This paper presents an experimental and numerical simulation to investigate a hybrid vertical axis wind turbine model highly efficient which can be worked at low wind speed by studying the aero dynamic characteristics of four models of hybrid VAWTs. The hybrid WT consists of the SWT having two blades and the DWT type straight having two blades. Four models were constructed to study experimentally and numerically to choose the best model. Two mode ls were DWT in the upper and SWT in the lower, also two models were SWT in the upper and DWT in the lower. The phase stage angle between the turbines is 0o and 90o. The experimental and numerical results showed that the performance of hybrid WT where DWT i n the upper and SWT in the lower with phase stage 90o is better than in the other models, it can be started to work at a wind velocity of 2.2 m/s. At the wind velocity 3 m/s, the values of the parameters are the rotational speed (198 rpm), the C P (0.3195), the C T (0.2003), the TSR (1.6) and self -starting rotation at this value of wind velocity (3 m/s). The efficiency of extracting the wind power by hybrid WT is (51.2 %). Keywords: Hybrid wind turbine, Vertical axis wind turbine, Low wind speed . Cite this Ar ticle: Nawfal M. Ali, Dr. Abdul Hassan A. K. and Dr. Sattar Aljabair , The Effect of Darrieus and Savonius Wind Turbines Position on the Performance of the Hybrid Wind Turbine at Low Wind Speed . International Journal of Mechanical Engineering and Technology 11(2), 2020, pp. 56-72. http://www.iaeme.com/IJ MET/issues.asp?JType=IJMET&VType=11&ITy pe=2 NOMENCLATURE P Static pressure A Hybrid swept area (m2) T Dynamic torque (N.m) As Swept area of turbine (m2) Ta Ambient temperature (K) Cp Power coefficient Tor Torque (N.m) CT Torque coefficient TSR Tip spe ed ratio c Darrieus blade chord (mm) w Distance between turbines (mm) d Savonius blade chord (mm) ⃗ Relative velocity of fluid D Savonius rotor diameter (mm) DWT Darrieus wind turbine e Shaft distance (mm) HWT Hybrid wind turbine The Effect of Darrieus and Savonius Wind Turbines Position on the Performance of the Hybrid Wind Turbine at Low Wind Speed http://www.iaeme.com/ IJMET/index. asp 57 editor@iaeme.com F Force (N) RANS Reynolds Averaged Navier -Stokes h Savonius blade height (mm) SST Shear Stress Transport H Darrieus blade height (mm) SWT Savonius wind turbine N Rotational speed (rpm) VAWT Vertical axis wind turbine R Darrieus rotor radius (mm) WT Wind Turbine Rg Gas constant (287 J/kg.K) ⃗⃗⃗ Rotational speed (rpm) Position vector  Viscosity (Pa.s) rp Radius of pulley (mm) ρ Air density (kg/m3) Patm Atmospheric pressure (Pa)  Angular velocity (rad/s) PAV Available power in the wind (W) Sui Centrifugal and Coriolis force PT Power produced from turbine (W) τij Average shear stress 1. INTRODUCTION: In recent years, the wind energy has become one of the most economical renewable energy technolog ies. Today, electricity -generating wind turbines employ proven and tested technology and provide a secure and sustaina ble energy supply. At good, windy sites, the wind energy can already successfully compete with conventional energy production. Many countries have considerable wind resources, which are still untapped. So , the wind energy has to compete with many other ene rgy sources in the future. To compete with others , it had to cope with the least affirmative conditions to maximum positive conditions. The usual challenge for the wind turbine is performing at low wind speed [1]. A hybrid vertical axis wind turbine (HVAWT ) is designed by the combination of two types of vertical turbine Darrieus and Savonius (on a common shaft) that can work in sites having low wind velocity. The benefits of this design are starting at low wind speeds and obtain a good efficiency. This mode l can be used to the wind power available on the highways caused by the movement of the vehicles. [2] studied the characteristics of the parameters by numerical simulation for a hybrid wind turbine (3 blades Savonius and 3 blades Darrieus) with various ove rlap conditions and compared with previous experimental results. The results showed that the maximum C P (0.377) is at an overlap ratio of 20%, maximum C T (1.271) at an overlap