Renewable and Sustainable Energy Reviews 89 (2018) 281–291

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Renewable and Sustainable Energy Reviews

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A critical review of vertical axis for urban applications T ⁎ Rakesh Kumara, , Kaamran Raahemifarb, Alan S. Funga a Department of Mechanical and Industrial Engineering, Ryerson University, Toronto, ON, Canada M5B 2K3 b Department of Electrical & Computer Engineering, Ryerson University, Toronto, ON, Canada M5B 2K3

ARTICLE INFO ABSTRACT

Keywords: Wind energy is one of the most promising resources for power generation, and rapid growth Wind energy has been seen in its acceptance since 2000. The most acceptable classification for wind turbines is by its axis of Vertical axis wind orientation: Horizontal Axis Wind Turbines (HAWT) and Vertical Axis Wind Turbines (VAWT). HAWTs are used Blade aerodynamics in many countries for medium-to-large scale power projects, and most commercial installations around the globe Power coefficient are solely based on these turbines. On the other hand, HAWTs are not recognized as a viable option to harness Field performance the energy of the wind in urban areas, where the wind is less intense, much more chaotic and turbulent. VAWTs Commercial development are suggested as a better choice for cities and isolated semi-urban areas. Several attributes have been suggested for the large-scale deployment of VAWTs, e.g., good performance under the weak and unstable wind, no noise and safety concerns, and aesthetically sound for integration in urban areas. Significant research has been published on technology and resources assessment methodologies, and this review paper is a modest attempt to highlight some of the major developments of VAWTs, with a focus on the integration with urban infrastructure. Several recommendations have been drawn based on the state-of-the-art information on the subject for future studies and acceptance of wind turbines in the urban areas. It was concluded that further research is critical in making VAWTs a viable, dependable, and affordable power generation technology for many low and decentralized power applications.

1. Introduction the environmental concerns [3,4]. The development of an efficient wind turbine (WT) and resource Global warming, energy scarcity, rapid depletion of fossil fuels and assessment methodology for the urban areas are crucial to increasing exponential growth in the energy demand in several developing coun- the penetration of technology in cities and semi-urban tries has created an excellent opportunity for large-scale acceptance of areas [5,6]. Researchers, designers, and project developers have often renewable energy technologies. Wind energy has become one of the recommended the installation of small-scale WTs over and around high- fastest emerging renewable energy technologies, with the total capacity rise buildings as a potential power solution for incorporating sustain- by the end of 2016 reaching 487 GW (about 4% of global ) ability, and to support the transition to the future net-zero energy [1]. The technology and manufacturing infrastructure of wind turbines buildings [7–10]. On the other hand, wind patterns in urban environ- have advanced enough for further rapid deployment. As per the 2013 ments are much more chaotic and full of turbulence. The traditional IEA roadmap, about 18% of global electricity needs will be met by wind methods used for determining wind-energy resource have limitations, energy by 2050 [2]. Most of the progress was anticipated on large especially when applied to more complex urban situations. Site wind onshore and offshore projects (MW capacity) far from urban areas, resource measurements require time and money that are often not where the wind is most intense, consistent and unperturbed. On the feasible for small-scale power projects. A precise tool for the estimation other hand, there is considerable wind in the urban areas with a sig- of wind resources in the built-up areas is one of the fundamental re- nificant potential for power, viz. road dividers, side of railway tracks, quirements for the expansion of wind energy in urban regions [11,12]. top and around the high-rise buildings. The development of an efficient Furthermore, Horizontal Axis Wind Turbine (HAWT) is relatively wind energy system closer to the point of use meets the local power ineffective in urban situations and face local resistance due to noise, demand, minimizes the use of diesel/gas-based electricity generation, aesthetic, visual and public safety concerns [13]. Alternatively, Vertical reduces the strain on the existing grid infrastructure, incorporates the Axis Wind Turbine (VAWT) has been predicted as a potential solution sustainability in the cities, supports the local economy, and addresses for the implementation of WTs in urban and semi-urban areas [14,15].

