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VAWT vs. HAWT technology

Manufacturing Group | November 10, 2011

How do wind-sail-type vertical axis wind turbines compare to conventional horizontal axis wind turbines (HAWT)?

VAWT vs. HAWT technology

By Richard Halstead, President, Empire Magnetics Inc., Rohnert Park, CA www.empiremagnetics.com; sales@empiremagnetics.com

Design of the wind-sail turbine technology is with a tip speed ratio (TSR) of 2.4 as indicated by the green triangle in the first diagram, as compared to a TSR of approximately seven for a typical horizontal axis wind turbines (HAWT). The slower rotational speed of the simple, constant section Wind-sail airfoils greatly reduces the noise produced as blades slice through the air. Other benefits include reduced surface wear on airfoils from particle impact, reduced structural stress, increased bearing life, simplified design, and reduced manufacturing costs. These all come at a modest reduction of peak efficiency in high wind conditions. The result is a lower cost/kWh relative to turbines with complex propeller designs (HAWT) plus significant environmental benefits including far less noise and a more bird friendly design. 

Fig 1 – Design Point Table. Courtesy of Glen Dahlbacka, PhD, Lawrence Berkeley National Laboratory.


 

There are several different vertical axis wind turbines (VAWT) designs that fall into two general groups: drag devices are the first group and lift devices are the second group. Drag devices are systems that block the wind in some manner.

 

 

 

 

Figure 2Fig 2 – Drag-type Savonius wind turbine design. Courtesy of Toshihiro Oimatsu. http://www.flickr.com/photos/72396314@N00. Retrieved from Wikipedia on November 1, 2011.

 

Figure 3Fig 3 – Drag-type wind capture with Savonius rotor. Courtesy of Ugo14. Retrieved from Wikipedia on November 1, 2011 - http://en.wikipedia.
org/wiki/File:Savonius-rotor_en.svg#filelinks.

 

 

 

 

 

Since drag based designs rotate with the wind they cannot move faster than the wind, whereas lift based designs, (airfoils) rotate against the wind and move faster than the wind. The drag design may be very rugged and therefore suitable for difficult situations, however the efficiency of wind power conversion in drag devices is approximately half of the more efficient lift designs. 

Wind-sail turbines are of the more efficient lift type design, where the turbine rotates against the wind. This alone places the performance of the Wind-sail VAWT ahead of more than 50% of the VAWT’s on the market today. The airfoils of the Wind-sail technology have a unique and carefully modeled and tested shape (patent published). This patent pending airfoil shape results in a self-starting turbine at very low wind speed, but also has a feature that, when properly controlled, allows the turbine to operate without producing excess power. There is a subtle but important difference between not producing power and having to dispose of power once produced. That difference translates into significant cost saving in the overall system.

Figure 4Figure 4 (Courtesy of SRC Vertical) shows computer calculation of power output from a 3kW Wind-sail-type turbine versus wind speed. Field-testing of prototype units verifies the aerodynamic modeling. By capping the power output, Wind-sail type technology captures the available wind power over 95% of the time without having to pay a premium for equipment to handle extreme power peaks that rarely occur.

In reviewing various lift type VAWT designs, the airfoils were a significant portion of the cost in early designs, so early designs tried to minimize the number of blades in the turbine assembly. The earliest full Darrieus design had only two airfoils. This design is not self-starting, as there are two positions where the turbine does not produce any power. Only after it starts spinning does the Darrieus design have sufficient momentum to carry it through these two positions. In addition a large range of harmonic frequencies are excited as the wind speed varies from the hub to the maximum diameter. Later designs, known as H Darrieus, had two, three, four, or more blades.

Two blade designs suffer because they are not self-starting, and the torque pulses caused as the blades cut across the wind introduce huge stresses into the structure causing it to vibrate and materials to fatigue. The three blade design reduces the amplitude of the power pulses and helps make the unit self-starting, but there are still structural problems and low power output.

The four-blade design reduces power pulses and improves self-starting, but the blade ends are unsupported so centrifugal forces deflect the blade ends effectively reducing their life. The three-blade design improves support of the airfoils, but significantly increases the drag as the support arms cut through the air.