ratio of 30% and maximum TSR (0.414) at an overlap ratio of 16.2%. [3] investiga ted the parameters of a combined VAWT , including dynamic torque, starting torque coefficients and power performance that was simulated and analyzed. DWT is straight -bladed and airfoil type NACA0012 the number of blades is 4, SWT with 2 buckets. The results showed that the power produces at all TSR s, that means the power performance for the combined VAWT improved at a low rotational speed field. The dynamic torque performance increased at the low TSR. [4] studied the performance of a hybrid VAWT by experimen tal work procedure. DWT H -type with three blades and airfoil was DU06W200 combined with an SWT that has two conventional buckets. The results depicted that the combining between Darrieus and Savonius WT make s an efficient hybrid WT, it has improved the self-starting with a higher power coefficient. [5] investigated aerodynamics and the starting performance for a hybrid VAWT with the assistance of CFD simulations. Hybrid VAWT consists of DWT H-type with 3 blades and airfoil NACA0021, SWT with 3 blades and di fferent distance s between blade and center of rotation (d = 0, 50, 100, 150) mm, the wind velocity is 9 m/s. The results showed that the C P of the hybrid VAWT decreases when increasing the distance, while the starting torque can be improved. The position o f the Savonius blade for the installation is very important . [6] studied the design of hybrid VAWT that can be started at low wind speed (4.8 m/s) and obtained a good efficiency. The results showed that the efficiency of hybrid VAWT reached up to 37% at a TSR of 0.9 and with no overlap. When increasing TSR, the C P and overall efficiency increase. TSR and C P decrease when the wind velocity increases . The effect of geometrical and operational parameters (C P), (C TS) and (C T) on the performance of Nawfal M. Ali, Dr. Abdul Hassan A. K. and Dr. Sattar Aljabair http://www.iaeme.com/ IJMET/index. asp 58 editor@iaeme.com hybrid WT was studied by [7]. Four models of HWT were taken to experimental tests. Wind velocity range in experimental tests was from (0 -12) m/s. The results showed that the turbine power of the model (3 blades Savonius with 3 blades Darrieus) is very good at wind velo city range 2 -8 m/s compared with the other models, also the C P for this model is 0.23. [8] studied the evaluation of the optimal design of Darrieus vertical axis wind turbine by CFD analysis and experimental tests, through analyzing six models of Darrieus wind turbines, number of blades and tip speed ratio. The airfoil profile used in the turbine blades was DU06W200 and constant geometry dimensions to turbines. The types of six models of DWT were straight, twisted 70o and helical 120o, all types with two an d three blades. The results showed that Darrieus WTs straight types can be self -started at the wind velocity 3 m/s, but the twisted types start ed over 5 m/s and the helical types start ed over 6 m/s. The performance of Darrieus WT with 2 blades rotor is bet ter than the other models. At low wind velocity (3 m/s) the value of C P is (0.2495), the C T (0.174), the rotational speed (198 rpm) and can be self -started at this wind velocity. The efficiency of extracting the w ind power by Darrieus WT is ( 39 %). [9] used Savonius WT with semi -circular profile blades and single -stage ha ving a various number s of blade (two, three and four ). The fourth model has two blades with two stages , the phase angle between stages one and two is 90o. The overlap distance is zero for a ll models. The experimental and numerical results show ed that the performance of Savonius WT ha ving two blades is better than the other models at low wind speed. The maximum experimental C p and C T for two blades rotor are 0.23 at TSR 0.685 and 0.402 at TSR 0.175 , respectively. The efficiency of extracting the w ind power by Savonius WT is (38.81 %). In this study, a comparison was execute between four models of hybrid wind turbine to study the behavior and performance of these models at wind speed range (3 -7.65) m/s, especially at (3 m/s) with different tip speed ratios. Experiments and numerical analyses were carried out to choose the best model that can be work ed at the low wind velocity conditions (3 m/s). 