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| Fig. 1: Horizontal Axis Wind Turbine Diagram. (Source: Wikimedia Commons) |
Wind turbines were first built in 1887 by American inventor Charles F. Brush. The devices were revolutionary in their utilization of natural phenomenons, like the wind. Wind turbines operate relatively simply: wind provides propellors with kinetic energy, causing them to move around a rotor, which spins a generator and converts mechanical energy to electrical energy - electricity. [1] There are two primary types of wind turbine: horizontal axis and vertical axis. Horizontal axis wind turbines (HAWTs) are generally considered "traditional" and have the axis of the rotors rotation parallel to the wind stream and a vertical propellor that spins perpendicularly to the direction of the wind. [2] Vertical axis wind turbine (VAWT) blades rotate around a vertical axis and have a main rotor shaft that is arranged vertically. Figs. 1 and 2 illustrate the two turbines.
Horizontal axis wind turbines have various hub heights, with the average hub height of land-based turbines at 94 meters (308 feet) and between 100 and 150 meters for offshore turbines. [3] The reason for these superb heights is that wind speeds are greater at higher altitudes because there is less friction that air particles face. An average onshore wind turbine has a capacity between 2.5 and 3 MW, which can produce over 6 million kWh in a year. The average monthly energy consumption per household in the US is 886 kWh. [4] We can use the data we have collected to calculate the number of households one onshore wind turbine in the US can power:
| 6 × 106 kWh y-1 886 kWh month-1 household-1 × 12 month y-1 |
= | 564 Households |
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| Fig. 2: Vertical Axis Wind Turbine Diagram. (Source: Wikimedia Commons) |
Modern, lift-type, vertical axis wind turbines were first introduced by French engineer Georges Jean Marie Darrieus in 1931, but were not developed commercially until the 1970s. [5] VAWTs produce significantly less power than HAWTs in a per turbine capacity. VAWTs are much smaller than HAWTs both in their hub height, radius of rotation, and turbine length, meaning they can be clustered much more closely together whilst maintaining higher efficiency collectively. To expand, HAWTs cannot be clustered closely together because they are so large and because their efficiency reduces significantly from the wind drag that each cause. For example, to maintain high flow velocities of 95% and to reduce aerodynamic interferences between HAWTs, turbines should be placed at a distance of 14D behind one another - meaning 14 times turbine diameter length. [6,7] VAWTs, in comparison, require a distance of just 6D to recover wind velocity to 95%. Resultantly, VAWTs have been found to have much greater power densities than HAWTs - meaning the amount of power generated in a given area of land. Modern wind farms generally produce 2 to 3 W of power per square meter, whilst VAWTs produce a daily mean power density of approximately 18 W per square meter - which is 6 to 9 times the power density of modern HAWT farms. [8] It has been found that under ideal spacing conditions, VAWTs are capable of producing a power density of 30 W m-2. [8]
If we use this value of 30Wm-2, we can calculate how much land is needed for VAWTs to produce as much power as one HAWT.
| 2.5 × 106 W 30 W m-2 |
= | 8.33 × 104 m2 | = | (289 m)2 |
One HAWT takes up substantially more space than this given the large rotor diameter and large distance that is required between turbines. If we consider a HAWT with a rotor diameter of 127 meters - which is roughly average for an onshore turbine in 2021 - there would have to be 1778 meters between it and the next turbine to maintain 95% flow efficiency. Hence, the ability for VAWTs to be packed more closely to each other means that they act as a viable alternative to HAWTs.
© Jawad Jafar. 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] J.F. Manwell, J. G. McGowan, and A. L. Rogers, Wind Energy Explained (Wiley, 2009).
[2] M. M. M. Saad and N. Asmuin, "Comparison of Horizontal Axis Wind Turbines and Vertical Axis Wind Turbines," IOSR J. Eng. 4, 27 (2014).
[3] L. Fingersh, M. Hand and A. Laxson, "Wind Turbine Design Cost and Scaling Model," U.S. National Renewable Energy Laboratory, NREL/TP-500-40566, December 2006.
[4] "Electric Power Annual 2021," U.S. Energy Information Administration, November 2022.
[5] H. J. Sutherland, "A Retrospective of VAWT Technology," Sandia National Laboratories, SAND2012-0304, January 2012.
[6] M. Kinzel, Q. Mulligan, and J. O. Dabiri, "Energy Exchange in an Array of Vertical Axis Wind Turbines," J. Turbul. 13, 1 (2012).
[7] M. Brown, "Vertical Axis Wind Turbines," Physics 240, Stanford University, Fall 2016.
[8] J. 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).
