An overview of aerodynamic performance analysis of vertical axis wind turbines

Abstract

In this paper, an attempt has been made to highlight major developments of vertical axis wind turbines (VAWTs) in the last few decades. The effects of various design parameters such as airfoil, number of blades, solidity, aspect ratio, blade helicity, and overlap ratio have been critically analyzed. Wind energy is the most promising renewable, cost-effective, efficient, and accessible source for both domestic and commercial applications. Horizontal axis wind turbines are highly developed and are being used for medium-to-large scale power projects. VAWT are considered viable options for urban and semi-urban areas. These turbines have several characteristics, such as omnidirectional, power generation in weak and unstable winds, esthetically sound, safety, and low noise. Darrieus turbines with a fixed blade-type have starting problems at low wind speeds. Savonius turbines have good starting capability; however, their power coefficients are lower than other types of VAWT. To overcome the shortcomings of conventional wind turbines, an innovative engineering solution was sought in the design of hybrid VAWT. The analysis revealed that hybrid wind turbines have addressed the deficiencies to an extent; however, the overall performance is still less than that of conventional wind turbines. Several recommendations have been made based on state-of-the-art information from the perspective of future studies and acceptability. It was concluded that vast opportunities for wind turbine applications are available in urban areas; however, further academic research is required on critical aspects such as self-starting at low wind speeds, efficiency, structural reliability, design improvement for aerodynamic performance, and wind resource assessment.

Keywords

Aerodynamics darrieus-turbine hybrid VAWT savonius-turbine vertical axis wind turbine and wind energy

Introduction

Since the beginning of the industry setup, the consumption of energy has increased rapidly in the world. It is expected that energy resources based on fossil fuels are not sufficient to meet the global energy requirements. Furthermore, fossil fuel-based energy is not considered to be a good choice because of high carbon emission, non-renewable and climate warming, etc.¹ The industrialized setup has undergone significant unfavourable changes, which have a significant negative influence on the environment and the available energy resources. Additionally, rising energy consumption points to both an energy crisis and environmental problems. Today, approximately 80% of the overall global energy is produced from fossil fuels; therefore, the quantity of these resources available on earth is limited, and a gradual decline has been observed over time.² The possibility of reducing fossil fuels has directed interest in renewable energy sources. To stop further depletion of natural resources and severe environmental deterioration, potential technology systems must consider sustainable growth principles and accomplishment criteria in technological processing, outcome, and operating limits.³ Various worldwide climate shifts have been brought on by the excessive release of greenhouse gases like carbon dioxide.⁴ According to the perspective of world energy technology, greenhouse gas emissions from different energy sectors will be significantly increased in the next few decades without the development and implementation of alternative energy sources. The analysis shows that approximately 1200 grams of CO $_{2}$ is emitted by coal fires for every kWh of electricity generation. Therefore, it can be stated that equivalent emissions can be avoided by producing each unit from wind energy.⁵ Figure 1 shows the reduction in carbon emissions from various energy systems. To overcome these emission effects, a great revolution in the era of energy technology is required, including energy efficiency, nuclear energy, renewable energy sources, and fossil-fuel decarburization. Many researchers are focusing on renewable energy sources that are environmentally friendly, safe, and secure.^6,7 Various alternative energy production sources are available, some of which are solar, wind, geothermal, hydropower, and biomass. Among these available energy sources, wind energy is found to be the most promising, long-term, and good alternative option, with the advantages of low environmental effects, cost efficiency, and sustainable solution.^3,8,9 Significant benefits of wind energy include preserving fossil fuels, preventing acid rain, and lowering greenhouse gas emissions into the environment.¹⁰ An increasing trend has also been observed in its growth and acceptability since 2000.¹¹

Figure 1.

Reduction in CO $_{2}$ emission from various energy systems. A very less contribution can be seen from wind, solar and hydro systems, however, they have potential to minimize the emission effects.⁶

The energy produced from renewable or natural resources can be termed green energy. The negative impacts caused by this energy are less than those of the other systems. Wind energy can be harnessed using rotating devices and converted into electrical energy for household or commercial applications.¹² It is a low-cost solution in terms of maintaining existing systems, using renewable energy, and utilizing upcoming technology.^13,14 A graphical representation of renewable energy depicting the need to harness ongoing issues is shown in Figure 2. Wind energy is an emerging technology for electricity production and has become very important in the last few decades. The installed capacity of wind energy has nearly doubled every three years during the past two decades of the twenty-first century.¹⁵ By the end of 2016, the total installed capacity had increased to 487 GW (4% of global electrical power) and this number is gradually increasing. Infrastructures and technology have rapidly advanced. Wind energy is considered to be a mature and convincing renewable source of energy. In 2019, an accumulative power capacity of approximately 651 GW was produced from wind energy.¹⁶ According to another survey conducted in 2013, by the end of 2050 wind energy technology will meet approximately 18% of global power requirements.¹¹ Extensive efforts have been made to improve wind energy technology aiming to decrease the import of energy from other countries and to meet the power requirements. Research by different countries has been carried out to identify favorable sites for wind velocities. According to the worldwide wind energy report, the power generation capacity of wind energy systems can be extended to 2000,000 MW by the end of the year 2025. Substantial wind with the potential of power is found near highways, urban areas, high-rise buildings, and railway tracks. Wind potential is available with varying intensities in different areas. An efficient power-producing device is required to capture wind energy that must cater to environmental changes, turbulence, and speed ranges.^6,17

Figure 2.

Graphical representation of renewable energy depicting its need to address ongoing issues. The figure shows that renewable energy sources have the features to provide secure, safe, sustainable, and economical power. (Reprint with written permission from Elsevier).¹⁸

Wind energy scenario and its national need

As the earth’s average temperature and global warming are continuously increasing owing to the use of fossil fuels. According to a survey, by the end of 2030, approximately 50% of the world’s energy consumption will increase. An urgent solution that considers safety and environmental friendliness is required to address these ongoing energy issues.^19,20 In response to growing environmental concerns, renewable energy options were developed.²¹ The determination of accurate wind speed data along with turbulence level for any site is critical for the effective performance of wind turbines. Prediction of wind profiles in urban locations is more difficult than in rural areas because of barriers.^22–25 Conventional techniques for electricity generation are mostly based on natural gas and oil. Despite the consumption of fossil fuels, low electricity production is a serious concern that hinders the development of a country. Electricity produced by wind resources is more efficient, safe, and economical than fossil fuels. The cost factor and air pollution are associated drawbacks of power generation by these existing means. The installed capacity of wind power generation in some countries is shown in Figure 3. The figure shows that China, USA, and Germany are leading with an installed wind power production capacity of more than 1000 GW, whereas a very small contribution can be seen from Brazil and Pakistan. Efforts are required from such countries to contribute to green energy production. A comparison of wind energy potential and installed power capacity in South Asian countries is presented in Table 1. The data shows that besides having great potential for wind power, a very low installed wind power capacity can be seen in some countries such as Pakistan, Afghanistan, and Bhutan. By taking initiative and introducing effective policies, encouraging wind farm firms can greatly help re-power their energy capacity. Research in this area is crucial to make it an affordable, viable, and dependable power solution for off-grid sites and various domestic as well as commercial applications.¹¹The growth of an efficient wind energy-production mechanism incorporates sustainability in various regions, minimizes the strain on the available grid setup, strengthens the economy, and meets the electricity requirements.

Figure 3.

Installed capacity of wind power generation of different countries. Besides having the great potential of wind energy in Pakistan and Brazil, power produced by this renewable source is very less as compared to other countries. (Reprint with written permission from Elsevier).¹⁸

Table 1.

Comparison of installed capacity and potential of wind power in South Asian countries.²⁶

S No	Country	Installed Capacity (MW)	Wind Potential (MW)
1	Pakistan	792	346,000
2	Sri Lanka	146	24,000
3	India	32,878	102,778
4	Bangladesh	3	20,000
5	Afghanistan	0	158,000
6	Nepal	0	3,000
7	Maldives	1	288
8	Bhutan	1	63,895

The utilization of wind flow patterns for electricity generation has been a challenging task for engineers, developers, researchers, and significant work has been done in the last few decades. During the research, attention was given to computational programs, resource assessment techniques, environmental aspects, blade materials, aerodynamics, test methods, modeling, and simulation.

This review paper highlights some important findings of VAWTs from published papers, technical reports, and manufacturer’s literature. Information related to the limitations and scope of different VAWTs was evaluated in this study. Some ongoing development and commercial activities in the field of wind turbines are also highlighted. It was observed that vast opportunities for wind turbine applications are in urban areas; however, further academic research is required on critical aspects such as design improvement for aerodynamic performance, cost reduction, and resource assessment for urban operating conditions.

In the second section, the classification of wind turbines with respect to the axis of rotation, aerodynamic forces, and power production capacity have been discussed. Some important parameters involved in the design and analysis of wind turbines have also been highlighted. In the third section, the aerodynamic performance analysis of the Darrieus VAWT is presented. Recent studies using experimental and computational techniques, including the effects of various design parameters, have been highlighted. The fourth section focuses on the design and development of Savonius wind turbines. The performance analysis of Savonius wind turbines based on rotor design modifications, number of blades, and overlap ratio was summarized. In the fifth section, recent progress in the development of wind turbines in the form of hybrid VAWTs is discussed. The importance of hybrid VAWT and the combined effect of various Darrieus and Savonius parameters has been emphasized. Based on the findings of different design configurations of wind turbines, analysis and discussion are provided in the sixth section. In the seventh section, conclusions are drawn based on the analysis of various wind turbines. The analysis provides meaningful results to explore further design modifications to improve the performance parameters.

Novelty of the research

This paper provides a thorough analysis of the literature on different wind turbine designs. With a focus on the key characteristics, benefits and drawbacks, and continuing research, a modest attempt has been made to highlight some of the significant advances of VAWTs. On the basis of the state-of-the-art information, some recommendations have been made for further research and the acceptance of wind turbines. Further academic research is necessary to make VAWTs feasible, reliable, and inexpensive power generating technology for numerous low and decentralized power applications. The paper will also serve as an information hub by citing the most pertinent sources for each project, making information about these projects more accessible.

Wind turbines and their classification

Wind is produced as a result of uneven heating of the earth from solar energy, which leads to a change in atmospheric pressure. Wind energy can then be harnessed by an appropriate device called a wind turbine. As a result of wind flow, the shaft starts rotating and a generator attached to the shaft starts producing electricity that can be used for household applications through the grid station. Wind turbines are used to transform wind kinetic energy into electrical power, which is a clean way to produce electricity.²⁷ However, there are some issues to use this technology, such as initial development expenses, installation capacity, especially for large communities, the requirement of large wind farms, and the availability of wind energy sites.²⁸ To enhance the utilization of renewable wind sources, awareness of available potential must be transmitted at all levels.^23–29 Considering the potential power solution, the installation of small-scale wind turbines over high-rise structures to sustain the transition phase has been recommended.^30–33 Wind turbines can be classified based on their power capacity, aerodynamic forces, and axis of rotation. Based on the axis of rotation, there are two major types: horizontal axis wind turbines (HAWTs) and VAWTs.³⁴ A sketch diagram depicting shaft and rotor orientation of the HAWT and VAWT is shown in Figure 4. The flow chart of various configurations of wind turbines based on their axis of rotation, aerodynamic forces, and power production capacity is shown in Figure 5.

Figure 4.

Orientation of shaft and rotor in HAWT and VAWT.³⁵

Figure 5.

Classification of wind turbines based upon the axis of rotation, aerodynamic forces, and power capacity.¹¹

The HAWTs have been widely used for decades. The rotating axis of the HAWT is parallel to the wind direction and horizontal to the ground.³⁶ On the contrary, the shaft of the VAWT remains perpendicular to the ground. Both HAWTs and VAWTs are used in various sizes (up to 100 m rotor diameter) and power production capacity (up to 3 MW). The small turbines may be used for auxiliary power sources, household applications and power traffic signal lights, etc. whereas; large turbines are used for domestic power supply as well as at an industrial level. Although massive wind farms currently produce the majority of the world’s wind energy, the number of small size wind turbine installations has been rising in recent years.³⁷ Over the past few decades, the size of wind turbines has amplified to a scale of 100. HAWTs are highly developed and these are being utilized all over the world but recent research and development work in the field of VAWTs can lead the wind energy technology in the near future.⁶ HAWTs are not omnidirectional and for better power output; they must change their direction with the wind. Also, for wind direction alignment, directional control mechanism must be installed on it.³⁸ HAWTs are widely used for most commercial applications around the world and most medium to large scale projects are based on such turbines.³⁹ For small-scale utilization and urban areas having turbulent or intense wind patterns, these are not considered viable options. VAWTs are considered to be a potential option for semi-urban areas and cities.^40,41 Based on aerodynamic behavior, VAWT can be further categorized into two subgroups, drag-based VAWTs (Savonius) and lift-based VAWTs (Darrieus). In both types of turbines, the corresponding lift or drag force is used to extract power from the available wind.¹¹ To improve the wind turbine’s performance parameters, several techniques based on numerical as well as experimental investigations have been recommended in the literature.^42–44

Important parameters of wind turbine

The performance of both HAWT and VAWT depends on design parameters which have been discussed in subsequent sections.

Power

Theoretical available power in the wind can be determined by Eq 1.⁴⁵

P = \frac{1}{2} ρ A V^{3}

(1)

where

A

is the swept area of the turbine,

ρ

is the air density and

V

is wind velocity. Output power produced by the generator or wind energy captured by the turbine blades can also be determined by Eq 2.⁴⁵

P = T ω

(2)

where

ω

is the rotational velocity and

T

is the torque produced by the wind turbine.

Power coefficient

It is the ratio of the power generated by the wind turbine to the power available in the wind (Eq 3).

