Effect of some design parameters: A performance test on VAWT

Scientific Study, 2018
76 Pages, Grade: 2



Chapter 1


1.1 Wind in world around us

1.2 Growth of wind power in India
1.3 Wind Turbines
1.3.1 Types of Wind Turbine
1.3.2 Early start of VAWT and HAWT
1.4 Performance map of Various Windmill Rotors:
1.5 Advantages of VAWT and HAWT
1.5.1 VAWT advantages
1.5.2 VAWT disadvantages
1.5.3 HAWT advantages
1.5.4 HAWT disadvantages
1.6 Applications of VAWT

Chapter 2

Literature Review
2.1 Preceding Researches
2.2 Outcome from the literature review:

Chapter 3

Experimental set-up and procedure
3.1 The subsonic wind tunnel and its calibration
3.2 Two-scoop rotor
3.3 Profiled blades
3.4 Blow down of wind tunnel
3.5 Performance parameters of Wind Turbine

Chapter 4

Results and Discussion
4.1 Forward Curved Blade (3 Full Blades System)
4.2 Forward Curved Blade (6 Full Blades System)
4.3 Forward Curved Blade (6 Half Blades Blades)
4.4 Comparison between 3-full Blades and 6-half Blades Forward Curved Profiles
4.5 Mixed Full and Half Blade System
4.6 Two Scoop Blades (Savonius Type)

Chapter 5

Conclusions and future scope
5.1 Two scoop blades
5.2 Forward curved blade
5.3 Future scope of work


About the Main Authors

Dr. S. K. Dhiman is a faculty of Mechanical Engineering and is working as Assistant Professor in Birla Institute of Technology, Mesra, Ranchi (India). He has been involved in teaching and research since 2000 in the field of engineering and technology. He has published many of his research work in international journals as well as presented in international conferences. He has been deeply involved in research work in the fields of fluid dynamics and heat transfer applications of engineering and technology.


The purpose of this book is to bridge the acquaintance breach of those students of engineering who are at their induction stage as well as involved in the experimental research on the performance parameter of Wind turbine. However, this book is also helpful to other students of PG or Ph.D. level. I have felt that the students at their PG or Ph.D. level have lacuna of writing their dissertation reports. Nevertheless, they also have insufficient exposure of writing contents with reasoning, representing the figures and tables, presenting the present and past scenario of research work they are undergoing or have undergone. They should at least know the main contents of doing the research and representing them.

This book has been written in the manner that a dissertation of PG level or Ph.D. level may be written. In this book I have included the research work carried out by my two students namely Mr. Gaurav Priyadarshi and Mr. Ajay Kumar Kapardar at their PG levels. The references in this book have been included from the past two decades except few, however, most of the literatures are from past five years. This brings about the ideas of writing reports in present days.

I am thankful to Dr. Vivek Gaba, Assistant Professor, National Institute of Technology, Raipur (India) for the helpful discussions carried out during the writing of this book. I am also thankful to Birla Institute of Technology, Mesra, Ranchi (India) whose laboratory facilities have been utilized for conducting the experiments.

Last but not the least, I am extremely thankful to my loving wife and daughters.

Dr. S. K. Dhiman


Nature has given unlimited resource for the generation of power harmlessly to benefit the environment and living beings. Wind is one of such major sources. Since centuries, power in wind has been extracted to meet the time-to-time technological need for the survival of lives on earth. For extracting power available in wind, called as dynamic power, a rotor is required which takes support on bearings and on which blade are to be fixed. These blades encounter the wind and moves by the air drag acts on it. Such an assembly has been called as turbine because its rotation can be utilized to develop electricity, move the piston of a pump, etc. Its shaft may be kept horizontal or vertical while the researchers have developed various shapes of blade as well as their mounting techniques so that the extent of generated power may be increased to its maximum possibility.

Under the similar efforts, the Finnish Engineer S. J. Savonius [1] has developed a rotor system, which eats the air from all the directions since its rotor shaft was kept vertical. Such a rotor was called S-rotor, as shown in Fig.1.1, consisted with two semi-circular scoops, however, it rotated at low speed and it showed low aerodynamic efficiency relative to Darrieus or propeller type rotors. Its principle of operation was the drag difference between the convex and concave scoops of S-rotor. Savonius rotor traced the power coefficient of 0.37 although Savonius reported it as 0.31. Various investigators had attempted according to S-rotor but did not observe the equivalent performance as reported by Savonius. The VAWT Savonius rotor did not get acceptance in electric power generation because of low aerodynamic efficiency compared to lift type mills. Nevertheless, its low cost and simple construction hailed the water pumping application. The researchers using three scoops and two-stages also investigated Savonius rotor. Together with low speed, the non-uniformity of the torque was the key problem. The reason was twofold. The major problem was wide torque variation causing vibrations in the assembly and material failure. Other problem was negative or very low static torque at some angular locations.

