Four Decades Of Research Into The Augmentation Techniques Of Savonius Wind Turbine Rotor

Transcript

Journal of Energy Resources Technology. Received November 05, 2017; Accepted manuscript posted December 19, 2017. doi:10.1115/1.4038785 Copyright (c) 2017 by ASME Four Decades of Research into the Augmentation Techniques of Savonius Wind Turbine Rotor Nur Alom Trainee Teacher Department of Mechanical Engineering  National Institute of Technology Meghalaya Shillong –  Shillong –  793003,  793003, India E-mail: [email protected] Ujjwal K. Saha Professor Department of Mechanical Engineering Indian Institute of Technology Guwahati Guwahati- 781039, India E-mail: [email protected] Abstract The design and development of wind turbines is increasing throughout the world to offer electricity without paying much to the global warming. The Savonius wind turbine rotor, or simply the Savonius rotor, is a drag-based device that has a relatively low efficiency. A high negative torque produced by the returning blade is a major drawback of this rotor. Despite having a low efficiency, its design simplicit y, low cost, easy installation, good starting ability, relatively low operating speed and independency to wind direction are its main rewards. With the goal of improving its power coefficient (C  ( C  P ), ), a considerable amount of investigation has JERT-17-1620 Alom 1 Journal of Energy Resources Technology. Received November 05, 2017; Accepted manuscript posted December 19, 2017. doi:10.1115/1.4038785 Copyright (c) 2017 by ASME  been reported in the past few decades where various design modifications are made by altering the influencing parameters. Concurrently, various augmentation techniques have also  been used to improve the rotor performance. performance . Such augmenters reduce the negative torque and improve the self-starting capability while maintaining a high rotational speed of the rotor. The C  P  of   of the conventional Savonius rotors lie in the range of 0.12-0.18, however, with the use of augmenters, it can reach up to 0.52 with added design complexity. This article attempts to give an overview of the various augmentation techniques used in Savonius rotor over the last four decades. Some of the key findings with the use of these techniques have been addressed and makes an attempt to highlight the future direction of rese arch. Keywords: Savonius rotor, blade profiles, augmentation techniques, torque coefficient, power coefficient, tip-speed ratio. 1. Introduction The Savonius rotor is a sort of vertical-axis wind turbine (VAWT ). ). A conventional turbine rotor, mounted on a rotating shaft or framework, consists of several semicircular blades. The rotor system may either be ground stationed or fastened in a  floating system. The system. The Savonius rotor, invented by the Finnish the  Finnish engineer  Sigurd  Sigurd Johannes Savonius in 1925 [1, 2], is one of the simplest type of wind turbine. Aerodynamically, it Aerodynamically, it is a drag a drag based  based device, and consists of two or three scoops (also known as buckets or blades). The top view of a 2-bladed rotor looks like an ‘S’  shape in cross section [3-4] [3-4].. The rotor blades experience less drag when moving against the wind than when it moves with the wind due to their curved shape. The differential drag force makes the rotor to spin. Since the Savonius rotor is a drag-based machine, it extracts lesser wind energy than a similarly sized lift-based devices like Darrieus rotor and horizontal-axis wind turbine ( HAWT ) [5] [5].. The  HAWT s are actually acknowledged for their reasonably higher C  P  than  than the Savonius VAWT s, s, and fundamentally have been used for power JERT-17-1620 Alom 2 Journal of Energy Resources Technology. Received November 05, 2017; Accepted manuscript posted December 19, 2017. doi:10.1115/1.4038785 Copyright (c) 2017 by ASME  been reported in the past few decades where various design modifications are made by altering the influencing parameters. Concurrently, various augmentation techniques have also  been used to improve the rotor performance. performance . Such augmenters reduce the negative torque and improve the self-starting capability while maintaining a high rotational speed of the rotor. The C  P  of   of the conventional Savonius rotors lie in the range of 0.12-0.18, however, with the use of augmenters, it can reach up to 0.52 with added design complexity. This article attempts to give an overview of the various augmentation techniques used in Savonius rotor over the last four decades. Some of the key findings with the use of these techniques have been addressed and makes an attempt to highlight the future direction of rese arch. Keywords: Savonius rotor, blade profiles, augmentation techniques, torque coefficient, power coefficient, tip-speed ratio. 1. Introduction The Savonius rotor is a sort of vertical-axis wind turbine (VAWT ). ). A conventional turbine rotor, mounted on a rotating shaft or framework, consists of several semicircular blades. The rotor system may either be ground stationed or fastened in a  floating system. The system. The Savonius rotor, invented by the Finnish the  Finnish engineer  Sigurd  Sigurd Johannes Savonius in 1925 [1, 2], is one of the simplest type of wind turbine. Aerodynamically, it Aerodynamically, it is a drag a drag based  based device, and consists of two or three scoops (also known as buckets or blades). The top view of a 2-bladed rotor looks like an ‘S’  shape in cross section [3-4] [3-4].. The rotor blades experience less drag when moving against the wind than when it moves with the wind due to their curved shape. The differential drag force makes the rotor to spin. Since the Savonius rotor is a drag-based machine, it extracts lesser wind energy than a similarly sized lift-based devices like Darrieus rotor and horizontal-axis wind turbine ( HAWT ) [5] [5].. The  HAWT s are actually acknowledged for their reasonably higher C  P  than  than the Savonius VAWT s, s, and fundamentally have been used for power JERT-17-1620 Alom 2 Journal of Energy Resources Technology. Received November 05, 2017; Accepted manuscript posted December 19, 2017. doi:10.1115/1.4038785 Copyright (c) 2017 by ASME generation [6] [6].. However, the Savonius VAWT s have various important rewards than the  HAWT s owing to their lesser fixing and preservation costs, and the direction independency [7-12].. Additionally, these rotors also do not need a yaw control mechanism and over speed [7-12] controller [13 [13]. ]. These benefits make them attractive and appropriate for many applications. But the main disadvantage of the Savonius rotor is that it produces negative torque in some rotational cycle of the rotor, and as a result, the net positive torque of the rotor gets reduced [14-17 14-17]. ]. To improve its performance, various blade profiles such as semicircular [18-22], Bach [23], [23], Benesh  Benesh [24], [24],   twisted [25], elliptical [23, 26-27], fish-ridged rotor [28] [28],, modified Bach type [14], Bronzinus [29], airfoil shape blade [30], [30], multiple quarter [31 [31], ], multiple miniature semicircular [32 [32], ], and spline  spline  [33] have been evolved. Besides using these blade  profiles, the various augmentation techniques have also been used to decrease the negative torque produced by the rotor. Several such techniques find their applications, notable among them are V-shape wedge deflector, curtains, concentrated and oriented jets, multi-staging, nozzle, venting slot, deflecting plate, guide vane and others [4, others  [4, 8-11, 14-16]. 1.1 Aim of the present study Since its inception, several wind tunnel experiments have been carried and are being conducted to estimate the performance characteristics of Savonius rotor. The main objectives in these studies have been to optimize the various parameters of the rotor for attaining suitable design configurations. It is only during the last few decades that the investigators have started following numerical studies with various numerical methods, optimization techniques [34 [34]] and soft-computing techniques to arrive at the same objectives. Though the experimental researches have exposed more precise findings, however, the numerical researches have provided the liberty to conduct an extensive study with reduced experimental intimidations and costs. Recently, Akwa et al. [17] and [17]  and Roy and Saha [18] have [18]  have provided a complete knowledge and beneficial evidence on the various aspects of Savonius rotors. Till JERT-17-1620 Alom 3 Journal of Energy Resources Technology. Received November 05, 2017; Accepted manuscript posted December 19, 2017. doi:10.1115/1.4038785 Copyright (c) 2017 by ASME date, various turbulence models and soft-computing techniques have been used by various researchers to improve the efficiency of the rotor. As stated previously, various blade profiles have been developed, and at the same time, various augmentation techniques have also been employed to improve the all-round performance of the rotor. Uniting the past experimental and numerical investigations, this review work tries to make an analysis on the various augmentation techniques applied [35-47] and makes recommendation of the future studies. 1.2 Evolution of Savonius blade profiles To improve the performance of Savonius rotors, few noticeable investigations are found on the use of dissimilar category of blade shapes such as conventional semicircular ,  Bach, Benesh,  twisted, elliptical, fish-ridged rotor, modified Bach, Bronzinus, airfoil shape blade, multiple quarter, multiple miniature semicircular  and spline. All these blade profiles are illustrated in Fig.1. The performance indices of these blade profiles are shown in Table 1. Rudimentary investigation with an elliptical profile has demonstrated its energy capturing  potential, hence, an improved C  P   than the conventional rotor [23].  