How A Wind Turbine Works - The Basics
A turbine is a device that harnesses kinetic energy from a moving fluid like air or water, and converts it to a usable force for doing work. Turbines usually consist of angled blades mounted to spinning rotors that act like small sails to harness the force of wind or water, but some designs are bladeless and use forms of motion other than rotation for capturing and converting energy, like a wave turbine that uses a system of paddles or levers and hydraulics to harness the oscillating motion of waves. However designed, the turbine that harnesses the energy is connected to a gearbox or directly to a pump, electric generator, or some other piece of equipment or tool that's sized to match the power of the turbine and do the actual work that's intended, such as pumping water, milling, or generating electricity.
Drag Vs Lift
Objects in the path of a fluid stream experience a force called drag. Some of the earliest wind turbine designs utilized this force and are known as vertical axis wind turbines (VAWT). While drag is easy to harness, VAWT's are limited to performing at much lower rpm's than horizontal axis wind turbines (HAWT) because as the blades rotate, a portion of the rotor is always travelling upwind in order to turn the blades into a better position to harness the downwind drag again. If the blades have an airfoil profile and are angled properly, then the drag force can be converted to a lift force, which always works at right angles to the wind direction. This means that a VAWT with airfoil blades can harness more energy during a rotation than a strictly drag based VAWT, but still not quite as much as a HAWT where the blades spin at right angles to the wind during their entire rotation instead of trying to fight the wind head on.
The blade lift configuration utilized by HAWT’s gives an advantage of increased rotational speed in relation to the incoming wind speed when compared to VAWT’s. The blade tips on a HAWT can safely travel up to 7 times faster than the incoming wind speed, whereas the blades on a VAWT will never travel faster than wind speed due to it less efficient drag based design.
Tracking
Tracking refers to how a wind turbine follows the direction of the wind. A VAWT doesn’t require a tracking mechanism because it can harness wind from any direction, so it can take better advantage of low level turbulent (aka 'dirty') winds where they'll work more consistently and produce more torque than a HAWT that always needs to face in the direction of the oncoming wind. HAWTs therefor require a tail to track the wind and minimize effects from turbulence, so they're better suited for clean wind environments where turbulence is minimal and production will be more consistent because the turbine won't be constantly spun around from one direction to another. They also require some form of overspeed protection because they can spin so fast in high winds that they'll tear themselves apart without speed control. For DIY turbines, this is typically done with a furling tail that both tracks the wind and furls to turn the rotor out of the wind when the speed gets dangerously high.
Furling can be achieved a few ways, but the most common method is to mount the tail to the turbine frame on a hinge with a compound angle, and use the drag force of the wind and gravity to operate it automatically. The hinge in a furling tail mechanism is angled to the back and side at certain degrees to cause the wind to lift the tail and subsequently turn the rotor out of facing the direction of the wind when its force is strong enough to overcome the weight of the tail. When the wind speed decreases, the tail drops and the rotor turns back into the wind again. This is a popular method because it's relatively simple and inexpensive, and it doesn't shut the turbine down completely - it just limits the amount of wind hitting the blades at an optimum angle and thus limits the rotor rpm and power output. Factors such as tail size, weight, and hinge angle determine at what wind speed the furling moment occurs.
Wind Power
With conventional bladed turbines, many factors come into play when it comes to how much power they'll harness and output, but the two main ones are the speed of the wind and the swept area of the blade rotor. A general rule is that doubling the swept area will double the power potential, but doubling the wind speed will increase the power potential by a factor of 8, because power increases with the cube of the wind speed.
Swept area refers to the area that the blades occupy when rotating. For example, if the area of a circle is = Pi * radius^2, then a HAWT with a 4 meter diameter rotor will have a swept area of:
A = 3.1459(2 x 2)
= 3.1459 x 4
= 12.58 square meters
The amount of energy that can be extracted from a wind stream by a wind turbine can be calculated as follows:
Power = 1/2ξpAV^3
where:
p = density of the air in the wind stream in kg per cubic meter, typically 1.23kg/m^3
A = rotor swept area in square meters = πr^2
V^3 = wind velocity in meters per second cubed
ξ = turbine coefficient = ~25-35%
Example using a 2.4m diameter rotor with a 30% coefficient, performing in a 5 mps wind stream:
Swept area = πr^2
= 3.1459(1.2 x 1.2)
= 3.1459 x 1.44
= 4.5 sq.m
Power = 1/2(0.3 x 1.23 x 4.5 x 5^3)
= 0.5(0.3 x 1.23 x 4.5 x 125)
= 0.5 x 207.6
= 103.8 watts
According to Betz's law , no turbine can capture more than 59.3% of the kinetic energy in a wind stream. The factor 16/27 (0.593) is known as Betz's coefficient, or the Betz limit. Practical utility-scale wind turbines achieve ~75% of the Betz limit on avg, or 35-40% efficiency. A reasonable approximation for a coefficient for a well built DIY turbine would be around 30%, and is incorporated and expressed in the above power equation as ‘ξ’.
Swept area refers to the area that the blades occupy when rotating. For example, if the area of a circle is = Pi * radius^2, then a HAWT with a 4 meter diameter rotor will have a swept area of:
A = 3.1459(2 x 2)
= 3.1459 x 4
= 12.58 square meters
The amount of energy that can be extracted from a wind stream by a wind turbine can be calculated as follows:
Power = 1/2ξpAV^3
where:
p = density of the air in the wind stream in kg per cubic meter, typically 1.23kg/m^3
A = rotor swept area in square meters = πr^2
V^3 = wind velocity in meters per second cubed
ξ = turbine coefficient = ~25-35%
Example using a 2.4m diameter rotor with a 30% coefficient, performing in a 5 mps wind stream:
Swept area = πr^2
= 3.1459(1.2 x 1.2)
= 3.1459 x 1.44
= 4.5 sq.m
Power = 1/2(0.3 x 1.23 x 4.5 x 5^3)
= 0.5(0.3 x 1.23 x 4.5 x 125)
= 0.5 x 207.6
= 103.8 watts
According to Betz's law , no turbine can capture more than 59.3% of the kinetic energy in a wind stream. The factor 16/27 (0.593) is known as Betz's coefficient, or the Betz limit. Practical utility-scale wind turbines achieve ~75% of the Betz limit on avg, or 35-40% efficiency. A reasonable approximation for a coefficient for a well built DIY turbine would be around 30%, and is incorporated and expressed in the above power equation as ‘ξ’.