Development of High Performance Airfoils for Application in Small Wind Turbine Power Generation

Department of Mechanical Engineering, Kwame Nkrumah University of Science and Technology, Kumasi, Ghana Department of Mechanical Engineering, Kumasi Technical University, P. O. Box 854, Kumasi, Ghana The Brew-Hammond Energy Center, Kwame Nkrumah University of Science and Technology, Kumasi, Ghana Faculty of Environmental Sciences and Natural Resource Management, Norwegian University of Life Sciences, Ås, Norway


Introduction
In many developing countries, access to sustainable energy is a challenge despite abundant renewable energy resources. In sub-Saharan Africa for example, it is estimated that over 600 million people do not have access to electricity [1]. Development of renewable energy resources in the region has been very low compared to other developed regions in the world [2,3]. Fossil-fuel-based power generation systems have been the main technologies for electricity generation [4]. The increasing effect of climate change and greenhouse gas emissions on the environment, as a result of electricity generation from fossil fuels, is driving a paradigm shift to more environmentally friendly and clean energy resources including solar and wind energy [5][6][7][8][9][10].
Wind energy is a major clean energy resource which has been demonstrated to be able to produce both small-scale and large-scale energy [8,[11][12][13]. However, small-scale wind energy generation requires the use of small wind turbine blades made of special airfoils with good aerodynamic performance under low wind speed conditions [14][15][16][17].
For small wind turbine applications, low wind speeds are generally characterized by airflows with Reynolds numbers (Re) below 500,000. Under this low Re condition, improperly designed blade airfoils encounter deterioration in aerodynamic performance which adversely affects the operating efficiency of the wind turbine [18][19][20][21]. As such, airfoils designed for large-scale wind turbines are not necessarily suitable for small-scale wind turbines [19,20,22,23]. The airfoil aerodynamic parameters of interest in low wind speed airfoils are the lift-to-drag ratio (L/D), lift coefficient, stall angle, stall performance, and drag bucket performance [22,[24][25][26][27].
Some airfoils have been developed for low wind speed applications. Gigueŕe and Selig developed and tested four airfoils (SG6040, SG6041, SG6042, and SG6043) for low Re small wind turbine applications [28]. The study recorded improvement in airfoil performance with increasing Re. At Re = 500,000, SG6040, SG6041, SG6042, and SG6043 airfoils had maximum L/D performances of 87, 84, 106, and 125, respectively.
Giguere and Selig studied fifteen airfoils for aerodynamic performance under low Re conditions [29]. The study recorded improvement in aerodynamic performance with increasing Re and relatively better performance for the thinner airfoils. At Re = 300,000, the SD7032 airfoil had the highest maximum L/D of 83, while the S823 airfoil had the lowest value of 63.
Singh et al. developed the AF300 low Re airfoil by geometrically modifying other airfoils using XFOIL and tested it numerically at Re between 38,000 and 205,000 [16]. The aerodynamic performance of the airfoil improved with increase in Re. At Re = 100,000, the AF300 airfoil had maximum L/D of 54 and stall angle of 14°. The study concluded that AF300 is a good airfoil for the design of small wind turbine blades.
Selig and McGranahan studied the aerodynamic performance characteristics of E387, FX63-137, S822, S834, SD2030, and SH3055 airfoils under low Re conditions between 100,000 and 500,000 [15]. At Re of 100,000, SD2030 had the highest maximum L/D of 53 and a stall angle of 12°. However, at Re of 500,000, FX63-137 had the highest maximum L/D of 108 and a stall angle of 12°.
Henriques et al. designed and tested the "T.Urban 10/193" airfoil in XFOIL for application in small horizontal axis wind turbines for urban environment [30]. At Re = 60,000, the maximum L/D and stall angle were 18 and 8°, respectively. However, at Re = 500,000, the maximum L/D and stall angle increased to 95 and 12°, respectively. The "T.Urban 10/193" airfoil was concluded to be a good airfoil for small wind turbine application.
From available literature, a number of studies have been conducted on airfoils for low Re applications in small wind turbines. However, there is no reported study on the aerodynamic performance parameters of airfoils based on thickness-to-camber ratio (t/c). In this work, three new airfoils are developed based on a range of optimal airfoil t/c and their aerodynamic performance characteristics are determined for low wind speed and low Re applications. These airfoils, which have high L/D, were subsequently used to develop 3-bladed wind turbine rotors, and their power coefficients were determined based on Blade Element Momentum Theory (BEMT). The methodology used in this study is presented in Section 2.

