Four Decades of Research into the Augmentation Techniques of Savonius Wind Turbine 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 Four Decades of Research into the Augmentation Techniques of Savonius Wind Turbine Rotor Nur Alom Trainee Teacher d ite Department of Mechanical Engineering ed National Institute of Technology Meghalaya py Shillong – 793003, India Co E-mail: [email protected] Ujjwal K. Saha ot tN Professor rip Department of Mechanical Engineering sc Indian Institute of Technology Guwahati Guwahati- 781039, India nu E-mail: [email protected] Ma ed Abstract pt The design and development of wind turbines is increasing throughout the world to offer ce electricity without paying much to the global warming. The Savonius wind turbine rotor, or Ac 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 simplicity, 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 (CP), a considerable amount of investigation has JERT-17-1620 Alom 1 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 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. Such augmenters reduce the negative torque and improve the self-starting capability while maintaining a high rotational speed of the rotor. The CP of the conventional Savonius rotors lie in the range of 0.12-0.18, however, with the d use of augmenters, it can reach up to 0.52 with added design complexity. This article attempts ite to give an overview of the various augmentation techniques used in Savonius rotor over the ed last four decades. Some of the key findings with the use of these techniques have been py addressed and makes an attempt to highlight the future direction of research. Co Keywords: Savonius rotor, blade profiles, augmentation techniques, torque coefficient, power coefficient, tip-speed ratio. ot tN 1. Introduction r ip The Savonius rotor is a sort of vertical-axis wind turbine (VAWT). A conventional turbine sc rotor, mounted on a rotating shaft or framework, consists of several semicircular blades. The nu rotor system may either be ground stationed or fastened in a floating system. The Savonius Ma rotor, invented by the Finnish engineer Sigurd Johannes Savonius in 1925 [1, 2], is one of the ed simplest type of wind turbine. Aerodynamically, it is a drag 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 pt an ‘S’ shape in cross section [3-4]. The rotor blades experience less drag when moving ce against the wind than when it moves with the wind due to their curved shape. The differential Ac 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]. The HAWTs are actually acknowledged for their reasonably higher CP than the Savonius VAWTs, and fundamentally have been used for power JERT-17-1620 Alom 2 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 generation [6]. However, the Savonius VAWTs have various important rewards than the HAWTs 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 controller [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 d rotational cycle of the rotor, and as a result, the net positive torque of the rotor gets reduced ite [14-17]. To improve its performance, various blade profiles such as semicircular [18-22], ed Bach [23], Benesh [24], twisted [25], elliptical [23, 26-27], fish-ridged rotor [28], modified py Bach type [14], Bronzinus [29], airfoil shape blade [30], multiple quarter [31], multiple Co miniature semicircular [32], and spline [33] have been evolved. Besides using these blade ot profiles, the various augmentation techniques have also been used to decrease the negative tN 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, ip nozzle, venting slot, deflecting plate, guide vane and others [4, 8-11, 14-16]. r sc 1.1 Aim of the present study nu Since its inception, several wind tunnel experiments have been carried and are being Ma conducted to estimate the performance characteristics of Savonius rotor. The main objectives ed in these studies have been to optimize the various parameters of the rotor for attaining pt suitable design configurations. It is only during the last few decades that the investigators ce have started following numerical studies with various numerical methods, optimization Ac techniques [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 Roy and Saha [18] have provided a complete knowledge and beneficial evidence on the various aspects of Savonius rotors. Till JERT-17-1620 Alom 3 Downloaded From: http://energyresources.asmedigitalcollection.asme.org/ on 12/26/2017 Terms of Use: http://www.asme.org/about-asme/terms-of-use ip Rudimentary investigation with an elliptical profile has demonstrated its energy capturing r sc potential.4038785 Copyright (c) 2017 by ASME date. [27] optimized the elliptical profile numerically and found a performance gain of 18. design simplicity.asmedigitalcollection.18% than the semicircular- ed bladed Savonius rotor. Alom et al. As stated previously. multiple miniature semicircular and spline.1115/1. Bronzinus. Thus. The aspect ratio (AR =H/D) of the rotor is defined as the ratio JERT-17-1620 Alom 4 Downloaded From: http://energyresources. overlap ratio. modified Bach. an improved CP than the conventional rotor [23]. fish-ridged rotor. various augmentation techniques have also been employed to improve the all-round performance of the rotor. Benesh. airfoil shape blade. The performance indices of these blade profiles are shown in Table 1. All these blade profiles are tN illustrated in Fig. its performance can be improved by optimizing the basic parameters like aspect ratio.asme. Bach. few noticeable investigations are found on Co the use of dissimilar category of blade shapes such as conventional semicircular. doi:10. hence.2 Evolution of Savonius blade profiles py To improve the performance of Savonius rotors. 2017.asme. and low cost. gap ratio. Accepted manuscript posted December 19.3 Geometric parameters Ac Despite having a low efficiency. and number rotor blades [25]. elliptical. this review work tries to make an analysis on the various d augmentation techniques applied [35-47] and makes recommendation of the future studies.7% over the semicircular profile [26].Journal of Energy Resources Technology. an elliptical profile of different design has shown a performance improvement of Ma 10. 2017. various turbulence models and soft-computing techniques have been used by various researchers to improve the efficiency of the rotor. ce 1. twisted. ot multiple quarter. In a separate numerical nu study.1. various blade profiles have been developed. it clear that the elliptical-bladed rotor can be a strong contender pt in the future designs of Savonius rotor. However. Received November 05.org/ on 12/26/2017 Terms of Use: http://www. At a later stage.org/about-asme/terms-of-use . ite ed 1. the Savonius rotor has become popular for its good starting ability. Uniting the past experimental and numerical investigations. and at the same time. Reynolds number. Received November 05. thereby reducing the net power of rotor [17].org/ on 12/26/2017 Terms of Use: http://www. doi:10.4038785 Copyright (c) 2017 by ASME of rotor height (H) to the overall diameter of the rotor (D). 2. A small rotor diameter always causes a fast diverging of airflow. 18]. 18]. 2017. Accepted manuscript posted December 19. This is mainly caused by the improved pressure on the concave part of the py returning blade due to the flow through the overlap distance [17. keeping the pressure difference between concave and Ma convex side of the buckets at satisfactory levels over the height of the rotor [15]. This is illustrated in Fig. and on the other Ac hand. The Reynolds number is the most important non-dimensional parameter for defining the flow ed characteristics of fluid flow conditions. The gap ratio (εs = e/s) Co of the rotor is defined as the ratio of separation gap between the rotor blades (s) to the chord ot length of the blade (d).asme. whereas the rotational speed of the rotor decreases. the wind does not tN strike properly on the concave side of returning blade. When the diameter of the rotor increases.asme. When the spacing between the blades are large. 