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 100 revolutions per minute. Everything under 100 rpm means huge additional resource investments into rare earth magnets and loads of copper (windings).

Main focus will be on 3 types:

• C-Rotor and further development based on it, with less parts if possible
• H-Rotor with NACA profiles or airfoil shaped wings
• V-rotor with airfoil wings

Additionally, there should be a design for a very simple H-Rotor made of half DN100-PE-tubes (standard sewer piping tubes) as wings, preferably three or 3 x n wings for very low resource budget projects.  

The standardization of the system and compatibility of components offers a perfect test environment for different rotor types to see how comparable rotor-surfaces will perform with different rotor-types in the same environment.

Example for a classification in Germany, Berlin, Cologne, Karlsruhe: The mean wind speed is classified above IEC class IV with an average value of 2.3 - 3.6 m/s at ground level (equals a mast height of 10 m or below) without any obstacles.

IEC classes are realistic for higher wind zones, industrial wind turbines usually sit at >50 m. We are safe with an IEC class IV design.

Each assembly should have a rotor surface no larger than 4m² to avoid possible legal restrictions in Europe. A wind surface of 4 m² equals a 2 m diameter VAWT rotor with a height of 2 m.

Each assembly should be matte painted in order to avoid irritation of people in the neighborhood by re-occurring reflections of direct sunlight by shiny rotating blades into the surrounding area. Additionally, the color itself should be visually in-obtrusive, to reduce the risk of people objecting to the turbines in the neighborhood, just because of prominent visual distractions. The following matrix gives a good overview, to choose from a commonly used range of colors for specific locations, to make it as invisible as possible.

: Define a common rotor to generator-shaft interface for easy rotor type interchangeability

• C-type requires lower wind speed, creates higher torque at lower wind speeds
• Usable bandwidth of wind speed is higher
• : Plans, Calculations & Prototype Images

Sources: https://github.com/apollo-ng/eXperimental-Turbines

Print two times and stack on each other approach. For <1W systems.

Sources: https://github.com/apollo-ng/eXperimental-Turbines

Corrections and additional approaches are always welcome.

Power in the wind
<x 20> P_{wind} = A_{wind} * (v_{wind})^3 * 1-2 \rho </x>

• <x 12>P_{wind}</x> → Available power in the wind, as kinetic energy in Watt
• <x 12>A_{wind}</x> → Area of surface (turbine/sail etc.) in m²
• <x 12>V_{wind}</x> → Wind speed in m/s
• <x 12>\rho</x> → (rho) Density of air (about 1.2 Kg/m³)

Example: Test Turbine MK7

<x 20> (0.75 * 0.8) * 5.4^3 * 1-2 \rho = 56.68 W </x>

Estimated Wind-Power conversion (mechanical)

<x 20> P_{mech} = P_{wind} * Conv_{Eff} </x>

You can watch these calculations in action, applied to reference wind speed measurements on the Wind Power VFCC Dashboard

A tuned VAWT probably has a best-case efficiency of 40%, while a simple drag-based turbine with no optimization nor special aerodynamics may have an efficiency of about 20%.