I have been attempting to optimize the amount of voltage that can be generated from a CD Wind Turbine with a Lasko 20-inch 3-Speed Box Fan to spin the blades on all three speed settings. The blades that I am currently using are 13.5 cm long, and 4 cm at the base and 2.5 cm at the tip, with a very small curve (around 1-5 degrees). So far, I have managed to generate the greatest amount of voltage (around 2.5 volts) with a two blade setup.
From here, is there anything I could do to improve the design of the airfoil to generate more voltage? I know that longer blades tend to perform better than shorter ones (I'm planning to increase the length of the blades to around 17.5 cm for the next design), but how should I change the width and/or the curve of the blade for better performance on all three voltages?
The theory to design the blades of a wind turbine is given in my public report KD 35: "Rotor design and matching for horizontal axis wind turbines". This report can be copied for free from my website:
www.kdwindturbines.nl at the menu KD-reports. If you study KD 35, you should answer the questions as given in KD 196.
A problem with your rotor is that it is very small, resulting small chords and therefore in a low local Reynolds number at a low tip speed ratio and at low wind speeds (see formula 5.5). Normal airfoils perform badly at low Reynolds numbers. A 7.14 % cambered airfoil performs rather well at low Reynoilds numbers. Characteristics of this airfoil are given in my public report KD 398.
The aerodynamic theory gives the optimum blade geometry for a certain rotor diameter, a certain number of blades, a certain design tip speed ratio and a certain airfoil used at a certain value of the lift coefficient. However, if you use a generator with a peak on the cogging torque, you need a rotor with a high starting torque coefficient. But the higher the design tip speed ratio, the lower the starting torque coefficient. So it might be that if you chose a high design tip speed ratio to get a high voltage at a certain rotational speed, the rotor won't start because the starting torque coefficient is too low.