2017. JERT-17-1620 Alom 5 Downloaded From: http://energyresources. the produced torque also increases. the separation of boundary layer takes place on the lower side of returning blade of ce the rotor. The plates at the rotor tops avoid the escape of wind from nu the concave side of the rotor blades. Because of the consequence of blade tips.1115/1. the torque coefficient of the rotor can be enhanced as r ip much as by 25%. and vice versa [17.Journal of Energy Resources Technology. 2 is the simplest attachment that can be sc added to improve its performance. A rotor with an overlapping proves to have a better starting characteristic than the one without ed overlapping. the Savonius rotors have low losses at high ARs [15]. The overlap ratio ( = e/d) is defined as the ratio of d ite overlap distance between the two blades (e) to the chord length of the blade (d). 18]. It is reported that when the Reynolds number pt increases. the lift force augments the power of the rotor when the rotor angles are oriented at 0⁰ or 180⁰ [17.org/about-asme/terms-of-use . By keeping a proper gap ratio. An end plate as shown in Fig. This reduces the drag force on the returning blade considerably.asmedigitalcollection. 2017. 33. It has been proved Co that a turbine can have the maximum possible CP of 59. a significant dimensionless parameter for relating the performance of a Savonius rotor. 15. However. This limit is termed as the Betz Limit. Received November 05. the net rotor torque is termed as static torque which is mostly ed responsible for the starting capability of the rotor.4 Performance parameters The performance of the Savonius rotor is estimated from the power and torque coefficients [11. It is important to find the optimum TSR to get the maximum power output of the JERT-17-1620 Alom 6 Downloaded From: http://energyresources. 48-49].4038785 Copyright (c) 2017 by ASME 1.asmedigitalcollection.asme. and with the help of numerical py techniques that solve the conservation equations of the wind flow [7. The power coefficient (CP) 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 d Pturbine T  s 2 NT Cp    ite Pavailable 1  AV 3 60  1  AV 3 (1) 2 2 ed The CP is usually estimated from field or wind tunnel tests. ot tN The torque coefficient (CT) is defined as the ratio of the actual torque produecd by the turbine ip (Tturbine) to the theoretical torque available in the wind (Tavailable) and can be expressed by r sc Tturbine Tturbine F  rp CT    nu Tavaialble 1  AV 2 R 1  AV 2 R (2) 2 2 Ma Under static condition.3%. Accepted manuscript posted December 19.org/about-asme/terms-of-use . at rotating condition. doi:10.org/ on 12/26/2017 Terms of Use: http://www. the net pt rotor torque is termed as dynamic torque and is mostly responsible for its power converting ce capability [18]. The high static torque coefficient of the Savonius rotor plays a crucial role in Ac improving the starting capability of vertical-axis Darrieus rotor [18. The tip speed ratio (TSR). 2017.1115/1. 50. 35].asme. is defined as the ratio of rotor tip speed (u) to the free stream wind speed (V) [52].Journal of Energy Resources Technology. 51]. though the recent rotors for pt electricity generation are of high-speed lift-type. the CT decreases. doi:10.org/ on 12/26/2017 Terms of Use: http://www. have low-speed drag-based rotors. it becomes essential that the generator driving shaft works at a significant speed (1000 or 1500 rpm). 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 py upper and lower blade surfaces (Fig. In tN the lift-based turbines. 18].and drag-type devices. Moreover. In comparison to the lift-based VAWTs. with the increase of TSR. These turbines equipped with an Ma 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].asme. the pressure differential between the blade surfaces creates the ip aerodynamic lift that causes the turbine to rotate. their vertical rotational axis allows them to be nu installed in multiple configurations in a restricted place.asme. 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].4038785 Copyright (c) 2017 by ASME rotor. ot whereas H.5 Aerodynamic parameters – Drag and lift d ite The drag force (D) is usually defined as the force parallel to the direction of the incoming ed airflow. When the swept area is same. JERT-17-1620 Alom 7 Downloaded From: http://energyresources. for electricity generation. the r drag-based VAWTs have shown better self-starting capabilities. However. 2017. Accepted manuscript posted December 19.1115/1. are lift-based VAWTs.org/about-asme/terms-of-use . The windmills and ed pumping devices. their efficiencies are sc found to be lower [17. composed of airfoil shaped blades. the performance is optimum at the intermediate range of TSR [26]. however. 1. Received November 05. Savonius and Sistan rotors are drag-based VAWTs.and Darriues rotors. in general. However. the VAWTs are Co classified into lift. the power ce extracted by a lift-based rotor is generally greater than the power extracted from a drag-based Ac rotor. 2017. and therefore. 3). the revolving speed of the rotor decreases. Based on the rotor blade design. It is found that with the addition of load.Journal of Energy Resources Technology.asmedigitalcollection. Ogawa ed et al. An augmenter concentrates the wind flow and increases the mass flow through its area [16]. the negative drag of the rotor is reduced [58] (Fig. this limit can be exceeded by an augmentation system.243. 4d). al. The use of V- Ac shaped wedge deflector (Fig. [56] and Huda et al. the negative drag Co of the rotor is decreased by avoiding the air from striking the returning blade of the rotor. [57] also used the deflector plate (Fig.4038785 Copyright (c) 2017 by ASME 2. The wind pressure ed exerted to the concave part of the returning blade of a Savonius rotor produces a high py negative torque and this drops its total power. multi-staging.[8] used a convergent nozzle (Fig. concentrator. flaps and guide vanes and others (Fig. curtain plates.org/ on 12/26/2017 Terms of Use: http://www.4a) and reported a maximum CP of 0. nu It was sometime around 1978 that Alexander and Holownia [46] used a combination of flat Ma and a circular shields (Fig. ot Hitherto. however. obstacle shield.1115/1.34 (Fig. tN twisted blades. 2017. Accepted manuscript posted December 19. By means of an augmenter. a slight increase in the ite incoming wind speed can significantly improve the turbine performance. 2017. 4f) at the front of advancing blade of a 6-bladed Savonius rotor to enhance the power extraction at low wind speeds. flat plate ip shield. windshields. Received November 05. doi:10. Since the power generated by a wind d turbine is proportional to the cube of the incoming wind speed. The starting capability of the Savonius rotor is improved with the aid of these techniques.212 and 0. The use of two-stage rotor (Fig. Morcos et.4c) and reported a maximum pt CP of 0. 4b). several augmentation techniques like V-shaped deflector.7% more power than a standard rotor without a deflector [21]. When multiple flaps are used in a rotor blade instead of ce one without the flaps. respectively.asmedigitalcollection. 4e) at the upstream of the rotor harnessed about 19.asme. Shikha et al.Journal of Energy Resources Technology. venting slot.3%. valve. nozzle.21. 4g) developed an JERT-17-1620 Alom 8 Downloaded From: http://energyresources. Augmentation techniques The Betz limit shows the maximum productivity of a wind turbine to be 59.org/about-asme/terms-of-use .asme. [55] also used a wind shields in front of the rotor and reported a maximum CP of 0. 