Increasing the power output of the Darrieus Vertical Axis Wind Turbine

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1 Increasing the power output of the Darrieus Vertical Axis Wind Turbine R. Ramkissoon 1 and K. Manohar 2* 1,2 Mechanical and Manufacturing Engineering Department, The University of the West Indies, St. Augustine, Trinidad and Tobago, West Indies. ABSTRACT 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. Keywords: Vertical Axis wind turbine, VAWT, Straight Bladed, Darrieus 1. INTRODUCTION The straight bladed Darrieus vertical axis wind turbine (VAWT) is very attractive for its low cost and simple design. Here in the Caribbean there is little use of wind turbines with more emphasis on solar energy. This research is generally towards sensitizing the general population to the possible use of wind turbines for the power generation (Chinchilla et al 2011) Research has shown that properly designed wind turbines has the potential to compete with other renewable sources of energy and can be economically feasible (Lowson et al 1994). Increasing the power output of these VAWT can increase its attractiveness as an option for power generation. The overall performance of a rotor is mainly influenced by: rotor geometry, rotational speed, airfoil shape, mean angle of attack, amplitude, and oscillation of the instantaneous angle of attack, Reynolds number, the turbulence levels and type of motion of the blades (Paraschivoiu 2002). * Corresponding author: Krishpersad.manohar@sta.uwi.edu

2 Parasitic losses can mainly be attributed to drag and frictional losses. The main drag losses occur at the blade and supporting struts. The function of the supporting struts is to stabilize the blades, reduce operating mean and fatigue stresses in the blades and influence some natural frequency of the rotor. The design of the struts involves a trade-off between aerodynamic and structural properties. There are three main types of support: overhang, cantilever and simple support In this study to increase the power output of the straight bladed VAWT different methods were tried. Varying angle of attack, use of the Mechanical Turbulator and drag reduction of the supporting struts were tested. 2. STRAIGHT-BLADED VAWT The Darrieus type VAWT was invented by French engineer Georges Jean Marie Darrieus in 1925 and it was patented in the USA in 1931 (Darrieus 1931). It comes in two configurations, namely egg-beater (or curved-bladed) and straight-bladed. 2.1 Straight-Bladed VAWT Applications VAWT can be used in a variety of applications, namely: (a) Grid connected: Wind turbines are most effective at supplying centralized electric power. Electricity from large clusters of interconnected wind turbines is fed into the local distribution grid and sold to local utility companies (b) Dispersed grid connected: Wind turbines are often used to produce electricity for homes, business and farms already connected to the utility grid (c) Remote stand alone systems: For sites a half mile or further from the utility grid, small wind turbines can provide a cost effective source of energy. Remote applications include rural residences, water pumping and telecommunications. Batteries are often used to store excess electricity, and many systems use a diesel generator or solar panels as a back-up system to provide electricity during low wind periods. 2.2 Operation The Darrieus-type straight bladed VAWT is designed with two or more airfoil blades vertically mounted on a rotating shaft or framework (Fig 1) (Science direct). As the rotor spins, the airfoils move forward through the air in a circular path. As the blades rotate it experiences a head-on air flow (headwind). Relative to the blade, when the oncoming airflow is added vectorially to the prevailing wind direction, the resultant airflow creates a varying positive angle of attack with rotation (Amin et al 2007). This generates a net force (lift force) pointing obliquely forwards along a certain 'line-of-action'. This force projected about the center of rotation, i.e. the turbine axis, gives a positive torque to the shaft, thus helping it to rotate in the direction it is already travelling. As the airfoil moves around the back of the apparatus, the angle of attack changes to the opposite sign, but the resultant force is still oblique to the direction of rotation, since the wings are symmetrical and the pitch angle is zero (Fig. 1) (Science direct).