C_{p} = \frac{P}{\frac{1}{2} ρ A V^{3}}

(3)

The relationship between the efficiency of the rotor or power coefficient (C $_{p}$ ) and torque produced by the wind turbine can be expressed by Eq 4.⁴⁵

C_{p} = \frac{T ω}{\frac{1}{2} ρ A V^{3}}

(4)

The maximum power that can be extracted from an available wind stream is associated with the Betz limit. This limit shows that a turbine cannot get over 59.3% of the power in an uninterrupted stream tube of air.^46,47

Torque

The amount of torque produced by the wind turbine can be estimated by Eq 5.⁴⁸

T = \frac{1}{2} ρ A R C_{t} V^{2}

(5)

where

ρ

is the air density,

A

is the swept area of the turbine,

R

is the rotor radius,

C_{t}

is the torque coefficient and

V

is wind velocity.

Tip speed ratio

The power coefficient can be used to calculate the aerodynamic efficiency of wind turbines based upon the tip speed ratio (TSR) that is stated by Eq 6.⁴⁹

λ = \frac{ω R}{V}

(6)

where

R

is the radius of the turbine blade and

ω

is rotational velocity. TSR is a parameter used to estimate turbine efficiency. Generally, the TSR range varies from 0 to 15 depending upon the tip speed and incoming wind velocity. Savonius turbines operate at low TSR ranges (0–3), whereas the TSR range of Darrieus turbines can be increased up to 8. Generally, Darrieus turbines operate at TSR between 4 and 6.⁵⁰ From the literature, the lower range of TSR can be differentiated as a value less than 3 (TSR

\leq

3).⁵¹

Solidity

It is defined as the measurement of VAWT occupied area and can be calculated from the ratio of blade face area to the rotor swept area.⁵² It also depends on the chord length ( $c$ ), the number of blades ( $N$ ), and the radius ( $R$ ) of the blade. The basic expression is given by Eq 7.

σ = \frac{N c}{R}

(7)

Solidity is used to determine the rotational velocity at which the turbine’s maximum performance is achieved. Generally, it varies from 0 to 1 depending upon the radius of the blade, chord length, and the number of blades. The maximum C $_{p}$ values of most of the turbines are achieved at a solidity range of 0.3–0.4.^48,53

Swept area

It is the area through which incoming wind flows. It may be circular in the case of HAWT or a product of base and height. Output power is directly proportional to the swept area.

Airfoil

The type of airfoil has a great effect on power production. In the past, most of the turbines were designed with NACA series symmetric airfoil,⁵⁴ but for VAWTs these airfoils were having starting issues. Due to more tangential thrust of cambered airfoils at low angle of attack (AoA) compared to that of symmetric airfoils, they have the potential to overcome these issues.⁵⁵

Chord length

The chord length of the blade is another important parameter. Solidity depends upon chord length and affects the aerodynamic performance of the wind turbine. At the lower TSR range, an increase in power output has been observed by using a higher solidity ratio (large chord length with a smaller diameter) as compared to the higher TSR range.⁴⁸

Number of blades

The number of blades plays a vital role in the design process and most of the existing configurations are installed with three blades. There are several design configurations for the number of blades required for a suitable turbine, depending on the aerodynamic efficiency, available cost, and reliability factor. However, with an increase in the number of blades, to balance the rotor design, efficiency gets reduce and a complicated dynamic stability is required to the blade hinge to alleviate the loads. One reason for the selection of three blades is that a wind turbine having two blades causes noise problems and that having four blades add weight penalty.⁶ The second reason for three-blade rotor selection is relatively more energy extraction from the wind sites followed by the conversion by the installed generators into electricity for domestic as well as commercial projects.^6,56

Design velocity

It is the velocity for which the wind turbine is required to be designed. Most of the time, the operational or average wind velocity is considered to be the design velocity.⁵⁷

Aspect ratio

It is the ratio of blade height (h) and rotor radius (R) as given in Eq 8.

A R = \frac{h}{R}

(8)

The low aspect ratio (AR) of a wind turbine is generally from 0.05 to 1, whereas the high aspect ratio is more than 1.2. The variation of aspect ratio causes a change in Reynolds number. With the decrease in AR, the Reynolds number increases which improve the performance of a wind turbine.⁵⁸

Cut-in & Cut-out speed

The lowest speed at which a turbine begins to generate electricity is known as the ’cut-in speed’. It can vary from 0.1 to 10 m/sec depending upon turbine self-starting capability. Cut-out speed is the maximum speed constraint to ensure system preservation and safety above which the turbine will cease generating electricity. It can vary from 10 to 40 m/sec depending upon the specification of the turbine.⁵⁹

Horizontal axis wind turbines

Horizontal axis wind turbines are commonly used all over the world. In this category, the axis of rotation remains horizontal or parallel with the ground. Generally, these turbines are designed with three blades and installed at a height (approx. $\geq$ 30 feet). Moreover, these are installed away from built-up areas to get maximum amount of steady wind. Due to more theoretical efficiency, the world is still focusing on these types of turbines.^60,61 A schematic showing major components of HAWT is shown in Figure 6. Due to more power output and better performance, large-scale projects and developments are based on these types of turbines. However, a decline has been observed in their performance during operation in unsteady and low wind speed areas.¹¹ Usually, these types of turbines are installed with a self-starting and yawing mechanism to keep the blades in the wind. The power output highly depends on the average wind speed.⁶² Under steady wind conditions, the ideal aerodynamic efficiency of such turbines is quite good i.e. 40–55%.⁶³ Performance parameters of different configurations of HAWTs are given in Table 2. It can be seen that some configurations have good-rated power as well as low cut-in speed. The remote areas and offshore sites have undisturbed and clean wind; therefore, these turbines are suitable options in such areas. However, due to their high cut-in speed and noise problems, these are not considered efficient designs in urban areas.^11,64 In HAWTs, the rotor is installed at the upper portion of the tower pointing towards the incoming wind, and its position is controlled by wind sensors coupled with a yawing mechanism.⁶⁵ Most of the turbines have a gearbox that is used to convert the low rotation of blades into a higher rotation and the same is more suitable to drive electrical generators.⁶⁶ These turbines cannot be installed close to each other because of wake influence on the downstream turbine. The appropriate distance between the two turbines is about 3–6 times’ of turbine diameter. The velocity of the flow decreases as it passes through the turbine, but the turbulence intensity increases significantly. The strong wake effects are found close to the hub center and decrease towards the blade tip.⁶⁷ HAWTs are very sensitive in terms of wind directions and turbulence effects. However, these are efficient to produce wind energy from the available amount of wind.⁶⁵ Some of the drawbacks of HAWTs are the large space requirement for blade operation, dependency on wind direction, obstruction for aircraft/birds, requirement of huge tower construction to support the heavy blades/generator, and maintenance issues due to design configurations.^33,68 However, the design of HAWTs allows to extract the maximum energy from the wind with full rotation under steady flow conditions.⁶⁹

Figure 6.

Major components of HAWT. Nacelle contains most of the components installed behind the rotor that rotates with the help of yawing mechanism to align with the incoming airflow.⁷⁰

Table 2.

Performance parameters of different configurations of HAWTs.^59,71

S No	Turbine type	Radius (m)	Cut-in speed (m/sec)	Cut-out speed (m/sec)	Rated power (watt)
1	Swift	1.04	3.4	65	1500
2	Ampair	0.86	3.0	–	600
3	Proven Energy,P11	2.75	3.5	–	6,000
4	Fortis	2.50	3.5	–	2500
5	Skystream	1.86	3.5	–	2400
6	Xzeres	1.80	2.2	–	2500
7	Bergey	3.50	2.5	–	10,000
8	Proven Energy,P7	1.75	3.5	–	2500

Vertical axis wind turbines

The axis of rotation in VAWTs remains perpendicular to the wind as well as the ground. The primary advantages of VAWTs are acceptance of wind from any direction (omnidirectional), low noise pollution, high starting torque, single moving part, compact size, free from the yawing mechanism, respond well to variation in wind direction, the capability to operate in a broad range of wind speed and turbulence, design simplicity and capability to give direct rotary drive.⁷² Moreover, the uniform section, untwisted, and straight blades of VAWTs are easy to fabricate.^73–75 They have better adaptable features to the unsteady wind and low environmental impact. VAWTs also have a strong ability to resist wind along with low cut-in speed and are quite easy to integrate with urban infrastructure due to design simplicity.^11,52 Because most of the components of VAWTs are installed on the ground that leads to ease of installation and maintenance activities. The most common components of VAWT required during the design and fabrications process are the rotor, hub, blades, tower, shaft, generator, link rod, and foundation. Schematic showing these components of VAWT is given in Figure 7.^49,76 Straight-bladed VAWT type consists of untwisted and uniform cross-sections which make them easy to extrude. On the contrary, blades of HAWTs should have tapered and non-uniform cross-sections to get optimum performance parameters.^13,77 In the recent past, the performance of VAWT has improved a lot due to research and advancements in rotor technology. These enhancements helped to increase performance over a broad operational range in challenging urban environment.^78–81 Various configurations of VAWTs based upon aerodynamic forces are shown in Figure 8. A detailed discussion of these configurations is given in detail in subsequent sections. These may be divided into two categories based on the aerodynamic forces that are exerted on the turbine blades i.e. lift-based (Darrieus) and drag-based (Savonius) VAWT. In Darrieus turbines, lift force on each blade produces positive torque while the negative torque from drag force counters with it. The overall contraction between these two opposing torques defines the turbine output torque. In contrast, the drag force of the Savonius turbines produces power. Compared to Savonius turbines, Darrieus turbines may have higher maximum efficiency.³⁶ Darrieus turbines can extract more power per unit area from the wind, whereas Savonius turbines are simpler in design, but their efficiency is very low.⁸²

Figure 7.

Major components of VAWT, the incoming airflow is perpendicular to the rotating shaft and turbine blades. Most of the components are installed on the ground that leads to ease of installation and maintenance activities.⁷⁶

Figure 8.

Various configurations of vertical axis wind turbines based on aerodynamic forces. Savonius turbine is a drag force-based turbine, whereas Helical, H-Rotor, and Darrieus are lift force-based turbine. Hybrid VAWT is a combination of lift and drag-based turbines.

A comparative study of HAWT and VAWT was carried out including various aspects of operation, maintenance, advantages, and applications.⁵² The summary based on some of the salient features of both types of wind turbines showing their merits and demerits is given in Table 3.

Table 3.

Comparison between VAWT and HAWT.^{51,52,83–85}

Parameter	VAWT	HAWT
Wind direction	Independent	Dependent
Applications suitability	Small scale applications	Large scale applications
Self-starting	No	Yes
Maintenance and installation cost	Low	High
Power production	Less	More
Space requirement for the same amount of power	Less	More
Environmental conditions	Can withstand extreme weather conditions	Cannot withstand extreme weather conditions because of their design
Yaw mechanism	Not required	Required
Tower	Not required	Required
Guy wires	Optional	No
Noise	Low-Moderate	High
Obstruction of birds/aircraft	No	Yes
Efficiency	40–50%	40–55%
Operating wind speed limits	2–65 m/s	6–25 m/s
Generator and accessories location	On-ground	In the nacelle, high up from ground level
Manufacturing cost	Low due to design simplicity	High due to complex blade profile
Capability to withstand higher wind speeds	During gusty weather conditions, this offer operational safety	Comparatively less safety advantage

Darrieus wind turbines

Darrieus wind turbines were invented by a French Engineer G. J. Mary Darrieus. The lift-based wind turbines consist of two or more blades.⁸⁶ These airfoil shape blades are installed with the centralized rotating shaft. The flow of wind over the airfoil surface creates a lift force that pulls the rotor blades. At high rotating speeds, Darrieus VAWTs can generate more power than drag-based turbines.⁸⁷ The curved or egg beater shape wind turbines commercially installed earlier in California allow minimizing the bending stresses produced on the blades.⁸⁸ H-Rotor turbine is the simplest type of Darrieus VAWT consisting of straight blades. This configuration may be of fixed or variable pitch blades. Investigations have shown that fixed blade designs have difficulty in starting, particularly in low wind ranges and for such configurations, an external power source or starting generator is required to initiate the turbine’s spinning. On the contrary, starting problems can be resolved by using variable pitch configuration, however, the design of such turbines is complicated which restricts their utilization for small-scale applications. Initially, most of the configurations were designed with symmetric airfoils which were incapable to self-start. From an acceptance perspective, the self-starting mechanism holds fundamental importance.⁷⁷ Efforts have been made on these turbines to enhance the aerodynamic efficiency by increasing the lift and decreasing the drag effects.^83,89 The working principle of the Darrieus turbine is shown in Figure 9.

Figure 9.

Top view of Darrieus turbine showing its working principle. Shaft and blades rotate under the action of lift force that results in electricity generation.⁸⁶

Aerodynamics of straight-bladed VAWT

Despite having the design simplicity, the aerodynamic analysis of the straight-bladed VAWT is very important. The associated flow velocities both in upwind and downwind sides of the turbine are shown in Figure 10.⁹⁰

Figure 10.

Diagram showing forces and associated flow velocities acting on a blade profile. (Reprint with written permission from Elsevier).⁹¹

Variation of local relative velocity

Considering the axial flow occurring on the turbine blade, the velocity component in normal (V $_{n}$ ) and chord (V $_{c}$ ) direction can be determined from Eq 9 and 10.

V_{c} = R ω + V_{a} \cos θ

(9)

V_{n} = V_{a} \sin θ

(10)

where

θ

represents the azimuth angle, V

_{a}

is the component of axial velocity on the rotor,

R

is the turbine’s radius, and

ω

is the rotational velocity . The relative flow velocity (

W

) can be determined from the figure as shown by Eq 11.