⁎ Corresponding author. E-mail addresses: [email protected] (R. Kumar), [email protected] (K. Raahemifar), [email protected] (A.S. Fung). https://doi.org/10.1016/j.rser.2018.03.033 Received 29 September 2016; Received in revised form 3 February 2018; Accepted 14 March 2018 1364-0321/ © 2018 Elsevier Ltd. All rights reserved. R. Kumar et al. Renewable and Sustainable Energy Reviews 89 (2018) 281–291

Nomenclature COE cost of energy CFD computational fluid dynamics

CP wind power coefficient/coefficient of performance/power CUE Center for Urban Energy coefficient (it is a ratio of electricity produced by the wind GHG greenhouse gas turbine to the total energy available in the wind) HAWT horizontal axis wind turbine TSR tip speed ratio (it is the ratio of the peripheral speed of the LCOE levelized cost of electricity tip of the blade to the wind speed) NSERC Natural Sciences and Engineering Research Council β overlap ratio (the overlap ratio β of Savonius rotor is ex- RPM rotation per minute pressed as: β = a/2 R, where a is the overlap distance and WT wind turbines R is the radius of the blade circle) VAWT vertical axis wind turbine WWEA World Wind Energy Association Acronyms and abbreviations

AEO annual energy output

The VAWTs have a relatively low environmental impact and better VAWT. In HAWTs, the rotor axis is parallel to the ground and in the adaptable characteristics to the unsteady wind of urban terrains. These direction of the wind. These turbines are usually equipped with self- turbines can produce electricity from any direction with low cut-in starter and to turn the blades towards the wind. The energy wind speed and are relatively simple in design to integrate with urban outputs of these turbines depend on the site average wind speed and the buildings and infrastructure. turbulence in the wind [16]. The ideal aerodynamic efficiency of these The use of chaotic wind flow to generate electricity has been a turbines is reported in the range of 40–55% under steady wind [17]. challenge to the researchers, developer, planner, and considerable work HAWTs are widely used in large applications in remote and has been done on the subject in the recent decades. The studies were offshore locations where the clean and the undisturbed wind are focused on the turbine aerodynamics, blade materials, modelling, si- available. In contrast, HAWTs are not considered an effective design for mulation, test methods, performance validation, grid integration, and urban environments due to the high value of cut-in wind speed, chaotic environmental aspects. Several other studies were emphasized on the nature of the wind and the public perception against these big machines development of an effective wind resources assessment methodology, [18]. Instead, VAWTs axis of rotation is perpendicular to the directions equipment, and computational programs. This review paper presents of wind and ground. These turbines are relatively simple and do not some significant findings on VAWTs from published resources, gov- need any yaw system and a self-starting mechanism (except, Darrieus ernment reports, non-profit organization publications, and manufac- turbines). VAWTs have low cut-in wind speed and noise level and can turer's literature. The manuscript has evaluated the information ob- be installed in the urban areas that restrict the installation of a tall jectively and presents the scope and limitations of VAWTs structure. The low wind energy conversion efficiency is an area of development. The paper also highlights commercial activities on concern of VAWTs [19]. VAWTs in different parts of the world. It was noted that the opportu- Another classification of wind turbines is based on the aerodynamic nities of urban wind turbines are enormous; however, it is evident that forces on the blade surface: drag and lift forces. In drag-based WTs, the further research is critical to improving turbine designs, reducing cost, aerodynamic force is in the direction of air flow, while, in the lift type and making available resource assessment tools for urban conditions. machines the force is perpendicular to the flow of the wind. HAWTs and Darrieus turbines (discussed later) are solely lift based machines, while, 2. Classification of wind turbines Savonius turbines (discussed later) and some other new designs of VAWT are based on the drag forces. The drag-based turbines are simple ffi There are several ways the modern WTs can be classified, such as in design but have a rather poor e ciency. On the other hand, the lift- based on the orientations of the axis of rotation, drag or/and lift forces based turbines are complex and extract more energy from the wind per on the blades, and the power capacity of turbines (Fig. 1). Based on the unit swept area [20]. Castelli and Benini [21] presented a comparison axis of rotation, there are two main types of WTs: the HAWT and of the annual energy output (AEO) of two vertical-axis wind turbines

Fig. 1. Classifications of wind turbines.