Methodology
2.1. Design of the Airfoils and Rotor. In this study, XFOIL version 6.99 was used as the tool for the airfoil design optimiza-tion and simulations. XFOIL is a popular simulation tool for the general design of airfoils [31]. It has simulation capabilities for the determination of airfoil characteristics such as shape, lift, drag, and stall. XFOIL has been used by several researchers for the design and test of airfoils and has been found to produce results that agree well with wind tunnel results [30,32,33].
To undertake the present study, the SG6043 airfoil reported by [28] was selected as the base airfoil for development of the new airfoils due to its good aerodynamic performance in low Re conditions. The geometry of the base airfoil was redesigned in XFOIL by modifications of the thickness and camber configurations to form new airfoils. At each configuration of t/c, the corresponding new airfoil was tested for maximum L/D at Re between 100,000 and 500,000. Afterwards, the optimal zone of t/c for the highest maximum L/D was used to determine the geometrical characteristics of the newly developed airfoils.
The newly developed airfoils were tested for aerodynamic performance parameters (lift, drag, L/D, stall, and drag bucket) and compared with the base airfoil at Re = 300,000. This Re was chosen based on average wind speed data of 5 to 6.1 m/s at 12 metres above ground level for Ghana as reported by the Energy Commission [34]. Equation (2) was also used in the determination of the Re based on common small wind turbine rotational speed range of about 400 RPM to 1,500 RPM as reported by [35][36][37][38]. The procedure used for the new airfoil designs in XFOIL is summarized in Figure 1.
The airfoil panel nodes used for the computation were 160 coordinate points. The maximum iteration limit used to obtain steady state solution for the XFOIL analysis was 1,000. The flow conditions were configured for viscous flow at Re = 300,000 and for angles of attack ranging from 0°t o 20°. Three new airfoils labelled EYO7-8, EYO8-8, and EYO9-8 were subsequently developed, and the performance 2 Journal of Energy parameters are presented in Section 3. BEMT was applied to the three new airfoils to develop 3-bladed 6-metre-diameter wind turbine rotors, and preliminary assessments of their power coefficients were evaluated. The power coefficient data was used to estimate the power generation of each wind turbine rotor based on typical average wind speed conditions in Ghana.

Mathematical Framework.
The Re for the airflow over the airfoil is expressed mathematically in Equation (1) [39]. For a typical airfoil in a wind turbine blade section, the relationship between the free stream airflow speed and the relative airflow speed at the airfoil is expressed in Equation (2) [29]. The airfoil lift and drag forces are expressed in Equations (3) and (4), respectively [40]. The lift-to-drag ratio is also expressed in Equation (5).
2.3. Blade Element Momentum Theory. Blade Element Momentum Theory (BEMT) uses airfoil information for the design of wind turbine blades for wind power extraction.
BEMT is used extensively for wind turbine blade design and testing. This has been mainly due to the agreement of the theory with experimental data. Several studies on wind turbine rotor design have demonstrated the robustness of BEMT in wind turbine rotor analysis [33,35,[41][42][43]. The BEMT equations used for the rotor development are expressed in Equations (6)-(10) [40]. The power generation (P) from the turbine rotor is computed from Equation (11) based on the free airstream properties (ρ and U), the projected rotor area (A), and the power coefficient C p (Equation (10)) [44].
2.4. Airfoil Design Optimization. In the present study, the airfoil design optimization was based on the lift-to-drag ratio (L/D) performance for different airfoil t/c. From the preliminary results for the design optimization, Figure 2 shows the variations in maximum L/D with t/c at Re between 100,000 and 500,000. From Figure 2, the maximum L/D had a dome-shaped relation with t/c at each Re, and the L/D performance generally improved with increase in Re. The highest point of the maximum L/D for each Re occurred in about the same range between t/c of 0.85 and 1.50. However, outside this t/c range, the maximum L/D is observed to decrease. In the design of the airfoils in this study, t/c in the range of 0.85-1.50 was considered in determining the thicknesses and camber of the airfoils.
Based on the preliminary findings presented above, the optimal zones chosen for the development of the new airfoils with improved aerodynamic performances were 7% c, 8% c, and 9% c thicknesses and 8% c camber. These thickness and camber characteristics fall within the t/c range of 0.85-1.50. Three airfoils were developed in this zone and were labelled EYO7-8, EYO8-8, and EYO9-8. The geometrical parameters and profiles of these airfoils are presented in   Figure 3(a), respectively. For the purpose of comparison, the different shapes of the airfoils have been overlapped in Figure 3(b).
From Figure 3, the main difference between the base airfoil and the newly developed EYO-Series airfoils is in the geometrical characteristics of the camber and thickness profiles.
The new EYO-Series airfoils have relatively higher cambered and thinner designs compared with the base airfoil. The performance characteristics of the newly designed airfoils as well as the base airfoil were determined at Re of 300,000, as already indicated above, based on average wind speed data of 5 to 6.1 m/s at 12 metres above ground level

