A Review about the Straight- Bladed Vertical Axis Wind Turbine (SB-VAWT) and its Performance by IJIRST

IJIRST 窶的nternational Journal for Innovative Research in Science & Technology| Volume 1 | Issue 6 | November 2014 ISSN (online): 2349-6010

A Review about the Straight-Bladed Vertical Axis Wind Turbine (SB-VAWT) and its Performance Khemraj Patel Student Department of Mechanical Engineering Kirodimal Institute of Technology Raigarh (C.G.) INDIA

Arti Tirkey Student Department of Mechanical Engineering Kirodimal Institute of Technology Raigarh (C.G.) INDIA

Yamini Sarthi Student Department of Mechanical Engineering Kirodimal Institute of Technology Raigarh (C.G.) INDIA

Prakash Kumar Sen Faculty Department of Mechanical Engineering Kirodimal Institute of Technology Raigarh (C.G.) INDIA

Shailendra Kumar Bohidar Faculty Department of Mechanical Engineering Kirodimal Institute of Technology Raigarh (C.G.) INDIA

Abstract This paper tries to present a review about the straight bladed vertical axis wind turbine (SB-VAWT), which is used to convert the sustainable energy of wind to an electrical energy. This also include about the materials used for blades of SB-VAWT and the performance of SB-VAWT. Performance of SB-VAWT is depends upon the thickness of blade, number of blades, setting angle of blade and blade arrangements in one or more stages. This paper focused on SB-VAWT, which is less popular as compared to horizontal axis wind turbine (HAWT). Keywords: SB-VAWT, VAWT, HAWT, CB-VAWT _______________________________________________________________________________________________________

I. INTRODUCTION In present, sustainable energy is very popular all over the world, either in developed or developing countries. Now days, consumption of non-renewable source of energy is very high and because of this, it getting lower and lower every day. Wind energy is one of the most popular and cheapest sources of sustainable energy and the wind energy is converted into electrical energy by means of a wind turbine either HAWT or VAWT. The operation of wind turbine is depends on the average velocity of wind, which is generally differs place to place on earth. The average velocity of wind must be at optimum level to initiate the turning of blade. The average velocity required for VAWT is very low and VAWT may be of straight or curved blade turbine, according to the shape of the blade. For a small scale wind power generation, straight-bladed vertical axis wind (SB-VAWT) turbine is more popular because of its simple in design, low cost and also good maintenance [1]. SB-VAWT is one of the simplest types of wind turbine, which is very simple and uncomplicated mechanically. In design of SB-VAWT, materials used for blade is a very important parameter, the material for blade should have long life at lowest cost as much as possible. Al blades are mostly used for VAWT, but it having poor fatigue properties. Therefore selection of blade material at low cost for SB-VAWT is very crucial. Blade, supporting strut and central column are three major components of SBVAWT as shown in Fig. 1 [2]. At present, most of the commercial VAWT systems on the market have the output power of 1ﾃｷ5 kW and blade chord length of 0.2ﾃｷ0.3 m. They are designed for wind speed between 3ﾃｷ15 m/s and its associated Reynolds numbers are low (Re < 105). There are few research publications that focus on optimal structure of VAWT with straight blades [3-4]. As pointed out by the First International Conference on Wind Turbine Noise [5], the development of onshore wind power results in wind turbines closer to habitations, leading to the possibility of noise problems, such that frequent objections raised in planning procedures concern noise and vibration. As observed by Raciti Castelli [6], the vertical axis wind turbine has an inherently non-stationary aerodynamic behavior, mainly due to the continuous variation of the blade angle of attack during the rotation of the machine: this peculiarity involves the continuous variation both of the relative velocity with respect to the blade profile and although to a lesser extent of the corresponding Reynolds number. This phenomenon, typical of slow rotating machines, has a significant effect both on the dynamic loads acting on the rotor and on the generated power and, therefore, on performance.

