Author Topic: Christmas Windmill Time (Read 28150 times)

taylorp035 · « **on:** December 19, 2019, 09:27:50 PM »

It's been a while since I've posted and done anything with my windmills, but I wanted to get back to making some new blades over Christmas break. I think everyone has their talent or favorite part when making a windmill and mine is definitely the blades.

This time around I wanted to do a bit better and make a set of blades with something that closer resembles a high lift airfoil instead of having a flat wind facing face and a sharp front edge (school textbook airfoil). The rest of blades were have been historically pretty good with an exponential taper and twist, all done by hand. For my new blades, I would like to make a mini set on my CNC mini-mill (5"x6"x12" + rotary axis) as well a hand carved larger set for my treadmill motor. The mini set will probably be 10" long blades and the larger blades will be in the 25-30" range to hit a desired 455 RPM cut in. If the mini set goes well, I may CNC the large blades in several steps.

I found in the past my blades would struggle to get going and then would rocket up quick in speed and then seemingly not do good at high speeds. I was reading Sparweb's blade aerodynamics page and realized I made the same mistake he mentions in that my blades had way t0o high of an angle of attack.
http://www.sparweb.ca/1_Blades/Aero.html

So I did a bit of math to figure out the apparent wind angle on the blade based on the TSR and where you are along the length of the blade. Then I picked an angle of attack I wanted based on looking at a few dozen lift coefficient graphs. I picked an angle a bit below the peak lift such that the blade would provide more lift once it really ran into the load from the motor or the wind speed picked up... maybe this is the wrong way of thinking, but I liken to having more torque down low in the RPM's from a turbo charged diesel engine. If did this wrong, let me know. I have some time since I'm still trying to get my CAD software back up and running due to a Windows 7 update issue.

Hopefully I can break my old record of 13.3 amps going into a 12v lawnmower battery. I now have a ~5ah, 14.4 volt LiFe battery that used to be in my daily driver as a replacement for the 65 lb Pb battery to play around with as well as a pile of other 3S 5ah LiPo packs.

SparWeb · « **Reply #1 on:** December 20, 2019, 01:41:14 AM »

Hi Taylor,
Happy to see that you are well and still experimenting!

That's a heck of a twist profile - the blades will look swoopy and fun. On the other hand, I'm looking at the "455 RPM" cut-in and it doesn't seem right. Can you confirm the open-circuit voltage per RPM of your generator?

When looking at CL, I always want to check the Reynold's number before judging what's realistic. I'm not sure you can get it to 1.4 at this scale. But XFOIL has provisions for dealing with RN, so just confirm for me that you can expect that. Also, let's look at the drag polar for this airfoil - it's pretty thick which means it will have a higher profile drag.

BTW, I've refined my work on blade twist by using a spreadsheet I got from Hugh Piggot and adding my own "twist". Let me know if you'd like to take a look and I'll post an attachment or find some way to get you a copy.

Adriaan Kragten · « **Reply #2 on:** December 22, 2019, 07:53:27 AM »

The aerodynamic theory how to design a windmill rotor is given in chapter 5 of my public report KD 35 which can be copied for free from my website www.kdwindturbines.nl at the menu KD-reports. I give an example for a blade designed with the optimum lift coefficient and for a blade designed with a constant chord. Constant chord blades are much simpler to manufacture and the rotor can still have a high maximum Cp if the correct airfoil is used.

A problem with making small blades with small chords is that the Reynolds value becomes very small at low wind speeds. The Reynolds value can be calculated for every blade section using formula 5.5 of KD 35. Normal aerodynamic airfoils get very high drag coefficients if the Reynolds value becomes less than about 10^5. So it is useless to use a very advanced airfoil which may have perfect characteristics at high Reynolds values if it is used at low Reynolds values. For low Reynolds values, one can better use the 7.14 % cambered plate airfoil (see KD 398) as this airfoil has still rather low Cd values at low Reynolds values. This is because the sharp airfoil nose makes the boundary layer turbulent and this turbulence prevents stalling at moderate angles of attack.

taylorp035 · « **Reply #3 on:** December 30, 2019, 07:51:13 PM »

I took the windmill out a couple of days ago and decided my modified 46" diameter blade set doesn't want to get the treadmill motor spinning at all from a stop. Even spinning it by hand to get it moving didn't want to make it keep going in ~8 mph winds. Of course it wasn't very windy by the time I got it outside...