In a separate numerical study, an elliptical profile of different design has shown a performance improvement of 10.7% over the semicircular profile [26].  At a later stage, Alom et al. [27]  optimized the elliptical profile numerically and found a performance gain of 18.18% than the semicircular bladed Savonius rotor. Thus, it clear that the elliptical-bladed rotor can be a strong contender in the future designs of Savonius rotor. 1.3 Geometric parameters Despite having a low efficiency, the Savonius rotor has become popular for its good starting ability, design simplicity, and low cost. However, its performance can be improved by optimizing the basic parameters like aspect ratio, overlap ratio, gap ratio, Reynolds number, and number rotor blades [25]. The aspect ratio ( AR =H/D) of the rotor is defined as the ratio JERT-17-1620 Alom 4 Journal of Energy Resources Technology. Received November 05, 2017; Accepted manuscript posted December 19, 2017. doi:10.1115/1.4038785 Copyright (c) 2017 by ASME of rotor height ( H ) to the overall diameter of the rotor ( D). This is illustrated in Fig. 2. Because of the consequence of blade tips, the Savonius rotors have low losses at high  ARs [15]. A small rotor diameter always causes a fast diverging of airflow. When the diameter of the rotor increases, the produced torque also increases, whereas the rotational speed of the rotor decreases, and vice versa [17, 18]. The overlap ratio (   = e/d ) is defined as the ratio of overlap distance between the two blades (e) to the chord length of the blade (d ). A rotor with an overlapping proves to have a better starting characteristic than the one without overlapping. This is mainly caused by the improved pressure on the concave part of the returning blade due to the flow through the overlap distance [17, 18]. The gap ratio (ε s = e/s) of the rotor is defined as the ratio of separation gap between the rotor blades ( s) to the chord length of the blade (d ). When the spacing between the blades are large, the wind does not strike properly on the concave side of returning blade, ther eby reducing the net power of rotor [17]. By keeping a proper gap ratio, the torque coefficient of the rotor can be enhanced as much as by 25%. An end plate as shown in Fig. 2 is the simplest attachment that can be added to improve its performance. The plates at the rotor tops avoid the escape of wind from the concave side of the rotor blades, keeping the pressure difference between concave and convex side of the buckets at satisfactory levels over the height of the rotor [ 15]. The Reynolds number is the most important non-dimensional parameter for defining the flow characteristics of fluid flow conditions. It is reported that when the Reynolds number increases, the separation of boundary layer takes place on the lower side of returning blade of the rotor. This reduces the drag force on the returning blade considerably, and on the other hand, the lift force augments the power of the rotor when the rotor angles are oriented at 0⁰ or 180⁰ [17, 18]. JERT-17-1620 Alom 5 Journal of Energy Resources Technology. Received November 05, 2017; Accepted manuscript posted December 19, 2017. doi:10.1115/1.4038785 Copyright (c) 2017 by ASME 1.4 Performance parameters The performance of the Savonius rotor is estimated from the power and torque coefficients [11, 15, 33, 48-49]. The power coefficient (C  P ) of the Savonius rotor is defined as the ratio of the power generated by the rotor to the available wind power and is given by C  p   Pturbine  P available  T   s 1 2   AV 2 NT   3 60  1 2 3  AV  (1) The C  P   is usually estimated from field or wind tunnel tests, and with the help of numerical techniques that solve the conservation equations of the wind flow [7, 35]. It has been proved that a turbine can have the maximum possible C  P   of 59.3%. This limit is termed as the Betz Limit. The torque coefficient (C T)  is defined as the ratio of the actual torque produecd by the turbine (T turbine) to the theoretical torque available in the wind (T available) and can be expressed by C T   Tturbine T avaialble  T turbine 1 2  2   AV R  F  r  p 1 2 2  AV R (2) Under static condition, the net rotor torque is termed as static torque which is mostly responsible for the starting capability of the rotor. However, at rotating condition, the net rotor torque is termed as dynamic torque and is mostly responsible for its power converting capability [18]. The high static torque coefficient of the Savonius rotor plays a crucial role in improving the starting capability of vertical-axis Darrie us rotor [18, 50, 51]. The tip speed ratio (TSR), a significant dimensionless parameter for relating the performance of a Savonius rotor, is defined as the ratio of rotor tip speed ( u) to the free stream wind speed (V ) [52]. It is important to find the optimum TSR to get the maximum power output of the JERT-17-1620 Alom 6 Journal of Energy Resources Technology. Received November 05, 2017; Accepted manuscript posted December 19, 2017. doi:10.1115/1.4038785 Copyright (c) 2017 by ASME rotor. It is found that with the addition of load, the revolving speed of the rotor decreases, and therefore, with the increase of TSR, the C T   decreases. However, the performance is optimum at the intermediate range of TSR [26]. 1.5 Aerodynamic parameters  –  Drag and lift The drag force ( D) is usually defined as the force parallel to the direction of the incoming airflow; whereas the lift force ( L) is defined as the force perpendicular to the direction of incoming airflow [53] and is a consequence of pressure differential spreading between the upper and lower blade surfaces (Fig. 3). Based on the rotor blade design, the VAWT s are classified into lift- and drag-type devices. Savonius and Sistan rotors are drag-based VAWT s; whereas H- and Darriues rotors, composed of airfoil shaped blades, are lift-based VAWT s. In the lift-based turbines, the pressure differential between the blade surfaces creates the aerodynamic lift that causes the turbine to rotate. In comparison to the lift-based VAWT s, the drag-based VAWT s have shown better self-starting capabilities, however, their efficiencies are found to be lower [17, 18]. Moreover, their vertical rotational axis allows them to be installed in multiple configurations in a restricted place. These turbines equipped with an energy storage system can be used at the top of buildings or communication towers or at the hilly locations for decentralized small-scale electricity generation [5]. The windmills and  pumping devices, in general, have low-speed drag-based rotors, though the recent rotors for electricity generation are of high-speed lift-type. When the swept area is same, the power extracted by a lift-based rotor is generally greater than the power extracted from a drag-based rotor. However, for electricity generation, it becomes essential that the generator driving shaft works at a significant speed (1000 or 1500 rpm). This together with the much higher aerodynamic efficiency of lift-based rotor indicates that the drag-based rotors are not favored for electricity production [54]. JERT-17-1620 Alom 7 Journal of Energy Resources Technology. Received November 05, 2017; Accepted manuscript posted December 19, 2017. doi:10.1115/1.4038785 Copyright (c) 2017 by ASME 2. Augmentation techniques The Betz limit shows the maximum productivity of a wind turbine to be 59.3%, however, this limit can be exceeded by an augmentation system. An augmenter concentrates the wind flow and increases the mass flow through its area [16]. Since the power generated by a wind turbine is proportional to the cube of the incoming wind speed, a slight increase in the incoming wind speed can significantly improve the turbine performance. The wind pressure exerted to the concave part of the returning blade of a Savonius rotor produces a high negative torque and this drops its total power. By means of an augmenter, the negative drag of the rotor is decreased by avoiding the air from striking the returning blade of the rotor. The starting capability of the Savonius rotor is improved with the aid of these techniques. Hitherto, several augmentation techniques like V-shaped deflector, nozzle, multi-staging, twisted blades, valve, curtain plates, windshields, obstacle shield, venting slot, flat plate shield, concentrator, flaps and guide vanes and others ( Fig. 4) have been used to improve the C  P . It was sometime around 1978 that Alexander and Holownia [46] used a combination of flat and a circular shields (Fig.4a) and reported a maximum C  P  of 0.243. Morcos et. al. [55] also used a wind shields in front of the rotor and reported a maximum C  P of 0.34 (Fig. 4b). Ogawa et al. [56] and Huda et al. [57] also used the deflector plate (Fig.4c) and reported a maximum C  P   of 0.212 and 0.21, respectively. When multiple flaps are used in a rotor blade instead of one without the flaps, the negative drag of the rotor is reduced [58] (Fig. 4d). The use of Vshaped wedge deflector (Fig. 4e) at the upstream of the rotor harnessed about 19.7% more  power than a standard rotor without a deflector [21].  Shikha et  al.