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Fig.1.1 Savonius rotor (S-rotor)

1.1 Wind in world around us

There is fast development in wind control improvement all inclusive. This usage of wind for power age is growing rapidly, due to a great extent to innovative enhancements, industry development and an expanding worry with nursery outflows related with consuming of non-renewable energy sources. The Association of Wind Energy Generation has anticipated this pattern will proceed as there is much chance to develop this asset globally. Given the colossal wind assets, just a little bit of the utilizable wind potential is being used directly. Electrical industry directions and Government, and government motivating forces, have a substantial part in deciding how rapidly wind power will be received. In Ontario, Canada, for example, government activities, for example, the Better Buildings Partnership in Toronto energize and encourage the improvement of little turbines in building plans. This specific program offers motivations up to $13,000 to urge private designers to "fabricate green". Over the U.S. in 2006, wind turbine establishment limit has developed from around 9,000 MW to 11,600MW. European nations have additionally broadly bridled this energy asset. Germany, Denmark, and Spain are outstanding clients of wind control. Denmark produces 20% of its power through wind turbines. The UK has the biggest wind energy assets and it is set for substantial development of this perfect energy source by exploiting the European market economies of scale to cut down the cost of wind energy. Introduced overall limit of wind control before the finish of 2007 was almost 100,000 MW. Compelling strategies will help enhance the motivating forces and guarantee which wind power may contend fairly along other fuel sources in power showcase.

1.2 Growth of wind power in India

In India, the development of windmills was commenced in early 1990s while the rate of development has so elevated that the present day installed capacity of windmill units has exceeded 32.3 GW, as shown in Fig.1.2. The power generation through wind has occupied almost 55% of entire renewable capacity in India. Global Wind Energy Council (GWEC) has published in their 2016 report that by 2016, India ranked 4th around the globe in terms of total installed wind power capacity among the total installed capacity of 487 GW around the globe. In accordance with National Institute of Wind Energy (NIWE) of India, the onshore wind potential is more than 300 GW at about 100 m above the ground level, however, only a fraction of it has been tapped.

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Fig.1.2 Growth of installed capacity of Windmills in India

The Ministry of New and Renewable Energy (MNRE), Government of India, has now aimed to large-scale commercialization of cost effective generation of grid-quality wind power. It has initiated comprehensive programme of wind resource assessment, research and development, implementation of demonstrating projects to build awareness, development of infrastructural capability, capacity to manufacture along with installation, operation and maintenance of wind turbines and conducive policy formulation [40]. An abundance of data is being collected via more than 800 wind-monitoring stations installed all over India. The majority of potential wind exists in seven states of India. The power potential at 100 m height in windy states has been tabulated in Fig.1.3.

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Fig.1.3 Wind power potential in India at 100 m above ground level [40]

Present day wind machine is capable of generating 3MW. More than 20 distinct manufacturers in India are manufacturing more than 50 diverse models of windmills. The current annual production capacity of domestic wind turbines is higher than 32000 MW as shown in Fig.1.4. The present day focus is to promote the technology for low wind regimes of India. India is now becoming the leading manufacturer of windmills and its components exporting windmills to US, Australia, Europe, Brazil, and Asian countries due to low cost of production around the globe. Owing to strong manufacturing sector about 70-80% windmill installations are being carried out indigenously. The government of India has set a target of achieving 60,000 MW by 2022 by adding capacity around 30,000 MW in the upcoming 5 years with annual addition of capacity of 6,000 MW.

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Fig.1.4 Year-wise cumulative wind power installed capacity in India (MW) (MNRE [40])

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1.3 Wind Turbines

Machine used to transform wind energy into electrical energy is called a wind turbines. In preceding history, wind turbines were known by windmill, wind turbines have been used for about 2000 years for the generation of electrical energy. Wind mills were basically used for grinding of cereals and for pumping water. Those windmills were the Persian windmills as shown in Fig.1.5 and were used around 1000 BC. They worked by blocking of wind by half of the sail. Wind-turbines are in used for power generation converting dynamic energy in wind since 1920. However, they were properly utilized after 1980 as working substance for the energy generation.