So today I did a generation study of the motor on my lathe while charging a 2ah 18650 lithium cell that was almost empty. I didn't have a RPM measurement under load, so it might push an extra amp or two at the listed RPM. I was able to get 7 amps into the cell at 614 RPM (no load RPM/volt used to calculate this speed)... there were a few losses through my alligator clips and ammeter setup, so I did some rough math to correct it based on resistance.

Doing some calculations with a TSR of 7 and a 6 foot diameter, you get the power curve shown. I hand drew the red line on there as the blades overpower the treadmill motor with a higher TSR. I might get 20 watts at 10 mph. To get the same power for a 8 foot diameter, I would need 13 mph. My wind site is really terrible since this is just a science project and I don't have it above the trees.

I have 14 of these 2ah cells that I harvested from a bad 6ah 20 / 60 volt Dewalt pack. I have a friend who might give me a few more to play with. If this works decently and I make some nice battery holders + have a battery management system, I may try purchasing some higher capacity cells for $2-3 a piece (~$200/kwh). Finding 1S battery management systems that handle more than 10 amps is proving to be elusive. The moment being, the BMS will have to be my eyeballs on a volt meter. I think it would be cool to get this to the point where I could easily charge two cell phones everyday.

SparWeb · « **Reply #4 on:** December 30, 2019, 09:02:32 PM »

There are a variety of cheap/simple ways to set up a tach, especially if you're testing on a lathe. My method was to use a bicycle speedometer. There's a way to enter a wheel diameter value that it will multiply by the pulse count to give you a number equal to the RPM.

If you have a lathe to drive the generator for tests, then you can probably find a way to measure torque at the same time as power output. Torque X RPM = Power input.

Then you'd have a firm power input curve that you can use to match the blade TSR and diameter.

taylorp035 · « **Reply #5 on:** December 30, 2019, 10:08:19 PM »

This motor has been tested quite a few times with different methods. I dug up some data using a Bridgeport and a load cell + lever arm + optical tachometer from 2016. This was the lowest RPM it was tested at.

I figure I'm at about 1.6 ohms, which based on the old data means about 10.5 amps at 880 RPM (or so I think...). It lines up with the data I have from today's lathe test if you were to extrapolate it. The resistance was calculated and not measured for this chart.

Efficiency is going to probably be in the 30-50% range at such a low RPM and voltage now that I look at it. The goal of this isn't to make the most efficient windmill system, but to make the most power with a set of blades that are big enough to get the thing spinning without a hurricane force wind. I'm half tempted to give up the higher amps for something that starts up the easiest (8ft vs 6 ft). I also am thinking of making the blades extra wide and have no taper for better start up performance.

For fun, the peak efficiency of the motor was measured at about 3500 RPM pushing ~500 watts at 83% efficiency. It still was holding 80% at 1000 watts. I've seen 16+ amps at 125 volts dc (2 kw) of electrical power out of it and it probably has done more when I wasn't looking. 4000 RPM with (10) 300W light bulbs hooked up was pushing 19 amps at 91 volts (1729 watts). The poor motor was smoking after we completed a half hour of testing at 100 rpm increments in a 10hp CNC mill. It had been used to test Briggs and Stratton's for about 8 years after it being in my windmills for 2 years. It was rated at 4,000 RPM, but I know it's seen at least 5,500 when we had a really hot 68 cc engine hooked up to it at WOT.

SparWeb · « **Reply #6 on:** December 31, 2019, 01:57:18 AM »

OK
Don't make 8-foot blades turn at 800 RPM with just 200W mechanically holding them back - things will fly apart!
Your generator can do much more than just charge a few 18650 batteries. It's got the guts to drive much more than that.
Unfortunately, what I see in the numbers you're writing is that it runs at speeds almost 10x faster than you'd want the wind turbine to run.
You want to see peak efficiency at 350 RPM, not 3500.

taylorp035 · « **Reply #7 on:** January 05, 2020, 08:20:18 PM »

Time for an update. I worked on the storage side for this project. I spent about 4 days tearing apart the 60v 6ah Dewalt pack, cleaned up the cells, ranked the cells by their capacities and then made a battery holder for it all. Soon I'll get 4 more 2600 mah cells from a friend. If it all works well and I want more, I can make another tray and triple my capacity for ~$60 worth of cells (~62 ah).