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(VAWTs) for the low-wind urban site of Trieste, Italy. One turbine was turbines [29]. Most commercial developments around the globe are based on drag force, while other was lift-driven. As expected, the higher solely based on HAWTs due to their better all-around performance. AEO was recorded for the lift-driven turbine. Nevertheless, the drag- Though, the performance of these turbines diminishes sharply in the driven turbine provides the advantages of self-starting capability and low and unsteady wind causing a major hindrance in their acceptance higher power output at low wind speeds. in urban areas. Ahmed and Cameron [13] reviewed the wind power To overcome individual deficiency of Darrieus and Savonius tur- technologies and concluded that even though a greater public aware- bines, two designs have been combined into a single hybrid design. The ness has been seen in the recent years about the significance of wind hybrid designs of Darrieus and Savonius are driven by both drag and lift power as a sustainable source, the issues regarding noise, land use, forces and have better efficiency and self-starting capability. Sharma avian deaths, electromagnetic interference and visual effects must be et al. [22] have designed and analyzed the performances of three- resolved to ensure continued public and political support for the ac- bladed hybrid turbine with Darrieus mounted on the top of Savonius ceptance of HAWTs. Several NIMBY (Not-In-My-Back-Yard) groups are rotor. They varied the overlap ratios of two design from 10.8% to working in many countries against the installation of large wind tur- 25.8%. The power and coefficients increase with an increase in bines due to unbearable noise in the surrounding areas [30]. Although, the overlap ratios to a maximum value and then decrease, giving an the techno-economic viability of wind power projects is getting better optimum value of overlap (16.8%) and Tip Speed Ratio (TSR) (0.604). each year in terms of the generation cost per kW; however, advance The maximum power coefficient corresponding to the optimum overlap research on urban HAWTs will make them a feasible option for power and TSR was recorded 0.53. Ghosh et al. [23] carried out computational generation in these areas. This paper has mainly reviewed the design, analysis of a combined three bladed Darrieus-Savonius (Fig. 2) and performance, and commercial aspects of VAWTs, and any reference to found higher power coefficient, which varied between 0.18 and 0.53 HAWTs are merely for a performance comparison. compared to Savonius rotor (0.15–0.38). The hybrid Darrieus-Savonius rotor could be a solution for the city environment to extract more en- 2.2. Vertical axis wind turbines (VAWT) ergy from the unpredictable wind. In recent years, the hybrid design of VAWTs has seen more accep- VAWT is a turbine whose axis of rotation is pointed in vertical di- tance in the urban areas due to better aerodynamic performance and rection. VAWTs can produce electricity from wind of any direction with self-starting capability [24]. One of the design of hybrid VAWTs is DS- low cut-in wind speed. These turbines are significantly quieter than the 700, developed and manufactured by Hi-VAWT Technology Corp. traditional HAWTs, lightweight, and can be easily integrated into Taiwan [25]. DS-700 consists of an S-type of Savonius rotor with three buildings. These turbines are based on the aerodynamic drag forces, blades of Darrieus to maximize the performance (Fig. 3). DS-700 except Darrieus types. The research efforts on these turbines are focused can generate electricity from the wind at relatively low cut-in wind on increasing the aerodynamic efficiency by reducing the drag effect speed (2 m/s) and achieves rated capacity (700 W) at 10 m/s. and increasing the lift forces [31,32]. It was predicted that they could fi A project on the performance and bene ts of small-scale hybrid be an efficient solution for the built-up areas where the wind is un- VAWTs in urban areas was undertaken by the Authors of this review stable. The designs of VAWT can be categorized into two groups, viz. paper at Ryerson University, Toronto, Canada [26]. DS-700 hybrid Savonius and Darrieus. VAWT was chosen for this study. Long-term performance testing is underway on this turbine at Kortright Center, Woodbridge, Ontario, Canada [27]. The project was supported by NSERC Canada and Hydro 2.2.1. Savonius types VAWTs – One through a collaborative research grant. The following broad ob- Finnish engineer Savonius [33 35] had invented this type turbine in jectives have been envisaged for this investigation: (1) build reliable 1920, principally based on drag force. This is the simplest WT with low performance data on the techno-economic potential of micro hybrid cut-in wind speed, it usually consists of two half cylinders facing op- VAWT in urban and semi-urban conditions, (2) create a credible per- posite directions in such a way that they have formed almost an S-shape formance evaluation tool (program) through system modelling and si- (Fig. 4). The main drawback of this design is poor aerodynamic per- mulations to estimate the influence of site-specific variables, and (3) to formance in comparison to other turbines. Savonius might be a pre- ffi see the potential benefits of micro hybrid VAWT under a microgrid ferred choice if power reliability is more important than turbine e - platform at residential level with complementary technology (Solar PV ciency or COE. ff and battery back-up). The outcomes of this research will be published in Concentrated e orts have been made on making Savonius as an the future publications. Another classification of wind turbines is in terms of rotor diameter and power capacity. Although, there are no universally accepted limits for segmentations of wind turbines for this type classification [28]. Usually, the power capacity of small wind turbines varies from 50 W to 15 kW (rotor diameter 0.5–10 m) and mainly used for low demand applications (battery charging, domestic consumption, cottages, etc.). While, wind turbines of 15 kW and higher capacities (rotor diameter 10 m and more) are designed and installed for commercial purpose and usually connected to the grid. The small wind turbines are relatively more expensive in terms of cost of electricity (COE) per unit swept area.

