THE EFFECT OF BLADE ANGLE AND SIZE ON WIND TURBINE PERFORMANCE

Transcription

1 THE EFFECT OF BLADE ANGLE AND SIZE ON WIND TURBINE PERFORMANCE Anish Bhattacharya, 8 th Grade, Unity Point School District 140 PURPOSE We have experienced a major fall in the economy in the recent past. We need something to help boost the economy. Our non-renewable sources of energy are also running out. We desperately are in need of an alternate renewable clean resource for energy. In doing this project, the researcher hopes to find the most efficient combination of blade angle and width on a wind turbine. HYPOTHESIS It is hypothesized that a blade angle of 45 degrees and a width of 2 cm will produce more usable energy than any other combination. THE OPERATING PRINCIPLES OF WIND TURBINES The Lift and Drag Aerodynamic Principles of a Rotor Blade When a gust of wind passes over the airfoil shaped blade, it passes faster over the upper or curved part of the airfoil. This causes a low-pressure area above the airfoil. The difference in pressure between the top and bottom surfaces results in a force, called aerodynamic lift. If this were an - 1 -

2 aircraft wing, this force would cause the airfoil to rise, lifting the airfoil, and therefore the aircraft, off the ground. Since the blades of a wind turbine are permanently attached to the hub at its center, the lift force causes rotation about the hub. In addition to lift force, a drag force perpendicular to the lift force impedes rotor rotation. The prime objective in a wind turbine design is to have a relatively high lift-to-drag ratio, and this ratio can be varied depending on the length of the blade to maximize the turbine s energy output at various wind speeds. Picture from: Basic Aerodynamic Operating Principles of Wind Turbines, 2008, [Online] PAST RESEARCH Summarization The researcher in conducted a similar study as this one. In it, the researcher studied the effect of blade length as well as the number of blades on the performance of a wind turbine. The amount of voltage produced and the revolutions per second of the turbines were used as indicators. The oncoming wind speed was also varied. The independent variables were wind speed, blade length, and blade number. The different wind speeds were 17.7 km/h, 22.5 km/h, and 29 km/h. The three blade lengths were 8.5 cm, cm, and 18 cm. The width of the blades was kept constant, 3.5 cm. For each blade length, there were three turbines, each with a different number of blades, 2, 3, or 4. All tests were conducted indoors to reduce interfering wind and other elements. The same environment was used for each test. Conclusion & Theories of Past Research It was found that if a wind turbine has larger blades, it will produce a lower voltage and RPS than the same turbine with smaller blades. It was also found that the 8.5cm bladed - 2 -

3 turbine produced more than double the voltage than the 18cm bladed one. Since larger blades are heavier, it is more difficult to spin. This causes slowing down, and also produces less voltage. The data acquired for the smaller-bladed turbines show that they spin faster, and produce a greater voltage, thus supporting this idea. A greater number of blades increase the weight to be turned by the turbine. On the other hand, more blades provide a greater available surface area for the wind to push, so it would produce more turning power. Having fewer blades could be beneficial because it will not be as heavy, and will be easier to turn than a greater number of blades, but it will also be somewhat inefficient because it produces less turning power. Using all the information above, the researcher concluded that (1) a four-bladed turbine should have about double the weight to turn compared to a two-bladed turbine of the same size; (2) but a four-bladed turbine has nearly double the pushing power compared to a two-bladed turbine of the same size. Therefore, the energy produced from a four-bladed turbine and a two-bladed turbine should be nearly equal. This is supported by some of the data. However, it is not supported by the remaining data and this reason could possibly be experimental error as discussed below. In the voltage and RPS graphs of the large bladed turbines, the graphs are not similar. The voltage for the four-bladed test was lower than expected. This could be the result of experimental error and/or incorrect data. The medium bladed turbines show that the data gathered from the two-bladed turbine are quite different from that of the four-bladed turbine. It was expected that these data would be similar for reasons previously explained. The reason for this difference may be the result of an error during the construction of the turbines. In the two-bladed turbine, one of the - 3 -