[8]  used a convergent nozzle (Fig. 4f ) at the front of advancing blade of a 6-bladed Savonius rotor to enhance the  power extraction at low wind speeds. The use of two-stage rotor (Fig. 4g) developed an JERT-17-1620 Alom 8 Journal of Energy Resources Technology. Received November 05, 2017; Accepted manuscript posted December 19, 2017. doi:10.1115/1.4038785 Copyright (c) 2017 by ASME improved torque and power coefficients in comparison to a single-stage rotor [59]. Twisted  bladed (Fig. 4h) rotor has proved to have a better self-starting capacity than the semicircular  bladed rotor [25]. Again, the use of valves (Fig. 4i) in semicircular blades reduces the negative torque on the rotor [60].  Circular windshield (Fig. 4j) has been also employed to reduce the wind pressure that exerts on the returning (or driven) blade of the rotor [61]. The use of curtain plate (Fig. 4k ) at the rotor front allows a maximum amount of wind to impinge on the advancing blade thereby reducing the negative torque [10]. It is reported that the use of obstacle shield (Fig. 4l) at the front of returning blade improves the rotor performance up to 30% [9]. Golecha et al. [62] used a deflector plate (Fig. 4m) in front of the advancing blade and reported a 50% increase in performance than the semicircular bladed rotor. It was also reported that with the use of shield (Fig. 4n) in a 6-bladed Savonius rotor, the C  P  could reach upto 0.52 [37]. Abraham et al. [11] studied the effect of venting (Fig. 4o) on a Savonius rotor  both experimentally and numerically to reduce drag on the returning blade. Roy et al. [14] employed concentrators (Fig. 4p) in the rotor front and reported a maximum C  P   of 0.32. Guide vane (Fig. 4q) also improves the performance of the rotor [15]. The conveyor-deflector curtain (Fig. 4r ) in a conventional Savonius rotor improved the C  P   up to 0.30 [30]. The summary of various augmentation techniques employed till date with their corresponding C  pmax is shown in Table 2 in a chronological manner. These techniques are discussed briefly in the following section. 2.1 Wind shields The obstacle shields are usually installed ahead of the returning blade of the rotor. This may  be of flat or circular type or both to decrease the active pressure on it. Alexander and Holownia [46] performed experiments in a low-speed wind tunnel and found a performance improvement 74% with a shielded Savonius rotor of high  AR (Fig. 4a). Morcos et. al. [55] also used similar type of shields to cover the returning blade of the rotor and reported a JERT-17-1620 Alom 9 Journal of Energy Resources Technology. Received November 05, 2017; Accepted manuscript posted December 19, 2017. doi:10.1115/1.4038785 Copyright (c) 2017 by ASME maximum C  P  of 0.34 (Fig. 4b). Hu et al. [61] carried out a numerical simulation using  RNG k-ε  turbulence model around a conventional Savonius rotor with a circular shield. The simulation was carried out for different inclination angle, β   = -90ᵒ, -45ᵒ, 0ᵒ, 15ᵒ, 30ᵒ, 45ᵒ and 60ᵒ  to optimize  β  (Fig. 4j), and found an improvement of 107% at  β  = 30ᵒ. Similarly, Mohamed et al.  [9, 63]  used the obstacle shield (Fig. 4l) in front of the returning blade to reduce the negative torque of the rotor. It was reported that optimally placed ( β   = 100.83ᵒ) obstacle shield improves the C  P  by 27.3% for the 2-bladed system at TSR= 0.7 (Fig. 5). 2.2 Deflector plates Usually, the deflecting plate is placed in front of the returning blade (Fig. 4c) to reduce the reverse force acting on it [56, 57, 62]. In this regard, Ogawa et al. [47, 56] carried out several wind tunnel experiments with a rotor set at  =0.20, and by varying the deflector angle ( β ) in the range 0-75ᵒ. They found the C  P  to improve by 27% at β  = 30ᵒ (Fig. 6). Interestingly, in the recent past, the deflector plates have also been used in water turbine applications where Golecha et al. [62] performed experiments with a modified Savonius rotor in an open water channel at a Reynolds number of 1.32 x 10 5. Eight various location of the plate was used by varying the geometric parameters viz., X 1, X2, and Y1, and by fixing Y 2 at 145 mm (Fig. 4m); whereas X1, X2, and Y1 are varied in the range of 135 – 230 mm, 135 – 230 mm and 0 – 108 mm, respectively. Thus, the deflector plate angle ( β ) is influenced by these parameters. The least value is selected such that the plate does not block the end plates of the turbine during rotation. It was reported that the deflector plate located at the optimum location (X 1=152 mm X2 =135 mm, Y 1= 55 mm and β = 101ᵒ) improved the C  P  up to 50% at TSR= 0.82. 2.3 Slatted blades In 1991, Reupke and Probert [58] proposed the practice of multiple flaps instead of using a continuous rotor blade (Fig. 4d). These flaps are open when moving into the wind, thus JERT-17-1620 Alom 10 Journal of Energy Resources Technology. Received November 05, 2017; Accepted manuscript posted December 19, 2017. doi:10.1115/1.4038785 Copyright (c) 2017 by ASME reducing the negative drag force on the rotor blades. The flaps are hinged in place of the curved parts of the blades to augment its harnessing effectiveness. The flaps open automatically when the rotor advance towards the wind thereby exerting more wind pressure on the advancing blade. Due to this, the static torque of the rotor is enhanced significantly. The investigation has been made with sixteen-hinged and thirty-two-hinged flaps in a 2 bladed rotor system. This rotor system demonstrated a better static torque than the conventional Savonius rotor; however, its performance was found inferior. The efficiency of the flapped system (modified Savonius) was found to be 5% as compared to the efficiency of 18% of a conventional system (Fig. 7). The modified system, thus, was found unacceptable for harnessing power. Tabassum and Probert [64] has used four hinged flaps in a Bach type rotor and found an improvement of 35% in the static torque in comparison to the original rotor of similar geometry under identical wind speed of 6.67 m/s (Fig. 8). The torque  produced in the complete rotation is found to be positive, which is not the case with the semicircular-bladed rotor without flaps. This reduced the amplitude of oscillation in the average torque produced during the complete rotation of the turbine. 2.4 V-shaped deflectors In practice, the V- shaped deflector is placed in front of the Savonius rotor ( Fig.4e), so that the wind flow resistance is encountered by the returning blade of the rotor. A series of wind tunnel experiments have been carried out by varying the deflector wedge semi-angle between 5-45ᵒ. With the optimally inclined deflector, the rotor extracts about 20% more power than the conventional Savonius rotor (Fig. 9). Such an important enhancement, achieved by a simple design, recommends that the practice of partly blocked wedges is extremely suitable. When the deflector plate is placed in the optimal location with wedge semi-angle of 37 ᵒ, the rotor operates over a wider range of TSR [21]. JERT-17-1620 Alom 11 Journal of Energy Resources Technology. Received November 05, 2017; Accepted manuscript posted December 19, 2017. doi:10.1115/1.4038785 Copyright (c) 2017 by ASME 2.5 Nozzles The application of nozzle is another idea to magnify the wind velocity before it encounters the blades of a Savonius rotor [8]. When a convergent nozzle (Fig. 4f ) is employed, the negative torque of the rotor is reduced and the effective wind speed is augmented. Wind tunnel experiments with five nozzle models are conducted for 2-, 4- and 6-bladed conventional Savonius rotor having overlap ratios of 1/3 and 1/6. The wind velocity at the nozzle inlet is varied from V1 = 0.6 to 0.9 m/s to amplify the outlet velocity to V 2 = 3 to 3.5 m/s when the length of the nozzle is 55 cm. When the nozzle length is increased to 80 cm, the inlet velocities are varied from V1 = 0.6 to 0.8 m/s to obtain outlet velocities from V 2 = 2 to 2.9 m/s. The 6-bladed Savonius rotor is found to enhance the power extraction at low wind speed under the application of convergent nozzle at the rotor front. 2.6 Multi-staging The conventional Savonius rotor mainly has two disadvantages on torque characteristics. Firstly, it has a large fluctuation of torque at some initial rotation of the turbine, and secondly, it has some angular positions where the torque becomes negative or even very small thereby reducing the rotor performance. As a result, the starting torque of a conventional Savonius rotor would be so low that the rotor cannot start on its own. Hence, to improve its static torque characteristics, staging of rotor (Fig. 4g) has been done [4, 43, 57, 59, 65] . As the staging of rotor is increased from 1 to 2, the C  P   becomes higher; but when the number of staging is increased from 2 to 3, the C  P  reduces due to the increased inertia of the rotor. Wind tunnel experiments demonstrates the optimal number of staging of the rotor to be 2 [ 65]. The C  P  for 2- and 3-stages conventional rotor is found to be 0.29 and 0.23, respectively [65]. This is accomplished by setting the phase lift at an angle 90ᵒ to each other for the 2-stage, and at 120ᵒ  for the 3-stage rotor, as shown in (Fig. 10). These arrangements increase the starting JERT-17-1620 Alom 12 Journal of Energy Resources Technology. Received November 05, 2017; Accepted manuscript posted December 19, 2017. doi:10.1115/1.4038785 Copyright (c) 2017 by ASME capability of the rotor. Hayashi et al. [4] noticed that a lower peak C  P   of a 3-stage rotor in comparison to its corresponding single-stage rotor. Staging results in the reduction of  AR of the individual stages of a 3-stage rotor as compared to that of a single-stage design. The 3stage rotors are better at low wind speeds as they have the uniform coefficient of static torque. Experimentally it has been reported that the multi-staging has shown a reduction of  power and dynamic torque for the same rotor. Thus, the multi-staging of rotors seems to  provide a better starting ability at low speeds with some reduction in performance [66]. 