Results and Discussions
3.1. Comparative Lift Performance of EYO-Series Airfoils and SG6043 Base Airfoil. The lift coefficient performance curves of the SG6043 and EYO-Series airfoils at Re = 300,000 are presented in Figure 4. The variation of maximum lift coefficient (C L ) for the airfoils at the same Re is also presented in Figure 5, in addition to the stall angles. From Figures 4 and 5, it is observed that the new EYO-Series airfoils have relatively higher lift coefficients than the SG6043 base airfoil. Although the new airfoils had similar lift performance trends for up to 12°angle of attack, the EYO9-8 airfoil had relatively better lift performance for angles of attack beyond 12°. The EYO8-8 and EYO9-8 airfoils had the highest maximum lift coefficient of 1.81 while the SG6043 airfoil had the lowest maximum lift coefficient of 1.63. The EYO7-8 airfoil had maximum lift coefficient of 1.77.
From Figure 5, the SG6043 and EYO9-8 airfoils had the highest stall angle of 15°. The EYO7-8 and EYO8-8 airfoils had stall angles of 12°and 14°, respectively. Again, among the new airfoils and from Figure 4, EYO9-8 had the most gradual stall performance. Gradual stall is desired in airfoil design in order to prevent the sudden loss of lift which could adversely affect wind turbine performance.  5 Journal of Energy of the base airfoil and EYO-Series airfoils at Re = 300,000 is presented in Figure 6.

Comparative
From the results of Figure 6, the EYO-Series airfoils have relatively better L/D performance than the SG6043 airfoil, especially for angles of attack between 0°and 9°. EYO7-8 airfoil had the highest maximum L/D of 134 while SG6043 had the lowest value of 117. EYO8-8 and EYO9-8 also had maximum L/D values of 131 and 127, respectively. The maximum L/D for all four airfoils occurred at 5°angle of attack.

Comparative Drag Buckets of EYO-Series Airfoils and
SG6043 Base Airfoil. The drag buckets of the SG6043 and EYO-Series airfoils at Re = 300,000 are presented in Figure 7. In airfoil design, the drag bucket gives a graphical indication of how the drag coefficient extends over a range of lift coefficients.
From Figure 7, the EYO-Series airfoils had better performing drag buckets than the SG6043 airfoil. Although the initial increases in lift occurred steadily with no corresponding   AF300 airfoil was developed for low Re wind turbine application. Maximum L/D and stall angle of 54 and 14°, respectively, were obtained at Re = 100,000.