2. HYBRID WIND TURBINE (HWT) PERFORMANCE The perf ormance of Hybrid VAWTs is explained by (C T), (C P), () given by [3,7 ]. Fig ure (1) shows the geometry and dimensions of the hybrid wind turbine: Figure 1 The geometry and dimensions of the hybrid wind turbine The swept area for HWT (A) is = ∑ + = 2RH + Dh (1.1)  = (1.2) The Effect of Darrieus and Savonius Wind Turbines Position on the Performance of the Hybrid Wind Turbine at Low Wind Speed http://www.iaeme.com/ IJMET/index. asp 59 editor@iaeme.com = (1.3) = = = =  (1.4) In this study, four models are taken to stud y and make a comparison between them to find out the best model of HWT, the study condition is at low wind velocity (3 m/s). 3. HYBRID WIND TURBINE MODELS The four models of experiments tests are named a s the following , the dimensions of HWTs are listed in the table (1).  Case 1 represented the h ybrid WT that consists of Darrieus WT upper and Savonius WT lower with phase stage 90o. Figure (2 -a) shows the geometry of Case 1.  Case 2 represented the h ybrid W T that consists of Darrieus WT upper and Savonius WT lower with phase stage 0o. Figure (2 -b) depicts the geometry of Case 2.  Case 3 represented the h ybrid WT that consists of Savonius WT upper and Darrieus lower with phase stage 90o. Figure (3 -a) shows the geometry of Case 3.  Case 4 represented the h ybrid WT that consists of Savonius WT upper and Darrieus lower with phase stage 0o. Figure (3 -b) reveals the geometry of Case 4. Table 1 : The dimensions of hybrid WTs (all Cases) Number of blades D (mm) d (mm) h (mm) R (mm) c (mm) e (mm) w (mm) 2-Savonius 2-Darrieus 350 175 380 250 100 25 70 (a) (b) Figure 2 Hybrid WT type Darrieus (upper) and Savonius (lower) model: (a) phase angle 0o and (b) phase angle 90o . Nawfal M. Ali, Dr. Abdul Hassan A. K. and Dr. Sattar Aljabair http://www.iaeme.com/ IJMET/index. asp 60 editor@iaeme.com (a) (b) Figure 3 Hybrid WT type Savonius (upper) and Darrieus (lower) model: (a) phase angle 0o and (b) p hase angle 90o 4. THE EXPERIMENTAL WORK 4.1. Experimental Setup Experiments were conducted using a subsonic wind tunnel under an open type test section, shown in fig. (4 - a,b,c) with the test section contain ing the turbine model and the measurement devi ces assembly with a cross -section of frontal view of 1m x 1.25m. Double butterfly valve was used to control and regulate the airflow rate, and the airspeed range was (0 - 9) m/s. The turbine models were fixed at 250 mm between the rotor shaft and exit of t he wind tunnel. Digital tachometer used to measure the rotational speed of the wind turbine. Digital force gauge was utilized to measure the force (F) produced from the rotor shaft, the torque can be calculated by: Tor = F x r p (1.5) Where, the radius of pulley (rp) = 50 mm. The static pitot tube was used to measure the airflow speed in the wind tunnel, it was connected w ith a macroscopic manometer. The digital thermometer was utilized to measure the ambient temperature. The pressure gauge was used to measure atmospheric pressure. The air density is given by: (1.6) The Effect of Darrieus and Savonius Wind Turbines Position on the Performance of the Hybrid Wind Turbine at Low Wind Speed http://www.iaeme.com/ IJMET/index. asp 61 editor@iaeme.com Figure 4-a A subsonic wind tunnel Figure 4-b: Schematic diagram for wind tunnel with the dimensions details Figure 4-c: The test section rigs with the turbi ne model and dimensions details Nawfal M. Ali, Dr. Abdul Hassan A. K. and Dr. Sattar Aljabair http://www.iaeme.com/ IJMET/index. asp 62 editor@iaeme.com 5. NUMERICAL MODEL To obtain a successful design and suitable geometry for the VAWT rotor, modeling styles should be applied. ANSYS -CFX software was used in this study for simulation and validation. The results were compared with experimental and numerical data, and the error was assessed. Transient conditions were considered for these modeling. Figure (5 -a) shows the 3D computational domain for the turbine rotor model. Two parts are separated by a sliding interface in the do main. Wind tunnel testing zone represents the stationary, that is the first part of the domain. The dimensions