Thursday, August 30, 2012

Methods of obtaining uniform (smooth) torque in Savonius wind turbine


In this post, I’m going to discuss the different methods and ideas to get a uniform torque through the whole revolution (cycle) of Savonius or any drag type VAWT. Some of these methods can be applied and the other may be only theoretical.
Consider the following relations to extract all the available methods to obtain a smoother torque operation.




1.   Using helical (twisted) profile Savonius

The common two-bladed Savonius wind turbine with 180 degrees angle of twist (like helix® wind turbine) will produce a uniform torque through the whole cycle. This method is the best method –till now- to smoothen torque. This method looks like placing a large (infinite) number of conventional Savonius turbines with infinitesimal height one above another, and each turbine is angularly-shifted (rotated about axis of rotation). Theoretically (as I claim, and neglecting the effect of top and bottom tips), a general relation which relates number of blades with angle of twist may be used:

Considering a Savonius bucket (semi-circular) element with infinitesimal height, the previous relation (formula) came from the fact that we want -at any angular position of the turbine- to find buckets with phase angles ranging from 0 to 360 degrees. Or more precisely, and in other words, for any angular position of the turbine we want to find blade patterns ranging from 0 to 360.



Note: increasing number of blades has certain limits. Performance of wind turbine deteriorates at high number of blades. Also, the geometry of the blade constrains the maximum number of blades.
2.   Variable radius (force arm)
In this method, the blade will slide out/in from/to the center of rotation in order to compensate decrease/increase of aerodynamic force to get a uniform torque through the whole cycle. In fact this method is completely idiot because it has the following disadvantages:
§  This method will work properly –theoretically- for only one wind direction (and its opposite direction), and so the main advantage of VAWT of receiving wind from any direction with the same performance is no more available

§  Sliding motion in and out is going to change the primary gap between inner blade tips which affects heavily the performance and noise level of the turbine

§  This method requires a complex mechanism (based on the mathematical model of the aerodynamic force) to give a variable radius. In most cases, aerodynamic force does not change linearly with angular position.

§  The use of certain mechanism will limit the RPM of the turbine and make it more noisy and difficult to maintain.
3.   Variable drag (projected) area
As a concept, it looks like the previous method. In this method we increase/decrease the area of the blade (in drag type VAWT) as the torque arm decreases/increases. This method also has the same disadvantages of the previous method. When drag area increases/decreases in a radial direction, torque radius also is going to increase/decrease and this is how two methods can be combined in one.
4.   Variable blade profile
It is some kind of foolish to think about this method, but it is just an option to imagine. In this method we change blade profile and so drag coefficient as the blade rotates. Blade profiles with high drag coefficients will be used at smaller torque radii, and vise versa. In addition to the disadvantages of methods 2 and 3, a changing blade profile may be considered as a science fiction as it is impossible to achieve.
5.   Controlling inlet wind speed
In fact, inlet wind speed can be controlled with different methods based on the imagination of the designer, but I’m going to assume that a flat shield plate is used to do this job. Controlling the angle of this shield plate is going to affect and control air speed over the advancing blade. The control of shield plate angle may be achieved by using stepper/servo motor or by linking the shield plate with the rotor by a mechanism to update plate angle passively without the need of external control. This method requires some setup for a given (fixed) wind direction, and this makes the turbine no more useful when wind comes from another directions.
6.   Controlling rotational speed of the wind
By controlling rotational speed of the turbine at different angular positions, we can control the relative velocity of air with respect to blades. At positions of high torque values increase turbine’s rpm, and at the positions of low torque values decrease turbine’s rpm.
7.   Changing inlet wind direction with time
When blade rotates, deviate the direction of inlet air keeping its speed the same.
As the turbine revolves, change the inlet wind direction in order to make the turbine exposed to the same aerodynamic force for the whole cycle.
8.   Controlling inlet air density
This method also is just an imagination in a science fiction movie. In this method increase/decrease air density when torque arm decreases/increases. Air density may be controlled by spraying a water mist in the inlet air (just imagine).
9.   Array of Savonius wind turbines with angular (phase) shift
Imagine a linear array of simple Savonius wind turbines that are connected (linked) using a large belt or sprocket chain, and this belt drives a central generator. Also, imagine that those turbines have different phase angles for a given wind direction. Although this method requires a farm of wind turbines, and has many disadvantages like the use of belts or chains, positioning problems of these turbines, and not-operating with the same performance for all wind directions, but it conceptually works.