2.1. Horizontal axis wind turbines (HAWT)

HAWT is considered a traditional design of WTs and somewhat re- sembles the historical . These turbines have two or three blades and are usually installed far from the built-up areas at a higher height to take advantage of the strong and steady wind. In order to reduce the installation, operation, and maintenance costs of projects based on HAWTs, an incredible growth has been seen since 1980 in the Fig. 2. Prototype of 3 bladed Darrius-Savonius turbine. (Reprint with written size of rotor diameter and the corresponding power capacity of these permission from Elsevier: Ghosh et al. [23]).

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Fig. 3. Commercial DS-700 hybrid VAWT and performance curve. (Reprint with written permission from Hi-VAWT Technology [25]). efficient design of WTs for urban areas, which include modifications in discrepancy in the experimental data and computational studies were the rotor design, blade shape, blade overlap, and the number of blades. caused by the flow characteristics of the wind, turbine geometry, and These turbines exhibit uniqueness in terms of rotor design and torque the adopted simulation methodology. Recently, a study was carried out production. The power coefficient (Cp) varies with the configurations of at Ryerson University about the shape of Savonius blade [42]. Savonius the rotor design. Roy and Saha [36] conducted several wind tunnel was tested with four blade configurations for the rotational perfor- experiments with a two-bladed Savonius turbine, mainly planned for mances. The experiments were conducted for the curved, straight, small-scale power applications. Simultaneous tests were also performed aerofoil, and twisted blade. The straight blade design was found to have with other blade configurations to make a comparison of performances. the least rotational performance (RPM) while the best performance was A notable gain of 34.8% was recorded for two-bladed Savonius rotor. In recorded for the twisted blade type (Fig. 5). a subsequent study, Roy and Saha [37] applied computational methods to study the operating parameters of Savonius. It was noted that the 2.2.2. Darrieus type VAWT differential drag force spins the turbine, but also primary reason for G.J.M. Darrieus [35,43], a French engineer, invented and patented poor aerodynamic effect. Efforts have also been made to minimize the this machine in 1931. Darrieus have better aerodynamic performance, drag effect by varying the blade numbers, orientation, and blades low cost, and simpler design than the Savonius. These machines are overlapping [38,39]. The variation in the blade numbers influence the designed based on the lift force and can extract more energy from the Cp, and the best performance was seen with three blades. wind per unit swept area. The of Darrieus moves forward in a The other notable studies on Savonius turbines examine the effect of circular path through the air and relative motion between the direction end plates, aspect ratio, bucket spacing, bucket overlap, blade numbers, of wind flow and the blade airfoil creates a small net force in forward rotor stages, buckets and rotor shapes, rotor shaft and other accessories direction, which creates a positive torque to the rotor (Fig. 6). Darrieus [40,41]. Investigation was also conducted to establish the Reynolds turbine supports blades in a way that minimizes bending stress, and the number, turbulence scales, and stators parameters. Some important force on the blades is solely due to tension. These turbines are available conclusions were drawn from all these investigations. An all-inclusive in two and three thin curved or straight blades with airfoil cross section analytical model is needed to predict the performance. Several shapes or constant chord length. Darrieus turbines suffer a major drawback as of the rotor of Savonius are a good feature of this turbine. The they require an external power source to start spinning since their

Fig. 4. A sectional view of Savonius turbine and operational mechanism. (Reprint with written permission from Elsevier: Jin et al. [35]; Hamdann et al., [104].

