Open Access proceedings Journal of Physics: Conference series

Aerodynamic study of innovative small wind turbine

N Constantin1 and A Dragomirescu2,

1 Professor, University Politehnica of Bucharest, Department of Strength of Materials, 313 Splaiul Independentei, 060042 Bucharest, Romania

2 Assistant Professor, University Politehnica of Bucharest, Department of Hydraulics, Hydraulic Machinery, and Environmental Engineering, 313 Splaiul Independentei, 060042 Bucharest, Romania

E-mail: andrei.dragomirescu@upb.roAbstract. In this paper the authors present results of an aerodynamic study carried out for a wind turbine runner with high solidity. The runner is intended to equip a small horizontal axis wind turbine, part of an integrated installation meant to provide renewable energy to customers looking for increased energy autonomy. The turbine is designed to harvest wind energy with high efficiency, in a large range of wind speeds. The results of the study prove that, to a large extent, the design solution was adequate to face the proposed efficiency targets.1. Introduction

Small wind turbines account for considerable number of variants, in which innovation plays an important role in achieving high efficiency of wind power to mechanical energy conversion. Apparently, this tendency was more present in conceiving vertical axis wind turbines (VAWT), while for horizontal axis wind turbines (HAWT) the three blade concept [1], common with that used for big wind turbines, seems to have been solidified. Nevertheless, small HAWT still require creative approaches for achieving high efficiency and reliable operation during their service life, in various atmospheric conditions.

There are two main aerodynamic challenges for small HAWT: the appropriate selection of the number of blades for ensuring high conversion rates in specific average atmospheric conditions and adequate control of the blade pitch for reaching the same target in conditions of rapid changes in wind speed, or for providing structural safety in extreme wind speed conditions. All authors agree that the number of blades has to be increased in areas with average to small wind velocities, on the expense of lower tip speed ratio (TSR), a parameter directly linked with the conversion rates [2, 3]. Concerning the blade pitch, the considerably less expensive passive/automatic control is generalized for small HAWT [1, 3], in various variants, generally using the centrifugal forces. In many cases, constant pitch is chosen for structural simplicity and related low final cost.

The present study aimed at the design of a small HAWT, able to deliver energy in low wind conditions and up to 4 kW of electrical power in high wind conditions. Additionally, the design had in view survivability in operating conditions up to the wind speed limit historically recorded across Romania, of about 30 m/s. The aerodynamic design had also to be correlated with structural and manufacturing requirements, aiming at a high in-service reliability, easy transport and mounting, and low manufacturing, mounting and operational costs.2. Runner geometry

In order to meet the aforementioned requirements, the solution of a six-blade ducted runner was adopted (figure 1), with passive blade pitch control, ensured by springs placed at both blade ends, balancing the aerodynamic forces. While the pitch will be quasi-constant along the blade, the springs will allow for it to change depending on wind speed.

This high solidity turbine design, with low aspect ratio values for the blades is expected to offer the possibility to harvest energy starting from wind speeds of 3-4 m/s. The ducted design is expected to increase the conversion coefficient, by improving the air flow across the runner in line with the basic assumption considered by A. Betz in his reference study [4], and to help in stabilizing the turbine in variable wind conditions.

The chosen airfoil for the blades is RAF6, with 12% relative thickness (figure 2). This airfoil presents some clear advantages: attractive aerodynamic characteristics in terms of lift, drag and moment coefficients, and a flat inner surface that will considerably ease the blade manufacturing.The blade spar was pushed forward in order to ease the positioning of the blades in feathering position in high winds, thus assuring structural protection of the whole installation in extreme weather conditions.3. Aerodynamic study

The runner investigated has a blade tip diameter . At the hub, the diameter is . The chord length of the RAF6 airfoil, , remains constant and equal to 400mm along the entire blade. Air properties were calculated for an atmospheric pressure of 101325 Pa and a temperature of 15C. The target power of the turbine is of 4 kW. The aerodynamic study aimed at assessing blade loads and turbine performance for wind speeds ranging from 2 m/s to 30 m/s, runner speeds ranging from 5 rpm to 200rpm, and pitch angles ranging from 20 to 80.

Essential for the aerodynamic study are the polar diagrams of the airfoils. Such diagrams could not be found in the literature for the wide ranges of angles of attack and Reynolds numbers that result from the wind speeds, runner speeds, and pitch angles considered. Therefore, we relied on the program JavaFoil [5] to obtain the polar diagrams. The results obtained for Reynolds numbers ranging from 5104 to 5106 and angles of attack ranging from -20 to 60 are presented in figure 3.

