Wind Energy Potential for Small-Scale Wind Concentrator Turbines *Ashraf Amer1), Ahmed Hamza H. Ali2), Yehia ElMahgary3), Shinichi Ookawara4), Mahmoud Bady5) 1), 2), 3), 5)
Department of Energy Resources and Environmental Engineering, Egypt-Japan University of Science and Technology, Alexandria, 21934, Egypt 4) Department of Chemical Engineering, Graduate School of Science and Engineering, Tokyo Institute of Technology, 152-8552, Japan 1)
[email protected] ABSTRACT The main aim of this study is to develop and to design a wind concentrator for smallscale turbine, to operate mainly at low and medium wind speeds. The computational fluid dynamics (CFD) simulations are used to investigate the wind flow patterns for different types and configurations of concentrators. Wind concentrators of different profiles, with and without cylindrical part and diffusers, and of different inlet to outlet diameter ratios are examined to attain maximum wind velocity value inside the concentrator and to estimate its location which represents the optimum place of the turbine rotor. The results show that the optimum converging diameter ratio d1/d2 is 1.5, converging length to cylinder diameter ratio l1/d2 is 1.33 and cylindrical section length to diameter ratio l2/d2 is 0.5, respectively. The optimal shape for the wind concentrator is the concave with a radius of curvature R1/d2 of 3.289. The free stream wind velocity is accelerated from 5 m/s to 11.64 m/s , consequentially the available wind energy capture at the turbine is increased by a factor (2.328)2. 1. INTRODUCTION As the fossil resources of Egypt are dwindling and could be exhausted in a decade or less, Egypt has no alternative, but to turn to renewable and nuclear energy. One of the most important future renewable energy sources of Egypt is wind energy which is also one of the main renewable energy sources worldwide. The wind pattern along the Red Sea and Mediterranean Sea coasts are suitable for harnessing wind energy, particularly on Suez Gulf where the average wind speed is over 10 m/s as seen in Fig. 1 (Mortensen et al. 2006). Egypt has already established wind farm in Zaafarana, along the Suez Gulf of approximately 370 MW, thus saving approximately one million tons of CO2/year. The main strategy of utilizing the wind energy in Egypt is mainly focused on such a large scale power generation. While for remote and small community’s areas apart from the strong wind regions, there is a need of developing a small and powerful wind turbine that can supply these areas with electric energy. Therefore, the main
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objective of this study is to develop wind concentrator turbine that can work at medium and low wind speed for power generation in remote and small communities’ areas in Egypt. Among others, wind energy technologies have developed rapidly and are about to play a big role in a new energy field. However, in comparison with the overall demand for energy, the scale of wind power usage is still small. Therefore, the introduction of a new wind power system that produces higher power output even in those areas with lower wind speeds and complex wind patterns is strongly desired (Ohya et al. 2008).
Fig. 1 Wind resource map of Egypt (Mortensen et al. 2006) The main component of extracting the energy from wind is the turbine. One of the differences between large- and small-scale wind turbines is that small-scale wind turbines are generally located where the power is required, often within a built environment, rather than where the wind is most favorable. In such location, the wind is normally weak because of the presence of buildings and other adjacent obstructions. In order to yield a reasonable power output from a small-scale wind turbine located in this environment, and to justify such an installation economically, the wind energy input to the turbines have to be concentrated for higher energy capture, particularly at low wind speeds. This means that small-scale turbines need to be specifically designed to work effectively in low wind resource areas (Wang et al. 2008). The techniques of the wind concentrator is attractive in low and medium wind speed due to the fact that the power produced from wind energy is proportional to the incident
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airspeed raised to the power three and the diameter to the power two. Therefore, any small increase in the incident wind velocity yields a large increase in the energy output from the turbine. The main idea of wind concentrator is to increase the wind power in front of the wind turbine in sites with weak to normal wind speeds conditions through overcoming the starting torque. In the literature, there can be seen many studies that attempted to exploit this relationship. Such studies mainly added a diffuser to the wind turbine from the early 1980s (Gilbert and Foreman 1983) to the beginning of this century (Phillips et al. 2000) (Hansen et al. 2000). More recently, (Ohya et al. 2002; 2004) have produced an effective wind-acceleration system which contains a large diffuser with a flange creating a large separation in the flow. This configuration generates a low-pressure region behind it which assists the turbine in capturing more wind energy compared to a system with a diffuser on its own. From experimental research conducted on this system, it has been shown that a diffuser–shrouded wind turbine generates more power compared to a bare wind turbine; with a power coefficient four times higher (Abe et al. 2005). Other research groups (Matsushima et al. 2006) have found that the wind speed in a diffuser is significantly influenced by the length and the expansion angle of the diffuser and with an optimum design can create a wind speed improvement of 1.7 times. Throughout the literature, it can be noted that the previous study abovementioned focused on the operation of wind concentrator in the higher wind speed range, while the applicability of the diffuser in the normal wind speed is not clarified. Moreover, it is difficult for small scale wind turbines to operate in weak airflows; it beforehand faces the difficulty in starting the rotor system at low airspeeds. In the literature, there can be seen some works on the start-up of wind turbine (Wright and Wood, 2004; Mayer et al., 2001; Wood, 2001; Ebert and Wood, 1997; Clausen and Wood, 2000). Many researchers as (Ohya et al. 2002, 2004, 2008; Abe et al. 2005; Abe et al. 2005; Phillips et al. 2000; Hansen et al. 2000) have studied the flow fields behind the small-scale wind turbine with a flanged diffuser in