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Development of small wind turbines for moving vehicles: Effects of flanged diffusers on rotor performance
Experimental Thermal and Fluid Science 42 (2012) 136–142
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Experimental Thermal and Fluid Science journal homepage: www.elsevier.com/locate/etfs
Development of small wind turbines for moving vehicles: Effects of flanged diffusers on rotor performance T.Y. Chen ⇑, Y.T. Liao, C.C. Cheng Department of Aerospace Engineering, Tamkang University, Taiwan, ROC
a r t i c l e
i n f o
Article history: Received 16 September 2011 Received in revised form 15 March 2012 Accepted 1 May 2012 Available online 17 May 2012 Keywords: Shrouded small horizontal-axis wind turbine Flanged diffuser Wind tunnel test
a b s t r a c t The main object of this research is to develop a shrouded, small, horizontal-axis wind turbine for moving vehicles. Specifically, this study investigates the effects of flanged diffusers on rotor performance of small (30 cm rotor diameter) wind turbines with different rotor solidities (20–60%) and wind speeds (10–20 m/ s). The experiments are conducted in a wind tunnel with and without a flanged diffuser. Results show that the flanged diffuser may significantly increase the power output, torque output, and rotor rotational speed of the wind turbine, largely depending on rotor solidity and wind speed. The higher the solidity and wind speed are, the smaller the effect of the flanged diffuser is. The 30%- and 40%-solidity rotors generate the largest power and torque outputs, respectively, while the 60%-solidity rotor has the lowest rotor rotational speed among the test rotors. These results provide some useful information when considering rotor-generator matching problems and the selection of rotor solidity for moving vehicles. This study also shows that a small wind turbine has the characteristics of low torque and high rotor rotational speed, and high rotor solidity for maximum power output compared to a conventional large wind turbine. Ó 2012 Elsevier Inc. All rights reserved.
1. Introduction The development and application of renewable, clean energy have become a very important issue in recent years due to the serious effects of global warming and rapid depletion of fossil fuel. Wind energy technologies have become one of the fastest growing energy sources in the world. Many factors play a role in the design of a horizontal-axis wind turbine (HAWT), including rotor aerodynamics, generator characteristics, rotor-generator matching, electrical output control and so on. Rotor aerodynamics plays a particularly important role in wind energy extraction. Rotor aerodynamics of wind turbines have been the subject of much research. Duquette and Visser [1] examined the effect of rotor solidity and blade number on aerodynamic performance, revealing that the range of tip speed ratio for maximum CP (power coefficient) varies strongly with solidity and weakly with blade number. Higher than traditional solidities and blade numbers result in higher CP. Increasing the solidity from the conventional 5–7% to 15–25% yields higher CP,max. Lanzafame and Messina [2] studied the performance of a double-pitch wind turbine with non-twisted blades. Results show that a non-twisted blade rotor has 15% less power output than a twisted one. The wind turbine they designed can effectively increase rotor power output compared with the traditional non-twisted blade rotor. ⇑ Corresponding author. E-mail address: [email protected] (T.Y. Chen). 0894-1777/$ - see front matter Ó 2012 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.expthermflusci.2012.05.001
Diffuser augmented wind turbines (DAWT) were a hot topic at the 1979 Wind Energy Innovative Systems Conference [3]. The diffuser or flanged diffuser generates separation regions behind it, where low-pressure regions appear to draw more wind through the rotors compared to a bare wind turbine. Wind power generation is proportional to the cube of the wind speed. A large increase in power output can be achieved with a slight increase in the velocity of the approaching wind to a wind turbine. Van Bussel’s study [3] shows that the power augmentation is proportional to the mass flow generated at the nozzle of the DAWT. Such mass flow augmentation can be achieved through two basic principles: increase in the diffuser exit ratio and/or by decreasing the negative back pressure at the exit. Comparison with a large amount of experimental data found in literatures shows that power augmentation factors above three have never been achieved. Matsushima et al. [4] studied the effect the diffuser’s shape had on wind speed. Results show that the wind speed in diffuser is greatly influenced by the length and expansion angle of the diffuser, and maximum wind speed increases 1.7 times with appropriate diffuser shape. Abe et al. [5] investigated the flow fields of a small wind turbine with a flanged diffuser, showing that the power coefficient of the shrouded wind turbine is about four times that of a bare wind turbine. Ohya et al. [6] examined the optimal form of the flanged diffuser, and demonstrated that power augmentation by a factor of about four to five, compared to a bare wind turbine. Wang et al. [7] investigated a convergent–divergent scoop effect on the power output of a small wind turbine. Results show
T.Y. Chen et al. / Experimental Thermal and Fluid Science 42 (2012) 136–142
current study investigates the aerodynamic characteristics of small, horizontal-axis wind turbine for moving vehicles. Thus, more than three-blade rotors (high-solidity rotors) will be adopted in the study. A series of studies are currently conducted to develop a shrouded, small, horizontal-axis wind turbine for moving vehicles, including the studies of turbine rotor performance, rotor-generator matching and electric power-output control. The generated electricity can be used to charge batteries to increase the endurance of electric vehicles, to supply electric power to vehicles to save energy. It can also provide DC power to charge portable electronics, to water electrolyzers for hydrogen production. This study applies a flanged diffuser on a small (30 cm rotor diameter) wind turbine, and focuses on the flanged diffuser’s effects on turbine power output, torque output, rotor rotational speed, and compares the rotor performance with different blade numbers and wind speeds.
