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Bouw je eigen windmolen van PVC

Windmolen van PVCDit artikel vormt een stap voor stap handleiding voor het bouwen van de wieken voor een kleine windmolen. Na het lezen van dit artikel ben je in staat om zelf de wieken van een eenvoudige windmolen te maken met behulp van PVC. De uitleg is voorlopig alleen nog in het Engels beschikbaar, maar voel je vrij om een vertaling aan te leveren. Daarnaast ben ik benieuwd naar de ervaringen van mensen die hem daadwerkelijk gemaakt hebben.

Dit artikel is gebaseerd op het "Excelbook" "PVC blade design" waarin Noam Dotan in een aantal eenvoudige stappen uitlegt hoe je van PVC een windmolen kunt maken. Noam is verbonden aan het projectteam Comet-ME. Dit een organisatie die tot doel heeft om doormiddel van eenvoudige stand-alone energieproductie, op een milieuvriendelijke manier mensen in de Palestijnse gebieden sociaal en economisch onafhankelijk te maken.

Suggested PVC blade design

  1. The angle of attack (usually called alpha - α) is taken as "the best" ratio of lift to drag forces. Note that the optimal blade width depends on this value. "The best" means the value for which rotor efficiency (also called coefficient of performance) Cp is highest.
    Alpha usually has a value between 3 and 5 degrees. Experimentation with different configurations for the PVC blades showed that the "best blades are designed for 4 degrees angle of attack".
  2. PVC pipe diameter must be in de order of 1/5 of blade length. Thus, if you design a 1 meter blade you should choose about 20 cm diameter (or 8 inches). Use a pipe wall thickness of around 6 mm.
  3. The rib length, which is a segment of the pipe circumference, must not exceed 1/4 of the circumference; otherwise drag losses become dominant and degrade the performance. 
  4. Due to PVC strength limitations, you are restricted to about 1.5 meter maximal blade length, which will limit your turbine to about 1 KW, for 9-10 m/s wind speeds.
  5. You must protect the PVC with a protective paint against UV, otherwise the blades will not last more than a year. UV breaks up PVC!

Guidelines for the design

  1. Define the blade length, angle of attack and tip speed ration (TSR).
  2. Define the required chord length either by exact calculation or by approximation.
    1. Noam's choice is a tapered profile blade. Define the chord lengh at each station by calculation the tip width and setting the root width to 1/4 of pipe circumference. The calculation of the blade width at the tip is carried out considering the TSR, the number of blades and the lift coefficient (e.g. using Piggott's formulas). As you can see in the graph below, for most of the blade length these are good approximations to the Piggott values; especially away from the root.
    2. another choice is to calculate the chord for every station, e.g. using the Piggott formulas. In this case, if and when the chord lengths exceed 1/4 of pipe circumference, keep the chord length equal to 1/4 of pipe circumference.
  3. Calculate the build angle.
  4. Define the rib by solving the required chord for the required build angle.

Designing the blade

The blade length from the root (center of the hub) to the tip is divided into 20 stations (Figuur 2). At each of these stations the chord ribs L1 and L2 are calculated.

Graph
Figuur 2: template for blade design

The procedure is as follows:

  1. The chord length at the tip (station 20) is calculated in green cell L59, that can be found in de appendix (see link below)  using the formulas of Piggott.
  2. This chord length at the tip, and the chord length at the root (1/4 of circumference at root) are used to calculate the taper angle in cell F18 (appendix). 
  3. Using the relations given above, angles θ, φ, φ1, and φ2 are calculated for each station and from these values, the tapered chard length and the rib lengths L1 and L2 are calculated (XXX).
  4. On a piece of paper a straight line equal in length to the blade length R is drawn. The 20 stations are marked as shown in column C (appendix).
  5. The rib lengths are marked at each station, the points are joined with a smooth line and the resultant shape is cut off to make the template.
  6. Draw a straight line along the pipe; the best way to do it is to place the pipe on a flat surface (the floor for instance), place a straight ruler along its side and draw a line.
  7. Place the template so that the "line of connection" drawn on the template coincides with the line drawn on the pipe.
  8.  

Template vleugel
Figuur 3: template van de wiek

     

  1. Use adhesive tape to fix the template to the pipe and a permanent marker to draw a line around the template.
  2. The template should include the position of the holes through which the blade will be connected to the blade holder (hub). Marking the position of the holes on the pipe will ensure all blades have identical hole positions.
  3. Drill a 10mm hole at one side of the tip to allow the jig saw to make the sharp turn around the tip of the blade.
  4. Remove the template and cut the pipe along the lines marked, try not to cut inside the line.
  5. Us a a wood plane to correct any deviations of the jig saw from the line and to shape the leading and tail edges of the blade.

The blade is ready for assembly. Make sure that all blades used on the same turbine are as close to identical as possible. A good anti-UV varnish coationg is still needed.

Vleugel uit PVC
Figuur 4: wiek uit PVC

 

"Last but not least: PVC production process is a very bad process for the people who do it and for the world in general. It is strongly advised to try and use scrap pipes and not a brand new one."

Calculations and formulas

De exacte afmetingen en vrom van de wiek kan met een aantal formules berekend worden. Hieronder vind je eerst de definieties van de gebruikte symbolen en vervolgens de bijbehordende formules. Wil je de wiek ook in de praktijk gaan bouwend dan kun je onderaan deze pagina de op Excel gebaseerde template downloaden. Deze rekent alles voor je uit, wel zo handig.

Definitions

v = speed of a point on the blade
ω = angular speed
r = distance from blade root along the blade to full blade radius R; i.e. radius from rotor center to point on blade
λ = tip speed ratio
C = chord length
φ = build angle (see notes above), it dictates the angle of the chord relative to the hub. As can be seen from the diagram, φ is also equal to the angle between the incoming wind and the perpendicular bisector of chard C.
The effective wind velocity at a point on the blade is the relative velocity between the actual wind velocity and the velocity of that blade point.
θ = effective wind derection along the blade
α =
angle of attack = angle between the blade chord and the effective wind velocity.
Schematische weergave van een windmolenwiek
Figuur 5: schematische weergave van de wiek

Formulas

v = ω * r
λ = V / v = ω * R / v
tan θ = vwind / vblade = (ω * r) = R / (λ * r)
φ = θ - α

Take into account Betz law, the wind speed is reduced by factor of 2/3

tan θ = (2/3) * Rp / (λ * r)

sin φ2 = (C / 2) / Rp
φ1 = φ2 - φ

L1 = π * R * (φ2 + φ) / 180

L2 = π * R * φ1 / 180

wiek van windmolen
Figuur 6: schematische weergave van de wiek (2)

Bestanden

Excel documenttemplate

 

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