Vertical wind turbines are characterized by a rotor axis positioned vertically (upright axis).
The first wind turbines built by humans had a vertical axis: the so-called Persian windmill dates back to the 7th century. Among vertical wind turbines, there are various designs such as the Darrieus rotor and the Savonius rotor, as well as hybrid forms.
Over time, wind turbines with a horizontal rotor axis have become dominant. However, interest in vertical wind turbines remains strong today, as the unique and often aesthetically pleasing design of the rotors sparks curiosity among many people. New manufacturers are entering the market.
But to what extent can vertical-axis turbines meet the demands of modern energy technology? Can buildings be efficiently powered with them? This article explains the key facts about small vertical wind turbines.
Function and Designs of Vertical Wind Turbines
There are several fundamental designs of vertical small wind turbines, which will be introduced below. In practice, one encounters many variants and hybrid forms—the ingenuity of developers knows no bounds when it comes to vertical wind turbines.
Savonius Rotors
Savonius rotors are known as drag-type turbines because the entire rotor surface offers resistance to the wind and is essentially pushed away by it. Savonius rotors move slowly, with the rotor reaching at most the prevailing wind speed. Among all small wind turbine types, Savonius rotors have the lowest efficiency rates and therefore produce significantly less electricity compared to other design types.
Darrieus Rotors
Darrieus rotors are lift-based turbines. Unlike drag-type turbines, the rotor blades are not positioned completely perpendicular to the wind. Instead, the wind flows over the rotor blade, generating lift—similar to the way an airplane wing works. These are classified as high-speed rotors: the rotational speed of the rotor can be significantly higher than the wind speed. This is crucial for achieving high efficiency in a wind turbine (power coefficient). All modern wind turbines are lift-based, including those with a horizontal rotor axis.
H-Rotor
A common design type of the Darrieus rotor is the H-rotor. Unlike the classic model with curved blades, the airfoils in this design are straight. The H-rotor has a larger surface area facing the wind and therefore, in theory, offers higher performance than rotors with curved blades.
The following animation shows the different rotor types in motion:
From left to right:
- Vertical Savonius rotor,
- Horizontal wind turbine,
- Vertical Darrieus H-rotor.
(Source: By Ssgxnh (Own work), via Wikimedia Commons)
Efficiency and Performance
The theoretical efficiency of wind turbines is expressed using the so-called power coefficient (also known as the Betz limit or specific power yield). The maximum power coefficient is 59%. This value describes the rotor’s ability to convert the kinetic energy of the wind. The overall efficiency is lower due to losses in the gearbox and generator.
Vertical wind turbines have a lower power coefficient than turbines with a horizontal axis. According to the current state of technology, horizontal-axis turbines can achieve a power coefficient of around 50%, while vertical-axis turbines reach a maximum of 40%. This clearly highlights the key difference between the two designs: horizontal-axis wind turbines typically generate more electricity than vertical ones.
However, the decisive factor for economic viability is not the efficiency itself, but rather the levelized cost of electricity (LCOE)—that is, the cost per kilowatt-hour of electricity generated. Even in this regard, horizontal turbines come out on top: when it comes to both efficiency and market maturity, small horizontal wind turbines currently hold a clear advantage. This design has become dominant in multi-megawatt turbines for good reason.
Prices and Costs of Vertical Wind Turbines
The purchase prices of vertical wind turbines are generally not lower than those of horizontal wind turbines. Across the entire small wind turbine market, there are significant variations in prices and specific costs per kilowatt of capacity, depending on the manufacturer and wind turbine model. The price range is between $3,000 and $10,000 Dollar per kilowatt of installed capacity.
Due to aerodynamic disadvantages, system control and storm protection are more complex for vertical wind turbines. For example, horizontal wind turbines can be turned out of the wind as a technically elegant form of storm protection—something that is not possible with vertical turbines. Therefore, vertical wind turbines require special technical solutions for system regulation, which is reflected in the construction costs.
Profitability Calculation Example
Let’s assume two small 6 kW wind turbines—one with a vertical rotor axis and one with a horizontal rotor axis—both cost $5,000 per kilowatt of capacity. The total cost for each turbine is therefore $30,000. Based on TÜV-certified annual energy yields for two commercially available wind generators at an average annual wind speed of 5 m/s (a good inland location), the following values can be expected:
- Vertical small wind turbine (6 kW): approx. 3,900 kWh per year
- Horizontal small wind turbine (6 kW): approx. 9,500 kWh per year
The horizontal wind turbine is significantly more economical. Over a 20-year operating period, the differences in the cost of generated wind electricity are substantial:
- Vertical wind turbine: $0.38 per kWh
- Horizontal wind turbine: $0.16 per kWh
While this example uses realistic values, it cannot be generalized to all small wind turbines available on the market. The wind strength at the installation site is the decisive factor for actual energy yields.