of the wind tunnel (stationary domain) are 4 m x 4 m x 8 m. Figure (5 -b) displays the mesh on the rotating domain, the mesh on the rotor and the mesh on the endplate in the rotor, also it appears the mesh near the blade showing the boundary layers. The flow around the rotor is turbulent, thus the simulation of CFD around the rotor is complex. Simulations of CFD were applied to solve the cases based on 3D steady finite volume incompressible Reynolds Averaged Navier -Stokes (RANS) equations. Controlling the equations of turbulence flow are continuity and Navier - Stokes equations, these equations in conservative forms are [10,11]: = 0 (1.7) ( ) ̅ ( ̅̅̅ ́ ́ ) (1.8) Where: = - [2 ⃗⃗ ⃗ ⃗⃗ ( ⃗⃗ )] (1.9) ̅̅̅ = - ( ̅̅̅ ̅̅̅ ) (1.10) To solve the case of the flow field around the rotor numerically, the turbulence models were added in RANS CFD solvers. The best model is that give s accepted results and agreement with the experimental results. 5.1. Boundary Conditions The boundary conditions in simulations of this study are:  Wind velocity at the entrance stationary domain is 3 m/s , and distance from the axis of the rotational domain is 1.5 m.  The pressure in the outer face is equal to the atmospheric pressure.  The blade of the rotor is set as a non -slip smooth wall. To obtain good results and more strictly study the flow in the boundary layers of rotor blade in this study, a prismati c mesh was applied on the sides of rotor blades to obtain correctly the boundary layer. Accordingly, the density of meshes was higher near the wall of rotor blades than the other parts .  The walls (sides and top) of the stationary domain are set as opening to the atmospheric pressure .  The base of the stationary domain is set as a non -slip smooth wall.  The SST k - turbulence model is applied in numerical simulation. 5.2. Turbulence Models The precedent studies used a 3D SST k - turbulence model which agrees with the experimental work of [1 2,13,14]. This model employs to analyze the transient forces that affect DWTs. The advanced turbulence models used in this work need a very fine mesh near the wall so that Y+ < 5 [1 5,16], to get a good result. So in this st udy, the SST k - turbulence model was applied in numerical simulation. The Effect of Darrieus and Savonius Wind Turbines Position on the Performance of the Hybrid Wind Turbine at Low Wind Speed http://www.iaeme.com/ IJMET/index. asp 63 editor@iaeme.com 6. RESULTS AND DISCUSSION The effect of model types and phase angle between the turbines is shown in Figs. ( 6.1) to (6.9). The power coefficient (C P) value increases with the increase of TSR value for all cases at constant wind velocity (3 m/s) for experimental results . Fig ure (6.1) illustrates the relationship between the C P of the HWT cases with various values of TSR for the experimental results at wind velocity (3 m/s), the C P for ca se 1 is higher than other cases at TSR more than 1.4. The relation between C P and wind velocity (u) is evinced in Fig. (6.2) , the CP values increase s with increas ing (u) value for all cases, and it is noted that the case s 1 and 2 can be work ed at wind velo city 2.2 m/s (self -starting). The C P value of case 1 is higher than case 2 for all (u) values. Figure (6.3-a and b) shows a comparison of the relation C P - TSR at constant wind velocity (3 m/s) between the experimental and numerical results for cases 1 and 3, respectively, the value s of C P for experimental and numerical results are convergent values. Table (2) lists the experimental results of the (C P) at various wind velocit ies for HWT cases, from the table , it is noted that cases 1 and 2 can be worked self-starting at wind velocity 2.2 m/s and this is the improvement on HWT. Table 2 The C P at various wind velocity for HWT cases HWT cases Wind velocity (m/s) 2.2 3 4 4.5 5.15 6.45 7.65 1 0.1965 0.3195 0.3243 0.3392 0.3442 0.3691 0.3897 2 0.1001 0.2440 0.2489 0.2530 0.3217 0.3541 0.3649 3 ---- 0.3242 0.3336 0.3436 0.3482 0.3681 0.3886 4 ---- 0.2672 0.2709 0.2768 0.3051 0.3379 0.3679 The torque coefficient (C T) value at the experimental results increases with increas ing the TSR value for all cases at con stant wind velocity (3 m/s ). Fig ure (6.4-a) manifests a