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Fig.1.5 Persian windmills

1.3.1 Types of Wind Turbine

The wind turbines are broadly classified in two types based on how the axis of the shaft is placed. They are:

1. The Vertical Axis Wind Turbine (VAWT)

2. The Horizontal Axis Wind Turbine (HAWT)

1.3.2 Early start of VAWT and HAWT

Vertical Axis Wind Turbine (VAWT)

VAWT (Vertical Axis Wind Turbine) has come into existence from the work of Savonius who was from Finland. Its rotor was S-shaped and had the principle rotor shaft organized vertically. Since then, VAWT has been under investigation for the improvement of its performance parameters. In these turbines, the generator and the gearbox are placed near the ground. The main advantage of Vertical Axis Wind Turbines is that it eats the air from all the directions. Thus, it does not require any yaw mechanism.

Horizontal Axis Wind Turbine (HAWT)

When the concept of wind turbine has reach Europe, there the idea had changes in the form of horizontal shaft and vertically spinning wheel in late twelfth century. Such machines were initially appeared in France and the in England. There the provision were developed to mount the gearbox and the generator to the wind turbine height, however a yawing mechanism was need to direct the spinning to face the prevailing wind in a cross-flow manner. Then after, the development of Horizontal Axis Wind Turbine (HAWT) started.

Various VAWT: Figures 1.6 a-g shows the mechanism of various type of VAWT rotors.

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Fig.1.6 (a-g) Mechanism of various VAWT

Characteristic of Wind

1. Wind speed increases roughly as (Height from the ground) 1/7. Typical tower heights are roughly 20-50 meters high.
2. Energy-patter factor: It is the ratio of actual energy in prevailing wind to energy calculated from the cube of mean wind speed. This factor is always greater than unity which means that the energy estimates based on mean (hourly) speed are pessimistic.

Characteristic of Savonius rotor

1. Self-Starting
2. Low Speed
3. Low Efficiency
4. Torque is produced by pressure difference between two-sides of the half facing the wind.
5. It needs a large surface area.

Characteristic of Darrieus rotor

1. Not Self-Starting
2. High Speed
3. High Efficiency
4. Potentially Low Capital Cost.
5. It needs much less surface area

1.4 Performance map of Various Windmill Rotors:

MM El-Wakil [42] explained that rigorous treatment of power extraction from the wind by propeller-type wind turbine shows that the power coefficient strongly dependent on blade to wind speed ratio, that it reaches its maximum value of about 0.6 only when the maximum blade speed i.e. blade speed at tip, is about 6 or 7 times the wind speed, that it drops rapidly at blade tip-to-wind speed ratios below about 2.0. Fig.1.7 shows the power coefficient for an ideal propeller-type wind turbine and various other wind turbines.

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Fig.1.7 Power coefficient of various windmills v/s tip-Speed Ratio of the blade [41]

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Fig.1.8 Comparison between different types of wind turbine [43]

1.5 Advantages of VAWT and HAWT

1.5.1 VAWT advantages

Vertical axis wind turbines have some advantages and disadvantages over horizontal axis wind turbine.

1. It works under lower wind velocity (about 3 m/s) and capable of working on high wind speeds (about 42 m/s).
2. Works on all wind directions: VAWT is able to catch wind of all directions, so called Omni-directional, without any requirement of sophisticated, heavy and expensive directional equipment.
3. It shows better efficiency when wind directions is axial to blades.
4. Turbine length: VAWT turbine is smaller compared to HAWT.
5. Maintenance and transportation is easy while the construction, maintenance and transportation costs are VAWT is low.
6. Operation: Performance of VAWT when wind velocity is low is better than HWAT because of low weight of blades.

1.5.2 VAWT disadvantages

- Due dragging force most of the VAWT are only half efficient.
- Issues related to vibration are caused due air flow between ground and other objects.
- Guy wire is required to hold it up (it is almost obsolete and heavy).
- Due to location near the ground it may not be able to produce energy as much as HAWT at same place and height.

