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| Fig. 1: Vertical-Axis Wind Turbines. (Source: Wikimedia Commons) |
Modern wind energy systems are predominantly based on horizontal-axis wind turbines (HAWTs) and vertical-axis wind turbines (VAWTs). Globally, roughly 90% of current installed wind turbines are of the HAWT type, making them the mainstream choice for utility-scale power generation. [1] HAWTs employ a horizontal rotor axis with lift-based blades and typically achieve higher peak aerodynamic efficiency in large wind farms. In contrast, VAWTs have a vertical rotor axis with blades rotating in a horizontal plane. They can operate using drag forces, lift forces, or a combination of both, and are often favored in specific applications due to the lower tip speeds, reduced acoustic noise, simpler maintenance, and better tolerance of multidirectional and turbulent winds. These characteristics make VAWTs attractive for urban environments and small-scale power systems. [2] Fig. 1 shows different types of vertical-axis wind turbines. Fig. 2 shows a wind farm with horizontal-axis wind turbines.
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| Fig. 2: Horizontal-Axis Wind Turbine. (Source: Wikimedia Commons) |
The blades of HAWTs are cantilevered from the hub and support the weight as well as aerodynamic loads, which requires large, tapered, and sometimes twisted composite blades. A heavy nacelle at the top of the tower houses the gearbox, generator, and yaw system, resulting in high structural loads on both the tower and the foundation. In comparison, Darrieus-type VAWTs, including curved-blade and straight-blade H-rotor designs, use lift-based blades that are connected to a central shaft via support arms. This allows simpler structural layouts and permits the generator to be located at ground level, eliminating the need for a nacelle and yaw mechanism. Curved-blade Darrieus rotors are more complex to manufacture due to the required blade curvature, whereas straight-blade H-rotors feature constant blade cross sections along the length and are easier to mass-produce. Savonius rotors simplify the structure, employing drag-based half-cylinder buckets that are easy to fabricate but suffer from low aerodynamic efficiency. Overall, HAWTs have more complex and costly construction, H-rotors Darrieus VAWTs offer a simpler structural concept, and Savonius rotors represent the structurally simplest but least efficient option. [3]
The core physical metric for comparing wind turbines is power density, defined as the power produced per unit of swept area. For a wind turbine operating in a given wind field, the power density can be written as:
| P/A | = | (1/2) ρv3 Cp |
Because the air density and wind speed are determined by the site, differences in power density between turbines operating in the same environment arise primarily from the power coefficient Cp, which aggregates the essential aerodynamic characteristics of the machine. Experiments and literatures show relatively consistent Cp ranges for different turbine configurations. Table 1 summarizes representative values.
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| Table 1: Representative Cp ranges for common HAWT and VAWT configurations. Data from Bhutta et al. [3] |
These ranges indicate that for the same swept area and wind speed, HAWTs extract a larger fraction of the winds kinetic energy than either Darrieus or Savonius VAWTs, leading to higher fundamental power density.
At a representative wind speed of v = 8 m/s and an air density of ρ = 1.225 kg/ m3, a HAWT with Cp = 0.50 yields a power density of
or 156 W/ m2. Under identical conditions, a Darrieus VAWT with Cp = 0.35 produces about 109 W/ m2, while a Savonius rotor with C p = 0.25 yields roughly 78 W/m 2 . These values come directly from the power-density equation and typical values above.
Wind turbine spacing influences the power output from the wind turbines. For HAWTs, wind tunnel experiments show that downstream turbines at a 5D spacing can experience power losses of up to 40%, and even at larger spacings of 7D to 11D, losses remain around 25 to 30%. [4] Such results indicate slow wake recovery in HAWT arrays and justify conventional wind farm layouts with downstream spacing of approximately 7D to 10D.
By contrast, counter-rotating VAWT arrays can operate effectively at much smaller spacings. [5] The wake interaction patterns and vortex dynamics can redistribute momentum such that the incoming flow to neighboring turbines is maintained or enhanced. As a result, VAWT arrays can achieve significantly higher power density at the wind farm scale, with field measurements suggesting potential order-of-magnitude increases compared to HAWT farms under optimized layouts. [5]
We have compared HAWTs and VAWTs through the lens of power density, examining structural characteristics, single-turbine aerodynamic efficiency, and wind farm scale spacing behavior. HAWTs achieve higher power coefficients and thus higher power density for a given swept area and wind speed, which underlies the dominance in global wind farms. VAWTs offer structural simplicity and superior performance in turbulent or multidirectional winds and can exploit compact array layouts to increase farm-level power density. Nonetheless, the lower single-turbine efficiency of VAWTs limits the competitiveness in large wind farms. Overall, HAWTs remain the preferred choice for large-scale wind energy extraction, while VAWTs provide valuable advantages in small-scale and urban applications.
© Xiao Huang. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] C. S. Bang,, Z. A. Rana, and S. A. Prince, "CFD Analysis on Novel Vertical Axis Wind Turbine (VAWT)," Machines 12, 800 (2024).
[2] C. Sarmiento-Laurel et al., "3D Numerical Analysis of an H-rotor Darrieus Vertical Axis Wind Turbine in Transient Regime," Energy 324, 135849 (2025).
[3] M. M. H. Bhutta et al., "Vertical Axis Wind Turbine: A Review of Various Configurations and Design Techniques," Renew. Sustain. Energy Rev. 16, 1926 (2012).
[4] S. McTavish, D. Feszty, and F. Nitzsche, "A Study of the Performance Benefits of Closely-Spaced Lateral Wind Farm Configurations," Renew. Energy 59, 128 (2013).
[5] J. O. Dabiri, "Potential Order-of-Magnitude Enhancement of Wind Farm Power Density Via Counter-Rotating Vertical-Axis Wind Turbine Arrays," J. Renew. Sustain. Energy 3, 043104 (2011).
