Optimized Blade Design for Homemade Windmills

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The intent of this project, created in collaboration with Mech425, is to identify the best angle for flat, uniform blades in relationship to the air stream to convert the most amount of energy into rotational motion. The primary use for flat blade windmills would be for people with limited access to tools or homemade devices when simplicity is preferred.

  • The project has been selected to provide support to individuals looking to generate electricity by harvesting the wind
  • The target audience are people who can not afford commercially available models and have chosen to build their own

Windmills have many functions and can be operated wherever there is access to wind. Windmills use their blades, or sails, to convert the energy in wind into rotational motion. This rotational motion can either be used for direct work or converted again into electricity. Originally, windmills were used to perform the grinding at mills. Today, they are still used for this purpose but have extended their range of uses to pumping water and primarily for electricity generation, In lesser economically developed countries, the electricity generated by homemade windmills are often used to charge batteries and cell phones or operate lighting, radios and irrigation pumps.

Modern, commercially available wind turbines are tailored to address specific wind speeds and are capable of generating megawatts of electricity from each turbine. However, homemade solutions are often low-tech and have undergone little scrutiny in terms of optimization. This report intends to identify to best angle to tilt the blades in relation to the oncoming wind and what length of blade is best suited for electricity generation.

William Kamkwamba is a fantastic example of who could benefit from the analysis presented in this report. His ambition for a better life and access to scrap materials was transformed into a working device that provides both light and irrigation to his community and inspiration to the rest of the world. As more people begin to develop solutions for their own energy needs, there is great value in optimizing these devices to maximize their social benefit.

William's story can be viewed below:

-- Benefits of Flat Blades --

Flat blades are less common than other designs but offer significant benefits, especially in low income or remote areas. The following is a list of benefits offered by using flat blades:

  • Easy to build
  • Less design and local knowledge required
  • Less machinery is required during construction compared to a curved design
  • Less time is required for construction purposes
  • Easier to ensure conformity among the blades

-- Engineering Calculations --

Power Available to the Turbine

The amount of power passing through the turbine blade area is primary dependent on the velocity of the wind and to a lesser extent, the area of the blades. To quantify the energy in the wind, we must first consider the wind to be a fluid flowing through the blades in a cylindrical shape.

The kinetic energy stored in the wind can be found according to Bournoulli's equation:

KE = 1 / 2(m * v2)

In order to find the energy in the wind, we must find the mass of the cylinder. This is based on the volume of the cylinder multiplied by the density of the fluid:

m = π * V

The total volume of the fluid that is represented by cylindrical column is:

V = A * L

We can calculate the area of the cylinder's base by:

A = 1 / 4(π * D2)

The length of the cylinder represents the amount of fluid that has passed through the windmill's swept area. This is calculated by multiplying the velocity of wind by time:

L = v * t

This can be simplified as follows:

KE = 1 / 8(ρ * π * D2) * v3 * t

Finally, the power in the wind is simply the energy per unit of time

P = π / 8(ρ * D2 * v3)

As demonstrated, the power in the wind highly related to the velocity of the wind and to a lesser extent, the diameter of the turbine blades

Maximum Possible Efficiency

The Betz limit was developed by Albert Betz and seeks to determine the maximum possible energy that can be derived by a device from a stream of fluid, flowing at a given speed. In the case of windmill, the maximum theoretical efficiency of a thin rotor can be found based on the following assumptions:

  • The rotor is considered ideal, having an infinite number of blades and no drag.
  • The flow into and out of the rotor is axial and in accordance conservation equations.
  • The fluid is modeled based on incompressible flow.

The Betz limit has been able to predicted the maximum value for the power coefficient to be 0.593. This means that the theoretical limit of power removed from from the fluid is 59.3%, although current commercial wind turbines are able to achieve 40 - 50% conversion efficiency.[1]

Optimal Angle of blades

The angle that the windmill blades are tilted compared to the stream of fluid will determine how much energy can be converted into rotational motion and then be captured by the system for meaningful work. The amount of force is calculated by finding the wind pressure.

The wind pressure exerted by the wind is given by: P = 1 / 2(1 + c) * ρ * v2

  • where c is a constant and equals 1.0 for long flat plates.