the higher wind speed range, while few of them (Wang et al. 2008) have studied the effect of concentrator on the flow field in front of and behind the turbine and the output power. Although there can be seen many trials to install the wind concentrator for large scale turbines in the literature, it was found to make the whole system bulky and cost-ineffective compared to those large-scale turbines installed at high wind speed sites. Small scale turbine with concentrator has been investigated in different designs configurations such as Savonius rotors, impulse turbine, and so on as (Kho et at. 2009; Orosa 2010; Anzai et al. 2009; Ladvyk 1977; Meheen 2008; Sabzevari 1978). In the present study, computational fluid dynamics (CFD) simulations are used to investigate the wind flow patterns for different types and configurations of wind concentrators. Wind concentrators with different profiles, with and without cylindrical section and diffusers, and with different inlet to outlet diameter ratios are examined to find the optimum place for the turbine generator and to attain the minimum wake losses. Moreover, to find the find the optimal design of concentrator for maximizing the wind energy captured by wind turbine. 2. METHODOLOGY
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The commercial CFD package (ANSYS-Fluent), which is based on finite volume method, is employed to perform a detailed 3D analysis of the flow pattern around and inside the wind concentrator in a stationary reference frame. Both the κ -ε RNG and the κ–ω SST turbulent models are used with a constant axial wind velocity at the farupstream. 2.1. Wind Concentrator Design The design of wind concentrator includes converging part, cylindrical part and diverging part as shown in Fig. 2. R1
R2
t
Outlet
Inlet d1
d2
l1
d3
l2
l3
Fig. 2 Wind Concentrator arrangement 2.2. Wind Concentrator Configurations The design parameters of the examined wind concentrators in dimensionless form are shown in Table 1. These parameters are d1/d2, d3/d2, l1/d2, l2/d2, l3/d2, R1/d2 and R2/d2, where R1 and R2 are radii of curvature of converging and diverging part, respectively as shown in Fig. 2. Table 1 Geometric parameters of examined contractors Case (1) (2) (3) (4) (5) (6) (9) (10)
Converging Part d1/d2 l1/d2 shape cone
cone cone cone cone
2
2 3 2.5 1.5
1
1 1 1 1
R1/d2
Cylinder d2[mm] l2/d2 300 300 300 600 300 300
Diverging part d3/d2 l3/d2 shape
2.5 0 1 0 0 0
cone cone
2 2
1 1
4
(12) (19) (22) (26) (11) (21) (24) (25) (27) (28) (30) (18) (35) (36) (37) (39) (40) (7) (13) (23) (33) (14) (15) (20) (34) (31) (43)
concave cone cone cone convex cone cone cone cone cone cone concave concave concave concave concave concave concave convex cone convex cone inside cylindrical shell
cone cone concave cone concave
specified profile 1.25 1.5 0.333 2 1.25 specified profile 1.5 1.5 0.667 2 1.25 1.25 2 1 1 specified profile 1.5 specified profile 1.25 1 specified profile 1.5 specified profile 1.5 specified profile 2 1 specified profile 1.75 1.5 2.99 1.5 2.22 1 1.5 1.5 2.99
300
1
300
1
300
1
300
1
300
1
300
1
1 2 1.5 1.5 1.5
1 0.333 specified profile 1 1 1.333 3.289
300
1
300 300 300
0.167 1 0.5
convex cone cone cone convex cone cone cone cone cone cone convex convex concave convex convex convex cone cone cone cone cone inside cylindrical shell
cone cone convex cylinder convex
1.5
1
1.5
1
1.5
1
1.5
0.5
2
0.5
1.5
1
2
1
1.5 1 1 1 2 1.333
2.3. The Computational Model The computational model which includes the wind concentrator and the fluid domain is developed using Ansys-Fluent. The domain has a cylindrical shape as shown in Fig. 3 and it has a diameter of 16.67 times that of the cylinder part of the concentrator, and a length of 33.33 times that of the concentrator, in order to investigate the effects of the wind concentrator on the airflow passing through it which still remains unknown (Wang et al. 2008). The inlet and outlet boundaries of the domain are defined as velocity inlet and pressure outlet, respectively, and the outer domain wall are defined as slip wall with momentum flux = 0. An unstructured mesh consisted of tetrahedral elements are used to mesh the domain, resulting in 1,949,921. Both κ -ε RNG with standard wall function and κ -ω SST turbulence models are adopted for all the simulations. 2.4. Boundary Conditions As the inlet boundary condition, a uniform wind flow Uo of 5 m/s, or 10 m/s is specified at the upstream-end of the domain. As the outlet boundary, a specific static pressure (atmospheric pressure, 101,325 Pa) is set at the downstream-end of the
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domain, with backflow conditions to minimize convergence difficulties. No-slip condition is defined for the wind concentrator walls. Ideal-slip or Euler-slip is specified for the side-wall of the domain which might be useful to diminish the influence from artificial boundaries. 2.5. Computational Grid The pre-processor Ansys-Meshing is used to build a tetrahedral (unstructured) mesh of approximately 1.9 million volumes. Around the concentrator walls, the grid size is 5 mm (fine grid) as shown in Figs. 4 and 5. To model the turbulent boundary layer near concentrator wall, five layers of inflation are used with a growth rate 1.05. Coarse grids of 750 mm were used for the rest of the computational domain. The computations are carried out on a super-computer that is consisted of 112 cores for parallel processing. Upstream end
Downstream end
Fig. 3 The cylindrical domain
Fig. 4 Fine mesh of 5 mm size near the concentrator walls
Fig. 5 Cross section through the wind concentrator to show the fine mesh near concentrator wall 6
3. RESULTS AND DISCUSSION The main aim of this study is to find the optimal concentrator configuration that maximizes wind speed to gain the maximum wind at turbine location in order to gain the maximum wind energy captured by the wind turbine. 3.1. Conical Wind Concentrator Wind concentrators which are consisted of conical converging part, are examined at first without addition of cylindrical or diverging parts. The axial air velocity distribution of the conical concentrator is presented in Fig. 6 for inlet wind speed of 5 m/s. The results show that, the converging diameter ratio d1/d2 affects the centerline velocity distribution. The inlet airflow velocity of the cone is decreasing by increasing the converging ratio because of increasing the flow resistance of the converging duct. The maximum obtained velocity ratio of Ux/Uo is 1.303 at an axial distance x of 1.668 times d2 for a converging ratio d1/d2 of 3 (case 6) as shown in Fig.6 and for wind speed of 5 m/s.
