Any figures on blade performance increase

[gotwind2]

Has anyone got any evidence of how much a set of blades with an airfoil can improve the efficiency of a wind generator compared to say a PVC prop? - I think I have read a 20% increase somewhere,I have seen mine really flying and producing power adequately just in my back garden.

I suspect it would be very difficult to offer exact results, but someone here might have a rough figure on the possible performance increase.

Ben.

[DamonHD]

And I am interested in including VAWT in that question, even drag-type.

Rgds

Damon

[Flux]

I think you will struggle to find an answer to this, there are so many factors that cloud the issue.

Are you really trying to compare pvc blades or curved surface blades in general.

Most of this relates to small machines and they are somewhat at a disadvantage in low winds as small generators have different quirks compared with large ones.

Small electrical machines generally are lower in efficiency and factors such as cogging, iron loss and bearing friction become of far greater effect.

Much of the differences in performance are related to the type of generator employed and the relative cost of the generator. It doesn't make a lot of sense to make the most efficient possible tiny machine if the market can't stand the cost.

Much of the characteristics of commercial machines is significantly affected by cost.

Similarly blades made of curved sheet that is rigid may not compare directly with flexible pvc pipe.

The type of blade you show should compare very closely with the metal TLG and similar products in low wind. The comparison may not apply in high winds.

Small machines often need low tsr blades to start and the speed of alternators ( or generators) is not as big an issue as with lager prop diameters. At low tsr, drag is not such a big factor and the high lift of curved blades sometimes has considerable advantage. Sailwing and curved metal sheet put up quite a good performance in low wind areas if you are not disadvantaged by their low speed. They don't do so well in high winds.

Even at higher tsr things are not always simple, the Bergey pulltruded blade is not so far removed from this case. The early Freelite blade was pressed from a piece of aluminium alloy sheet, the later ones were carved from wood with a simple aerofoil shape and I am not aware of any drastic change in performance and they were running at tsr8 or more.

I think the issues you have to sort out are rather more concerned with the matching of prop to generator over the speed range and the behaviour of flexible materials compared with rigid ones.

A well matched single surface blade may well equal a true aerofoil or beat it under suitable conditions of tsr and load matching.

I think that PVC will come out badly in high winds and in the larger sizes even under favourable conditions but it may not be totally related to the profile.

Flux

[feral air]

Damon, at least for s-vawts, pvc is so cheap and easy to work with that I wouldn't bother with anything else for small machines. "Perfect" vanes may get you a 3-5% increase but at what cost (time*money)? You'd be better off tweaking the alt and/or spending a little more on the bearings, imo. You should quit stalling and make a "quick vawt" already.

As far as pvc over airfoils for hawts, if it was me I would assume a 15% difference. I know jack about 'em, basically, but 20% seems like a lot.

[ghurd]

I think it is probably over 20% different.

But, like Flux said, it depends on what they are expected to do.

Most of my little stuff cogs. Sometimes PVC blades get to charging speed before better low TSR blades get turning much.  When the better blades get going, they go a lot faster in the same wind, even with more load.

Some really small stuff (like steppers), peak output is limited by total resistance, so higher speed doesn't matter once that RPM is reached. The PVC blades gets them going earlier, and at enough RPM to peak on output, so faster won't help anyway.

Just observations.

G-

[DamonHD]

I'm working up to it!  I'm going to try a simple plastic bottle VAWT first, probably not even with a generator underneath.  Today has been another <3mph wind day though!

I have also bought myself the PVC pipe to try to hack some better blades out of for my HAWT.

But this is all outside my comfort zone.  B^>

Rgds

Damon

[feral air]

I didn't qualify why I would assume 15%...because I can't go higher than 35ft without a permit and the winds here are generally low to moderate. pvc blades could conceivably outperform airfoil blades for me, overall.

..and in the one place where I do get high winds, they're so high that I wouldn't even try to use pvc. In that case airfoils would be 120% more efficient.

[SparWeb]

I think there is a simple way to compare them: with the Lift/Drag ratio.

Propeller blades, wing, wind turbine blades, they are all airfoils with an angle of attack, producing aerodynamic lift and drag.  Whether the aerodynamic energy is going in or out depends on whether you're in a plane or a windmill.

Both the curved pipe section and a "true" airfoil have a coefficient of lift and a coefficient of drag at any angle of attack.  Where an efficient wing may have a L/D ratio greater than 10 (maybe greater than 20), a poor wing will have a L/D ratio less than 2 or 3.  The same goes for a propeller or a windmill blade.

