Performance charts -solidity/torque/yaw

[Stewart Corman]

Since the Dual rotor thread was getting bogged down in equations and calculations, I decided to open a new thread here with some info I found.

One question was "how much does a turbine slow down when it is furled ?".
In the tiltup design, a simple pull cable can be used to manually furl the head in the tilted position. In that regard, measurements of rpm and efficiency vs angle to the wind, can be done at wind speeds lower than furl by simply pulling on that cable in increments

298

In my redesign, I plan to have a tilt stop at 60 degrees.
From the above chart, the different models show a significant decrease.
There is an added benefit when using a flat, non-twisted airfoil, that the drag goes up significantly with rotation speed, since more of the blade is NOT designed to be optimized on purpose ..this is called "stall-regulated design" ..it is the exact opposite of the huge commercial "pitch regulated blades" whose design is to optimized at any/all wind speeds ..it is a known fact that fat blades have lots of drag at high rotation speeds

Then comes the choice of how many blades, how solid should they be, and what TSR should they run at.

299

The commercial units such as a Bergey, use 3 long thin blades at TSR=7 ...this is great when you have 25mph blasts and don't care about wasting 80%+ of the windpower, because what is left is more than enough to fry the generator

Go to Hugh's site and you see nice fat roots and thin tips
http://www.scoraigwind.com/nirvana/page2.htm

... that is high solidity ...then look at Hugh's tutorial and he clearly states that the most efficient turbine is Claus Nybroe's Windflower with 12 thin blades and TSR = 3.6.

For those of us in marginal wind sites, the best compromise is five or six fairly wide blades giving high solidity and TSR around 4. Life is a compromise!

BUT , the more efficient it is , the lower the TSR, the lower the rpm, the higher the thrust on the tower

300

luckily, if we design a large 18 foot turbine for 12mph WS, we can choose the generator to handle juice at 15+mph and furl <20mph, so as not to have it run at high rpm, nor make loud noise (WS^5 or 13% of noise at TSR=6), nor have high vibration components, nor have the high stresses in the root, nor high centripetal forces trying to tear it apart ...If we are lucky, it might be so inefficient at higher wind speed , that it never goes any faster than at 15mph??

All the above is based upon the original Betz presumptions of a single disk analysis and the reduction of windspeed to 1/3 caused by the turbine interacting with the oncoming wind ...

Stew Corman from sunny Endicott

[Sean Skirvin]

"There is an added benefit when using a flat, non-twisted airfoil, that the drag goes up significantly with rotation speed, since more of the blade is NOT designed to be optimized on purpose ..this is called "stall-regulated design" ..it is the exact opposite of the huge commercial "pitch regulated blades" whose design is to optimized at any/all wind speeds ..it is a known fact that fat blades have lots of drag at high rotation speeds"

It was my understanding that a "pitch-regulated" blade was a "stall-regulated" blade, they just stay in the max output (or rated RPM) until the generator is maxed. Once they get past the rated output the blades are stalled to regulate their output. The advantage of having a twist-tapered blade is during the stall periods the stress is more evenly distributed, and the stall is more predictable along the span of the blade.

It has also been my understanding that mega-watt blades are generally thicker proportionally than small wind systems. This is more expressed in the inboard 0%-60% of the blade span. This is mainly due to the structural capacity of a thicker airfoil section is beneficial with a where the loads are highest. While most smaller wind turbine manufacturers seem more interested in squeezing every last watt out of a given diameter. With the exception of Bergey, Jacobs, and most home builders who do not use twist-tapered designs.

[Stewart Corman]

Sean,
we are probably getting in symantics here on who is using what definitions.
IMHO, a fixed pitch design is usually considered as "stall-regulated" , while the commercial scale, pitch regulated are actually computer controlled to get the rpm nearly constant at decent WS, because that is their optimum power transmission to the grid ...so they design the blade at that optimium AOA/TSR:

http://www.horizonwind.com/about/ftkc/howdoeswindturbinework.aspx
On a stall-regulated wind turbine, the blades are locked in place and do not adjust during operation. Instead the blades are designed and shaped to increasingly “stall” the blade’s angle of attack with the wind to both maximize power output and protect the turbine from excessive wind speeds.The crux of a non-twist design for low WS is to get max power out while still functioning in a laminar flow condition which implies low TSR and fairly low RE#.
As turbulent flow is achieved in much higher winds, the drag gets bigger dramatically at the root, so the tip does not dominate anymore, the TSR actually goes down and the polar plot shows you how fast the L/D ratio changes for the worse as AOA changes (is a function of TSR only!).