4) have been used to improve the r sc C P. 4q) also improves the performance of the rotor [15]. [14] ip employed concentrators (Fig.asme. 2017. the use of valves (Fig. doi:10. Again.org/ on 12/26/2017 Terms of Use: http://www. These techniques are discussed briefly ed in the following section. 2017. Abraham et al. [62] used a deflector plate (Fig.4038785 Copyright (c) 2017 by ASME improved torque and power coefficients in comparison to a single-stage rotor [59]. 4o) on a Savonius rotor both experimentally and numerically to reduce drag on the returning blade.1 Wind shields Ac The obstacle shields are usually installed ahead of the returning blade of the rotor. Golecha et al. 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. r sc Guide vane (Fig. 4i) in semicircular blades reduces the negative torque on the rotor [60].asmedigitalcollection. pt ce 2.org/about-asme/terms-of-use .asme. 4k) at the rotor front allows a maximum amount of wind to impinge ite on the advancing blade thereby reducing the negative torque [10]. The conveyor-deflector nu curtain (Fig.32. It was also ot reported that with the use of shield (Fig. The Ma summary of various augmentation techniques employed till date with their corresponding Cpmax is shown in Table 2 in a chronological manner. Received November 05. 4m) in front of the advancing blade Co and reported a 50% increase in performance than the semicircular bladed rotor. The d use of curtain plate (Fig. al. Roy et al. [11] studied the effect of venting (Fig. 4a). It is reported that the use of ed obstacle shield (Fig.1115/1. 4n) in a 6-bladed Savonius rotor. 4h) rotor has proved to have a better self-starting capacity than the semicircular bladed rotor [25]. Morcos et. This may be of flat or circular type or both to decrease the active pressure on it. 4l) at the front of returning blade improves the rotor performance up to py 30% [9]. [55] also used similar type of shields to cover the returning blade of the rotor and reported a JERT-17-1620 Alom 9 Downloaded From: http://energyresources. 4j) has been also employed to reduce the wind pressure that exerts on the returning (or driven) blade of the rotor [61]. 4r) in a conventional Savonius rotor improved the CP up to 0. Twisted bladed (Fig.Journal of Energy Resources Technology. 4p) in the rotor front and reported a maximum CP of 0.30 [30]. Accepted manuscript posted December 19.52 [37]. the CP could reach tN upto 0. Circular windshield (Fig. org/about-asme/terms-of-use . The least pt value is selected such that the plate does not block the end plates of the turbine during ce rotation. 63] used the obstacle shield (Fig. the deflector plates have also been used in water turbine applications where r sc Golecha et al. 6).3% for the 2-bladed system at TSR= 0. and Y1. in the ip recent past. 2017. the deflecting plate is placed in front of the returning blade (Fig.32 x 105. In this regard. 135–230 mm and 0–108 mm. These flaps are open when moving into the wind. It was reported that the deflector plate located at the optimum location (X1=152 mm Ac X2 =135 mm. -45ᵒ. and by fixing Y2 at 145 mm (Fig.34 (Fig. X2. X1.Journal of Energy Resources Technology. 15ᵒ. 2..asmedigitalcollection.asme. 4c) to reduce the reverse force acting on it [56. X2. [9. 57. [47. Received November 05. Y1= 55 mm and β = 101ᵒ) improved the CP up to 50% at TSR= 0. 0ᵒ. Mohamed et al. 4d).82.20.4038785 Copyright (c) 2017 by ASME maximum CP of 0. 2017. thus JERT-17-1620 Alom 10 Downloaded From: http://energyresources. ed py 2.3 Slatted blades In 1991. 56] carried out several ot wind tunnel experiments with a rotor set at =0. doi:10. 45ᵒ and 60ᵒ to optimize β (Fig. 62].org/ on 12/26/2017 Terms of Use: http://www. The simulation was carried out for different inclination angle.2 Deflector plates Co Usually. Reupke and Probert [58] proposed the practice of multiple flaps instead of using a continuous rotor blade (Fig. It was reported that optimally placed (β = 100. They found the CP to improve by 27% at β = 30ᵒ (Fig.asme. Eight various location of the plate was used by Ma varying the geometric parameters viz. Thus. 4l) in front of the returning blade to d reduce the negative torque of the rotor. Similarly.83ᵒ) ite obstacle shield improves the CP by 27. [62] performed experiments with a modified Savonius rotor in an open water nu channel at a Reynolds number of 1. the deflector plate angle (β) is influenced by these parameters. 4j). Accepted manuscript posted December 19. [61] carried out a numerical simulation using RNG k-ε turbulence model around a conventional Savonius rotor with a circular shield. and Y1 are varied in the range of 135–230 mm. and found an improvement of 107% at β = 30ᵒ. Interestingly. Hu et al.7 (Fig. whereas X1. β = -90ᵒ. 5). and by varying the deflector angle (β) in tN the range 0-75ᵒ.1115/1. 30ᵒ. 4m). 4b). ed respectively. Ogawa et al. nu 2. JERT-17-1620 Alom 11 Downloaded From: http://energyresources. the V. Tabassum and Probert [64] has used four hinged flaps in a Bach type ot rotor and found an improvement of 35% in the static torque in comparison to the original tN rotor of similar geometry under identical wind speed of 6. 7). 9). A series of wind pt tunnel experiments have been carried out by varying the deflector wedge semi-angle between ce 5-45ᵒ. This reduced the amplitude of oscillation in the r sc average torque produced during the complete rotation of the turbine. 2017.org/ on 12/26/2017 Terms of Use: http://www.4e). The investigation has been made with sixteen-hinged and thirty-two-hinged flaps in a 2- d bladed rotor system. When the deflector plate is placed in the optimal location with wedge semi-angle of 37ᵒ.67 m/s (Fig.asmedigitalcollection. 8). This rotor system demonstrated a better static torque than the ite conventional Savonius rotor. With the optimally inclined deflector. The efficiency of ed the flapped system (modified Savonius) was found to be 5% as compared to the efficiency of py 18% of a conventional system (Fig.shaped deflector is placed in front of the Savonius rotor (Fig. Due to this. achieved by a simple design. the rotor extracts about 20% more power than Ac the conventional Savonius rotor (Fig. The torque produced in the complete rotation is found to be positive. the static torque of the rotor is enhanced significantly.asme. which is not the case with the ip semicircular-bladed rotor without flaps.asme.Journal of Energy Resources Technology.1115/1. however. Received November 05. Such an important enhancement. Accepted manuscript posted December 19. recommends that the practice of partly blocked wedges is extremely suitable. The modified system. thus.4038785 Copyright (c) 2017 by ASME reducing the negative drag force on the rotor blades. so that ed the wind flow resistance is encountered by the returning blade of the rotor. The flaps are hinged in place of the curved parts of the blades to augment its harnessing effectiveness.4 V-shaped deflectors Ma In practice. doi:10. The flaps open automatically when the rotor advance towards the wind thereby exerting more wind pressure on the advancing blade. was found unacceptable Co for harnessing power. 2017. the rotor operates over a wider range of TSR [21]. its performance was found inferior.org/about-asme/terms-of-use . and secondly. but when the number of ce staging is increased from 2 to 3. Wind tunnel experiments with five nozzle models are conducted for 2-. Accepted manuscript posted December 19. 59.1115/1. to improve its static ed torque characteristics. Hence. r sc Firstly. the CP reduces due to the increased inertia of the rotor. 2017. respectively [65]. it has a large fluctuation of torque at some initial rotation of the turbine. As a result. These arrangements increase the starting JERT-17-1620 Alom 12 Downloaded From: http://energyresources.and 3-stages conventional rotor is found to be 0.6 to 0. the starting torque of a conventional Savonius rotor would be so low that the rotor cannot start on its own. 2017.org/about-asme/terms-of-use . 