that the scoop boosts the airflow speed and increases the power output of 2.2 times with the same swept area. Bet and Grassmann’s [8,9] results show that the power of a wind turbine is increased by a factor of two, through a wing structure placed at some distance around the turbine. Framlpvoc and Vrsalovic [10] designed a ring wing and placed it around a wind turbine, with its lower-pressure side pointed towards the center. The lift force on every part of the wing is directed radically centripetally. This forces a greater air mass flow to pass through the turbine and increases power efficiency up to 3.5 times. The investigations into the effect of diffusers on the performance of hydrokinetic turbines also received attention [11,12]. Similar to the results in DAWTs, these studies show that the use of diffusers is able to augment the power output. These previous studies focused on relatively large rotor diameters and low speeds, revealing that all the different-type diffusers are able to augment the power output. None of these researches discussed the diffuser effects on torque output and rotor rotational speed, which may be important to a small-size wind turbine. One of the conclusions from the 1979 Wind Energy Innovative Systems Conference was that power augmentations are possible by diffusers, but economic application of DAWT’s seemed not feasible because of the high costs of the configuration [3]. A technology status review by Khan el al. [13] also doubt whether duct augmentation is worth attempting in hydrokinetic turbines. In addition, the field tests conducted by Matsushima et al. [4] showed when the wind direction changes frequently, the diffuser may not make effective use of wind energy. Although several small DAWTs have recently entered the market [14,15], they are not designed for moving vehicles. A moving vehicle can easily induce high-speed wind, and usually induces more stable wind than natural wind. When applying a wind turbine in vehicles, the rotor diameter must be small for installation and drag considerations. Also, the wind turbine can be installed in right positions to avoid or minimize additional aerodynamic drag [16,17]. Thus, the DAWT is very suitable for moving vehicle applications either from economic or wind-source consideration. The rotor performance of a small DAWT may be different from a conventional large DAWT. The three-blade rotor has been adopted extensively for large commercial horizontal-axis wind turbines. However, rotors with more than three blades have been used in small (rotor diameter of a few meters) commercial horizontal-axis wind turbine [14,15]. The results by Duquette and Visser [1] indicated that higher than traditional solidities and blade numbers result in higher CP for small rotor diameters. Increasing the solidity from the conventional 5–7% to 15–25% yields higher CP,max. The
2. Experimental setup and methods Figs. 1 and 2 present a schematic of the wind tunnel system and photographs of the primary experimental setup utilized in this study, respectively. A 9 m long, open-circuit wind tunnel was developed for the present study. The test section of the wind tunnel is 1.3 m high, 1.3 m wide and 3 m long. A 60 hp blower was used to drive the airflow, with the maximum air speed 25 m/s in the test section. The flow turbulence intensity was less than 1%, and the flow uniformity was greater than 99%. Turbine blades were attached to a cone hub with a base diameter of 6 cm, which was connected to a measurement apparatus, including a torque sensor, a rotational-speed sensor, and a magnetic particle brake by Chain-Tail Co., LTD, Taiwan [18]. The torque sensor measures the torque range from 0 to 1 N-m with the uncertainty of 0.1%, the rotational-speed sensor measures the rotor speed up to 6000 rpm (revolution per minute) with the uncertainty of 1%. The magnetic particle brake utilizes electromagnetic powder to transmit torque, which simulates loadings on the rotor. When the voltage is applied to the brake, the torque is generated, which can be adjusted by the applied voltage. The measurement apparatus was placed on a support located in the middle of the tunnel test section. A pitot-static tube, placed 1 m upstream of the rotor, measured the total and static pressures of moving air by a digital pressure sensor. Local pressure, temperature and humidity were also measured to account for density variation during each run. The free-stream air velocities were calculated using Bernoulli’s equation. Repeatability tests indicate that the uncertainty due to the measurement system on rotor performances is less than 6%.
digital pressure meter
Pitot-static tube inlet
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torque and rotating speed measurement systems blade
support
Fig. 1. A schematic of the wind tunnel system.
blower
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30 cm diameter 6-blade rotor
1.3m x 1.3 m Test section
hub
blade
support
(a) The cross-sectional view of the test section torque & rotational speed sensor 6-blade rotor
magnetic-particle brake
connector Fig. 4. The Cp-TSR relations for rotors with different solidities at 12 m/s without a flanged diffuser.
(b) The measurement systems
Air in
flange
diffuser
(c) The flanged diffuser Fig. 2. Photographs of the primary experimental setup.
root, respectively, where b is the angle between the blade chord line and rotor rotating plane. The NACA 4415 airfoil has good aerodynamic performance [19], and a twisted blade yields better performance than a non-twisted one [2]. The 14-cm blades were inserted into the cone hub, resulting in the rotor diameter of 30 cm. Each blade has root and tip chord lengths of 2.15 cm and 3.31 cm, respectively. The solidity of each blade was around 5%. Thus, a six-blade rotor had a solidity of 30% and a twelve-blade rotor had a solidity of 60%. The solidity is defined as the ratio between the total blade area and the rotor swept area. The flanged diffuser (Fig. 2C) had a 30-cm inlet diameter, 10-cm length, 30° diffusion angle and 3-cm flange height. The inlet to outlet area ratio of the diffuser is around 52%. This geometry is not optimized for maximum wind turbine power output. It is a size consideration for installation. Also, this study mainly focuses on flanged diffuser’s effects on rotor performance with different rotor solidities and wind speeds. The rotor was placed in the middle of the diffuser for the study. No tunnel blockage correction was made
Fig. 3. The Cp-TSR relations for 30%-solidity rotor at different wind speeds without a flanged diffuser.
The blade cross-sectional profile was NACA 4415 and each blade was twisted 35° with pitch angles (b) of 5° and 40° at blade tip and
Fig. 5. The Cp-TSR relations for 30%-solidity rotor at different wind speeds with a flanged diffuser.
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T.Y. Chen et al. / Experimental Thermal and Fluid Science 42 (2012) 136–142 Table 1 The CP,max, CT,max and RPM at CP,max for 30%-solidity rotor. Wind speed
10 m/s
12 m/s
14 m/s
16 m/s
18 m/s
20 m/s
Flanged diffuser
No
Yes
No
Yes
No
Yes
No
Yes
No
Yes
No
Yes
CP,max CT,max RPM at CP,max
0.231 0.069 1557
0.609 0.187 1703
0.269 0.095 1845
0.674 0.247 2039
0.298 0.117 2085
0.714 0.265 2311
0.322 0.138 2307
0.730 0.274 2689
0.358 0.150 2495
0.756 0.285 2948
0.401 0.165 2681
0.773 0.290 3251
Flanged diffuser increases CP,max and CT,max by CP,max CT,max
164% 170%
151% 158%
since the maximum tunnel blockage ratio of the rotor and flanged diffuser is less than 10% [20]. 3. Results and discussion 3.1. Rotor performance without a flanged diffuser Fig. 3 presents typically the relations between the power coefficient (CP) and the tip speed ratio (TSR) for 30%-solidity rotor (six-blade) at different wind speeds without the flanged diffuser, where CP ¼ P=ð0:5qpR2 U30 Þ, P is actual power output, q is air density, R is rotor radius, U0 is free-stream wind speed, TSR is the ratio between the blade tip speed and wind speed. This figure shows that CP increases with wind speed, indicating the higher the wind speed, the higher the turbine’s efficiency. High wind speed means high air-collision frequency with turbine blades, resulting in high energy extraction. The Cp,max and TSR for the investigated small wind turbine (ex. Cp,max = 0.269 at 12 m/s) are smaller than a conventional (large) wind turbine which usually has Cp,max around 0.3–0.5 at 12 m/s. The smaller the wind turbine, the less air collides with the blade, resulting in low energy extraction. The distributions of the torque coefficient (CT) vs. TSR for different wind speeds of 30%-solidity rotor are qualitatively similar to Cp vs. TSR, where CT ¼ T=ð0:5qpR3 U20 Þ, T is the torque output. The CT,maxs for different wind speeds are summarized in Table 1, which shows the CT,max increases with wind speed and the values are small, compared to conventional wind turbines, because of the short rotor diameter. Fig. 4 presents CP vs. TSR for rotors with different solidities at 12 m/s without a flanged diffuser. Results show the 40%-solidity rotor has highest Cp,max among the test rotors, but the differences are not large among different rotor solidities. Similar results were obtained at other wind speeds. Conventional wind turbines usually have higher CP,max for rotor solidity of less than 10%. Duquette and Visser [1] also showed the solidity of 15–25% yielded higher CP,max with rotor radius of 1 m. These results suggest that the smaller the rotor diameter, the larger the solidity for a higher CP,max. There exists an optimum wind-turbine solidity range for the best wind-energy extraction. If the solidity is too small, not much wind energy can be captured. On the contrary, if the solidity is too large, the blades may blur, less air particles pass through the blades, and less wind energy is captured. Fig. 4 also reveals that the 60%-solidity rotor has the lowest TSR among the test rotors, indicating it has the lowest rotor rotational speed to achieve CP,max.