W = \sqrt{{V_{n}}^{2} + {V_{c}}^{2}}

(11)

The non-dimensional form of relative flow velocity can also be determined using free stream velocity as given in Eq 12.

\frac{W}{V_{\infty}} = \sqrt{((1 - a) \sin θ)^{2} + (λ + (1 - a) \cos θ)^{2}}

(12)

where

λ

is TSR and

a

is the axial induction factor which depends on free stream velocity (V

_{\infty}

) and induced velocity (V

_{a}

) defined by Eq 13.

a = \frac{V_{\infty} - V_{a}}{V_{\infty}}

(13)

Variation of local angle of attack

From Figure 10, it can be observed that the local angle of attack depends on the velocity component as given in Eq 14.

α = \arctan (\frac{V_{n}}{V_{c}})

(14)

By expressing the above equation in terms of induced velocity and TSR, Eq 15 can be obtained.

α = \arctan (\frac{(1 - a) \sin θ}{λ + (1 - a) \cos θ})

(15)

Considering the pitching angle (

γ

) of the blade, the above expression can be modified as Eq 16.

α = \arctan (\frac{(1 - a) \sin θ}{λ + (1 - a) \cos θ}) - γ

(16)

Variation of normal and tangential force

The resultant components of the lift and drag in normal direction can be expressed as coefficient of the normal force (C $_{n}$ ). Moreover, resultant components in tangential direction can be expressed as coefficient of the tangential force (C $_{t}$ ) as given in Eq 17 and 18, respectively.

C_{n} = C_{l} \cos α + C_{d} \sin α

(17)

C_{t} = C_{l} \sin α - C_{d} \cos α

(18)

The expressions of overall tangential and normal forces are given in Eqs 19 and 20, respectively.

F_{t} = \frac{1}{2} C_{t} ρ c H W^{2}

(19)

F_{n} = \frac{1}{2} C_{n} ρ c H W^{2}

(20)

where

H

is the height of the turbine,

ρ

is the air density and

c

is the chord length of the blade. Different aerodynamic models such as cascade, vortex, and momentum models have been developed to determine these forces.⁷⁷

Power and Torque

The tangential and normal components of acting forces for arbitrary azimuth (orbital) location are considered as a function of the orbital position. The average value of tangential force (F $_{t a}$ ) can be determined from Eq 21.

F_{t a} = \frac{1}{2 π} \int_{0}^{2 π} F_{t} (θ) d θ

(21)

For the

N

number of blades, the total torque (

Q

) and total power (

P

) can be determined from Eqs 22 and 23, respectively.

Q = N . F_{t a} . R

(22)

P = Q . ω

(23)

Performance analysis of Darrieus wind turbines

To optimize the design configurations and to enhance power output, extensive research work has been carried out by various scientists in the last few decades. Darrieus wind turbines have self-starting issues and starting torque, as well as the efficiency of these turbines, needs to be improved for utilization in large-scale projects. Efforts have been made to overcome the shortcomings of conventional Darrieus VAWT through innovative designs.^92,93 Self-starting issues have also been tried to resolve by the development of tailored airfoils.^94–97 To study unsteady and complex flow problems related to wind turbines, computational fluid dynamics (CFD) is considered to be an effective engineering tool and it has been widely accepted in industrial sectors and academic fields.^98–101 CFD simulations are helpful to simulate the fluid flow, optimize and verify the performance of design before costly prototypes and physical tests.^102–105 In some cases, the 2D simulation approach may be insufficient as the simplified geometry generally suggests an over prediction of the results due to the absence of the detached tip vortices which renders 3D flow.¹⁰⁶ Moreover, the appropriate aerodynamic simulation of VAWT carries a substantial challenge. There is a cyclical variation in azimuth direction in the AoA during the turbine operation. At the low TSR range, when the value of the AoA exceeds transiently beyond the airfoil static stall angle, a dynamic stall can occur. Ahmad et al.¹⁰⁷ studied the effect of design parameters of H-rotor VAWT using one factor at a time and the design of experiments (DOE) techniques. The analysis was performed in a QBlade software followed by numerical simulations of the optimized design. The analysis revealed of approx. 8.43% improvement in the maximum C $_{p}$ of the optimized wind turbine. Altmimi et al.¹⁰⁸ optimized the design parameters of VAWT using QBlade software. The analysis was performed under different environmental conditions for various design factors such as the number of blades to find the optimal values of efficiency of VAWT. Maeda et al.¹⁰⁹ performed an experimental testing and CFD analysis of a straight-bladed VAWT to determine the aerodynamic performance characteristics. Results showed that there was a decrease in fluid force in the span-wise direction by excluding the support structure. Furthermore, by reducing 3D effects on the fluid flow around the support structure and rotor blade, the C $_{p}$ was significantly improved (approx. 20%). Roy et al.¹¹⁰ performed experimental testing based on the double multiple streamtube model of VAWT with flexible and rigid-blades configurations. Passively-morphing VAWT resulted in improvement of maximum C $_{p}$ and operating range of TSR by 8.9% and 40.3%, respectively than rigid bladed VAWT. Zhu et al.¹¹¹ performed the numerical analysis of straight-bladed VAWT with bionic blades. The analysis was carried out at various TSRs at a wind speed of 8 m/sec. The results revealed an improvement in aerodynamic performance parameters as compared to standard blade configuration. Hameed and Afaq⁵⁷ performed numerical analysis using blade element momentum theory. During the study, the effect of aspect ratio, solidity, and pressure coefficient was focused on. The analysis was carried out with various thickness values (1–5 mm). Blade parameters were optimized to increase structural strength, and to reduce bending stresses as well as deflection. The values of maximum deflection and stress started decreasing from solid hollow cross-section up to a wall thickness of 2 mm; however, beyond that value, an increasing trend was observed. The maximum C $_{p}$ value was obtained as 0.43 at a solidity of 0.24 and TSR of 4.1. Lee and Lim⁴⁸ performed numerical analysis and wind tunnel testing to study the flow behavior in VAWT. Three different cases with solidity values of 0.4, 0.6, and 0.8 having thickness ratios of 0.15, 0.18, and 0.21, respectively were analyzed. The results revealed that NACA0015 airfoil made a significant difference in the lift and drag force values at various angles of attacks as compared to NACA0018 and NACA0021. Moreover, large chord length and small diameter increased the power performance in the lower TSR range (TSR $\approx 1.2$ ). Moreover, the maximum value of output power was found at TSR of 1.6 with a helical angle of 0 $^{0}$ (straight-bladed) and a pitch angle of $- 2^{0}$ . Howell et al.⁵¹ analyzed small-scale VAWTs by using combined computational and experimental techniques. During the study, the effect of surface roughness on performance characteristics was analyzed. Results showed that below wind speed of 4.31 m/sec, performance degradation was observed by smooth surface finish, however, the performance was improved above this value. The maximum value of C $_{p}$ was obtained as 0.24 at 5.45 m/sec.

Mitchell et al.¹¹³ performed a computational analysis of VAWT to improve the self-starting capability by using a novel vented airfoil. The results showed an improvement in the tangential force at high AoA (>90 $^{0}$ ). Sabaeifard et al.⁵³ studied the effect of airfoil type, the number of blades, solidity on C $_{p}$ value. Transient simulations were performed to observe the variations of C $_{p}$ with rotational velocity. The maximum value of C $_{p}$ was obtained as 0.32 and 0.36 (wind tunnel and CFD simulations, respectively) at TSR of 3.5 from the optimized turbine by using DU 06-W-200 airfoil and solidity of 0.3. Block diagram of wind tunnel testing showing its various components and working sequence is shown in Figure 11.¹¹² Mohammed et al.⁹⁰ also performed numerical simulations using multiple stream tube theory to study the effect of solidity, Reynolds number, and blade profile on the performance characteristics. It was found that low digit symmetric airfoil provides better performance in high blade speed range than high digit symmetric airfoil. A positive slope of output power was observed with an increase in Reynolds number within the blades’ speed range of 1–12 m/sec, however, it remained unchanged with a further increase in Reynolds number. Over a certain range of solidity, the steepened behavior of power performance around the peak was observed for straight-bladed VAWT.⁵⁴ Li et al.¹¹⁴ applied two-dimensional airfoil theory to study the aerodynamic performance of a VAWT with high TSR. Analytical relationships of various design parameters such as pitch angle, AoA, TSR, etc. were developed. Furthermore, a dynamic-pitching method was used to optimize the instantaneous torque as a function of pitch angle. Syawitri et al.¹¹⁵ studied the performance of H-rotor VAWT by using different turbulent flow models. The analysis was carried out for various TSRs ranging from 1.44 to 3.3. In both medium and high TSR ranges, about 3% fewer discrepancies were observed for the transition shear-stress transport model with a compromise of 25% extra computational time.

Figure 11.

Block diagram of wind tunnel testing system. The figure shows the mechanism to capture the airflow by the turbine rotating the shaft. The rpm and torque are measured by the sensors, which can then be used to determine the performance parameters.¹¹²

Gulve et al.³⁸ studied the influence of drag force on turbine efficiency. The blades were modified with a J-type Darrieus turbine. The drag-based turbine was analyzed at various rpm settings (30–97 rpm). The blades were fabricated using a GI sheet with a wooden frame. Results showed that turbine efficiency was decreased by 1.5% due to frictional losses and manufacturing flaws. Cambered airfoil plays a significant role in self-starting at a low TSR range (0–2). Beri et al.⁵⁵ investigated the influence of asymmetric airfoil on self-starting of VAWT by using the moving mesh technique. The blades were designed with NACA2415 airfoil and unsteady flow around the VAWT was investigated. The results were obtained at various TSR values and a great impact of using cambered airfoil on self-starting of VAWT was observed. The torque values were found positive at TSR $\leq$ 1.5. However, a reduction in C $_{p}$ (about 15–20%) was observed as compared to a symmetrical airfoil. The twist angle and curvature of the blade surface also affect the performance parameters. Hussen et al.¹³ carried out a study to evaluate the performance of VAWT under different environmental conditions. Three-dimensional static analysis using straight and twisted blade configurations were performed at different wind velocities ranging from 3–15 m/s. The different blade materials i.e., aluminum, glass fiber, and carbon fiber were used. Results revealed that an average C $_{t}$ can be increased up to 8.75% by using twisted blades with an added advantage of a low-speed startup. A comparative study of straight and helical Darrieus turbines was carried out by Alaimo et al.¹¹⁶. Two-dimensional dynamic analysis was carried out at various TSR and results in the form of lift, drag, and torque coefficients were obtained. Moreover, flow behavior over the rotor blade with respect to azimuth position for a range of revolution was analyzed. Subsequently, 3D static and dynamic analysis were also carried out and results showed an increase in C $_{t}$ (approx. 8–10%) values by using helical blades. Cheng et al.¹¹⁷ also performed a 2D and 3D analysis of similar configurations. The results showed that blades having helicity (about 70 $^{0}$ ) produces less noise and has better aerodynamic performance. The findings also showed the strong effect of TSR on power output due to wake interaction and variation in the AoA. In the azimuth position of 0 $^{0}$ to 180 $^{0}$ , the power output value was increased with TSR due to a higher AoA. Su et al.¹¹⁸ performed experimental testing of VAWT to determine the influence of array configuration. Various arrangements including longitudinal and transversal were considered. An improvement of about 8.2% in maximum C $_{p}$ was found for the transversal arrangement compared to that of an isolated turbine with in-between rotor spacing of 2.4D. Hassanpour and Azadani¹¹⁹ optimized the horizontal and vertical distance between a pair of VAWT. The computational analysis revealed an improvement in the power output (approx. 26.6%) for a pair of wind turbine as compared to a single wind turbine. Peng et al.¹²⁰ performed wind tunnel testing to study the turbulence flow field of VAWT. Various turbulence levels were created and analysis was performed to isolate the effect of aspect ratio, pitch angle, and solidity ratio. The efficiency of VAWT for optimized turbulence was improved from 7.9% to 38% as compared to smooth flow. Aihara et al.¹²¹ studied the effect of the tower and strut on the performance of VAWT. Computational analysis was performed for three different cases and results revealed a decrease in C $_{p}$ by 43% which leads to proper modeling of the strut for accurate prediction of the performance. Scheurich et al.¹²² simulated the wake aerodynamics of both helical and straight bladed turbines using a vorticity transport model. The analysis shows that a rotor with helical blades provides steady torque to the rotating shaft. Moreover, localized perturbations to the aerodynamic loading can be found as a result of interaction between vortices and blades in the wake region therefore, a suitable 3D model is essential for this purpose. Analysis of Darrieus turbines by different techniques is summarized in Table 4.

Table 4.

Summary of selected studies on the straight and helical-bladed Darrieus wind turbines.

