**4. VAWT design**

VAWT design correlates geometrical characteristics of the rotor with the Power Coefficient (22) of the turbine. The influence of the main aerodynamic design parameters is compared with the operation of turbines [20]. This section presents the considerations and parameters necessary for the construction of VAWT turbines. The design procedure taking aerodynamics into account can be expressed as follows:


#### **4.1 Application and desired power**

The VAWT turbines have different applications [21] to generate electricity, pump water, purify and/or desalinate water by reverse osmosis, heating, and cooling using vapor compression heat pumps, mixing and aerating bodies of water; and heating water by fluid turbulence. Rathore et al. [22] suggests VAWT use on highways, in which vehicles travel at high speed in both directions producing an

acceleration of the surrounding wind that can be used by turbines located in the separators.

To do so, the power (*PT*) required for the application is selected. This *PT* is given for a particular velocity (*V*), area (*A*), density of the air (*ρ*), probable Power Coefficient (*Cp*) and efficiencies of the mechanical components (gearbox, generator, etc.) (η) as following in the Eq. (16) [6].

$$P\_T = C\_p \,\,\eta \,\,\frac{1}{2} \rho V^3 A \tag{16}$$

#### **4.2 Geometrical aspects**

Among the main aspects of VAWT turbines are the chord length (c), rotor height (H), rotor diameter (D), and aerodynamic airfoil (**Figure 10**).

## *4.2.1 Height/diameter ratio (Φ = H/D)*

The relation Φ is analyzed from the turbine shape indicating the visual proportions of the turbine. On the other hand, for a fixed sweep area, low Φ values are characteristic of turbines in which optimal flow conditions are obtained in the aerodynamics airfoil, due to large diameters that increase the peripheral speed. On the contrary, high values of Φ can be related to turbines where blade efficiency is preferred [20].

The Darrieus rotor has low aspect ratios to minimize the length of the blade and the center column for a given swept area. If the Φ is increased, then the rotor speed increases (to maintain the same relative wind speed and tip speed ratio), and torque decrease if power is constant [11].

#### *4.2.2 Chord/diameter ratio (ξ = c/D)*

High ξ values indicate that chord length is increased to improve the Reynolds number, while low values relate to rotors in which the relative wind speed increases proportionally to the relative wind speed on the aerodynamic airfoil [20].

**Figure 10.** *Schematic view of the architecture of the Darrieus turbines [23].*

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

#### *4.2.3 Rotor swept area (A)*

The swept area of the turbine (**Figure 10**), corresponds to the amount of air that is dragged by the turbine blades. In particular, the larger sweep areas guarantee fewer demanding limits of the turbine radius, therefore a high peripheral speed is obtained leading to a good Reynolds number on the blades.

The energy capture is proportional to the swept area and the cube of wind velocity. It is important to identify an equilibrium between energy capture and the cost of the swept area, a bigger area means more manufacturing cost of the turbine. The parameters Φ and ξ are geometric parameters that allow modifying the swept area of the turbine, they are directly related to the design of VAWT turbines.

### *4.2.4 Number of blades*

According to Paraschivoiu [11] for given solidity, it is structurally advantageous to have fewer blades of a larger chord rather than more blades of a smaller chord. This is due to the bending stresses which are dependent on the square of the chord size whereas the aerodynamics loads are dependent on only the first power of the chord. For these reasons, the VAWT have generally two or three blades, but each design is unique for each application, therefore, it's important to analyze the relationship between the geometric parameters as the solidity and the *Cp* of the turbine. **Table 3**, can show some advantages for two and three blades in a VAWT.

#### **4.3 Airfoil selection**

The VAWT blades' performance depends largely on the airfoil behavior, which is selected or designed in terms of the wind flow conditions of the feasible location [1].

Employing CFD modeling, Garcia Rodriguez [1] found that the DU06W200 airfoil aerodynamics performance is larger than NACA0018 under the Chicamocha's canyon wind energy conditions. **Table 4** summarizes the calculated


#### **Table 3.**

*Advantages of two or three blades [11].*


#### **Table 4.**

*Lift and drag coefficients of the airfoils NACA0018 and DU06W200 under Chicamocha's canyon wind speed [1].*

aerodynamic coefficients of the most feasible point, proving the advantage of considering the DU06W200 airfoil [1].
