The largest wind turbines
Claims are sometimes made that a new wind turbine is the biggest yet and produces more power than any other. Such claims need to be carefully considered and compared with verified performance data. The latest claimant (in 2011) for the title of “largest wind turbine” appears to be the Enercon E-126, a 3-bladed rotor of 127 m diameter and rated at 7.58 MW. The hub height was stated to be 135 m.
The previous record holder was the 5 MW REpower Systems wind turbine installed at Brunsbu¨ttel in Schleswig-Holstein, Germany (October 1, 2004), according to a report in Renewable Energy World (November-December 2004). The three-bladed rotor again has a tip diameter of 126.3 m (blade length 61.5 m, maximum chord 4.6 m) and a hub height of 120 m.
The various speeds and rotor speed range quoted (data that may be useful in problem solving) are
The chief factors that influence the higher output of the Enercon E-126 turbine seems to be the increased hub height and possibly the windier location of the site.
A factor of some importance in getting higher performance from HAWT’s is the enormous length of the individual blades and transporting them to the site of use often along roads with unhelpful bends. Until recently, the world’s longest blades were around 80-100 m. Now blades are being made of carbon fiber (instead of fiberglass) in shorter pieces which are joined up together. This advance makes possible individual offshore wind turbines with an output capacity of 8-10 MW.
What are the limits on the size of wind turbine
A recent investigation by Ceyhan (2012) has been made in a project called UPWIND in which several aspects of the design have been investigated. Due to the increased blade size, 252 m diameter, the local Reynolds number values along the blade were found to be as high as 25 3 106 while the local wind speeds remain constant. Further progress on this advanced project is hampered by the lack of performance data for aerofoils operating at such high Reynolds numbers. The author of this seminal work concludes that high quality measurements will be the key to obtaining cost effective and reliable designs for very large offshore wind turbines.
Final remarks
This chapter has given an introduction to the aerodynamics of HAWTs. In order to model the performance of a HAWT, it was necessary to give a brief introduction to the mathematics of wind speed probability. By combining the statistical method of probability theory with the important but relatively simple BEM method, the performance of a HAWT over a wide range of wind speeds can be determined.