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101 renewable - small wind turbines

Article Details

Last Updated
7th of January, 2020

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Wind turbines or wind generators transform  wind's  kinetic energy  into electrical energy.  Turbines can be divided into “lift” or  “drag” machines according to which force component  it is in use as “motive force”.  In the “lift” turbines,  the wind flows on both side of the blade surface,  thus creating at the upper surface a depression area and a  pressure area on the lower surface.

The pressure difference between the upper and the lower surface of the wind blade it is called the aerodynamic lift.  Lift force on the wings of an airplane can lift it from the ground and support it in flight, whereas in a wind turbine since the blades are bound to the ground, it determines the rotation over the hub axis. At the same time with the lift force, a drag force is generated, opposed to the motion (direction of rotation) and  perpendicular to the lift force. For a correctly designed wind turbine, the ratio lift-drag is high.  A wind generator requires a minimum wind velocity (cut in speed) of 3-5 m/s. To deliver the nameplate capacity  we need a wind velocity of 12-14 m/s. At high speeds, usually exceeding 25 m/s (cut-off speed) the turbine is blocked by the braking system for safety reasons. The block can be carried out by means of real mechanical brakes which slow down the rotor or for variable pitch blades, “hiding” the blades from the wind by putting them in the so-called “flag” position. The wind velocity it is different  based on the environmental conditions and  the height where we will achieve the optimal wind turbine speed.

The graphic bellow depicts that  placing  the wind turbine in less crowded areas is optimal for wind turbines and the structural cost of the turbine pole it will have a smaller  footprint.


Wind Turbines main advantages:~

The main advantages of the wind plants summarized:
• distributed generation;
• effective conversion of the wind energy into electrical energy (59% theoretical efficiency);
• lack of emission of polluting substances;
• saving on fossil fuels;
• reduced service and maintenance costs, there are no costs for the fuel supply;
• easy  to dismantle the wind turbines at end of its life (in average 25 years);
• the power generation capacity of the wind turbine ranges from few hundreds of Watts to Mega Watts.

Wind Turbines constructive classification:~

Wind turbines can be divided according to their constructive technology into two macro-families:
• Vertical Axis Wind Turbines - VAWT
• Horizontal Axis Wind Turbines – HAWT

VAWT turbines, which constitutes 1% of the turbines installed at the present  time are divided into:
Savonius turbines
• Darrieus turbines
• Hybrid turbines, Darrieus-Savonius

HAWT turbines, which constitutes 99% of the turbines  installed at the present  time are divided into:
• upwind turbines.
• downwind turbines.

About 99% of the installed horizontal axis wind turbines are three-blade turbines, whereas 1% are two-blade turbines.

Horizontal axis wind turbines: Upwind horizontal axis wind turbines, called so because the wind meets first the rotor than the tower, have a higher efficiency than downwind machines since there is no aerodynamic interference with the tower. On the other hand, they have the drawback that they are not self-aligning in the direction of the wind and therefore they need a tail vane or a yaw system. Upwind horizontal axis turbines are affected by the negative effects of the interaction tower-rotor, but are intrinsically self-aligning and have the possibility to use a flexible rotor to withstand strong winds.


The main options in a wind turbine design include:
• number of blades (commonly  two or three).
• rotor orientation (upwind or downwind of the tower).
• blade material, construction method, and profile.
• power control via aerodynamic control (stall control)  or variable-pitch blades (pitch control);.
• fixed or variable rotor speed.

Wind turbine blades:~

The blades are the components which interact with the wind and are designed with an airfoil to maximize the aerodynamic efficiency.

The typical form of a blade and its transversal sections shows that  the blade winds up and the total angle between the root and the tip is about 25°. The cross-sectional area of the blade is quite large to get the high stiffness necessary to withstand the variable mechanical loads under normal operation.  In fact, the wind exerts an unsteady  force, both for the fluctuations due to the turbulence, as well as for  the higher speed as a function of altitude. Those forces are the wear and tear of the wind turbine blades.

During rotation, a blade in a high position becomes subject to a stronger wind action in comparison with the intensity of the wind in a lower position. Finally, the centrifugal force due to rotation exerts traction on the different sections of the blade and the weight of the blade itself creates a bending  moment on the root which alternates at each rotation. Blades are made from light materials, such as fiber-reinforced plastic materials, which have good properties of resistance to wear and tear.


Many propeller type wind turbines have blades with airplane-like cross sections. This form allows airplanes to gain lifting power and keep flying. However, the wind needs to blow against the cross section at a certain angle -- inflow range -- to gain the lift. When the wind blows outside of this range, the lifting power is not be created and causes a stall condition -- unable to stay afloat.

During strong winds, blade slows the rotation, changes the wind inflow angle, and shifts to stall mode.

Why is stall mode necessary?  Because lifting power-type propellers usually  rotate faster as the wind speed increases. When the speed is too fast, the propeller may eventually break from its own centrifugal force or wind pressure. As the rotation speed approaches sonic speed, shock waves may  occur.

With conventional wind power generation, damage from strong wind is prevented by changing the pitch (attachment angle) of the propeller or deflecting the wind turbine (furling). On the other hand, has adopted the stall operation based on the following development concept.

Blades featuring carbon fiber technology has become "ultra-light", "highly rigid", and "longer lasting", capable of enduring stress from strong winds.

Fewer parts improve the machine's reliability. Compared with methods such as pitch angle change and furling structure, apparently uses fewer parts, thus less chance of failure.The simple structure with fewer parts reduces costs.

Steady and continuous power generation in high wind conditions:~

If within the stall range, can suppress the rapid acceleration of the rotation speed and maintain a relatively stable rotation. However, the trade-off for stall mode's stable operation with restrained rotation is less readiness to the wind and a drop in power generation efficiency. High winds with the average wind speed of over 12m/s for several tens of seconds.

Wind Generator Breaks:~

Nearly all wind turbines employ mechanical brakes mounted on the drive train, in addition to an aerodynamic brake. In many  cases, mechanical brakes can stop the rotor under adverse weather conditions besides being used as “parking” brakes to keep the rotor from turning when the turbine is not operating.

There are two types of mechanical brakes in common usage:
• disc brakes;
• clutch brakes;

Asynchronous Generator:~

It is essentially an induction three-phase motor characterized by a synchronous speed which depends on the number of poles and on the network frequency. If the mechanical torque acting on the rotor shaft is motive instead of resistant and makes the rotation speed to increase and exceed the synchronous speed, the asynchronous machine stops working as a motor and starts working as a generator, thus putting electrical energy into the grid. The relative difference between the synchronous speed and the effective rotation speed is called slip (s), which is negative when the machine is operated as a generator. In traditional asynchronous generators with squirrel cage rotor (short-circuit rotor), the slip is about 1% so that such devices are actually considered as having constant rotation speed. The magnetizing current of the stator, which generates the rotating magnetic field in the air-gap, is supplied by the grid.

The type of towers used with small wind turbines:~

There are two main types of towers commonly used horizontally with axis wind turbines:
•free-standing lattice (truss);


The first wind turbines were erected on free-standing lattice towers, commonly used until the mid-1980s. Nowadays wind turbines are mostly of tubular type since they offer  a number of advantages in comparison with the truss one. In particular, tubular towers do not require many bolted connections which need to be periodically checked; they provide a protected area to access the turbine and climbing to the nacelle is made safer and easier thanks to internal stairway or lift in case of larger turbines. Furthermore, they are aesthetically more acceptable in comparison with truss towers.

About the author of this article:
View Michael Comsa's profile on LinkedIn

Visitor Comments
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