Wind Power

Part of Generators and Motors

Converting wind energy to electrical generation: turbine design fundamentals, siting, generator coupling, and managing the variability of wind.

Why This Matters

Wind turbines provide electricity from moving air — a resource available everywhere on Earth, though not always reliably or in useful quantities. For communities without accessible running water but with consistent wind, a wind turbine may be the primary or only practical source of renewable electricity. For communities that have water power, wind provides supplementary generation and geographical flexibility (towers can be placed where wind is best, not just where a stream happens to flow).

Wind energy technology spans from simple water-pumping windmills built with hand tools to sophisticated aerodynamic machines requiring precision manufacturing. A rebuilding civilization can access points along this spectrum and climb toward more capable designs as manufacturing improves. Understanding the physics and engineering at each level is the guide for that progression.

The Physics of Wind Power

Wind power is governed by the kinetic energy of moving air. The power available in a swept area A at wind speed v is:

P_available = ½ × ρ × A × v³

where ρ is air density (1.225 kg/m³ at sea level, standard conditions). Three critical observations from this formula:

Power scales with the cube of wind speed. Doubling wind speed multiplies available power by 8. This is the most important fact about wind power — small increases in average wind speed yield enormous increases in energy production. A site with 6 m/s average wind produces (6/5)³ = 1.73 times the energy of a 5 m/s site. Choose sites carefully.

Power scales with swept area (proportional to rotor diameter squared). Doubling rotor diameter quadruples swept area and available power. Larger rotors produce dramatically more power but require stronger structures and more careful blade design.

Not all power is extractable. The Betz limit states that the maximum extractable fraction of available wind power is 16/27 ≈ 59%. Good horizontal-axis wind turbines reach 40–50% of available power; simple home-built designs may reach 25–35%.

For practical sizing: a 2-meter diameter rotor at 5 m/s wind with 35% efficiency produces: P = 0.35 × 0.5 × 1.225 × π × 1² × 5³ = 135 W. Useful for battery charging. The same design at 8 m/s: P = 0.35 × 0.5 × 1.225 × π × 1² × 8³ = 553 W. Wind speed matters enormously.

Rotor Design Fundamentals

Horizontal-axis wind turbines (HAWT, the classic propeller type) are the most efficient and are the standard for electricity generation. The blades extract energy from the wind by generating aerodynamic lift — the same principle as aircraft wings, not the simple drag of a barn sail.

Blade geometry: effective wind turbine blades are airfoil-shaped, twisted along their length. The twist compensates for the fact that blade speed (and thus apparent wind direction) varies from root to tip. At the tip, blade speed is much higher than the wind speed; the blade must be pitched more nearly edge-on to the wind. At the root, blade speed is low; the blade must be pitched more face-on. The optimal twist varies continuously from root to tip — approximately 20–35° at the root to 0–5° at the tip for typical designs.

Tip speed ratio (TSR): the ratio of blade tip speed to wind speed. Most efficient HAWTs operate at TSR of 6–8. At this TSR, the blades move 6–8 times faster than the wind. TSR determines how fast the generator shaft must spin for a given wind speed and rotor diameter. At TSR=7 and wind 5 m/s with 1 m radius: tip speed = 35 m/s, rotor speed = 35/1 = 35 rad/s = 335 RPM. This is in a useful generator speed range without excessive step-up.

Blade materials: traditional options include carved wood (spruce, fir, pine), fiberglass-reinforced resin, and PVC pipe (for small turbines). Wood requires hours of skilled carving but produces excellent aerodynamic shapes and is fully repairable. PVC pipe blades are quick and simple to make, but the airfoil shape is limited by the pipe cross-section. For a first turbine, PVC is acceptable. For higher performance, carved wood is the artisanal gold standard.

Number of blades: most efficient turbines use 3 blades for minimum vibration and good efficiency. 2-blade designs are simpler but vibrate more and are dynamically more complex. More than 3 blades adds material and weight with diminishing aerodynamic return. For a first build, 3 blades.

Overspeed Protection: Non-Optional

Without overspeed protection, a wind turbine in high winds will accelerate to destructive speed, shaking itself apart or throwing a blade with lethal consequences. Every wind turbine must have an effective overspeed control built in from the start.

Furling (side furling): the turbine is hinged to turn sideways out of the wind at a set speed. As wind increases, the side force on the offset tail vane exceeds the force alignment, and the turbine furls 90° away from the wind. Simple, reliable, passive. The standard approach for small off-grid turbines.

Pitch control: blades are pivoted at their root and rotate to reduce pitch (reduce angle of attack) in high wind, spilling energy and reducing torque. More complex to build but gives smooth power control over a wider range. Standard on large turbines.

Passive stall: fixed-pitch blades aerodynamically stall at high wind speed (the airfoil exceeds its critical angle of attack and lift drops), limiting power naturally. This requires careful blade design to work correctly and is less controllable than active methods.

For a home-built turbine, side furling is the most achievable and reliable protection. Do not skip this and plan to manually shut down in storms — you cannot be there every time the wind picks up suddenly, and an unprotected turbine will eventually be destroyed.

Generator Selection and Coupling

Variable-speed wind turbine operation (speed follows wind speed) means the generator produces variable voltage and frequency if directly connected to the shaft. Options:

Direct AC generation at variable frequency: connect the turbine generator through a rectifier to a battery bank, then invert to AC. The battery bank buffers the variable generation and provides stable DC; the inverter produces stable AC from the DC. This is the standard approach for small off-grid systems and is well-suited to low-technology circumstances. Disadvantage: inverter complexity, battery cost, and battery cycle life.

Permanent magnet alternator (PMA): a multi-pole PM generator produces AC directly, easily rectified to DC. PMAs designed for wind turbines are available in standard rotor diameters. For a hand-built system, a PMA can be fabricated from salvaged neodymium magnets (from hard drives and motors) mounted on a rotor disk, with stator coils wound from copper wire on a fiberglass or resin-cast stator. This is a proven approach for small community wind turbines.

Induction generator with grid connection: if a grid exists, an induction machine oversped to above synchronous speed generates power back to the grid. Simple and efficient, but requires an existing grid.

Tower and Foundation

A wind turbine must be elevated to access stronger, less turbulent wind. Wind speed increases significantly with height above ground — doubling height typically increases average speed by 10–25% (Hellmann exponent effect), which means 30–95% more power.

Minimum tower height: 10 meters above any obstruction within 100 meters radius. Higher is better. For small turbines (rotor diameter under 3 m), a guyed pipe tower is practical: a central steel pipe mast, guyed with 3–4 wire ropes to ground anchors at radius equal to tower height. The base is a concrete foundation with a pivot point that allows the tower to be tilted up and down for installation and maintenance.

Tilt-up tower design: weld a pivot flange at the base of the mast and a matching base plate bolted to the foundation. Guy wires run from a point near the top of the mast to ground anchors. To raise: stand the mast up from horizontal using a gin pole (a short pole attached at the pivot, used to get mechanical advantage at low angles). To lower for maintenance: reverse the process. Never work on the turbine at height if the tower can be lowered — this simple design feature prevents most maintenance accidents.