2.7 Twisted blades The twisted bladed rotor (Fig. 4h) is used in order to reduce the negative torque and to improve the self-starting characteristics of a single-stage Savonius rotor system [6, 25, 60, 65-67]. Wind tunnel experiments are carried out for twisted bladed rotor at a fixed twist angle 10.28ᵒ  and varying the gap width (i.e., separation gap) from S   = 14 to 67 mm [25]. The aerodynamic performance of these blades has been evaluated based on starting torque, power output and the rotational speed at various twist angles and gap widths. Later experimental investigation with a twisted-bladed rotor (twist angle is 12.5ᵒ) shows a C  P  of 0.19 as opposed to Cp of 0.18 for the conventional Savonius rotor [60, 65]. Kamoji et al. [66] investigated a twisted-bladed rotor with a twist angle of 90ᵒ in a low-speed wind tunnel. The experiments were conducted by varying   from 0.0 to 0.16 and the  AR from 0.88 to 1.2. The maximum C  P  of the twisted bladed rotor ( AR=0.88) was found to be 0.179 at  = 0.0 when the Reynolds number was 1.5 x10 5. Experimental investigation demonstrated a higher potential of the twisted bladed rotor in terms of smooth running, higher C  P   and self-starting capability than that of the semicircular-bladed rotor (Fig. 11). 2.8 Valves JERT-17-1620 Alom 13 Journal of Energy Resources Technology. Received November 05, 2017; Accepted manuscript posted December 19, 2017. doi:10.1115/1.4038785 Copyright (c) 2017 by ASME This new concept (Fig. 4i) has been incorporated in a twisted bladed Savonius rotor and is named as the Valve-Aided Twisted Savonius (VATS ) rotor [60, 65]. The rotor with valves has  been tested in a low-speed wind tunnel to calculate its performance. The mechanism is found to be independent of wind directions, and shows suitable for large machines. When the blade advances towards the wind, the valve opens automatically due to the wind pressure and hence experiences a lower flow resistance. The valve gets closed automatically by the centrifugal force during the power-harnessing part of the c ycle. This technique significantly improves the static torque of the rotor. Keeping the simplicity of the rotor intact, the VATS   rotor can increase the power coefficient. VATS   mechanism also helps to make direction independence of the rotor. In addition to this, damage to the rotor at high speed can be reduced. In the mechanism of VATS , a small deflecting plate is hinged on the concave side of the rotor blade in front of a hole. When the wind is on the concave side, this deflecting plate is enforced to cover the hole. But when this blade returns with its convex side to the wind, the hole is uncovered, allowing more air to flow. As a result, it reduces the drag on the returning blade and increases the performance without significantly disturbing the simplicity of the rotor. It is found that when the blade is oriented at α = 0° (Fig. 4i), the valve is aligned with the wind and is thus oriented at α =0° in the coordinate system with the valve surface coinciding with the wind flow direction. Again, when the rotor is at α = 90°, the valve is perpendicular to the flow giving a maximum drag force. The orientation of α = 180° is similar to the orientation of α = 0°. The centrifugal force is self-regulating of orientation, and it is a function of the mass of the valve, its radius of rotation and the angular velocity. Properly aligned valves with minimum frictional losses would improve the performance of VATS   mechanism. Saha et al. [65] also used valves in twisted as well as in semicircular bladed rotors. The 2-stage 3-bladed Savonius rotor with valves has demonstrated higher C  P   than the rotor without valves (Fig. 12). JERT-17-1620 Alom 14 Journal of Energy Resources Technology. Received November 05, 2017; Accepted manuscript posted December 19, 2017. doi:10.1115/1.4038785 Copyright (c) 2017 by ASME 2.9 Guide box About a decade ago, Irabu and Roy [68] used the guide box tunnel augmentation technique to improve the output power and to prevent the rotor from a wind disaster. The guide box tunnel is a passage in which the test rotor is involved. In order to adjust the input power, the area ratio between the inlet and outlet is varied from 0.3 to 0.7 [68]. Various experiments were conducted at Reynolds number of 6.05 x10 4  and 9.08 x 10 4  to obtain the adequate configuration that would provide the maximum C  P . It was found that the maximum C  P   with guide box of the area ratio 0.43 was increased by 1.5 times in 3-bladed system, and 1.23 times in the 2-bladed system. Further, with the use of guide box tunnel there was no negative torque in the complete rotation of the rotor when the guide box entrance opening angle was in  between 60ᵒ to 90ᵒ. 2.10 Curtain plates Altan and Atilgan [10, 69]  used a novel arrangement of curtains at the rotor front with the intention of improving its performance by preventing the negative torque that opposes the rotor rotation. Experiments with three different curtains, oriented at varying inclinations ( Fig. 4k ) were carried out in a low-speed wind tunnel with   =0.15 and the gap distance of 2.6 cm. The highest rotor power has been found from curtain 1 ( α = 45ᵒ and β  = 15ᵒ) at around 8 W. There was 16% improvement of performance in case of curtain 1 as compared to curtainless rotor (Fig. 13). 2.11 Shield Sometime during 2011, Emmanuel and Jun [37] used a different type of shield arrangement (Fig. 4l) in a six-bladed Savonius rotors. This arrangement is slightly different from the types used by Alexander and Holownia [46], Morcos et al. [55] and Hue et al. [61]. The goal of the investigators [37] was to suppress the pressure exerted on the convex part of the rotor. In this JERT-17-1620 Alom 15 Journal of Energy Resources Technology. Received November 05, 2017; Accepted manuscript posted December 19, 2017. doi:10.1115/1.4038785 Copyright (c) 2017 by ASME connection, various configurations of the six-bladed rotor have been examined using 2D unsteady simulation with  RNG k-ε  turbulence model. In the study, the six-bladed rotor without shield is found to have lower efficiency but still higher than a conventional two bladed Savonius rotor. The six-bladed rotor with shields and with stator have indicated maximum C  P   of around 0.4 and 0.5, respectively, however this occurs at dissimilar TSRs (Fig.14). 2.12 Venting slots Abraham et al. [11] and Plourde et al. [70] have used venting slots which is found more effective and simpler in design than the valves used by the past investigators [65, 70]. The rotors have been tested without and with venting slots to minimize the thrust loading on the returning blade. Wind tunnel experimentations are carried out to determine the power vs. load  parameterized with various wind speeds. At each wind speed, the generator is connected to a resistive load that could be effortlessly varied. The load resistances have been varied from 20 Ω to approximately 1000 Ω to  determine the resulting power curve (Fig.15). They have observed a very weak dependence on electrical load for the unvented and uncapped case, however, the performance is found strongly linked to the electrical system for the capped and vented case. This suggests that the electrical system should be designed appropriately while linking to the rotor. Inspired by the work of Abraham et al. [11] and Plourde et al. [70], Alom and Saha [71] used the venting slots on a two-bladed semicircular Savonius rotor. In order to arrive at the optimum position of the venting slots, three different configurations are designed and tested numerically (Fig. 16). The 2D unsteady simulation is carried out using SST  k -ω turbulence model at    = 0.20. Among the configurations studied, Design -II at TSR = 0.80 has shown a maximum C  P  of 0.292 (Fig. 17). There is an improvement of 7.53% with this design over the JERT-17-1620 Alom 16 Journal of Energy Resources Technology. Received November 05, 2017; Accepted manuscript posted December 19, 2017. doi:10.1115/1.4038785 Copyright (c) 2017 by ASME semicircular profile. In Design-II, the magnitude of velocity on the concave side of returning  bucket is found more in comparison to the semicircular profile (Figure 18). 18). Thus, it is clear from this numerical investigation that the venting slots, as demonstrated in Fig. 16b, 16b, can be used in an advanced blade profile (such as the recent elliptical type) to bring more effectiveness into the rotor design. 2.13 Concentrators Roy et al. [14] have studied and investigated the performance and starting characteristics of Savonius rotor employing concentrators (Fig. ( Fig. 19), 19), a technique similar to those of nozzle (Fig. ( Fig. 4f ) and curtain plates (Fig. (Fig. 4k ). ). This augmenter is used so that the major portion of the wind is incident on the concave side of the rotor. The experiments are conducted in a low speed wind tunnel at the wind velocity of 6.2 m/s, where loads are applied progressively with respected to the corresponding rotational speeds. With the augmenters placed α = 40°, and β  and  β  =10°, the rotor obtains a peak C  P  of   of 0.32, a value competitive to that of a lift-type turbine. This shows an overall performance improvement of 47.5% as compared to a semicircular bladed Savonius rotor without concentrators concentrators (Fig. ( Fig. 20). 20). 