3 Figure 1. Schematic of 3 bladed Darrieus lift type turbine ( This creates a couplet of forces about the axis of rotation. Hence, the rotor spins at a rate unrelated to the wind speed, and usually many times faster (Manwell et al 2002). The energy arising from the torque and speed may be extracted and converted into useful power by using an electrical generator (Manohar et al 2007). 2.3 Aerodynamic Challenges of Straight Bladed VAWT Some of the challenges faced with the VAWT are (a) they operate at low Reynolds numbers where the blades are highly prone to separation (b) the blades produce fluctuating forces which can cause vibrations and dynamic stalling (c) deep stalling may occur at low tip speed ratios (d) most of the power extracted is on the upstream portion of the turbine and (e) VAWT suffer from parasitic losses (Paraschivoiu 1982). 3. TEST VAWT SPECIFICATION A vertical axis wind turbine was built for experimental testing. The airfoil section was designed in accordance with the NACA 0018 profile and drawn using the AutoCAD program and then electronically loaded into the CNC machine. This NACA 0018 profile was chosen for its good lift characteristics and flatwise strength (Timmer 2008). Six blade profiles were cut to specifications. Mechanical attachments were required at the top and bottom of the blades and as such aluminum was chosen as the material for these NACA 0018 profile cutouts. The inner profiles provided structural stability to maintain the airfoil shape. Hence, locally available Trinidad cedar wood was chosen for the other 4 NACA 0018 profile cut-outs due to its low density (340.2 kg/m 3 ) and easy machining ability. Local cedar has a cross-grain structure and a tensile strength of 7.7 MN/m 2 (Manohar et al 2004). The blade section was 305 cm long, 55 cm wide and 1.30 cm thick with solidity of 0.27 and an Aspect Ratio of A total of 3 blades were constructed and used. The VAWT diameter was 6.76 m and has a height of 3.05 m. An aluminum pipe was placed through the centre of all the equally spaced airfoil cutouts. This pipe served as the mounting supports for the blades. The blades were then formed by wrapping and riveting a 0.75 mm thick aluminum sheet around the blade profile. The maximum height of the VAWT is 365 cm. The picture below shows the actual VAWT built.

4 Figure 2. VAWT built and located at Manzanilla, Trinidad. 4. PRELIMINARY RESULTS The wind turbine was placed at Manzanilla, East Coast of Trinidad, approximately 550m from the sea coast. This site was generously offered for experimental testing of the wind turbine by the ministry of agriculture since no other site could have been occupied close to the sea coast. This beach is used for bathing and recreational activities by the general public. As such, several limitations were imposed on the testing phase of the project. The location was chosen to minimize the obstructions to the public and tree cutting were not permitted. The location, however, was not ideal for wind turbine testing due to the presence of several trees in close proximity to the turbine. Also, budget constraints limited the turbine to be placed close to the ground. Working within the constraints encountered, testing was conducted to gather preliminary data. Equipment used to obtain the wind speed and turbine rpm was the anemometer and a tachometer. The anemometer used was an Extech heavy duty Hot Wire Thermo-anemometer. It has an accuracy of +/- 3% for a wind speed range of 0.20 to 20 m/sec. The tachometer used was a DT-207L non-contact tachometer. It has an accuracy of +/- 1 rpm for a range of 6 to 8300 rpm. A low cost gearing mechanism was designed at the base of the turbine to power the automobile alternators. The mechanism comprised of a used automobile automatic transmission flex plate and a used automobile starter gear. The flex plate was mounted between the rotating hub and the turbine rotating shaft. The starter gear was matted into the flex plate (Figure 3) and connected to the alternators via pulleys and belts as shown in. An automobile front wheel hub with the breaking mechanism was used as the main bearing and the stopping mechanism.

5 Figure 3. Gearing system to power alternators Alternators connected via belts to the pulley of the gearing mechanism. The alternators are then electrically wired to 3 deep cycle batteries. The specifications for the batteries were 12 volt, 950 cranking amps and have a reserve capacity of 95 minutes. These batteries were used to provide power independent of the grid. An estimated mechanical efficiency of 70% was used to convert the rotational energy of the turbine to electrical energy; this value was chosen due to the amount of mechanical contact between the gears and the pulley system of the alternators (K. Tota-Maharaj et al 2012). The following graphs (Figs. 4 to 6) were generated Figure 4. Turbine RPM vs Wind Speed

6 Figure 5. Tip Speed Ratio vs Wind Velocity Figure 6. Power Generated vs Wind Speed 5. TECHNIQUES USED FOR TRYING TO IMPROVE OUTPUT POWER FROM THE VAWT The following techniques were attempted to improve the VAWT power output: 5.1 Varying the Angle of Attack of the Blades. Adjustment to the preset pitch angle of the airfoil, β (the angle with which the blade is mounted to the strut), causes changes to the performance of the turbine. Adjusting the blade preset pitch to a toe-out configuration for a VAWT then results in a range of angles of attack (α) on both the upwind and downwind blade passes (figure 7). This pitch angle, β, is defined as positive for toe-in configurations.

7 Figure 7: Apparent zero wind angle of attack (α o ) as a function of chord location (x/r) and preset pitch angle (β) (South and Rangi 1972). 5.2 Drag Reduction of the Blade Supporting Struts. The main purpose for the supporting struts is to attach the blades to the main shaft and provide mechanical support to the blades. Usually these struts have no aerodynamic characteristic to them. Struts commonly used are round pipes or flat metal plates. To fabricate a strut into an airfoil shape would be very costly indeed. It has been observed that in the case of VAWT the power losses caused by the strut can be as much as 26% (Worstell 1980) Figure 8: Picture of strut modification The original struts were modified by forming an additional component (Figure 8) which converts the round pipe to a shape resembling that in the figure 9 below which has a resistance of 15%.