12
-2
-1
0
x/d2 1
2
3
4
5
6 2.4
case2 (d2/d1=2, L1/d2=1, d2=300mm) case5 (d1/d2=2, L1/d2=1, d2=600mm) case6 (d1/d2=3, L1/d2=1, d2=300mm) case9 (d1/d2=2.5, L1/d2=1, d2=300mm) case10 (d1/d2=1.5, L1/d2=1, d2=300mm) Case 43 (optimum)
11 Velocity Magnitude Ux [m/s]
-3
10 9 8 7 6
2.2 2 1.8 1.6 1.4 1.2
5
1
4
0.8
3
0.6
2
0.4
1
0.2 -1.2
-0.9
-0.6
-0.3
0
0.3 0.6 Axial distance x [m]
0.9
1.2
1.5
Ux/Uo
-4
1.8
Fig. 6 Airflow velocity distribution along the central line parallel to the airflow inside conical wind concentrator of different converging ratio d2/d1 at inlet wind speed of 5 m/s
3.2. Effect of Adding the Cylindrical and Diverging Parts to Concentrator In order to enhance the flow characteristics and to accelerate the airflow velocity, cylindrical and diverging parts are added to the converging part. The axial air velocity distribution of concentrators with cylinder and/or diffusers is shown in Fig. 7. The presented results show that, the conical diffuser alone (Case 3) is better than conical concentrator (case 10) in accelerating airflow velocity that reaches 1.23 times the undisturbed wind velocity Uo, while it becomes 1.435 times Uo in case of addition of cylinder ahead of the same diffuser (case 4) as shown in Fig. 7, for inlet wind speed of 5 m/s. Moreover, the centerline velocity Ux increases to 2.085 times Uo by using a
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convex concentrator with cylinder and diffuser (case13) as shown in Fig. 7. Therefore, it is not recommended to use contractor without addition of cylinder or diffuser because of poor flow characteristics and, more investigation will be carried out throughout this study is to find the optimal design parameters d1/d2, d3/d2, l1/d2, l2/d2, l3/d2, R1/d2 and R2/d2 of the wind concentrator. x/d2
-2
-1
0
1
case1 (cylinder, L2/d2=2.5)
2
3
4
5
6
12
2.4
11
2.2
10
2
case4 (cylinder L2/d2=1 with conical diffuser L3/d2=1, d3/d2=2)
9
1.8
case10 (conical concentrator L1/d2=1, d1/d2=1.5)
8
1.6
7
1.4
6
1.2
5
1
4
0.8
3
0.6
2
0.4
1
0.2 -0.9
-0.6
-0.3
0
0.3 0.6 0.9 Axial distance x [m]
1.2
1.5
case3 (conical diffuser, L3/d2=1, d3/d2=2)
Ux/Uo
Velocitymagnitude Ux[m/s]
-3
case30 conical concentrator (L1/d2=1, d1/d2=1)+ cylinder (L2/d2=1)+ conical diffuser (L3/d2=1, d3/d2=1.5) case31 conical concentrator (d1/d2=1.5, L1/d2=1)+ cylinder (L2/d2=1)+ conical diffuser (d3/d2=1, L3/d2=1) case33 convex concentrator (R/d2=2.99,d2/d1=1.5, L1/d2=1) +cylinder(L2/d2=1)+ convex diffuser (d3/d2=1.5, L3/d2=1) case13 convex concentrator (R/d2=2.22,d3/d2=1.5, L3/d2=1)+cylinder (L2/d2=1) + convex diffuser (d3/d2=1.5, L3/d2=1) Case 43 optimal
1.8
Fig. 7 Airflow velocity distribution along the central line parallel to the airflow inside the wind concentrator at inlet wind speed of 5 m/s to study the effect of adding cylindrical and diverging part to the converging part
3.3. Effect of converging diameter ratio and shape of wind concentrator From the previous results, clearly, the converging diameter ratio d1/d2 of the concentrator has a significant effect on the velocity distribution and the location of the maximum velocity inside it. The effect of converging ratio d1/d2 on the axial air velocity distribution is shown in Figs. 8 through 13. The presented results show that, as the converging ratio d1/d2 decreases from 2 to 1.25, the corresponding maximum central line velocity ratio Ux/Uo increases from 1.4 to 1.95 inside the wind concentrator as it can be seen from Fig. 8. This is due to the decrease in the duct flow resistance which assists in capturing more air flow rate. Moreover, the converging shape affects the value of maximum central line velocity. The results show that, the concentrator of concave shape (case 22) is better than conical shape (case 26) for the same converging ratio d1/d2 of 1.25, and the corresponding Ux/Uo increases from 1.95 to 2.15, respectively, as it can be seen from Fig. 8. This is due to due to that, the further decrease in duct flow resistance. Moreover, the results show that, the effect of changing concentrator length to cylinder diameter ratio l1/d2 leads to a slight increase of the maximum value of Ux/Uo. It is increased from 2.12 to 2.20 by increasing the ratio l1/d2 from 0.67 to 1 (case 11 and case7, as shown in Figs. 9 and 10), respectively.