Advantages of Vertical Wind Turbines
- Better suited for locations with turbulent wind conditions (e.g., cities).
- Easy maintenance, since components requiring frequent servicing—like the generator—are located close to the ground.
- Often lower noise emissions. However, this depends on the specific case, as some horizontal wind turbines can also be quiet.
- Very popular and widely accepted, which can be beneficial during the permitting process.
Permitting Advantages
Not only among some potential small wind turbine operators, but also within certain building authorities and other regulatory bodies, there is a belief that vertical wind turbines are better suited for urban areas. This perception can lead to advantages in obtaining a building permit—based on the notion:
A vertical wind turbine may be approved, while a horizontal turbine might not get permission.
One frequently cited reason is noise: It is generally assumed that vertical turbines are quieter. However, modern horizontal turbines are not necessarily louder.
Another, possibly more important, reason why vertical wind turbines are sometimes favored during the permitting process is the visual impact on the landscape or cityscape. Some people perceive large wind turbines in wind farms as a visual intrusion into the natural landscape. This subjective sense of visual disturbance is often associated only with horizontal wind turbines. In other words, vertical wind turbines seem to benefit from a “design bonus” in the eyes of some individuals.
Strictly speaking, the appearance of a wind turbine should not matter during the approval process. What truly counts are facts such as:
- Noise and shadow emissions
- Total height of the system
- Rotor diameter
- The turbine’s safety systems
- Relevant inspection or certification documents
It should also be noted that all small wind turbines—regardless of whether they are vertical or horizontal—have no impact on the landscape when compared to large wind turbines in wind farms, which now often exceed 200 meters in total height
Disadvantages of Vertical Small Wind Turbines
- Low efficiency and lower energy yields.
- Significantly higher cost of electricity production (lower economic viability).
- Few independent references (tests, certifications).
- High vibrations and resonances.
- Use of taller masts requires additional effort (guy ropes, robust construction, etc.).
The decisive disadvantage of vertical wind turbines is their lower economic viability, which ties into the question: What is the cost per kilowatt-hour of wind electricity produced? The cost of electricity production for vertical wind turbines is generally significantly higher than for horizontal turbines (see example above).
Another issue is the high vibrations, which can cause resonance in the mast. As a result, vertical wind turbines can only be installed on taller masts (above 20 meters) with special effort. However, this becomes a necessity in many inland locations due to moderate wind conditions.
Manufacturers and Providers of Vertical Wind Turbines
Vertical wind turbines are still a niche market within the small wind sector. The majority of manufacturers continue to offer small wind turbines with horizontal rotor axes. According to a study by the World Wind Energy Association (WWEA) in 2012, 74% of the 327 small wind turbine manufacturers worldwide produced turbines with horizontal axes. Although the study is outdated, it’s likely that the market conditions are still similar today.
Those who monitor the market regularly will notice that many vertical turbine manufacturers do not remain in the market for long. Looking at the global inventory of certified small wind turbines, almost all models have a horizontal rotor axis. In the certification process, extensive tests are conducted, and the performance curve is measured.
According to meticulous market analyses for the small wind market report, vertical wind turbines are consistently a marginal presence in the market. This is mainly because there are very few independent references available for vertical axis turbines, such as test field results, measured performance curves, or certifications.
However, research and the market for vertical wind turbines should definitely be kept under observation. Numerous startups and universities worldwide are working on new concepts. The interest in vertical wind turbines remains high: with a successful technology, significant profits can be made.
Vertical Wind Turbines for Single-Family Homes?
Some suppliers of vertical wind turbines make bold promises regarding the electricity supply for private homes. The question is, how much electricity can a small vertical wind turbine produce annually, and whether the cost of the produced electricity is reasonably affordable.
Consumers need to be cautious: Single-family homes in residential areas often have limited wind availability. This is especially true for central locations, where surrounding buildings and vegetation block the wind. In such cases, the viability of a small wind turbine should be seriously questioned. Whether the turbine is to be placed on the roof or in the garden, if there is little wind, it is better to choose the most efficient wind turbine type, which is the horizontal wind turbine.
Some potential buyers are adamant: It must be a vertical wind turbine. However, those who are seriously considering the operation of a small wind turbine should also look into horizontal axis turbines and be open to all types of rotor designs.