If anyone want to pursue this line of reasoning, I can add detail later.

[wooferhound]

PVC blades are like 10w 40w motor oil, it all depends on how fast the wind is blowing. PVC blades Start easy in Low wind and operate well at low wind speeds. But as the blades spin faster the curve of the pipe becomes the problem and they Self Ferl. The curve of the blade is a Resistance to the wind and as the speed increases, so does the drag. PVC Blades reach a maximum speed and won't go much faster, and they get louder with a Swooshing sound and sometimes a Helicopter like beating sound.

So the TSR depends on the wind Speed . . .

[finnsawyer]

While maximizing the L/D ratio might be good for an airplane wing where the entire wing has the same width and speed through the air, it might not work for a windmill blade.  To begin with, the maximum value of L/D might be at an angle too close to stall, 10 degrees or more.  Since most people design their blades for a constant AOA along the length of the blade, the AOA is determined by conditions at the tip, where supposedly most of the power is generated.  For a TSR of 7, for instance, an AOA of 4 degrees is usually used, which would not be anywhere near the the optimum angle.  I would, however, be interested in what you think you could accomplish by optimizing L/D along the length of the blade or whether it could be done.    

[SparWeb]

There are principles common to aircraft propellors, helicopter rotors, and wind-mill blades, and make the aircraft example relevant.

If you really study it, the terms "TSR" and "angle of attack" really refer to the same thing.  Mathematically, TSR = 1/alpha.

Unfortunately, the two terms are measured from opposite points of view.  The outside observer can easily measure TSR by watching the blades spin and measuring wind speed.  The blade, on the other hand, sees a particular angle of attack which depends on a lot more factors.

Twist is the basic feature that one uses to control the angle of attack of the blade.  The tip moves fastest, the root slowest.  You can assure a constant angle of attack along the whole windmill blade by twising the blade.

Do we want a blade with all points at a constant angle of attack?  Maybe.  Once the blade isn't working at EXACTLY the conditions you designed it for (speed/load/wind), it won't have exactly the same angle of attack along the span any more.  Is this detrimental?

I don't have all the answers.

But speaking qualitatively, the LIFT drives the blade around giving you power.  DRAG pulls the blade back, reducing its efficiency.  You definitely want to maximize Lift and minimize Drag.  So L/D-max is a good place to be.

I just can't prove it's the best.

This stuff constantly rattles around in my head, and someday I'll try to work it out.

[finnsawyer]

It is easy to get confused.  I was confused for a time about the direction of lift.  In wind tunnel tests the drag is measured in the direction of the (apparent) wind and the lift at right angles to it, which is a valid procedure.  You only need two components of force at right angles to determine the net force on an object.  At the tip of the blade for a TSR of 7 the angle of the apparent wind to the direction of rotation is about 8 degrees.  The lift is always at right angles to the apparent wind and a small fraction of that appears in the direction of rotation.  While the fraction remains constant, the value of that component of lift can be increased by increasing the angle of attack.  In fact one could go right past 8 degrees to 10 degrees, and the direction of lift won't change according to the wind tunnel tests.  But one would then get a larger component of lift in the direction of rotation, allowing a narrower blade and less drag.  The problem is that at start with a 10 degree AOA the blade tip will want to go backwards.  Well, most people put very little twist in their blades, so a large AOA at the tip probably wouldn't work for them.  Also, with very little twist, as one moves toward the root the AOA will eventually exceed 12 degrees, resulting in a large increase in drag for those sections.  So, the outer parts of the blade will be using power to drag the inner parts through the air.  If one designs the blade with the proper twist down to the root, one might be able to increase the AOA at the tip and still get reasonable start up characteristics since the blade twist may exceed 45 degrees toward the root.  At the same time one would be reducing drag and improving performance.

[SparWeb]

Most wind turbine books touch on the topic only lightly and don't provide enough information to work with.  Gary Johnson's book, for example, doesn't mention the thrust load from lift or drag, and repeats the old lie that the air "has farther to go" on the lift side.  The conversion of the Lift force into input Power is left to the reader's imagination.

Aerodynamics books, on the other hand, are mostly concerned with aircraft and helicopters, making the wind and power arrows point in the wrong direction!

[finnsawyer]

"The conversion of the Lift force into input Power is left to the reader's imagination."