8
Velocity magnitude Ux [m/s]
10 9 8 7
-1
0
3
4
5
6
case19 (conical concentrator d1/d2=1.5, conical diffuser) case22 (conical concentrator d1/d2=2, conical diffuser) case26 (conical concentrator d1/d2=1.25 , conical diffuser) case12 (concave concentrator d1/d2=1.25, convex diffuser) case 43 (Optimal Case)
2.4
12
2.2
11
2
10
1.8 1.6 1.4
9 8 7
-3
-2
-1
0
1
2
3
4
5
6
2.4
case21 (conical concentrator d1/d2=1.5,conical diffuser) case24 (conical concentrator d1/d2=2,conical diffuser) case25 (conical concentrator d1/d2=1.25, conical diffuser) case11 (concave concentrator d1/d2=1.5, convex diffuser) case 43 (optimal shape)
2.2 2 1.8 1.6 1.4
6
1.2
5
1
4
0.8
3
0.6
0.4
2
0.4
0.2
1
0.2
6
1.2
5
1
4
0.8
3
0.6
2 1 -1.2 -0.9 -0.6 -0.3 0 0.3 0.6 0.9 1.2 1.5 1.8 Axial Distance x [m]
Fig. 8 Airflow velocity distribution along the central line parallel to the airflow inside wind concentrator to study effect of d1/d2 for d3/d2=1.5, l1/d2=0.33, l2/d2=1, l3/d2=1 at inlet wind speed of 5 m/s
-1.2 -0.9 -0.6 -0.3 0 0.3 0.6 0.9 1.2 1.5 1.8 Axial Distance x [m] Fig. 9 Airflow velocity distribution along the central line parallel to the airflow inside wind concentrator to study effect of d1/d2 for d3/d2=1.5, l1/d2=0.667, l2/d2=1, l3/d2=1 at inlet wind speed of 5 m/s
Figs. 10 and 11 show the results of decreasing the diffuser length to cylinder diameter ratio l3/d2 from 1 to 0.5, which is followed by, the maximum value of Ux/Uo slightly increases from 2.2 to 2.24 (case 7 and case 18), respectively. While for the concave concentrator with convex diffuser, the results shown in Fig. 12, clearly indicate that, the optimum converging ratio d1/d2 is 1.5 at the corresponding maximum value of Ux/Uo of 2.27 (case 37). In addition, the concentrator radius of curvature R1 affects the air velocity distribution inside the wind concentrator as shown in Fig. 13. Clearly, the results from this figure show that, increasing the radius of curvature R1/d2 from 2.22 to 2.99, the corresponding maximum value of Ux/Uo increases from 2.1 to 2.2 (case 13 and case 7), Moreover, the concave concentrator (case 7) has better Ux/Uo than convex one (case 33) of the same radius of curvature R1/d2 of 2.99 as seen from results in Fig. 13. 3.4. Effect of Converging Length of the Concentrator The effect of converging length to cylinder diameter ratio l1/d2 of the concentrator on the axial velocity distribution inside it, corresponding to different values of converging diameter ratio d1/d2, diffuser diameter ratio d3/d2, cylinder length to diameter ratio l2/d2 and diffuser length to cylinder diameter ratio l3/d2 is shown in Figs 14 through 17. The presented results of the conical concentrator with conical diffuser of d1/d2=1.25, d3/d2=1.5, l2/d2=1, l3/d2=1, indicate that the maximum value of Ux/Uo increases from 1.75 to 2.05 by decreasing l1/d2 of the contractor from 1 to 0.33, as it can be seen from Fig.
9
Ux / Uo
11
-2
Velocity magnitude Ux [m/s]
12
-3
x / d2 -4
Ux/ Uo
-4
x/d2 1 2
14. However, changing the contcentrator shape fromconical to concave and the diffuser shape from conical to convex (case 12) shown in Fig. 14, the results indicate that, the maximum value of Ux/Uo increases to 2.15 which is better value than that of case 26. 3
4
5
6
10 9 8 7 6
-4 2.4
12
2.2
11
2
10
1.8 1.6 1.4 1.2
-3
-2
-1
0
1
2
3
4
5
6 2.4
case18 (concave concentrator d1/d2=1.5 , convex diffuser) case35 (concave concentrator d1/d2=1.25, convex diffuser) case36 (concave concentrator d1/d2=1.5, concave diffuser) case 43 (optimal shape)
9 8 7
2.2 2 1.8 1.6 1.4
6
1.2
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1
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3
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0.2 -1.2 -0.9 -0.6 -0.3 0
-1.2 -0.9 -0.6 -0.3 0 0.3 0.6 0.9 1.2 1.5 1.8 Axial distance x [m]
-1
0
x / d2 1 2
3
4
5
12 11
Velocity magnitude Ux [m/s]
10
-3
6
case37 (d1/d2=1.5) case39 (d1/d2=2) case40 (d1/d2=1.75) Case 43 (optimal)
-1
0
1
x/ d2 2
3
4
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6
12
2.4
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2
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9
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-2
2.4
0.2 -1.2 -0.9 -0.6 -0.3 0 0.3 0.6 0.9 1.2 1.5 1.8 Axial distance x [m]
Fig. 12 Airflow velocity distribution along the central line parallel to the airflow inside wind concentrator (concave contractor with convex diffuser) to study effect of d1/d2 for d3/d2=2, l1/d2=1, l2/d2=1, l3/d2=0.5 at inlet wind speed of 5 m/s
Velocity magnitude Ux [m/s]
-2
Fig. 11 Airflow velocity distribution along the central line parallel to the airflow inside wind concentrator to study effect of d1/d2 for d3/d2=1.5, l1/d2=1, l2/d2=1, l3/d2=0.5 at inlet wind speed of 5 m/s
Ux / U o
-3
0.3 0.6 0.9 1.2 1.5 1.8
Axial Distance x [m]
Fig. 10 Airflow velocity distribution along the central line parallel to the airflow inside wind concentrator to study effect of d1/d2 for d3/d2=1.5, l1/d2=1, l2/d2=1, l3/d2=1 at inlet wind speed of 5 m/s