I CNC cut some battery holders and made some rubber backed aluminum tabs to contact all the cells at once. Then I tried it out on the lathe with the treadmill motor afterwards. The cells really hold the voltage well as I was hoping and it did 6.4 amps at 4.14 volts at ~600 rpm. Again, it will be a hair better once I don't have high resistance jumper leads in the system and a volt meter.

I ordered two cheap Chinese 10A 1S battery management units and a 40A 3S BMS once I'm ready to consume the power. I'll make my 3S 6P battery holder while I wait for them to travel on the slow boat over here. I've been recently busy fighting my 4S 12ah LiFe battery in my 1999 BMW M3 that I'm using instead of the ~60 lbs piece of lead that was in it. I've been running it for 6 months and ~10k miles with a 100ma balancer permanently hooked to it, but after flattening the battery for the 5th time, I realized that some cells were over charged and others had almost no charge... it's been quite the project to get it properly working and probably deserves a thread since it's a bit different.

Some pictures:

SparWeb · « **Reply #8 on:** January 07, 2020, 01:01:18 AM »

Quote

I've been recently busy fighting my 4S 12ah LiFe battery in my 1999 BMW M3 that I'm using instead of the ~60 lbs piece of lead that was in it. I've been running it for 6 months and ~10k miles with a 100ma balancer permanently hooked to it, but after flattening the battery for the 5th time, I realized that some cells were over charged and others had almost no charge... it's been quite the project to get it properly working and probably deserves a thread since it's a bit different.

Please!

Bruce S · « **Reply #9 on:** January 07, 2020, 09:57:46 AM »

Quote from: SparWeb on January 07, 2020, 01:01:18 AM

Quote
I've been recently busy fighting my 4S 12ah LiFe battery in my 1999 BMW M3 that I'm using instead of the ~60 lbs piece of lead that was in it. I've been running it for 6 months and ~10k miles with a 100ma balancer permanently hooked to it, but after flattening the battery for the 5th time, I realized that some cells were over charged and others had almost no charge... it's been quite the project to get it properly working and probably deserves a thread since it's a bit different.

Please!

What SparWeb said!

taylorp035 · « **Reply #10 on:** January 11, 2020, 02:57:25 PM »

So I just took my 46" diameter blade set back outside since we were seeing 20+ mph winds today. The performance was super disappointing as it was really struggling to get the tip speed ratio up and didn't really want to start spinning. I've definitely seen my 8 foot blades spin faster than this. So I think I've decided that a set of 7 diameter blades that are extra wide (maybe 5") and have a steeper angle of attack would be appropriate to meet my rpm goals. The charge cut it did seem reasonable though, even with the 1v drop through the diode. At a nice 10 mph breeze if the blades actually perform well, I think I will see 1-3 amps, which would charge a set of batteries in a day or so.

Even in the highest gusts today, the generator really over powered the blades by a huge margin.

Criger · « **Reply #11 on:** January 13, 2020, 10:10:11 AM »

That's certainly counter-intuitive, isn't it? At least it seems that way to me. I would have thought the 46-inch diameter blades would get much better performance than that.

SparWeb · « **Reply #12 on:** January 13, 2020, 09:13:10 PM »

Could you take a picture of the blades, Taylor? I'm puzzled by this, too.
Something should be happening.
If it's the blades I can (sort of) see in your profile picture, the solidity ratio is very small, which is a big detriment to start-up, but at 20MPH should not be a problem.

That said...
"...a set of 7 diameter blades that are extra wide (maybe 5") ..." For a 7-foot diameter blade, 5 inch chord is not "extra wide". 5 inch chord is barely wide enough at the root for such a diameter. For that diameter, the tips should not be less than 2" either.

So on both counts I have some questions about the solidity ratio of your rotors.

