17-11-2012, 02:42 PM
Wind Turbine Blade Aerodynamics
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Wind turbine blades are shaped to generate the maximum power from the wind at the minimum cost. Primarily the design is driven by the aerodynamic requirements, but economics mean that the blade shape is a compromise to keep the cost of construction reasonable. In particular, the blade tends to be thicker than the aerodynamic optimum close to the root, where the stresses due to bending are greatest.
The blade design process starts with a “best guess” compromise between aerodynamic and structural efficiency. The choice of materials and manufacturing process will also have an influence on how thin (hence aerodynamically ideal) the blade can be built. For instance, prepreg carbon fibre is stiffer and stronger than infused glass fibre. The chosen aerodynamic shape gives rise to loads, which are fed into the structural design. Problems identified at this stage can then be used to modify the shape if necessary and recalculate the aerodynamic performance.
The Wind
It might seem obvious, but an understanding of the wind is fundamental to wind turbine design. The power available from the wind varies as the cube of the wind speed, so twice the wind speed means eight times the power. This is why sites have to be selected carefully: below about 5m/s (10mph) wind speed there is not sufficient power in the wind to be useful. Conversely, strong gusts provide extremely high levels of power, but it is not economically viable to build machines to be able to make the most of the power peaks as their capacity would be wasted most of the time. So the ideal is a site with steady winds and a machine that is able to make the most of the lighter winds whilst surviving the strongest gusts.
As well as varying day-to-day, the wind varies every second due to turbulence caused by land features, thermals and weather. It also blows more strongly higher above the ground than closer to it, due to surface friction. All these effects lead to varying loads on the blades of a turbine as they rotate, and mean that the aerodynamic and structural design needs to cope with conditions that are rarely optimal.
Number of blades
The limitation on the available power in the wind means that the more blades there are, the less power each can extract. A consequence of this is that each blade must also be narrower to maintain aerodynamic efficiency. The total blade area as a fraction of the total swept disc area is called the solidity, and aerodynamically there is an optimum solidity for a given tip speed; the higher the number of blades, the narrower each one must be. In practice the optimum solidity is low (only a few percent) which means that even with only three blades, each one must be very narrow. To slip through the air easily the blades must be thin relative to their width, so the limited solidity also limits the thickness of the blades. Furthermore, it becomes difficult to build the blades strong enough if they are too thin, or the cost per blade increases significantly as more expensive materials are required.
Blade section shape
Apart from the twist, wind turbine blades have similar requirements to aeroplane wings, so their cross-sections are usually based on a similar family of shapes. In general the best lift/drag characteristics are obtained by an aerofoil that is fairly thin: it’s thickness might be only 10-15% of its “chord” length (the length across the blade, in the direction of the wind flow).
Blade planform shape
The planform shape is chosen to give the blade an approximately constant slowing effect on the wind over the whole rotor disc (i.e. the tip slows the wind to the same degree as the centre or root of the blade). This ensures that none of the air leaves the turbine too slowly (causing turbulence), yet none is allowed to pass through too fast (which would represent wasted energy). Remembering Betz’s limit discussed above, this results in the maximum power extraction.
Rotational speed
The speed at which the turbine rotates is a fundamental choice in the design, and is defined in terms of the speed of the blade tips relative to the “free” wind speed (i.e. before the wind is slowed down by the turbine). This is called the tip speed ratio.
High tip speed ratio means the aerodynamic force on the blades (due to lift and drag) is almost parallel to the rotor axis, so relies on a good lift/drag ratio. The lift/drag ratio can be affected severely by dirt or roughness on the blades.