-4
Ux / Uo
0
9
1.8
8
1.6
7
1.4
6
1.2
5
1
4
case23 (conical concentrator)
3
case7 (concave concentrator R=896 mm)
0.6
2
case13 (convex concentrator R=665.66 mm)
0.4
0.8
case33 (convex concentrator R=896 mm)
Case 43 (optimum)
1
0.2 -0.9 -0.6 -0.3
0 0.3 0.6 0.9 Axial distance x [m]
1.2
1.5
1.8
Fig. 13 Airflow velocity distribution along the central line parallel to the airflow inside wind concentrator to study effect of converging shape for d1/d2=1.5, d3/d2=1.5, l1/d2=1, l2/d2=1, l3/d2=1 at inlet wind speed of 5 m/s
10
Ux / U o
-1
Velocity mangnitude Ux[m/s]
-2
case23 (conical concentrator d1/d2=1.5 , conical diffuser) case27 (conical concentrator d1/d2=1.25,conical diffuser) case28 (conical concentrator d1/d2=2, conical diffuser) case30 (conical concentrator d1/d2=1,conical diffuser) case7 (concave d1/d2=1.5, convex diffuser) case13 (convex concentrator d1/d2=1.5,convex diffuser) case 43 (optimal shape)
11
Velocity magnitude Ux [m/s]
-3
x / d2
Ux / Uo
-4 12
x / d2 1 2
Increasing the converging diameter ratio d1/d2 to 1.5 as presented in Fig. 15 while keeping the remaining parameters d3/d2, l2/d2 and l3/d2 unchanged, a new result is obtained. The results indicate that, the maximum value of Ux/Uo is increased from 1.74 to 2.06 by increasing converging length to cylinder diameter ratio l1/d2 from 0.33 to 1 for the conical contcentrators with conical diffusers (cases 19, 21 and 23) as the converging diamter ratio d1/d2 increased from 1.25 to 1.5. In addition, changing the concentrator shape from conical to concave while keeping other parameters unchanged, the results show that, the maximum value of Ux/Uo is increased to 2.19 as it can be seen from Fig. 15 (case 7), this is due to streamlining of the incoming flow and avoiding of sharp corners. The effect of increasing the diameter converving ratio d1/d2 to 2 for the conical concentrator with conical diffuser on the ratio Ux/Uo is shown in Fig. 16. The presented results show that, all maximum values of Ux/Uo are decreased to 1.38, 1.41 and 1.61 corresponding to l1/d2 of 0.33, 0.667 and 1, respectively. This is due to the increase in contraction resistance of the concentartor. Moreover, increasing the diffuser diameter ratio d3/d2 to 2, the maximum values of Ux/Uo are decreased to 1.27 and 1.45 corresponding to l1/d2 of 0.33 and 1, respectively as it can be seen from Fig. 17. This is due to theseparation effect which happens in diffuser by increasing its diameter ratio d3/d2.
-2
-1
0
1
x/ d2 2
3
4
5
6
-3
7
-2
-1
0
1
x/ d2 2 3
4
5
6
7
2.4
12
2.4
11
2.2
11
2.2
10
2
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2
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3
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2
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1
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1
-0.9 -0.6 -0.3
0 0.3 0.6 0.9 1.2 Axial distance x [m]
1.5
1.8
case25 (L1/d2=0.667, conical concentrator with conical diffuser) case26 (L1/d2=0.333,conical concentrator and diffuser) case27 (L1/d2=1, conical concentrator and diffuser) case12 (L1/d2=0.33, concave concentrator with convex diffuser) Case 43 (optimal)
2.1
Velocity Magnitude Ux [m/s]
9
Ux / U o
Velocity Magnitude Ux [m/s]
12
9
1.8
8
1.6
7
1.4
6
1.2
5
1
4
0.8
3
0.6
2
0.4 0.2 -0.9 -0.6 -0.3
0 0.3 0.6 0.9 1.2 1.5 1.8 2.1 Axial Distance x [m]
case19 (L1/d2=0.33, conical concentrator with conical diffuser) case21 (L1/d2=0.667, conical concentrator with conical diffuser) case23 (L1/d2=1, conical concentrator with conical diffuser) case7 (L1/d2=1, concave concentrator with convex diffuser) case11 (L1/d2=0.667, convex concentrator with convex diffuser) Case 43 (optimum case)
Fig. 15 Airflow velocity distribution along the Fig. 14 Airflow velocity distribution along the central central line parallel to the airflow inside wind line parallel to the airflow inside wind concentrator concentrator to study effect of converging length to study effect of converging length to cylinder to cylinder diameter ratio l1/d2 for d1/d2=1.5, diameter ratio l1/d2 for d1/d2=1.25, d3/d2=1.5, d /d =1.5, l /d 3 2 2 2=1, l3/d2=1 at inlet wind speed of 5 l2/d2=1, l3/d2=1 at inlet wind speed of 5 m/s m/s
11
Ux / U o
-3
-2
-1
0
3
4
5
6
7
12 11 10 9 8 7 6 5 4 3 2 1
2.4 2.2 2 1.8 1.6 1.4 1.2 1 0.8 0.6 0.4 0.2 -0.9 -0.6 -0.3
case22 (L1/d2=0.333)
Ux/ Uo
Velocity magnitude Ux [m/s]
-3
x / d2 1 2
case24 (L1/d2=0.667) case28 (L1/d2=1) Case 43 (optimum)
0 0.3 0.6 0.9 1.2 1.5 1.8 2.1 Axial Length x [m]
Fig. 16 Airflow velocity distribution along the central line parallel to the airflow inside wind concentrator (conical concentrator with conical diffuser) to study effect of converging length l1/d2 for d1/d2=2, d3/d2=1.5, l2/d2=1, l3/d2=1 at inlet wind speed of 5 m/s -2
-1
0
1
x/d2 2
3
4
5
6
7
12 11 10 9 8 7 6 5 4 3 2 1
2.4 2.2 2 1.8 1.6 1.4 1.2 1 0.8 0.6 0.4 0.2 -0.9 -0.6 -0.3