8 Figure 9: Resistance to flow by different shapes ( 5.3 Drag Reduction on the rotor blades (Using the Mechanical Turbulator). The additional drag, which arises from laminar separation bubbles, can be eliminated, by avoiding them or by reducing their size. Forced transition by artificial disturbances, using a mechanical turbulator as in this case, is one way of achieving this. This device will usually be attached just before the region of laminar separation and has to introduce enough disturbances to cause transition into the turbulent state, before the laminar separation can occur. The mechanical turbulator used in this case was a tape with bumps evenly spaced out on its surface. The upset bumps on the trip tape are spaced 0.5 cm apart and height of 0.2 cm. Figure 10 below shows a picture of the tape used Figure 10: Picture of the trip tape used for turbulator. 6. RESULTS USING THE DRAG REDUCTION TECHNIQUES In these tests positive and negative values of the angle of attack was represented as shown in the diagram below.

9 The results shown on Figure 11 indicate that there was increased power produced by the turbine after 4.45 m/s. Very low overall power generation was seen with this modification to the blade s angle of attack Figure 11: Power produced vs Wind velocity at 10 Degrees Angle of Attack The results shown on Figure 12 indicate that there was increased power produced by the turbine after 3.75 m/s. Very low overall power generation was seen with this modification to the blade s angle of attack Figure 12: Power produced vs Wind velocity at -10 Degrees Angle of Attack With the addition of the turbulator on the blade, the results shown in Figure 13 indicated that the maximum power produced by the VAWT was 672 Watts at a wind speed of 5 m/s. A wind speed range of 2 m/s to 5 m/s produced 200 Watts and 600 Watts, respectively.

10 Figure 13: Power produced vs Wind velocity using the Turbulator Figure 14: Power produced vs Wind velocity with the modified strut. Results using the modified strut, Figure 14, showed within the wind speed range 3.4 m/s to 7.0 m/s power output of 460 Watts to 760 Watts, respectively. A maximum power of 826 Watts was observed at 6.3 m/s. 7. RESULTS AND DISCUSSION This paper s main objective was to optimize the VAWT to increase its power output using different techniques.

11 Figure 15: Power produced vs Wind velocity at different Angle of Attacks From figure 15 that the power produced at 0 degrees Angle of attack is greater than the other Angle of Attack. This increase or decrease in the blade s Angle of Attack caused the VAWT to go into the dynamic stall region earlier than it normally does. All VAWT blades undergo dynamic stalling at certain angle of attacks, but with this adjustment to the angle of attack, dynamic stalling occurs earlier. This occurrence of dynamic stalling occurring earlier than when the blade s angle of attack is 0 degrees, causes the VAWT to lose momentum and doesn t have the required torque to generate power at the particular wind speed. When in this region the turbine cannot produce maximum power and the air flow leaves the blade surface, thus giving low turbine power output Figure 16: Power produced vs Wind velocity with and without Turbulator attached From figure 16 that turbine s power output at the various wind speeds was the same with and without the turbulator attached to the blade. When the Turbulator was fixed to the turbine blades surface, it became ineffective as the Angle of Attack changed resulting in the laminar separation point changing as well.

12 Figure 17: The power produced by the VAWT with the strut modified & original. The results shown in Figure 17 indicated that the VAWT produced more power with the strut modified to reduce drag. The original strut was a 2.5 round pipe. From Figure 9 it can be seen that the round pipe has a resistance of 50%, as compared to a flat plate which has 100% resistance to flow. The original strut was modified by forming an additional component which converted the round pipe to a shape with a resistance of approximately 15%. This modification caused an increase in the turbine s power of approximately 17% within a wind speed range of 3.5 m/s to 5 m/s. 8. CONCLUSIONS Experimental results from the optimization of the Straight-Bladed Vertical Axis Wind Turbine indicated: 1. The varying of the angle of Attack form 0 degrees to 10 and -10 degrees has no significant effect on increasing the output power from vertical axis wind turbine. Varying the angle of Attack from 0 degrees caused a degradation of the turbine s output power. 2. The addition of the mechanical turbulator to the turbine blade had no effect on the vertical axis wind turbine power output. As the turbine rotated the position of the laminar separation bubble changed on the upper and lower surface of the blade as a result of the changing angle of attack and this rendered the turbulator useless. 3. Modification of the original strut of the VAWT showed to be quite an improvement on the turbine s power output. This modified strut had less resistance to flow and increased the power output of the wind turbine. The modification caused an increase in the turbine s power of approximately 17% for wind speeds within the range of 3.5 m/s to 5 m/s. COMPETING INTERESTS Authors have declared that no competing interests exist.

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