10). the py inlet velocities are varied from V1 = 0. the CP becomes higher. doi:10. When the nozzle length is increased to 80 cm. nu it has some angular positions where the torque becomes negative or even very small thereby Ma reducing the rotor performance. 43. As the pt staging of rotor is increased from 1 to 2. and at 120ᵒ for the 3-stage rotor.org/ on 12/26/2017 Terms of Use: http://www.and 6-bladed d ite conventional Savonius rotor having overlap ratios of 1/3 and 1/6. the negative torque of the rotor is reduced and the effective wind speed is augmented.9 m/s to amplify the outlet velocity to V2 = 3 to 3. staging of rotor (Fig.asme. 4. 65].Journal of Energy Resources Technology. ot tN 2.6 to 0. The CP for 2. Received November 05. Wind Ac tunnel experiments demonstrates the optimal number of staging of the rotor to be 2 [65].5 Nozzles The application of nozzle is another idea to magnify the wind velocity before it encounters the blades of a Savonius rotor [8].asme.8 m/s to obtain outlet velocities from V2 = 2 to Co 2.6 Multi-staging ip The conventional Savonius rotor mainly has two disadvantages on torque characteristics. 4g) has been done [4. as shown in (Fig.asmedigitalcollection.4038785 Copyright (c) 2017 by ASME 2. The wind velocity at the ed nozzle inlet is varied from V1 = 0.23. This is accomplished by setting the phase lift at an angle 90ᵒ to each other for the 2-stage.9 m/s. 57. 4f) is employed. 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.5 m/s when the length of the nozzle is 55 cm.29 and 0. When a convergent nozzle (Fig. 65].0 to 0.asmedigitalcollection. tN 65-67]. Accepted manuscript posted December 19.4038785 Copyright (c) 2017 by ASME capability of the rotor. 25. Thus.5ᵒ) shows a CP of 0.org/about-asme/terms-of-use .Journal of Energy Resources Technology. 4h) is used in order to reduce the negative torque and to ot improve the self-starting characteristics of a single-stage Savonius rotor system [6.88) was found to be 0. 2017. power sc output and the rotational speed at various twist angles and gap widths. The 3- stage rotors are better at low wind speeds as they have the uniform coefficient of static torque. Kamoji et al.0 when the Reynolds ce number was 1.179 at = 0.org/ on 12/26/2017 Terms of Use: http://www. 2. [66] investigated a twisted-bladed rotor with a twist angle of 90ᵒ in a low-speed wind tunnel.18 for the conventional Savonius rotor [60.asme. Wind tunnel experiments are carried out for twisted bladed rotor at a fixed twist angle ip 10.asme.28ᵒ and varying the gap width (i. 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. doi:10. Experimentally it has been reported that the multi-staging has shown a reduction of d power and dynamic torque for the same rotor. 60. [4] noticed that a lower peak CP of a 3-stage rotor in comparison to its corresponding single-stage rotor. 2017.5 x105. Hayashi et al.e. the multi-staging of rotors seems to ite provide a better starting ability at low speeds with some reduction in performance [66]. higher CP and self-starting capability than that of the semicircular-bladed rotor (Fig..7 Twisted blades Co The twisted bladed rotor (Fig. The experiments ed were conducted by varying  from 0. Later experimental nu investigation with a twisted-bladed rotor (twist angle is 12. separation gap) from S = 14 to 67 mm [25].2.88 to 1. The r aerodynamic performance of these blades has been evaluated based on starting torque. Experimental investigation demonstrated a higher potential of the Ac twisted bladed rotor in terms of smooth running. The maximum CP pt of the twisted bladed rotor (AR=0. Received November 05. ed py 2.19 as opposed Ma to Cp of 0. 11).8 Valves JERT-17-1620 Alom 13 Downloaded From: http://energyresources.1115/1.16 and the AR from 0. allowing more air to flow. Again. 12). Accepted manuscript posted December 19. [65] also used valves in twisted as well as in semicircular bladed rotors. doi:10.4038785 Copyright (c) 2017 by ASME This new concept (Fig. The orientation of α = 180° is similar to the orientation of pt α = 0°. and it is a function of the mass ce of the valve. damage to the rotor at high speed can be reduced. it reduces the drag on the returning blade r sc and increases the performance without significantly disturbing the simplicity of the rotor. The rotor with valves has been tested in a low-speed wind tunnel to calculate its performance.asme. But when this blade returns with its convex side to the wind. VATS mechanism also helps to make direction independence Co of the rotor.1115/1. In the ot mechanism of VATS. JERT-17-1620 Alom 14 Downloaded From: http://energyresources. Properly aligned valves with Ac minimum frictional losses would improve the performance of VATS mechanism. the valve is perpendicular to the ed flow giving a maximum drag force. The centrifugal force is self-regulating of orientation. In addition to this. this deflecting plate is enforced to cover the hole. The 2-stage 3-bladed Savonius rotor with valves has demonstrated higher CP than the rotor without valves (Fig. When the blade advances towards the wind. a small deflecting plate is hinged on the concave side of the rotor blade tN in front of a hole. the valve is aligned with the wind Ma and is thus oriented at α =0° in the coordinate system with the valve surface coinciding with the wind flow direction. the valve opens automatically due to the wind pressure and hence d experiences a lower flow resistance. its radius of rotation and the angular velocity. when the rotor is at α = 90°. 2017. the VATS rotor can py increase the power coefficient. The valve gets closed automatically by the centrifugal ite force during the power-harnessing part of the cycle. 2017. Received November 05. 4i) has been incorporated in a twisted bladed Savonius rotor and is named as the Valve-Aided Twisted Savonius (VATS) rotor [60. 4i). Saha et al.asmedigitalcollection. The mechanism is found to be independent of wind directions.asme. Keeping the simplicity of the rotor intact.Journal of Energy Resources Technology. As a result.org/about-asme/terms-of-use . 65]. It is nu found that when the blade is oriented at α = 0° (Fig. and shows suitable for large machines. When the wind is on the concave side.org/ on 12/26/2017 Terms of Use: http://www. the hole is ip uncovered. This technique significantly improves the ed static torque of the rotor. In order to adjust the input power. ed There was 16% improvement of performance in case of curtain 1 as compared to curtainless pt rotor (Fig. 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. Experiments with three different curtains. Various experiments were d ite conducted at Reynolds number of 6. It was found that the maximum CP with guide box of the area ratio 0.org/ on 12/26/2017 Terms of Use: http://www. the area ratio between the inlet and outlet is varied from 0. 2017. ot tN 2. Received November 05.asme.05 x104 and 9. with the use of guide box tunnel there was no negative Co torque in the complete rotation of the rotor when the guide box entrance opening angle was in between 60ᵒ to 90ᵒ. 69] used a novel arrangement of curtains at the rotor front with the r sc intention of improving its performance by preventing the negative torque that opposes the nu rotor rotation. This arrangement is slightly different from the types used by Alexander and Holownia [46].1115/1. The guide box tunnel is a passage in which the test rotor is involved.org/about-asme/terms-of-use .Journal of Energy Resources Technology. The highest rotor power has been found from curtain 1 (α = 45ᵒ and β = 15ᵒ) at around 8 W. doi:10. 