MattM · « **Reply #13 on:** January 13, 2020, 10:32:53 PM »

More stiffness, less angle! You will believe.

taylorp035 · « **Reply #14 on:** January 18, 2020, 10:16:45 AM »

My old blades were 3" wide at the root and 1-3/8" at the tips. They don't have much of an angle at the tips. I originally had 6 of these blades on one hub to help have more startup power, but it didn't work very well either. I took some pictures of it.

I plan on doing a 4 degree tip angle and 4" wide for the whole blade (no taper) and 42" long. The boards are glued together and ready to be carved. They are 4.125" wide x ~2.25" thick in total, made of ash.

taylorp035 · « **Reply #15 on:** January 19, 2020, 10:53:41 AM »

I made a wood cross section template and laid out my first board so I know where to cut. Time to carve!

SparWeb · « **Reply #16 on:** January 19, 2020, 04:44:54 PM »

Enjoy!

taylorp035 · « **Reply #17 on:** January 19, 2020, 09:24:49 PM »

Progress photo. I'm thinking this is going to take a good 10 hours per blade to carve by hand. I'm not using any power tools. I have a draw knife, a few handled scrapers, various scraping cards and a barrel shaped wood plane for helping with the inverted curvature.

I do have to say, I like the shape. I'm thinking of making a Youtube video of how to carve a windmill blade using #2 while trying out my new Go-Pro. It might get a decent enough number of views I think if I do a good job.

MagnetJuice · « **Reply #18 on:** January 19, 2020, 09:36:13 PM »

Looking good!

I think a YouTube video of carving wood blades should be popular. Also very useful for the viewers.

Ed

taylorp035 · « **Reply #19 on:** January 24, 2020, 11:15:09 PM »

Another ~2 hours of carving. It's starting to look pretty good.

gsw999 · « **Reply #20 on:** January 26, 2020, 08:36:24 AM »

Nice work !!!

taylorp035 · « **Reply #21 on:** January 26, 2020, 12:09:44 PM »

Something I learned yesterday that I didn't think about is my single cross section template technically needs to be longer as I go to the steeper angles if I want the tip and tail of the airfoil to be at each side of my board. Making a handful of templates down the length would of been a good idea before starting.

I put another 2 hours into blade #1 yesterday and hopefully this afternoon I'll finish it up after a few more hours. I'm figuring it will have about 10 hours into carving it. To make it smoother, I think I will have to break out the DA sander to get the ripples out of it since the spokeshave and other tools don't do a very good job over the grain boundaries in the wood.

SparWeb · « **Reply #22 on:** January 27, 2020, 12:12:38 AM »

As you've probably seen, I used several templates to get the twist to progress consistently, so if it's not too late you can add one or two to the mid-span.
They do look good indeed.
I believe you will have plans to taper the tips, but a hint, if you would like one: wait until the blades are mounted to your hub before tapering the tips. The flat end of the blade you have now is useful when you are checking/adjusting the tracking of the blades. You want them all to pass through exactly the same plane. You can also measure the angle of each tip and make sure you've made good on that.

I think I can see the texture of the grain in your photos. Sanding is inevitable, no matter what material you use. There's still a bit of grain texture in my cedar blades.

taylorp035 · « **Reply #23 on:** March 17, 2020, 09:18:12 PM »

Blade #2 has been completed. I put one of the old blades in there for size... clearly this will get moving a bit better in low wind speeds.

I also wired up the electronics for both charging the cells (two 10A 1S li-ion BMS units) and for discharging (40A 3s BMS unit). I tried it out on my lathe and it worked perfectly by cutting off at 4.200 volts. It's super satisfying to watch 6A being pumped into the cells and it hold 4.14volts for several minutes before making it's way up to 4.2v.

ElSuperFantasma · « **Reply #24 on:** March 17, 2020, 09:34:13 PM »

Beautiful blades! Hope you did not suffer from allergies after doing all the sanding on them. I learnt my lesson after I almost gave myself pneumonia from all the cedar dust while sanding them down to shape.

Always use a good respirator.

Again, nice craftsmanship on those blades.

TSG

SparWeb · « **Reply #25 on:** March 18, 2020, 01:33:49 AM »

ESF,
Sorry to hear you are affected by cedar dust. That doesn't sound fair.