139% 126%
126% 99%
111% 90%
93% 76%
increases. The rotor rotational speeds at CP,max are 2039 rpm and 3251 rpm for the investigated wind speeds of 10 m/s and 20 m/s, respectively. The power output is up to 260 W for the wind speed of 20 m/s. These results indicate that a small wind turbine has much higher rotor rotational speed, compared to a conventional wind turbine, which is the main cause of the power output. The flanged diffuser’s effects on torque output are qualitatively similar to those of power output. The CT,maxs for different wind speeds are shown in Table 1. The CT,maxs increase, but also not largely, with the wind speed. For example, the CT,maxs are approximately 0.25 and 0.29 for the wind speeds of 12 m/s and 20 m/s, respectively.
3.3. Comparisons of rotor performance with/without a flanged diffuser There are apparent differences between the rotors with and without a flanged diffuser. First, the CP,maxs for the rotor with a flanged diffuser are much larger than those without one. A preliminary numerical investigation using FLUENT software studied flow fields around the investigated flanged diffuser. Fig. 7 presents the contours of velocity magnitude in the vicinity of a flanged diffuser for the free-stream velocities of 10 m/s and 20 m/s. The separation regions are clearly seen inside the diffusers and behind the flanges, which create pressure differences between the inlet and outlet of the flanged diffusers. More air is drawn into the flanged diffusers and the flows accelerate inside them. Since the power output is cubed proportional to the wind speed, the CP,maxs for the rotor with a flanged diffuser should be much larger than those
3.2. Rotor performance with a flanged diffuser Figs. 5 and 6 present CP vs. TSR and P vs. RPM (rotor rotational speed), respectively, for 30%-solidity rotor at different wind speeds with a flanged diffuser. The CP,maxs also increase, but not largely, with the wind speed. For example, the CP,maxs are approximately 0.674 and 0.773 for the wind speeds of 12 m/s and 20 m/s, respectively. The rotor rotational speed largely increases as the wind speed
Fig. 6. The power-RPM relations for 30%-solidity rotor at different wind speeds with a flanged diffuser.
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Fig. 7. Contours of velocity magnitude (m/s) for 10 m/s (a) and 20 m/s (b) free-stream velocities in the vicinity of a flanged diffuser.
without one. It is also noted from Fig. 5 that the CP,maxs exceed Betz’ limit (0.593) for all investigated wind speeds. Second, the CP,maxs and CT,maxs for rotors without a flanged diffuser are much dependent on wind speed compared to rotors with a flanged diffuser. For example, for rotors without a flanged diffuser, the CP,max is 0.269 at 12 m/s, and 0.401 at 20 m/s; the CT,max is 0.095 at 12 m/s, and 0.165 at 20 m/s, see Table 1. The increase in CP,max and CT,max are around 49% and 73.7%, respectively. For rotors with a flanged diffuser, the CP,max is 0.674 at 12 m/s, and 0.773 at 20 m/s; the CT,max is 0.247 at 12 m/s, and 0.29 at 20 m/s. The increase in CP,max and CT,max are only around 14.7% and 17.4%. In the case of rotors without a flanged diffuser, wind energy extraction and torque production by the rotor owe solely to the blade’s effect. The efficiency of energy extraction and the torque production largely increase with wind speed since the rotor rotational speed at high wind speed is much greater than that at low wind speed. Consequently, CP,max and CT,max are largely dependent on wind speed. On the other hand, wind energy extraction and torque production by the rotor is due to the effects of both the blade and flanged diffuser for rotors with a flanged diffuser. Fig. 7 reveals that the maximum flow velocities inside the diffusers are about 15 m/s and 30 m/s for the 10 m/s and 20 m/s free-stream velocities, respectively. The flanged diffuser increases wind speed inside the diffuser by around 50% for both free-stream velocities. Since the power output is cubed proportional to the wind speed,
the flanged diffuser has an important effect on CP and CT. Fig. 7 also shows that the separation regions and the flow accelerations behaviors inside diffusers are very similar between the 10 m/s and 20 m/s free-stream velocities. This result suggests that the flanged diffuser effect on rotor performance is less dependent on wind speed. The combined effects of the blade and flanged diffuser on energy extraction and torque production cause the CP,maxs and CT,maxs for rotors with a flanged diffuser decreases dependence on wind speed.