comparison of the relation C T - TSR at constant wind velocity (3 m/s) between the HWT cases for the experimental results. Fig ure (6.4-b) shows a comparison of the relation C T with various wind velocit ies between the HWT cases for the experimental results, the value of the C T increases to a certain extent, then the value decreases and increases again for all cases . Figure (6.5-a and b) elucidates a comparison of the relation C T - TSR at constant wind ve locity (3 Figure 5-a : The 3D computational domain for the stator and turbine rotor. Figure 5-b: The mesh of rotating domain Nawfal M. Ali, Dr. Abdul Hassan A. K. and Dr. Sattar Aljabair http://www.iaeme.com/ IJMET/index. asp 64 editor@iaeme.com m/s) between the experimental and numerical results for cases 1 and 3 respectively, the value s of C T for experimental and numerical results are convergent values. Table ( 3) list s the experimental results of the (C T) at various wind velocit ies for HWT cases, also cases 1 and 2 can be work ed at wind velocity 2.2 m/s. Table 3 The C T at various wind velocit ies for HWT cases HWT cases Wind velocity (m/s) 2.2 3 4 4.5 5.15 6.45 7.65 1 0.1665 0.2003 0.2003 0.1923 0.1888 0.1947 0.2043 2 0.1329 0.1772 0.1778 0.1746 0.1835 0.1945 0.2015 3 ---- 0.1885 0.2136 0.1884 0.1835 0.1921 0.2114 4 ---- 0.1675 0.1773 0.1543 0.1630 0.1783 0.2021 The value of C T for case 1 is higher than in other cases for experimental results at wind velocity (3 m/s). Performanc e parameters in terms of (C P, C T) for hybrid WT case 1 are shown in Fig. (6.6 - a and b) re lated to TSR for the experimental and numerical results. Figure (6.6-a) shows the experimental results, both C T and C P values increase with the increase of TSR value. Figure (6.6-b) demonstrates the numerical results, both C T and C P values increase with increasing the TSR value to a certain value and then the y decrease with increas ing the TSR. Figure (6.7-a) represent s the relationship between the rotational speed (N) a nd wind speed (u) for the HWT cases, it is noted that HWT cases 1 and 2 can be work ed at the wind speed 2.2 m/s. The rotational speed (N) increase s with the wind speed (u) increase for all cases. Figure (6.7-b) shows a comp arison of the relation N - TSR at constant wind velocity (3 m/s) between the experimental and numerical results for cases 1, the value s of N for the experimental and numerical results are convergent. Table ( 4) lists the experimental results of the (N) at various wind velocit ies for HWT ca ses. Table 4 The rotational speed (N) at various wind velocit ies (u) HWT cases Wind velocity (m/s) 2.2 3 4 4.5 5.15 6.45 7.65 1 102 198 249 305 375 485 575 2 65 168 215 250 360 465 545 3 ---- 210 240 315 390 490 550 4 ---- 195 235 310 385 485 545 The rotational speed (N) is in (rpm). The HWT case 1 and 2 can be work ed at a wind velocity of 2.2 m/s, but case 1 has better performance than case 2. Figure (6.8-a) shows a comparison of the relation C P - TSR at constant wind velocity (2.2 m/s) betwee n the experimental and numerical results for case 1, the value s of C P for experimental and numerical results are convergent. Figure (6.8-b) depicts a comparison of the relation C T - TSR at constant wind velocity (2.2 m/s) between the experimental and numer ical results for case 1. Performance parameters in terms of (C P, C T) for hybrid WT case 1 are shown in Fig. ( 6.9- a and b) related to TSR for the experimental and numerical results at constant wind velocity 2.2 m/s. Figure (6.9-a) shows the experimental r esults, both C T and C P values increase with the increase of TSR value. Figure (6.9-b) exhibits the numerical results, both C T and C P values increase with increas ing the TSR value. From the results in the tables ( 2, 3 and 4 ), it is shown that the performan ce of HWT (case 1) is better than in other cases, it can be started to work at the wind velocity of 2.2 m/s. When The Effect of Darrieus and Savonius Wind Turbines Position on the Performance of the Hybrid Wind Turbine at Low Wind Speed http://www.iaeme.com/ IJMET/index. asp 65 editor@iaeme.com the wind velocity is 3 m/s , the values of the parameters for HWT case 1 are the rotational speed (198 rpm), the C P (0.3195), the C T (0.2003), the TSR (1.6) and self -starting rotation at this value of wind velocity (3 m/s). The values of the parameters for HWT case 1 at wind velocity 2.2 m/s are the rotational speed (102 rpm), the C P (0.1965), the C T (0.1665), the TSR (1.18) and self -starting rot ation at this value of wind velocity (2.2 m/s). The efficiency o f extracting the wind power by HWT is 51.2 % . 