S No	Authors (Analysis Technique)	Geometric Parameters	C $_{p}$	Cut-in speed (m/sec)	Comments
1	Kavade and Ghanegaonkar, 2017¹²³ (Exp $^{c}$ )	NACA0021 Diameter = 600 mm Height = 600 mm Chord length = 95 mm	0.21	4.0	A significant contribution of number of blades to performance parameters has been observed. Output power of Darrieus turbine with three blades is 15–20% more as compared to four blades.
2	Andrew Shires, 2013¹²⁴ (Num $^{b}$ and Exp $^{c}$ )	NACA0015 Diameter = 19.5 m Chord = 1.02 m	–	4.3	Aerodynamic analysis of novel V-shaped VAWT has been carried out using multiple streamtube theory. The results revealed a reasonable agreement with measured data.
3	Beri et al., 2011⁵⁵ (CFD $^{a}$ )	NACA2415 Chord = 1 m Radius = 4 m	0.27	4.0	The effect of cambered airfoil on performance parameters was investigated. The self-starting capability was improved, however, a reduction in C $_{p}$ (about 15–20%) was observed as compared to a symmetrical airfoil.
4	Maleki et al., 2021¹²⁵ (CFD $^{a}$ )	NACA0018 S815 Chord = 0.075 m Radius = 0.59 m Height = 1.18 m	0.34	–	The modified turbine resulted in good potential with an improvement of 13.3% in self-starting capability. Moreover, the C $_{p}$ of the novel configuration is 4.5 times greater than that of the S815 airfoil turbine at the TSR of 2.5.
5	Kanyako and Janajreh, 2014¹²⁶ (CFD $^{a}$ )	NACA0015 NACA0018 Chord = 0.4 m Diameter = 2.5 m Height = 3 m	0.35	3.0	Comparative analysis shows that symmetric airfoil blades are not self-starting. Moreover, NACA0018 airfoil resulted in more C $_{p}$ (approx. 5%–6%) at a TSR of 3 than NACA0015 airfoil.
6	Sabaeifard et al., 2012⁵³ (CFD $^{a}$ )	NACA0018 DU 06-W-200	0.36	–	Effect of airfoil type, number of blades, solidity on C $_{p}$ was investigated. The turbine designed with three DU 06-W-200 airfoil blades showed a 9% increase in efficiency as compared to the NACA0018 airfoil. Moreover, the self-starting capability was also improved at a solidity value of 0.35.
7	Ardaneh et al., 2022¹²⁷(Num $^{b}$ )	NACA0021 Chord = 0.3 m Height = 1.15 m Radius = 0.99 m Pitch angle = 0 $^{0}$ , $- 2^{0}$	0.25	–	The modified wind turbine with a pitch angle of $- 2^{0}$ resulted in an improvement of torque coefficient of 13.65% at a TSR of 0.89.
8	Clarke et al., 2014¹²⁸ (CFD $^{a}$ )	DU 06-W-200 Chord = 1.6 m Height = 20 m Diameter = 16 m Aspect ratio = 1.25	0.37	4.4	A large-scale turbine for 100 kW was designed by modelling in QBlade. The software gave results of C $_{p}$ with about 20% overestimation as compared to the experimental data.
9	Divakaran et al., 2021¹²⁹ (CFD $^{a}$ )	NACA0015 Helix Angle = 60 $^{0}$ , 90 $^{0}$ Chord = 210 mm Diameter = 2.7 m	0.40	–	The results revealed better performance parameters for a 60 $^{0}$ helical-bladed turbine as compared to the other designs.
10	Cheng et al., 2017¹¹⁷ (CFD $^{a}$ )	Number of blades = 4 NACA0018 Chord = 0.1 m Radius = 0.21 m Height = 0.54 m	0.24	3.9	The low or high TSR value can lead to a deterioration of the power output of the helical VAWT. Blades having helicity (about 70 $^{0}$ ) produced less noise and the maximum power output obtained at TSR value of 1.8.
11	Scheurich et al., 2010¹²² (CFD $^{a}$ )	NACA0015 Number of blades = 3 C/R = 0.15 Aspect ratio = 20	0.30	4.1	Helical turbine blades deliver steady torque to the rotating shaft. Moreover, localized perturbations to the aerodynamic loading can be found as a result of interaction between vortices and blades in the wake region.

Computational fluid dynamics (CFD).

Numerical analysis (Num).

Experimental testing (Exp).

Findings

Following are some of the findings of Darrieus wind turbines based upon the investigations carried out in the last few years.

Darrieus wind turbines have self-starting issues. Therefore, for low wind speed areas, these may not be a feasible option.³⁸

At low TSR (0.8-1.2), the C $_{p}$ increases with an increase in solidity, however, at high TSR range (2.2–2.6), due to an increase in drag force, C $_{p}$ gets decrease.^53,90

Surface roughness also affects the performance characteristics. At low wind speeds ( $\leq$ 3 m/sec) performance degradation may be observed by smooth surface finish, however, the performance gets improved above this range.⁵¹

Variation of thickness ratio may not impart a significant difference in performance characteristics, however, higher drag force by thick airfoil can degrade the C $_{p}$ value at higher TSRs ( $\geq$ 3).⁴⁸

For a certain wind speed range, with an increase in Reynolds number, output power gets increased however, it remains unchanged with further increase in Reynolds number.^54,90

At high wind speeds (>3 m/sec), low digit symmetric airfoil provides better performance than high digit symmetric airfoil.⁹⁰

Cambered airfoil plays a significant role to improve self-starting capability; however, overall performance may be reduced.⁵⁵

Blade helicity can contribute to improving self-starting capability. However, the maximum C $_{p}$ value of helical VAWTs is less than straight bladed turbines.¹¹⁷

At high wind speeds ( $\geq$ 3 m/sec), these turbines provide better performance parameters. Therefore, these can be a viable option for large-scale projects. Moreover, within the available space, a greater number of similar VAWTs can be installed as compared to HAWTs that results in more power output.¹²⁸

Different design modifications contribute to performance enhancement; therefore, critical analysis is required to explore the optimum geometric parameters. Investigations may be carried out to improve self-starting capability, operation at low wind speeds, starting torque, and further enhancement of efficiency.

Savonius wind turbines

Savonius turbines are drag force-driven turbines with some integral augmentation of rotor performance accessible due to the air flows transversely besides individual vane and mutual coupling of both halves of the rotor. These types of turbines were first invented by a Finnish engineer S.J. Savonius in the year of 1929. The blades of this turbine are simplest and rugged. These turbines are easy to manufacture, last longer in extreme weather conditions, and require less maintenance. Aerodynamic drag force is used for the rotation of blades to produce electrical power. The working principle of the Savonius turbine is shown in Figure 12. The parameter $e$ indicates the overlap ratio, which is estimated with respect to the rotor diameter. Moreover, $D$ represents the total diameter of the turbine.¹² The rotor consists of two or three half cylinders/blades facing opposite installed in single or multi-stage compositions. Blades are installed in such a way that they almost form an S-shape. Each part of the blade’s cup gets the wind, turns the shaft, and subsequently brings the opposite cup into the wind flow. This process is repeated till the wind is available in the vicinity. Hence the rotation of the shaft is used to drive a small generator or produce electricity. Normal and tangential forces are generated as a result of wind strikes on the turbine blades.¹³⁰ The main features of these types of wind turbines are self-starting capability at low wind velocity and high starting torque. Typically, the C $_{p}$ of these types of turbines is 0.15–0.25 which is less as compared to other types of VAWTs. Therefore, these turbines are mostly used for wind speed measuring instruments or low power household applications.^77,83 In recent years, the popularity of such turbines has gradually increased. The reason for the increase in demand for such turbines is to fulfill electricity requirements, particularly in urban areas.⁷³ Savonius turbines are not stable and the design is also not strong enough to handle the heavyweight of turbine blades hence, large-scale models cannot be constructed. However, these turbines are more effective at low wind speed than high wind speeds and they can operate well in turbulent winds. The drag-based turbine should have the capability to harness energy from the non-directional wind at low cut-in speed, to makes it a better choice for various urban applications.²⁸

Figure 12.

Working principle of Savonius turbine, a drag force-based turbine. Blades rotate under the action of drag force that results in electricity generation. (Reprint with written permission from Elsevier).⁸⁶

Performance analysis of Savonius wind turbines

In the last few decades, efforts have been made to improve the efficiency of these turbines that include blade shape, rotor design modifications, number of blades, and overlap ratio.^131–133 Performance analysis of Savonius turbines has been carried out to explore their efficient design configurations. Torres et al.¹³⁴ performed numerical analysis of Savonius VAWT to improve its aerodynamic performance. The Kriging technique was used to optimize the design parameters followed by a response surface methodology. The optimized parameters resulted in a maximum C $_{p}$ of 0.21 at a wind speed of 12 m/sec. Savonius turbines having two and three-blade configurations have been analyzed to study the effect of overlap ratio. The results revealed better performance characteristics with an optimum overlap ratio at low wind speeds.^135,136 Additionally, the presence of curved guide vanes or twisted drag blades can enhance performance characteristics.^137,138 Comparison of two and three-blade configurations based on experimental testing showed that two-bladed configurations had better C $_{p}$ values.¹³⁹ Dhote et al.⁷³ designed the S-shaped Savonius turbines to study the electricity economics of household applications. During the analysis, the average monthly electricity consumption was calculated and found to be 328.69 kWh/month. Aerodynamics analysis was carried out and the maximum drag coefficient was obtained as 1.50. Subsequently, structural analysis was also carried out and total deformation was found to be 2.11 mm. During the research, it was concluded that Savonius VAWT is a good option as it operates well in low wind conditions (approx. 2–4 m/sec). CFD analysis reported that favorable variations of velocity/pressure across the rotor cause high performance.¹⁴⁰ Shah et al.¹⁷ developed a MATLAB algorithm to compare the rotational performances of four types of Savonius rotor blades namely curved, straight, airfoil, and twisted. Results revealed that twisted blade design can provide good performance among all while the straight blade is found to be less effective. The economic performance analysis of said turbine was also carried out and the annual electricity cost per saving ratio was found about $$ 847$ .

Bouzaher¹⁴¹ studied the performance parameters of Savonius VAWT by using flexible blades. The variation of torque during expansion and contraction of blades during the complete cycle resulted in an improvement of torque coefficient by 112% and 6% for the highest and lowest expansion amplitude, respectively.

Dunn et al.²⁸ investigated the performance of roof-mounted Savonius turbine to determine its feasibility. The S1223 airfoil blades were designed with and without enclosure configurations and rotor-shrouds were analyzed to optimize the efficiency. Experimental analysis of the scaled-down model was carried out based upon the wind data obtained from the wind simulation program. Additionally, the rotational velocity and voltage variations were measured using rpm meter and step generator, respectively. The results showed that the turbine with the enclosure had about 73% more power than without an enclosure. The C $_{p}$ value for split Savonius without enclosure, with 90 $^{0}$ enclosure and four-bladed turbines with 90 $^{0}$ enclosure were found as 0.102, 0.237, and 0.222, respectively. Ali¹³⁹ performed a comparative study of the Savonius turbine with two and three-blade configurations. The maximum C $_{t}$ value of turbine with two blades was found more. The most probable reason for such difference is that with an increase in the number of blades, drag surfaces also increase which results in reverse torque. Moreover, the net torque generated by the rotation of turbine blades is decreased.

Aboujaoude et al.¹⁴² modified the Savonius wind turbine by using the optimized axisymmetric deflectors. The results revealed an improvement of C $_{p}$ along with starting torque by 30% over a broad range of wind speeds.

Kamoji et al.¹⁴³ studied the influence of aspect ratio on performance parameters. They examined the turbines with different aspect ratios between 0 and 1. The design with an aspect ratio of 0.7, blade arc angle of 124 $^{0}$ , and an overlap ratio of 0.0 had the highest C $_{p}$ . Overlap ratio also affects performance characteristics as it can augment the overall output power. Akwa et al.¹⁴⁴ performed CFD analysis to study the effect of overlap ratio. The maximum C $_{p}$ value was obtained at buckets overlap of 0.15 at TSR of 1.25. Besides these analyses, studies have also been carried out to analyze the effect of rotor shape, end plates, supporting accessories, rotor shaft, bucket spacing, and rotor stages, etc.^145,146 Analysis of Savonius wind turbines by different techniques is summarized in Table 5. It has been concluded that an analytical model is required to predict overall performance characteristics.¹⁴⁵ Studies have also been carried out to establish the relationship between TSR and overlap ratio.¹³⁷ Comparison of performance characteristics of conventional and helical blades Savonius rotor,¹⁴⁷ aerodynamic performance evaluation due to the effect of aspect ratio, numerical simulations, CFD modeling, and analysis.^148–150

Table 5.

Summary of selected studies on Savonius wind turbines.

S No	Authors (Analysis Technique)	Geometric Parameters	C $_{p}$	Cut-in speed (m/sec)	Comments
1	Dunn et al., 2013²⁸ (Exp $^{c}$ )	S1223 airfoil	0.23	2.5	Turbine with the enclosure can produce more power (approx. 60–80%) than without an enclosure. Moreover, the C $_{p}$ value of Savonius turbine with 90 $^{0}$ enclosure can be 6–20% more as compared to split or without enclosure.
2	Kavade and Ghanegaonkar, 2017¹²³ (Exp $^{c}$ )	Rotor diameter = 600 mm Height = 600 mm Shaft diameter = 380 mm	0.19	2.1	Number of blades has significant contribution to performance parameters. Output power of Savonius turbine having two blades is 25–45% more as compared to three or four blades.
3	Saad et al., 2021¹⁵¹ (CFD $^{a}$ )	Aspect ratio = 1-4 Number of stages = 1-4 Twist angle = 0 $^{0}$ -180 $^{0}$	0.261	–	The rotors were designed for different aspect ratios along with multi-stages and the maximum C $_{p}$ of 0.261 was obtained for the two-stage configuration.
4	Ferrari et al., 2016¹⁵² (Num $^{b}$ and Exp $^{c}$ )	Diameter = 0.90 m Overlap ratio = 0.2 End plates diameter = 1 m	0.20	–	With increase in aspect ratio, moment coefficient (C $_{m}$ ) was increased up to 0.498. The maximum C $_{p}$ was obtained as 0.20 at optimized aspect ratio of 1.1 and TSR of 0.8.
5	Al-Ghriybah et al. 2021¹⁵³ (CFD $^{a}$ )	Diameter = 0.100–0.188 m Spacing b/w inner blades = 0.005–0.02 m Number of inner blades = 2 Angle of inner blades = 100 $^{0}$ –180 $^{0}$	0.175	–	The results revealed improvement in C $_{p}$ by 32.9% at TSR of 0.7, inner blade spacing of 0.005 m, and inner blade angle of 100 $^{0}$ . Furthermore, the maximum value of C $_{t}$ was obtained as 0.435 at a TSR of 0.4 and inner spacing of 0.005 m.
6	Tjahjana et al. 2021¹⁵⁴ (Exp $^{c}$ )	Diameter = 400 mm Height = 400 mm Number of blades = 2	0.138	–	Wind turbine with an overlap ratio of 10% shows the better performance and the maximum C $_{p}$ was obtained for the slotted gap of 3 mm at Reynolds number of $1.44 \times 10^{4}$ .
7	Akwa et al., 2012¹⁴⁶ (Exp $^{c}$ )	Semicircular profile bucket	0.17	2.8	With increase in overlap ratio, C $_{p}$ and C $_{t}$ values were increased. The maximum C $_{p}$ and C $_{t}$ value was obtained as 0.17 and 0.26 at overlap ratio of 0.15.
8	Ali, 2013¹³⁹ (Exp $^{c}$ )	Diameter = 100 mm Height = 200 mm Thickness = 0.079 mm End plates diameter = 210 mm End plates thickness = 1.59 mm	0.21	2.3	Savonius turbine having two blades produced 19% more C $_{p}$ value at low TSR of 0.8 as compared to three-bladed turbine.
9	Kamoji, 2009¹⁴³ (Exp $^{c}$ )	Overlap ratio = 0.0 AR = 0.77 Arc angle = 135 $^{0}$ Shape factor = 0.2 End plate parameter = 1.33	0.22	2.9	Turbines with different low aspect ratios ranging from 0 to 1 were analyzed. The maximum C $_{p}$ was obtained at aspect ratio of 0.7. The reason is that at low aspect ratio, an increase in Reynolds number improves the overall performance.