Horizontal Axis Wind Turbine (HAWT)

Level axis wind turbines (HAWT) have the primary rotor shaft and electrical generator at the highest point of a pinnacle to drive an electrical generator, also must be pointed to wind. Little turbines are pointed away a straight forward wind, which vast turbines by and large utilize a wind sensor combined along a servo engine. All have a gearbox that transforms moderate revolution of edges to a speedier turn such is appropriate. After all pinnacle produces turbulence behind it; the turbine is generally pointed upwind of the pinnacle. Turbine cutting edges are forming solid to keep sharp edges from being pushed to pinnacle away high winds. Also, sharp edges are put an extensive separation after pinnacle also are infrequently inclined up a little sum. Downwind machines are worked, notwithstanding issue of turbulence, since all needn't bother with an extra system as keeping them in accordance along wind, also in light of fact that in high winds the sharp edges can be permitted to twist which diminishes their cleared territory and in this manner their wind protection. After all cyclic (such is dull) turbulence can be prompt exhaustion disappointments most HAWTs are upwind machines.

1.5.3 HAWT advantages

- Variable blade pitch, which gives the turbine blades the optimum angle of attack. Allowing the angle of attack to be remotely adjusted gives greater control, so the turbine collects the maximum amount of wind energy for the time of day and season.
- The tall tower base allows access to stronger wind in sites with wind shear. In some wind shear sites, every ten meters up, the wind speed can increase by 20% and the power output by 34%.
- High efficiency, since the blades always move perpendicularly to the wind, receiving power through the whole rotation. In contrast, all vertical axis wind turbines, and most proposed airborne wind turbine designs, involve various types of reciprocating actions, requiring airfoil surfaces to backtrack against the wind for part of the cycle. Backtracking against the wind leads to inherently lower efficiency.
- The face of a horizontal axis blade is struck by the wind at a consistent angle regardless of the position in its rotation. These results in a consistent lateral wind loading over the course of a rotation, reducing vibration and audible noise coupled to the tower or mount.

1.5.4 HAWT disadvantages

- The tall towers and blades up to 90 meters long are difficult to transport. Transportation can now cost 20% of equipment costs.
- Tall HAWTs are difficult to install, needing very tall and expensive cranes and skilled operators.
- Massive tower construction is required to support the heavy blades, gearbox, and generator.
- Reflections from tall HAWTs may affect side lobes of radar installations creating signal clutter, although filtering can suppress it.
- Their height makes them obtrusively visible across large areas, disrupting the appearance of the landscape and sometimes creating local opposition.
- Downwind variants suffer from fatigue and structural failure caused by turbulence when a blade passes through the tower's wind shadow (for this reason, the majority of HAWTs use an upwind design, with the rotor facing the wind in front of the tower).

- HAWTs require an additional yaw control mechanism to turn the blades toward the wind.

1.6 Applications of VAWT

- Vertical Axis Wind Turbine or better abbreviated as VAWT successfully finds application in many areas ranging from power generation to power lamps. These are described in few details in the following points below: -
- Power Generation: - It is quite known and common that wind turbines have been used for power generation throughout the world. They are commercially available for electricity generation
- Water Pumping: - VAWT have used as mechanical application for pumping of water.
- In Telecommunications/Grid Monitoring: VAWT or Vertical Axis Wind Turbine are gaining wide popularity in the field of telecommunications since they are used for monitoring of mobile stations in Korea, China, etc. other countries.
- Wind & Solar Lamps: - Wind solar hybrid system for parking lot, etc. are being developed in various countries like Japan, etc. and their usage is mostly based on VAWT.
- Polar Research Station System: - VAWT have found their usage in Antarctica as for generation of power supply.
- Energy Recovery Systems: - There have been various researches going on throughout the world where VAWT are used as systems that are helpful in recovering energy.
- Increase in Power Generation System: - VAWT are used as substitute for gaining more power generation etc.

Chapter 2 Literature Review

2.1 Preceding Researches

A Finnish Engineer S. J. Savonius [1] started the early work of vertical Axis Wing Turbines (VAWT) in 1931. Then after, it had taken focus towards the eating of air from all the direction without turning the windmill. Since then various researchers have been involved to extract maximum energy available in the prevailing wind. Out of those some have been summarized here.

Work of Menet [2] deals with the conception of a small Savonius rotor (i.e. of low power) for local production of electricity thereby developing a prototype of a complete electromechanical system. Manet chose an optimized configuration for prototype. Low technicality and high efficiency of the Savonius rotors were achieved for local production of electricity.

Mahmoud et al. [3] have done experiments on Savonius rotor with different geometriesof 2, 3 and 4 blades and found that the rotor with two blades is comparatively efficient than 3 and 4 blades rotor. Providing end plates further raises the efficiency. Withou overlap ratio performance is better compared to with overlap ratio. Power increases with the increase of aspect ratio.