Taylor,
I agree, they look good so far.
I once new a guy who loved to build custom cars. He often said building the left side was original and always felt like fun. Then building the right side was too much the same, and always felt like work.

taylorp035 · « **Reply #26 on:** March 31, 2020, 09:58:41 PM »

Blade #3 is about 75% done. Tonight, we bored out the hole on the hub since it was off center by about 0.100" and put a plug in it.... even though I pre-drilled it on the CNC machine, my drill press wandered that much!!! Also trued up the blade mating face. This will be important considering how heavy these new blades are.

taylorp035 · « **Reply #27 on:** April 02, 2020, 10:15:24 PM »

Blades are finished being carved. Holes have been drilled to mount them to the hub. Now I'm lacking the proper bolts, but I'll make due with what I have until I can get to Home Depot so I can get it balanced. I plan on rounding off the blade tips.

Mary B · « **Reply #28 on:** April 03, 2020, 02:14:33 PM »

Leave them square and add a winglet across the end. Then try rounded tips after a months running... Or not LOL I like to experiment!

taylorp035 · « **Reply #29 on:** April 11, 2020, 06:19:08 PM »

I'm not sure how other people balance their windmills, but I made a giant structure with some ground steel rails to let a temporary shaft roll on it.

I first found the heaviest and lightest blades. I then carved more wood off the heaviest until it was the same weight as the lightest such that the middle weight blade would be hanging straight down. Then I carved the middle weight blade.
Here's a progression over 6-7 hours of balancing.

By the end, I think I had it balanced within a gram or two at the tip.

Now I need to make all the blades sweep in the same plane, as they are about 1" off on the tips. When I spun it up with 14 volts it was shaking pretty good (along with moving a lot of air!). I also need to cut the key way on the shaft and make sure it isn't going to move axially on the shaft. Tomorrow should be a good test day with 8-14 mph winds and a balmy 58 F degrees that might melt the 3" of snow we got two days ago.

ElSuperFantasma · « **Reply #30 on:** April 11, 2020, 08:03:03 PM »

This is what I used to balance the blades on the toy I built and came out with almost perfect results. There is no vibration at all.

I had to remove the seals to the bearings and washout the grease so that they would freely spin as Adriaan Kragten suggested in one of his replies in an old thread.

*Thanks Adriaan!

Those blades are beautiful and I'm sure they will perform beautiful.

TSG

taylorp035 · « **Reply #31 on:** April 12, 2020, 05:45:21 PM »

The maiden voyage was completed today. It started up in really light winds as I was hoping for. I'm thinking 4-6 mph winds. It also starts charging in 6-ish mph winds (best guess). In 8-10 mph winds, it's dumping a good 0.5-2 amps. I saw it get up to 5.25 amps at one point in a ~15-18 mph wind gust (should be 500-600 rpm). The 10A single cell li-ion BMS unit was working well, which cut off at about 4.15 volts. I was dissapointed that they didn't act like a diode, so I will need to put on in-line so it doesn't back feed the dc motor.

The blades seemed pretty balanced since it wasn't shaking the 10' ladder too much. I still need the blades to be in the same plane. This might improve the high speed performance a bit. Once I test that, I might even drop the pitch of the blades by a degree and/or sand the tips of the blades so it goes a hair faster. But I don't want to do that until I watch it charge over a longer period of time.

I was also pleased with how much rotational inertia these blades have, as they keep on going even if there is a small dip in the wind speed. The charging current was super smooth. Another thing to note as expected was the charging load had almost no noticeable impact on the speed of the blades... not surprising when you have 10-25 watts of charging load vs. 200-500 watts of shaft power.

For people who don't read this whole thread, the purpose of this windmill is to charge 18650 li-ion cells in low wind speeds, not be the most efficient windmill. The generator efficiency is only about 25% at 4 volts.

SparWeb · « **Reply #32 on:** April 12, 2020, 10:17:23 PM »

Looks great!
Your Christmas Windmill... christened at Easter.

I do different things to balance my blades, but the method you used is good. Thanks for the photos.

It would make me feel better if you used two hub plates, on the front and back. Thrust pushes the blades back, and in your current attachment they can bend back toward the tower (or ladder as the case may be).

You're going to need a new avatar picture.

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Author Topic: Christmas Windmill Time (Read 28150 times)