Computational fluid dynamics (CFD).

Numerical analysis (Num).

Experimental testing (Exp).

Findings

Following are some of the findings of Savonius turbines based upon the investigations carried out in the last few years.

Savonius VAWT is a good option in low wind speed operations (2–4 m/sec) that extends their applications for measuring instruments or household applications.⁷³

The blades of this turbine are simple, easy to manufacture, last longer in extreme weather conditions, and require less maintenance.⁸²

Self-starting capability of these turbines is good as these can start at low wind speeds (2–3 m/sec).^73,123

These have high starting torque that allows them to generate power at low TSR.^133,135

The C $_{p}$ of these types of turbines is less (0.15–0.25) as compared to other types of VAWTs.¹⁵⁵

For small-scale applications, these can be an economical option.²⁸

Turbines having twisted blades have better performance characteristics (6–10%) as compared to curved, straight, or airfoil-based rotors.¹⁷

Enclosure contributes to improve overall efficiency of Savonius wind turbine. The analysis revealed that the turbine with the enclosure installed at 90 $^{0}$ to wind direction has the capability to produce more power than without an enclosure.²⁸

Number of blades can affect the performance characteristics. Turbines having two blades have better C $_{t}$ value as compared to three or more blades due to more drag produces that leads to reverse torque.¹²³

At the lower range of aspect ratio (0.5) maximum C $_{p}$ increases with an increase in aspect ratio, however, after certain limits (0.5–0.7), a decline may be observed.^143,152

Turbine designed with optimum overlap ratio (approx. 15.0%–16.8%) can augment the overall output power.^146,156

The overall performance of these turbines is less as compared to other conventional VAWTs. Therefore, further investigations in the area of design optimizations are required to critically analyze the shortcomings of existing designs.

The characteristics i.e., self-starting capability, design simplicity, high starting torque, enclosure, and shroud can be embedded with the Darrieus wind turbine to design a modified hybrid VAWT.

Hybrid vertical axis wind turbines

To overcome the shortcomings of conventional designs, an innovative technical approach is sought in the design of VAWT. The advantages of both Darrieus and Savonius turbines can be combined to design a hybrid VAWT. This turbine is based on both lift and drag forces to get the maximum benefits at various wind speed ranges. The hybrid turbine may be designed by combining straight-bladed Darrieus with Savonius, Helical Darrieus with Savonius, or Helical Savonius with Helical Darrieus. Generally, Darrieus or lift-based turbine is attached at the outer side to extract maximum wind energy, whereas Savonius or drag-based turbine is installed at the inner side close to the rotating shaft to give self-start at low wind speed. Furthermore, some hybrid turbines have also been designed with tandem arrangements depending upon the type of turbine to get the maximum advantages.¹⁵⁷ A two-dimensional top view of hybrid VAWT is shown in Figure 13.

Figure 13.

Two-dimensional view of hybrid VAWT; a lift-drag force-based turbine. The rotor rotates under the combined effect of lift and drag forces to get advantages of both types of turbines.⁸³

Why Hybrid VAWT?

The design and development of VAWT is a stimulating field and involves state-of-the-art engineering ideas to achieve promising wind energy outcomes. It requires a systematic approach, encompassing efficient design, and detailed aerodynamic analysis such as CFD, wind tunnel testing, and empirical engineering design tools. Previous studies of conventional VAWTs have investigated the improvement in one of the performance parameters by compromising the other i.e., efficiency of the designed turbine was declined as compared to conventional Darrieus VAWT, however, the self-starting capability was improved. The design and development of hybrid VAWT is a modern innovative idea that combines the features of both the Darrieus and Savonius VAWT. It exhibits the potential to overcome the shortfalls of conventional designs. A pictorial view representing salient features of hybrid VAWT is shown in Figure 14. The hybrid VAWT is an active area of research in the field of engineering. A limited number of researchers have endeavored to present the conceptual and preliminary design of hybrid VAWTs. Several design configurations have been studied by combining conventional turbines. Most of the representative designs are still in the development phase and only a few experimental prototypes have been operationally verified. This area of research needs further exploration and improvement. Further academic research is required on critical aspects such as cut-in speed, self-starting, efficiency, performance analysis, and structural reliability, etc. The hybrid VAWT can prove to have a force multiplier effect by providing a safe, effective, and cost-efficient power-producing solution. Moreover, hybrid VAWT has the potential to perform well in gusty wind areas, self-start at low wind speeds, and produce more power as compared to conventional wind turbines.^83,158

Figure 14.

A hybrid VAWT is an attractive alternative for many household and commercial applications because of its distinctive features such as good starting torque, self-starting capability, good efficiency, low cut-in speed, operate well under steady as well as turbulent flow areas, and over a wind range of wind speeds.

Performance analysis of Hybrid VAWTs

Hybrid VAWTs are considered to be a viable option in urban areas. Research in designs of hybrid VAWTs has been carried out by various researchers to address the deployment in industrial setup and to enhance operational capability.⁸⁴ In this regard, prospects have also been documented for the Research and Development of VAWTs.¹⁵⁹ Some innovations in designs of VAWT i.e. power window,¹⁶⁰ dual VAWT,¹⁶¹ and Variable-Geometry-Oval-Trajectory Darrieus turbines¹⁶² have been analyzed. The hybrid turbine can be a viable solution in urban areas to extract power from unpredictable weather conditions. Comparison of lift and drag-based VAWTs show that the C $_{p}$ of a lift-based turbine is higher than the counterpart, however, one of the drawbacks of such turbines is low starting torque due to which these cannot be self-started at low wind speeds. On the other hand, drag-based turbines have a good self-starting capability. Hybrid VAWTs have some challenges i.e., design optimization of a more complex system, vortex shedding, and relatively lower performance at high TSR values due to the generation of drag forces by the Savonius turbine. However, modified hybrid VAWT can address the shortcomings of conventional VAWT, i.e., high C $_{p}$ at a higher TSR range and self-starting characteristics. To improve self-starting capability, primary torque, and to obtain an extended operational range, several investigations have been carried out.^163–167 The idea was to explore the design configurations to obtain optimum design and performance parameters.

Ahmad et al.¹⁵⁵ proposed a novel design configuration of hybrid VAWT. The design parameters of symmetric and cambered airfoil-based double-Darrieus hybrid VAWT were optimized using the DOE. The maximum C $_{p}$ for the optimized turbine was obtained as 0.49 at a TSR of 3.

Kou et al.¹⁶⁸ performed experimental testing to investigate the self-starting capability and output power of hybrid VAWT. The turbine was designed with an H-blade rotor consisting of NACA0012 airfoil blades mounted on the vertical shaft separated by 120 $^{0}$ and a Savonius turbine consisting of two semi-circular cylindrical blades attached at 90 $^{0}$ to each other. Experiments were performed at various wind speeds ranging from 0 to 12 m/sec. The results revealed that the designed turbine started at 1.4 m/sec (3.3 m/sec before Darrieus VAWT). Moreover, hybrid turbines produced about 11.8% more power than conventional straight-bladed VAWT designed with similar configurations. Thus, features of both lift and drag-based turbines were obtained that led to better wind energy utilization. Kavade and Ghanegaonkar¹²³ studied the performance characteristics of hybrid VAWT by keeping the same aspect ratio. Experimental testing of turbines with two, three, and four blades were carried out. At low TSR, a turbine with three blades resulted in a better C $_{p}$ value. Additionally, comparative analysis of Darrieus, Savonius, and hybrid VAWT was also carried out to study the effect of operational and geometric parameters. The results revealed that the hybrid turbine gave a higher value of C $_{p}$ of 0.23 as compared to Darrieus (0.21) and Savonius (0.19) turbine. Tripathi et al.¹⁶⁹ combined Savonius and Darrieus VAWT to overcome the drawbacks of individual turbines. The numerical analysis was performed at various TSRs with an average wind speed of 9 m/sec. The results revealed an improvement of C $_{p}$ by 8.39% and 5.66% at TSRs of 1.4 and 1.3, respectively. Computational analysis of a hybrid VAWT with a two-bladed Savonius and a three-bladed Darrieus turbine was carried out by Hosseini et al.⁷⁸ According to the data, the conventional Darrieus turbine’s highest C $_{p}$ value was 0.48 at a TSR of 2.5. The modified hybrid turbine, however, demonstrated the ability to start on its own with a maximum C $_{p}$ value of 0.41 at the same TSR; however, the conventional turbine suffered with start-up torque constraints. The hybrid designs showed improvements in operational range and turbine efficiency which help the energy sustainability expectations.

Sharma et al.¹⁵⁸ investigated the effect of overlap ratio on performance of hybrid VAWT. The turbine consisting of three blades was analyzed by varying overlap ratio of 10.8-25.8%. The maximum C $_{p}$ of 0.53 was obtained at an optimum value of overlap ratio (16.8%) and TSR of 0.604. The analysis showed that with an increase in overlap ratio, the resultant C $_{p}$ was declined, however, the starting torque value was improved. The C $_{p}$ of the turbine at a low TSR range with three blades was found to be more than others. Investigation revealed that hybrid VAWT has the potential to produce better efficiency as compared to conventional VAWTs. Ghosh et al.¹⁷⁰ performed CFD analysis of a similar hybrid turbine to predict the maximum C $_{p}$ value. The variation in the results was found between 0.18 and 0.53.

Sun et al.⁸³ investigated aerodynamic performance characteristics of the lift-drag-based turbine using experimental and computational techniques. The study was focused to analyze the effect of distance between drag blade and central rotating shaft. Four different models were prepared by installing a drag type blade at a distance of 0, 50, 100, and 150 mm respectively, away from the spindle. The results showed that the turbine model having a drag blade close to the rotational center had more C $_{p}$ (about 0.33) as compared to the others. Furthermore, a significant improvement was observed in the starting torque value. The C $_{p}$ value obtained was less as compared to the conventional Darrieus VAWT. The most probable reason for the poor C $_{p}$ for the model far away from the center is the low-pressure difference on lift type blade than the conventional turbine. Additionally, the wake phenomenon and velocity field in the vicinity can also affect the performance characteristics of hybrid VAWT.

To analyze the effect of the shroud, hybrid VAWT was designed and experimentally tested by Machlin.¹² The wind velocity data was taken by using a wind anemometer. Based upon the available wind data, the hybrid turbine was designed with lift and drag-based components. The shroud in the drag-based part was also incorporated for protection as well as to avoid unwanted disturbances. The design optimization was carried out based on the characteristics of the urban environment and wind pattern. The project was accomplished by using several blade designs considering the efficiency factor, cost, noise, reliability, vibrations, aesthetics, and ease of manufacturing. Results revealed that the designed turbine impart an improvement in performance characteristics.

The starting torque of Savonius wind turbines is usually high while the peak C $_{p}$ value is very low. On the other hand, the Darrieus VAWT’s static torque output is insufficient for starting. A hybrid VAWT that incorporates the Savonius rotor into a Darrieus VAWT with the same axis has been developed to effectively utilize the advantages of both types of turbines.¹⁷¹ Analysis of hybrid wind turbines by various researchers employing different techniques is summarized in Table 6.

Table 6.

Summary of selected studies on Hybrid wind turbines.