Investigation of Altan et al. [4] have introduces a new curtaining arrangement to improve the performance of Savonius wind rotors. The curtain arrangement was placed in front of the rotor preventing the negative torque opposite the rotor rotation. The geometrical parameters of the curtain arrangement were optimized to generate an optimum performance. The rotor with different curtain arrangements was tested out of a wind tunnel, and its performance was compared with that of the conventional rotor. The maximum power coefficient of the Savonius wind rotor was increased to about 38.5% with the optimum curtain arrangement. The experimental results showed that the performance of Savonius wind rotors could be improved with a suitable curtain arrangement.

Gupta et al. [5] have proposed a combined Savonius–Darrieus type vertical axis wind rotor. They suggested two types of models, one simple Savonius and the other combined Savonius–Darrieus wind rotors. The Savonius rotor was a three-bucket system having provisions for overlap variations, while the Savonius–Darrieus rotor was a combination of three-bucket Savonius and three- bladed Darrieus rotors with the Savonius placed on top of the Darrieus rotor. The overlap variation was made in the upper part, i.e. the Savonius rotor only. These were tested in a subsonic wind tunnel available in the department. From the investigation, it was seen that with the increase of overlap, the power coefficients start decreasing. The maximum power coefficient of 51% was obtained at no overlap condition. However, while comparing the power coefficients (Cp) for simple Savonius-rotor with that of the combined configuration of Savonius–Darrieus rotor, it was observed that there is a definite improvement in the power coefficient for the combined Savonius–Darrieus rotor without overlap condition. Combined rotor without overlap condition provided an efficiency of 0.51, which was higher than the efficiency of the Savonius rotor at any overlap positions under the same test conditions.

In order to decrease this variation in static torque from 0o to 360o, a helical Savonius rotor with a twist of 90o was proposed by Kamoji et al. [6]. The tests on helical Savonius rotors were conducted in an open jet wind tunnel. Coefficient of static torque, coefficient of torque and coefficient of power for each helical Savonius rotor were measured. The performance of helical rotor with shaft between the end plates and helical rotor without shaft between the end plates at different overlap ratios of 0.0, 0.1 and 0.16 was compared. Helical Savonius rotor without shaft was also compared with the performance of the conventional Savonius rotor. The results indicate that all the helical Savonius rotors have positive coefficient of static torque at all the rotor angles. Helical rotor without shaft at an overlap ratio of 0.0 and an aspect ratio of 0.88 was found to have almost the same coefficient of power when compared with the conventional Savonius rotor. Correlation for coefficient of torque and power was developed for helical Savonius rotor for a range of Reynolds numbers studied.

Fernando and Modi [7] have described a detail mathematical model based on the discrete vortex method for predicting the performance of a Savonius wind turbine. The method attempts to represent the complex unsteady flow field with separating shear layers in a realistic fashion consistent with the available computational tools. Important steps in the numerical analysis of this challenging problem were discussed at some length and a performance evaluation algorithm established. Of considerable importance was the effect of computational parameters such as number of elements representing the rotor, time-step size, location of the nascent vortices, etc., on the accuracy of results and the associated cost.

Saha et al [8] conducted a series of wind tunnel tests to assess the aerodynamic performance of single, two and three-stage Savonius rotor using both the semicircular and twisted blades in either case. All the rotors were provided identical stage aspect ratio keeping the projected area identical for each rotor. Experiments were conducted for the different parameters to be optimized such as the number of stages, number of blades (two and three) and geometry of the blade (semicircular and twisted). In addition, they attempted to investigate the performance of two-stage rotor system by introducing valves on the concave portion of the blade.

Altan et al. [9] have introduced a new curtaining configuration by placing before of the rotor to prevent the negative torque counter to the rotor rotation. Optimum performance was obtained by optimizing geometrical parameters. Various curtain arrangements were tested at the exit of the wind tunnel and thereby its performance parameter was compared with conventional rotor. Power coefficient was shown as increased by 38.5% with a suitable curtain arrangement.

Gupta et al. [10] have conducted a subsonic wind tunnel test on Savonius rotor and Savonius–Darrieus rotor. The overlap ratio was varied in only Savonius part. Power and torque coefficients were estimated with overlap as well as without overlap. Effects of overlap on Power coefficient were explained. A 51% maximum coefficient of power was extracted from without overlap condition. from the Savonius–Darrieus rotor.