2.14 Guide vane The main idea of using guide vane in Savonius rotor is to improve the wind harvesting capacity of incoming air at the cost of structural complexity. Three designs, as illustrated in Fig. 21, 21, have been investigated by El-Askary et al. [15 15]. ]. The purpose is to minimize the negative torque and increase the exerted positive torque by guiding the incoming air effectively and smoothly. In this context, the Design-III is found to give an adequate developing length and reduced entrance effect. Numerical analysis using  FVM  solver   solver Ansys Fluent with SST k-ω k-ω turbulence model is carried out for each of the design. As seen from Fig. 22,, the Design-III shows a peak C  P   of 0.52 at TSR  22 TSR  = 2.2. The novel designs needs more JERT-17-1620 JERT-17-1620 Alom 17 Journal of Energy Resources Technology. Received November 05, 2017; Accepted manuscript posted December 19, 2017. doi:10.1115/1.4038785 Copyright (c) 2017 by ASME special treatments from the point of noise generation as they produced robust vortex shedding and large eddies behind and around the rotors. 3. Wind tunnel tests at a glance Wind tunnel experiments of model turbines represent an inexpensive and effective way for examining the wind turbine aerodynamics saving expenses, time, and uncertainties related to full-scale experimentation. As evident from the present review work, wind tunnels (both open and closed-circuit types) have been used extensively to evaluate the performance characteristics of augmented Savonius rotors. Tests have been carried out by employing multi-staging, venting slots, oriented jet, wind shields, deflector plates, valves either in front of the returning blade or inbuilt into the rotor blades. The summary of these tests in augmented rotors is shown in Table 3. 4. Numerical studies at a glance The flow field around a Savonius rotor is time dependent in nature; and flow separation and vortex formation are common phenomena. Therefore, the complex unsteady flow characteristics around the rotor is often impossible to explore over the classical aerodynamic tools such as blade element theory. Several numerical techniques such as  FVM , finite difference method (FDM ) and finite element method (FEM ) have been used for discretizing the governing equations around the rotor, however, the  FVM   is preferred due to complex numerical geometry. On the other hand, FVM  hand,  FVM  based  based commercial codes (e.g., ANSYS Fluent, CFX , Star CCM+) have CCM+) have shown an outstanding potential for predicting the flow behavior and  performance of Savonius rotor. In the numerical methods, the selection of turbulence models (S-A  S-A  model, realizable k-ϵ, k-ϵ, standard k-ϵ, k-ϵ, RNG k -ε, k -ω transition , k-ω k-ω SST, v 2-f ) and the selection grid size around the rotor are the most important criteria [62-80] [62-80].. The various numerical methods used in augmented Savonius rotors are summarized in Table 4. JERT-17-1620 JERT-17-1620 Alom 18 Journal of Energy Resources Technology. Received November 05, 2017; Accepted manuscript posted December 19, 2017. doi:10.1115/1.4038785 Copyright (c) 2017 by ASME 5 Conclusions Wind turbine designers are always being challenged to search for the resolution to use a smaller wind rotor in harvesting a higher power output while maximizing the cost saving and simplifying the structural complexity. Lesser self-starting capacity, poor starting torque, and lesser coefficient of power, are some of the main drawbacks of Savonius VAWTs. VAWTs. It is proven that the augmentation techniques increase the self-starting capability and C  P   of Savonius rotors. This review article makes an attempt to analyze the four decades of research into the augmented Savonius rotors. The key findings along with direction of research are summarized below: o The augmentation techniques (with additional cost and complexity to the rotor system) enhance the self-starting capability, amplify the wind speed, improve the visual impact, prevent blade cracking, and stop bird assaults. Other advantages include mounting of additional features to the system such as rainwater harvester and solar panel. o The augmenters such as V-shaped wedge deflector, curtain, obstacle shield, shields reduce the exerted wind pressure on the returning blade of rotor and hence raise the net positive torque. With the use of deflector plate at the rotor front, the C  P   can be enhanced up to 20-50% than a rotor without the deflector. On the other hand, the use of shield in a six-bladed Savonius rotor can improve the C  P   up to 0.50. These augmenters do not offer much structural complexity to the rotor system. o With the employment of guide box tunnel and convergent nozzle, the C  P   of a semicircular-bladed rotor may increase up to 1.5 and 3 times. The convergent nozzle cuts down the negative torque and increases the wind harvesting capacity of the turbine rotor. The guide box increases the rotor system complexity resulting a lesser JERT-17-1620 JERT-17-1620 Alom 19 Journal of Energy Resources Technology. Received November 05, 2017; Accepted manuscript posted December 19, 2017. doi:10.1115/1.4038785 Copyright (c) 2017 by ASME C  P , however, the nozzle makes the rotor lesser complex with a gain in C  P . An optimally designed guide vane can bring a maximum C  P   of 0.52, however, there is a chance of strong vortex shedding and high wake is generated around and behind the rotor leading to high noise generation. o The use of hinged flaps in a Bach type Savonius rotor can increase the static torque by 35% relative to the one without flaps. However, the hinged flaps increase the structural design complexity of the rotor system. o The venting-slots, if properly designed and oriented, can raise the C  P   by 7.5% over the conventional rotor without slots. The performance is found to be maximum when the venting slot is oriented at 30ᵒ above and below the central axis of the rotor blade. The venting slots are easier to be incorporated in rotor blades. o Among the blade profiles evolved, the elliptical-bladed Savonius rotor has proved to harness wind energy more efficiently. The gain in C  P   for an optimally designed elliptical-bladed rotor profile is found 18.18% higher than a semicircular-bladed Savonius rotor. o The foregoing analysis suggests the use of deflector plate(s), valves and especially the venting slots in an elliptical-bladed rotor to improve the C  P   without bringing much complexity to the turbine system. The location of augmenters in the elliptical-bladed rotor blades can be optimized with the help of numerical methods followed by wind tunnel experiments. JERT-17-1620 Alom 20 Journal of Energy Resources Technology. Received November 05, 2017; Accepted manuscript posted December 19, 2017. doi:10.1115/1.4038785 Copyright (c) 2017 by ASME Nomenclatures  Latin letters  A Swept area (m2)  AR Aspect Ratio C  P  Power coefficient C T  Dynamic torque coefficient C TS  Static torque co-efficient  D Rotor diameter (m)  D Drag force (N)  DO End plate diameter of the rotor (m) d Chord length of the blade (m) e Overlap distance between rotor blades (m)  H Rotor height (m) k Turbulence kinetic energy (m2/s2)  L Lift force (N)  N Rotor rotational speed (rpm) n  Number of time step  P available Power available in the wind (W)  P turbine Power produced by the turbine rotor (W) S Separation gap/ gap width (m) T available Theoretical torque available in the wind (N.m) T turbine Actual torque produced by the turbine rotor (N.m) T S  Static torque (N-m) u Rotor tip speed (m/s) V Wind velocity (m/s) JERT-17-1620 Alom 21 Journal of Energy Resources Technology. Received November 05, 2017; Accepted manuscript posted December 19, 2017. doi:10.1115/1.4038785 Copyright (c) 2017 by ASME Greek letters α, β  Angle of curtain plate (degree) δ Overlap ratio ε Energy dissipation rate ε s Gap ratio θ  Rotor blade angle (degree)  µ Dynamic viscosity (N-s/m2) ν Kinematic viscosity (m2/s)  ρ Density of air (kg-m3/s) ω Specific dissipation rate  Abbreviations  ANN Artificial Neural Network CFD Computational Fluid Dynamics  FDM Finite Difference Method  FEM Finite Element Method  FVM Finite Volume Method  HAWT Horizontal Axis Wind Turbine  RANS Reynolds Averaged Navier Stokes  RNG Renormalized  RSM Response Surface Method SIMPLE Semi-Implicit Method for Pressure Linked Equations SIMPLEC SIMPLE Consistent S-A Spalart and Allmaras sc Semicircular JERT-17-1620 Alom 22 Journal of Energy Resources Technology. Received November 05, 2017; Accepted manuscript posted December 19, 2017. doi:10.1115/1.4038785 Copyright (c) 2017 by ASME SST Shear Stress Transport TSR Tip Speed Ratio VATS Valve-Aided Twisted Savonius tw Twisted VAWT Vertical Axis Wind Turbine wv With Valves References 1. Jian, C., Kumbernuss, J., Linhua, Z., Lin, L., and Hongxing, Y., 2012, “Influence of   phase-shift and overlap ratio on Savonius wind turbine’s performance,” ASME J. Sol. Energy Eng., 134(1), p. 11016. 2. Zhou, T., and Rempfer, D., 2013, “Numerical study of detailed fl ow field and  performance of Savonius wind turbines,” Renew. Energy , 51, 373– 381. doi:10.1016/j.renene.2012.09.046. 3. Modi, V., and Fernando, M., 1989, “On the performance of the Savonius wind turbine,” ASME J. Sol. Energy Eng., 111(1), pp. 71 – 81. 4. Hayashi, T., Li, Y., and Hara, Y., 2005, “Wind T unnel tests on a different phase three-stage Savonius rotor,” JSME Int. J. Ser. B, 48(1), pp. 9 – 16. 5. Tummala, A., Velamati, R. K., Sinha, D. K., Indraja, V., and Krishna, V. H., 2016, “A review on small scale wind turbines,” Renew. Sustain. Energy Rev., 56, pp. 1351 –  1371. 6. Damak, A., Driss, Z., and Abid, M. S., 2013, “Experimental investigation of helical Savonius rotor with a twist of 180ᵒ,” Renew. Energy, 52, pp. 136 – 142. 7. Fujisawa, N., and Gotoh, F., 1994, “Experimental   study on the aerodynamic 548  performance of a Savonius rotor,” ASME J. Sol. Energy Eng., 116(3), pp. 148 – 152. JERT-17-1620 Alom 23 Journal of Energy Resources Technology. Received November 05, 2017; Accepted manuscript posted December 19, 2017. doi:10.1115/1.4038785 Copyright (c) 2017 by ASME 8. Shikha, Bhatti, T. S., and Kothari, D. P., 2003, “Wind energy conversion systems as a distributed source of generation,” J. Energy Eng., 129(3), pp. 69 – 80. 