* Corresponding author: E-mail: Krishpersad.manohar@sta.uwi.edu

1

Increasing the power output of the Darrieus 2

Vertical Axis Wind Turbine 3

4

R. Ramkissoon1 and K. Manohar2* 5

6 1,2

Mechanical and Manufacturing Engineering Department, The University of the West 7 Indies, St. Augustine, Trinidad and Tobago, West Indies. 8

E-mail: Krishpersad.manohar@sta.uwi.edu 9 10 11 12 .13 ABSTRACT 14

15

The Darrieus Vertical Axis Wind Turbine is a versatile method of generating power in the

Caribbean. The cost, reliability and power produced are of paramount importance in the

success of these wind turbines. This study analyzed different methods of improving the

output power of a Vertical Axis Wind Turbine. In the case study a Vertical Axis Wind Turbine

was built using the NACA 0018 airfoil type for the blade profile. The turbine consisted of

three blades of length 3.05 meters and had a diameter of 6.10 meters. Experimental results

showed that drag reduction on the strut arms of the blades increased the power output

greater than any other method tested. The Vertical Axis wind turbine power output increased

by approximately 27% in some cases using a strut modifier to decrease the drag component

of the blade’s strut.

16 Keywords: Vertical Axis wind turbine, VAWT, Straight Bladed, Darrieus 17 18 19

1. INTRODUCTION 20 21 The straight bladed Darrieus vertical axis wind turbine (VAWT) is very attractive for its low 22 cost and simple design. Here in the Caribbean there is little use of wind turbines with more 23 emphasis on solar energy. This research is generally towards sensitizing the general 24 population to the possible use of wind turbines for the power generation. 25 26 Research has shown that properly designed wind turbines has the potential to compete with 27 other renewable sources of energy and can be economically feasible (Lowson et al 1994). 28 Increasing the power output of these VAWT can increase its attractiveness as an option for 29 power generation. 30 31 The overall performance of a rotor is mainly influenced by: rotor geometry, rotational speed, 32 airfoil shape, mean angle of attack, amplitude, and oscillation of the instantaneous angle of 33 attack, Reynolds number, the turbulence levels and type of motion of the blades 34 (Paraschivoiu 2002). 35 36

Parasitic losses can mainly be attributed to drag and frictional losses. The main drag losses 37 occur at the blade and supporting struts. The function of the supporting struts is to stabilize 38 the blades, reduce operating mean and fatigue stresses in the blades and influence some 39 natural frequency of the rotor. The design of the struts involves a trade-off between 40 aerodynamic and structural properties. There are three main types of support: overhang, 41 cantilever and simple support 42 43 In this study to increase the power output of the straight bladed VAWT different methods 44 were tried. Varying angle of attack, use of the Mechanical Turbulator and drag reduction of 45 the supporting struts were tested. 46 47 48

2. STRAIGHT-BLADED VAWT 49

50 The Darrieus type VAWT was invented by French engineer George Jeans Mary Darrieus in 51 1925 and it was patented in the USA in 1931 (Darrieus 1931). It comes in two configurations, 52 namely egg-beater (or curved-bladed) and straight-bladed. 53 54

2.1 Straight-Bladed VAWT Applications 55

56 VAWT can be used in a variety of applications, namely: (a) Grid connected: Wind turbines 57 are most effective at supplying centralized electric power. Electricity from large clusters of 58 interconnected wind turbines is fed into the local distribution grid and sold to local utility 59 companies (b) Dispersed grid connected: Wind turbines are often used to produce electricity 60 for homes, business and farms already connected to the utility grid (c) Remote stand alone 61 systems: For sites a half mile or further from the utility grid, small wind turbines can provide a 62 cost effective source of energy. Remote applications include rural residences, water 63 pumping and telecommunications. Batteries are often used to store excess electricity, and 64 many systems use a diesel generator or solar panels as a back-up system to provide 65 electricity during low wind periods. 66

2.2 Operation 67

The Darrieus-type straight bladed VAWT is designed with two or more airfoils blades 68

vertically mounted on a rotating shaft or framework (Fig 1) (Darrieus Wind Turbine 2007). 69

As the rotor spins, the airfoils move forward through the air in a circular path. As the blades 70 rotate it experiences a head-on air flow (headwind). Relative to the blade, when the 71 oncoming airflow is added vectorially to the prevailing wind direction, the resultant airflow 72 creates a varying positive angle of attack with rotation. This generates a net force (lift force) 73 pointing obliquely forwards along a certain 'line-of-action'. This force projected about the 74 center of rotation, i.e. the turbine axis, gives a positive torque to the shaft, thus helping it to 75 rotate in the direction it is already travelling. As the airfoil moves around the back of the 76 apparatus, the angle of attack changes to the opposite sign, but the resultant force is still 77 oblique to the direction of rotation, since the wings are symmetrical and the pitch angle is 78 zero (Fig. 1) (Darrieus Wind Turbine 2007). 79

