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Savonius Wind Turbine
Brief description
A Savonius Wind Turbine is a drag-type vertical-axis wind turbine (VAWT) invented by Finnish engineer Sigurd Johannes Savonius in 1922. Featuring two or three scoop-shaped semi-cylindrical blades mounted on a vertical shaft, it rotates under aerodynamic drag regardless of wind direction. Because of its extreme mechanical simplicity, self-starting capability under low wind speeds, and high torque at low rotational speeds, the Savonius turbine is an essential low-tech device for mechanical pumping, ventilation, and decentralized off-grid power generation.
Use / Function
- Mechanical Water Pumping: Drives positive displacement pumps or deep-well piston pumps directly without complex speed-increasing gearboxes.
- Low-Wind Power Generation: Rotates low-speed permanent magnet alternators or dynamos in environments with gentle or turbulent wind currents.
- Aeration and Agitation: Powers mechanical agitators for composting, wastewater aeration, or agricultural grain mixing.
- Ventilation: Rotates rooftop exhaust extractors to draw hot air and fumes out of agricultural structures and workshops.
- Scale: Small domestic or farm-scale installations (50W–1000W mechanical or electrical equivalent).
Operating principle
The Savonius rotor operates primarily on differential aerodynamic drag combined with cross-flow impulse:
- Differential Drag Force: When wind blows across the vertical axis, it hits both the concave inner side and convex outer side of the scoop blades. The concave scoop experiences high aerodynamic drag ($C_d \approx 1.2$), while the convex scoop presents a smooth profile with significantly lower drag ($C_d \approx 0.4$).
- Torque Generation: The net difference in drag forces creates a strong rotational torque around the central vertical axis, causing the turbine to self-start even in light breezes.
- Cross-Flow Recirculation: Air striking the concave scoop flows around its inner curve and is redirected across the central gap into the opposing scoop, providing an auxiliary impulse boost that enhances efficiency.
- Omnidirectional Operation: Because the scoops are arranged symmetrically around a vertical axis, the turbine operates efficiently regardless of wind direction without needing a tail-vane or yaw mechanism.
How to create it
Vertical-Axis Savonius Rotor Construction
- Fabricate the Scoop Blades: Cut a cylindrical metal drum or heavy PVC pipe longitudinally into two halves, or curve sheet metal into two semi-cylindrical scoops.
- Prepare the Shaft and Hub Plates: Cut two circular end plates from thick Wood or Steel plate. Drill a central hole for the vertical shaft.
- Mount the Scoops with Overlap: Secure the scoops between the top and bottom end plates in an offset “S” shape. Position them so that the inner edges overlap by approximately 15% to 20% of the scoop diameter, leaving a central air gap for cross-flow recirculation.
- Construct Support Frame and Bearings: Mount the vertical shaft on top and bottom low-friction bearings anchored to a sturdy Wood or Steel frame. Use a thrust bearing at the base to support the total weight of the rotor.
- Connect Power Take-Off: Attach a pulley, sprocket, or direct coupling to the bottom shaft to drive a mechanical pump or low-speed Iron permanent-magnet generator.
Materials needed
- Scoops & End Plates: Sheet metal, halved metal drums, or thick outdoor Wood plywood.
- Vertical Shaft: Solid Steel rod or structural pipe.
- Frame & Bearings: Structural Iron or wooden support frame, ball bearings, or brass sleeve bearings.
- Hardware: Bolts, lock nuts, bracket clamps, and drive belts/pulleys.
Variants and improvements
- Helical Savonius Turbine: Twisted scoop blades (typically 90° twist) that eliminate torque fluctuation during rotation, providing smooth continuous torque and reduced vibration.
- Multi-Stage Savonius: Two or more Savonius rotors stacked vertically on the same shaft, offset by 90 degrees to eliminate dead spots in rotation.
- Deflector Shield Augmentation: Wind deflectors positioned upstream to shield the returning convex blade from counter-drag, significantly boosting power coefficient ($C_p$).
Limits and risks
- Lower Aerodynamic Efficiency: Maximum power coefficient ($C_p \approx 0.15 - 0.20$) is significantly lower than lift-based horizontal axis wind turbines ($C_p \approx 0.45$).
- High Material Mass per Output Unit: Requires more physical material per watt generated compared to slender high-speed blades.
- Dynamic Structural Stress in High Winds: Large surface area experiences significant lateral aerodynamic forces during severe storms, requiring robust guy wires and frame anchoring.