9. Mohamed, M. H., Janiga, G., Pap, E., and Thévenin, D., 2010, “Optimal blade shape of a modified Savonius turbine using an obstacle shielding the returning blade,” Renew. Energy, 52(1), pp. 236 – 242. 10. Altan, B. D., and Atilgan, M., 2010, “The use o f a curtain design to increase the  performance level of a Savonius wind rotors,” Renew. Energy, 35(4), pp. 821 – 829. 11. Abraham, J. P., Plourde, B. D., Mowry, G. S., Minkowycz, W. J., and Sparrow, E. M., 2012, “Summary of Savonius wind turbine development and future applications for small-scale power generation,” J. Renew. Sustain. Energy, 4(4). 12. Amano, R. S., 2017, “Review of Wind Turbine Research in 21st Century,” ASME J. Energy Resour. Technol., 139(5), p. 50801. 13. Dossena, V., Persico, G., Paradiso, B., Battisti, L., Dell’Anna, S., Brighenti, A., and Benini, E., 2015, “An Experimental Study of the Aerodynamics and Performance of a Vertical Axis Wind Turbine in a Confined and Unconfined Environment,” ASME J. Energy Resour. Technol., 137(5), p. 51207. 14. Roy, S., Mukherjee, P., and Saha, U. K., 2014, “Aerodyn amic performance evaluation of novel Savonius-style wind turbine under oriented jet,” Proceedings of the ASME 2014 Gas Turbine India Conference GTINDIA2014, ASME, pp. 1 – 7. 15. El-Askary, W. A., Nasef, M. H., AbdEL-hamid, A. A., and Gad, H. E., 2015, “Harvesting wind energy for improving performance of Savonius rotor,” J. Wind Eng. Ind. Aerodyn., 139, pp. 8 – 15. 16. Wong, K. H., Chong, W. T., Sukiman, N. L., Poh, S. C., Shiah, Y.-C., and Wang, C.T., 2017, “Performance enhancements on vertical axis wind turbines using flow JERT-17-1620 Alom 24 Journal of Energy Resources Technology. Received November 05, 2017; Accepted manuscript posted December 19, 2017. doi:10.1115/1.4038785 Copyright (c) 2017 by ASME augmentation systems: A review,” Renew. Sustain. Energy Rev., 73(February), pp. 904 – 921. 17. Akwa, J. V., Vielmo, H. A., and Petry, A. P., 2012, “A review on the performance of Savonius wind turbines,” Renew. Sustain. Energy Rev., 16(5), pp. 3054 – 3064. 18. Roy, S., and Saha, U. K., 2013, “Review of experimental investigations into the design, performance and optimization of the Savonius rotor,” Proc. Inst. Mech. Eng. Part a-Journal Power Energy, 227(4), pp. 528 – 542. 19. Savonius, S.J., 1930, “The S-rotor and its applications. Mechanical Engineering,” http://www.google.co.in/patents/US1766765. 20. Modi, V. J., Roth, N. J., and Fernando, M. S. U. K., 1984, “Optimum -configuration studies and prototype design of a wind-energy- operated irrigation system,” J. Wind Eng. Ind. Aerodyn., 16(1), pp. 85 – 96. 21. Shaughnessy, B. M., and Probert, S. D., 1992, “Partially-blocked Savonius rotor,” Appl. Energy, 43(4), pp. 239 – 249. 22. Promdee, C., and Photong, C., 2016, “Effects of wind angles and wind speeds on voltage generation of Savonius wind turbine with double wind tunn els,” Procedia Comput. Sci., 86(March), pp. 401 – 404. 23. Kacprzak, K., Liskiewicz, G., and Sobczak, K., 2013, “Numerical investigation of conventional and modified Savonius wind turbines,” Renew. Energy, 60, pp. 578 –  585. 24. Benesh, A.H, Ave, S.A. Dak, P.S., 1996, “Wind turbine with Savonius -type rotor,” (1996) 88. 25. Grinspan, A. S., Saha, U. K., and Mahanta, P., 2004, “Experimental investigation of twisted bladed savonius wind turbine rotor,” Int. Energy J., 5(1), pp. 1 – 9. JERT-17-1620 Alom 25 Journal of Energy Resources Technology. Received November 05, 2017; Accepted manuscript posted December 19, 2017. doi:10.1115/1.4038785 Copyright (c) 2017 by ASME 26. Banerjee, A., Roy, S., Mukherjee, P., and Saha, U. K., 2014, “Unste ady flow analysis around an elliptic-bladed Savonius-style wind turbine,” Proceedings of the ASME 2014 Gas Turbine India Conference GTINDIA2014, ASME, pp. 1 – 7. 27. Alom, N., Kolaparthi, S. C., Gadde, S. C., and Saha, U. K., 2016, “Aerodyn amic design optimization of elliptical-bladed Savonius-style wind turbine by numerical simulations,” Volume 6: Ocean Space Utilization; Ocean Renewable Energy, ASME,  p. V006T09A009. 28. Song, L., Yang, Z.-X., Deng, R.-T., and Yang, X.- G., 2013, “Performance and structure optimization for a new type of vertical axis wind turbine,” Proceedings of the 2013 International Conference on Advanced Mechatronic Systems, IEEE, pp. 687 – 692. 29. Gerardo, G., and Molfino, R., 2014, “From Savonius to Bronzinus : A comparison among vertical wind turbines,” Energy Procedia. 50, 10– 18. doi:10.1016/j.egypro.2014.06.002. 30. Tartuferi, M., Alessandro, V.D., Montelpare, S., and Ricci, R., 2015, “Enhancement of Savonius wind rotor aerodynamic performance : a computational study of new  blade shapes and curtain systems,” Energy. 79, 371– 384. doi:10.1016/j.energy.2014.11.023. 31. Sharma, S., and Sharma, R. K., 2017, “CFD investigation to quantify the effect of layered multiple miniature blades on the performance of Savonius rotor,” Energy Convers. Manag., 144, pp. 275 – 285. 32. Sharma, S., and Sharma, R. K., 2016, “Performance improvement of Savonius rotor using multiple quarter blades - A CFD investigation,” Energy Convers. Manag., 127,  pp. 43 – 54. JERT-17-1620 Alom 26 Journal of Energy Resources Technology. Received November 05, 2017; Accepted manuscript posted December 19, 2017. doi:10.1115/1.4038785 Copyright (c) 2017 by ASME 33. Mari, M., Venturini, M., and Beyene, A., 2017, “A novel geometry for vertical axis wind turbines based on the savonius concept,” ASME J. Energy Resour. Technol. Trans. ASME, 139(6), pp. 1 – 9. 34. Derakhshan, S., Tavaziani, A., and Kasaeian, N., 2015, “Numerical Shape Optimization of a Wind Turbine Blades Using Artificial Bee Colony Algorithm,” ASME J. Energy Resour. Technol., 137(5), p. 51210. 35. Fujisawa, N., Ishimatsu, K., and Kage, K., 1995, “A Com parative study of navierstokes calculations and experiments for the Savonius ro tor,” J. Sol. Energy Eng., 117(4), p. 344. 36. D’Alessandro, V., Montelpare, S., Ricci, R., and Secchiaroli, A., 2010, “U nsteady aerodynamics of a Savonius wind rotor: A new computational approach for the simulation of energy performance,” Energy, 35(8), pp. 3349 – 3363. 37. Emmanuel, B., and Jun, W., 2011, “Numerical study of a six-bladed Savonius wind turbine,” ASME J. Sol. Energy Eng., 133(4), p. 44503. 38. Irabu, K., and Roy, J. N., 2011, “Study of direct force measurement and characteristics on blades of Savonius rotor at static state,” Exp. Therm. Fluid Sci., 35(4), pp. 653 – 659. 39. Coughtrie, A. R., Borman, D. J., and Sleigh, P. A., 2013, “Effects of turbulence modelling on prediction of flow characteristics in a bench-scale anaerobic gas-lift digester,” Bioresour. Technol., 138, pp. 297 – 306. 40. Gupta, A. K., 2015, “Efficient wind energy conversion: evolution to modern des ign,” ASME J. Energy Resour. Technol., 137(5), p. 51201. 41. Baz, A. M., Mahmoud, N. A., Hamed, A. M., and Youssef, K. M., 2016, “Optimization of two and three rotor Savonius wind turbine,” Proceedings of ASME JERT-17-1620 Alom 27 Journal of Energy Resources Technology. Received November 05, 2017; Accepted manuscript posted December 19, 2017. doi:10.1115/1.4038785 Copyright (c) 2017 by ASME Turbo Expo 2015: Turbine Technical Conference and Exposition GT2015, ASME,  pp. 1 – 11. 42. Caboni, M., Sergio Campobasso, M., and Minisci, E., 2016, “Wind turbine design optimization under environmental uncertainty,” ASME J. Eng. Gas Turbines Power , 138(8), p. 82601. 43. Frikha, S., Driss, Z., Ayadi, E., Masmoudi, Z., and Abid, M. S., 2016, “Numeric al and experimental characterization of multi-stage Savonius rotors,” Energy, 114, pp. 382 –  404. 44. Gad-el-Hak, M., 2016, “Nine decades of fluid mechanics,” ASME J. Fluids Eng., 138(c), pp. 1 – 10. 45. Ducoin, A., Shadloo, M. S., and Roy, S., 2017, “Direct Numerical simulation of flow instabilities over Savonius style wind turbine blades,” Renew. Energy, 105, pp. 374 –  385. 46. Alexander, A. J., and Holownia, B. P., 1978, “Wind tunnel tests on a Savonius rotor,” J. Ind. Aerodyn., 3, pp. 343 – 351. 47. Ogawa, T., Yoshida, H., and Yokota, Y., 1989, “Development of rotational speed control systems for a Savonius-type wind turbine,” ASME J. Fluids Eng., 111(1), p. 53. 48. Van Treuren, K. W., 2015, “Small-Scale Wind Turbine Testing in Wind Tunnels Under Low Reynolds Number Conditions,” ASME J. Energy Resour. Technol., 137(5), p. 51208. 49. Ohya, Y., Miyazaki, J., Göltenbott, U., and Watanabe, K., 2017, “Power Augmentation of Shr ouded Wind Turbines in a Multirotor System,” ASME J. Energy Resour. Technol., 139(5), pp. 51202 – 51212. JERT-17-1620 Alom 28 Journal of Energy Resources Technology. Received November 05, 2017; Accepted manuscript posted December 19, 2017. doi:10.1115/1.4038785 Copyright (c) 2017 by ASME 50. Gupta, R., Biswas, A., and Sharma, K. K., 2008, “Comparative study of a three  bucket Savonius rotor with a combined three-bucket Savonius-three-bladed Darrieus rotor,” Renew. Energy, 33(9), pp. 1974 – 1981. 51. Dobrev, I., and Massouh, F., 2011, “CFD and PIV investigation of unsteady flow through Savonius wind turbine,” Energy Procedia, 6, pp. 711 – 720. 52. Naccache, G., and Paraschivoiu, M., 2017, “Development of the dual vertical axis wind turbine using CFD,” J. Fluids Eng., 139(December), pp. 1 – 17. 53. Alejandro Franco, J., Carlos Jauregui, J., Carbajal, A., and Toledano-Ayala, M., 2017, “Shape Morphing Mechanism for Improving Wind Turbines Performance,” ASME J. Energy Resour. Technol., 139(5), p. 51214. 54. Walker, J. F., and Jenkins, N., 1997 “ Wind energy technology,”  John Wiley and Sons, Chichester, England. 