The force of the wind against the windmill blade is based on the wind pressure multiplied by the area of the blade facing the oncoming flow. In the event that the blade is tilted at an angle to the oncoming airstream, then the area of the blade exposed to the fluid is reduce by a factor of sinθ. As such, the wind pressure calculation is multiplied by A * sinθ to obtain the force of the wind on the blades

In addition, the force of the wind converted into rotational motion is related to the angle of the blade in relationship to the oncoming fluid flow. This relationship is given by a factor of cosθ.

Furthermore, the blades will encounter a drag coefficient related to the angle of the blades as they rotate in their own axis perpendicular to the oncoming flow of fluid. This drag coefficient will be represented by D * cosθ.

Therefore, the combined calculation to determine the force balance on the blades is:

F = ρ * v2 * A * sinθ * cosθ * D * cosθ

An important relationship to note is that between force and θ. The combined force balance indicates a relationship between force and sinθ * cosθ * cosθ.

As a result, the optimal tilt of the blades would provide an angle to the airflow such that sinθ * cosθ * cosθ is a maximum. This value has been presented in the graph below to show how the value changes as θ is adjusted. 

Blade Angle cos cos sin.jpg

The angle is adjusted in radians and seems to indicate a maximum value at approximately 0.62 radians, or roughly 35.5 degrees. This translates in a maximum conversion of 38.5% of the wind force into rotational motion. Therefore, the blades should be tilted at an angle of roughly 35.5 degrees from the oncoming air stream to obtain the optimal amount of energy using flat blade windmills.

-- Regional Considerations --

The target regions for this technology are locations where people have limited access to tools or supplies, such as Sub Sahara Africa. In addition, the area must have access to a reasonable wind source and access to certain key resources. These resources include many technologically advanced materials such as generators and motors. However, these materials can be found in local garbage sources and meet the fundamental requirements. While many people are unable to afford these items, there are many broken down cars and appliances that would be sufficient.

In terms of the social impact, he ability to access electricity is a powerful device that has the ability to help raise communities out of poverty. Conversely, it has been repeatedly shown to create a social divide that widens the economic divide. It is important that energy is handles with respect and that it is not used as a tool to further impoverish the unfortunate.

-- Materials --

William Kamkwamba was able to build his windmill using: [2]

  • Tractor fan
  • Shock absorber
  • Bicycle frame
  • PVC pipe
  • Bicycle generator
  • Bamboo poles
  • Bicycle dynamo
  • Rubber belt
  • Pulleys
  • Bike chain ring
  • Piston
  • Copper wire

If the intent is to store electricity, then these additional materials are required:[3]

  • Deep cycle batteries 12V (If the user intends to store electrical energy)
  • Charge controller to regulate how much the battery charges
  • DC/AC Converter
  • Bridge rectifier (to ensure electricity flows into batteries)

-- Tools --

In rural parts of the world, people are forced to be more creative with the resources available. Here are a couple examples of how this is the case:[4]

  • The flat blades can be created by cutting a PVC pipe lengthwise, using a saw or similar device, and then heating the pipe over a fire. Once hot, it can be molded into a long, flat blade.
  • Washers can be made by hammering bottle caps to be flat and then punching a hole through the middle.

In these circumstances, nails, rocks, fire and wood become the tools to build the wind turbine. Steel or rocks can be used as hammers and bicycle spokes can be scraped along a rock to create a flat edge and plastic bags can be melted to build a handle around one end. Furthermore, drills can be created made out of a maize-cob as a handle and an extruding nail. The nail can then be heated over an open fire until it becomes red hot and then be used to penetrate through certain materials.

-- Skills and Knowledge --

In order to benefit from the energy generating potential of wind turbines, it is important to understand how much wind is available at a given location. A Beaufort scale provides an indication of wind speeds based on various visual clues. While these clues offer indications of the wind speed on the ground, there is likely a greater amount of energy in the wind as the altitude increases. This is based on the boundary layer that develops on the Earth's surface as a result of various obstructions on the ground. The Beaufort scale is provided below:[5]

Beaufort scale indicating wind speeds

In order to obtain a more complete list of physical identifying factors, please follow this link.