S No	Authors (Analysis Technique)	Geometric Parameters	C $_{p}$	Cut-in speed (m/sec)	Comments

1	Ahmad et al., 2022¹⁵⁵ (CFD $^{a}$ )	NACA0018 DU 06-W-200 Rotor diameter = 3.854 m Rotor height = 3.12 m Chord = 0.546 m	0.491	2.81	The performance parameters of double-Darrieus hybrid VAWT with symmetric and cambered airfoil were improved by 10–14% as compared to conventional VAWTs.
2	Kavade and Ghanegaonkar, 2017¹²³ (Exp $^{c}$ )	NACA0021 Rotor diameter = 600 mm Rotor height = 600 mm Chord = 95 mm Savonius diameter = 500 mm Savonius heights = 600 mm	0.23	3.0	The C $_{p}$ value of hybrid turbine with three Savonius blades and three airfoil blades was about 6–10% higher than two or four blades combinations.
3	Sun et al. 2016⁸³ (CFD $^{a}$ )	NACA0018 Diameter = 3 m Chord = 0.5 m	0.33	3.95	Turbine having drag-blades installed close to central rotating shaft had more C $_{p}$ (about 0.33) as compared to the others. The reason for poor performance in the case of blade installed away from the center was the low-pressure difference on lift type blade.
4	Abdelsalam et al., 2021¹⁷² (CFD $^{a}$ )	NACA0021 Rotor diameter = 600–1200 mm Rotor height = 1000 mm Chord = 98 mm	0.49	–	Hybrid wind turbine with radius ratio of 0.43 and attachment angle of 30 $^{0}$ resulted in a best performance.
5	Hosseini and Goudarzi, 2019⁷⁸ (CFD $^{a}$ )	NACA0021 Aspect ratio = 1.5 Solidity = 0.5 End plates dia = $1.1 \times$ Bach Rotor diameter = 100 mm Bach arc angle = 124 $^{0}$ Material = Al 1060 alloy	0.41	4.15	The modified hybrid turbine exhibited the self-starting capability (3-3.5 m/sec) with a maximum C $_{p}$ value of 0.41 at TSR of 2.5.
6	Hosseini and Goudarzi, 2018¹⁵⁷ (CFD $^{a}$ )	Bach height = 0.525 m Bach diameter = 0.75 m Darrieus height = 4.50 m Darrieus diameter = 3 m NACA0021 Hybrid height = 4.50 m Hybrid diameter = 3.0 m	0.35	3.85	By incorporating a switch mechanism in hybrid VAWT, self-starting speed along with C $_{p}$ value was improved about 0.5 m/sec and 5%, respectively.
7	Asadi and Hassanzadeh, 2021¹⁷³ (CFD $^{a}$ )	NACA0018 Number of blades = 2 Attachment angle = 0 $^{0}$ , 45 $^{0}$ , 90 $^{0}$	0.375	–	Among the hybrid and non-hybrid configurations, the hybrid rotor with an attachment angle of 45 $^{0}$ resulted in maximum C $_{p}$ at TSRs of 1.5 and 2.5.
8	Andrews et al., 2019¹⁷⁴ (CFD $^{a}$ and Exp $^{c}$ )	Helical height = 0.8 m Helical diameter = 0.5 m Helical chord = 0.15 m Savonius height = 0.25 m Savonius diameter = 0.5 m	0.12	4.2	Very low value (0.12) of C $_{p}$ was obtained with the helical-Savonius design. The most probable reason was frictional, environmental or mechanical losses.
9	Gupta and Biswas, 2011¹³⁶ (CFD $^{a}$ )	Width = 12 mm Thickness = 3 mm Diameter = 8 cm Height = 10 cm	0.36	4.1	Two-dimensional analysis using CFD simulations was carried out at various overlap ratios. The Coanda flow was occurred that migrated towards upstream side and hence augmented power was increased (approx. 5%).

Computational fluid dynamics (CFD).

Numerical analysis (Num).

Experimental testing (Exp).

Findings

Following are some of the findings based upon investigations carried out by various researchers.

The hybrid VAWT is a viable solution in urban areas to extract power from unpredictable weather conditions.^11,13,116

The shortcomings of conventional VAWTs i.e., low C $_{p}$ at a higher TSR range (>3) and self-starting characteristics can be addressed by designing hybrid VAWT.^155,163

Hybrid VAWT designed with same geometric configurations as of individual conventional turbines can provide overall better performance characteristics; however, a small compromise of C $_{p}$ may be observed.^78,157

At higher TSRs (>3), the efficiency of hybrid VAWT is less than straight-bladed turbine due to drag induced by the Savonius turbine. Moreover, the Savonius part imposes a negative torque on the hybrid turbine and acts as a brake to reduce the power produced by the Darrieus rotor.⁷⁸

Overlap ratio has a significant contribution in performance characteristics. With an increase in overlap ratio, the resultant C $_{p}$ gets declined; however, the starting torque value is improved. A turbine having an optimum overlap ratio (approx. 15.0%-16.8%) can produce better output power.¹⁷⁵

Number of blades also affects the overall performance. The C $_{p}$ value of the turbine at a low TSR range with three blades was found to be more than others.¹²³

The turbine model having Savonius blades installed close to the central rotating shaft can produce more C $_{p}$ as compared to the blades at far away distance.⁸³

Shroud installed on conventional Savonius turbine or modified hybrid turbine can improve performance parameters as it can avoid unwanted disturbances.¹²

The advantages of both Darrieus and Savonius can be obtained by designing an efficient hybrid VAWT. This combined turbine has improved the performance characteristics but the overall efficiency is still less than conventional HAWTs.⁷⁸

Further investigations in the area of design optimizations are required to critically analyze the shortcomings of hybrid VAWTs.

Analysis and discussion

Analysis of various configurations of HAWT and VAWT showed that both types of turbines have unique features. HAWTs are large power-producing turbines; however, there are some drawbacks related to design, structure, environmental conditions, installation, maintenance, and noise. On the other hand, VAWTs are low-power-producing turbines and are mostly used for small-scale projects. However, owing to certain features such as low cost, noise, ease of maintenance and operation in both smooth and gusty wind conditions, these have been widely accepted for most applications. The cut-in speed and C $_{p}$ are the most important parameters that determine the overall performance characteristics of a turbine. The maximum C $_{p}$ and cut-in speed for various design configurations are shown in Figure 15. The figure shows that HAWTs have self-starting capability at low cut-in speeds. The maximum C $_{p}$ values of some of the turbines were also good. The analysis of VAWTs shows that Savonius turbines have good starting ability; however, the maximum C $_{p}$ values are very low. H-Rotor has good C $_{p}$ ; however, its cut-in speed is high, which is why they lie on the right side of the figure. Helical turbines show some improvement in cut-in speed values, but their efficiency has been compromised. Lastly, hybrid turbines showed improvements in both parameters. Hybrid VAWTs can fall in the favorable regions. Further investigations are required to improve the self-starting capability and maximum C $_{p}$ value to extend the utilization of VAWTs in large-scale projects.

Figure 15.

Analysis of maximum C $_{p}$ vs cut-in speed, HAWTs have shown overall good performance, Savonius turbines (on the left bottom side) show low cut-in speed, however, their maximum C $_{p}$ is also less, H-Rotor turbines (lies on the right extreme side) show good C $_{p}$ results but their cut-in speed is also high. Helical and Hybrid fall in-between regions show improvement in both the parameters. Any turbine that lies close to the favorable region will have overall good performance characteristics.

Conclusion

Wind turbine design and development have been at the forefront of engineering innovation for the past few decades. Wind turbine technology is a stimulating field that involves state-of-the-art engineering ideas to achieve promising energy efficiency. It requires a systematic approach, encompassing efficient design and detailed aerodynamic analysis, such as computational fluid dynamics, experimental testing, and empirical engineering design tools. This review paper highlights some important findings of vertical axis wind turbines from published papers, technical reports, and the manufacturer’s literature. Some ongoing development and commercial activities in the field of wind turbines are also highlighted in this paper.

HAWTs are highly developed and used in steady wind conditions where a constant stream and appropriate direction of the wind are available. Vertical axis wind turbines have certain advantages such as omnidirectional, easy maintenance, independence of yaw mechanism, operation in turbulent flow and low wind speed areas, high operating wind speed limits, and low noise signature. Compared to Darrieus turbines with fixed blade types, Savonius turbines can start up quickly, however, their power coefficient is 0.12–0.25, which is less than that of other types of VAWTs. The low C $_{p}$ values restrict their utilization for wind speed measuring instruments or household applications. Hybrid VAWT is a combination of Darrieus and Savonius wind turbine to overcome self-starting and efficiency problems. This design is considered a viable solution for extracting power from complicated weather conditions in urban areas. The studies show that the conventional Darrieus turbine gives a maximum C $_{p}$ value of 0.3–0.5 at various TSRs; however, the modified hybrid turbines exhibit a self-starting capability with a maximum C $_{p}$ value of 0.22–0.41.

The design and development of hybrid vertical axis wind turbine is an active area of research in the field of engineering. This modern concept of high-performance and state-of-the-art designs can render promising features. Several design configurations have been studied by combining conventional turbines. Based on studies and state-of-the-art information, it can be concluded that a hybrid VAWT is an attractive choice for the present circumstances. Research in this area is crucial for making it an affordable, viable, and dependable power solution for off-grid sites and various domestic and commercial applications. It was also observed that vast opportunities for wind turbine applications are in urban areas; however, further academic research is required on critical aspects such as design improvement for aerodynamic performance, cost reduction, and resource assessment for urban operating conditions.

Scope of further studies

The design and development of wind turbines is an active area of research in the field of engineering. The overall efficiency of the VAWT is still less than that of the conventional HAWT. From the perspective of the future growth of VAWTs, attention may be given to explore ways to improve the power coefficient and self-starting capability. Hybrid VAWTs have the potential to be improved by incorporating the features of existing conventional VAWTs. In this regard, the double-Darrieus hybrid VAWT concept can be explored by replacing the Savonius turbine with a cambered airfoil blade at the inner side, and optimization of design parameters such as the distance from the central rotating shaft and pitch angle. The conceptual design of an innovative configuration of a Double-Darrieus hybrid VAWT is shown in Figure 16.

Figure 16.

A conceptual design of an innovative configuration of double-Darrieus hybrid VAWT. In order to address the shortcomings of the existing hybrid VAWT, the Savonius turbine has been replaced with a cambered airfoil-based Darrieus turbine.

Footnotes

Data Availability

The data that supports the findings of this study are available within the article.

Declaration of conflicting interests

The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The authors received no financial support for the research, authorship, and/or publication of this article.

ORCID iD

Muhammad Ahmad

Muhammad Ahmad holds a master's degree in aerospace engineering. His area of research is renewable energy focusing on wind energy, design optimization, computational fluid dynamics, and experimental testing of wind turbine.

Aamer Shahzad holds a doctoral degree in mechanical engineering. He has been involved in the research of bio-inspired and bio-mimicking flapping wings, with an emphasis on improving aerodynamic performance in hovering flight. His research interests include flapping wing propulsion, vertical axis wind turbines, bladeless technologies and VTOL UAVs.

M. Nafees Mumtaz Qadri holds a doctoral degree in mechanical engineering. His area of research is computational & experimental fluid dynamics, flapping wing aerodynamics, FSI and flow control.

References

Wang

, et al. Distribution parameter-determining method comparison for airborne wind energy potential assessment in the eastern coastal area of china. Sust Energy Technol Assessm 2022; 52: 102161.

Kalair

Abas

Hasan

, et al. water, energy and food nexus of indus water treaty: Water governance. Water-Energy Nexus 2019; 2: 10–24.

Saidur

Rahim

Islam

, et al. Environmental impact of wind energy. Renew Sust Energy Rev 2011; 15: 2423–2430.

Bashir

Khan

. Renewable energy sources: A study focused on wind energy. In Mitigating Climate Change, pages 99–118. Springer, 2022.

Şahin

. Progress and recent trends in wind energy. Prog Energy Combust Sci 2004; 30: 501–543.

Islam

Mekhilef

Saidur

. Progress and recent trends of wind energy technology. Renew Sust Energy Rev 2013; 21: 456–468.

Thresher

Robinsion

Veers

. Wind energy technology: current status and r&d future. Technical report, National Renewable Energy Lab.(NREL), Golden, CO (United States), 2008.

Amano

. Review of wind turbine research in 21st century. J Energy Resour Technol 2017; 139: 2–9.

Lian

Zhang

, et al. A review on recent sizing methodologies of hybrid renewable energy systems. Energy Convers Manage 2019; 199: 112027.

10.

Adem Çakmakçı

Hüner

. Evaluation of wind energy potential: a case study. Energy Sourc, Part A: Recove, Utiliza Environ Effects 2022; 44: 834–852.

11.

Kumar

Raahemifar

Fung

. A critical review of vertical axis wind turbines for urban applications. Renew Sust Energy Rev 2018; 89: 281–291.

12.

Machlin

. Design of an alternative hybrid vertical axis wind turbine. 2014.

13.

Hussen

MDS

Rambabu

Ramji

, et al. Design and analysis of vertical axis wind turbine rotors. Int J Recent Technol Mech Electr Eng 2015; 2: 54–62.

14.

Toja-Silva

Colmenar-Santos

Castro-Gil

. Urban wind energy exploitation systems: Behaviour under multidirectional flow conditions-opportunities and challenges. Renew Sust Energy Rev 2013; 24: 364–378.

15.

Singh

Rizwan

Malik

, et al. Wind energy scenario, success and initiatives towards renewable energy in india-a review. Energies 2022; 15: 2291.

16.

Jung

Schindler

. On the influence of wind speed model resolution on the global technical wind energy potential. Renew Sust Energy Rev 2022; 156: 112001.

17.

Shah

Kumar

Raahemifar

, et al. Design, modeling and economic performance of a vertical axis wind turbine. Energy Reports 2018; 4: 619–623.

18.

Nazir

Mahdi

Bilal

, et al. Environmental impact and pollution-related challenges of renewable wind energy paradigm–a review. Sci Total Environ 2019; 683: 436–444.

19.

Aized

Ilahi

Qureshi

, et al. Potential analysis of kinetic energy harvesting system from national highways of pakistan. Pakistan J Eng Appl Sci 2017; 21: 1–7.

20.

Khan

Mehmood

Shahzad

, et al. Evaluation of wind energy potential alongside motorways of pakistan. Asian J Appl Sci Eng 2014; 3: 375–381.

21.

Charabi

Abdul-Wahab

Al-Mahruqi

, et al. The potential estimation and cost analysis of wind energy production in oman. Environ Dev Sust 2022; 24: 5917–5937.

22.