3.4. Results of rotor performance for different rotor solidities Since the rotor performances of other rotor solidities are qualitatively similar to that of the 30% rotor solidity, the performance figures are not presented. Tables 1–4 summarize the CP,max, CT,max and rotor rotational speed at CP,max for the 30%-, 40%-, 45%- and 60%-solidity rotors under different test conditions, respectively. These tables show that the values of Cp,maxs for different solidity rotors without the flanged diffuser are very closed to one another at the same wind speeds. It is known that the Cp is related to the air particle collision rate to the blade, which should be closely related to the wind speed and rotor solidity. The Cp,maxs for different solidity rotors without the flanged diffuser are largely dependent on the wind speed but less on the rotor solidity. This may be due
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T.Y. Chen et al. / Experimental Thermal and Fluid Science 42 (2012) 136–142 Table 2 The CP,max, CT,max and RPM at CP,max for40%-solidity rotor. Wind speed
10 m/s
12 m/s
14 m/s
16 m/s
18 m/s
20 m/s
Flanged diffuser
No
Yes
No
Yes
No
Yes
No
Yes
No
Yes
No
Yes
CP,max CT,max RPM at CP,max
0.235 0.133 1262
0.525 0.274 1450
0.272 0.158 1450
0. 595 0.305 1727
0.309 0.180 1685
0.667 0.34 1995
0.331 0.203 1900
0.695 0.368 2256
0.364 0.227 2135
0.734 0.385 2480
0.405 0.252 2381
0.755 0.409 2651
Flanged diffuser increases CP,max and CT,max by CP,max CT,max
124% 106%
118% 93%
115% 88%
110% 81%
101% 69.6%
86.5% 62.3%
Table 3 The CP,max, CT,max and RPM at CP,max for 45%-solidity rotor. Wind speed
10 m/s
Flanged diffuser
No
Yes
12 m/s No
Yes
14 m/s No
Yes
16 m/s No
Yes
18 m/s No
Yes
20 m/s No
Yes
CP,max CT,max RPM at CP,max
0.232 0.144 1192
0.487 0.276 1430
0.265 0.167 1380
0. 52 0.301 1678
0.298 0.188 1602
0.574 0.33 1905
0.323 0.210 1800
0.602 0.358 2056
0.364 0.238 2010
0.663 0.379 2250
0.405 0.261 2180
0.712 0.401 2445
Flanged diffuser increases CP,max and CT,max by CP,max CT,max
110% 91.6%
96.2% 80.2%
92.6% 75.5%
86.4% 70%
82% 59.2%
75.8% 53.6%
Table 4 The CP,max, CT,max and RPM at CP,max for 60%-solidity rotor. Wind speed
10 m/s
12 m/s
14 m/s
16 m/s
18 m/s
20 m/s
Flanged diffuser
No
Yes
No
Yes
No
Yes
No
Yes
No
Yes
No
Yes
CP,max CT,max RPM at CP,max
0.228 0.169 1077
0.428 0.275 1295
0.262 0.187 1305
0.461 0.295 1450
0.293 0.202 1508
0.499 0.311 1626
0.321 0.225 1581
0.529 0.338 1775
0.351 0.252 1635
0.552 0.363 1884
0.399 0.285 1770
0.595 0.395 1995
Flanged diffuser increases CP,max and CT,max by CP,max CT,max
88% 62.7%
76% 57.8%
to the small rotor diameter, and thus, the collision rate is dominated by the wind speed. Table 1 indicates that the flanged diffuser increases CP,max by 164% at 10 m/s and 111% at 18 m/s, The flanged diffuser’s effect apparently plays a more important role in wind energy extraction than the blade effect for the 30%-solidity rotor at wind speeds less than 20 m/s. Table 2 shows that the flanged diffuser increases CP,max by 124% at 10 m/s and 101% at 18 m/s. The flanged diffuser’s effect plays slightly more important role in wind energy extraction than the blade effect for the 40%-solidity rotor at wind speeds less than 20 m/s. Tables 3 and 4 show that the power and torque augmentations are mostly less than 100%. In this case, the blade effect plays a more important role in wind energy extraction and torque production than the flanged diffuser’s effect for the 45%- and 60%solidity rotors. These tables also indicate that the flanged diffuser’s effects on CP,max and CT,max decrease as the rotor solidity and wind speed increase. The flanged diffuser’s effect on the 30%-solidity rotor is the largest among the test rotors. Also, the flanged diffuser increases the rotor rotational speed for all the test rotors. A flanged diffuser creates a pressure difference between the inlet and outlet of it. Placing rotating blades inside the diffuser will induce drag, reducing this pressure difference. The higher the rotor solidity and rotational speed are, the smaller this pressure difference is. It is noted that the bare rotor of 30% solidity has the fewest blades, but the highest rotor rotational speed, among the test bare rotors. This result suggests that flanged diffuser’s effect on rotor performance is
70.3% 54%
64.8% 50.2%
57.3% 44%
49.1% 38.6%
more dominated by the rotor solidity than the rotor rotational speed. Comparison of these tables shows that the 60%-solidity rotor without a flanged diffuser has the lowest rotational speed and the highest torque output among the test rotors, which confirms the existing knowledge. As noted above, the flanged diffuser’s effect on rotor performance decreases as the rotor solidity increases. The 60%-solidity rotor with a flanged diffuser has the lowest rotational speed, but do not have the largest CT,maxs, among the test rotors. The 40%-solidity rotor without a flanged diffuser has less torque outputs than the 60%-solidity rotor, but it has larger flanged diffuser’ effect than the 60%-solidity rotor. The combined effect causes that the 40%-solidity rotor has the largest CTs. It has indicated in this paper that a small wind turbine has the characteristics of low power and torque outputs, and high rotor rotational speed compared to a conventional wind turbine. A generator usually requires a large torque to generate high electric power, and low rotational speed for dynamic balance consideration and so on. Thus, the torque output and rotor rotational speed of a small wind turbine should play a more important role in rotor-generator matching than the power output. This study provides some useful information when considering the rotor-generator matching problems. The 30%-solidity and 40%-solidity rotors generate the largest power and torque outputs, respectively, but also have high rotor rotational speeds. The 60%-solidity rotor has the smallest power and torque outputs but also the lowest rotational speed among the test rotors. The selection of rotor solidity should depend
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on the generator characteristics, safety and dynamic balance considerations, and so onto achieve an optimal performance for moving vehicles. 4. Conclusions This paper studies the flanged diffuser’s effects on rotor performance of small wind turbines with rotor solidities between 20% and 60% at wind speeds between 10 m/s and 20 m/s. Results show that the flanged diffuser may significantly increase the power and torque outputs, and rotational speed of the wind turbine, depending on the rotor solidity and wind speed. The higher the solidity and wind speed are, the smaller the flanged diffuser’s effect is. The flanged diffuser’s effect on the 30%-solidity rotor is the largest among the test rotors. For the 30%- and 40%-solidity rotors, the flanged diffuser’s effect plays a more important role in wind energy extraction than the blade’s effect at wind speeds less than 20 m/s. For the 45%- and 60%-solidity rotors, the blade’s effect, in general, plays a more important role in wind energy extraction than the flanged diffuser’s effect. Results of this study also show that the 30%- and 40%-solidity rotors generate the largest power and torque outputs, respectively, while the 60%-solidity rotor has the lowest rotor rotational speed among the test rotors. The selection of rotor solidity should depend on the generator characteristics, safety and dynamic balance considerations, and so onto achieve an optimal performance for moving vehicles. This study also shows that the rotor solidity to achieve highest CP of the investigated small wind turbine is around 35–40%, which is different from a conventional large wind turbine of less than 10%. Duquette and Visser [1] showed the solidity of 15–25% yielded higher CP,max with rotor radius of 1 m. These results suggest that the smaller the rotor diameter, the larger the rotor solidity should be adopted for a higher CP,max. Acknowledgment This research was sponsored by the National Science Council of the Republic of China under contract NSC98-2221-E-032-042MY3.
