Thin or fat blades for my ~5 W turbine

seanspotatobusiness · July 2015

I'm making a very small turbine for keeping my phone and external battery charged when I go hiking/camping. I live in Scotland where the wind is much more reliable than the sun!

I could really use some advice regarding the blades. I intend to buy some blades rather than attempt to make them myself since they're pretty cheap and will be relatively well-made/balanced.

Here are some options on eBay:

http://www.ebay.co.uk/itm/121693415808

http://www.ebay.co.uk/itm/121678487367

http://www.ebay.co.uk/itm/121167148897

I'm going to be combining the blades with a stepper motor I'll salvage from a broken printer (I don't have this yet but not sure that matters).

Thanks in advance for any insight you can offer.

waynefromnscanada · July 2015

Hi "seanspotatobusiness",
In general more and bigger blades provide quicker startup in very low wind speeds. BUT - - - low wind speeds do not provide usable power.
Yes, it might be nice to see a turbine turning at low wind speed, and that's fine if a positive visual experience is what one wants. However, if useful energy is the desire, go with the fewest blades and only those with an airfoil (airplane wing) design. Another thing to consider is that the more blades, the more drag, thus lower efficiency and slower RPM.
There have been experiments with single blade turbines, one single blade with a counter weight where the other blade would be, and they are very efficient and high speed, unfortunately, like two blade turbines, they suffer great vibration and stress when changing direction to follow changing wind direction. Thus 3 blade turbines are the most used as they are far smoother when changing direction. The first blade design you list, 4 blades, is the only one of the three I would even consider. The other two are typical fan blades and although they will turn in the wind, their efficiency will be in the basement when used as a wind turbine. Beyond that, one might think that the space between a turbine with only 3 blades is wasted space that could be used to catch wind if there were more blades. While that may seem reasonable, in reality a lot of blades tend to push the wind out around the turbine instead of passing through the blades. In fact, even a fast turning 3 blade turbine can appear as an almost solid disk to the wind, but not nearly as bad as a turbine with more blades.

seanspotatobusiness · July 2015

Thanks a lot for the quick response, Wayne! I really appreciate it. I thought I'd be waiting a long while. I'll see if I can find a three-blade version of the RC plane propeller. Thanks again, Sean.

BB. · July 2015

None of those are going to give you any useful amount of power... And using a stepper motor in sun/rain/etc. is probably not going to last very long either.

First warning, I am not a big fan of small wind. I, personally, believe it is almost useless (expensive to build tower/electronics, unreliable, subject to weather/lightning, etc.).

Just to look at a ~1 foot diameter wind turbine:

http://en.openei.org/wiki/Small_Wind_Guidebook/How_Much_Energy_Will_My_System_Generate

To get a preliminary estimate of the performance of a particular wind turbine, use the formula below (assume good wind, tower 20+ meters high, non-turbulent airflow).
AEO = 0.01328 D² V³

Where:
AEO = Annual energy output, kWh/year

D = Rotor diameter, feet

V = Annual average wind speed, mph

AEO = 0.01328 * (1 foot)² * (10 mph)³ = 13.28 kWH per year
13.28 kWH per year * 1,000 Watts/kW * 1/365 days per day = 36.4 Watt*Hours per day (guess at average harvest)
13.28 kWH per year * 1/365 days per year * 1,000 watts/kW * 1/24 hour per day = 1.5 Watt average power.

Realistically, in heavy wind (25 mph), you might see 15 watts. peak???

Using the Solarelectrichandbook for a fixed array for Aberdeen: [h=3]Aberdeen
Average Solar Insolation figures[/h] Measured in kWh/m²/day onto a solar panel set at a 33° angle from vertical:
(For best year-round performance)

Jan
Feb
Mar
Apr
May
Jun

1.00

1.88

2.60

3.44

4.08

3.93

Jul
Aug
Sep
Oct
Nov
Dec

3.75

3.50

2.96

2.01

1.16

0.76

So, if we assume 0.76 hours of "noon time equivalent sun per day (yep, that is terrible sun all year round):

36.4 Watt*Hours per day * 1/0.52 off grid solar efficiency * 1/0.76 hours of sun (December average) = 92 Watt solar panel

Even in the worst (average December) month, a ~92 Watt panel will produce as much (or more) than your ~1 food diameter wind turbine (really rough guesses).

Now, 36 WH per day is around 3 AH on a 12 volt battery...

The question is, "how much power do you need to generate to have a successful system"?

And to answer your original question--A big issue with wind turbine blade design (besides the diameter or "swept area") is the speed (RPM) of the turbine. A slow turbine with "fat blades" vs a fast turning turbine with thinner blades, etc....

If you still want to design your own turbine for fun... Here are some good starting links:

www.otherpower.com (good forum for DIY Wind Power)
Hugh Piggott - Scoraig Wind Electric site for tons of info (from mike90045)
Scoraig Wind "Recipe Book" for DYI Turbines
www.greenpowertalk.org (added from "russ"--Like here but more wind/less solar)

People do build good DIY (do it yourself) wind turbines--But they are typically not small, and need to be mounted on well designed towers >=60 feet (20 meters) in good wind, clear of obstructions (trees, houses, etc.).

-Bill

seanspotatobusiness · July 2015

Thanks, Bill. I'll look into this a bit deeper. I don't really need a lot from this turbine. I'd be happy with at least 2.5 W since it's just to keep a USB battery charged. I don't need anything above 5 W because the battery only accepts 1 A at 5 V. The wind speeds on the hills I've been camping on tend to be 15-18 mph and one advantage of the turbine is that it can keep going while I sleep.

BB. · July 2015

In the end, go ahead and experiment. The blades are cheap, and if you have access to a small lathe to make hubs/adapters--Have fun.

You can get a small/inexpensive DC AH/WH meter (there are lots of similar devices out there these days) and try different configurations.

One suggestion--Go fly a kite. Where the kite flies in clean air (no turbulence) is the minimum altitude where the wind turbine should be installed. Turbulent air has much less energy and is much harder on the equipment.

-Bill

seanspotatobusiness · July 2015

Regarding the blades, I'm now looking at something like:

http://www.hobbyking.com/hobbyking/store/__8113__Master_Airscrew_propeller_16x10_Pusher_.html

which is the largest reasonably-priced three-bladed RC propeller I can find or:

http://www.ebay.co.uk/itm/331608016545?_trksid=p2055119.m1438.l2649&ssPageName=STRK%3AMEBIDX%3AIT

which may be better or far worse?

BB. · July 2015

I cannot really tell you which is better... Much of it depends on the stepper motor you will be using.

With many permanent magnet type motors, they have a "cogging" effect. On some, it can take a fair amount of torque to get the motor turning--So you may need a larger diameter/wider set of blades to get the started (in reasonable levels of wind). And there is the RPM you need to spin the motor to get useful amount of power for your needs.

-Bill

Thin or fat blades for my ~5 W turbine

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