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| Fig. 1: Array of horizontal axis wind turbines at Power County Wind Farm, Idaho. (Source: Wikimedia Commons) |
As with many other sources of renewable energy, there is a large amount of interest and research in wind power: extracting kinetic energy out of the wind with a turbine and converting it into electrical energy with a generator. Although wind power does not represent a large proportion of the world's energy budget, it is substantial in an absolute sense. By 2015, the total investment in wind power was 329 billion USD and capacity had reached 432.9 GW. [1] Wind power generation is clean, in that it does not modify the chemical composition of the environment, and renewable, but it is not without drawbacks. Major criticisms of wind power include the potential for ecological damage from birds and bats colliding with turbine blades, and concern that natural aesthetic of the landscape is negatively impacted by large numbers of wind turbines. [2] This is exacerbated by the fact that most commercial wind turbines cannot be packed closely together; wind farms take up much more land per Watt produced than other power sources, like natural gas or coal power plants.
Almost all wind turbines currently in operation are horizontal axis wind turbines (HAWTs), such as those shown in Fig. 1. HAWTs are well-researched, both academically and commercially, which has lead to development of turbines with efficiencies that approach the Betz limit, the theoretical maximum efficiency of a wind turbine. [3] However, this efficiency drops when the wind turbines are placed close together in a wind farm. It can take somewhere on the order of 20D (twenty times the diameter of the turbine's swept area) for the the velocity downstream of a horizontal axis wind turbine to return to its far-field value, although most wind farms are spaced closer than this. [4] This means that wind farm designers must trade-off between using a large area of land and operating the turbines at peak efficiency.
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| Fig. 2: A modern vertical axis wind turbine. (Source: Wikimedia Commons) |
An alternative to the horizontal axis wind turbine is the vertical axis wind turbine (VAWT), such as that shown in Fig. 2. While the concept of the VAWT (much like the HAWT) is not a modern development, large scale commercial VAWTs came out of research at Sandia National Labs beginning the 1970's. [5] In a partnership with Sandia National Labs, the FloWind Corporation developed and installed many VAWTs in California, before widespread fatigue failure in the the turbine blades drove the company out of business years later. This was due to materials and manufacturing methods that were not well-suited for the fatigue loads the turbines experienced, which were not well understood at the time. [5] This drove a perception in the industry that VAWTs were prone to fatigue.
Recent resarch has shown that VAWTs can be packed significantly closer than HAWTs with less drop in efficieny, making them much better suited for wind farms. [6] Experimental data shows wind speed recovering to 95% of the far field wind speed within 6D. [7] Even if individual VAWTs are less efficient than HAWTs, the tighter spacing of counter-rotating turbines allows VAWT farms to have higher power densities. While a modern HAWT farms produce 2-3 W per square meter, field experiments with VAWT farms show a potential production of 30 W per square meter. [8] This could potentially address one of the major drawbacks of wind power: that it uses a large amount of space.
In additon to densely packed land-based wind farms, there has also been renewed interest in VAWT for use in offshore wind farms. [5,9] While HAWTs typically place the transmission and generator high in the air (close to the axis of rotation), VAWTs can locate these heavy components lower down. This can make maintenence easier and safer, and placing components under the water level improves the stability of the system. Another advantage of VAWTs for offshore farms is the the vertical axis of symmetry: the direction of the wind is irrelevant. [5]
While VAWTs are less developed and are less commerically available, their potential certainly warrants addition research.
© Matthew Brown. 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] "Global Wind Report Annual Market Update 2015," Global Wind Energy Council, 2015.
[2] D. Lloyd, "Wind Energy: Advantages and Disadvantages," Physics 240, Stanford University, Fall 2014.
[3] J. Johansen et al., "Design of a Wind Turbine Rotor for Maximum Aerodynamic Efficiency," Wind Energy 12, 261 (2009).
[4] M. Kinzel, D. B. Araya, and J. O. Dabiri, "Turbulence in Vertical Axis Wind Turbine Canopies," Phys. Fluids 27, 115102 (2015).
[5] H. J. Sutherland, D. E. Berg, and T. D. Ashwill, "A Retrospective of VAWT Technology," Sandia National Laboratories, SAND2012-0304, January 2012.
[6] I.D. Brownstein, M. Kinzel, and J. O. Dabiri, "Performance Enhancement of Downstream Vertical-Axis Wind Turbines," J. Renew. Sustain. Energy 8, 053306 (2016).
[7] M. Kinzel, Q. Mulligan, and J. O. Dabiri, "Energy Exchange in an Array of Vertical-Axis Wind Turbines," J. Turbul. 13, 1 (2012).
[8] 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).
[9] V. Troutman, "Offshore Wind Energy is Critical for the Future of Renewable Energy in the U.S.," Physics 240, Stanford University, Fall 2016.