I guess I've gone further than the books, since I've developed the equation:

    FlxVr = (0.59)x3.14xrhoxRxVi^3,

where Fl is the component per unit length of the lift force in the direction of rotation at radius R and Vr is the speed of the blade section at radius R.  The 0.59 is the Betz Limit, since there is no point trying to get more power out than you are entitled to.  The rest of the equation is from the power flow due to the incident air in an annular ring of width dr at radius R, namely P =0.5xrhoxVi^3x(6.28xRxdr).  The total lift force will also have the term dr from the length in the plan form of the air foil, but the dr will cancel out from both sides of the equation.  Using this equation we can then solve for the width of the blade as a function of R based on our AOA profile and the resulting values for CL.  One can then calculate the drag effects using Cd.  Note that the component of the drag force per unit length in the direction of rotation must be subtracted from the corresponding lift force to calculate the actual torque or power out.  We will, of course, get a number less than that predicted by the Betz' Limit.  I presented a derivation of this equation in one of my comments.  If you find it and see any discrepencies let me know.  I'm doing this from memory.  Maybe I should have put it in a diary.  As I suggested, if you wish to continue this discussion perhaps you should start a new thread of better yet a diary, where you will then have all this in one place.

[SparWeb]

"...I presented a derivation of this equation in one of my comments.  If you find it ..."

Yes, I think we discussed this before, too.  Both times, I'm immersed in too many things to take a "break" and hack out equations.  That's a "winter project"!

I'm not sure, by the way, that it's smart to start with the Betz limit in your analysis.  One formula won't do it for this task - you must break the problem into all of the important pieces, describe each part mathematically, and then re-assemble them with all the assumptions, input variables, and boundaries intact.

There's a reason the people that do this work at universities!

[finnsawyer]

"There's a reason the people that do this work at universities!"

Apparently they're not doing the work.  It is easy for people to overlook various aspects of a situation.  In case you haven't noticed, people tend to have a strong herd instinct.  It shows up here all the time, and also manifests itself at the university level.  So, everybody ends up taking the same approach.  

As far the Betz limit, it is either right or wrong.  If it is right then there is no point trying to oversize the blades to get more power.  If it is wrong, then time will reveal that fact.

You seem to think that you can somehow come up with a mathematical solution that encompasses all the physical processes that affect a windmill's performance, plug it into a computer and, "voila", the ultimate windmill.  It doesn't work that way.  How are you going to deal with the effects of the hub (turbulence going up the blade, supposedly) on the air flow or turbulence at the tip?  Engineering has always involved trial and error.  One tries to find and derive mathematical relationships that help in the solution to the problem at hand.  That's what the equation does.  Given the value of the incident wind at a given radius and the characteristics of the air foil you can determine the dimensions of the airfoil at that radius to match the available power.  What more do you want?  You can fiddle with the AOA or maximize lift to drag ratio, but the mill must still react reasonably to changing wind conditions and load conditions.  The power of the equation is that it can accommodate an incident wind profile that changes along the blade length.  If you add a nose cone you will have changed that profile.  And what about the vortices off of the tip?  So, you go in steps.  I realize that doesn't suit the behavior here where people just want to build a working mill (35% efficient).  Maybe it's up to the university people to do it, but are they listening?    

[SparWeb]

"...plug it into a computer and, "voila", the ultimate windmill..."

Well, that is why I didn't bother before just building my machine.  Like the "herd".

But I do find math fun, and just attempting to perform performance analysis is helpful in learning how to strike the balance.

BTW, don't underestimate the power of analytical computer software, either.

I have actually analyzed helicopter rotor power and the analysis does quite closely predict the engine power required for flight.  The analysis was done from first principles, with only a little help from empirical data.  Not good enough to bet a million dollars, but it suited the limited purpose at the time.

A backyard windmill is also a limited purpose.  So any computer model that is close by 25% is worth something...  Do it for fun, not for profit, that's all.

There are some folks who have gone to the trouble of making these kinds of analyses available in "calculators" on web pages.  These are handy, and some are pretty good.  But if I want to "see inside", I am happy to do the work myself.

[finnsawyer]

It is my understanding that an airplane propeller can be 83% efficient, which would imply that it is a simpler and different problem than the windmill blade that takes power from the wind.  Presumably that also would be true of the helicopter blade.

The Betz Limit would imply that there are factors that limit the power that an air foil can get from the wind, although the equations for lift have no such constraints.  If one ignores Betz, then one may have created a blade that is too wide, resulting in too much drag and poor performance, as well as other effects,such as wake interference and excessive turbulence at the tip.  One should really try identical mills differing only in blade widths to find the best solution.  One can then compare that to the dictates  of Betz.  The equation, by the way, would give you the total blade width.  You divide that by the number of blades to get the actual width per blade.