case15 (L1/d2=1)
Ux/Uo
Velocity magnitude Ux [m/s]
-3
case20 (L1/d2=0.333) Case 43 (optimum case)
0 0.3 0.6 0.9 1.2 1.5 1.8 2.1 Axial Distance x[m]
Fig. 17 the airflow velocity distribution along the central line parallel to the airflow inside wind concentrator (conical contractor with conical diffuser) to study effect of converging length l1/d2 for d1/d2=2, d3/d2=2, l2/d2=1, l3/d2=1 at inlet wind speed of 5 m/s
3.6. Effect of Cylindrical Section Length to Diameter Ratio The effect of cylindrical part length to diameter ratio l2/d2 is shown in Fig. 18. The presented results show that, the optimum cylinder length l2 is 0.5 times cylinder diameter d2 (case 18) for concave contractor with convex diffuser which results a velocity ratio Ux/Uo of 2.05. Moreover, for any selected value of the cylinder length to diameter ratio l2/d2 ,except the optimum value l2/d2 which is 0.5, the maximum value of Ux/Uo will decrease, as it can be seen from Fig. 19, for case 34 (l2/d2 = 0.1667) and
12
case 33 (l2/d2 = 1) of the same shape. This is due to the increase in friction length for case 34 (l2/d2 = 1), while for case 33 (l2/d2 = 0.5), the flow is not fully developed inside the cylindrical part. 12 11 10 9 8 7 6 5 4 3 2 1
0
1
x/d2
2
3
4 2.4 2.2 2 1.8 1.6 1.4 1.2 1 0.8 0.6 0.4 0.2
case23 (L2/d2=1, conical concentrator with conical diffuser) case33 (L2/d2=1, concave concentrator with convex diffuser) case13 (L2/d2=1, convex concentrator with convex diffuser) case18 (L2/d2=0.5, concave concentrator with convex diffuser) case34 (L2/d2=0.1667, concave concentrator with convex diffuser) case36 (L2/d2=0.5, concave concentrator with concave diffuser) Case 43 (optimum shape)
-0.3
0
0.3 0.6 Axial Distance x [m]
0.9
Ux/Uo
Velocity Magnitude Ux [m/s]
-1
1.2
Fig. 18 Airflow velocity distribution along the central line parallel to the airflow inside wind concentrator (different shapes) to study effect of cylinder length ratio l2/d2 for d1/d2=1.5, d3/d2=1.5, l1/d2=1, l3/d2=1 at inlet wind speed of 5 m/s
3.7. Optimal Configuration for the Wind Concentrator From the previously presented results, clearly, the optimal wind concentrator configuration is shown in Fig. 19. This is based on the CFD results that are presented from Fig. 6 throughout Fig. 18. These results show that the optimum converging diameter ratio d1/d2 is 1.5, concentrator length to cylinder diameter ratio l1/d2 is1.33 and the cylinder length to diameter ratio l2/d2 is 0.5. In addition, the optimal concentrator shape is the concave with a radius of curvature to cylinder diameter ratio R1/d2 of 3.289, while for the diffuser is the convex. 950.00
R687
R986.95
150.00
600.00
475.00
375.00
41.11° 325.00
375.00 21.24°
420.00
440.00
450.00
.96
400.00
300.00
400.00
Fig. 19 Optimal wind concentrator configuration (case 43) 13
3.8. Detailed Results for the Optimal Wind Concentrator Configuration Presentation of results of the all examined different cases of the wind configuration concentrator is shown in Fig. 20; clearly the presented results indicate that, the case 43 is the optimum one. Fig. 21 shows the air velocity distribution along central line parallel to the flow direction for the optimal wind concentrator configuration by using SST k-w turbulent model for the boundary condition of an undisturbed inlet wind velocity Uo of 5 and 10 m/s, respectively. The results show that, the central line velocity is accelerated from 5 m/s to 11.313 m/s (Ux/Uo = 2.263) for undisturbed inlet wind velocity Uo of 5 m/s and from 10 m/s to 24.53 m/s (Ux/Uo = 2.453) for Uo of 10 m/s. While, Fig. 22 show the distribution of absolute pressure along central line parallel to the flow direction for the optimal wind concentrator configuration (case 43) by using SST k-w turbulent model for undisturbed inlet wind velocity Uo of 5 m/s. from this figure clearly, the absolute pressure is dropped from 101325 Pa to 101258 Pa inside the wind concentrator and it is recovered to 101325 Pa far away from the wind concentrator. The local airflow velocity profile at the location of maximum velocity for the optimal concentrator configuration (case 43) for the case of an undisturbed inlet wind velocity Uo of 5 m/s is shown in Fig. 23, at which the value of velocity at the centerline is 11.313 m/s. While, the airflow velocity profile at the outlet of the optimal wind concentrator configuration (case 43) for undisturbed wind velocity Uo of 5 m/s is shown in Fig. 24 at which, the centerline velocity at the concentrator outlet is 6.14 m/s. The contours of velocity magnitude for the whole domain for the case of optimal wind concentrator configuration for undisturbed inlet wind velocity Uo of 5 m/s are shown in Fig. 25 and 26. While Fig. 27 shows the contours of absolute pressure for undisturbed inlet wind velocity Uo of 5 m/s. The velocity vectors map of the optimal wind concentrator configuration (case 43) is shown in Fig. 28 for undisturbed inlet wind velocity Uo of 5 m/s. From this figure, it can be seen that, a recirculation zone which happens at the outlet edge of the wind concentrator due to the separation effect. 2.4