4 blades was glued crookedly, which did make a difference in the data collected with it. Another example of possible data error could be the three-bladed turbine of the largest size. In this turbine, the spaces between the blades were uneven, which may have caused it to produce less voltage than the two and four-bladed ones. This section (Past Research) ends here. The description of the study is continued below. METHODS OF PROCEDURE In the procedure, the first step was to build the turbines with the unique sizes. This was done using 1 mm thick basswood. A 2 cm long triangle shaped formation was added at the end of each blade, and this part would be inserted in each foam hub. The hubs of the turbines were made from foam eggs, and after cutting, they had a diameter of 4 ½ cm and a height of 3 cm. Next, the cut blades were secured at the required angle 30, 45, or 60 degrees in each hub with a protractor and Elmer s glue. Glass tubing with a diameter of 5 mm and a thickness of 1 mm was cut into sections of 1.75 cm. Using Gorilla Glue these were permanently placed at the rear of the hub. The finished turbine was then attached to a plastic gearbox (the glass tubing attached turbine to gearbox), which was attached to a large metal ring stand and was placed in front of a wind tunnel. The generator (1.5 volt motor) was then attached to the gearbox. This was next attached to a voltmeter to measure the energy output. Next, a photogate was attached to the small metal ring stand and the LabPro Interface, which was connected to the computer. The photogate would measure the turn frequency of the turbine. After this, the program required to measure the turn frequency was opened on the computer. The wind tunnel was turned onto low power, where it would be emitting a wind speed of 17.7 km/h. The voltage reading on the voltmeter was recorded, as well as five - 4 -

5 readings for the turn frequency (revolutions per second). These five readings served as multiple trials for each turbine. I then turned the wind tunnel to medium and high power, and recorded the same data. I used the same method to make and test the other turbines, and recorded the same data. Mr. Blair assisted me to assemble the apparatus for gathering the data. RESULTS Discussion This results section contains Tables 1-4 and Figures 1-6. Statistical tests were conducted to measure statistical significance of the data. The t- and p-values for the different sized turbines are shown in Tables 1-4. These tests are discussed in greater detail in a later paragraph. Tables 1, 2, and 3 show the statistics calculated for the small, medium, and large bladed turbines, respectively. Table 4 combines the averages of the three sizes for an overall comparison. Figures 1-6 present the energy outputs and turn frequencies in bar graphs. Figures 1-2, 3-4, and 5-6 hold these data of the small, medium, and large bladed turbines. Tests were performed not only on blade size, but also on blade angle, shown in individual tables and figures. For each combination of blade size and angle, five different values were collected to measure the mean turn frequency (RPS) using three different wind speeds. For these wind speeds, the voltages produced by the wind turbines were also recorded. In order to reduce experimental error, each turbine was tested in exactly the same method, using the same apparatus. Error was also controlled by using the same environment for testing. Oncoming wind interfering from other directions was prevented by performing all tests indoors. Let: v 1 = difference in voltage output between averages of small and medium bladed turbines - 5 -

6 v 2 = difference in voltage output between averages of medium and large bladed turbines v 3 = difference in voltage output between averages of small and large bladed turbines Using the mean turn frequency from the different combinations of blade sizes and angles, statistical tests were conducted, namely t-tests, to find if the means were significantly different or not. These t-tests were conducted using a computer program called MINITAB. Using the mean and variance of the data, the t-statistics and p-values were found for each pair of size and angle of blade combinations (Tables 1-4). If the t-statistics were between -2 and +2 (approximately), then the p-values would be greater than.05. However, if the t- statistics would either be less than -2, or greater than +2 (approximately), then the p-values would be less than or equal to.05. Furthermore, when the p-values are less than or equal to.05, then the data in question would be considered as statistically significant (or sufficient to conclude that the means significantly differ). If the p-values are greater than.05, then the data would be considered as statistically insignificant (or not sufficient to conclude that the means significantly differ)

7 T-Statistics and P-Values Calculated with Different Blade Combinations Table 1 Table 2 Table 3 Blade Angle Comparison 30 degrees vs. 45 degrees 45 degrees vs. 60 degrees 30 degrees vs. 60 degrees Blade Angle Comparison 30 degrees vs. 45 degrees 45 degrees vs. 60 degrees 30 degrees vs. 60 degrees Blade Angle Comparison Blade Size: 2.0cm x 8.5cm Wind Speeds km/h T=24.60 T=25.16 T=21.64 T=16.92 T=14.79 T=14.98 T=41.25 T=47.67 T=37.60 Blade Size: 4.5cm x 8.5cm Wind Speeds km/h T=9.19 T=9.57 T=18.29 T=12.33 T=21.90 T=30.89 T=18.90 T=18.33 T=34.00 Blade Size: 7.0cm x 8.5cm Wind Speeds km/h degrees vs. 45 degrees T=4.57 P= T=7.25 P= T= degrees vs. 60 degrees T=10.65 T=17.18 T= degrees vs. 60 degrees T=20.05 T=13.28 T=32.70 Table 4 Comparisons of Blade Sizes (using averages of frequencies for three wind speeds) Blade Size Comparison Blade Angle 30 degrees 45 degrees 60 degrees 2.0cmx8.5cm vs. 4.5cmx8.5cm T=-6.56 T=-8.28 T=0.14 P= cmx8.5cm vs. 7.0cmx8.5cm T=2.38 P= T=-0.02 P= T=-4.43 P= cmx8.5cm vs. 7.0cmx8.5cm T= T=-7.75 T=-4.26 P=