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Fig. 5. A sectional view of tested blade designs and variations of RPM in blade design with wind speed [42]. starting torque is very low. aerodynamic performance, blade design, modelling, prototype devel- To overcome some of the shortcomings of Darrieus turbines, some opment, feasibility and environmental analysis [46–48]. Computational modifications were performed to make these machines more versatile in and experimental methods were used to establish the performance all situations. A timeline of the developments of various Darrieus de- parameters, e.g., wind characteristics, blade design, rotational speed, signs is given in Fig. 7. Most of these developed configurations have TSR, mechanical torque, power coefficient, power and energy outputs. high COE and weak financial prospect in the present market situation. Amongst other approaches, the computational methods based on The requirements of large concrete foundation and high cost of man- computational fluid dynamics (CFD) are extensively used. While, ex- ufacturing made them less cost-competitive relatively. Musgrove perimental methods, based on field measurements and wind tunnel [44,45] conducted a systematic investigation on the blade cross-section experiments, are relatively straightforward, but limited in scope due to at University of Reading, UK. Musgrove's efforts led to the design of higher cost and longer time frame. A combined approach has also been Darrieus with straight blade, which is known as H-blade configuration. suggested to serve better to optimize the performance. The next few The design and manufacturing process of H-blade is relatively simple sections describe some major research on VAWTs in the recent years. compared to the curved blades, and the performance is better than the conventional design. In recent decades, Musgrove's designs were further 3.1. Wind resources and performance assessment evolved into fixed-pitch straight-bladed H-rotor and is available in several variations: Articulating, Tilted, and Helical H-rotor. But, the The precise data of the wind speed and level of turbulence is crucial ff acceptance of any versions of Darrieus depends on their cost-e ec- for the successful design and performance prediction of WTs for any tiveness ($/kWh), reliability and self-starting capability at low wind location. As mentioned earlier, predicting wind behavior in urban en- speeds. vironments is more challenging than in rural areas where there are fewer obstructions [49–52]. It is extremely difficult to forecast how the 3. Performance of VAWT wind behaves in these areas, resulting in either over or under estima- tion of wind resources. A thorough understanding of available wind Despite VAWTs potential having been recognized for more than two resource in these areas is fundamental for the justification of a long- decades, the research that has been carried out on these turbines as term investment for any types of WTs. The obstructions (e.g., buildings, compared to more popular HAWT is limited, which is considered a tress, electric poles/wire) in built-up areas prevent the turbines from major hindrance in their development. The work has been done on capturing enough wind. The intensity of the turbulence depends

Fig. 6. A design of Darrieus turbine with the direction of lift force on the blade surface. (Reprint with written permission from Elsevier: Jin et al. [35]).

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Fig. 7. A timeline of the growth of Darrieus turbines. (Reprint with written permission from Elsevier: Tjiu et al. [43]). significantly on the surface roughness. It was estimated that urban areas estimators, but in actual measurements, they failed to meet the an- have twice the conventional surface layer turbulence than that of rural ticipated outputs. The study concluded that a better methodology is sites. As the energy in the wind is a function of the cube of the wind needed for built-up areas. speed; therefore, a fractional change in the wind speed causes a sig- Kosasih and Hudin [60] have studied the impact of turbulence in- nificant impact on the overall power outputs. tensity on the efficiency of WTs in urban settings. They compared the Cp The available wind resource data, from various sources, to de- and TSR of a turbine with/without a wind diffuser, and with a varied termine power potential for a site has limitations, especially when ap- degree of turbulences. The study concluded that the presence of diffuser plied to more complex situations. Wind maps are not built for wind doubled the Cp, while an increase in the turbulence reduced the Cp. The project site selection, and even the ones with the highest resolution still reduction in the Cp has been observed faster at higher TSR. overlook local effects [53]. RETScreen [54] and other resources do not In another significant research, Sunderland et al. [61] have studied provide enough information to project developers on how to modify the impact of urban and rural conditions on the available wind re- these resources for urban sites, leading to poor site selection and a sources and subsequent effects on the net energy outputs of a small negative opinion of the technology [55]. Long-term site wind mea- wind system. Three international sites were selected for this study, viz., surements require time and money that often are not justifiable for Ireland, Sri Lanka, and the UK. As expected, the wind was more con- small scale projects. In recent years, several studies for the estimation of sistent for rural sites for all three country locations. They evaluated the site-specific urban wind resources have been reported in literature, project significance (frequency) in terms of capital cost, wind speed, however a detailed discussion on all these methods is out of scope of and interest rate. As illustrated in Fig. 9, the primary contributing factor this review paper [56–58]. It was concluded from all these studies that to justify a project is local wind speed, whereas the capital investment a better understanding of wind resource is crucial in built-up areas. The and interest rate play relatively secondary roles in the decision-making existing methods are based on many assumptions and have limitation in process. The COE was estimated as high as four times for a rural site complex terrains. than the COE of an urban site (Fig. 10). The study concluded that Drew et al. [59] investigated the effectiveness of two urban wind average wind speed is a prerequisite for the justification of any urban resources assessment methods, often used by project developers in the wind power project. UK to predict the outputs of small wind turbines of 91 sites, viz., NOABL and Energy Saving Trust wind estimator. It was noticed (Fig. 8a and b), these tools overestimate the site wind resources by 23% (NOABL) and 3.2. Modelling, simulation and experimental studies on VAWTs 18% (Energy Saving Trust). The techno-economic studies on several projects has shown significant errors in the esti- In the recent decades, a broad understanding has been reached mation of the site power outputs and project feasibility. All these pro- about the capability of CFD modelling in the prediction of wind pat- jects were predicted as being economically viable based on these terns at the urban sites. A progression in CFD methodologies holds fu- ture potential for the accurate predictions of wind resource in complex

