Advantages of WindStor® Wind Turbine

A reliable affordable electricity supply is a cornerstone of social and economic development worldwide. Historically, most power generation has used various forms of fossil fuel, but growing concern about future exhaustion of supply and possible environmental damage has spurred development of alternative technologies. In the context of wind energy, the "propeller-type" horizontal axis wind turbine (HAWT) has supplanted the ancestral vertical axis wind turbine (VAWT). Recent comparisons of HAWT and VAWT technologies have focused on outdated VAWTs, ignoring important new breakthroughs in the field. The following report prepared by Analytical Design Service Corp describes the advantages of the improved Darrieus type of VAWT - WindStor®.

Vertical Axis Wind Turbine – "VAWT"

Beginning with George Darrieus' patent in 1929, the symmetrical airfoil VAWT has received considerable attention because it has the advantage of omni-directionality. Figure 1 shows a three-blade VAWT. Lift & drag on this airfoil depends on a complex interplay of rotational and wind velocity components. Paraschivoiu was first to develop an effective "stream tube" analysis of VAWTs, and his work was subsequently refined and verified by Worstel, Berg and others at Sandia National Laboratories. Our innovative design builds upon work by Berg and Paraschivoiu with improvements to address energy output efficiency, vibration, durability and manufacturability (i.e. cost).

Most commercial VAWTs use three straight vertical blades because of ease of fabrication. However, the horizontal supporting struts introduce ruinous parasitic drag, so this report concentrates on three self-supporting curved blades. Since the largest stress in a blade arises from centrifugal load, it is essential that there not be any bending at high rotations. By imagining a chain in place of the blade, the optimal shape is seen to be (approximately parabolic) troposkein. The fourth blade in a four-blade VAWT interferes with airflow to the other blades, while two-blade VAWTs are not self-starting. Hence the focus of this report on a three-curved-blade strut-less tower-mounted VAWT.

Advantages of VAWTs

• VAWTs are omni-directional, so they do not suffer from the 20.56% "gyroscopic" penalty on HAWTs.
• VAWTs have better power output at lower wind speeds. Wind speeds follow a Rayleigh distribution in which milder winds predominate. See Figure 2.
• Plastic-clad curved steel blades are stiff, fatigue-free, and strong in high winds. Unlike HAWTs, they are supported at both ends and the self-supporting VAWT blade design reduces parasitic losses by eliminating all struts.
• VAWTs are ideally suited to direct-drive generators, which circumvent the reliability problems of speed reducers at high power.
• Catastrophic high-wind failure can be prevented by a relatively inexpensive failsafe shaft caliper brake.

Conclusions

• Being omni-directional, VAWTs avoid the known 20.56% "gyroscopic" penalty associated with HAWTs, which fail to track the wind. This offsets the theoretical slight aerodynamic disadvantage of the symmetrical VAWT airfoil compared to asymmetrical HAWT airfoils.
• Strut-less self-supporting curved steel blades, mounted on a full-height center post, raise the performance of VAWTs to new levels.
• VAWTs can have less expensive blade and drive-train components, especially when a direct-drive generator replaces the conventional speed reducer.

 

Fig. 1 WindStor® three-blade VAWT (tower not shown)

Environment - Noise & Emissions

WindStor®'s design includes fewer moving parts that increase performance while decreasing noise levels. A 2MW horizontal axis wind turbine having a sound level of around 40 dB at 400 meters away is significantly lower than average street traffic noise estimated to be 80 to 90 dB. Note dB are on a logarithmic scale, they do not add up linearly, therefore doubling the noise level adds only 2 to 3 dB. It is often difficult to measure sound from modern wind turbines above wind speeds of 8 m/s because the background wind generated noise masks the wind turbine noise above 8m/s wind speeds. Also, vertical axis wind turbines, like WindStor®, have less noisy components and consequently, the noise level is lower. WindStor® is a variable speed wind turbine and therefore has better control over noise emissions.
WindStor® components that could produce noise consist of a fan and a generator, similar to that of an air conditioning system which consists of the same components. Blade and gear noise is negligible compared to a HAWT by reason of the design difference. WindStor® blades rotate around the mast avoiding the compressed air "whump" sound emitted by HAWT blades as they sweep by the mast. Given these differences, the noise impact of WindStor® will be negligible to non-measurable, considering other noises indigenous to urban environments.

Without consuming any natural resources or emitting any greenhouse gases:

• 1-MW of power generated from a wind turbine displaces 1,800 tons of carbon dioxide each year (equivalent to planting a square mile of forest), based on the current average U.S. utility fuel mix
• To generate the same amount of electricity as a single 1-MW turbine using the average U.S. utility fuel mix would mean emissions of 9 tons of sulfur dioxide and 4 tons of nitrogen oxide each year
• To generate the same amount of electricity as a single 1-MW wind turbine for 20 years would require burning 29,000 tons of coal (a line of 10-ton trucks 11 miles long) or 92,000 barrels of oil
• To generate the same amount of electricity as today's U.S. wind turbine fleet (6,740 MW) would require burning 9 million tons of coal (a line of 10-ton trucks 3,437 miles long) or 28 million barrels of oil each year
• 100,000 MW of wind energy will reduce CO2 production by nearly 150 million tons annually

Safety

Because WindStor® will be located close to urban activities, it is imperative that it is a safely operating device. WindStor® was designed to withstand the harsh environments of the Arctic. Often located where access for repairs and maintenance are limited to weeks a year, failure proof design of WindStor® and its components is paramount. This quality and structural integrity requirement for the North integrated well into safety for Urban sites. WindStor® will incorporate three braking systems, vibration sensors for immediate shutdown, double attachment blades with anchor cables at the connection points and joints, and UL, RF, IEEE / ANSI electrical certifications. WindStor's eight safety advantages over horizontal turbines include:

Steel blades securely attached at both ends
Internal safety cable
Remote monitoring
Disc brake system
Motor Brake
Load resistor brake
Vibration sensors
Integrated smart controller