Page 26 - Energize Issue 1 2023

TECHNICAL

Disadvantages Turbines relying on drag, such as Savonius models, cannot
VAWTs often have less rotation efficiency than HAWTs. Due to the spin more quickly than the wind blows, and are thus limited by
rotor design, not all the blades on the vertical axis rotor receive the wind speed. There are many variations of the basic SWT, all
incoming wind at the same time. In fact, only the wind-facing of which attempt to improve on the basic design. In the basic
blades are driven by the wind to turn while the others are simply two-blade version, torque varies from maximum to zero during a
following along. During rotation, vertical axis rotors are also faced rotation cycle, which makes power generation erratic. This effect,
with more drag — or aerodynamic resistance — on the blades. known as torque ripple, is a major problem with the basic SWT.
This is especially so on Savonius wind turbines because they have There have been numerous attempts to improve the SWT,
wider blade surfaces. involving different blade shapes, multiple blades, wind deflectors,
The efficiency of different types of wind turbines is shown guide vanes, helical blades and multi stage units. The most
in Figure 1. On average, the efficiency of a horizontal axis wind common variations are discussed here.
turbine is between 40% and 50%, meaning the turbine is able to
convert 40% to 50% of the kinetic energy it receives into actual Overlapping blades
electrical power. On the other hand, a Savonius vertical axis The most basic variation is shown in Figure 4. The two blades
wind turbine has an average efficiency of 10% to 17%; while the overlap with a gap between the blade ends, creating a cross flow
Darrieus vertical axis wind turbine reaches 30 to 40% efficiency. of air which improves efficiency.
A small number of blades and low rotation speed allow a lot of
wind to pass through the turbine without encountering the blades. Multi-bladed version
Higher speed and more blades would mean more interaction, but Multiple blade versions have been developed that attempt to
a limit is reached where the rotating blades effectively form a solid reduce torque ripple and smooth out power production. Most
wall to the wind, resulting in no aerodynamic effect being possible.

Savonius model
The basic form of the Savonius WT (Figure 2) consists of two
cups or blades mounted on opposite sides of a shaft. The drag on
the open cup is high and that on the reverse cup is low, giving a
rotational force.

Figure 3: Torque ripple with basic SWT 1

Figure 1: Wind turbine efficiencies 1

Figure 2: Basic version of Savonius WT 1 Figure 4: Overlapping blades 8

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