Due to their design, small wind turbines with a vertical axis appear futuristic and often attract the attention of the viewer. But what about the facts, what are the technical differences between conventional wind turbines with a horizontal axis?
In the case of large wind turbines in the megawatt class, a type of construction has prevailed over the decades that characterizes the appearance of a “normal” wind turbine: turbines with a horizontal axis and three rotor blades that point against the wind by means of yaw tracking.
There is a much broader range of designs for small wind turbines. A completely different type of construction is represented by the so-called vertical wind turbines, the rotor axis of which is set up vertically to the ground. But what are the technical differences between horizontal wind turbines, and what are the advantages and disadvantages?
The decisive disadvantage of vertical wind turbines is the lower level of efficiency and the associated cost-effectiveness.
Disadvantages of vertical small wind turbines
Lower efficiency
A decisive disadvantage of vertical small wind turbines is their lower efficiency. While horizontal systems have achieved a so-called specific output yield of up to 50%, the value for vertical runners is a maximum of 40%. Horizontal systems often produce significantly more electricity than vertical systems. Savonius rotors in particular are characterized by low efficiency. In comparison, vertical Darrieus rotors have a higher current yield because they are additionally driven by a buoyancy effect. This causes the rotor to turn faster.
Lower profitability
The most serious disadvantage can be derived from the lower efficiency and the low electricity yields: The cost of the kilowatt hour of electricity produced by wind energy is usually significantly higher for vertical wind turbines than for horizontal wind turbines.
High vibrations and loads
Vertical systems can cause strong resonance.
Low overall height
The rotors are located close to the ground due to adverse vibrations. Horizontal wind turbines can reach higher air layers through a pole or tower.
Advantages of vertical wind turbines
No wind tracking is necessary
Vertical small wind turbines do not have to be equipped with a controller that turns the rotor into the wind. Horizontal turbines usually have a wind vane and a tracking system.
More suitable for turbulent wind conditions
At locations with strong air turbulence, systems with a vertical axis often work more reliably than horizontal rotors. In the case of permanently turbulent wind conditions, however, the suitability of the location must be questioned.
Easy maintenance
The high-maintenance components such as the generator are located at ground level. In the case of horizontal systems, the machine parts are located in the gondola. If the pole cannot be tilted, a hoist or crane is required.
Low blade speeds and lower noise levels
Depending on the design, vertical wind turbines can be quieter than horizontal wind turbines and are therefore more suitable for residential areas and cities. However, there are also comparisons between different types of wind turbines with the result of lower sound propagation of the horizontal rotors.
No shadow punch
Systems with a horizontal rotor axis can cause unpleasant shadows when the sun is low in the sky, but this can almost be ruled out with vertical systems.
Higher acceptance by some approval authorities
Some building authorities show greater acceptance of vertical wind turbines, combined with a greater willingness to issue a building permit.
Dissemination of construction types
According to a study by the University of Birmingham for wind energy and energy system technology, horizontal rotors with three rotor blades are also currently the most widespread in small wind turbines. Of a total of 118 system types examined, 88% had a horizontal axis and 12% a vertical axis. Among the horizontal rotors, those with three rotor blades were the most widespread.
The regularly updated small wind turbine market report lists only high-quality small wind turbines based on market analyses. In the market analysis published in March 2020, vertical small wind turbines were included for the first time. A wind turbine model even has an official certification according to the Japanese small wind standard.
Outlook
The small wind industry is still in its infancy when compared to the photovoltaic industry, which went into mass production years ago. The lower efficiency of vertical systems does not mean that the concept cannot survive in the long term. In the photovoltaics industry, a technology with relatively low efficiencies has been able to establish itself for a long time. Solar power module manufacturer First Solar has maintained a leading market position despite the fact that its thin-film modules were significantly less efficient than traditional silicon solar cell modules. The reason for the strong market position was significant production cost advantages so that the specific investment costs and the resulting electricity generation costs were correspondingly low.
Most small wind projects are currently being implemented with horizontal turbines since the higher levels of efficiency have a positive effect on profitability. Modern horizontal runners are state-of-the-art. But the momentum in the segment of vertical wind turbines is great, and there is some evidence that this design will increasingly assert itself on the market.
