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Study suggests arrays of closely-spaced counter-rotating vertical-axis wind turbines could enhance power density of wind farms by up to an order of magnitude
18 July 2011
Research by Prof. John Dabiri at Caltech suggests that using counter-rotating vertical-axis wind turbines (VAWTs) arrayed in layouts that enable them to extract energy from adjacent wakes and from above the wind farm could potentially achieve power densities (watts of power per square meter of land area) an order of magnitude greater than those of wind farms consisting of horizontal-axis wind turbines (HAWTs).
Moreover, Dabiri notes in a paper in press in the Journal of Renewable and Sustainable Energy, this improved performance does not require higher individual wind turbine efficiency, only closer wind turbine spacing and a sufficient vertical flux of turbulence kinetic energy from the atmospheric surface layer.
Modern wind farms comprised of horizontal-axis wind turbines (HAWTs) require significant land resources to separate each wind turbine from the adjacent turbine wakes. This aerodynamic constraint limits the amount of power that can be extracted from a given wind farm footprint. The resulting inefficiency of HAWT farms is currently compensated by using taller wind turbines to access greater wind resources at high altitudes, but this solution comes at the expense of higher engineering costs and greater visual, acoustic, radar and environmental impacts.
...To maintain 90 percent of the performance of isolated HAWTs, the turbines in a HAWT farm must be spaced 3 to 5 turbine diameters apart in the cross-wind direction and 6 to 10 diameters apart in the downwind direction. The power density of such wind farms, defined as the power extracted per unit land area, is between 2 and 3 W m-2.
Wind turbines whose airfoil blades rotate around a vertical axis (i.e. vertical-axis wind turbines; henceforth, VAWTs) have the potential to achieve higher power densities than HAWTs. This possibility arises in part because the swept area of a VAWT rotor (i.e. the cross-sectional area that interacts with the wind) need not be equally apportioned between its breadth—which determines the size of its footprint—and its height. By contrast, the circular sweep of HAWT blades dictates that the breadth and height of the rotor swept area are identical. Therefore, whereas increasing HAWT rotor swept area necessarily increases the turbine footprint, it is possible to increase the swept area of a VAWT independent of its footprint, by increasing the rotor blade height...The power density of the VAWT design is more than three times that of the HAWTs, suggesting that VAWTs may be a more effective starting point than HAWTs for the design of wind farms with high power density.—Dabiri 2011
Dabiri hypothesized that counter-rotating arrangements of VAWTs can benefit from constructive aerodynamic interactions between adjacent turbines, thereby mitigating reductions in the performance of the turbines when in close proximity. By accommodating a larger number of VAWTs within a given wind farm footprint, the power density of the wind farm is increased.
Dabiri and his team have been conducting a field study at an experimental two-acre wind farm in the Antelope Valley of northern Los Angeles County, California. Dabiri’s experimental farm—the Field Laboratory for Optimized Wind Energy (FLOWE)—houses 24 10-meter-tall, 1.2-meter-wide VAWTs. Half a dozen turbines were used in a 2010 field test.
The turbines were a modified version of a commercially available model (Windspire Energy Inc.) with 4.1-m span airfoil blades and a 1200 W generator connected to the base of the turbine shaft. Three of the turbines rotated around their central shaft in a clockwise direction (e.g. from a top view) in winds above 3.8 m s-1 ; the other three rotated in a counter-clockwise direction when the wind speed exceeded the same threshold (which Dabiri calls the cut-in wind speed).
Averaged over the 48.6-m2 footprint of the six-turbine VAWT array, the daily mean power density produced by the array varied from 21 to 47 W m-2 at wind speeds above cut-in and 6 to 30 W m-2 overall. This performance significantly exceeded the 2 to 3 W m-2 power density of modern HAWT farms, despite the relatively low mean wind speed during this set of field tests (5.7 m s-1), Dabiri noted.
To extrapolate the present measurements to larger VAWT farms, we considered the present VAWT diameter (1.2 m) and inter-turbine spacing (4 diameters), and we made conservative estimates for both the total aerodynamic loss in the array (10 percent) and the capacity factor (i.e. the ratio of actual power output to the maximum generator power output; 30 percent). The calculated power density for a VAWT farm with these parameters is approximately 18 W m-2. This performance is 6 to 9 times the power density of modern wind farms that utilize HAWTs.
Furthermore, it is straightforward to compute combinations of VAWT rated power output and turbine spacing that can achieve 30 W m-2 (i.e. 10 times modern HAWT farms) by using 1.2-m diameter VAWTs like those studied here. Higher VAWT rated power outputs can be achieved by taller turbine rotors than the 4.1-m structures used in these experiments, and by connecting the turbine shaft to larger generators. Indeed, in initial field tests with 6.1-m tall rotors, the captured wind power exceeded the capacity of the 1200 W generator on each turbine.—Dabiri 2011
Having every turbine turn in the opposite direction of its neighbors, the researchers found, also increases their efficiency, perhaps because the opposing spins decrease the drag on each turbine, allowing it to spin faster (Dabiri got the idea for using this type of constructive interference from his studies of schooling fish).
We’re on the right track, but this is by no means “mission accomplished”. The next steps are to scale up the field demonstration and to improve upon the off-the-shelf wind-turbine designs used for the pilot study.—John Dabiri
This summer, Dabiri and colleagues are studying a larger array of 18 VAWTs to follow up last year’s field study.
Dabiri JO (2011) Potential order-of-magnitude enhancement of wind farm power density via counter-rotating vertical-axis wind turbine arrays. Journal of Renewable and Sustainable Energy, in press. [preprint]
A new approach to wind energy (John O. Dabiri presentation, Dec 2010)
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