Design Requirements and Constraints

Operate in typical urban wind speeds (3–6 m/s)

Compact and lightweight for window mounting

Safe, low-noise operation

Manufacturable using 3D-printed components and off-the-shelf hardware

Output conditioned to regulated 5 V USB via power electronics

Rotor, Blade, and Hub Design

Rotor Configuration:

A three-blade horizontal-axis rotor with a diameter of 0.5 m was selected to balance efficiency, structural stability, and vibration reduction. Increasing rotor diameter significantly improves power capture, and 0.5 m represents the smallest practical size capable of producing watt-level power in urban wind while remaining suitable for window mounting.

Blade Geometry:

Each blade was designed with a profile that expanded at the center and then tapered down to the tip. They were fashioned off of images and dimensions of in use wind turbine blades that have a curved almost tear drop like side that faces the wind and then a flat back.

Hub Design:

The hub was sized to provide sufficient structural rigidity while accommodating blade mounting hardware and a central shaft interface. A hub diameter of approximately 60 mm and thickness of 45 mm were selected to resist bending moments transmitted from the blades while remaining compact and lightweight. Symmetric blade mounting ensures rotor balance and reduced vibration.

Shaft and Bearing System

A 10 mm steel shaft was selected to transmit torque from the rotor to the generator while maintaining a large safety margin against shear and bending.

Generator Selection Trade Study

Two generator options are currently under evaluation:

Option 1: NEMA 17 Stepper Motor

Advantages: Generates usable voltage at relatively low rotational speeds; inexpensive and readily available; simple mechanical integration.

Disadvantages: High cogging (detent) torque increases startup resistance, potentially preventing rotation in low wind conditions; reduced efficiency due to electromagnetic drag.

Option 2: Brushless DC Outrunner Motor

Advantages: Very low cogging torque enables easier startup in low and turbulent wind; smoother rotation; higher efficiency potential.

Disadvantages: Requires additional power electronics for rectification and voltage regulation; slightly increased system complexity.

The final generator selection will be based on experimental evaluation of startup behavior, generated voltage versus rotational speed, and overall energy capture under realistic urban wind conditions. This tradeoff highlights the balance between low-speed voltage generation and mechanical startup torque that dominates small-scale wind energy systems.

Takeaways

Power capture at small scales is strongly limited by rotor size and wind speed, making startup torque a critical design consideration.

Mechanical isolation of the generator using bearings significantly improves system durability and efficiency.

Generator selection involves tradeoffs between electrical output characteristics and mechanical resistance, particularly at low rotational speeds.