While the success of vertical wind turbines or “egg beater” turbines for electricity generation is extremely limited, there are circumstances under which they can outperform horizontal axis or “propeller” turbines.
- In the first place vertical axis wind turbines are easier to maintain than their horizontal counterparts because their moving parts a located near the ground. This is due to the vertical wind turbine’s configuration, which is something like an ordinary windmill lying on its back with its “face” to the sky.
The airfoil or rotor blades are connected by arms to a shaft that rests on a bearing and drives a generator below, usually with a gearbox. As the rotor blades are vertical, a yaw device is not needed, making it more cost effective than a horizontal axis turbine.
A yaw device assists the blades of the horizontal axis wind turbine to face into the wind.
- Second, vertical wind turbines have a higher airfoil pitch angle, giving improved aerodynamics while decreasing drag at low and high pressures.
- Third, vertical axis wind turbines are not tall (remember they are like a horizontal turbine lying down) and this configuration can be useful where laws do not permit very tall structures.
- Fourth, small vertical turbines are quite easy to transport and install.
- Fifthly, they do not need a free-standing tower so they are less expensive and stronger in the high winds that are closer to the ground.
- Sixth, they have a lower tip speed ration (TPR) so they are less likely to break in high winds.
- Seventh, they don’t need to be pointed into the wind. They can turn regardless of wind direction.
- Eighth and most importantly, and this is the main area in which vertical wind turbines can out-perform the horizontal type, if vertical turbines are placed on the ground on high prominence (mesas, hilltops, ridge-lines, etc.) or in passes, they can produce more power than horizontal vertical axis turbines.
The benefits of vertical axis wind turbines must, however, be weighed against their significant disadvantages. Most types of vertical turbines produce energy at only 50% of the efficiency of horizontal turbines due to their drag action. They are limited in terms of height. They also need to be constructed on relatively flat land, unlike the horizontal types.
Most vertical wind turbines have low starting torque and may require energy to start the turning
In addition, vertical wind turbines requiring guyed wires to hold them in place put stress on the bottom bearing as all the weight of the rotor is on the bearing. Guyed wires attached to the top bearing increase downward thrust in wind gusts. Solving this problem requires a superstructure to hold a top bearing in place in guyed wire models to eliminate the downward thrusts when there is gusting wind.
While the parts of installed vertical wind turbines are located on the ground, this means that they are under the weight of the structure above it, which can make changing parts near impossible without dismantling the structure, depending on the design of the structure. More info