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Experimental Thermal and Fluid Science journal homepage: www.elsevier.com/locate/etfs
Development of small wind turbines for moving vehicles: Effects of flanged diffusers on rotor performance T.Y. Chen ⇑, Y.T. Liao, C.C. Cheng Department of Aerospace Engineering, Tamkang University, Taiwan, ROC
a r t i c l e
i n f o
Article history: Received 16 September 2011 Received in revised form 15 March 2012 Accepted 1 May 2012 Available online 17 May 2012 Keywords: Shrouded small horizontal-axis wind turbine Flanged diffuser Wind tunnel test
a b s t r a c t The main object of this research is to develop a shrouded, small, horizontal-axis wind turbine for moving vehicles. Specifically, this study investigates the effects of flanged diffusers on rotor performance of small (30 cm rotor diameter) wind turbines with different rotor solidities (20–60%) and wind speeds (10–20 m/ s). The experiments are conducted in a wind tunnel with and without a flanged diffuser. Results show that the flanged diffuser may significantly increase the power output, torque output, and rotor rotational speed of the wind turbine, largely depending on rotor solidity and wind speed. The higher the solidity and wind speed are, the smaller the effect of the flanged diffuser is. The 30%- and 40%-solidity rotors generate the largest power and torque outputs, respectively, while the 60%-solidity rotor has the lowest rotor rotational speed among the test rotors. These results provide some useful information when considering rotor-generator matching problems and the selection of rotor solidity for moving vehicles. This study also shows that a small wind turbine has the characteristics of low torque and high rotor rotational speed, and high rotor solidity for maximum power output compared to a conventional large wind turbine. Ó 2012 Elsevier Inc. All rights reserved.
1. Introduction The development and application of renewable, clean energy have become a very important issue in recent years due to the serious effects of global warming and rapid depletion of fossil fuel. Wind energy technologies have become one of the fastest growing energy sources in the world. Many factors play a role in the design of a horizontal-axis wind turbine (HAWT), including rotor aerodynamics, generator characteristics, rotor-generator matching, electrical output control and so on. Rotor aerodynamics plays a particularly important role in wind energy extraction. Rotor aerodynamics of wind turbines have been the subject of much research. Duquette and Visser [1] examined the effect of rotor solidity and blade number on aerodynamic performance, revealing that the range of tip speed ratio for maximum CP (power coefficient) varies strongly with solidity and weakly with blade number. Higher than traditional solidities and blade numbers result in higher CP. Increasing the solidity from the conventional 5–7% to 15–25% yields higher CP,max. Lanzafame and Messina [2] studied the performance of a double-pitch wind turbine with non-twisted blades. Results show that a non-twisted blade rotor has 15% less power output than a twisted one. The wind turbine they designed can effectively increase rotor power output compared with the traditional non-twisted blade rotor. ⇑ Corresponding author. E-mail address: [email protected] (T.Y. Chen). 0894-1777/$ - see front matter Ó 2012 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.expthermflusci.2012.05.001
Diffuser augmented wind turbines (DAWT) were a hot topic at the 1979 Wind Energy Innovative Systems Conference [3]. The diffuser or flanged diffuser generates separation regions behind it, where low-pressure regions appear to draw more wind through the rotors compared to a bare wind turbine. Wind power generation is proportional to the cube of the wind speed. A large increase in power output can be achieved with a slight increase in the velocity of the approaching wind to a wind turbine. Van Bussel’s study [3] shows that the power augmentation is proportional to the mass flow generated at the nozzle of the DAWT. Such mass flow augmentation can be achieved through two basic principles: increase in the diffuser exit ratio and/or by decreasing the negative back pressure at the exit. Comparison with a large amount of experimental data found in literatures shows that power augmentation factors above three have never been achieved. Matsushima et al. [4] studied the effect the diffuser’s shape had on wind speed. Results show that the wind speed in diffuser is greatly influenced by the length and expansion angle of the diffuser, and maximum wind speed increases 1.7 times with appropriate diffuser shape. Abe et al. [5] investigated the flow fields of a small wind turbine with a flanged diffuser, showing that the power coefficient of the shrouded wind turbine is about four times that of a bare wind turbine. Ohya et al. [6] examined the optimal form of the flanged diffuser, and demonstrated that power augmentation by a factor of about four to five, compared to a bare wind turbine. Wang et al. [7] investigated a convergent–divergent scoop effect on the power output of a small wind turbine. Results show
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current study investigates the aerodynamic characteristics of small, horizontal-axis wind turbine for moving vehicles. Thus, more than three-blade rotors (high-solidity rotors) will be adopted in the study. A series of studies are currently conducted to develop a shrouded, small, horizontal-axis wind turbine for moving vehicles, including the studies of turbine rotor performance, rotor-generator matching and electric power-output control. The generated electricity can be used to charge batteries to increase the endurance of electric vehicles, to supply electric power to vehicles to save energy. It can also provide DC power to charge portable electronics, to water electrolyzers for hydrogen production. This study applies a flanged diffuser on a small (30 cm rotor diameter) wind turbine, and focuses on the flanged diffuser’s effects on turbine power output, torque output, rotor rotational speed, and compares the rotor performance with different blade numbers and wind speeds.