Fig. 8. A comparison of the average wind speed prediction and the measured annual mean wind speed for 91 sites of UK : (a) NOABL method; (b) Energy Saving Trust Method. (Reprint with written permission from Elsevier: Drew et al. [59]).

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carried out by Jin et al. [35]. The studies on Darrieus was categorized into three parts: computational aerodynamics, CFD modelling, and experimental methods. All three approaches were compared in terms of their capability in the evaluation of turbine performance parameters. The computational aerodynamic methods were useful for the evalua- tion of the impact of an airfoil, solidity, Reynolds number, aerodynamic load [70], and analysis of low-frequency noises from the turbines [71]. While, the CFD modelling methods provide better aerodynamic per- formance and can be applied for the design evaluation in steady and unsteady wind [72], symmetrical and asymmetrical blade shapes [73], airfoil in the freezing conditions [74], and for the evaluation of dy- Fig. 9. The relative significance (frequency) for a wind project of mean wind namic stall phenomena effect [75]. Several other mathematical algo- fi speed, capital cost and interest rate. (The gure was recreated from the source rithms were also developed to study other aspects of turbines and data with written permission from Elsevier: Sunderland et al. [61]). summarized in Fig. 11. The experimental methods were more reliable for the evaluation of aerodynamic performance [76], surface roughness [77], and efficiency [78]. However, the experimental methods are expensive, limited in scope, and not suitable in many complex situations. Two experimental approaches were used for turbine performance assessments: wind tunnel method and particle image velocimetry (PIV). The wind tunnel method is extensively used and is often applied to measure the aero- dynamic performance, efficiency, and surface roughness of WTs. In wind tunnel tests, the wind is blowing through the tunnel and passes over the turbine blades, and the rotation per minute (RPM) is recorded to obtain the turbine's characteristic curve. An advancement in CFD modelling and availability of better computing has reduced the use of Fig. 10. COE associated with the wind electricity generation for urban and wind tunnel methods [79,80]. Even though, the modelling results must rural sites of three countries. (The figure was recreated from the source data be verified with wind tunnel test or some other approach. Contrary to with written permission from Elsevier: Sunderland et al. [61]). the wind tunnel test is the particle image velocimetry (PIV), in which the wind flow field around turbine blades are examined in a dynamic fl terrains [62–64]. The aerodynamic and economic performances of site- stall state by ow visualization and measurement [81]. specific small VAWTs have been studied by a research group in Iran Howell et al. [82] performed wind tunnel experiments on a small [65]. The study has shown that a site-specific design could help to scale Darrieus and compared the experimental results with 2D and 3D improve energy yield and reduce the manufacturing cost. The max- CFD modelling. The Cp and TSRs for experimental and modeled values imum benefits were reported for the sites with low turbulence. Bhutta are shown in Fig. 12 (for selected two wind speeds 4.31 m/s and 5.07 fi fi et al. [66] reviewed several design techniques along with their re- m/s). Although a signi cant error ( ± 20%) was xed for experimental results, a reasonable agreement was seen between 3D CFD modelling spective performance results and found that the Cp is varied with tur- bine configurations and can be optimized with respect to TSR. In recent results and experimental data. The vast gap in the experimental data years, several research articles have been published on the techno- and 2D modelling indicates that 2D modelling is probably useful for economic potential of VAWTs in urban and rural areas [67–69]. It was qualitative performance prediction. The results also implied that 3D fl found that the absence of robust methodologies to find an optimal lo- modelling predicts more precisely the quantitative air ow behavior cation for built-up areas and dearth of reliable performance prediction around the blades. It was concluded either method can be used to im- models are some of the hindrances in the progress of these turbines. In prove the design, but, the current focused was on aerodynamic per- the subsequent sections, some of the research methods are described. formance. A significant drawback of Darrieus is their inability to self-start and dependency on an external power source. A self-starting mechanism 3.2.1. Performance of Darrieus turbines under typical wind conditions holds fundamental importance for their Darrieus was promoted as a potential alternative to HAWTs for future acceptance. Numerous blade configurations were studied to urban applications. A review of research methods on these turbines was

