Germany/TiVA

Mechanics

Forces

<m>F_{pole} = \frac{1}{2} \times \rho \times C_d \times A_{wind} \times v_{wind}^2</m>

<m>\rho</m> Density of air = about 1.2 Kg/m³
<m>C_d</m> Coefficient of drag = 1.0 (cylinder Re > 100)
<m>A_{wind}</m> Area of turbine
<m>v_{wind}</m> Wind speed in m/s

A board of spruce ready to be worked on with a planer.

After the first few steps of planing the edges.

Work in progress photo of a wooden NACA airfoil made with a table saw and a planer. The wood (spruce) used here is of rather low quality and split up at the trailing edge.

Rotors

Compared to drag-only type rotors (Savonius), the lift-only type rotors (Darrieus) haven proven to be generally less suitable for low wind environments. However, the maximum speed of drag-only type rotors is always lower than a comparable lift-only type rotor, because a lift-only type rotor can rotate faster than the wind speed at the tips but with less torque. A drag-only type rotor can develop more torque, even at early stages in low wind conditions, but that would require a very specific and resource-intensive generator to accommodate for the very low rotational speed. A typical low end for a direct driven axial flux permanent magnet alternator with many poles is about 150 revolutions per minute. Everything under 150 rpm means huge additional resource investments into rare earth magnets and loads of copper (windings).

For the very small TiVA, the main focus will be on two types, both of them mounted either on a H (with arms) or V (with a base mount) shaped rotor:

1. A lift-only type wing profile. The wings are formed by one (NACA) profiled element or segments of pipes, e.g. made of DN100-PE-tubes (standard sewer piping in Germany)
2. The Van Canstein wing form and further derivatives based on it, with less parts if possible.

C-Type Rotor

The "Van Canstein" wing form is a special type of H-rotor with a combined lift-and-drag-wing.

H-Type Rotor

• may be the simplest design, very simple wing forms are possible.

Complex Darrieus rotor: wings in helix-form, spiraled, lift-type Simpler H-rotor: wings straight. May even be without any profile. Lift-type or Drag-type or lift-drag-type -> C-rotor

C-type vs simple H-type

con C-type, pro H-type:

• C-type requires two parts to form a wing -> more material
• wing tip has to be bent into an aerodynamic shape -> more complexity, especially at the mounting points
• upper wind speed limit is lower

pro C-type, con H-type:

• C-type requires lower wind speed, creates higher torque at lower wind speeds
• usable bandwidth of wind speed is higher

NACA0018 profiled straight wing fabrication process

One idea is to make a wing out of two symmetrical pieces. One half of the profile could be milled out of wooden sheet material or planed out of a pre-cut board by hand. Half of a profile can be hold down and clamped because one side will still be flat. The two halves are then glued together.

The pictures at the right show a simple profile made out of thin boards. They are cut out of a sheet with a table saw and then planed by hand.