Selig and McGranahan, 2004 [15]
FX63-137 airfoil was tested for low Re performance at Re = 500,000. Maximum L/D and stall angle obtained were 108 and 12°, respectively. Henriques et al., 2009 [30] T.Urban 10/193 airfoil was developed for low Re wind turbine application and tested at Re = 500,000. Maximum L/D and stall angle obtained were 95 and 12°, respectively.
Limin et al., 2011 [47] The seagull airfoil was developed for small wind turbine application and tested at Re = 100,000, 350,000, and 600,000. Maximum L/D obtained were 38, 52, and 57, respectively. Shah et al., 2014 [27] UBD6166 airfoil was developed for small wind turbine application and tested at Re = 100,000. Maximum L/D of 66.9 was obtained.
Wata et al., 2011 [17] SG6043_Eppler 442 airfoil was developed for small wind turbine application and tested at Re = 205,000. Maximum L/D obtained was 80. Among the EYO-Series airfoils, the performance differences mainly occur beyond the drag coefficient of 0.05, where EYO9-8 has the best drag bucket performance, followed by EYO8-8 and EYO7-8.

Airfoil Performance Summary.
To ascertain the performance of the EYO-Series airfoils in comparison with other airfoils developed by different researchers for low Re and small wind turbine applications, the L/D at Re = 300,000 are presented in Figure 8 which is adapted from a study by Gigueŕe and Selig [28]. From the comparative results of Figure 8, the EYO-Series airfoils outperform the SG604x airfoil family and the others by relatively higher L/D performances which occur at a relatively higher lift coefficient of about 1.6. Again, the EYO-Series airfoils, with their high lift-to-drag ratios, outperform other developed low Re airfoils presented in Table 2. These performance improvements of the new airfoils are due to the optimal t/c employed in their design. The zone for the t/c between 0.85 and 1.50 leads to the development of relatively thin and cambered designs which are generally known as features that improve airfoil aerodynamic performance [23,45,46].

Power Generation Potential.
To be able to estimate the potential power generation from the performance characteristics of the newly developed EYO-Series airfoil designs, power coefficient curves were computed for 3-bladed 6 m diameter rotors made from each airfoil and are presented in Figure 9. The power coefficient for all three rotors had similar dome-shaped relationship with tip-speed ratio. The maximum power coefficient for all three rotors occurred at about the same tip-speed ratio of 4.5. However, the EYO7-8 rotor had the highest maximum power coefficient of 0.371, while the EYO9-8 rotor had the lowest maximum power coefficient of 0.358. The EYO8-8 rotor had maximum power coefficient of 0.366. Based on typical average wind speed in Ghana of 6.1 m/s reported by the Ghana Energy Commission, the estimated power production from the EYO7-8, EYO8-8, and EYO9-8 rotors according to Equation (11) is 1488 W, 1468 W, and 1435 W, respectively. This potential power generation from the small wind turbines meets the power demand of most residential facilities in developing countries as reported in the works of [48][49][50][51].

Conclusions
In the present study, three new airfoils (EYO-Series) for small wind turbine applications were designed and tested in XFOIL for aerodynamic performance at Re = 300,000. The airfoils were subsequently used to develop and test 3-bladed 6 m diameter wind turbine rotors for power generation. From the results, the following key findings were made: Abbreviations a: Axial induction factor a′: Angular induction factor α: Angle of attack ρ: Air density c: Airfoil chord μ: Dynamic viscosity ν: Kinematic viscosity Ω: Angular velocity ϕ: Relative airflow angle θ p : Pitch angle σ r : Chord solidity λ: Tip-speed ratio λ r : Local speed ratio λ h : Blade root speed ratio A: Rotor cross-sectional area Airfoil span L: Lift force L/D: Lift-to-drag ratio m: Metre P: Rotor power r: Radial airfoil location Re: Reynolds number t/c: Thickness to camber ratio U: Free stream airflow speed U rel : Relative airflow speed % c: Percentage of chord.

Data Availability
The data used to support the findings of this study are available from the corresponding author upon request.

Additional Points
Further Studies. The present study has analysed the performance of newly developed airfoil designs based on an optimal thickness-to-camber ratio (t/c) of 0.85-1.5, for low Reynolds number applications, using XFOIL. The results have shown that better performance of the airfoil is obtained within this optimal t/c range. Future work of constructing prototypes of the airfoil designs based on the optimal t/c and testing them in a wind tunnel is worth investigating.

Conflicts of Interest
The authors declare that there is no conflict of interest regarding the publication of this paper.