More blades give a closer [cost] shave

October 1, 2011

Responding to Professor Michael J Economides' recent condemnation of alternative energy efficiencies and economics (OE August), serial oil industry inventor Benton Baugh turns his attention to the renewables sector with this firmly tongue-in-cheek (‘but true,' he adds) piece for OE on small-scale test results indicating how the output of wind energy devices could potentially be quadrupled.

Wind farm in Hawaii

The photograph shows a wind farm in Hawaii, probably the only state in the US which has to import 100% of its fossil fuels. They are not generating power and are in need of repair. It is not clear what the situation is; however, it would tend to teach that wind energy is not actually economic – except as a venture based on government grants. Once the grant money runs out, they are abandoned.

The development of substantially more efficient wind energy can be a key component in US energy independence and a greener environment. The same would apply for the entire world. Fossil fuels are now and will continue to be critical to civilization as we know it. Literally, when fossil fuels are gone planes will no longer fly.

Conventional wisdom says to make wind turbines with two or three blades. If you want more power you simply make the blades longer. Blades are now being designed to be as long as a football field. This increases capacities, but with the following unintended consequences:

  • substantial cost increases on the blades and the taller towers to support them;
  • the tip speed of the blades is so fast that birds cannot see them, and when installed on bird migration paths can cause substantial bird kill; and
  • it is hard to imagine that you can accelerate the tips of the blades to a speed which makes them invisible and still harvest net energy. You would get some energy from the tips and you would spend energy to push the tips at great speeds.

The conventional three blade turbine (occasionally there will be only two blades) is what can be considered as the base case.

Figure 1 Design 1

Figure 1 will increase the capacity to harvest energy from the wind by an extra 300%. That's it. Add nine more blades.

Staying with the same tower and the same blade length it becomes four times as efficient. The pipe to make the tower may need to have a slightly thicker wall, but it will not be significant. Most of the work of the tower is to hold itself up.

This change is so simple that a small child can understand it. Who it is not clear to is industry experts. I personally proposed this to a past chair of the ASME Wind Energy Division several years ago, and only received a blank stare. I have shown this to hundreds of individuals over the years and have had no disagreement. Finally, a college professor gave a reason: he said that all the formulas were set up for two or three blades so what could he do?

I suspect that the problem stems from propeller designers drawing on blade designs based on powered propellers such as aircraft propulsion.

In an airplane you may be putting hundreds of horsepower into blades plowing into the wind. In this case if you have too many blades, one will be in the wake of the preceding blade giving it bad flow characteristics.

When the wind is powering the blades the dynamics are very different. You are much less likely to have the wake of one blade interfering with the next blade. The wake will be moving away from the blades at the speed of the wind.

The challenge I regularly put forward is for a person to go to a farmer who has 12 blades on a windmill and tell him he will get more power if he removes nine of them. I have gotten no takers.

Figure 2Design 2

Design 2 was manufactured and tested against the professionally designed propeller of the Sterling engine shown below. The ‘crankshaft' was removed and replaced with a 0.062in Tig wire to produce a straight shaft for the propeller to drive. The propeller is 7.990in long.

The Sterling engine was mounted as shown and a nylon thread of 0.008in diameter glued to the axle, giving a moment arm of 0.035in to the centerline of the thread as seen in Figure 2.

Figure 3

The Sterling engine was mounted on a board and a single nail weighing 0.0165oz was tied to the thread. A three-speed Galaxy box fan with five 18in blades was placed in front of it and moved until the commercial propeller barely would pick up the single nail on high speed and would not pick it up on medium speed. This is shown in Figure 3. The resultant distance between the fan blade and commercial propeller was 27.5in. The torque produced by the propeller in this condition was 0.035*0.0165 = 0.0005775ozin, or 3.008*10^(-6)ft-lbs.

The fan speed on the low setting was 4.2mph, on the medium 4.8 mph, and on the high speed 5.5mph.

This design was found at Make-a-Pinwheel and is shown in Figure 4: scaling the drawing to being 7.990in corner to corner, it fits in the same space as the commercial propeller.

The photographs (shown below) show the pinwheel which is taped to the front of the propeller for a mounting.

The results of testing with the fan in the same position were:

low speed 28 nails lifted
medium speed 42 nails lifted
high speed 60 nails lifted

The photographs show the pinwheel which is taped to the front of the propeller for a mounting.

With the pinwheel installed with no modifications it outperformed the commercial blade by a factor of 60:1. In all fairness, commercial wind farms usually use three blades rather than the two blades on the Sterling engine, so it really only outperformed by a factor of 40:1. There were two additional observations made in this study. On the website where the design was garnered, there was a picture of a small boy with a pinwheel indicating the use of construction with eight blades rather than the four used in testing. Would using a double pinwheel be 80 times as efficient?

Secondly, the blades on the Galaxy box fan had a scooping profile or a curved surface rather than the flat surface of the farmer's windmills. It may be interesting to see how much this adds to the efficiency of wind energy harvesting.

Maybe time to get a new formula? OE

Benton F Baugh PhD PE

About the Author:

Mechanical engineer Benton F Baugh PhD PE, has been inventing things since high school and his first commercial success – a drilling choke – dates back to 1965. He started his own Houston-based consultancy, Radoil, in 1981 and today lays claim to 107 patents. His latest effort, the Shearable Drill Collar, was featured exclusively in OE July 2011.

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