Velocity ratio Ux/Uo
2.2 2 1.8 1.6 1.4 1.2 1
Fig. 20 Maximum central line velocity to the undisturbed wind velocity ratio for different configurations of wind concentrator at inlet wind speed of 5 m/s
14
25
101330
20
101320
Case43 k-w SST for Uo=10m/s
Absolue Pressure [Pa]
Velocity magnitude Ux [m/s]
Case43 k-w SST for Uo=5 m/s
15 10 5
101310 101300 101290 101280 101270 101260
0
101250
-0.9 -0.6 -0.3
0 0.3 0.6 0.9 1.2 1.5 1.8 Axial Distance x [m]
Fig. 21 Air velocity distribution along central line parallel to the flow direction for the optimal wind concentrator (case 43) by using SST k-w turbulent model for undisturbed inlet wind velocity Uo of 5 and 10 m/s
-2
-1
Velocity magnitude [m/s]
Velocity magnitude [m/s]
3
7
12 10 8 6 4
6 5 4 3 2
2
1
0 -0.1
2
Fig. 22 Absolute pressure distribution along central line parallel to the flow direction for the optimal wind concentrator (case 43) by using SST k-w turbulent model for undisturbed inlet wind velocity Uo of 5 m/s
14
-0.2
0 1 Axial Distance x [m]
0 0
0.1
0.2
Radius r [m]
Fig. 23 The airflow velocity distribution along the central line cross the location of maximum velocity for the optimal concentrator configuration (case43) for undisturbed inlet wind velocity Uo of 5 m/s
-0.4
-0.2
0
0.2
0.4
Radius r [m]
Fig. 24 Airflow velocity distribution along the central line cross the outlet of the optimal wind concentrator configuration (case43) for undisturbed inlet wind velocity Uo of 5 m/s
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Fig. 25 Contours of velocity magnitude for the optimal wind concentrator (case 43) inside the computational domain for undisturbed inlet wind velocity Uo of 5 m/s
Fig. 26 Contours of velocity magnitude for the optimal wind concentrator configuration (case 43) for undisturbed inlet wind velocity Uo of 5 m/s
Fig.27 Contours of absolute pressure for the optimal wind concentrator configuration (case 43) for undisturbed inlet wind velocity Uo of 5 m/s
16
Fig. 28 Velocity vectors for the optimal wind concentrator configuration (case43) for undisturbed inlet wind velocity Uo of 5 m/s
12
14
11
12
Velocity magnitude [m/s]
Velocity magnitude [m/s]
3.9. Comparison Between Turbulent Models SST k-ω and RNG k-ε Results for the optimal wind concentrator configuration (case 43) In order to clarify the difference between the two turbulent models SST k-ω and RNG k-ε that are used in this study, a comparison is made between them for undisturbed inlet wind velocity Uo = 5 m/s in order to check the accuracy of CFD obtained results as shown in Figs. 30 and 31. From these figures clearly, the maximum central line velocities are 11.64 m/s and 11.313 m/s for RNG k-ε and SST k-ω turbulent models, respectively. The results show that, RNG k-ε turbulent model predicts slightly higher values than that of SST k-ω turbulent model.
10 9 8 7 6
Case43 k-e, RNG
5
Case43 k-w, SST
10 8 6 4
k-e yy
2
k-w yy
0
4 -0.1
0
0.1
0.2
0.3
0.4
x [m]
0.5
0.6
0.7
0.8
Fig. 29 Air velocity distribution along central line parallel to the flow direction for the optimal wind concentrator configuration (case 43) using SST k-ω and RNG k-ε turbulent models for undisturbed inlet wind velocity Uo = 5 m/s
-0.2
-0.1
0 Radius r [m]
0.1
0.2
Fig. 30 Airflow velocity distribution along the central line cross the location of maximum velocity for the optimal concentrator (case 43) using SST k-ω and RNG k-ε turbulent models for undisturbed inlet wind velocity Uo = 5 m/s
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4. Conclusions The main aim of this study was to find the optimal wind concentrator configuration that maximizes wind speed to gain the maximum wind at turbine location in order to gain the maximum wind energy captured by the small-scale wind turbine to operate mainly at low and medium wind speeds. The commercial CFD package (ANSYSFluent), was employed to perform a detailed 3D analysis of the flow patterns around and inside different types and configurations of wind concentrator in a stationary reference frame. Both the κ -ε RNG and the κ–ω SST turbulent models were used with a constant axial wind velocity at the far-upstream. Wind concentrators of different profiles, with and without cylindrical part and diffusers, and with different inlet to outlet diameter ratios were examined to estimate the optimum place of the turbine generator and to attain minimum wake losses. The obtained results showed that the optimum converging diameter ratio d1/d2 is 1.5, concentrator length to cylinder diameter ratio l1/d2 is1.33 and the cylinder length to diameter ratio l2/d2 is 0.5. In addition, the optimal concentrator shape was the concave with a radius of curvature to cylinder diameter ratio R1/d2 of 3.289, while for the diffuser was the convex. The free