Li and Hayashi [11] have conducted an investigation to reduce the discrepancy in torque in a Savonius rotor They also investigated to overcome the starting characteristics via three stages Savonius bucket blades arranged at 120o phase shift between the adjacent stages. A subsonic wind tunnel was utilized. The variations of static and dynamic torque per one revolution of the three-stage rotor were investigated to smoothen it. The torque characteristics with the guide vanes were also investigated. With the guide vanes, the torque coefficient was increased but exhibited low tip speed ratio while the torque coefficient was decreased exhibiting high tip speed ratio.

Fujisawa and Gotoh [12], have conducted experiments by means of measurements of pressure over the surface of the blade and by means of flow visualization to study power performance of Savonius rotor. They illustrated the formation of low pressure regime towards the convex part and called Coanda-like flow pattern via visualization. They noticed a two-dimensional flow field near the rotor because the coefficients of power and torque when estimated via measurement of pressure at the centerline of the blade closely resembled with the total torque measured.

In a subsonic wind tunnel, Sargolzaei and Kianifar [13] have experimentally predicted the torque and power factor of wind turbines utilizing the artificial neural networks (ANNs). The data were collected for 7 prototypes of Savonius rotors. The main input parameter to be predicted in neural network is the tip speed ratio (TSR). They simulated torque and the power factor for different tip speed ratios as well as the different blade angles. The simulations were performed to show the capabilities of ANNs to predict and estimated power and efficiency of the rotor.

In the work of Tabassum and Probert [14] the portions of each Bach-type blade of a unoriginal Savonius rotor have been substituted by four flaps. While the flap faced the wind it opened, and reduced the soothe drag on the blade as result the average static torque nearly increased by 35% compared to that of conventional rotor of alike geometry. Both the batch –type as well as conventional rotor have given wind of 6.67 m/s. There was no existence of negative torque in batch-type rotor which not the same in conventional rotor.

Mojola [15] has examined the performances characteristics of Savonius windmill rotor under field conditions. The speed, torque and power of the rotor were tested at numerous speeds of wind by varying the overlap ratios. Some design criteria were established by fully representing and discussing the performance data.

Irabu and Roy [16] have conducted experiments for the measurement and clarification of quantifiable disparities of force of steadily and uniformly flowing fluid on three classes of blade arrangement on a Savonius rotor. By splitting both the end-disks of the blades into two half parts, the two blades were independent on each other and the one blade attached to both the half end-disks was only connected to the axis rod of the rotor, and the other blade connecting to the other half end-disks was free from the axis rod. Therefore, the lift and drag forces acting on the previous blade were measured directly at each phase angle in a short period and each overlap-ratio of the two blades using the force balance meter. The rotor was provided 160 mm diameter and 160 mm height keeping in wind stream at Re = 0.64x105. The drag coefficients were calculated. For the tests overlap ratio was kept zero.

Saha and Rajkumar [17] have done experiments twisted on twisted blades with three blades. The wind speeds were kept low and the experiments were conducted in wind tunnel. The angle of twist was given 1o. The performance of twisted blade was compared with conventional rotor of semi-circular blades. Starting characteristics, static torque and rotational speed were the performance parameters tested. Twisted blades, as compared to the conventional bladed rotor, established smooth running, higher efficiency, and self-starting capability. Experiments were also conducted to optimize the angle of twist.

Li et al. [18] conducted experimental tests on subsonic wind tunnel on straight-bladed vertical axis wind turbine (VAWT) to assess the phenomenon of stalling linked with unsteady flow around the air foil. To examine the flow, tuft flow visualization on the internal surface of the blade were done at low Reynolds numbers. Numerical examination was also carried out in a two-dimension domain utilizing standard k-ε and k-ω SST turbulence models for the stalling phenomenon and flow structures around the airfoil.

Bhutta et al. [19] have exposed the fact that the two blades rotor is relatively more effective compared to the arrangement of three and four blades in Savonius rotor. The relative effectiveness of two blades can be improved by putting end plates on the two-blade system. In addition, two-stage compared to the single stage rotor has higher performance. They also conducted tests on overlap rations as well as aspect ratios and established that output increases with aspect ratio. Static torque and power coefficient were mainly judged.