2017. oriented at varying inclinations (Fig. and 1.9 Guide box About a decade ago.15 and the gap distance of 2.3 to 0. Emmanuel and Jun [37] used a different type of shield arrangement (Fig. Ma 4k) were carried out in a low-speed wind tunnel with  =0.08 x 104 to obtain the adequate ed configuration that would provide the maximum CP. In this JERT-17-1620 Alom 15 Downloaded From: http://energyresources. Accepted manuscript posted December 19. [55] and Hue et al. Morcos et al.5 times in 3-bladed system.asmedigitalcollection.11 Shield Sometime during 2011.6 cm. 13).23 py times in the 2-bladed system. The goal of the investigators [37] was to suppress the pressure exerted on the convex part of the rotor.asme.43 was increased by 1.10 Curtain plates ip Altan and Atilgan [10. ce Ac 2. Further.4038785 Copyright (c) 2017 by ASME 2. [61].7 [68]. 4l) in a six-bladed Savonius rotors. Received November 05. [70] have used venting slots which is found more Co effective and simpler in design than the valves used by the past investigators [65. Among the configurations studied.org/ on 12/26/2017 Terms of Use: http://www. At each wind speed. The load resistances have been varied from 20 r sc Ω to approximately 1000 Ω to determine the resulting power curve (Fig.14). [11] and Plourde et al. The six-bladed rotor with shields and with stator have indicated maximum CP of around 0.53% with this design over the JERT-17-1620 Alom 16 Downloaded From: http://energyresources. Design-II at TSR = 0.4 and 0.80 has shown a maximum CP of 0.asme.5. pt ce Inspired by the work of Abraham et al. load tN parameterized with various wind speeds. They have nu observed a very weak dependence on electrical load for the unvented and uncapped case.1115/1. 17). In order to arrive at the optimum position of the venting slots.asmedigitalcollection. 2017. [70]. Ma however. doi:10.12 Venting slots py Abraham et al. ite ed 2.Journal of Energy Resources Technology. the performance is found strongly linked to the electrical system for the capped and vented case. however this occurs at dissimilar TSRs d (Fig. Alom and Saha [71] used Ac the venting slots on a two-bladed semicircular Savonius rotor. Wind tunnel experimentations are carried out to determine the power vs. This suggests that the electrical system should be designed appropriately while ed linking to the rotor. In the study. The 2D unsteady simulation is carried out using SST k-ω turbulence model at  = 0. three different configurations are designed and tested numerically (Fig. various configurations of the six-bladed rotor have been examined using 2D unsteady simulation with RNG k-ε turbulence model. 16). the generator is connected to a ip resistive load that could be effortlessly varied.org/about-asme/terms-of-use . There is an improvement of 7. Accepted manuscript posted December 19. 2017. the six-bladed rotor without shield is found to have lower efficiency but still higher than a conventional two- bladed Savonius rotor.20.4038785 Copyright (c) 2017 by ASME connection. [11] and Plourde et al.292 (Fig. respectively.15).asme. 70]. The rotors have been tested without and with venting slots to minimize the thrust loading on the ot returning blade. 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). Thus, it is clear from this numerical investigation that the venting slots, as demonstrated in Fig. 16b, can be used in an advanced blade profile (such as the recent elliptical type) to bring more effectiveness into the rotor design. d ite 2.13 Concentrators ed Roy et al. [14] have studied and investigated the performance and starting characteristics of py Savonius rotor employing concentrators (Fig. 19), a technique similar to those of nozzle (Fig. Co 4f) and curtain plates (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 ot wind tunnel at the wind velocity of 6.2 m/s, where loads are applied progressively with tN respected to the corresponding rotational speeds. With the augmenters placed α = 40°, and β ip =10°, the rotor obtains a peak CP of 0.32, a value competitive to that of a lift-type turbine. r sc This shows an overall performance improvement of 47.5% as compared to a semicircular- nu bladed Savonius rotor without concentrators (Fig. 20). Ma 2.14 Guide vane The main idea of using guide vane in Savonius rotor is to improve the wind harvesting ed capacity of incoming air at the cost of structural complexity. Three designs, as illustrated in pt Fig. 21, have been investigated by El-Askary et al. [15]. The purpose is to minimize the ce negative torque and increase the exerted positive torque by guiding the incoming air Ac 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 Ansys Fluent with SST k-ω turbulence model is carried out for each of the design. As seen from Fig. 22, the Design-III shows a peak CP of 0.52 at TSR = 2.2. The novel designs needs more JERT-17-1620 Alom 17 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 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 d examining the wind turbine aerodynamics saving expenses, time, and uncertainties related to ite full-scale experimentation. As evident from the present review work, wind tunnels (both open ed and closed-circuit types) have been used extensively to evaluate the performance py characteristics of augmented Savonius rotors. Tests have been carried out by employing Co 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 ot augmented rotors is shown in Table 3. tN ip 4. Numerical studies at a glance r sc The flow field around a Savonius rotor is time dependent in nature; and flow separation and nu vortex formation are common phenomena. Therefore, the complex unsteady flow Ma 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 ed difference method (FDM) and finite element method (FEM) have been used for discretizing pt the governing equations around the rotor, however, the FVM is preferred due to complex ce numerical geometry. On the other hand, FVM based commercial codes (e.g., ANSYS Fluent, Ac CFX, Star 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 model, realizable k-ϵ, standard k-ϵ, RNG k-ε, k-ω transition, k-ω SST, v2-f) and the selection grid size around the rotor are the most important criteria [62-80]. The various numerical methods used in augmented Savonius rotors are summarized in Table 4. JERT-17-1620 Alom 18 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 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. It is proven d ite that the augmentation techniques increase the self-starting capability and CP of Savonius ed 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 py summarized below: Co o The augmentation techniques (with additional cost and complexity to the rotor ot system) enhance the self-starting capability, amplify the wind speed, improve the tN visual impact, prevent blade cracking, and stop bird assaults. Other advantages ip include mounting of additional features to the system such as rainwater harvester and r sc solar panel. nu o The augmenters such as V-shaped wedge deflector, curtain, obstacle shield, shields Ma 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 CP can be ed enhanced up to 20-50% than a rotor without the deflector. On the other hand, the use pt of shield in a six-bladed Savonius rotor can improve the CP up to 0.50. These ce augmenters do not offer much structural complexity to the rotor system. Ac o With the employment of guide box tunnel and convergent nozzle, the CP 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 Alom 19 Downloaded From: http://energyresources.asmedigitalcollection.asme.org/ on 12/26/2017 Terms of Use: http://www.asme.org/about-asme/terms-of-use ip o Among the blade profiles evolved. Ma o The foregoing analysis suggests the use of deflector plate(s). Accepted manuscript posted December 19. An optimally designed guide vane can bring a maximum CP of 0. however.5% over Co the conventional rotor without slots. the nozzle makes the rotor lesser complex with a gain in CP.4038785 Copyright (c) 2017 by ASME CP. can raise the CP by 7.asme. ot tN The venting slots are easier to be incorporated in rotor blades. JERT-17-1620 Alom 20 Downloaded From: http://energyresources.52. if properly designed and oriented. 