80 81

Figure 1. Schematic of Darrieus lift type turbine (Darrieus Wind Turbine 2007). 82

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This creates a couplet of forces about the axis of rotation. Hence, the rotor spins at a rate 84 unrelated to the wind speed, and usually many times faster. The energy arising from the 85 torque and speed may be extracted and converted into useful power by using an electrical 86 generator (Manohar et al 2007). 87

2.3 Aerodynamic Challenges of Straight Bladed VAWT 88

Some of the challenges faced with the VAWT are (a) they operate at low Reynolds numbers 89 where the blades are highly prone to separation (b) the blades produce fluctuating forces 90 which can cause vibrations and dynamic stalling (c) deep stalling may occur at low tip speed 91 ratios (d) most of the power extracted is on the upstream portion of the turbine and (e) they 92 suffer from parasitic losses. 93 94

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3. TEST VAWT SPECIFICATION 96

97 A vertical axis wind turbine was built for experimental testing. The airfoil section was 98 designed in accordance with the NACA 0018 profile and drawn using the AutoCAD program 99 and then electronically loaded into the CNC machine. This NACA 0018 profile was chosen 100 for its good lift characteristics and flatwise strength (Timmer 2008). Six blade profiles were 101 cut to specifications. Mechanical attachments were required at the top and bottom of the 102 blades and as such aluminum was chosen as the material for these NACA 0018 profile cut-103 outs. 104 105 The inner profiles provided structural stability to maintain the airfoil shape. Hence, locally 106 available Trinidad cedar wood was chosen for the other 4 NACA 0018 profile cut-outs due to 107 its low density (340.2 kg/m

3) and easy machining ability. Local cedar has a cross-grain 108

structure and a tensile strength of 7.7 MN/m2 (Manohar et al 2004). 109

110 The blade section was 305cm long, 55cm wide and 1.3cm thick with solidity of 0.27 and an 111 Aspect Ratio of 5.58. A total of 3 blades were constructed and used. The VAWT diameter 112 was 6.76 m and has a height of 3.05 m. An aluminum pipe was placed through the centre of 113 all the equally spaced airfoil cutouts. This pipe served as the mounting supports for the 114 blades. The blades were then formed by wrapping and riveting a 0.75 mm thick aluminum 115 sheet around the blade profile. The picture below shows the actual VAWT built. 116 117

118 Figure 2. VAWT built and located at Manzanilla, Trinidad. 119

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4. PRELIMINARY RESULTS 121

The wind turbine was placed at Manzanilla, East Coast of Trinidad, approximately 150 feet 122 from the sea coast Equipment used to obtain the wind speed and turbine rpm was the 123 anemometer and a tachometer. The anemometer used was an Extech heavy duty Hot Wire 124 Thermo-anemometer. It has an accuracy of +/- 3% for a wind speed range of 0.20 to 20 125 m/sec. The tachometer used was a DT-207L non-contact tachometer. It has an accuracy of 126 +/- 1 rpm for a range of 6 to 8300 rpm. A mechanical efficiency of 70% was used to convert 127 the rotational energy of the turbine to electrical energy (K. Tota-Maharaj et al 2012). 128 129 The following graphs (Figs. 3 to 6) were generated. 130 131

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Figure 3. Turbine RPM vs Wind Speed 133

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Figure 4. Tip Speed Ratio vs Wind Velocity 135

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Figure 5. Power Generated vs Wind Speed 138

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Figure 6. Coefficient of performance vs. tip speed ratio 141

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5. TECHNIQUES USED FOR TRYING TO IMPROVE OUTPUT POWER FROM 143

THE VAWT 144

The following techniques were attempted to improve the VAWT power output: 145

5.1 Varying the Angle of Attack of the Blades. 146

Adjustment to the preset pitch angle of the airfoil, β (the angle with which the blade is 147 mounted to the strut), causes changes to the performance of the turbine. Adjusting the blade 148 preset pitch to a toe-out configuration for a VAWT then results in a range of angles of attack 149 (α) on both the upwind and downwind blade passes (figure 7). This pitch angle, β, is defined 150 as positive for toe-in configurations. 151

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155 156 157 158 159 160 161 162 163 164 165 Figure 7: Apparent zero wind angle of attack (αo) as a function of chord location (x/r) 166

and preset pitch angle (β) (South and Rangi 1972). 167 168

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5.2 Drag Reduction of the Blade Supporting Struts. 170

The main purpose for the supporting struts is to attach the blades to the main shaft and provide 171

mechanical support to the blades. Usually these struts have no aerodynamic characteristic to 172

them. Struts commonly used are round pipes or flat metal plates. To fabricate a strut into an 173

airfoil shape would be very costly indeed. It has been observed that in the case of VAWT the 174

power losses caused by the strut can be as much as 26% (Worstell 1980). 175

176 Figure 8: Picture of strut modification 177

178 The original strut was modified by forming an additional component (Figure 8) which 179 converts the round pipe to a shape resembling that in the figure 9 below which has a 180 resistance of 15%. 181