When I was visiting Allison, he showed me an output for a 2bladex4rotor model which had a max eff at 12mph, which dropped by 1/3 at 20mph ..so if you furled at 16mph, the power should go up by 2.37x, but knock off 1/3 eff and lower TSR and you get only a minmal increase of the rpm ..ideally as the power goes up by WS^3, the efficiency/TSR falls so that a 2x increase in WS only gives a doubling of rpm, not 8x. This occurs when the efficiency goes from 40% to 10%.

When you use the twist design that Sandia developed ..their purpose was to maximize power, but that occurs at high WS ie 25mph+...Not the paradigm I am looking at ....furthermore, look at the problems that Dave B had with his 18footer (Wincharger blade design) when he started cranking 330rpm in 30+mph winds ..not good!

Stew

[Sean Skirvin]

The crux of a non-twist design for low WS is to get max power out while still functioning in a laminar flow condition which implies low TSR and fairly low RE#.
As turbulent flow is achieved in much higher winds, the drag gets bigger dramatically at the root, so the tip does not dominate anymore, the TSR actually goes down and the polar plot shows you how fast the L/D ratio changes for the worse as AOA changes (is a function of TSR only!).

When I was visiting Allison, he showed me an output for a 2bladex4rotor model which had a max eff at 12mph, which dropped by 1/3 at 20mph ..so if you furled at 16mph, the power should go up by 2.37x, but knock off 1/3 eff and lower TSR and you get only a minmal increase of the rpm ..ideally as the power goes up by WS^3, the efficiency/TSR falls so that a 2x increase in WS only gives a doubling of rpm, not 8x. This occurs when the efficiency goes from 40% to 10%.

When you use the twist design that Sandia developed ..their purpose was to maximize power, but that occurs at high WS ie 25mph+...Not the paradigm I am looking at ....furthermore, look at the problems that Dave B had with his 18footer (Wincharger blade design) when he started cranking 330rpm in 30+mph winds ..not good!

Stew
IMHO the crux of a non twist tapered blade is usually to save money on construction and possibly some structural benefits. Also, the purpose of a twist tapered blade is to maximize power up to the rated speed, because once you are past that (which is usually around 25MPH) their is ample energy to be harnessed and it actually needs to be reduced through stalling, by pitching the blades to a less efficient AOA.

[Stewart Corman]

Sean,
I am mostly in agreement with your comment.

Bergey makes a non-twist because their "pultuded" manufacturing technique is basically a high tech materials extrusion process.

Jacobs on the other hand makes units from 23 to 31 feet in diameter (10->20KW) still using a non-twist blade (fiberglass on 20KW only?)
BTW, they have added sophistication since the 50 year old models:

Speed Control
Centrifugal variable pitch governor (Loaded 25 - 30 mph, Unloaded 15 - 20 mph).
Automatic furling - hypoid gear drive (40 - 45 mph) .AND, they no longer have a flat face but something closer to the Wncharger that Dave B is flying.

I don't think that cost has anything to do with twist or not.



Also, the purpose of a twist tapered blade is to maximize power up to the rated speed, because once you are past that (which is usually around 25MPH) their is ample energy to be harnessed and it actually needs to be reduced through stalling, by pitching the blades to a less efficient AOA.This thread IS talking about fixed pitch turbines!
MY point is related to your comment here, but in reverse. Your twist design gets to be VERY efficient in higher winds when you don't have the capability of "pitching" as in a fixed pitch design. Just look at the polar plots and L/D when the RE# really start shooting up. Runaway is the biggest problem most on the Otherpower board are faced with, especially if generator fries or transmission fails ie if my chain breaks. So , a flat profile + thick root becomes far more self stall-regulating in high winds, whereby they perform with superior efficiency on low winds, where we can't afford to waste the energy available. None of the commercial units give enough KW output at 12mph avg WS to give a reasonable ROI. I can throw away all the KW at 20mph+, because I won't see it for more than 2% of the operating time. Safe operation in high wind is my only concern.

Stew

[Sean Skirvin]

I also agree mostly with your last comment. While non twist tapered blades will perform well at low WS, a properly proportioned fully TT blade should perform better than a non TT blade, even at low WS. I also think that most of the problems with the otherpower designs stems from improper furling ie. tails are too heavy or angled wrong. Though I personally think their is issues with the flat resin stator design as far as heat dissipation/warping and possible UV cracking, they can perform quite well for extended periods of time if they are furl at the proper times, and the gen is matched to the blades well.

I agree with you strongly on the benefit of a thick inboard section/root on any blade, strait or TT. I also agree that one way to improve the total output per dollar for wind turbines is to increase the proportion of swept area to the max gen output to increase the output in the low WS.