QED.

[finnsawyer]

     In other words, you don't have a clue what is going on.  Look like it's time to get some books and do some studing!  The action of pressure isn't that difficult to understand.

[fabricator]

Nope, I don't think so, good luck with your problem though.

[spottrouble]

Ok, here is my stupid question for the day.....
Every water pumper I've ever seen uses sheetmetal for the blades, either flat or with a little curvature, yet not an airfoil. Has anyone ever built a multi-bladed (more than 4 blades) mill using airfoils? I'm guessing speed would still be low due to increased drag, but does it increase torque over the non-airfoil design?

Kristi

[fabricator]

Yes they used more blades/more lifting power, you need less blades with an air foil  for more speed less torque for an electrical generating mill.

[bob g]

jumping in the sandpile...

forget bernoulli!!

take another look at newton, it is now the accepted  theory of lift.

do the math if you like, and i think you will find that bernoulli can not explain the amount of power that is seen by a cambered wing section used as a rotor blade.

bernoulli also does not explain the low pressure region just ahead of a wing section, which combined with newtons theory high pressure region trailing behind the blade equate to the amount of torque or power realized by wind machines.

ducking for cover now...

bob  g

[spottrouble]

Yes they used more blades/more lifting power, you need less blades with an air foil  for more speed less torque for an electrical generating mill.

I'm not quite sure that answered my question, seems you just gave the standard canned response to wind water pumpers vs wind turbines. Seems to me there were several airplanes with 6 bladed props, and I'm thinking a few helicopter designs with 6 or 8 blades too. Chris'O has pretty well blown the "no gearbox" arguement out of the water, and he is even running a 4 bladed prop, not sure of the results though. I guess we'll just have to see how many blades Chris is willing to try , I'm guessing there is a point of diminishing returns.

Kristi

[bob g]

Kristie

there is a world of difference between an airplane prop, helicopter rotor, ship screw and a rotor made to extract power from the wind.

the rotor made  for extracting power from the wind relates better to an airplane wing rather than the other examples that are optimized to deliver power to the wind/air.

the reason you don't see many multiblade rotors on windmachines is because each blade leaves turbulence behind it for the next blade to deal with. turbulence equates to lost lift, loss of lift relates to reduced power.

the best design in theory therefore is a single blade rotor, however from a practical standpoint the counterweight needed is problematic.
the next best is the double blade rotor, the problem with it is vibration and gyroscopic issues as the rotor changes with wind direction and issues with tower shading.
that is why the next best is the three blade, it doesn't have the shading issues, less vibration, no gyro issues, although it loses a bit to turbulence. it is the best all around compromise.

remember power is made from torque and rpm, 

multibladed props make lots of low wind torque, but very low rpm
the issues with turbulence limit their ability to utilize higher winds effectively therefore they cannot spin as fast as a 3blade prop.

the three blade prop, will make very high rpm's relative to the multiblade prop, even though the torque is lower the speed more than makes of for the loss of torque. the end result is higher rpm even with lower torque equates to more shaft horsepower.

this doesn't consider issues with the generator, most of which make more power the faster they can be driven, very low speed generators such as something suitable for a multiblade prop would be very large in order to have the rim speed of the field fast enough to make more power... then you still have the limited amount of power the multiblade prop is able to deliver.

i can't remember for sure, but many of the old water pumpers only made something less than one hp at the shaft at rated windspeed. that is something less than 745 watts max output. then you have to deal with generator efficiency so you would be doing good to get half that "if" and "only if" you either have a very large generator  or a gear train to increase the output speed to something useful for a common aircore generator.  remember the gear train introduces losses of its own, so maybe you end up with a couple hundred watts output in 8-10mph winds and the machine will not make more power in higher winds, because it is unable to utilize the increased windspeed effectively.

maybe you already know all this? if so maybe it will be useful to someone else.

bob g

[finnsawyer]

     Kristi, your question is exactly why I posed my little problem.  You don't need an air foil to get power out.  And, Bob g. there will be a sweet spot (Rpm) where the air can flow past the blades with no turbulence, since there will be no interaction with the blades.  Of course, it would help if the blades were streamlined and the proper twist were applied.  But I wanted to keep the problem simple. 

[jvnn]

And to think I clicked on this to see who is talking about boats...