Mukulo

Ngaruiya

Kamau

. Determination of wind energy potential in the mwingi-kitui plateau of kenya. Renew Energy 2014; 63: 18–22.

23.

Bahaj

Myers

James

PAB

. Urban energy generation: Influence of micro-wind turbine output on electricity consumption in buildings. Energy Build 2007; 39: 154–165.

24.

Emejeamara

Tomlin

Millward-Hopkins

. Urban wind: Characterisation of useful gust and energy capture. Renew Energy 2015; 81: 162–172.

25.

Roth

. Review of atmospheric turbulence over cities. Q J R Meteorol Soc 2000; 126: 941–990.

26.

Irfan

Zhao

Z-Y

Mukeshimana

, et al. Wind energy development in south asia: Status, potential and policies. In 2019 2nd International Conference on Computing, Mathematics and Engineering Technologies (iCoMET) ; 1–6: IEEE: 30-31 January 2019.

27.

Valipour

Singh

. Global experiences on wastewater irrigation: challenges and prospects. In Balanced urban development: options and strategies for liveable cities, pages 289–327. Springer, Cham, 2016.

28.

Dunn

Sarkar

Deisadze

. Vertical axis wind turbine evaluation and design. 2013.

29.

Heagle

ALB

Naterer

Pope

. Small wind turbine energy policies for residential and small business usage in ontario, canada. Energy Policy 2011; 39: 1988–1999.

30.

Grieser

Sunak

Madlener

. Economics of small wind turbines in urban settings: An empirical investigation for germany. Renew Energy 2015; 78: 334–350.

31.

Shu

Chen

. Performance assessment of tall building-integrated wind turbines for power generation. Appl Energy 2016; 165: 777–788.

32.

Yang

A-S

Y-M

Wen

C-Y

, et al. Estimation of wind power generation in dense urban area. Appl Energy 2016; 171: 213–230.

33.

Ishugah

Wang

, et al. Advances in wind energy resource exploitation in urban environment: A review. Renew Sust Energy Rev 2014; 37: 613–626.

34.

Bianchi

Mantz

De Battista

. The wind and wind turbines. Wind Turbine Control Systems: Principles, Modelling and Gain Scheduling Design, pages 7–28, 2007.

35.

Schubel

Crossley

. Wind turbine blade design. Energies 2012; 5: 3425–3449.

36.

Sharma

. Recent development in the field of wind turbine. Materials Today: Proceedings 2022; 64: 1512–1520.

37.

Wang

Geoffroy

Bonhomme

. Urban form study for wind potential development. Environ Plan B: Urban Analy City Sci 2022; 49: 76–91.

38.

Gulve

Barve

. Design and construction of vertical axis wind turbine. Int J Mech Eng Technol (IJMET) 2014; 5: 148–155.

39.

Muratoğlu

DEMİR

. Numerical analyses of a straight bladed vertical axis darrieus wind turbine: Verification of dms algorithm and qblade code. Eur J Tech (EJT) 2019; 9: 195–208.

40.

Khorsand

Kormos

MacDonald

, et al. Wind energy in the city: An interurban comparison of social acceptance of wind energy projects. Energy Res Soc Sci 2015; 8: 66–77.

41.

Simic

Havelka

Vrhovcak

. Small wind turbines–a unique segment of the wind power market. Renew Energy 2013; 50: 1027–1036.

42.

Alom

Saha

.: Four decades of research into the augmentation techniques of savonius wind turbine rotor. Journal of Energy Resources Technology, 140(5), 2018.

43.

Jackson

Amano

. Experimental study and simulation of a small-scale horizontal-axis wind turbine. J Energy Resour Technol 2017; 139: 051207(1–19).

44.

Mari

Venturini

Beyene

. A novel geometry for vertical axis wind turbines based on the savonius concept. J Energy Resour Technol 2017; 139: 061202 (1–9).

45.

Keerthana

Sriramkrishnan

Velayutham

, et al. Aerodynamic analysis of a small horizontal axis wind turbine using cfd. J Wind Eng 2012; 9: 14–28.

46.

Mathew

. Wind energy: fundamentals, resource analysis and economics. Berlin, Heidelberg: Springer, 2006.

47.

Suffer

Usubamatov

Quadir

, et al. Modeling and numerical simulation of a vertical axis wind turbine having cavity vanes. In 2014 5th International Conference on Intelligent Systems, Modelling and Simulation 2014; 479–484, IEEE.

48.

Lee

Y-T

Lim

H-C

. Numerical study of the aerodynamic performance of a 500 w darrieus-type vertical-axis wind turbine. Renew Energy 2015; 83: 407–415.

49.

Hasan Shahariar

Hasan

. Design & construction of a vertical axis wind turbine. In 2014 9th International Forum on Strategic Technology (IFOST), pages 326–329. IEEE, 2014.

50.

DeCoste

McKay

Robinson

, et al. Vertical axis wind turbine. Department of Mechanical Engineering Dalhousie University, pages 1–77, 2005.

51.

Howell

Qin

Edwards

, et al. Wind tunnel and numerical study of a small vertical axis wind turbine. Renew Energy 2010; 35: 412–422.

52.

Eriksson

Bernhoff

Leijon

. Evaluation of different turbine concepts for wind power. Renew Sust Energy Rev 2008; 12: 1419–1434.

53.

Sabaeifard

Razzaghi

Forouzandeh

. Determination of vertical axis wind turbines optimal configuration through cfd simulations. In International Conference on Future Environment and Energy, volume 28, pages 109–113, 2012.

54.

Roh

S-C

Kang

S-H

. Effects of a blade profile, the reynolds number, and the solidity on the performance of a straight bladed vertical axis wind turbine. J Mech Sci Technol 2013; 27: 3299–3307.

55.

Beri

and Yao

. Effect of camber airfoil on self starting of vertical axis wind turbine. J Environ Sci Technol 2011; 4: 302–312.

56.

Ghasemian

Ashrafi

Sedaghat

. A review on computational fluid dynamic simulation techniques for darrieus vertical axis wind turbines. Energy Convers Manage 2017; 149: 87–100.

57.

Hameed

Afaq

. Design and analysis of a straight bladed vertical axis wind turbine blade using analytical and numerical techniques. Ocean Eng 2013; 57: 248–255.

58.

Brusca

Lanzafame

Messina

. Design of a vertical-axis wind turbine: how the aspect ratio affects the turbine’s performance. Int J Energy Environ Eng 2014; 5: 333–340.

59.

Hyams

. Wind energy in the built environment. In Metropolitan Sustainability, pages 457–499. Elsevier, 2012.

60.

Aho

drew Buckspan

Laks

, et al. A tutorial of wind turbine control for supporting grid frequency through active power control. In 2012 American Control Conference (ACC) 20123120–3131, IEEE.

61.

Collecutt

Flay

RGJ

. The economic optimisation of horizontal axis wind turbine design. J Wind Eng Ind Aerodyn 1996; 61: 87–97.

62.

Carrasco

Galván

Portillo

. Wind turbine applications. In Power Electronics Handbook. Elsevier, 2011; 791–822.

63.

Tummala

Velamati

Sinha

, et al. A review on small scale wind turbines. Renew Sust Energy Rev 2016; 56: 1351–1371.

64.

Liu

S-Y

Y-F

. Wind energy applications for taiwan buildings: What are the challenges and strategies for small wind energy systems exploitation? Renew Sust Energy Rev 2016; 59: 39–55.

65.

Saad

MMM

Asmuin

. Comparison of horizontal axis wind turbines and vertical axis wind turbines. IOSR J Eng (IOSRJEN) 2014; 4: 27–30.

66.

Paulides

JJH

Encica

Jansen

, et al. Small-scale urban venturi wind turbine: Direct-drive generator. In 2009 IEEE International Electric Machines and Drives Conference 2009; 1368–1373, IEEE.

67.

Tang

Lam

K-M

Shum

K-M

, et al. Wake effect of a horizontal axis wind turbine on the performance of a downstream turbine. Energies 2019; 12: 2395.

68.

Nikam

Kherde

. Literature review on design and development of vertical axis wind turbine blade. Int J Eng Res Appl (IJERA) ISSN 2015; 1: 2248–9622.

69.

Winslow

. Urban wind generation: Comparing horizontal and vertical axis wind turbines at clark university in worcester, massachusetts. 2017.

70.

Nayar

Islam

Dehbonei

, et al. Power electronics for renewable energy sources. In Power electronics handbook, pages 723–766. Elsevier, 2011.

71.

Peacock

Jenkins

Ahadzi

, et al. Micro wind turbines in the uk domestic sector. Energy Build 2008; 40: 1324–1333.

72.

Yuyi

Decai

Liang

, et al. The design of vertical axis wind turbine rotor for antarctic. Inf Technol J 2013; 12604.

73.

Dhote

Bankar

. Design analysis and fabrication of savonius vertical axis wind turbine. Int Res J Eng Technol 2015; 2: 2048–2054.

74.

Salyers

. Experimental and numerical investigation of aerodynamic performance for vertical-axis wind turbine models with various blade designs. 2016.

75.

Feng

Y-H

Zhao

C-Y

, et al. Experimental and numerical investigation on aerodynamic performance of a novel disc-shaped wind rotor for the small-scale wind turbine. Energy Convers Manage 2018; 175: 173–191.

76.

Tjiu

Marnoto

Mat

, et al. Darrieus vertical axis wind turbine for power generation i: Assessment of darrieus vawt configurations. Renew Energy 2015; 75: 50–67.

77.

Islam

Ting

DS-K

Fartaj

. Aerodynamic models for darrieus-type straight-bladed vertical axis wind turbines. Renew Sust Energy Rev 2008; 12: 1087–1109.

78.

Hosseini

Goudarzi

. Design and cfd study of a hybrid vertical-axis wind turbine by employing a combined bach-type and h-darrieus rotor systems. Energy Convers Manage 2019; 189: 49–59.

79.

X-X

Liu

C-H

Leung

DYC

, et al. Recent progress in cfd modelling of wind field and pollutant transport in street canyons. Atmos Environ 2006; 40: 5640–5658.

80.

McIntyre

Lubitz

Stiver

. Local wind-energy potential for the city of guelph, ontario (canada). Renew Energy 2011; 36: 1437–1446.

81.

Pope

Dincer

Naterer

. Energy and exergy efficiency comparison of horizontal and vertical axis wind turbines. Renew Energy 2010; 35: 2102–2113.

82.

Bedon

Antonini

EGA

De Betta

, et al. Evaluation of the different aerodynamic databases for vertical axis wind turbine simulations. Renew Sust Energy Rev 2014; 40: 386–399.

83.

Sun

Chen

Cao

, et al. Research on the aerodynamic characteristics of a lift drag hybrid vertical axis wind turbine. Adv Mech Eng 2016; 8: 1687814016629349.

84.

Bhutta

MMA

Hayat

Farooq

, et al. Vertical axis wind turbine–a review of various configurations and design techniques. Renew Sust Energy Rev 2012; 16: 1926–1939.

85.

Liu

Lin

Zhang

. Review on the technical perspectives and commercial viability of vertical axis wind turbines. Ocean Eng 2019; 182: 608–626.

86.

Jin

Zhao

Gao

, et al. Darrieus vertical axis wind turbine: Basic research methods. Renew Sust Energy Rev 2015; 42: 212–225.

87.

MacPhee

Beyene

. Recent advances in rotor design of vertical axis wind turbines. Wind Eng 2012; 36: 647–665.

88.

Sutherland

Berg

Thomas

. Ashwill td. a retrospective of vawt technology. Sandia Report 2012

89.

Wekesa

Wang

Wei

, et al. Experimental and numerical study of turbulence effect on aerodynamic performance of a small-scale vertical axis wind turbine. J Wind Eng Ind Aerodyn 2016; 157: 1–14.

90.

Mohammed

Ouakad

Sahin

, et al. Vertical axis wind turbine aerodynamics: summary and review of momentum models. J Energy Resour Technol 2019; 141: 050801 (1–10).

91.

Mitchell

Ogbonna

Volkov

. Aerodynamic characteristics of a single airfoil for vertical axis wind turbine blades and performance prediction of wind turbines. Fluids 2021; 6: 257.

92.

Kumar

Ajit

Srikanth

, et al. On the mathematical modelling of adaptive darrieus wind turbine. J Power Energy Eng 2017; 5: 133–158.

93.

Kumar

Anbazhagan

Srikanth

, et al. Optimization, design, and construction of field test prototypes of adaptive hybrid darrieus turbine. J Fund Renew Energy Appl 2017; 7: 245.

94.

Kumar

Surya

MMR

Sin

, et al. Design and experimental investigation of airfoil for extruded blades. Int J Adv Agric Environ Eng (IJAAEE) 2017; 3: 2349–1523.

95.

Kumar

Surya

MMR

Srikanth

. On the improvement of starting torque of darrieus wind turbine with trapped vortex airfoil. In 2017 IEEE International Conference on Smart Grid and Smart Cities (ICSGSC) 2017120–125, IEEE.

96.

Kumar

Kulkarni

Srikanth

, et al. Performance assessment of darrieus turbine with modified trailing edge airfoil for low wind speeds. Smart Grid Renew Energy 2017; 8: 425–439.

97.

Kumar

Rashmitha

Srikanth

, et al. Wind tunnel validation of double multiple streamtube model for vertical axis wind turbine. Smart Grid Renew Energy 2017; 8: 412–424.

98.

Trivedi

Cervantes

Dahlhaug

. Experimental and numerical studies of a high-head francis turbine: A review of the francis-99 test case. Energies 2016; 9: 74.

99.

Trivedi

Cervantes

Gandhi

. Investigation of a high head francis turbine at runaway operating conditions. Energies 2016; 9: 149.

100.

Laín

García

Quintero

, et al. Cfd numerical simulations of francis turbines. Revista Facultad de Ingeniería Universidad de Antioquia 2010; 24–33.

