For the disadvantages of the HAWT, most turbines are not efficient when it is installed close to the ground where there isn’t much wind. Also, the tall tower and long blades are very expensive and difficult to transport and install. Another disadvantage of HAWT is that it is difficult to perform repairing operations to HAWT because the gearbox, generator and the rotor are all placed at the top of the tower.
Friday, March 13, 2009
Advantage of Vertical Wind Turbine
The advantage of VAWT is that it doesn’t need to always be faced against the wind’s direction like the HAWT. The blades of the vertical axis wind turbine can intercept the wind from any directions because of the blade design. The VAWT are usually of smaller scale turbine than most HAWT and therefore it is easier to be transported and installed. The VAWT can also be use in residential area on roofs because of its smaller size. It is also easier to repair VAWT because the gearbox and generator are placed at the bottom of the turbine.
Disadvantage of Vertical Wind Turbine
The disadvantage of the vertical wind turbine comparatively to the horizontal wind turbine is the efficiency. In some cases, the efficiency of the energy produced of a VAWT can be less than the half of the production of a HAWT. The VAWT designs are also limited in height because of the sweep area available. The VAWT can only be installed on flat surfaces. Many VAWT types need to be started manually by giving an initial momentum or by an automatic starting mechanism because they have low starting torque.
Aerodynamics of Rotor Blades
The aerodynamics principles of the two types of wind turbine are different. The HAWT and VAWT can have many different configurations of blades; therefore the aerodynamic concept may be different. In most cases, HAWT blades are aerodynamically based on the lift. The higher the lift forces, the higher power we get. On the other hand, the VAWT is mostly based on the drag forces.
Aerodyanamics of Horizontal Wind Turbines
The horizontal wind turbine types of blades are usually made of two or three airfoils such as a propeller. In these types of blades, it is the lift force which makes the rotor turn. As shown in figure 4, when the wind hits the airfoil, the wind gets separated into two. It will pass over the top and the bottom side of the airfoil. Since the top distance is longer, the wind velocity will increase, therefore creating lower pressure on the top of the airfoil. The lower portion will therefore have a higher pressure than the top of the airfoil and this difference in pressure will result in the force known as the lift.
Principles of HAWT Aerodynamic LiftL=1/2ρV^2ACL
Where
ρ=Density of air [kg/m^3]
V=Velocity of the wind [m/s]
A=Surface area [m^2]
CL=Lift coefficient
The drag force acts perpendicular to the lift force due to the resistance of the airfoil from the wind and would counteract the rotation to rotor. The higher the lift-to-drag ratio, the higher the torque output would be for the wind turbine.
Aerodynamics of Vertical Wind Turbines
For the vertical axis wind turbines, there are more creativity and variety in the design of the blades. Some sub-types turbines use blades that are based on lift forces such as the Darrius. But most of the VAWT rely on drag force to rotate the rotor shafts such as the Savonius and the Zephyr types wind turbine. When the wind hits the blade, the resistance of the blade would create a force called drag. The drag applied on the blades would create a torque and rotate the rotor shaft. The drag force can be obtained from the below equation:
D=1/2ρV^2ACd
Where
ρ=Density of air [kg/m^3]
V=Velocity of the wind [m/s]
A=Surface area [m^2]
Cd=Drag coefficient
The drag coefficient is dependant on the geometry of the blade. The table below gives the coefficient of various shapes. In order to obtain a higher torque output, the drag must be maximized. The design of the blade must therefore be made of geometry of high drag coefficient such as a hollow semi-cylinder or a long flat plate at 90°. The blade must also be as large as possible since the drag is directly proportional to the surface area.
Drag Coefficient
Economy
Wind power is the world’s fastest-growing energy source and has an average annual growth rate of 29% over the last decade. There are many thousand of wind turbines operating in different parts of the world.
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