that the scoop boosts the airflow speed and increases the power output of 2.2 times with the same swept area. Bet and Grassmann’s [8,9] results show that the power of a wind turbine is increased by a factor of two, through a wing structure placed at some distance around the turbine. Framlpvoc and Vrsalovic [10] designed a ring wing and placed it around a wind turbine, with its lower-pressure side pointed towards the center. The lift force on every part of the wing is directed radically centripetally. This forces a greater air mass flow to pass through the turbine and increases power efficiency up to 3.5 times. The investigations into the effect of diffusers on the performance of hydrokinetic turbines also received attention [11,12]. Similar to the results in DAWTs, these studies show that the use of diffusers is able to augment the power output. These previous studies focused on relatively large rotor diameters and low speeds, revealing that all the different-type diffusers are able to augment the power output. None of these researches discussed the diffuser effects on torque output and rotor rotational speed, which may be important to a small-size wind turbine. One of the conclusions from the 1979 Wind Energy Innovative Systems Conference was that power augmentations are possible by diffusers, but economic application of DAWT’s seemed not feasible because of the high costs of the configuration [3]. A technology status review by Khan el al. [13] also doubt whether duct augmentation is worth attempting in hydrokinetic turbines. In addition, the field tests conducted by Matsushima et al. [4] showed when the wind direction changes frequently, the diffuser may not make effective use of wind energy. Although several small DAWTs have recently entered the market [14,15], they are not designed for moving vehicles. A moving vehicle can easily induce high-speed wind, and usually induces more stable wind than natural wind. When applying a wind turbine in vehicles, the rotor diameter must be small for installation and drag considerations. Also, the wind turbine can be installed in right positions to avoid or minimize additional aerodynamic drag [16,17]. Thus, the DAWT is very suitable for moving vehicle applications either from economic or wind-source consideration. The rotor performance of a small DAWT may be different from a conventional large DAWT. The three-blade rotor has been adopted extensively for large commercial horizontal-axis wind turbines. However, rotors with more than three blades have been used in small (rotor diameter of a few meters) commercial horizontal-axis wind turbine [14,15]. The results by Duquette and Visser [1] indicated that higher than traditional solidities and blade numbers result in higher CP for small rotor diameters. Increasing the solidity from the conventional 5–7% to 15–25% yields higher CP,max. The
2. Experimental setup and methods Figs. 1 and 2 present a schematic of the wind tunnel system and photographs of the primary experimental setup utilized in this study, respectively. A 9 m long, open-circuit wind tunnel was developed for the present study. The test section of the wind tunnel is 1.3 m high, 1.3 m wide and 3 m long. A 60 hp blower was used to drive the airflow, with the maximum air speed 25 m/s in the test section. The flow turbulence intensity was less than 1%, and the flow uniformity was greater than 99%. Turbine blades were attached to a cone hub with a base diameter of 6 cm, which was connected to a measurement apparatus, including a torque sensor, a rotational-speed sensor, and a magnetic particle brake by Chain-Tail Co., LTD, Taiwan [18]. The torque sensor measures the torque range from 0 to 1 N-m with the uncertainty of 0.1%, the rotational-speed sensor measures the rotor speed up to 6000 rpm (revolution per minute) with the uncertainty of 1%. The magnetic particle brake utilizes electromagnetic powder to transmit torque, which simulates loadings on the rotor. When the voltage is applied to the brake, the torque is generated, which can be adjusted by the applied voltage. The measurement apparatus was placed on a support located in the middle of the tunnel test section. A pitot-static tube, placed 1 m upstream of the rotor, measured the total and static pressures of moving air by a digital pressure sensor. Local pressure, temperature and humidity were also measured to account for density variation during each run. The free-stream air velocities were calculated using Bernoulli’s equation. Repeatability tests indicate that the uncertainty due to the measurement system on rotor performances is less than 6%.
digital pressure meter
Pitot-static tube inlet
137
torque and rotating speed measurement systems blade
support
Fig. 1. A schematic of the wind tunnel system.
blower
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30 cm diameter 6-blade rotor
1.3m x 1.3 m Test section
hub
blade
support
(a) The cross-sectional view of the test section torque & rotational speed sensor 6-blade rotor
magnetic-particle brake
connector Fig. 4. The Cp-TSR relations for rotors with different solidities at 12 m/s without a flanged diffuser.
(b) The measurement systems
Air in
flange
diffuser
(c) The flanged diffuser Fig. 2. Photographs of the primary experimental setup.
root, respectively, where b is the angle between the blade chord line and rotor rotating plane. The NACA 4415 airfoil has good aerodynamic performance [19], and a twisted blade yields better performance than a non-twisted one [2]. The 14-cm blades were inserted into the cone hub, resulting in the rotor diameter of 30 cm. Each blade has root and tip chord lengths of 2.15 cm and 3.31 cm, respectively. The solidity of each blade was around 5%. Thus, a six-blade rotor had a solidity of 30% and a twelve-blade rotor had a solidity of 60%. The solidity is defined as the ratio between the total blade area and the rotor swept area. The flanged diffuser (Fig. 2C) had a 30-cm inlet diameter, 10-cm length, 30° diffusion angle and 3-cm flange height. The inlet to outlet area ratio of the diffuser is around 52%. This geometry is not optimized for maximum wind turbine power output. It is a size consideration for installation. Also, this study mainly focuses on flanged diffuser’s effects on rotor performance with different rotor solidities and wind speeds. The rotor was placed in the middle of the diffuser for the study. No tunnel blockage correction was made
Fig. 3. The Cp-TSR relations for 30%-solidity rotor at different wind speeds without a flanged diffuser.
The blade cross-sectional profile was NACA 4415 and each blade was twisted 35° with pitch angles (b) of 5° and 40° at blade tip and
Fig. 5. The Cp-TSR relations for 30%-solidity rotor at different wind speeds with a flanged diffuser.
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T.Y. Chen et al. / Experimental Thermal and Fluid Science 42 (2012) 136–142 Table 1 The CP,max, CT,max and RPM at CP,max for 30%-solidity rotor. Wind speed
10 m/s
12 m/s
14 m/s
16 m/s
18 m/s
20 m/s
Flanged diffuser
No
Yes
No
Yes
No
Yes
No
Yes
No
Yes
No
Yes
CP,max CT,max RPM at CP,max
0.231 0.069 1557
0.609 0.187 1703
0.269 0.095 1845
0.674 0.247 2039
0.298 0.117 2085
0.714 0.265 2311
0.322 0.138 2307
0.730 0.274 2689
0.358 0.150 2495
0.756 0.285 2948
0.401 0.165 2681
0.773 0.290 3251
Flanged diffuser increases CP,max and CT,max by CP,max CT,max
164% 170%
151% 158%
since the maximum tunnel blockage ratio of the rotor and flanged diffuser is less than 10% [20]. 3. Results and discussion 3.1. Rotor performance without a flanged diffuser Fig. 3 presents typically the relations between the power coefficient (CP) and the tip speed ratio (TSR) for 30%-solidity rotor (six-blade) at different wind speeds without the flanged diffuser, where CP ¼ P=ð0:5qpR2 U30 Þ, P is actual power output, q is air density, R is rotor radius, U0 is free-stream wind speed, TSR is the ratio between the blade tip speed and wind speed. This figure shows that CP increases with wind speed, indicating the higher the wind speed, the higher the turbine’s efficiency. High wind speed means high air-collision frequency with turbine blades, resulting in high energy extraction. The Cp,max and TSR for the investigated small wind turbine (ex. Cp,max = 0.269 at 12 m/s) are smaller than a conventional (large) wind turbine which usually has Cp,max around 0.3–0.5 at 12 m/s. The smaller the wind turbine, the less air collides with the blade, resulting in low energy extraction. The distributions of the torque coefficient (CT) vs. TSR for different wind speeds of 30%-solidity rotor are qualitatively similar to Cp vs. TSR, where CT ¼ T=ð0:5qpR3 U20 Þ, T is the torque output. The CT,maxs for different wind speeds are summarized in Table 1, which shows the CT,max increases with wind speed and the values are small, compared to conventional wind turbines, because of the short rotor diameter. Fig. 4 presents CP vs. TSR for rotors with different solidities at 12 m/s without a flanged diffuser. Results show the 40%-solidity rotor has highest Cp,max among the test rotors, but the differences are not large among different rotor solidities. Similar results were obtained at other wind speeds. Conventional wind turbines usually have higher CP,max for rotor solidity of less than 10%. Duquette and Visser [1] also showed the solidity of 15–25% yielded higher CP,max with rotor radius of 1 m. These results suggest that the smaller the rotor diameter, the larger the solidity for a higher CP,max. There exists an optimum wind-turbine solidity range for the best wind-energy extraction. If the solidity is too small, not much wind energy can be captured. On the contrary, if the solidity is too large, the blades may blur, less air particles pass through the blades, and less wind energy is captured. Fig. 4 also reveals that the 60%-solidity rotor has the lowest TSR among the test rotors, indicating it has the lowest rotor rotational speed to achieve CP,max.
