Investigating the Use of a Variable-Pitch Wind Turbine to Optimize Power Output Under Varying Wind Conditions. Galen Maly Yorktown High School

Background In 2001, 0.15 percent of electricity consumed in the U.S. came from wind turbines. If 0.6 percent of the land in the U.S. was used for wind farms, then those turbines could produce 15 percent of the electricity.

How Wind Turbines Work Wind turbines change the kinetic energy of the wind into electricity in a 2-step process. First, the kinetic energy of the wind is turned into rotational energy of the rotor by the blades. Second, rotational energy of the rotor is turned into electricity by a generator.

Lift vs. Drag The kinetic energy of the wind is harnessed through two forces, lift and drag Drag, the force of the real wind pushing on the blade gets the rotor spinning. After the rotor begins spinning, an induced wind occurs. The induced wind combines with the real wind to form the resulting wind. When the angle between the resulting wind and the blade is about 15 degrees, lift occurs.

Pitch Pitch is the angle of the wind turbine blade. Pitch can be measured as an angle or a unit of length (the distance the blade would travel were it to be spun 360 degrees through a solid like a screw) A fixed-pitch wind turbine is a wind turbine that cannot change the angle of its blades. A variable-pitch wind turbine has the ability to change the angle of its blades.

Purposes and Hypotheses The overall purpose was to determine if a variable-pitch wind turbine would be more effective than a fixed-pitch turbine in converting the kinetic energy of wind into electrical power. This overall purpose was divided into five objectives

Objective 1 To validate the previous year’s results by testing the power output from fixed-pitch blades with a more consistent higher-speed wind source (a wind tunnel). The hypothesis was that as pitch decreased and wind speed increased, output from the wind turbine would increase.

Objective 2 To measure the RPMs of the wind turbine rotor in order to determine the angle of attack of the resulting wind on the blade. The hypothesis was that the angle of attack would be in the range of 15 degrees.

Objective 3 To determine if electrical output of the wind turbine could be maximized by varying blade pitch. The hypothesis was that a variable-pitch wind turbine is able to achieve higher levels of output than a fixed-pitch wind turbine.

Objective 4 To see if it is possible to create a computer program to optimize the electrical output of a variable-pitch wind turbine through real- time measurement of RPMs. The hypothesis was that a program could be written that uses real-time RPM input to achieve optimized pitch for maximum power output.

Objective 5 To create a computer program that adjusts pitch in response to a given wind speed in order to optimize power output. The hypothesis was that a program could be written to do so.

Procedures: Programming 1.Set the pitch at a specific angle at the blade’s midpoint. 2.Slowly increased the pitch in 1-degree increments. 3.Read in the RPMs of the wind turbine and kept adjusting the blade pitch for a given wind speed until maximum RPMs were achieved. 4.Read in wind speed and set the pitch for maximum output using an equation derived from previous experimentation. Four programs were written in iC for use on a microprocessor attached to the turbine.

Procedures: Objective 1 To test the first hypothesis, voltage and RPM outputs were recorded at pitches of 15, 34, 45, and 60 degrees at several different wind speeds ranging from 4.5 to 14.3 m/s. This process was repeated for two trials, and results were gathered in a data table.

Procedures: Objective 2 The second hypothesis was tested by gathering data on the RPMs at the optimum pitch for several wind speeds. The RPMs were used to calculate the speed of the blade at its midpoint and, from that, the angle of attack on the blade.

Procedures: Objective 3 The third hypothesis was tested by increasing the pitch of the blade until the rotor would begin spinning. Then, the pitch was decreased towards a pitch of zero (an ability unique to variable-pitch wind turbines). The power output was recorded at every degree of pitch

Procedures: Objective 4 The fourth hypothesis was tested by using the program to optimize the pitch of the wind turbine at pitches from 4.5 to 14.3 m/s in both the wind tunnel and using a fan. The program was run twice (due to time limitations at the wind tunnel) for each wind speed, and results were collected. Regression statistical analyses for the data were conducted.

Procedures: Objective 5 The fifth hypothesis was tested by attaching a sensor to a Kestrel wind meter to measure wind speed and feeding that information to the Handyboard.

Results/Conclusions: Objective 1 Higher pitch blades begin to spin first at lower wind speeds, but at high wind speeds, lower pitch blades spin faster and produce higher output than higher pitch blades. A higher-pitch blade can translate more of the wind’s drag force into rotational motion, so higher-pitch blades are more effective at low wind speeds. Once the blade starts spinning, the blade’s rotational movement causes a head wind, and lift makes the blade spin faster. Once lift dominates drag, the lower-pitch blades can spin faster and produce more electricity.

Results/Conclusions: Objective 1 Effect of Wind Speed on Power Output Using Fixed-Pitch and Programmed Variable-Pitch Blades

Results/Conclusions: Objective 2 By knowing the RPMs of the turbine rotor shaft, the speed of the blade at any distance from the shaft can be computed. The speed of the real wind and induced wind can then be used to compute the velocity of the resulting wind on the blade. The angle of attack can then be computed based on the blade’s pitch angle. The angles of attack at the midpoint were mostly between 10 and 20 degrees, with an average of 14.9 degrees. Hence, the hypothesis was supported.

Results/Conclusions: Objective 3 Effect of Varying Blade Pitch on Power Output When Wind Speed is Constant (6.3 m/s)

Results/Conclusions: Objective 4 For every wind speed, the maximum output discovered by the program was close to being equal to or higher than any other output from the wind turbine. In other words, a variable-pitch turbine can be programmed to constantly seek maximum power output.

Results/Conclusions: Objective 4 Effect of Wind Speed on Power Output Using Fixed-Pitch and Programmed Variable-Pitch Blades

Results/Conclusions: Objective 4 Effect of Wind Speed on Optimum Pitch

Results/Conclusions: Objective 5 This objective was not achieved because the Handyboard had insufficient processor power to be able to read every RPM of the wind meter as signaled by the sensor. The sensor could read in very low wind speeds accurately, but was completely unreliable at high wind speeds.

Results/Conclusions: Overview Variable-pitch wind turbines are more effective than fixed-pitch wind turbines at optimizing electrical power output due to the ability of variable-pitched wind turbines to start spinning at a high pitch and then decrease the pitch to optimize power output.

Results/Conclusions: Overview A fixed-pitch turbine is like a 21-speed bike stuck in one gear: It is either hard to get moving, or it will not move fast. A variable- pitch turbine can “switch gears” to take better advantage of drag, lift, and varying winds, and it is therefore about three times more powerful.