The Advantages of a Square Rigged Sail Design

Currently, the most common method for harnessing wind energy has been to use horizontal-axis wind turbines. However, they have certain inherent disadvantages. Vertical-axis wind turbines overcome some of these disadvantages but many designs introduce problems of their own. After summarising the disadvantages and advantages of past and current designs of both horizontal and vertical-axis turbines, the author introduces the idea of a square rigged sail wind turbine, which he feels has greater utility than previous designs.

By Jon Howard, Research Specialist, H Energy Innovations, USA

Description of 'Prior Art'
{access view=!registered}Only logged in users can view the full text of the article.{/access}{access view=registered}Wind power has been a source of energy for centuries; however, there have always been distinctly different approaches as to how to extract that energy. In particular, there have been both horizontal-axis and vertical-axis wind turbines. In these modern times, the most common method for harnessing wind energy has been to use a horizontal-axis wind turbine. While horizontal-axis wind turbines have been promoted as being the more efficient type compared to other methods, they present several disadvantages. For instance, horizontal-axis wind turbines have to be turned into the wind to start functioning. Also, they have a relatively high cut-in wind speed for operation and a low cut-out wind speed. This allows for only a relatively narrow window of operation, beyond which they are prone to damage. Another problem associated with the horizontal-axis design is that they optimally require a near gale force wind to produce power. Further, horizontal-axis wind turbines can be extremely high above a ground surface, making it difficult for technicians to perform repairs. Due to such heights, technicians are often exposed to grave risks as they provide maintenance in adverse weather conditions.

Alternatives to Traditional Designs
Vertical-axis wind turbines change the axis of rotation of the turbine. These offer an alternative to the traditional wind turbine. The vertical-axis wind turbines improve the safety of servicing and maintenance duties because services are performed much closer to the ground. In the 1920s, a French inventor by the name of Georges Jean Marie Darrieus designed a vertical-axis wind turbine that has been referred to as the ‘Darrieus design’ or ‘eggbeater’. The Darrieus design uses a series of sails that are fixed at a set angle and arranged symmetrically around a vertical axis. The symmetry of the sails provides a very effective means of generating a rotational force to the vertical shaft axis. These types of vertical-axis wind turbines are used today on tall buildings to utilise the high wind velocity found at higher altitudes. Unfortunately, sail fatigue, which causes premature failure of the system, is a common problem associated with the Darrieus design.

As an alternative to the Darrieus design, the US Patent No. 4,449,053, issued to Kutcher, shows a vertical-axis wind turbine that uses vertically positioned rotor blades. Blades are connected both at the top and bottom of a vertically extending rotor tube. While the Kutcher design reduces sail fatigue, the vertically positioned rotor blades do not easily capture wind at all angles, thereby reducing their effectiveness.

Other Vertical-Axis Designs
Another variation is the Giromill Cycloturbine, shown in US Patent No. 7,315,093, issued to Graham. The Giromill Cycloturbine has sails mounted such that the sails can rotate around an axis. The design of the Cycloturbine allows the sails to be pitched such that the sails are always at an angle relative to the wind. A main advantage to this design is that the torque generated remains almost constant over a fairly wide angle. Therefore, a Cycloturbine with three or four sails has a fairly constant torque. Predetermining the range of angles, the torque approaches a possible maximum torque, wherein the system generates more power. The system also has the advantage of being able to self start by pitching the down-wind moving sails flat to the wind to generate drag and start the turbine spinning at a low speed. One drawback to this design is that the sail pitching mechanism is complex and generally heavy, and a wind direction sensor must be added to the design in order to properly pitch the sails.

Current Designs
Currently, the commercial application of wind energy harnessing is primarily, if not exclusively, horizontal-axis wind turbines, even though vertical-axis wind turbines avoid most of the disadvantages inherent in the horizontal-axis design. For example, vertical-axis wind turbines are omni-directional and have a lower cut-in wind speed and higher cut-out speed, thus making the window of operation wider. Also, vertical-axis wind turbines can have components that need servicing located at the bottom end of the structure making access more convenient. Vertical-axis wind turbines also allow for lower-ratio gearboxes, which are less expensive and more efficient than the gearboxes needed to operate horizontal-axis wind turbines, Further, vertical-axis wind turbines are able to operate at a higher wind speed and at lower risk of suffering wind damage. Finally, vertical-axis wind turbines are more suited to a simpler design and construction.

Thus, there is a continuing need for a vertical-axis wind turbine that captures the inherent advantages of the vertical-axis design, yet improves upon the drawbacks of existing vertical-axis designs.

Summary of Findings
The conclusion that was drawn from studying previous designs in this field was that an improved version, without the drawbacks of the previous models, could be created by using reinforced square rigged sails fixed at a 90 degree angle, at the tip of parallel and horizontal yardarms. Such a design is described below.

Square Rigged Sail Wind Turbine
The suggested new wind turbine is a vertical axis turbine designed to generate electricity at both onshore and offshore locations. The turbine is driven by a square rigged sail conformation that can include one or more stacked sail assemblies. Each sail assembly includes a main shaft having a vertical axis of rotation, with each successive sail assembly in the stack sharing the main shaft. Each sail assembly includes one or more yardarms that extend horizontally from the main shaft. The sails are attached to the yardarms such that the main shaft is central and positioned between the sails.

From the top down in the blue diagram (figure 1), the second and fourth sail assemblies are obscured showing two sails only for each. Also, the entire rear vertical column, the vertical main shaft axis, and corner bracing on the lower three sail assemblies are obscured in this diagram.

Biography of the Author
Since 2007, Jon Howard has been a research specialist, for H Energy Innovations, in
southern California, USA, where extensive studies have been conducted in wind energy. Mr Howard’s research on wind energy development and studies in meteorology and the environment has led him to create a vertical-axis wind turbine with greater utility advantage than the previous designs.{/access}

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