Basics of Wind Turbine Tower
The tower of wind turbine is one of the very important component of wind turbine system. The tower of wind turbine is the only support available to the whole turbine system. Wind turbine tower supports the both static and dynamic load of the turbine. It also provides housing for different other mechanical and electrical components of the turbine [F. Knox, 2010]. The tower does not only bear the weight of the entire turbine but it also provide resistance against acute weather conditions .i.e. rain, storm, snow etc. Life of a wind turbine directly or indirectly depends upon its tower, therefore, the tower’s material properties such as strength, density and stiffness become important parameters in the design of a sustainable wind turbine system [J. Manwell, 2009]. The tower of wind turbine is also the most expensive part of the whole wind turbine structure due to its larger weight. For the sake of decreasing the expenses, the dimensions of tower are optimized in each case. In the process of optimization parameters like stiffness, strength etc. are taken into account to ensure the safety of whole system. Cost is usually lessened without compromising on strength and stiffness of the tower.
Diameter of Wind Turbine Tower
Tower of wind turbine supports the nacelle of the wind turbine that contains its blades, generator, gear assembly and other important components [F. Knox, 2010]. The turbine’s diameter should be selected in such a way that it has enough space to install nacelle of the turbine properly along with yawning mechanism so that nacelle can be rotated properly to make the turbine face the wind to produce maximum power. At the bottom, a turbine have transformer and other necessary electronic equipment as shown in the figure. There is a door at the bottom section of the tower to enter the tower for maintenance and other purposes. In the middle section there is a staircase which is built to allow workers to reach different sections of the system for maintenance purposes [J Manwell, 2009]. The tower height and weight of nacelle are also taken into account while deciding the tower’s diameter. If the size and weight of nacelle is greater, the tower’s diameter is also kept larger. Similarly if the tower has a large height, the diameter of the tower should also be kept larger to prevent its buckling. The requirements which have been discussed above are usually a concern when we are dealing with the larger turbines because for the smaller turbines the electrical and mechanical components are simple and can be installed on the outside. An NREL type horizontal axis wind turbine (HAWT) with the power rating of 5 MW has a diameter of 6 meters at the bottom and 3.87 meters at the top section [Kackman 2005]. Similarly a large horizontal axis wind turbine (HAWT) with the power rating of 10 MW has a diameter of 8.3 meters at the bottom and 5.5 meters at the top section [Bac 2013].
Wall Thickness of Wind Turbine Tower
Wall thickness of wind turbine is an important factor which affects the strength of the wind turbine tower. Increasing the wall thickness also increases the strength but it also increases its weight [Yang 2014]. We have a limit to which wall thickness of the tower can be increased or decreased to obtain the necessary strength [Huskey 2005]. A medium size turbine having power rating of 5MW must have 20mm of wall thickness to endure the stresses produced due to the nacelle’s weight and during operation. In the similar manner, the wall thickness for two turbines of 10 MW and 8 MW power rating must have a minimum of 38 mm and 36 mm respectively [Cian Desmond, 2016]. Stiffness rings, now a days, are being used to acquire the increased strength without increasing the thickness and weight of wind turbine tower’s wall [Yang 2014]. Stiffness rings are very efficient against compressive loads but less effective when it comes to bending stresses. Predicting the exact integer of stiffness rings for a specific wind turbine is very difficult therefore we have to compromise on the strength of tower and if we don’t, the cost of system becomes very high [Yang 2014]. Material used in the making of wind turbine tower greatly impacts the wall thickness of the tower. Any material whose stiffness and strength are high will require small thickness of wall as compared to the material which have low stiffness and strength to withstand the amount of operational and static load. Thus, wall thickness is a tradeoff between stiffness, density and strength [Gwon, 2011].
Height of Wind Turbine Tower
Dimensions of wind turbine, amount of power to be produced, available wind speed in the locality where wind turbine is being installed and area in the vicinity are important factors that have a role in deciding the height of wind turbine tower [F. Knox, 2010]. Wind turbines that are intended for small scale power production generally have small towers but in the urban areas where neighboring have obstacles i.e. buildings and houses etc. it is necessary to build a tower of large height to avoid shear that comes into play due to fluid and solid interaction. For large scale wind turbines there is a limitation to tower height due to large weight of the system. Height and type of wind turbine tower changes from turbine to turbine [Manwell 2009]. The tower height of a small scale wind turbine can vary from 15 meters to 25 meters but this limit is not fixed. Based on the wind speed available, tower’s height can be increased or decreased. It is also effected by the chord length of blades so there is a limit to tower height in case of medium and large size wind turbines. Tower height for small and large scale turbines is taken equal to the diameter of turbine or 1.5 times the diameter of wind turbine (Diameter of rotor). For medium size wind turbines of power rating of around 5MW the approximate tower height is around 80 meters which increase to above 100 meters for a turbine of power rating greater than 8 MW. For turbines that have power rating greater than 8MW, the tower height is increased to 120 meters [Cian Desmond 2016]. In addition to the factors discussed above the process of manufacturing and transporting the material to the site are also very important in deciding the dimensions of the tower. Obviously, it is difficult to build large towers and construction costs for building them is also very high.