7. CONCLUSION S The present study showed experimentally and numerically the effect of the combin ation between the two types of wind turbines (Savon ius and Darrieus). The main conclusions from the results are as the following:  The C P and CT values increase with increas ing the TSR value for all cases at constant wind velocity (3 m/s) for the experimental results.  The C P values increase with increas ing (u) value for all cases, case 1 and 2 (Darrieus at the upper and Savonius at the lower part with phase stage 90o and 0o, respectively) can be work ed at wind velocity 2.2 m/s (self -starting).  From t he comparison of the relation C T with various wind velocit ies between the HWT cases for the experimental results, the value of the C T increases to a certain extent, then the value decreases and increases again for all cases.  The value of C T for case 1 is higher than in other cases for the experimental results at w ind velocity (3 m/s).  The performance of the hybrid WT case (1) is better than the other cases. At low wind velocity (3 m/s) , the value of C P is (0.3195), the C T (0.2003), the rotational speed (198 rpm) and can be self -started at wind velocity 2.2 m/s.  The efficiency of extracting the wind power for the Hybrid WT case (1) is 51.2 %. Figure 6.1: Experimental relation between C p and TSR for all cases at a wind velocity of 3 m/s Nawfal M. Ali, Dr. Abdul Hassan A. K. and Dr. Sattar Aljabair http://www.iaeme.com/ IJMET/index. asp 66 editor@iaeme.com Figure 6.2: Experimental relation between Cp and u for all cases at various w ind velocit ies Figure 6.3-a: A comparison between experimental and numerical results for Case 1 Figure 6.3-b: A comparison between experimental and numerical results for Case 3 The Effect of Darrieus and Savonius Wind Turbines Position on the Performance of the Hybrid Wind Turbine at Low Wind Speed http://www.iaeme.com/ IJMET/index. asp 67 editor@iaeme.com Figure 6.4-a: Experimental relation between C T and TSR for all c ases at wind velocity 3 m/s Figure 6.4-b: Experimental relation between C T and TSR for all cases at various win d velocit ies Figure 6.5-a: A comparison (C T) between experimental and numerical results for Case 1 Nawfal M. Ali, Dr. Abdul Hassan A. K. and Dr. Sattar Aljabair http://www.iaeme.com/ IJMET/index. asp 68 editor@iaeme.com Figure 6.5-b: A comparison (C T) between experimental and numerical results for Case 3 Figure 6.6-a: Performance parameters in terms of (C p, CT) for Hybr id WT Case 1 (Experimental results) Figure 6.6-b: Performance parameters in terms of (C p, CT) for hybrid WT Case 1 ( Numerical results) The Effect of Darrieus and Savonius Wind Turbines Position on the Performance of the Hybrid Wind Turbine at Low Wind Speed http://www.iaeme.com/ IJMET/index. asp 69 editor@iaeme.com Figure 6.7-a: Relationship between the rotational speed (N) and wind velocity (u) (Experimental results) Figure 6.7-b: A comparison between experimental and numerical results for case 1 at constant wind velocity (3 m/s) Figure 6.8-a: A comparison between experimental and numerical results: the relation between C p and TSR for case 1 at wind velocity 2.2 m/s Nawfal M. Ali, Dr. Abdul Hassan A. K. and Dr. Sattar Aljabair http://www.iaeme.com/ IJMET/index. asp 70 editor@iaeme.com Figure 6.8-b: A comparison between experimental and numerical results: the relation between C T and TSR for case 1 at wind velocity 2.2 m/s Figure 6.9-a: Performance parameters in terms of (C p, CT) for HWT case 1 at wind velocity 2.2 m/s (Experimental results) Figure 6.9-b: Performance parameters in terms of (C p, CT) for HWT case 1 at wind velocity 2.2 m/s (Numerical results) The Effect of Darrieus and Savonius Wind Turbines Position on the Performance of the Hybrid Wind Turbine at Low Wind Speed http://www.iaeme.com/ IJMET/index. asp 71 editor@iaeme.com