8 Figure 1 Figures presenting data acquired from small-bladed turbines The Effect of Blade Angle on Turn Frequency Using Blades of 2.0cm x 8.5cm Frequency (RPS)iiiiii Blade Angle (degrees) 17.7 km/h 22.5 km/h 29.0 km/h Figure 2 The Effect of Blade Angle on Energy Output Using Blades of 2.0cm x 8.5cm Energy (Voltage)iiiiii Blade Angle (degrees) 17.7 km/h 22.5 km/h 29.0 km/h - 8 -

9 Figure 3 Figures presenting data acquired from medium-bladed turbines The Effect of Blade Angle on Turn Frequency Using Blades of 4.5cm x 8.5cm Frequency (RPS)iiiiii Blade Angle (degrees) 17.7 km/h 22.5 km/h 29.0 km/h Figure 4 The Effect of Blade Angle on Energy Output Using Blades of 4.5cm x 8.5cm Energy (Voltage)iiiiii Blade Angle (degrees) 17.7 km/h 22.5 km/h 29.0 km/h - 9 -

10 Figure 5 Figures presenting data acquired from large-bladed turbines The Effect of Blade Angle on Turn Frequency Using Blades of 7.0cm x 8.5cm Frequency (RPS)iiiiii Blade Angle (degrees) 17.7 km/h 22.5 km/h 29.0 km/h Figure 6 The Effect of Blade Angle on Energy Output Using Blades of 7.0cm x 8.5cm E nergy (Volta ge)iiiiii Blade Angle (degrees) 17.7 km/h 22.5 km/h 29.0 km/h

11 CONCLUSION The hypothesis stated that the turbine with a blade width of 2cm (length of 8.5cm is constant for all turbines) and a blade angle of 45 degrees would produce the most energy when compared to certain other combinations. Voltage output was used as an indicator. This hypothesis was rejected after interpreting the data. The turbine with a blade width of 4.5cm (medium size) and a blade angle of 30 degrees had the highest turn frequency and produced the most voltage. It was also noted that manipulating the width (and therefore the width:length ratio) and the angle of the blade significantly changed the amount of energy produced. It was found that if a turbine has narrower blades (the ratio width:length is relatively small) then it will produce lower voltage and a lower frequency. The turbines with a blade width of 2cm (Figure 2) produced significantly less voltage when compared to turbines with blade widths of 4.5 and 7cm (Figures 4 and 6). The turbines with 2cm blades also showed overall to have the least performance. This could be because of lack of surface area. The surface area is less when compared to the other turbines, so the wind has a lesser area to push, creating less lift. However, the lesser surface area also creates less drag. It is conjectured that the reduced drag, however, does not fully counteract the effect of lesser lift (the ratio of lift:drag is small). The data from the medium bladed turbines show that this size produced the most voltage and the highest turn frequency (Figures 3 and 4). This could be the result of greater surface area, and therefore greater lift. However, the greater surface area also results in greater drag. It is presumed that the increased drag has a smaller effect on the turbine than this increase in lift (lift:drag is high) when compared to the smaller bladed turbine

