However, it's not enough for the blades to be moved by the wind. Engineers must consider speed and drag in designing the blades to ensure the highest level of efficiency. For example, if too much drag is created by the obstruction of the blades the power yield will be a lot lower. If not enough drag is created, the blades could move too quickly, causing them to break the sound barrier. One of the biggest benefits of wind turbines is how quietly they operate.
If they broke the sound barrier, it might make residents near proposed wind farms more likely to oppose the implementation of the turbines. By and large, most wind turbines operate with three blades as standard. The decision to design turbines with three blades was actually something of a compromise. Because of the decreased drag, one blade would be the optimum number when it comes to energy yield.
However, one blade could cause the turbine to become unbalanced, and this is not a practical choice for the stability of the turbine. Similarly, two blades would offer greater energy yield than three, but would come with its own issues.
Two-bladed wind turbines are more prone to a phenomenon known as gyroscopic precession , resulting in a wobbling. Naturally, this wobbling would create further stability issues for the turbine as a whole. This would also place stress on the component parts of the turbine, causing it to wear down over time and become steadily less effective.
Any number of blades greater than three would create greater wind resistance, slowing the generation of electricity and thus becoming less efficient than a three blade turbine. For these reasons, turbines designed with three blades are the ideal compromise between high energy yield and greater stability and durability of the turbine itself. Despite the fact that three-bladed turbines have become the standard model of clean energy production in recent years, that doesn't mean they always will be.
Engineers are still working on better, more efficient designs for future energy generation efforts. One of the most popular proposed designs is a bladeless turbine. Though this might seem counter to the resistance needed in order to convert the wind's energy into electricity, there are actually a number of benefits to creating a turbine without blades.
One benefit is cost and maintenance. Yaw is what a wind turbine does when its head or nacelle turns to face the wind. Due to an effect called gyroscopic precession, when a 2-blade turbine yaws, it creates a wobble as the blades go from the vertical plane to the horizontal plane, which places additional stresses on the mounting bearings, wearing them down faster.
In turbines with 3 or more blades, the blade masses are distributed evenly across the horizontal and vertical planes, no matter what position the blades are in, and so they do not suffer this wobbling gyroscopic effect.
Once a turbine has been designed, it has to be manufactured, transported, and then assembled on site. This makes designing any turbine a compromise — balancing the practical manufacture, transportation and assembly constraints with theoretical optimal design specifications.
As mentioned earlier, the blades cannot be manufactured too thin, or they will break under high winds. This means that 3 blades is chosen — any less and the turbine will experience gyroscopic wobble; any more and the manufacturing costs will be higher. The roads have to be of sufficient quality to handle the weight of the specialist trucks and the blades themselves.
The logistics of turbine blade transportation planning has become a serious issue in recent times, being specifically addressed in the book Transport Infrastructure and Systems. Once the components have arrived at the construction site the main tower is erected first, and the nacelle mounted on top.
The blades are lifted and attached to the nacelle, which, allowing for 30m clearance, could be as high as m from the ground in the case of the Vestas V On-shore turbines use mobile crane platforms like the Liebherr LG, which has a maximum lift weight of tonnes and can lift to metres.
Off-shore turbines are assembled using specialist floating platforms with cranes mounted onto them. These platforms can be sailed like ships to the site, and then jack themselves out of the water to provide a stable base from which to assemble the turbine.
Theoretically, a turbine with no blades or blades of zero thickness! While blades of zero thickness are physically impossible to manufacture, at least one bladeless design has been brought to market. This does not have blades and a rotor like a traditional turbine, but rather it uses a process called vortex-induced resonance to extract kinetic energy from the wind using a vertical pillar.
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