Loganathan et al. [20] have investigated the effects of size of a multi blades type Savonious rotor on power. They designed three-blade system of 8, 16 and 24 blades, 300 mm diameter and 160 mm blade length and evaluated the power output with different wind speeds. The rotor diameter and blade lengths and radius were also doubled and then tested. With various experiments, they reported the power improvement of about 80% by multiplying the size and of about 50% by doubling the blade diameter and blade lengths and radius.

Loganathan and Mustary [21] again computationally investigated the VAWT based on performance by high aspect ratio by means of ANSYS CFX. Shape-design model and mechanism model of turbine blade were investigated. Computations were conducted to estimate drag force produced as an effect of blade shape. Coefficient of power of both models were compared. Ratio of “Turning to Reverse Force” was cast-off to correlate the distinct shapes of the turbine blade. Mechanism model attained 16.2% of power coefficient which was better compared to the Shape-design model as far as the harnessing of energy in the wind is concerned.

Wenehenubun et al. [22] have conducted experiments to judge the Savonius rotor performance by means of number of blades. Two, three and four blades were tested based on trends on power coefficient, tip-speed-ratio and torque where the free-stream wind velocity was varied. As conclusion, they have reported that:

1. Turbine rotation is greatly dependent on blade’s count.

2. Four-blade rotor produces more torque compared to two or three-blade rotor.
3. Performance of four-bladed rotor at lower tip-speed-ratio was better and that of three-bladed rotor at high tip-speed-ratio.

Molina et al. [23] tested an H-Darrieus VAWT with a turbulence intensity of 5% and compared the results with free-stream with no turbulence condition. They reported that turbulence intensity has high impact on turbine performance causing power drop at high speeds of rotation, special control is required due to troublesome conditions and high turbine vibration resulted.

Castelli et al. [24] showed a mode to prevent deviations in torque during the rotation of a VAWT in terms of increase in number of blades. 2D computations were performed on a straight-bladed Darrius-type rotor by taking 3, 4 and 5 numbers of rotor blades of the profile of NACA0025 airfoil. Varying the tip-speed-ratio by varying the blade numbers, the torque and power curves of the rotor were analyzed and compared.

Brusca et al. [25] have looked into the designing a VAWT to maximize coefficient of power. A numerical code of Multiple Stream Tube Model was made. It was shown that the solidity in rotor and Reynolds number influences the coefficient of power. It was also analyzed that aspect ratio influences the Reynolds number. It was suggested that turbine having low aspect ratio have advantages over theat with high aspect ratio.

Didane et al. [26] have employed the contra-rotating idea to a VAWT system. Torque and power coefficients were analyzed. They claimed that power improvement is threefold over the entire functioning ranges of wind speeds. It also improved the inherent complications of the Darrieus rotor of self-starting. It can work under high intensity of turbulence too.

Battisti et al. [27] Have compared the different small VAWT system with respect to performance and loads. A Blade Element-Momentum (BE-M) model was prepared to find aerodynamic loads and performed experiments on it. And some features of the rotor were changed viz. number of blades i.e. 2, 3, etc., airfoil camber line (symmetrical/asymmetrical), inclinations of the blades (straight/helical), etc. The effect of all such provisions on both the power and the thrust in terms of both the blades azimuthal condition and tip-speed-ratio were reported and discussed.

Bhuyan and Biswas [28] have highlighted the difficulty of self-starting and high performance of H-rotor due to symmetrical blades of NACA airfoil type, although it provides high coefficient of power. To overcome such difficulty, they investigated unsymmetrical cambered S818 airfoil type blades which were three in numbers in an H- rotor that showed self-starting at several azimuthal angles. For making self-starting H-rotor was made hybrid system with Savonius rotor. Experiments were conducted for Reynolds number of 1.44x105 to 2.31x105. Five distinct overlap ratios were used on Savonius part. Hybrid system performed better compared to non-hybrid. Power coefficient of 0.34, tip-speed-ratio of 2.29 at Re = 1.94x105 under optimum overlap ratio of 0.15 was obtained as the best performance.

Bianchini et al [29] have employed a Blade Element Momentum (BEM) approach to predict performance maps. Concentrating on the main streamlined necessities, a contrary conduct was found amongst cambered and symmetric, un cambered airfoils. For uncambered profiles, when the normal wind speed in the site builds up, the best solidity diminishes continually, increment in aspect ratio, while the Shape Factor U increments for medium– low normal wind speeds and afterward ends up stabilizing. This pattern is essentially because of the way that, by intensifying the wind speed, the relative velocity is expanded, and the Reynolds number is paced up. The chords can be then lessened, with outstanding benefits regarding blade's efficiency. An all-around characterized reliance on the tsr ratio was additionally seen: by diminishing the t/c ratio, the ideal solidity is continually lower while higher Aspect Ratios are ideal. With this determination, the ideal solidity is likewise extremely low.