55. Morcos, S. M., Khalafallah, M. G., and Heikel, H. A., 1981, “The effect of shielding on the aerodynamic  performance of Savonius wind turbines,” Intersoc. Energy Convers. Eng. Conf. 16th, Atlanta, GA, August 9-14, 1981, Proceedings. Vol. 2. (A82-11701 02-44) New York, Am. Soc. Mech. Eng. 1981, p. 2037-2040., pp. 2037  –  2040. 56. Ogawa, T., and Yoshida, H., 1986, “E ffects of a deflecting plate and rotor end plates on performance of Savonius type wind turbine,” Bulletin of J. S. M. E. 29, 2115–  2121. 57. Huda, M. D., Selim, M. A., Islam, A.K.M.S., Islam, M. Q., 1992, “The performance of an S-shaped Savonius rotor with a deflecting plate,” RERIC Int. Energy J. 14, 25 –  32. 58. Reupke, P., and Probert, S. D., 1991, “Slatted -blade Savonius wind-rotors,” Appl. Energy, 40(1), pp. 65 – 75. JERT-17-1620 Alom 29 Journal of Energy Resources Technology. Received November 05, 2017; Accepted manuscript posted December 19, 2017. doi:10.1115/1.4038785 Copyright (c) 2017 by ASME 59. Menet, J. L., 2004, “A double -step Savonius rotor for local production of electricity: A design study,” Renew. Energy, 29(11), pp. 1843 – 1862. 60. Rajkumar, M. J., and Saha, U. K., 2006, “Valve-aided twisted Savonius rotor,” Wind Eng., 30(3), pp. 243 – 254. 61. Hu, Y., Tong, Z., and Wang, S., 2009, “A new type of VAWT and blade optimization,” Int. Technol. Innov. Conf. 2009 (ITIC 2009), pp. 14 – 14. 62. Golecha, K., Eldho, T. I., and Prabhu, S. V., 2011, “Influence of the deflector plate on the performance of modified Savonius water turbine,” Appl. Energy, 88(9), pp. 3207 –  3217. 63. Mohamed, M.H., Janiga, G., Pap, E., and Thévenin, D., 2011, “Optimal blade shape of a modified Savonius turbine using an obstacle shielding the returning blade, ” Energy Convers. Manag., 52, 236 – 242. doi:10.1016/j.enconman.2010.06.070. 64. Tabassum, S. A., and Probert, S. D., 1987, “Vertical-axis wind turbine: A modified design,” Appl. Energy, 28(1), pp. 59 – 67. 65. Saha, U. K., Thotla, S., and Maity, D., 2008, “Optimum design configuration of Savonius rotor through wind tunnel experiments,” J. Wind Eng. Ind. Aerod yn., 96(8 –  9), pp. 1359 – 1375. 66. Kamoji, M. A., Kedare, S. B., and Prabhu, S. V., 2009, “Performance tests on helical Savonius rotors,” Renew. Energy, 34(3), pp. 521 – 529. 67. Lee, J. H., Lee, Y. T., and Lim, H. C., 2016, “Effect of twis t angle on the performance of Savonius wind turbine,” Renew. Energy, 89, pp. 231 – 244. 68. Irabu, K., and Roy, J. N., 2007, “Characteristics of wind power on Savonius rotor using a guide- box tunnel,” Exp. Therm. Fluid Sci., 32(2), pp. 580 – 586. 69. Altan, B. D., and Atilgan, M., 2008, “An expe rimental and numerical study on the improvement of the performance of Savonius wind rotor,” Energy Convers. Manag., JERT-17-1620 Alom 30 Journal of Energy Resources Technology. Received November 05, 2017; Accepted manuscript posted December 19, 2017. doi:10.1115/1.4038785 Copyright (c) 2017 by ASME 49(12), pp. 3425 – 3432. 70. Plourde, B., Abraham, J., Mowry, G., and Minkowycz, W., 2012, “Simulations of three-dimensional vertical-axis turbines for communications applications,” Wind Eng., 36(4), pp. 443 – 454. 71. Alom, N., and Saha, U. K., 2016, “ Numerical optimization of semicircular-bladed Savonius rotor using vent augmenters,” in: Proceedings of the ACGT2016   Asian Congress on Gas Turbines, November 14-16, IIT Bombay, Mumbai, India. 72. Kamoji, M.A., Kedare, S.B., and Prabhu, S.V., 2008, “Experiments investigations on single stage, two stages and three stages conventional Savonius rotor,” Int. J. Energy Res. 10: 877 – 895. 73. Roy, S. and Saha, U.K., 2013 “Review on the numerical investigations into the design and development of Savonius wind rotors,” Renew. Sustain. Energy Rev. 24, 73 – 83. 74. Kang, C., Liu, H., and Yang, X., 2014, “Review of fluid dynamics aspects of Savonius-rotor-based vertical-axis wind rotors,” Renew. Sustain. Energy Rev. 33, 499  – 508. 75. Song, C., Zheng, Y., Zhao, Z., Zhang, Y., Li, C., and Jiang, H., 2015, “Investigation of meshing strategies and turbulence models of computational fluid dynamics simulations of vertical axis wind turbines,” J. Renew. Sustain. Energy, 7(3), pp. 0 – 19. 76. Balduzzi, F., Bianchini, A., Maleci, R., Ferrara, G., and Ferrari, L., 2014, “Blade design criteria to compensate the flow curvature effects in H-darrieus wind turbines,” ASME J. Turbomach., 137(1), p. 11006. 77. Uemura, Y., Tanabe, Y., Mamori, H., Fukushima, N., and Yamamoto, M., 2017, “Wake Deflection in Long Distance from a Yawed Wind Turbine,” ASME J. Energy Resour. Technol. Trans. ASME, 139(5), pp. 1 – 9. JERT-17-1620 Alom 31 Journal of Energy Resources Technology. Received November 05, 2017; Accepted manuscript posted December 19, 2017. doi:10.1115/1.4038785 Copyright (c) 2017 by ASME 78. Rahman, M., Morshed, K.  N., Lewis, J., and Fuller, M., 2010, “Experimental and numerical investigations on drag and torque characteristics of three-bladed Savonius wind turbine,” IMECE2009, Vol 6, ASME, pp. 85 – 94. 79. Ferdoues, M. S., Ebrahimi, S., and Vijayaraghavan, K., 2017, “Multi -objective optimization of the design and operating point of a new external axis wind turbine,” Energy, 125, pp. 643 – 653. 80. Castelli, M. R., and Benini, E., 2012, “Effect of  blade inclination angle on a Darrieus wind turbine,” ASME J. Turbomach., 134(3), p. 31016. JERT-17-1620 Alom 32 Journal of Energy Resources Technology. Received November 05, 2017; Accepted manuscript posted December 19, 2017. doi:10.1115/1.4038785 Copyright (c) 2017 by ASME List of Figures Fig. 1 Various blade profiles used for Savonius rotors. Fig. 2 Basic parameters of Savonius rotor Fig. 3 Lift and drag force on Savonius rotor Fig. 4 Various types of augmentation techniques. Fig. 5 C  P  vs TSR for obstacle and without obstacle [9] Fig. 6 Cp vs TSR for various deflector azimuthal angle [47] Fig. 7 Cp vs TSR for various flaps [58] Fig. 8 Static torque vs angle of rotation for various flaps [64] Fig. 9 Cp vs various deflector plate angle[21] Fig. 10 Cp vs velocity for various configurstion [65] Fig. 11 RPM  vs velocity for various gap width of twisted bladed rotor [25] Fig. 12 C  P  vs velocity for various valve aided Savonius rotor [65] Fig. 13 Power vs RPM  for various curtain design [10] Fig.14 Variation of C  P  with TSR for various rotor configurations [37] Fig. 15: Variation of power vs wind speeds for a vented and c apped rotor [11]. Fig.16 Vents at three different positions on the semicircular-bladed profiles [71] Fig.17 Variation of C  P with TSR [71] Fig.18 Velocity contour of the conventional Savonius rotor [71]. Fig.19 Orientation of the concentrators [14] Fig.20 C  P  vs TSR at various orientations of the concentrators [14] Fig.21 Different guide vane designs by El-Askary et al. [15] Fig.22 C  P  vs TSR for various guide vane position [15] JERT-17-1620 Alom 33 Journal of Energy Resources Technology. Received November 05, 2017; Accepted manuscript posted December 19, 2017. doi:10.1115/1.4038785 Copyright (c) 2017 by ASME List of Tables Table 1 Performance of various Savonius rotor profiles Table 2 Various augmentation techniques and observation Table 3 Literature reported experimental work on augmentation techniques. Table 4 Literature reported numerical work on augmentation techniques. JERT-17-1620 Alom 34 Journal of Energy Resources Technology. Received November 05, 2017; Accepted manuscript posted December 19, 2017. doi:10.1115/1.4038785 Copyright (c) 2017 by ASME Table 1: Performance of various Savonius rotor profiles I nvestigator(s) Year Blade profi le C Pmax  Alexander and Holownia [46] 1978 Semicircular 0.147 Owaga et al. [47] 1984 Semicircular 0.17 Grinspan et al. [25] 2004 Twisted 0.1159 Kacprzak et al. [23] 2013 Bach 0.178 Song et al. [28] 2013 Fish-ridged 0.23 Kacprzak et al. [23] 2013 Elliptical 0.172 Banerjee et al. [26] 2014 Elliptical 0.27 Roy et al. [14] 2014 Modified Bach 0.30 Roy et al. [14] 2014 Roy profile (New) 0.31 Gerardo and Molfino [29] 2014 Bronzinus 0.25 Tartuferi et al. [30] 2015 Airfoil shape 0.22 Alom et al. [27] 2016 Elliptical 0.33 Sharma and Sharma [31] 2016 Multiple quarter 0.2266 semicircular Sharma and Sharma [32] 2017 Multiple miniature 0.226 semicircular Mari et al. [33] JERT-17-1620 2017 Spline Alom 0.2477 35 Downloaded From: http://energyresources.asmedigitalcollection.asme.org/ on 12/26/2017 Terms of Use: http://www.asme.org/about-asme/terms-of-use Journal of Energy Resources Technology. Received November 05, 2017; Accepted manuscript posted December 19, 2017. doi:10.1115/1.4038785 Copyright (c) 2017 by ASME Table 2: Various augmentation techniques and observation I nvestigators Year Augmenter used Observation Alexander and Holownia [46] 1978 Wind shields C  Pmax = 0.243 Morcos et al. [55] 1981 Wind shields C  Pmax = 0.34 Ogawa et al. [56] 1984 Deflector plate C  Pmax = 0.212 Reupke and Probert [58] 1991 Slatted blade C  Pmax = 0.18 Shaughnessy and Probert [21] 1992 V-shaped deflector 19.7% increase of C  P  from the conventional rotor. Huda et al. [57] 1992 Deflector plate 20% increase of C  P  from the conventional rotor. Shikha et al. [8] 2003 Nozzle Increase of wind speed by 2 to 3 times. Menet [59] 2004 Multi-staging Improved C  P  than the single stage rotor. Grinspan et al. [25] 2004 Twisted blade C  Pmax = 0.1159 Rajkumar and Saha [60] 2006 Valve Reduces negative torque. Irabu and Roy [68] 2007 Guide box tunnel Increase in C  P  by 1.5 times for 3-bladed and 1.23 times for 2-bladed rotors. Hu et al. [61] 2009 Circular shield Reduces wind pressure on the returning blade. Altan and Atilgan [10] 2010 Curtain design C  Pmax = 0.38 Mohamed et al. [9] 2011 Obstacle shielding C  Pmax = 0.258 Golecha et al. [62] 2011 Deflector plate 50% increase of C  P  from the conventional rotor. Emmanuel and Jun [37] 2011 Shield C  Pmax = 0.50 Abraham et al. [11] 2012 Venting slots Reduces negative torque. Roy et al. [14] 2014 Concentrators C  Pmax = 0.33 El-Askary et al. [15] 2015 Guide vane C  Pmax = 0.52 Tartuferi et al. [30] 2015 Conveyor-deflector curtain C  Pmax = 0.30 JERT-17-1620 Alom 36 Journal of Energy Resources Technology. Received November 05, 2017; Accepted manuscript posted December 19, 2017. doi:10.1115/1.4038785 Copyright (c) 2017 by ASME Table 3: Literature reported experimental work on augmentation techniques.  