139% 126%
126% 99%
111% 90%
93% 76%
increases. The rotor rotational speeds at CP,max are 2039 rpm and 3251 rpm for the investigated wind speeds of 10 m/s and 20 m/s, respectively. The power output is up to 260 W for the wind speed of 20 m/s. These results indicate that a small wind turbine has much higher rotor rotational speed, compared to a conventional wind turbine, which is the main cause of the power output. The flanged diffuser’s effects on torque output are qualitatively similar to those of power output. The CT,maxs for different wind speeds are shown in Table 1. The CT,maxs increase, but also not largely, with the wind speed. For example, the CT,maxs are approximately 0.25 and 0.29 for the wind speeds of 12 m/s and 20 m/s, respectively.
3.3. Comparisons of rotor performance with/without a flanged diffuser There are apparent differences between the rotors with and without a flanged diffuser. First, the CP,maxs for the rotor with a flanged diffuser are much larger than those without one. A preliminary numerical investigation using FLUENT software studied flow fields around the investigated flanged diffuser. Fig. 7 presents the contours of velocity magnitude in the vicinity of a flanged diffuser for the free-stream velocities of 10 m/s and 20 m/s. The separation regions are clearly seen inside the diffusers and behind the flanges, which create pressure differences between the inlet and outlet of the flanged diffusers. More air is drawn into the flanged diffusers and the flows accelerate inside them. Since the power output is cubed proportional to the wind speed, the CP,maxs for the rotor with a flanged diffuser should be much larger than those
3.2. Rotor performance with a flanged diffuser Figs. 5 and 6 present CP vs. TSR and P vs. RPM (rotor rotational speed), respectively, for 30%-solidity rotor at different wind speeds with a flanged diffuser. The CP,maxs also increase, but not largely, with the wind speed. For example, the CP,maxs are approximately 0.674 and 0.773 for the wind speeds of 12 m/s and 20 m/s, respectively. The rotor rotational speed largely increases as the wind speed
Fig. 6. The power-RPM relations for 30%-solidity rotor at different wind speeds with a flanged diffuser.
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Fig. 7. Contours of velocity magnitude (m/s) for 10 m/s (a) and 20 m/s (b) free-stream velocities in the vicinity of a flanged diffuser.
without one. It is also noted from Fig. 5 that the CP,maxs exceed Betz’ limit (0.593) for all investigated wind speeds. Second, the CP,maxs and CT,maxs for rotors without a flanged diffuser are much dependent on wind speed compared to rotors with a flanged diffuser. For example, for rotors without a flanged diffuser, the CP,max is 0.269 at 12 m/s, and 0.401 at 20 m/s; the CT,max is 0.095 at 12 m/s, and 0.165 at 20 m/s, see Table 1. The increase in CP,max and CT,max are around 49% and 73.7%, respectively. For rotors with a flanged diffuser, the CP,max is 0.674 at 12 m/s, and 0.773 at 20 m/s; the CT,max is 0.247 at 12 m/s, and 0.29 at 20 m/s. The increase in CP,max and CT,max are only around 14.7% and 17.4%. In the case of rotors without a flanged diffuser, wind energy extraction and torque production by the rotor owe solely to the blade’s effect. The efficiency of energy extraction and the torque production largely increase with wind speed since the rotor rotational speed at high wind speed is much greater than that at low wind speed. Consequently, CP,max and CT,max are largely dependent on wind speed. On the other hand, wind energy extraction and torque production by the rotor is due to the effects of both the blade and flanged diffuser for rotors with a flanged diffuser. Fig. 7 reveals that the maximum flow velocities inside the diffusers are about 15 m/s and 30 m/s for the 10 m/s and 20 m/s free-stream velocities, respectively. The flanged diffuser increases wind speed inside the diffuser by around 50% for both free-stream velocities. Since the power output is cubed proportional to the wind speed,
the flanged diffuser has an important effect on CP and CT. Fig. 7 also shows that the separation regions and the flow accelerations behaviors inside diffusers are very similar between the 10 m/s and 20 m/s free-stream velocities. This result suggests that the flanged diffuser effect on rotor performance is less dependent on wind speed. The combined effects of the blade and flanged diffuser on energy extraction and torque production cause the CP,maxs and CT,maxs for rotors with a flanged diffuser decreases dependence on wind speed.