stream wind velocity was accelerated from 5 m/s to 11.64 m/s by using RNG k-ε turbulent model , consequentially the available wind energy capture at the turbine was increased by a factor (2.328)2. While, for undisturbed wind velocity Uo of 10 m/s, the velocity ratio Ux/Uo was 2.453. A comparison was made between the two turbulent models SST k-ω and RNG k-ε that were used in this study in order to clarify the difference between them and to check the accuracy of the obtained results for undisturbed wind velocity Uo = 5 m/s. The maximum central line velocities were 11.64 m/s and 11.313 m/s for RNG k-ε and SST k-ω turbulent models, respectively. The results showed that, RNG k-ε turbulent model predicted slightly higher values than that of SST k-ω turbulent model. 5. RERFERENCES Abe, K., Nishida, M., Sakurai, A., Ohya, Y., Kihara, H., Wada, E., Sato, K. (2005), “Experimental and numerical investigations of flow fields behind a small wind turbine with a flanged diffuser”, Journal of Wind Engineering and Industrial Aerodynamics 93, 951–970. Anzai,A., Nemoto, Y., Ushiyama, I. (2009), “Wind Tunnel Analysis of Concentrators for Augmented Wind Turbines, Wind Engineering, 28 (5), online date 29 July. Clausen, P.H., Wood, D.H. (2000), “Recent advances in small wind turbine technology”, Wind Eng. 24 (3), 189–201 Ebert, P.R., Wood, D.H. (1997), “Observations of the starting behavior of a small horizontal-axis wind turbine”, Renew. Energy, 12 (3), 245–257. Gagnon F. (1978), “Wind Concentrator Turbine, Concepts and designs by Francois Gagnon”, ‘Goudreau Gage Dubuc’ of Montréal. U.S. patent (61/245 461). Gilbert, B.L., Foreman, K.M. (1983), “Experiments with a diffuser-augmented model wind turbine”, Trans. ASME, J. Energy Res. Technol., 105, 46–53. Hansen, M.O.L., Sorensen, N.N., Flay, R.G.J. (2000), “Effect of placing a diffuser around a wind turbine”, Wind Energy, 3, 207–213.
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Kho, J. (2009), “Startup Green Energy Tech Installs First Small-Wind Concentrators”, Wind Power Conference, Chicago. Ladvyk, J. (1977), “Horizontal Wind energy Concentrator”, US Patent (4045144). Matsushima, T., Takagi, S., Muroyama, S. (2006), “Characteristics of a highly efficient propeller type small wind turbine with a diffuser”, Renew. Energy, 31 (9), 1343–1354. Mayer, C., Bechly, M.E., Hampsey, M., Wood, D.H. (2001), “The starting behaviour of a small horizontal-axis wind turbine”, Renew. Energy, 22, 411–417. Meheen, H. J. (2008), “Wind Driven Venturi Turbine”, Patent, COLORADO SPRINGS, CO US IPC8 Class: AF03D900FI, USPC Class: 290 55. Mortensen, N.G., Hansen, J.C., Badger, J., Jørgensen, B.H., Hasager, C.B., Paulsen, U. S., Hansen, O. F., Enevoldsen, K., Youssef, l. G., Said, U. S., Moussa, A. A., Mahmoud, M. A., Yousef, A. E., Awad, A. M., Ahmed, M. A., Sayed, M. A. M., Korany, M. H., Tarad M. A. E. (2006), “Wind Atlas for Egypt: Measurements, microand mesoscale modeling”, Proceedings of the 2006 European Wind Energy Conference and Exhibition, Athens, Greece, February 27 to March 2. Orosa, J. A. (2010), “A Proposal for Wind-energy Conversion for Low Wind–speed Areas of India”, International Association for Energy Economics (IAEE Energy Forum) 25, 26, Second Quarter. Ohya, Y., Karasudani, T., Sakurai, A. (2002), “Development of high-performance wind turbine with brimmed diffuser”, J. Jpn. Soc. Aeronaut, Space Sci., 50, 477–482 (in Japanese). Ohya, Y., Karasudani, T., Sakurai, A., Inoue, M. (2004), “Development of highperformance wind turbine with a brimmed-diffuser: Part 2”, J. Jpn. Soc. Aeronaut. Space Sci., 52, 210–213 (in Japanese). Ohya, Y., Karasudani, T., Sakurai, A., Abe, K., Inoue, M. (2008), “Development of a shrouded wind turbine with a flanged diffuser”, Journal of Wind Engineering and Industrial Aerodynamics, 96, 524–539. Phillips, D.G., Richards, P.J., Flay, R.G.J. (2000), “CFD modeling and the development of the diffuser augmented wind turbine”, Proceedings of the Computational Wind Engineering, Birmingham, pp. 189–192. Sabzevari, A. (1978), “Power Augmentation in a ducted Savonius rotor”, Second International Symposium on Wind Energy Systems, vol.1, October 3rd-6th. Wang, F., Bai, L., Fletcher, J., Whiteford, J., Cullen, D. (2008), “Development of small domestic wind turbine with scoop and prediction of its annual power output”, Renewable Energy 33, 1637–1651. Wang, F., Bai, L., Fletcher, J., Whiteford, J., Cullen, D. (2008), “The methodology for aerodynamic study on a small domestic wind turbine with scoop”, Journal of Wind Engineering and Industrial Aerodynamics, 96, 1–24. Wright, A.K., Wood, D.H. (2004), “The starting and low wind speed behaviour of a small horizontal axis wind turbine”, J. Wind Eng. Ind. Aerodyn., 92, 1265–1279. Wood, D.H. (2001), “A blade element estimation of the cut-in wind speed of a small turbine”, Wind Energy, 25 (4), 249–255.
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