Fig. 11. Programs used for CFD modelling [35].

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Fig. 12. Modeled vs. experiments performance of Darrieus turbine. (Reprint with written permission from Elsevier: Howell et al. [82]). eliminate the necessity of an external power source [83,84]. Singh et al. Nasef et al. [92] have studied the impact of overlap ratios of the

[85] suggested by replacing conventional blades with three-bladed H- two-bladed rotor. The performances (Cp)offive two bladed rotors with type rotor equipped with unsymmetrical S1210 blades. They concluded different overlap ratios (0.0, 0.15, 0.20, 0.30 and 0.50) are compared. that increasing rotor solidity might be a potential solution to resolve the Fig. 16 shows the configurations of blades for three overlap ratios 0.0, self-starting requirement. The higher blade solidity also benefits in 0.15 and 0.30. The result indicates that (Fig. 17) the overlap causes an better turbine efficiency. Batista et al. [86] suggested an approach increase in the Cp up to the ratio of 0.15, and corresponding averaged based on the new blade design profile (named EN0005), which offered Cp equal to 0.21 was estimated at a TSR of 0.9. self-starting capability at a speed of as low as 1.25 m/s (Fig. 13). The In another study, Akwa et al. [41] have shown the variation of prototype has shown a stable performance under stress test mode with average values of rotor and power with the rotational speed of wind velocity as high as 25 m/s. The modelling and experimental re- Savonius rotor. At higher rotational speed, the tip speed of the blade is sults of this new blade design are promising; however, more field per- greater than the wind speed and causes a drop in the Cp. Other notable formance data is needed to make this a feasible solution. studies discussed the relation in the blade overlaps and cut-in wind speed [93], a comparative performance of helical design with conven- tional blades rotors [94], relative performance of single stage rotor to 3.2.2. Performance of Savonius turbines 2nd and 3rd stage rotors [95],effect of aspect ratios on the aero- The other traditional design of VAWTs is the Savonius. As men- dynamic performance, and CFD modelling and simulation [96–99]. The tioned, the Savonius rotor is a drag-type machine, consisting of two or focus of these studies was to improve the aerodynamic efficiency of more blades (Fig. 4). These turbines have achieved some level of ac- Savonius. ceptance due to their low cut-in wind speeds and self-starting capability but suffer from a low rotational performance and overall efficiency. Akwa et al. [41] performed a detailed study on Savonius turbines and 4. Commercial developments of small wind turbines concluded that a diversity of rotor configurations is a useful char- acteristic of this type machine. The aerodynamic performance is af- Two market segments exist for WTs: Large-scale WTs (based on fected by the geometry of the rotor, air flow characteristics, and op- HAWTs) and small-scale WTs (based on VAWTs). Growth has been seen erational conditions. The power coefficient of different designs of in the acceptance of HAWTs and VAWTs around the world; however, Savonius varied and evaluated in the range of 0.05–0.30. Roy and Saha the growth has been exponential for HAWTs, while, the acceptance of [36] reviewed the performance evaluation methods of these turbines. VAWTs is relatively modest [100]. As the recognition of VAWTs is ex- They concluded that the computational methods can be an effective panding to meet low power demands and decentralized power strategy for the advancement of Savonius design with minimum ex- pense. Efforts have also been made to increase the aerodynamic effi- ciency. The modifications in the blade shape, size, and orientation was also analyzed [87–90]. It was recommended that the design, aero- dynamic efficiency, and power coefficient could be improved by the selection of appropriate computational methods. Kacprzak et al. [91] have evaluated numerically the conventional and modified Savonius turbine rotors using quasi 2D flow method. Three rotor configurations, conventional, bach-type and elliptical shapes, were modeled with the use of ANSYS CFX. The Cp of all three designs was analyzed with respect to the TSR. All three configurations and the Cp are shown in Figs. 14 and 15, respectively. The major findings of this study were: (1) all selected rotor configurations have achieved maximum Cp at TSR around 0.8, (2) Cp of bach-type rotor is better than the other two rotor designs, the Cp of classical rotor is su- perior if TSR is more than 1.0, (3) the wake effect is limited in all three designs, and TSR can be used to predict the frequency of fluctuation in Fig. 13. New Darrieus VAWT design prototype for field performance. (Reprint the power outputs. with written permission from Elsevier: Batista et al. [86]).