Augmentation Techniques Multi-staging Rotor dimensions (H ×D) ( m×m) TSR No. of blades Hayashi et al. [4] 0.23×0.33 0.074 × 0.184 0.75 2 C T  is higher in the single stage rotor than the 3-stage rotor Frikha et al. [43] 0.20 × 0.173 -- 2 C  P  increases with the increase of number of stages. Menet [59] 0.1025 × 0.445 -- 2, 3 Golecha et al. [62] 0.170 × 0.245 0.82 2 Saha et al. [65] 0.173 × 0.109 0.122 × 0.077 0.10 × 0.063 -- 2, 3 Kamoji et al. [72] 0.208 × 0.208 0.226 × 0.113 0.289 × 0.096 0.83 2 Static C T  is lower in 3-stage rotor than the 1- and 2-stages rotor. 1.1 × 1.32 -- 2 Reduces the negative torque 0.230 × 0.209 0.70 2 Researcher(s) Observation Improved C  P   in 2-stage, 2-bladed rotor than the single stage rotor. The single-stage modified Savonius rotor is found better as compared to two- and three-stage modified Savonius rotors. C  Pmax = 0.29 for 2-stage, 2-bladed rotor. C  Pmax = 0.26 for 2-stage, 3-bladed rotor. C  Pmax = 0.23 for 3-stage, 2-bladed rotor. C  Pmax = 0.20 for 3-stage, 3-bladed rotor. Venting slot Abraham et al. [11] Oriented jet Roy et al. [14] Wind shields Alexander and Holownia [46] 0.46 × 0.19 0.72 2 C  Pmax = 0.234 Shaughnessy and Probert [21] 0.58 × 0.45 0.44 2 19.7% increase of C  P  from the conventional rotor. Ogawa et al. [56] 0.175 × 0.3 0.86 2 C  Pmax = 0.212 Huda et al. [57] 0.185 × 0.32 0.72 2 20% increase of C  P  from the conventional rotor. Golecha et al. [62] 0.170 × 0.245 0.82 2 Valve Rajkumar and Saha [60] 0.220 × 0.250 0.669 3 50% increase of C  P  from the conventional rotor. Reduces negative torque. Curtain Altan and Atilgan [69] 0.32 × 0.32 -- 2 Deflector plate C  Pmax = 0.23 The optimum curtain angle has been found as α = 45ᵒ and β = 15ᵒ JERT-17-1620 Alom 37 Downloaded From: http://energyresources.asmedigitalcollection.asme.org/ on 12/26/2017 Terms of Use: http://www.asme.org/about-asme/terms-of-use Journal of Energy Resources Technology. Received November 05, 2017; Accepted manuscript posted December 19, 2017. doi:10.1115/1.4038785 Copyright (c) 2017 by ASME Table 4: Literature reported numerical work on augmentation techniques. Augmentation Techniques Obstacle shield Researcher(s) Mohamed et al. [9] Abraham et al. [11] CF D  methodology  FVM  with 2D realizable k-ϵ  model 2D and 3D k-ω SST  model No. of Observation blades 2, 3 2 Venting slot C  Pmax = 0.258 Reduces the negative torque. 7.53% increase of C  P  Alom and Saha [71] 2D k-ω SST model 2 from conventional rotor without venting slots Guide vane Conveyordeflector El-Askary et al. [15] 2D k-ω SST  model 2 C  Pmax = 0.52 for DesignIII Fluent & Matlab, v2-f , Tartuferi et al. [30]  FVM  and RSM  turbulence flow 2 20% more power than the conventional rotor. Journal of Energy Resources Technology. Received November 05, 2017; Accepted manuscript posted December 19, 2017. doi:10.1115/1.4038785 Copyright (c) 2017 by ASME Table 4: Literature reported numerical work on augmentation techniques. Augmentation Techniques Obstacle shield Researcher(s) Mohamed et al. [9] Abraham et al. [11] CF D  methodology  FVM  with 2D realizable k-ϵ  model 2D and 3D k-ω SST  model No. of Observation blades 2, 3 2 Venting slot C  Pmax = 0.258 Reduces the negative torque. 7.53% increase of C  P  Alom and Saha [71] 2D k-ω SST model 2 from conventional rotor without venting slots Guide vane Conveyordeflector El-Askary et al. [15] 2D k-ω SST  model 2 C  Pmax = 0.52 for DesignIII Fluent & Matlab, v2-f , Tartuferi et al. [30]  FVM  and RSM  2 20% more power than the conventional rotor. turbulence flow Six-bladed rotor with Wind shield Enamuel and Jun [37] 2D RNG k-ε 2, 6 shield and stator is found better. The dynamic C T   and the Multi-staging Frikha et al. [43]  FVM  with 3D 2 modified k-ϵ  model C  P  enhanced as the number of stage increased. Curtain Altan and Atilgan [69]  FVM  with 2D standard k-ϵ  model The optimum curtain 2 angle has been found at α = 45ᵒ and β  = 15ᵒ JERT-17-1620 Alom 38 Journal of Energy Resources Technology. Received November 05, 2017; Accepted manuscript posted December 19, 2017. doi:10.1115/1.4038785 Copyright (c) 2017 by ASME Figure 1: Various blade profiles used for Savonius rotors. JERT-17-1620 Alom 39 Journal of Energy Resources Technology. Received November 05, 2017; Accepted manuscript posted December 19, 2017. doi:10.1115/1.4038785 Copyright (c) 2017 by ASME Figure 2: Basic parameters of Savonius rotor JERT-17-1620 Alom 40 Journal of Energy Resources Technology. Received November 05, 2017; Accepted manuscript posted December 19, 2017. doi:10.1115/1.4038785 Copyright (c) 2017 by ASME Figure 3: Lift and drag force on Savonius rotor  JERT-17-1620 Alom 41 Journal of Energy Resources Technology. Received November 05, 2017; Accepted manuscript posted December 19, 2017. doi:10.1115/1.4038785 Copyright (c) 2017 by ASME Figure 4: Various types of augmentation techniques. JERT-17-1620 Alom 42 Journal of Energy Resources Technology. Received November 05, 2017; Accepted manuscript posted December 19, 2017. doi:10.1115/1.4038785 Copyright (c) 2017 by ASME Figure 5: C  P  vs TSR for obstacle and without obstacle [9] JERT-17-1620 Alom 43 Journal of Energy Resources Technology. Received November 05, 2017; Accepted manuscript posted December 19, 2017. doi:10.1115/1.4038785 Copyright (c) 2017 by ASME Figure 6: Cp vs TSR for various deflector azimuthal angle [47] JERT-17-1620 Alom 44 Journal of Energy Resources Technology. Received November 05, 2017; Accepted manuscript posted December 19, 2017. doi:10.1115/1.4038785 Copyright (c) 2017 by ASME Figure 7: Cp vs TSR for various flaps [58] JERT-17-1620 Alom 45 Journal of Energy Resources Technology. Received November 05, 2017; Accepted manuscript posted December 19, 2017. doi:10.1115/1.4038785 Copyright (c) 2017 by ASME Figure 8: Static torque vs angle of rotation for various flaps [64] JERT-17-1620 Alom 46 Journal of Energy Resources Technology. Received November 05, 2017; Accepted manuscript posted December 19, 2017. doi:10.1115/1.4038785 Copyright (c) 2017 by ASME Figure 9: Cp vs various deflector plate angle[21] JERT-17-1620 Alom 47 Journal of Energy Resources Technology. Received November 05, 2017; Accepted manuscript posted December 19, 2017. doi:10.1115/1.4038785 Copyright (c) 2017 by ASME Figure 10: C  P  vs velocity for various configurstion [65] JERT-17-1620 Alom 48 Journal of Energy Resources Technology. Received November 05, 2017; Accepted manuscript posted December 19, 2017. doi:10.1115/1.4038785 Copyright (c) 2017 by ASME Figure 11: RPM  vs velocity for various gap width of twisted bladed rotor [25] JERT-17-1620 Alom 49 Journal of Energy Resources Technology. Received November 05, 2017; Accepted manuscript posted December 19, 2017. doi:10.1115/1.4038785 Copyright (c) 2017 by ASME Figure 12: C  P  vs velocity for various valve aided Savonius rotor [65] JERT-17-1620 Alom 50 Journal of Energy Resources Technology. Received November 05, 2017; Accepted manuscript posted December 19, 2017. doi:10.1115/1.4038785 Copyright (c) 2017 by ASME Figure 13: Power vs RPM  for various curtain design [10] JERT-17-1620 Alom 51 Journal of Energy Resources Technology. Received November 05, 2017; Accepted manuscript posted December 19, 2017. doi:10.1115/1.4038785 Copyright (c) 2017 by ASME Figure 14: Variation of C  P  with TSR for various rotor configurations [37] JERT-17-1620 Alom 52 Journal of Energy Resources Technology. Received November 05, 2017; Accepted manuscript posted December 19, 2017. doi:10.1115/1.4038785 Copyright (c) 2017 by ASME Figure 15: Variation of power vs wind speeds for a vented and capped rotor [11]. JERT-17-1620 Alom 53 Journal of Energy Resources Technology. Received November 05, 2017; Accepted manuscript posted December 19, 2017. doi:10.1115/1.4038785 Copyright (c) 2017 by ASME Figure 16: Vents at three different positions on the semicircular-bladed profiles [71] JERT-17-1620 Alom 54 Journal of Energy Resources Technology. Received November 05, 2017; Accepted manuscript posted December 19, 2017. doi:10.1115/1.4038785 Copyright (c) 2017 by ASME Figure 17: Variation of C  P  with TSR [71] JERT-17-1620 Alom 55 Journal of Energy Resources Technology. Received November 05, 2017; Accepted manuscript posted December 19, 2017. doi:10.1115/1.4038785 Copyright (c) 2017 by ASME Figure 18: Velocity contour of the conventional Savonius rotor [71]. JERT-17-1620 Alom 56 Journal of Energy Resources Technology. Received November 05, 2017; Accepted manuscript posted December 19, 2017. doi:10.1115/1.4038785 Copyright (c) 2017 by ASME Figure 19: Orientation of the concentrators [14] JERT-17-1620 Alom 57 Journal of Energy Resources Technology. Received November 05, 2017; Accepted manuscript posted December 19, 2017. doi:10.1115/1.4038785 Copyright (c) 2017 by ASME Figure 20: C  P  vs TSR at various orientations of the concentrators [14] JERT-17-1620 Alom 58 Journal of Energy Resources Technology. Received November 05, 2017; Accepted manuscript posted December 19, 2017. doi:10.1115/1.4038785 Copyright (c) 2017 by ASME Figure 21: Different guide vane designs by El-Askary et al. [15] JERT-17-1620 Alom 59