12 The largest bladed turbine produced slightly less voltage and had a lower frequency (Figures 5 and 6) than the medium bladed turbine. The researcher theorizes that this is because of the drag that the large blade size produces. The large size has more weight when compared to other sizes, but it also has a greater surface area which provides more lift. This lift partially balances out the added drag, but the data gathered shows that the lift:drag ratio is still slightly smaller than the medium bladed turbine. It was also found that the largest bladed turbine shook violently during testing. This shows that at high wind speeds, turbines with larger blades are unstable. This may reduce the performance of the turbine, and it may break down. The data acquired shows that v 2 is small; less than v 1 (definitions on pg. 5). This suggests that the optimal ratio of width to length of blade (ratio of lift:drag is relatively high) is close to the ratios presented with the medium and large turbines. These width to length ratios for the medium and large bladed turbines (reduced) are 9:17 and 14:17, respectively. In studying how the angle of blade affected the performance of each turbine, it was found that the turbines with a blade angle of 30 degrees produced the most voltage and the highest turn frequency (Figures 1-6). This could be because it produced more lift, because the wind created a higher pressure difference rather than blowing right past the blade. In the turbines with blade angles of 45 and 60, less of a pressure difference may have been the result of the air moving past the blade, with less hindrance. And although the effect is minimal (because wind is always blowing past the blade instead of staying still), it also had less drag, because when the turbine turned, it could easily cut through the resistant air beside the blade

13 It was also found that if the blades of a turbine are placed at a lesser degree (30 degrees), then the difference of voltage values at different wind speeds are greater than if the blades were at a steeper degree (45 and 60). The researcher estimates that since turbines with blades at a lesser degree produce more energy, the effects of small changes or modifications (such as wind speeds) are magnified. It was reasoned that since the voltage is in direct relation to the turn frequency, the pictorial part of the graphs (excluding scale) of them for the same blade size would look about the same. This is supported by Figures 1-6. In conclusion, it was found that two main factors, lift and drag, affected the overall performance of each turbine (for definitions refer to pg. 1). Lift is the result of a greater surface area and the angle of the blade. A greater surface area gives the wind more area to push, creating a higher pressure on that side of the blade compared to the other side. This lift turns the turbine when the blades are angled. It was also found that if the blades are angled to face more towards the wind source (lesser degree to the forward-facing hub surface), it will produce more lift than if the blades were angled away from the wind source (steeper degree to the hub surface), letting the wind blow past the blade. In addition, if the blades are angled at a lesser degree when the turbine is turning, then the blades cut through the resistant air more efficiently. The other effect, drag, is also the result of greater surface area and the angle of the blades. A greater surface area creates more drag because the resistant air has more area to push against. The angle of the blade also has a significant effect on drag. If the blade is placed at a steeper angle and the turbine is turning, it will be less aerodynamically efficient

14 (more drag) than if the blades were placed at a lesser angle. This is because more surface area is exposed to the resistant air. As mentioned in the Discussion section of the Results (pg. 5), statistical t-tests were conducted on the data collected. A great majority of the p-values collected with the tests are 0 or less than.05, meaning that the data are statistically significant, and therefore the means significantly differ. However, as proven by the t-tests, the 60 degree turbine with a blade width of 4.5cm did not perform as well as expected (Table 4). The data collected with this turbine is presented in Figure 3 and 4. This drop in performance is possibly due to human error when constructing the turbines. The turbine could have blades that were too steep or the glass tubing at the rear of the hub could be crooked. Another example of human error could possibly be presented in the 45 degree turbine with a blade width of 7cm. This turbine did unusually well, and it was later found that the blades were not accurately placed at 45 degrees, but rather closer to 30 degrees. Comparison with Past Research As explained previously in the Past Research section (pg. 2), the researcher in conducted a similar study regarding wind turbines. In this study, the length and number of blades were varied to find the most efficient combination. After further interpretation of the data, the width to length ratios of the blade sizes were figured, and are presented in the following table along with the ratios used in the study. Year of Study Ratios of different sizes (width:length) :17 (best) 4.7:17 3.3: :17 9:17 (best) 14:

15 In the recent study, the best ratio of blade was 9:17. This is supported by the earlier study, in which the best ratio was 7:17. When compared to the differences of the other ratios, the difference of these ratios is minimal. This suggests that the optimal width to length ratio (lift:drag ratio is highest possible) is relatively close to these two ratios, 7:17 and 9:17. Future extensions of this project could study how the angle in which the wind strikes the blades of a turbine, the materials used in constructing the blades, and how the Bernoulli Effect affects voltage produced. Acknowledgement I would like to thank Mr. Blair, my science teacher at school, who let me use his books for research, and assisted me while I was gathering my data. I would like to thank my father for teaching me the concepts of t-tests and p-values, and finding them using MINITAB. Mr. Blair and my parents helped to edit my paper. Most of all, I would like to thank them all for supporting me throughout my project