101.

Sommerfeld

Lain

. From elementary processes to the numerical prediction of industrial particle-laden flows. Multiphase Sci Technol 2009; 21: 123–140.

102.

Maître

Amet

Pellone

. Modeling of the flow in a darrieus water turbine: Wall grid refinement analysis and comparison with experiments. Renew Energy 2013; 51: 497–512.

103.

Balduzzi

Bianchini

Maleci

, et al. Critical issues in the cfd simulation of darrieus wind turbines. Renew Energy 2016; 85: 419–435.

104.

Kumar

Surya

Sivalingam

, et al. Computational optimization of adaptive hybrid darrieus turbine: Part 1. Fluids 2019; 4: 90.

105.

Abdul Akbar

Mustafa

. A new approach for optimization of vertical axis wind turbines. J Wind Eng Ind Aerodyn 2016; 153: 34–45.

106.

Laín

Cortés

López

. Numerical simulation of the flow around a straight blade darrieus water turbine. Energies 2020; 13: 1137.

107.

Ahmad

Shahzad

Akram

, et al. Determination of efficient configurations of vertical axis wind turbine using design of experiments. Proceedings of the Institution of Mechanical Engineers, Part A: Journal of Power and Energy, 2022: 09576509221095347.

108.

Altmimi

Alaskari

Abdullah

Alhamadani

Sherza

.: Design and optimization of vertical axis wind turbines using qblade. Applied System Innovation 2021; 4: 74.

109.

Maeda

Kamada

Murata

, et al. Wind tunnel and numerical study of a straight-bladed vertical axis wind turbine in three-dimensional analysis (part i: For predicting aerodynamic loads and performance). Energy 2016; 106: 443–452.

110.

Roy

Kincaid

Mahmud

, et al. Double-multiple streamtube analysis of a flexible vertical axis wind turbine. Fluids 2021; 6: 118.

111.

Zhu

Guo

Zhang

, et al. Numerical study of aerodynamic characteristics on a straight-bladed vertical axis wind turbine with bionic blades. Energy 2022; 239: 122453.

112.

. Straight-bladed vertical axis wind turbines: history, performance, and applications. In Rotating Machinery. IntechOpen, 2019.

113.

Mitchell

Ogbonna

Volkov

. Improvement of self-starting capabilities of vertical axis wind turbines with new design of turbine blades. Sustainability 2021; 13: 3854.

114.

Chopra

Zhu

, et al. Performance analysis and optimization of a vertical-axis wind turbine with a high tip-speed ratio. Energies 2021; 14: 996.

115.

Syawitri

Yao

Chandra

, et al. Comparison study of urans and hybrid rans-les models on predicting vertical axis wind turbine performance at low, medium and high tip speed ratio ranges. Renew Energy 2021; 168: 247–269.

116.

Alaimo

Esposito

Messineo

, et al. 3d cfd analysis of a vertical axis wind turbine. Energies 2015; 8: 3013–3033.

117.

Cheng

Liu

, et al. Aerodynamic analysis of a helical vertical axis wind turbine. Energies 2017; 10: 575.

118.

Meng

, et al. Wind tunnel experiment on the influence of array configuration on the power performance of vertical axis wind turbines. Energy Convers Manage 2021; 241: 114299.

119.

Hassanpour

Azadani

. Aerodynamic optimization of the configuration of a pair of vertical axis wind turbines. Energy Convers Manage 2021; 238: 114069.

120.

Peng

Zhong

, et al. Optimization analysis of straight-bladed vertical axis wind turbines in turbulent environments by wind tunnel testing. Energy Convers Manage 2022; 257: 115411.

121.

Aihara

Mendoza

Goude

, et al. A numerical study of strut and tower influence on the performance of vertical axis wind turbines using computational fluid dynamics simulation. Wind Energy 2022; 25: 897–913.

122.

Scheurich

Fletcher

Brown

. The influence of blade curvature and helical blade twist on the performance of a vertical-axis wind turbine. In 48th AIAA aerospace sciences meeting including the new horizons forum and aerospace exposition, 2010, 1579.

123.

Kavade

Ghanegaonkar

. Design and analysis of vertical axis wind turbine for household application. J Clean Energy Technol 2017; 5: 353–358.

124.

Shires

. Development and evaluation of an aerodynamic model for a novel vertical axis wind turbine concept. Energies 2013; 6: 2501–2520.

125.

Dastjerdi

Gharali

Al-Haq

, et al. Application of simultaneous symmetric and cambered airfoils in novel vertical axis wind turbines. Appl Sci 2021; 11: 8011.

126.

Kanyako

Janajreh

. Vertical axis wind turbine performance prediction for low wind speed environment. In 2014 IEEE Innovations in Technology Conference 2014; 1–10, IEEE.

127.

Ardaneh

Abdolahifar

Karimian

SMH

. Numerical analysis of the pitch angle effect on the performance improvement and flow characteristics of the 3-pb darrieus vertical axis wind turbine. Energy 2022; 239: 122339.

128.

Clarke

Hancox

Mackenzie

, et al. Design of a vertical-axis wind turbine. Group M11. Report 2014;

129.

Divakaran

Ramesh

Mohammad

, et al. Effect of helix angle on the performance of helical vertical axis wind turbine. Energies 2021; 14: 393.

130.

Khan

Bashar

Rahman

. Computational studies on the flow field of various shapes-bladed vertical axis savonius turbine in static condition. In ASME International Mechanical Engineering Congress and Exposition 2013; 56291: V06BT07A086. American Society of Mechanical Engineers.

131.

Kang

Liu

Yang

. Review of fluid dynamics aspects of savonius-rotor-based vertical-axis wind rotors. Renew Sust Energy Rev 2014; 33: 499–508.

132.

Heo

Choi

, et al. Cfd study on aerodynamic power output of a 110 kw building augmented wind turbine. Energy Build 2016; 129: 162–173.

133.

Muscoloa

Molfinob

. From savonius to bronzinus: a comparison among vertical wind turbines. Energy Procedia 2014; 50: 10–18.

134.

Torres

Marulanda

Montoya

, et al. Geometric design optimization of a savonius wind turbine. Energy Convers Manage 2022; 262: 115679.

135.

Morshed

Rahman

Molina

, et al. Wind tunnel testing and numerical simulation on aerodynamic performance of a three-bladed savonius wind turbine. Int J Energy Environ Eng 2013; 4: 18.

136.

Gupta

Biswas

. Cfd analysis of flow physics and aerodynamic performance of a combined three-bucket savonius and three-bladed darrieus turbine. Int J Green Energy 2011; 8: 209–233.

137.

El-Askary

Nasef

Abdel-Hamid

, et al. Harvesting wind energy for improving performance of savonius rotor. J Wind Eng Ind Aerodyn 2015; 139: 8–15.

138.

Ghatage

Joshi

. Optimisation of vertical axis wind turbine: Cfd simulations and experimental measurements. Can J Chem Eng 2012; 90: 1186–1201.

139.

Ali

. Experimental comparison study for savonius wind turbine of two & three blades at low wind speed. Int J Modern Eng Res (IJMER) 2013; 3: 2978–2986.

140.

Gupta

Sharma

. Flow physics of a three-bucket savonius rotor using computational fluid dynamics (cfd). Int J Res Mech Eng Technol 2011; 1: 46–51.

141.

Bouzaher

. Effect of flexible blades on the savonius wind turbine performance. J Brazilian Soc Mech Sci Eng 2022; 44: 1–16.

142.

Aboujaoude

Beaumont

Murer

, et al. Aerodynamic performance enhancement of a savonius wind turbine using an axisymmetric deflector. J Wind Eng Ind Aerodyn 2022; 220: 104882.

143.

Kamoji

Shireesh

Kedare

Prabhu

. Experimental investigations on single stage modified savonius rotor. Appl Energy 2009; 86: 1064–1073.

144.

Vicente Akwa

da Silva Júnior

Petry

. Discussion on the verification of the overlap ratio influence on performance coefficients of a savonius wind rotor using computational fluid dynamics. Renew Energy 2012; 38: 141–149.

145.

Kim

Cheong

. Development of low-noise drag-type vertical wind turbines. Renew Energy 2015; 79: 199–208.

146.

Akwa

Vielmo

Petry

. A review on the performance of savonius wind turbines. Renew Sust Energy Rev 2012; 16: 3054–3064.

147.

Jangid

Bera

Joseph

, et al. Potential zones identification for harvesting wind energy resources in desert region of india–a multi criteria evaluation approach using remote sensing and gis. Renew Sust Energy Rev 2016; 65: 1–10.

148.

Maeda

Kamada

Murata

, et al. Study on power performance for straight-bladed vertical axis wind turbine by field and wind tunnel test. Renew Energy 2016; 90: 291–300.

149.

Lee

J-H

Lee

Y-T

Lim

H-C

. Effect of twist angle on the performance of savonius wind turbine. Renew Energy 2016; 89: 231–244.

150.

Zhao

Zheng

, et al. Optimum design configuration of helical savonius rotor via numerical study. In Fluids Engineering Division Summer Meeting 2009; 43727: 1273–1278.

151.

Saad

Elwardany

El-Sharkawy

, et al. Performance evaluation of a novel vertical axis wind turbine using twisted blades in multi-stage savonius rotors. Energy Convers Manage 2021; 235: 114013.

152.

Ferrari

Federici

Schito

, et al. Cfd study of savonius wind turbine: 3d model validation and parametric analysis. Renew Energy 2017; 105: 722–734.

153.

Al-Ghriybah

Zulkafli

Didane

, et al. The effect of spacing between inner blades on the performance of the savonius wind turbine. Sust Energy Technol Assessments 2021; 43: 100988.

154.

Tjahjana

DDDP

Arifin

Suyitno

, et al. Experimental study of the effect of slotted blades on the savonius wind turbine performance. Theoretical Appl Mech Lett 2021; 11: 100249.

155.

Ahmad

Shahzad

Akram

, et al. Design optimization of double-darrieus hybrid vertical axis wind turbine. Ocean Eng 2022; 254: 111171.

156.

Kamoji

Kedare

Prabhu

. Experimental investigations on the effect of overlap ratio and blade edge conditions on the performance of conventional savonius rotor. Wind Eng 2008; 32: 163–178.

157.

Hosseini

Goudarzi

. Cfd and control analysis of a smart hybrid vertical axis wind turbine. In ASME Power Conference 2018; 51395: V001T06A027. American Society of Mechanical Engineers, 2018.

158.

Sharma

Biswas

Gupta

. Performance measurement of a three-bladed combined darrieus-savonius rotor. Int J Renew Energy Res (IJRER) 2013; 3: 885–891.

159.

Gorelov

Krivospitsky

. Prospects for development of wind turbines with orthogonal rotor. Thermophys Aeromechanics 2008; 15: 153–157.

160.

Jafari

SAH

Safaei

Kosasih

, et al. Power generation analysis of powerwindow, a linear wind generator, using computational fluid dynamic simulations. J Wind Eng Ind Aerodyn 2015; 147: 226–238.

161.

Naccache

. CFD based analysis and parametric study of a novel wind turbine design: the dual vertical axis wind turbine. PhD thesis, Concordia University, 2016.

162.

Ponta

Otero

Lago

. Innovative concepts in wind-power generation: The vgot darrieus. Wind Turbines 2011; 1: 137–162.

163.

Hansen

MOL

. Aerodynamics of wind turbines. London: Routledge, 2015.

164.

Chen

Guerrero

Blaabjerg

. A review of the state of the art of power electronics for wind turbines. IEEE Trans Power Electron 2009; 24: 1859–1875.

165.

Ekanayake

Holdsworth

, et al. Dynamic modeling of doubly fed induction generator wind turbines. IEEE Trans Power Syst 2003; 18: 803–809.

166.

Polinder

VanderPijl

FFA

DeVilder

G-J

, et al. Comparison of direct-drive and geared generator concepts for wind turbines. IEEE Trans Energy Convers 2006; 21: 725–733.

167.

Ekanayake

Jenkins

. Comparison of the response of doubly fed and fixed-speed induction generator wind turbines to changes in network frequency. IEEE Trans Energy Convers 2004; 19: 800–802.

168.

Kou

Shi

Yuan

, et al. Modeling analysis and experimental research on a combined-type vertical axis wind turbine. In 2011 International Conference on Electronics, Communications and Control (ICECC) 2011, 1537–1541, IEEE.

169.

Tripathi

Das

Aggarwal

, et al. Efficiency enhancement of a hybrid vertical axis wind turbine by utilizing optimum parameters. Materials Today: Proceedings 2022; 62(6): 3582–3588.

170.

Ghosh

Biswas

Sharma

, et al. Computational analysis of flow physics of a combined three bladed darrieus savonius wind rotor. J Energy Inst 2015; 88: 425–437.

171.

Zhao

Wang

, et al. A review: Approaches for aerodynamic performance improvement of lift-type vertical axis wind turbine. Sust Energy Technol Assessments 2022; 49: 101789.

172.

Abdelsalam

Kotb

Yousef

, et al. Performance study on a modified hybrid wind turbine with twisted savonius blades. Energy Convers Manage 2021; 241: 114317.

173.

Asadi

Hassanzadeh

. Effects of internal rotor parameters on the performance of a two bladed darrieus-two bladed savonius hybrid wind turbine. Energy Convers Manage 2021; 238: 114109.

174.

Andrews

AAE

Karthick

Jeraled

, et al. Experimental and numerical study on modified vertical axis wind turbine. In IOP Conference Series: Materials Science and Engineering2019; 574: 012007, IOP Publishing.

175.

Bhuyan

Biswas

. Investigations on self-starting and performance characteristics of simple h and hybrid h-savonius vertical axis wind rotors. Energy Convers Manage 2014; 87: 859–867.