A Review about the Straight-Bladed Vertical Axis Wind Turbine (SB-VAWT) and its Performance (IJIRST/ Volume 1 / Issue 9 / 008)

NOMENCLATURE VAWT HAWT SB-VAWT CB-VAWT FRP Al

Vertical axis wind turbine Horizontal axis wind turbine Straight blade Vertical axis wind turbine Curved blade Vertical axis wind turbine Fiber Reinforced Plastic Aluminum

II. SB-VAWT BLADE MATERIALS In design consideration, selection of blade material is very important parameter; the material should have low cost and high operating life. The smaller wind turbine blades are usually made of aluminum, or laminated wood [7]. Metals were initially a popular material because they yield a low-cost blade and can be manufactured with a high degree of reliability, however most metallic blades (like steel) proved to be relatively heavy which limits their application in commercial turbines [8]. In the past, laminated wood was also tried on early machines in 1977 [9]. There are many materials used for design of SB-VAWT, such as wood, Al and fiber glass composites. Wood, a naturally occurring composite material, is readily available as an inexpensive blade material with good fatigue properties [10]. Wood has been a popular windt urbine blade material since ancient time.Wood has relatively high strength-to-weight ratio, good stiffness and high resilience. The application of wood to large blades is hindered by its joining efficiency which in many cases has forced designers to examine other materials [8].

Fig. 1: The Main Components of a Typical SB-VAW

Aluminum blades fabricated by extrusion and bending are the most common type of VAWT materials. The early blades of Darrieus type VAWTs were made from stretches and formed steel sheets or from helicopter like combinations of aluminum alloy extrusions and fiberglass. It has been reported by Parashivoiu [11] that the former were difficult to shape into smooth airfoil, while the latter were expensive. The major problem that aluminum alloy for wind turbine application is its poor fatigue properties and its allowable stress levels in dynamic application decreases quite markedly at increasing numbers of or approximately twice the strength of low carbon steel [13]. In recent times, pultruded FRP blades have been preferred by one of the leading small HAWT type wind turbine manufacturer like Bergey [13] and a few other small wind energy conversion system [10]. SB-VAWT mostly uses two materials Al and FRP as the blade material. Cyclic stress applications when compared to other materials such as steel, wood or fiberglass reinforced plastics [10]. Now a day, fiber glass composite becomes very popular for selection of blade material of SB-VAWT. Composites constructed with fiberglass reinforcements are currently the blade materials of choice for wind turbine blades [8] of HAWT types. This class of materials is called fiberglass composites or fiber reinforced plastics (FRP). In turbine designs they are usually composed of E-glass in polyester, vinyl ester or epoxy matrix and Blades are typically produced using hand-layup techniques. Recent advances in resin transfer moulding and pultrusion technology have provided the blade manufacturers to examine new procedures for increasing the quality of the final product and reducing manufacturing costs [8]. The characteristics that make composites, especially glass fiber-reinforced and wood/epoxy composites, suitable for wind turbine blades are [12]:  low density;  good mechanical properties;  excellent corrosion resistance;  tailor ability of material properties; and

A Review about the Straight-Bladed Vertical Axis Wind Turbine (SB-VAWT) and its Performance (IJIRST/ Volume 1 / Issue 9 / 008)

 Versatility of fabrication methods. Pultruded FRP is very economical for manufacturing of SB-VAWT blade. Pultrusion is a continuous forming process that allows for a very high glass fiber content, which results in a very high strength, yet flexible rotor blade and the basic material strength is in the order of 100,000 psi (689.5 MPa)

III. REQUIRED PROPERTIES OF SB-VAWT BLADE MATERIAL SB-VAWT blades are exposed to diversified load conditions and dynamic stresses are considerably more severe than many mechanical applications. Based on the operational parameters and the surrounding conditions of a typical SB-VAWT for delivering electrical or mechanical energy, the following properties of the SB-VAWT blade materials are required [14]:  It should have adequately high yield strength for longer life;  It must endure a very large number of fatigue cycles during their service lifetime to reduce material degradation;  It should have high material stiffness to maintain optimal aerodynamic performance;  It should have low density for reduced amount of gravity and normal force component;  It should be corrosion resistant;  It should be suitable for cheaper fabrication methods;  It must be efficiently manufactured into their final form; and  It should provide a long-term mechanical performance per unit cost Among all these requirements, fatigue is the major problem facing both HAWTs and VAWTs and an operating turbine is exposed to many alternating stress cycles and can easily be exposed to more than 108 cycles during a 30 year life time. The sources of alternating stresses are due to the dynamics of the wind turbine structure itself as well as periodic variations of input forces [10].