3.4. Results of rotor performance for different rotor solidities Since the rotor performances of other rotor solidities are qualitatively similar to that of the 30% rotor solidity, the performance figures are not presented. Tables 1–4 summarize the CP,max, CT,max and rotor rotational speed at CP,max for the 30%-, 40%-, 45%- and 60%-solidity rotors under different test conditions, respectively. These tables show that the values of Cp,maxs for different solidity rotors without the flanged diffuser are very closed to one another at the same wind speeds. It is known that the Cp is related to the air particle collision rate to the blade, which should be closely related to the wind speed and rotor solidity. The Cp,maxs for different solidity rotors without the flanged diffuser are largely dependent on the wind speed but less on the rotor solidity. This may be due
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T.Y. Chen et al. / Experimental Thermal and Fluid Science 42 (2012) 136–142 Table 2 The CP,max, CT,max and RPM at CP,max for40%-solidity rotor. Wind speed
10 m/s
12 m/s
14 m/s
16 m/s
18 m/s
20 m/s
Flanged diffuser
No
Yes
No
Yes
No
Yes
No
Yes
No
Yes
No
Yes
CP,max CT,max RPM at CP,max
0.235 0.133 1262
0.525 0.274 1450
0.272 0.158 1450
0. 595 0.305 1727
0.309 0.180 1685
0.667 0.34 1995
0.331 0.203 1900
0.695 0.368 2256
0.364 0.227 2135
0.734 0.385 2480
0.405 0.252 2381
0.755 0.409 2651
Flanged diffuser increases CP,max and CT,max by CP,max CT,max
124% 106%
118% 93%
115% 88%
110% 81%
101% 69.6%
86.5% 62.3%
Table 3 The CP,max, CT,max and RPM at CP,max for 45%-solidity rotor. Wind speed
10 m/s
Flanged diffuser
No
Yes
12 m/s No
Yes
14 m/s No
Yes
16 m/s No
Yes
18 m/s No
Yes
20 m/s No
Yes
CP,max CT,max RPM at CP,max
0.232 0.144 1192
0.487 0.276 1430
0.265 0.167 1380
0. 52 0.301 1678
0.298 0.188 1602
0.574 0.33 1905
0.323 0.210 1800
0.602 0.358 2056
0.364 0.238 2010
0.663 0.379 2250
0.405 0.261 2180
0.712 0.401 2445
Flanged diffuser increases CP,max and CT,max by CP,max CT,max
110% 91.6%
96.2% 80.2%
92.6% 75.5%
86.4% 70%
82% 59.2%
75.8% 53.6%
Table 4 The CP,max, CT,max and RPM at CP,max for 60%-solidity rotor. Wind speed
10 m/s
12 m/s
14 m/s
16 m/s
18 m/s
20 m/s
Flanged diffuser
No
Yes
No
Yes
No
Yes
No
Yes
No
Yes
No
Yes
CP,max CT,max RPM at CP,max
0.228 0.169 1077
0.428 0.275 1295
0.262 0.187 1305
0.461 0.295 1450
0.293 0.202 1508
0.499 0.311 1626
0.321 0.225 1581
0.529 0.338 1775
0.351 0.252 1635
0.552 0.363 1884
0.399 0.285 1770
0.595 0.395 1995
Flanged diffuser increases CP,max and CT,max by CP,max CT,max
88% 62.7%
76% 57.8%
to the small rotor diameter, and thus, the collision rate is dominated by the wind speed. Table 1 indicates that the flanged diffuser increases CP,max by 164% at 10 m/s and 111% at 18 m/s, The flanged diffuser’s effect apparently plays a more important role in wind energy extraction than the blade effect for the 30%-solidity rotor at wind speeds less than 20 m/s. Table 2 shows that the flanged diffuser increases CP,max by 124% at 10 m/s and 101% at 18 m/s. The flanged diffuser’s effect plays slightly more important role in wind energy extraction than the blade effect for the 40%-solidity rotor at wind speeds less than 20 m/s. Tables 3 and 4 show that the power and torque augmentations are mostly less than 100%. In this case, the blade effect plays a more important role in wind energy extraction and torque production than the flanged diffuser’s effect for the 45%- and 60%solidity rotors. These tables also indicate that the flanged diffuser’s effects on CP,max and CT,max decrease as the rotor solidity and wind speed increase. The flanged diffuser’s effect on the 30%-solidity rotor is the largest among the test rotors. Also, the flanged diffuser increases the rotor rotational speed for all the test rotors. A flanged diffuser creates a pressure difference between the inlet and outlet of it. Placing rotating blades inside the diffuser will induce drag, reducing this pressure difference. The higher the rotor solidity and rotational speed are, the smaller this pressure difference is. It is noted that the bare rotor of 30% solidity has the fewest blades, but the highest rotor rotational speed, among the test bare rotors. This result suggests that flanged diffuser’s effect on rotor performance is
70.3% 54%
64.8% 50.2%
57.3% 44%
49.1% 38.6%
more dominated by the rotor solidity than the rotor rotational speed. Comparison of these tables shows that the 60%-solidity rotor without a flanged diffuser has the lowest rotational speed and the highest torque output among the test rotors, which confirms the existing knowledge. As noted above, the flanged diffuser’s effect on rotor performance decreases as the rotor solidity increases. The 60%-solidity rotor with a flanged diffuser has the lowest rotational speed, but do not have the largest CT,maxs, among the test rotors. The 40%-solidity rotor without a flanged diffuser has less torque outputs than the 60%-solidity rotor, but it has larger flanged diffuser’ effect than the 60%-solidity rotor. The combined effect causes that the 40%-solidity rotor has the largest CTs. It has indicated in this paper that a small wind turbine has the characteristics of low power and torque outputs, and high rotor rotational speed compared to a conventional wind turbine. A generator usually requires a large torque to generate high electric power, and low rotational speed for dynamic balance consideration and so on. Thus, the torque output and rotor rotational speed of a small wind turbine should play a more important role in rotor-generator matching than the power output. This study provides some useful information when considering the rotor-generator matching problems. The 30%-solidity and 40%-solidity rotors generate the largest power and torque outputs, respectively, but also have high rotor rotational speeds. The 60%-solidity rotor has the smallest power and torque outputs but also the lowest rotational speed among the test rotors. The selection of rotor solidity should depend
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on the generator characteristics, safety and dynamic balance considerations, and so onto achieve an optimal performance for moving vehicles. 4. Conclusions This paper studies the flanged diffuser’s effects on rotor performance of small wind turbines with rotor solidities between 20% and 60% at wind speeds between 10 m/s and 20 m/s. Results show that the flanged diffuser may significantly increase the power and torque outputs, and rotational speed of the wind turbine, depending on the rotor solidity and wind speed. The higher the solidity and wind speed are, the smaller the flanged diffuser’s effect is. The flanged diffuser’s effect on the 30%-solidity rotor is the largest among the test rotors. For the 30%- and 40%-solidity rotors, the flanged diffuser’s effect plays a more important role in wind energy extraction than the blade’s effect at wind speeds less than 20 m/s. For the 45%- and 60%-solidity rotors, the blade’s effect, in general, plays a more important role in wind energy extraction than the flanged diffuser’s effect. Results of this study also show that the 30%- and 40%-solidity rotors generate the largest power and torque outputs, respectively, while the 60%-solidity rotor has the lowest rotor rotational speed among the test rotors. The selection of rotor solidity should depend on the generator characteristics, safety and dynamic balance considerations, and so onto achieve an optimal performance for moving vehicles. This study also shows that the rotor solidity to achieve highest CP of the investigated small wind turbine is around 35–40%, which is different from a conventional large wind turbine of less than 10%. Duquette and Visser [1] showed the solidity of 15–25% yielded higher CP,max with rotor radius of 1 m. These results suggest that the smaller the rotor diameter, the larger the rotor solidity should be adopted for a higher CP,max. Acknowledgment This research was sponsored by the National Science Council of the Republic of China under contract NSC98-2221-E-032-042MY3.
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