Intuition would lead one to believe that more blades are better than fewer blades. However, rigorous studies by the Wind-sail aerodynamic engineering team at the Makeyev Design Bureau of the State Rocket Center (MSRC) in Miass, Russia proves this assumption is not true. As one adds more than three or four blades per plane (tier) to a given wind turbine design, the amount of drag increases faster than the amount of power the additional blade contributes. Therefore, in practice three to four blades per tier on a wind turbine provide the most efficient power-to-drag combination.

Figure 5a, below left, Figure 5b, below middle, Figure 6, below right

Figure 5aFigure 5bFigure 6

 

 

 

 

 

 

 

 

 

The Wind-sail design (Fig5a and 5b, Courtesy of SRC Vertical), with three blades on top and three blades on the bottom, optimizes the power-to-drag ratio in the airfoil section of the turbine. However, by offsetting the blades on two tiers, Wind-sail technology obtains six power pulses per revolution instead of just three. This design greatly reduces the harmonic stress in the assembly making it quieter and longer lasting, and provides self-starting at lower wind speeds. Figure 6 (courtesy of Russian State Rocket Center) shows the difference in amplitude, where the top line is the six-blade version of the Wind-sail-type turbine, and the bottom is the four-blade design.

The Gorlov-type VAWT seeks to address the same power pulse problems solved with the six blade offset in the Wind-sail design by using spiral curved blades. While the Gorlov design produces smooth driving power, the blades are significantly more complex and expensive to manufacture relative to the straight blades of the Wind-sail design.

Because the Wind-sail blades are of constant cross section, they can be easily and cheaply mass-produced by either extrusion or pultrusion processes. This feature allows easy adaption of the Wind-sail turbine to specific wind classes by simply changing blade lengths. For light wind regimes, the blades are longer to increase swept area, and for heavy wind regimes the blades are shorter to decrease swept area and prevent turbine damage during peak wind speeds.


Figure 7aFigure 7bFigures 7a (left) and 7b (right) (both courtesy of Russian State Rocket Center) show Wind-sail airfoil development, and figures 8a, 8b, and 8c (Courtesy of Russian State Rocket Center) show hydrodynamic testing, at the MSRC.

 

Figure 8a, left; Figure 8b, middle; FIgure 8c, right

Figure 8aFigure 8bFigure 8c

 

 

 

 

 

 

 

The support rings at the top and bottom of the Wind-sail-type VAWT turbine shown in figure 9 (Courtesy of Glen Dahlbacka, PhD, Lawrence Berkeley National Laboratory) provide needed support to the blades without a significant increase in the drag (Figure 10 shows a tower mounted Wind-sail with support rods; Courtesy of SRC Vertical). The ring is not cutting the air in the direction of rotation, as it is simply spinning in the space that the ring already occupies. The ring design is more expensive to make than a structure with support rods as shown on the right photo above, but there is a decrease in performance for the lower cost rod design due to increased drag and a less robust structure.

Figure 9, below left; FIgure 10, below right

Figure 9

With the smaller Wind-sail turbines below 100kW in rated output, the bearings are located at the middle of the turbine assembly, as opposed to a bearing at either end of the turbine. This location allows the use of one set of support arms and significantly reduces the aerodynamic drag as compared to two sets of support arms.

In figure 11 (below, Courtesy of Empire Magnetics), it is possible to see the three support arms that extend from the central hub of the Wind-sail turbine to the middle outer ring. Shrouding of these three arms reduces drag and incorporates a patented aerodynamic brake. As the turbine speed increases beyond a pre-determined point, the flaps begin to deploy with full flap extension at maximum speed.

Figure 11Hidden in the hub of Wind-sail VAWTs is a proprietary high efficiency direct drive alternator. There are no gears or mechanical devices to transmit the rotation of the turbine to the alternator, no mechanical moving parts to wear out, and no mechanical energy losses need considered. The use of this alternator makes Wind-sail technology more reliable and able to produce more power over the long term than other VAWT’s with standard alternator configurations.

Not visible is a unique dynamic control system that takes advantage of a design feature in the Wind-sail airfoils. This dynamic control system facilitates maximum power generation in light wind conditions, and prevents damage in the rare situations when high wind velocity could produce more power than the electrical system can handle. By giving up the power that occurs less than 1% of the time, Wind-sail is able to reduce the overall system cost while providing maximum power that is available 99% of the time.

For more info on Wind-sail technology visit www.wind-sail.com. For more information on the wind energy industry visit http://www.awea.org/learnabout/ and for a history see http://telosnet.com/wind/early.html.

 

 

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