182 Figure 9: Resistance to flow by different shapes. 183

184

5.3 Drag Reduction on the rotor blades (Using the Mechanical Turbulator). 185

The additional drag, which arises from laminar separation bubbles, can be eliminated, by 186 avoiding them or by reducing their size. Forced transition by artificial disturbances, using a 187 mechanical turbulator as in this case, is one way of achieving this. This device will usually be 188 attached just before the region of laminar separation and has to introduce enough 189 disturbances to cause transition into the turbulent state, before the laminar separation can 190 occur. 191 192 193 194 195 196 197

6. RESULTS USING THE DRAG REDUCTION TECHNIQUES 198

In these tests positive and negative values of the angle of attack was represented as 199

shown in the diagram below. 200

201 The results shown on Figure 10 indicate that there was increased power produced by the 202 turbine after 4.45 m/s. Very low overall power generation was seen with this modification to 203 the blade’s angle of attack. 204

205

Figure 10: Power produced vs Wind velocity at 10 Degrees Angle of Attack 206

The results shown on Figure 11 indicate that there was increased power produced by the 207 turbine after 3.75 m/s. Very low overall power generation was seen with this modification to 208 the blade’s angle of attack. 209

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Figure 11: Power produced vs Wind velocity at -10 Degrees Angle of Attack 211 212

With the addition of the turbulator on the blade, the results shown in Figure 12 indicated that 213 the maximum power produced by the VAWT was 672 Watts at a wind speed of 5 m/s. A 214 wind speed range of 2 m/s to 5 m/s produced 200 Watts and 600 Watts, respectfully. 215 216

217 Figure 12: Power produced vs Wind velocity using the Turbulator 218

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Figure 13: Power produced vs Wind velocity with the modified strut. 223 224 225

Results using the modified strut, Figure 13, showed within the wind speed range 3.4 m/s 226

to 7 m/s power output of 460 Watts to 760 Watts, respectively. A maximum power of 227

826 Watts was observed at 6.3 m/s. 228

229

7. RESULTS AND DISCUSSION 230

This paper’s main objective was to optimize the VAWT to increase its power output using 231 different techniques. 232

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Figure 14: Power produced vs Wind velocity at different Angle of Attacks 234 235

From figure 14 that the power produced at 0 degrees Angle of attack is greater than the 236 other Angle of Attack. This increase or decrease in the blade’s Angle of Attack caused the 237 VAWT to go into the dynamic stall region. When in this region the turbine cannot produce 238 maximum power and the air flow leaves the blade surface, thus giving low turbine power 239 output. 240 241

242 Figure 15: Power produced vs Wind velocity with and without Turbulator attached 243

244 From figure 15 that turbine’s power output at the various wind speeds was the same with 245 and without the turbulator attached to the blade. When the Turbulator was fixed to the 246 turbine blades surface, it became ineffective as the Angle of Attack changed resulting in the 247 laminar separation point changing as well. 248 249

250 Figure 16: The power produced by the VAWT with the strut modified & original. 251

252 The results shown in Figure 16 indicated that the VAWT produced more power with the strut 253 modified to reduce drag. The original strut was a 2.5” round pipe. From Figure 9 it can be 254 seen that the round pipe has a resistance of 50%, as compared to a flat plate which has 255 100% resistance to flow. The original strut was modified by forming an additional component 256 which converted the round pipe to a shape with a resistance of approximately 15%. 257 This modification caused an increase in the turbine’s power of approximately 17% within a 258 wind speed range of 3.5 m/s to 5 m/s. 259 260

8. CONCLUSIONS 261

Experimental results from the optimization of the Straight-Bladed Vertical Axis Wind Turbine 262 indicated: 263

1. The varying of the angle of Attack form 0 degrees to 10 and -10 degrees has no 264 significant effect on increasing the output power from vertical axis wind turbine. Varying 265 the angle of Attack from 0 degrees caused a degradation of the turbine’s output power. 266 This was due to the fact that the turbine experienced dynamic stall. 267 268 2. The addition of the mechanical turbulator to the turbine blade had no effect on the 269 vertical axis wind turbine power output. As the turbine rotated the position of the laminar 270 separation bubble changed on the upper and lower surface of the blade as a result of 271 the changing angle of attack and this rendered the turbulator useless. 272 273 3. Modification of the original strut of the VAWT showed to be quite an improvement on 274 the turbine’s power output. This modified strut had less resistance to flow and increased 275 the power output of the wind turbine. The modification caused an increase in the 276 turbine’s power of approximately 17% for wind speeds within the range of 3.5 m/s to 5 277 m/s. 278 279

COMPETING INTERESTS 280

281 Authors have declared that no competing interests exist. 282 283

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'Darrieus Wind Turbine’, 2007. Wikipedia, the free encyclopedia, 290

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