**5.1 Drag-based turbines**

The Drag-based turbines have the advantage of self-starting ability, and they are commonly found as small-sized turbines in urban and remote areas with relatively low wind speed. These turbines generally are not preferred due to high solidity, heavier weight, and low efficiency. One example of this turbine is the Savonius turbine [24], **Figure 12** shows characteristic parameters of a Savonius wind turbine with two semicircular airfoil blades.

The Savonius turbine produces high torque at low tip-speed ratios (λ) due to the large area facing the wind. The disadvantage with this turbine is that the same drag

#### **Figure 11.**

*Schematic view of different types of VAWTs from left to right: S-type Savonius wind turbine, straight-type, Troposkien-type, and helical-type Darrieus wind turbine [24].*

**Figure 12.** *Two bladed Savonius rotor [25].*

#### *Vertical Axis Wind Turbine Design and Installation at Chicamocha Canyon DOI: http://dx.doi.org/10.5772/intechopen.99374*

of the blades which is used to produce power also works against the turbine by the returning blades, reducing the power that can be obtained [15].

According to Zemamou et al. [25] the number of blades has an important impact on the turbine performance. For obtaining the highest value of the *Cp* under the same test condition, a Savonius turbine must have two blades as shown the **Figure 13**.

#### **5.2 Lift-based turbines**

The lift-type turbine consists of airfoil sections that capture the wind energy using the lift force. This lift force produces torque on a shaft, which can then be connected to a generator to produce electricity as power output [15]. The advantage of this configuration is their simple and extruded blades, hence lower manufacturing costs [24]. The straight type, Troposkien type, and helicoidal type are examples of this configuration.

#### *5.2.1 Straight type*

These blades usually are used in small-scale, fixed pitch, rooftop designs are commercially available for domestic and other applications. The straight blades have a high value of *Cp* (0.23). This configuration can have any number of blades, from one to a configuration of five. However, the most used are two-bladed (commonly called H-type turbines) or three-bladed [26]. In **Figure 14** it can be the straight blades with two and three blades.

According to Ali and Sattar Aljabair [27] this configuration is better than the type helicoidal at low wind velocity, also, the power coefficient values for DWTs straight model with 2 blades are higher than other models as can see in **Table 5**.

**Figure 13.** *The Cp variation with the TSR for two & three blades [25].*

#### **Figure 14.**

*Darrieus WT type straight blades with two and three blades [27].*


#### **Table 5.**

*The (CP) at various wind velocities for DWT models number [27].*

#### **Figure 15.**

*The numerical relationship between CP and TSR for the DWT models has 2 blades [27].*

## The Straight blades present a higher value of *Cp* compared to the others as shown the **Figure 15**.

#### *5.2.2 Troposkien*

The Troposkien architecture is characterized by hub-to-hub blades, this configuration offers a lower aerodynamic drag (compared to the H-shaped one), which minimizes the bending stress in the blades [28]. In **Figure 16** it can see the Troposkien type.

*Vertical Axis Wind Turbine Design and Installation at Chicamocha Canyon DOI: http://dx.doi.org/10.5772/intechopen.99374*

According to Battisti et al. [28] the Troposkien type is more efficient than the Hshaped configuration (two straight blades) at high values of TSR as can see in **Figure 17**. On other hand, for low values of TSR, the Troposkien present a lower *Cp* compared to the other type.

Quite similar behavior is registered for low wind velocities and a cut-in wind speed of 6–6.5 m/s is observed. For high values of wind velocity, the Troposkien is capable to generate significantly more power than the H-shaped configuration as shown in **Figure 18**. This quite different behavior could relate to the higher blade Reynolds number, which promotes an improved aerodynamic efficiency, in the investigation of [28], the radius of the Troposkien type is bigger than the H-shaped type to maintain the same rotor swept area. For this reason, the Troposkien type have a bigger *Rec* and consequently bigger efficient and more power generated.

**Figure 17.** *Rotor power coefficients, as a function of the equatorial tip speed ratio [28].*

**Figure 18.** *Power curves for the two analyzed rotor configurations [28].*

### *5.2.3 Helical type*

Helical H-rotor distributes the blade airfoil along the rotor perimeter uniformly, thus making the swept area as well as the blade sections constant to the wind in all cases of turbine rotation [29]. In **Figure 19** it can observe the Helical type.

Tjiu et al. [29] made a comparison was made between the helical, straight, and Troposkien types. The comparison was made using 3 blades using the NACA 0015 airfoil with a TSR of 5. The behavior can be seen in **Figure 20** where it can be observed that the Troposkien typology obtained the highest fluctuation with a Cp value of approximately 0.3, the straight blades typology had a fluctuation in the Cp of 0.2 and the lowest fluctuation was obtained by the helicoidal rotor with a variation of approximately 0.03 Cp. However, despite the benefits obtained, the helical blades are more expensive to manufacture, so depending on the desired application

**Figure 19.** *Helical design [30].*

*Vertical Axis Wind Turbine Design and Installation at Chicamocha Canyon DOI: http://dx.doi.org/10.5772/intechopen.99374*

**Figure 20.** *Power coefficient variations of a typical Troposkien rotor, H-rotor, and helical H rotor.*

and the available budget, a middle point must be chosen for the selection of the different types of rotors.

### **5.3 VAWT selection**

A critical factor in the feasibility of power generation with VAWT turbines is the self-starting of the turbine, according to Ali and Sattar Aljabair [27], at a wind speed of 3 m/s, the VAWT with airfoil DU06W200 has the capability of self-starting as seen in **Table 6**. The straight blade type has better performance because the turbine can self-start at lower wind velocity than the others turbines.

The straight blade configuration offers the flexibility to adjust the swept area. Rotor height and diameter can be independently adjusted to suit each design. In addition, this configuration is usually mounted on a tower, which provides higher stability, lower bending, and torsional stresses on the blades compared to the Troposkien topology. Similarly, the gravity-induced bending stress is lower in the straight-bladed configuration as they are stiffer with the same chord length and thickness as the blades of a Troposkien rotor. In addition, they are vertically


**Table 6.**

*The wind speed at which Darrieus WT models can be DWT auto-started [27].*


#### **Table 7.**

*Characteristics of the proposed VAWT designs [13].*

positioned and suspended by supports, so they are not subjected to constant bending stress due to gravity [31].

In his investigation Meana-Fernández et al. [13] proposes an optimized design for medium and low wind speed which presents a maximum ?? of 0.5798 and 0.5996 respectively, as observed in **Table 7**.

The type of blades used by [13] were straight blades, it is observed that the proposed design presents a good performance for both medium and low wind speed. It should be noted that for low speeds, as described throughout this section, straight blades perform well without the complexity of construction and high manufacturing cost of the helical type for example, or the instability and torsional stress produced by the Troposkien type.