To calculate lift, drag, and power at different radii from hub to blade tip, a blade was divided into 19 blade elements bounded by 20 equally spaced blade cross-sections. Such a cross-section with the corresponding kinematic and dynamic quantities is presented in figure 4. At the leading edge(index1), the tangential velocity (or blade linear velocity), , and the relative velocity, , have the the following expressions:

(1)

The angle of attack can be calculated with the formula

(2)

The Reynolds number at radius was defined as

(3)

where is the air kinematic viscosity. For the angles of attack and Reynolds numbers obtained at each radius, the lift, drag, and moment coefficients , , and respectively were calculated by linear interpolation based on the data supplied by JavaFoil. With these coefficients the lift, drag, and moment on the airfoil at radius were calculated with the usual formulas:

(4)

where is the air density. The axial and tangential forces, and respectively, have the expressions

(5)

and

(6)

The power on the blade cross-section is

(7)

The loads and power defined above are per blade unit length and can be used for the structural analysis of the blade. The axial force is important from the point of view of properly designing both the axial bearing of the turbine and the mast, while the tangential force is the one that produces power. The moment is essential for designing the springs that will assure the passive blade pitch control.

By integrating the power variation along the blade and multiplying with the number of blades, , the turbine power is obtained:

(8)

The integral was calculated with the Simpsons rule. Once the turbine power is known, the torque at the turbine shaft results:

(9)

Additionally, the power coefficient of the turbine can also be evaluated:

(10)

Some results obtained following the above workflow are summarized in figure 5 as variations of torque at turbine shaft and turbine power depending on runner speed and variation of power coefficient depending on turbine tip speed ratio (TSR). The results correspond to operating scenarios in which the pitch angle of the blades equals35. The results suggest that, even in low wind conditions, the turbine is able to deliver up to 0.5 kW when properly operated. The target power of 4 kW can be attained at wind speeds of about 10 m/s when the runner operates at about 80 rpm. The power coefficient could be as high as 0.4, which is not an unrealistic result considering the good performance of HAWTs.4. Conclusions

This study is part of a comprehensive approach meant to cover all aspects linked to the design, manufacturing, and maintenance of an integrated installation for producing renewable energy. It was intended to provide basic input data concerning the loads required by the structural analysis. The study also offers information for an adequate design of the passive blade pitch control. Next, it will serve as a valuable data basis meant to optimize the wind energy harvesting, along the whole chain: runner, direct drive electric generator, inverter, regulator, storage batteries, and end users.

In this theoretical stage, the main parameters characterizing the runner design adopted for this HAWT have encouraging spanning along the wide range of wind speeds in view to be harnessed. Subsequent in-situ measurements on the physical demonstrator will finally prove the validity of the chosen architecture of the wind turbine and the whole integrated design.5. Acknowledgment

The research work was performed under project 258/2014, financed by the Romanian Ministry of National Education through its dedicated body (MEN-UEFISCDI), in the frame of the Partnership in priority domains - PN II programme.6. References

[1] Hau E 2006 Wind Turbines: Fundamentals, Technologies, Application, Economics (Springer)[2] Burton T and Sharp D 2006 Wind Energy Handbook. (Chichester: John Wiley & Sons Ltd)[3] Ragheb M and Ragheb A M 2011 Wind turbines theory the Betz equation and optimal rotor tip speed ratio Fundamental and Advanced Topics in Wind Power ed R Carriveau (InTech)[4] Betz A 1966 Introduction to the Theory of Flow Machines (Oxford, New York: Pergamon Press)[5] Hepperle M 2007 JavaFoil http://www.mh-aerotools.de/airfoils/javafoil.htmFigure SEQ "Figure" \* MERGEFORMAT 1. Six-blade, high solidity ducted runner.

Figure SEQ "Figure" \* MERGEFORMAT 2. RAF6 airfoil with 12% relative thickness.

Figure SEQ "Figure" \* MERGEFORMAT 3. Polar diagrams of the RAF6 airfoil with 12% relative thickness.

Figure SEQ "Figure" \* MERGEFORMAT 4. Kinematic and dynamic quantities at a blade cross-section.

Figure SEQ "Figure" \* MERGEFORMAT 5. Performance curves of the turbine for QUOTE a = 35.

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