288 R. Kumar et al. Renewable and Sustainable Energy Reviews 89 (2018) 281–291

Fig. 14. Three rotor configurations of Savonius turbine: (a) classical design, (b) bach-type design, and (c) elliptical design. (Reprint with written permission from Elsevier: Kacprzak et al. [91]).

Fig. 15. Power coefficient vs. TSR of three Savonius rotors of Fig. 17. (This figure was recreated from the source data with written permission from Else- vier: Kacprzak et al. [91]). generation, the list of manufacturers and system integrators are also Fig. 17. Time average power coefficient vs. TSR for different overlap ratios of increasing. Some of the major suppliers of VAWTs turbines and com- two bladed Savonius, β represents the level of overlap. (Reprint with written ponents are; Arborwind, Astralux LTD/New Wind Turbine, Cleanfield permission from Elsevier: Nasef et al. [92]). Energy, Global Wind Group, Gual StatoEolien, Helix Wind, Hi-VAWT, McCamley, Ropatec, Sauer Energy, Urban Green Energy, VBINE Energy, energy extraction capability is significantly lower than the HAWTs. As Venger Wind, Windside, Windspire [101]. Even though the research, per the World Wind Energy Association (WWEA) report 2015 [103],at development, and deployment have demonstrated several benefits of the end of 2013, the total capacity of small wind turbine installations VAWTs, the market is still relatively small as compared to popular was estimated 755 MW (at least 870,000 small wind turbines were in- HAWTs. Gipe [102] in his review on household size turbines has re- stalled all over the world), about 41% of these turbines sited in China, ported that the maximum aerodynamic efficiency of VAWT design is 30% in the United States, 15% in the UK, and small fraction is in the about 15–25% lower than similar size HAWT designs. Thus, VAWTs rest of the world. Approximately 18% of wind turbine manufacturers

Fig. 16. Overlaps of two bladed Savonius rotors with wind velocity profile: (a) no overlap ratio, (b) 0.15 overlap ratio, and (c) 0.3 overlap ratio. (Reprint with written permission from Elsevier: Nasef et al. [92]).

289 R. Kumar et al. Renewable and Sustainable Energy Reviews 89 (2018) 281–291 were exclusively supplying VAWTs, while, 74% of manufacturers were www.gwec.net/global-figures/wind-in-numbers/〉; 2015 [Accessed 15 March supplying HAWTs, and a small number was producing both types of 2018]. [2] International Energy Agency. Technology roadmap wind energy, OECD/IEA-2013. turbines. The average power capacity of small VAWTs was estimated at Available on: 〈https://www.iea.org/publications/freepublications/publication/ 7.4 kW, while, the average power capacity of small HAWTs was found Wind_2013_Roadmap.pdf〉; 2013 [Accessed 24 July 2016]. much smaller. It was documented in the market projection report [3] Toja-Silva F, Colmenar-Santos A, Castro-Gil M. Urban wind energy exploitation systems: behavior under multidirectional flow conditions-opportunities and chal- (2015–2023), that without advancing economic competitiveness, it is lenges. 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