Computational work of Chen et al. [30] explains that the power yield of two straight-blade VAWT system is re-enacted using CFD and additionally investigated to improve accompanying the Taguchi technique. Five working elements of approaching stream angle, tip speed ratio, turbine separation, blade angle and rotational direction alongside four levels are considered to represent their effects on the execution of the dual turbine framework. The profile of degree demonstrates that the approaching stream angle and tip speed ratio assume critical parts in deciding power yield, while the blade angle nearly has no impact on the power output.

Tian et al. [31] reported that in order to increase the coefficient of power of Savonius turbine, some novel blade shapes must be considered. They conducted computational analysis to investigate the influence of blade fullness on power generation, which is a crucial blade factor for a Savonious rotor. The strategy of numerical scheme was made compatible with experimental data which were used to estimate the design parameters. A relation between the turbine performance and the blade fullness was obtained. The blade with fullness 1 revealed a power coefficient of 0.2573 showing 10.98% improvement over conventional Savonius rotor.

Delafin et al. [32] carried out an integral analysis for designing the VAWT on the basis of blades counts. Two, three and four bladed turbines were simulated by means of a free-wake vortex model. The effect of solidity on coefficient of power was presented while the momentary torque, thrust and lateral force of 2, 3, 4-bladed turbines were equated with a similar solidity. It was discovered that any blade increment fundamentally diminishes the torque, thrust and lateral force. Including a fourth blade furthermore cuts the ripples aside from the torque at low tip speed ratio.

Sharma and Sharma [33] have conducted computations to carryout performance based comparative investigation between a multiple quarter bladed Savonius rotor and a conventional Savonius rotor. The computations were conducted on commercial software-Ansys CFX. Numerical work was compared with the experimental work of Saha et al. Comparative study showed that multiple quarter bladed Savonius rotor has performed better compared to conventional rotor and provided the power coefficient of 0.2266 with free-stream wind velocity of 8.23 m/s showing 8.89% improvement.

Joo et al. [34] have conducted a 3D unsteady numerical investigation on vertical axis and 2 straight blades rotor. A correlation of streamlined qualities at different operational parameters was performed. The free stream course which had been moving towards the blade was extensively bowed after the communication between blade-to-blade and blade-to-free stream. Despite the fact that the maximum torque assessed with solidity, solidity alone does not enhance the execution of the H-Darrieus, the obstruction and collaboration may assist the torque increment. The obstruction created by the blades in the wind caused its revolution while the connection between the blades and the angle of attack altogether change the extent of tip-speed ratio and increased the power.

Kjellin et al. [35] have conducted tests on a 12KW vertical axis H-rotor. The estimation of aerodynamic parameters was done at various rotational speeds on the turbine amongst differing wind speeds various tip-speed-ratio. The highest power coefficient was obtained as 0.29 for a tip-speed-ratio close to 3.3.

Posa et al. [36] have conducted PIV investigation and computed via LES to investigate the wake structure behind a VAWT. Simulation was conducted at Re= 1.8x105. They conducted experiments and computations were extremely matched with incessant two-way feedback to produce the most insightful results. They investigated the wake structure dependency on tip-speed-ratio. They discussed instantaneous, ensemble-averaged and phase-averaged fields and as well as the coherent structures dynamics in the rotor region and downstream wake were discussed.

Nasef et al. [37] numerically conducted an aerodynamic study in a commercial software FLUENT over five Savonius rotors with two blades under static and dynamic condition and comparing four different turbulence models (RNG k–ε, Standard k–ε, Realizable k–ε and SST k–ω) and various overlap ratios (0, 0.15, 0.2, 0.3 and 0.5). The rotor was given different angle between 0o and 180o. The SST k–ω turbulence model provided the closest results. It was concluded that the improvement in coefficient of static torque may be achieved with the overlap ratio particularly on the returning blade. Under rotation condition maximum performance was achieved at the overlap ratio of 0.15.


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Effect of some design parameters: A performance test on VAWT
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Blade testing, VAWT, Some effects on performance
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S. K. Dhiman (Author), 2018, Effect of some design parameters: A performance test on VAWT, Munich, GRIN Verlag, https://www.grin.com/document/440975


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