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 elliptical-bladed Savonius rotor has proved to r sc harness wind energy more efficiently.1115/1. Received November 05.org/ on 12/26/2017 Terms of Use: http://www.asme. o The use of hinged flaps in a Bach type Savonius rotor can increase the static torque by d ite 35% relative to the one without flaps. however. doi:10. However.Journal of Energy Resources Technology. 2017. The location of augmenters in the elliptical-bladed ce rotor blades can be optimized with the help of numerical methods followed by wind Ac tunnel experiments.asmedigitalcollection. py o The venting-slots.18% higher than a semicircular-bladed nu Savonius rotor. valves and especially the ed venting slots in an elliptical-bladed rotor to improve the CP without bringing much pt complexity to the turbine system. The gain in CP for an optimally designed elliptical-bladed rotor profile is found 18.org/about-asme/terms-of-use . there is a chance of strong vortex shedding and high wake is generated around and behind the rotor leading to high noise generation. the hinged flaps increase the ed structural design complexity of the rotor system. 2017. Accepted manuscript posted December 19. Received November 05.org/about-asme/terms-of-use . 2017.Journal of Energy Resources Technology.asmedigitalcollection.m) TS Static torque (N-m) u Rotor tip speed (m/s) V Wind velocity (m/s) JERT-17-1620 Alom 21 Downloaded From: http://energyresources.1115/1.m) Ac Tturbine Actual torque produced by the turbine rotor (N.asme. 2017.org/ on 12/26/2017 Terms of Use: http://www.4038785 Copyright (c) 2017 by ASME Nomenclatures Latin letters A Swept area (m2) AR Aspect Ratio CP Power coefficient d CT Dynamic torque coefficient ite CTS Static torque co-efficient ed D Rotor diameter (m) py D Drag force (N) Co DO End plate diameter of the rotor (m) d Chord length of the blade (m) ot tN e Overlap distance between rotor blades (m) H Rotor height (m) ip k Turbulence kinetic energy (m2/s2) r sc L Lift force (N) nu N Rotor rotational speed (rpm) Ma n Number of time step Pavailable Power available in the wind (W) ed Pturbine Power produced by the turbine rotor (W) pt S Separation gap/ gap width (m) ce Tavailable Theoretical torque available in the wind (N.asme. doi:10. asme. doi:10. 2017.asme.org/about-asme/terms-of-use .4038785 Copyright (c) 2017 by ASME Greek letters α. β Angle of curtain plate (degree) δ Overlap ratio ε Energy dissipation rate εs Gap ratio d θ Rotor blade angle (degree) ite µ Dynamic viscosity (N-s/m2) ed ν Kinematic viscosity (m2/s) py ρ Density of air (kg-m3/s) Co ω Specific dissipation rate ot tN Abbreviations ANN Artificial Neural Network ip CFD Computational Fluid Dynamics r sc FDM Finite Difference Method nu FEM Finite Element Method Ma FVM Finite Volume Method HAWT Horizontal Axis Wind Turbine ed RANS Reynolds Averaged Navier Stokes pt RNG Renormalized ce RSM Response Surface Method Ac SIMPLE Semi-Implicit Method for Pressure Linked Equations SIMPLEC SIMPLE Consistent S-A Spalart and Allmaras sc Semicircular JERT-17-1620 Alom 22 Downloaded From: http://energyresources. 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India. and Yamamoto. 24. Y. “Numerical optimization of semicircular-bladed d Savonius rotor using vent augmenters. 2014..K.. L. Received November 05... 2013 “Review on the numerical investigations into the ot tN design and development of Savonius wind rotors. 1–9. 7(3). 443–454.. Roy. “Simulations of three-dimensional vertical-axis turbines for communications applications. “Experiments investigations on py single stage. S. Technol. Energy Resour. X. 137(1). 73–83. pt 76. Li. 3425–3432. Zhang. H. Kedare. 36(4). pp. 2008. and Prabhu. Energy. two stages and three stages conventional Savonius rotor. Alom.. Bianchini. Kang. Ferrara. pp. and Jiang. 2017. pp. IIT Bombay. ip 74. Abraham. JERT-17-1620 Alom 31 Downloaded From: http://energyresources.” J. pp. “Investigation of meshing strategies and turbulence models of computational fluid dynamics ed simulations of vertical axis wind turbines.. and Yang. Energy Co Res. Fukushima. Kamoji.” Renew.org/about-asme/terms-of-use .” Int.” Renew. Maleci. Energy Rev.. N.. 0–19.. C. U. Mowry. ed 72.Journal of Energy Resources Technology.. 2015.4038785 Copyright (c) 2017 by ASME 49(12).. and Minkowycz. Y. M. p. “Review of fluid dynamics aspects of r sc Savonius-rotor-based vertical-axis wind rotors. 2017. and Ferrari. Balduzzi. and Saha. 70.B. 499 nu –508. J..” Ac ASME J.asme.. F. H. C.. Liu.. Energy Rev.. B. N. C. 139(5). Zheng. Lewis. 79.. and Fuller. 2017. 31016. “Effect of blade inclination angle on a Darrieus ed wind turbine.” IMECE2009. S. M.. 2017. ASME. N. Received November 05.” ASME J. 2010. doi:10.. pp. K.” d Energy. Morshed.asme... Ebrahimi. “Multi-objective optimization of the design and operating point of a new external axis wind turbine. M. ite 80. M. “Experimental and numerical investigations on drag and torque characteristics of three-bladed Savonius wind turbine...Journal of Energy Resources Technology.org/ on 12/26/2017 Terms of Use: http://www. Rahman. R. S. Vol 6. 2012.4038785 Copyright (c) 2017 by ASME 78. pp. J...asme. K.. py Co ot tN r ip sc nu Ma ed pt ce Ac JERT-17-1620 Alom 32 Downloaded From: http://energyresources. Turbomach. and Benini. 643–653. E.1115/1. 125.org/about-asme/terms-of-use . 85–94. 134(3). 2017. M. p. Castelli. and Vijayaraghavan. Ferdoues. Accepted manuscript posted December 19.asmedigitalcollection. 17 Variation of CP with TSR [71] pt Fig.4038785 Copyright (c) 2017 by ASME List of Figures Fig. 9 Cp vs various deflector plate angle[21] ot Fig.18 Velocity contour of the conventional Savonius rotor [71]. Accepted manuscript posted December 19.19 Orientation of the concentrators [14] Ac Fig. 10 Cp vs velocity for various configurstion [65] tN Fig. Received November 05.org/ on 12/26/2017 Terms of Use: http://www.20 CP vs TSR at various orientations of the concentrators [14] Fig.asme. 2017.Journal of Energy Resources Technology. ite Fig.22 CP vs TSR for various guide vane position [15] JERT-17-1620 Alom 33 Downloaded From: http://energyresources. 11 RPM vs velocity for various gap width of twisted bladed rotor [25] ip Fig. [15] Fig. 12 CP vs velocity for various valve aided Savonius rotor [65] r sc Fig.1115/1. Fig. ce Fig. 5 CP vs TSR for obstacle and without obstacle [9] ed Fig. 1 Various blade profiles used for Savonius rotors. 8 Static torque vs angle of rotation for various flaps [64] Fig.asme. 3 Lift and drag force on Savonius rotor d Fig. doi:10. 4 Various types of augmentation techniques. Fig.asmedigitalcollection. 15: Variation of power vs wind speeds for a vented and capped rotor [11].21 Different guide vane designs by El-Askary et al. 7 Cp vs TSR for various flaps [58] Co Fig.org/about-asme/terms-of-use .16 Vents at three different positions on the semicircular-bladed profiles [71] ed Fig. 2 Basic parameters of Savonius rotor Fig. 6 Cp vs TSR for various deflector azimuthal angle [47] py Fig.14 Variation of CP with TSR for various rotor configurations [37] Ma Fig. 13 Power vs RPM for various curtain design [10] nu Fig. 2017. asme. ed py Co ot tN r ip sc nu Ma ed pt ce Ac JERT-17-1620 Alom 34 Downloaded From: http://energyresources.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. Accepted manuscript posted December 19. Received November 05. d ite Table 4 Literature reported numerical work on augmentation techniques. doi:10.asmedigitalcollection.asme. 2017.Journal of Energy Resources Technology. 