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There are two main types of wind turbines, vertical and horizontal, each of which have their advantages and disadvantages for different applications.
Vertical:
Do better with strong variable winds that are low to the ground because they are built near ground level and can take wind from all directions without adjusting. In these setups, the turbine blades rotate in the same plane as the wind is blowing.
Advantages
- Are typically built lower to the ground, so the cost, aesthetic, maintenance, permitting and other issues with tall towers don't exist.
- More aerodynamic than horizontal turbines
- Typically have a larger cross section swept for the same diameter, increasing the power potential
- Usually have lower start-up speeds, typically generating electricity at only 6 mph
- Less likely to break in high winds
- They can take wind from all directions without needing to turn
Disadvantages
- Most vertical turbines only produce energy at half the efficiency of horizontal turbines because of the additional drag the blades have as they rotate into the wind
- They are typically built close to the ground, where the wind speeds tend to be lower and power output cubes with wind speed
- While they can produce energy at lower speeds, they may require an energy input to begin turning
- Unless there is a superstructure supporting the top of the turbine, all of the weight is on the bottom bearing, which is under a lot of stress when there are wind gusts
- All of the mechanics of the system are on the ground under the turbine, so care must be taken in design to allow maintenance without dismantling the turbine.
Horizontal:
The more common wind turbine design, these are what were used as old fashioned windmills and make up most current wind turbines. They typically have 3 blades, although there can be more or less. More blades equates to more power, but more turbulence, and when taking into account that you need to have an odd number of blades, 3 appears ideal, although a number of different styles can be found. Thereason to not have an even number of blades is that it produces a cyclic twist that would tear the turbine apart at high wind speeds. Horizontal turbine blades rotate in a plane perpendicular to the movement of the wind and are typically pointed into the wind by computer-controlled motors, although there are some passive systems as well. The tip speeds can reach up to six times that of the wind speed. Turbine blades typically range from 65 to 130 feet long although they can be longer and are at the top of a tower that is typically 200 to 300 feet tall. A gear box typically ups the speed of the generator from the 10-22 rpm of the turbine blades. These turbines have a high wind shut down feature of some sort to avoid damage from high winds. These braking systems are described below.
Advantages
- Most turbines can adjust the blade pitch in addition to the direction to maximize the amount of energy pulled from the wind
- A tall tower allows for access to winds at higher elevations, which are typically faster. At some sites, wind speed can increase by 20% and power output by 34% for every 10 meter (33 foot) increase in elevation, and once again, power output cubes with wind speed. This means that, all else being equal, doubling the wind speed multiplies the power output by 8 times.
Disadvantages
- Horizontal turbines have trouble operating near the ground and in turbulent winds
- The tall towers and long turbine blades can be difficult and expensive to transport, with transport costing up to 20% of the equipment cost. They are also very difficult to install
- The height of the turbines allows them to be seen from great distances away
- Turbine must be turned into the wind, requiring additional controls and sometimes energy
- The turbine creates turbulence downwind
Braking:
As described above, wind turbines require a braking system for when the wind speed is too high. Below are the three common types of braking systems, some active, some passive, that will cause the the blades to rotate below maximum speed when the wind speed gets too high:
- Pitch control - The turbine monitors its power output to tell when rotation speeds may be reaching dangerous levels. At wind speeds over the acceptable maximum speed, the power output will be too high, at which point the controller tells the blades to alter their pitch so that they become unaligned with the wind. This slows the blades' rotation. Pitch-controlled systems require the blades' mounting angle to be adjustable.
- Passive stall control - The blades are mounted to the rotor at a fixed angle but are designed so that when the wind becomes too fast, the blades will twist. The blades are angled so that winds above a certain speed will cause turbulence on the upwind side of the blade, inducing stall by causing the blade's angle facing the wind becomes so steep that it starts to eliminate the force of lift, decreasing the speed of the blades.
- Active stall control - This system is a combination of the two previous systems. In this system, the blades' pitch is controllable, but instead of turning them out of the wind when the speed gets too high, this system turns the blades into an angle that will induce a stall.
Variables Involved in the Design of a Wind Turbine:
- The low wind speed that will cause the blades to begin spinning (depends on size, smaller blades, lower speed)
- The high wind speed before it brakes
- The number of blades
- The length of the blades
- The height of the tower (wind typically faster and more regular higher up, 12% more wind for double height)
- Whether or not turbine adjusts for the wind direction
- Whether or not a heating mechanism is needed at lower temperature to melt ice off of the turbines and to make sure readings on all of the equipment are correct.