1. VERTICAL AXIS WIND TURBINE:
Vertical axis wind turbines, as shortened to VAWTs, have the main rotor shaft arranged vertically.  
The main advantage of this arrangement is that the wind turbine does not need to be pointed into the wind. This is an advantage on sites where the wind direction is highly variable or has turbulent winds.
With a vertical axis, the generator and other primary components can be placed near the ground.
so the tower does not need to support it, also makes maintenance easier.
The main drawback of a VAWT generally create drag when rotating into the wind.

2. It is difficult to mount vertical-axis turbines on towers, meaning they are often installed nearer to the base on which they rest, such as the ground or a building rooftop.
The wind speed is slower at a lower altitude, so less wind energy is available for a given size turbine.
Air flow near the ground and other objects can create turbulent flow, which can introduce issues of vibration, including noise and bearing wear which may increase the maintenance or shorten its service life.
However, when a turbine is mounted on a rooftop, the building generally redirects wind over the roof and this can double the wind speed at the turbine. If the height of the rooftop mounted turbine tower is approximately 50% of the building height, this is near the optimum for maximum wind energy and minimum wind turbulence.

4. VAWT subtypes Darrieus wind turbine
Darrieus wind turbines are commonly called "Eggbeater" turbines, because they look like a giant eggbeater.
They have good efficiency, but produce large torque ripple and cyclic stress on the tower, which contributes to poor reliability.
Also, they generally require some external power source, or an additional Savonius rotor, to start turning, because the starting torque is very low.
The torque ripple is reduced by using three or more blades which results in a higher solidity for the rotor.
Newer Darrieus type turbines are not held up by guy-wires but have an external superstructure connected to the top bearing.

6. Savonius wind turbine
A Savonius is a drag type turbine, they are commonly used in cases of high reliability in many things such as ventilation and anemometers. Because they are a drag type turbine they are less efficient than the common HAWT.
Savonius are excellent in areas of turbulent wind and self.

9. Advantages
VAWTs are not affected by the direction of the wind, which is useful in areas where the wind changes direction frequently and quickly.
Unlike traditional horizontal axis wind turbines, a yaw mechanism is not needed to turn the wind turbine towards the wind
VAWTs are ideal for both rural and urban applications, including roof top installations. Depending on the shape of the roof, the wind flow over the roof can be concentrated, leading to an increased energy output
Simple to install and maintain
Quiet operation
Pleasant appearance

10. Disadvantages of vertical axis wind turbines
Some disadvantages that the VAWTs possess are VAWTs have very low starting torque, as well as dynamic stability problems.
Complicated in structure, failure rate high.
wind energy efficiency is low.
The VAWTs are sensitive to off-design conditions and have a low installation height limiting operation to lower wind speed environments.
The vertically oriented blades used in early models twisted and bent as they rotated in the wind. This caused the blades to flex and crack. Over time the blades broke apart and sometimes leading to catastrophic failure.