IV. PERFORMANCE OF SB-VAWT In design consideration, the performance or output of SB-VAWT depends upon the thickness of blades, blade arrangements in one or two stages, number of blades and the setting angle of blades. VAWT developed torque at low speeds is greater when the setting angle of blades is greater but the VAWT developed torque at higher speeds is greater when the setting angles of blades is less [15].

(a) Two-bladed VAWT (b) Three-bladed VAWT Fig. 2: Rotational speed of wind turbine [16]

From the figure above, it can be concluded that the SB-VAWT has a relatively low cut-in velocity as compared to curvedbladed VAWT irrespective the number of blades. The CB-VAWT with two and three blades has a cut-in velocity of about 7 m/s and 4 m/s respectively. The three blades VAWT has lower cut-in velocity as compared to the one with two blades. This observation can be explained as follows; the additional blade have larger wetted surface and hence giver higher torque, which finally results in a lower cut-in velocity. However, both cut-in velocities are quite high to be compared to the average wind velocity [16]. A VAWT should have good self-starting performance. A VAWT is considered to be self-starting only if can accelerate from rest to the point where it starts to produce a useful output or, more specific, if the rotor has accelerated from the rest to a steady speed that exceed the wind speed, i.e. the tip-speed ratio1 TSR > 1, when the transition from the drag driving to the lift driving take place [17, 18].

A Review about the Straight-Bladed Vertical Axis Wind Turbine (SB-VAWT) and its Performance (IJIRST/ Volume 1 / Issue 9 / 008)

Fig. 3: Torque â&#x20AC;&#x201C; rotational speed diagram for two stage three blades rotor with 6 degree pitch

Fig. 4: Torque â&#x20AC;&#x201C; rotational speed diagram for single stage six blades rotor with 3 degree pitch

A Review about the Straight-Bladed Vertical Axis Wind Turbine (SB-VAWT) and its Performance (IJIRST/ Volume 1 / Issue 9 / 008)

Fig. 5: Torque â&#x20AC;&#x201C; rotational speed diagram for two stage three

Fig. 6: Comparative torque â&#x20AC;&#x201C; rotational speed diagrams based on blades rotor with 3 degree pitch curve fit equations for the analyzed rotor configurations Setting angle of blade is also very important parameter for performance of SB-VAWT. Three different turbine rotor configurations which comprise two stages, each with three blades, arranged at 0 and 3 setting (pitch) angle degree and one stage with six blades arranged at 3 setting (pitch) angle degree were considered. As shown in Figure 3, 4, 5 and 6 [15].

V. CONCLUSION The vertical axis wind turbine (VAWT) is either of curvedbladed (CB) or straight-bladed (SB) type VAWT according to design configuration but the SB-VAWT is mostly used because of its simplicity, Whereas CB-VAWT is rarely used due to its manufacturing difficulties. It is concluded that, to efficiently and economically change the wind energy to electrical energy, the SB-VAWT is mostly used. There are different types of materials being used for the manufacture of SB-VAWT blades such as wood, fiberglass composites, Aluminum and Pultruded FRP. The performance of SB-VAWT affected by the blade thickness,

A Review about the Straight-Bladed Vertical Axis Wind Turbine (SB-VAWT) and its Performance (IJIRST/ Volume 1 / Issue 9 / 008)

blade arrangements in one or two stages, number of blades and setting angle of blades. The straight blades Darrieus turbine has self-starting Capabilities both at low Reynolds numbers and to moderate values of the solidity. The blade pitch angle has a great influence on the self-starting behavior of the turbine: higher values of the blade pitch angle are Favorable for a low speed wind starting. According to the results showed in Fig. 6, the VAWT developed torque at low speeds is greater when the setting angle of blades is greater but the VAWT developed torque at higher speeds is greater when the setting angles of blades is less.

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