A wind turbine is a rotating mechanism that converts the kinetic energy of the wind into mechanical energy. If mechanical energy is used directly in production (pump, millstone), then the mechanism is called a windmill. If the mechanism converts mechanical energy into electrical energy, it is called a wind generator, a windmill, or an air generator.
Construction of windmills requires places with constant strong winds. Analyzing these conditions, it is possible to estimate the amount of energy produced by the wind turbine. The most frequently used criterion for determining suitable places for construction — Wind Power Density WPD (English Wind Power Density — WPD). This is an estimate of the wind strength at this location, depending on the altitude. Maps are drawn with color markings for a certain area, for example “Average density of wind energy at a height of 50 meters”.
Depending on the direction of rotation, the turbine can be divided into two types:
- Horizontally rotating turbines are the more common type.
- Turbines rotating in the vertical direction are used relatively less.
In horizontal-axis wind turbines (HAWT) (English: Horizontal-axis wind turbines – HAWT) there is a rotor shaft directed into the wind and an electric generator at the top of the tower. Small turbines are driven by a conventional vane, while large turbines are driven by a wind sensor and servo motor. Most turbines have a gearbox. That gearbox converts the slow rotation of the blades into a fast rotation more suitable for an electric generator. Because the tower creates turbulence on its back side, the turbine is placed against the wind.
Turbine blades are manufactured from strong materials because strong winds can fold them towards the tower. At the same time, the wings are installed together from the tower and sometimes they are slightly bent up. Turbines facing the same direction as the wind can also be installed, as they do not require a special mechanism to capture the wind, and the blades can be folded, reducing resistance to strong winds. But because cyclic (repeated) twisting can cause material fatigue. Therefore, most HAWTs are against the wind.
- A variable wing flap that helps to choose the optimal angle between the aerodynamic surface and the surrounding wind.
- A tall tower allows stronger winds to find their way.
- High efficiency, because the wings always move perpendicular to the wind and receive continuous energy during rotation. In contrast, all vertical turbines include partial upwind motions.
- Towers and wings up to 90 meters are difficult to transport (transportation can add up to 20% of the total cost of the supply).
- High towers are difficult to install, the process requires very tall and expensive cranes and professional workers.
- A large tower is needed to hold the heavy wings, gearbox and generator.
- Reflections from high HAWT can cause radar clutter (although filtering can reduce these effects).
- The height of those turbines is very attractive and spoils the surrounding landscape.
- Turbines in the same direction as the wind suffer from flying and functional impairments due to the twisting that occurs at the back of the tower.
- A special mechanism is necessary to direct the wings against the wind.
In vertical axis wind turbines (VAWT), the rotor shaft is located vertically. The main advantage of this design is that there is no need to direct the turbine against the wind. This advantage is obvious in areas with very variable wind. In turbines with a vertical axis, the generator and gearbox can be placed close to the ground, and they do not need to be supported by the tower. Therefore, the operation of the turbine becomes easier. The disadvantage is that the wings rotate against the wind, and the torque is variable. At the same time, it is difficult to install vertical turbines on towers, and they are usually placed close to the foundation. For example, on the ground or on the roofs of buildings. Wind energy is weak at small heights. Torsion (turbulence) can occur in the air flow near the ground and other objects. This twisting causes vibration, which creates noise and shortens the service life. However, if the turbine is placed on the roof of the building (the building mainly attracts the wind to itself), it can capture twice as much wind. Optimally, the height of the roof should be approximately 50% of the height of the building to achieve the maximum wind force and minimum torsion. In the 1970s and 1980s, the Ministry of Energy conducted serious research on vertical axis turbines. Due to shortcomings, none of the species was successful in the market. “… vertical turbines cannot compete with horizontal turbines in the cost of electricity, reliability and economy of materials due to efficiency”. In addition, the statements of manufacturers of vertical turbines are unfounded or false.
The advantages of VAWT are used, the turbines are placed close to the ground and their operation becomes easier.
- Models with a fixed curvature of the rotor do not need a special mechanism for catching the wind.
- The initial speed of the wind in VAWT is lower than in HAWTs. Usually they generate electricity at a wind speed of 10 km/h.
- VAWT can be built where tall buildings are prohibited.
- Elevations and mountains located on the ground disperse the wind like a natural tunnel.
- VAWTs cause less noise.
- Many VAWTs can only produce 50% of the efficiency of HAWTs because their wings also rotate against the wind.
- VAWTs with holding extensions apply high pressure to the lower setpoint because the weight of the rotor is above that setpoint. Retaining extensions increase this pressure during sudden gusts of wind. To solve this problem, an add-on is needed to maintain the upper reference point.
- With poor technical planning, the repair of the VAWT can be difficult, since all the weight is placed on the foundation, and in order to change some parts, it is necessary to dismantle the entire structure.
- Since the rotors are located close to the ground where the wind speed is low, VAWT cannot produce the same power as horizontal turbines on the same foundation surface or at the same height.
- Since VAWTs are not widely used due to their serious shortcomings, they may seem unusual to those who are not familiar with wind power. This has led to a spate of lawsuits and investment scams surrounding VAWT over the past 50 years.