2017.1115/1.org/about-asme/terms-of-use .org/ on 12/26/2017 Terms of Use: http://www. [23] 2013 Elliptical 0.27 Roy et al.org/ on 12/26/2017 Terms of Use: http://www. [47] 1984 Semicircular 0. Received November 05.1159 ite Kacprzak et al. doi:10.2477 Ma ed pt ce Ac JERT-17-1620 Alom 35 Downloaded From: http://energyresources.asme. 2017.Journal of Energy Resources Technology. [30] 2015 ot Airfoil shape 0.30 Co Roy et al.4038785 Copyright (c) 2017 by ASME Table 1: Performance of various Savonius rotor profiles Investigator(s) Year Blade profile CPmax Alexander and Holownia [46] 1978 Semicircular 0.17 d Grinspan et al.33 Sharma and Sharma [31] 2016 Multiple quarter 0.22 tN Alom et al. [28] 2013 Fish-ridged 0.147 Owaga et al. [14] 2014 Roy profile (New) 0. Accepted manuscript posted December 19.25 Tartuferi et al. [26] 2014 Elliptical 0. [33] 2017 Spline 0. [14] 2014 Modified Bach 0.31 Gerardo and Molfino [29] 2014 Bronzinus 0. [23] 2013 Bach 0.226 sc semicircular nu Mari et al. 2017.172 py Banerjee et al.2266 ip semicircular r Sharma and Sharma [32] 2017 Multiple miniature 0. [25] 2004 Twisted 0. [27] 2016 Elliptical 0.org/about-asme/terms-of-use .23 Kacprzak et al.178 ed Song et al.1115/1.asmedigitalcollection.asme. [11] 2012 Venting slots Reduces negative torque. doi:10.258 ed Golecha et al.4038785 Copyright (c) 2017 by ASME Table 2: Various augmentation techniques and observation Investigators Year Augmenter used Observation Alexander and Holownia [46] 1978 Wind shields CPmax = 0.30 JERT-17-1620 Alom 36 Downloaded From: http://energyresources. [25] 2004 Twisted blade CPmax = 0. [62] 2011 Deflector plate 50% increase of CP from the conventional rotor.50 ce Abraham et al. Ac Roy et al.5 times r sc for 3-bladed and 1.asme. [56] 1984 Deflector plate CPmax = 0.34 d Ogawa et al. Ma Altan and Atilgan [10] 2010 Curtain design CPmax = 0. Accepted manuscript posted December 19. tN Grinspan et al.243 Morcos et al.23 times for 2-bladed rotors. Menet [59] 2004 Multi-staging ot Improved CP than the single stage rotor.7% increase of CP from ed the conventional rotor. 2017. [30] 2015 Conveyor-deflector curtain CPmax = 0. [9] 2011 Obstacle shielding CPmax = 0. [15] 2015 Guide vane CPmax = 0.org/about-asme/terms-of-use . Co Shikha et al. [55] 1981 Wind shields CPmax = 0.org/ on 12/26/2017 Terms of Use: http://www.asmedigitalcollection.1159 Rajkumar and Saha [60] 2006 Valve Reduces negative torque. nu Hu et al.212 ite Reupke and Probert [58] 1991 Slatted blade CPmax = 0. [8] 2003 Nozzle Increase of wind speed by 2 to 3 times.33 El-Askary et al. [57] 1992 Deflector plate 20% increase of CP from the conventional rotor.asme. py Huda et al.38 Mohamed et al. pt Emmanuel and Jun [37] 2011 Shield CPmax = 0.52 Tartuferi et al.18 Shaughnessy and Probert [21] 1992 V-shaped deflector 19. [61] 2009 Circular shield Reduces wind pressure on the returning blade.Journal of Energy Resources Technology.1115/1. Received November 05. ip Irabu and Roy [68] 2007 Guide box tunnel Increase in CP by 1. [14] 2014 Concentrators CPmax = 0. 2017. 173 -.org/about-asme/terms-of-use . [14] 0.122 × 0. Accepted manuscript posted December 19. [56] 0. [43] 0.212 Deflector plate d Huda et al. 2017.220 × 0.208 × 0.23 nu Oriented jet Roy et al.asme.29 for 2-stage.org/ on 12/26/2017 Terms of Use: http://www.226 × 0. 2017.445 -.32 × 0.230 × 0.45 0.074 × 0.32 0.289 × 0. of blades Observation Techniques (H×D) (m×m) d 0.58 × 0. 3-bladed rotor.245 0. [62] 0. Received November 05.170 × 0. p Reduces negative torque. 3 CPmax = 0.113 0. [65] 0.Journal of Energy Resources Technology.245 0.82 2 50% increase of CP from the conventional rotor. 3 rotor. [62] 0. 0.10 × 0.184 Frikha et al. 3-bladed rotor. [72] 0.23×0. [11] 1.72 2 20% increase of CP from the conventional rotor.asmedigitalcollection. 0. 0. 0.72 2 CPmax = 0.7% increase of CP from the conventional rotor.86 2 CPmax = 0.209 0.077 -. doi:10.1 × 1.173 × 0.20 for 3-stage. 2.175 × 0.26 for 2-stage.46 × 0.250 0. 2-bladed rotor.208 ip Static CT is lower in 3-stage rotor than the 1.23 for 3-stage.109 CPmax = 0. 2-bladed rotor. [4] 0.063 CPmax = 0.20 × 0.1025 × 0. [57] 0. Ogawa et al. Valve Rajkumar and Saha [60] 0.3 0.19 0.32 -.44 2 19.and three-stage modified Savonius rotors.096 r sc Venting slot Abraham et al.33 ye Hayashi et al. 2 15ᵒ Ac JERT-17-1620 Alom 37 Downloaded From: http://energyresources. 2 CP increases with the increase of number of stages. tN Saha et al.83 2 rotor.asme.185 × 0.4038785 Copyright (c) 2017 by ASME Table 3: Literature reported experimental work on augmentation techniques. d ite Augmentation Rotor dimensions Researcher(s) TSR No.1115/1. 2. p Co Improved CP in 2-stage. The single-stage modified Savonius rotor is found better as Multi-staging Golecha et al. ot CPmax = 0.and 2-stages Kamoji et al. 2 Reduces the negative torque CPmax = 0.669 3 ce The optimum curtain angle has been found as α = 45ᵒ and β = Curtain Altan and Atilgan [69] 0. 2-bladed rotor than the single stage Menet [59] 0.82 2 compared to two.32 -.234 Ma Shaughnessy and Probert [21] 0.75 2 CT is higher in the single stage rotor than the 3-stage rotor 0.70 2 Wind shields Alexander and Holownia [46] 0. te Golecha et al.170 × 0. nu The dynamic CT and the FVM with 3D 2 CP enhanced as the Multi-staging Frikha et al. 6 shield and stator is sc found better. doi:10. 3 CPmax = 0. [11] 2 model torque. Received November 05.Journal of Energy Resources Technology. ed Venting slot 7. Accepted manuscript posted December 19.org/about-asme/terms-of-use . [9] 2. 20% more power than tN Tartuferi et al. 2017.52 for Design- Guide vane El-Askary et al. ed The optimum curtain FVM with 2D standard Curtain Altan and Atilgan [69] 2 angle has been found at k-ϵ model pt α = 45ᵒ and β = 15ᵒ ce Ac JERT-17-1620 Alom 38 Downloaded From: http://energyresources. [15] 2D k-ω SST model 2 III ot Fluent & Matlab. v2-f.asme.asmedigitalcollection. [43] Ma modified k-ϵ model number of stage increased.53% increase of CP Alom and Saha [71] 2D k-ω SST model 2 from conventional rotor py without venting slots Co CPmax = 0.1115/1.asme. Augmentation No. turbulence flow ip Six-bladed rotor with r Wind shield Enamuel and Jun [37] 2D RNG k-ε 2. 2017. [30] FVM and RSM 2 deflector the conventional rotor.258 shield realizable k-ϵ model d 2D and 3D k-ω SST ite Reduces the negative Abraham et al.4038785 Copyright (c) 2017 by ASME Table 4: Literature reported numerical work on augmentation techniques.org/ on 12/26/2017 Terms of Use: http://www. of Researcher(s) CFD methodology Observation Techniques blades Obstacle FVM with 2D Mohamed et al. Conveyor. 2017. Accepted manuscript posted December 19.asme. 2017.4038785 Copyright (c) 2017 by ASME d ite ed py Co ot tN r ip sc nu Ma ed pt ce Ac Figure 1: Various blade profiles used for Savonius rotors.Journal of Energy Resources Technology.1115/1.asme.org/about-asme/terms-of-use . JERT-17-1620 Alom 39 Downloaded From: http://energyresources. Received November 05.asmedigitalcollection. doi:10.org/ on 12/26/2017 Terms of Use: http://www. org/ on 12/26/2017 Terms of Use: http://www. doi:10. 2017.1115/1.asmedigitalcollection.org/about-asme/terms-of-use .asme. Received November 05. 2017.Journal of Energy Resources Technology.asme.4038785 Copyright (c) 2017 by ASME d ite ed py Co ot tN Figure 2: Basic parameters of Savonius rotor r ip sc nu Ma ed pt ce Ac JERT-17-1620 Alom 40 Downloaded From: http://energyresources. Accepted manuscript posted December 19. 2017.asme.org/about-asme/terms-of-use .asme.asmedigitalcollection. doi:10. Received November 05.org/ on 12/26/2017 Terms of Use: http://www.1115/1.Journal of Energy Resources Technology. 2017. Accepted manuscript posted December 19.4038785 Copyright (c) 2017 by ASME d ite ed py Figure 3: Lift and drag force on Savonius rotor Co ot tN r ip sc nu Ma ed pt ce Ac JERT-17-1620 Alom 41 Downloaded From: http://energyresources. Accepted manuscript posted December 19. Received November 05.4038785 Copyright (c) 2017 by ASME d ite ed py Co ot tN r ip sc nu Ma ed pt ce Ac Figure 4: Various types of augmentation techniques. 2017.Journal of Energy Resources Technology.org/about-asme/terms-of-use . 2017.asmedigitalcollection. JERT-17-1620 Alom 42 Downloaded From: http://energyresources. doi:10.org/ on 12/26/2017 Terms of Use: http://www.asme.1115/1.asme. asme.org/ on 12/26/2017 Terms of Use: http://www. doi:10. 