Furthermore, to transmit the electricity generated to be used for applications such as operating lights, radios or charging batteries, then it is critical to have an understanding of electrical theory such as the required voltage and amperage to power the desired device.

-- Technical Specifications --

William was able to build his windmill based on the schematic shown below.[6] It was then mounted on a large tower he created out of wood. Overall, the machine is fairly simple in concept with the major limitations being available materials and limited access to tools. Through testing, William found that using his design, a four blade windmill was able to generate more power than its three blade counterpart.[7]

Schematic of William Kamkwamba's Windmill

-- Estimated Costs --

William has disclosed that his windmill cost approximately $15 to produce and that the bicycle generator was the most difficult to attain.[8] An estimate of the various costs for the components has been compiled based on relative accessibility. The costs have been contrived to total the $15 that William had to pay. This project would certainly not be desirable in a more economically developed country based on access to better tools to improve performance and the prohibitive costs to buy materials that only generate a minimal amount of energy. That said, the minimal energy generated by William Kamkwamba was able to change his village and provide simple luxuries that have offered the community hope and a new outlook on life.

Estimated costs for William Kamkwamba's windmill

In rural settings, the cost for the parts will vary significantly based on the materials available locally. Therefore, it is more appropriate to offer a range of anticipated costs based on the variability of how accessible certain materials are. The costs have not been calculated as the cost to purchase and send the materials would be astronomical and the true costs will have massive discrepancies based on the local and availability of materials.While this is purely an estimate, it offers an idea as to how much one might expect to pay for the parts.

Estimated range of costs for a flat blade windmill

As indicated, the range of costs is approximately $0 - $99 and provides a general range of project costs based on how much can be salvaged. Based on these estimates, William's $15 budget appears to be at the lower end of the range as he was able to find the majority of his materials from what others considered to be waste.

Beyond the initial cost, the power harnessed from the wind has an opportunity to become an income generating technology. Cellphones have provided jobs to many people who rent their phones to individuals looking to call neighboring markets to determine the price for various goods. Similarly, power can be sold to people looking to charge their cellphones or other batteries that can be used for lighting applications or to listen to radios. 

-- Common Mistakes --

There are many variations to the windmill design used by William Kamkwamba and refined throughout this section. However, there are some variations that are commonly used and have a negative impact on performance. One of these examples is using wood as the material to create blades. If avoidable, wood is a poor choice is it is a more heavy substance and requires more energy to begin rotation and achieve more frequent rotations.

Also, It is very important that blades are evenly shaped as this can cause a wobble to occur. The wobble will result in reduced performance and will shorten the windmill's life based on additional vibrations. The windmill blades should also be placed high above all other obstructions in order to obtain a more powerful and consistent wind stream. A good rule of thumb is to place the turbine twice as high as any nearby obstructions.

-- Other Designs --

If you happen to have access to additional equipment such as saws, then it may be possible to use the design shocased in the video below. Also, be sure to note that the wind turbine is able to pivot and uses a tail to direct the blades into the wind.


  1. Gorlov A.M., Silantyev V.M., Limits of the Turbine Efficiency for Free Fluid Flow, Journal of Energy Resources Technology - December 2001 - Volume 123, Issue 4, pp. 311-317.
  2. Kamkwamba, William. The Boy Who Harnessed the Wind. William Morrow, 2009.
  3. Make a Wind Turbine. Available at: http://makeawindturbine.com/ [Accessed April 9, 2010].
  4. The Doers Club. Available at: http://changeobserver.designobserver.com/entryprint.html?entry=10707.[Accessed April 4, 2010]
  5. The Beaufort Scale. Available at: http://gcaptain.com/maritime/blog/beaufort-scale-images/ [Accessed April 4, 2010].
  6. The Doers Club. Available at: http://changeobserver.designobserver.com/entryprint.html?entry=10707.[Accessed April 4, 2010]
  7. Kamkwamba, William. The Boy Who Harnessed the Wind. William Morrow, 2009.
  8. African Leadership Academy. Available at: http://www.alagapyear.org/community/african_students/williamk.htmlfckLR[Accessed April 16, 2010].