Mestre

Wind Turbine Blade Aerodynamics
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 thestresses 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 chosenaerodynamic 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 thewind speed means eight times the power. This is why sites have
to be selected carefully: below about 5m/s (0mph) 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 tovarying
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.
By extracting power, the turbine itself has an effect on the wind: downwind of the
turbine the air moves more slowly than upwind. The wind starts to slow down even
before it reaches the blades, reducing the wind speed through the“disc” (the imaginary circle formed by the blade tips, also called the swept area) and hence reducing
the available power. Some of the wind that was heading for the disc diverts around
the slower-moving air and misses the blades entirely. So there is an optimum amount
of power to extract from a given disc diameter: try to take too much and the wind will
slow down too much, reducing the available power.In fact the ideal is to reduce the
wind speed by about two thirds downwind of the turbine, though even then the wind
just before the turbine will have lost about a third of its speed. This allows a theoretical maximum of 59% of the wind’s power to be captured (this is called Betz’s limit).
In practice only 40-50% is achieved by current designs.

WE Handbook- 2- Aerodynamics and Loads 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...