2017.4038785 Copyright (c) 2017 by ASME d ite ed Figure 5: CP vs TSR for obstacle and without obstacle [9] py Co ot tN r ip sc nu Ma ed pt ce Ac JERT-17-1620 Alom 43 Downloaded From: http://energyresources.asme. Accepted manuscript posted December 19.asmedigitalcollection.org/about-asme/terms-of-use . 2017.1115/1.Journal of Energy Resources Technology. Received November 05. Accepted manuscript posted December 19.asme.Journal of Energy Resources Technology. doi:10.asmedigitalcollection.4038785 Copyright (c) 2017 by ASME d ite ed Figure 6: Cp vs TSR for various deflector azimuthal angle [47] py Co ot tN r ip sc nu Ma ed pt ce Ac JERT-17-1620 Alom 44 Downloaded From: http://energyresources. 2017. 2017.1115/1.org/ on 12/26/2017 Terms of Use: http://www.asme.org/about-asme/terms-of-use . Received November 05. 2017.org/about-asme/terms-of-use .org/ on 12/26/2017 Terms of Use: http://www.asme. doi:10. 2017.asme. Accepted manuscript posted December 19.Journal of Energy Resources Technology. Received November 05.1115/1.asmedigitalcollection.4038785 Copyright (c) 2017 by ASME d ite ed Figure 7: Cp vs TSR for various flaps [58] py Co ot tN r ip sc nu Ma ed pt ce Ac JERT-17-1620 Alom 45 Downloaded From: http://energyresources. org/ on 12/26/2017 Terms of Use: http://www.asmedigitalcollection.asme.asme.1115/1. Received November 05.4038785 Copyright (c) 2017 by ASME d ite ed py Figure 8: Static torque vs angle of rotation for various flaps [64] Co ot tN r ip sc nu Ma ed pt ce Ac JERT-17-1620 Alom 46 Downloaded From: http://energyresources.Journal of Energy Resources Technology. 2017.org/about-asme/terms-of-use . 2017. doi:10. Accepted manuscript posted December 19. 1115/1.org/about-asme/terms-of-use . Received November 05.org/ on 12/26/2017 Terms of Use: http://www.asme. 2017.asmedigitalcollection.asme.4038785 Copyright (c) 2017 by ASME d ite ed py Figure 9: Cp vs various deflector plate angle[21] Co ot tN r ip sc nu Ma ed pt ce Ac JERT-17-1620 Alom 47 Downloaded From: http://energyresources. doi:10. 2017. Accepted manuscript posted December 19.Journal of Energy Resources Technology. org/about-asme/terms-of-use . doi:10.asme. Received November 05. 2017.asmedigitalcollection.org/ on 12/26/2017 Terms of Use: http://www.asme. 2017.Journal of Energy Resources Technology. Accepted manuscript posted December 19.4038785 Copyright (c) 2017 by ASME d ite ed py Figure 10: CP vs velocity for various configurstion [65] Co ot tN r ip sc nu Ma ed pt ce Ac JERT-17-1620 Alom 48 Downloaded From: http://energyresources.1115/1. 1115/1. Accepted manuscript posted December 19.asme. doi:10.org/ on 12/26/2017 Terms of Use: http://www.Journal of Energy Resources Technology.4038785 Copyright (c) 2017 by ASME d ite ed py Figure 11: RPM vs velocity for various gap width of twisted bladed rotor [25] Co ot tN r ip sc nu Ma ed pt ce Ac JERT-17-1620 Alom 49 Downloaded From: http://energyresources.asme. 2017.asmedigitalcollection. Received November 05. 2017.org/about-asme/terms-of-use . 2017.1115/1.asmedigitalcollection.asme.Journal of Energy Resources Technology. doi:10.asme.org/about-asme/terms-of-use . 2017. Received November 05.org/ on 12/26/2017 Terms of Use: http://www. Accepted manuscript posted December 19.4038785 Copyright (c) 2017 by ASME d ite ed py Figure 12: CP vs velocity for various valve aided Savonius rotor [65] Co ot tN r ip sc nu Ma ed pt ce Ac JERT-17-1620 Alom 50 Downloaded From: http://energyresources. 1115/1. 2017.asmedigitalcollection. Accepted manuscript posted December 19. 2017. Received November 05.asme.org/about-asme/terms-of-use . doi:10.Journal of Energy Resources Technology.4038785 Copyright (c) 2017 by ASME d ite ed py Figure 13: Power vs RPM for various curtain design [10] Co ot tN r ip sc nu Ma ed pt ce Ac JERT-17-1620 Alom 51 Downloaded From: http://energyresources.asme.org/ on 12/26/2017 Terms of Use: http://www. doi:10. 2017.4038785 Copyright (c) 2017 by ASME d ite ed Figure 14: Variation of CP with TSR for various rotor configurations [37] py Co ot tN r ip sc nu Ma ed pt ce Ac JERT-17-1620 Alom 52 Downloaded From: http://energyresources.org/about-asme/terms-of-use . Accepted manuscript posted December 19.Journal of Energy Resources Technology.1115/1.asmedigitalcollection. 2017.asme.asme.org/ on 12/26/2017 Terms of Use: http://www. Received November 05. doi:10. 2017. Received November 05.asme.org/about-asme/terms-of-use .Journal of Energy Resources Technology.asme.4038785 Copyright (c) 2017 by ASME d ite ed py Figure 15: Variation of power vs wind speeds for a vented and capped rotor [11]. 2017.1115/1. Co ot tN r ip sc nu Ma ed pt ce Ac JERT-17-1620 Alom 53 Downloaded From: http://energyresources.asmedigitalcollection. Accepted manuscript posted December 19.org/ on 12/26/2017 Terms of Use: http://www. asmedigitalcollection.asme.asme.org/ on 12/26/2017 Terms of Use: http://www. Received November 05. 2017. Accepted manuscript posted December 19.1115/1. 2017.4038785 Copyright (c) 2017 by ASME d ite ed Figure 16: Vents at three different positions on the semicircular-bladed profiles [71] py Co ot tN r ip sc nu Ma ed pt ce Ac JERT-17-1620 Alom 54 Downloaded From: http://energyresources. doi:10.org/about-asme/terms-of-use .Journal of Energy Resources Technology. Accepted manuscript posted December 19.org/ on 12/26/2017 Terms of Use: http://www. 2017.asmedigitalcollection.asme. doi:10.Journal of Energy Resources Technology.asme. Received November 05.org/about-asme/terms-of-use . 2017.4038785 Copyright (c) 2017 by ASME d ite ed py Figure 17: Variation of CP with TSR [71] Co ot tN r ip sc nu Ma ed pt ce Ac JERT-17-1620 Alom 55 Downloaded From: http://energyresources.1115/1. Accepted manuscript posted December 19. Received November 05.org/about-asme/terms-of-use .org/ on 12/26/2017 Terms of Use: http://www. doi:10.asme.1115/1.4038785 Copyright (c) 2017 by ASME d ite ed Figure 18: Velocity contour of the conventional Savonius rotor [71]. 2017.asmedigitalcollection.asme. 2017. py Co ot tN r ip sc nu Ma ed pt ce Ac JERT-17-1620 Alom 56 Downloaded From: http://energyresources.Journal of Energy Resources Technology. doi:10.asme.org/about-asme/terms-of-use .org/ on 12/26/2017 Terms of Use: http://www. Received November 05. 2017. Accepted manuscript posted December 19.1115/1.Journal of Energy Resources Technology. 2017.asmedigitalcollection.asme.4038785 Copyright (c) 2017 by ASME d ite ed py Figure 19: Orientation of the concentrators [14] Co ot tN r ip sc nu Ma ed pt ce Ac JERT-17-1620 Alom 57 Downloaded From: http://energyresources. asme.org/about-asme/terms-of-use . 2017.org/ on 12/26/2017 Terms of Use: http://www. 2017.4038785 Copyright (c) 2017 by ASME d ite ed py Figure 20: CP vs TSR at various orientations of the concentrators [14] Co ot tN r ip sc nu Ma ed pt ce Ac JERT-17-1620 Alom 58 Downloaded From: http://energyresources.asmedigitalcollection.Journal of Energy Resources Technology. Received November 05. doi:10. Accepted manuscript posted December 19.asme.1115/1. Journal of Energy Resources Technology. Received November 05.org/ on 12/26/2017 Terms of Use: http://www. Accepted manuscript posted December 19.asme.asme. 2017.4038785 Copyright (c) 2017 by ASME d ite ed py Figure 21: Different guide vane designs by El-Askary et al.asmedigitalcollection. [15] Co ot tN r ip sc nu Ma ed pt ce Ac JERT-17-1620 Alom 59 Downloaded From: http://energyresources. 2017.1115/1. doi:10.org/about-asme/terms-of-use . asme. doi:10. 2017. Accepted manuscript posted December 19.1115/1.asmedigitalcollection.Journal of Energy Resources Technology. Received November 05.asme.org/about-asme/terms-of-use .org/ on 12/26/2017 Terms of Use: http://www.4038785 Copyright (c) 2017 by ASME d ite ed py Figure 22: CP vs TSR for various guide vane position [15] Co ` ot tN r ip sc nu Ma ed pt ce Ac JERT-17-1620 Alom 60 Downloaded From: http://energyresources. 2017.