A new report from the EU-funded UpWind project concluded that it is feasible to construct wind turbines to 20 MW, though doing so would involve more than just a simple scaling up of today's wind turbine components.
The dimensions on 20 MW turbines would be gigantic -- rotor diameters would be around 200 m, compared to some 120 m on today's 5 MW turbines. And 200 m is about the wingspan of the fictional Japanese mutated pterosaur called Rodan which entertained audiences in a 1956 sci-fi movie.
But 20 MW turbines couldn't be just scaled-up versions of smaller machines for the same reason that building-sized insects that populate bad sci-fi movies aren't found in real life: The mass of the creature would go up with the cube of its size, so a super-sized bug would be crushed by its own weight. And so too would 20 MW wind turbines that were just bigger versions of their smaller cousins.
So a number of innovations will be needed to make 20 MW turbines a reality. Researchers from the Netherlands' Energy Research Centre, the Danish Technical University Risoe DTU, and 44 other institutions that contributed to the report suggest that it will be necessary to lower fatigue loads on blades to allow construction of longer and lighter blades. Fatigue loads can be lowered by 10% through a fore-bending of them and using more flexible materials. Use of individual blade control could lower fatigue loads by another 20 to 30%. Researchers also suggest making blades in two sections (like airplane wings) so each can be controlled separately. This could lower fatigue loads by 15% and will simplify the task of transporting the blades.
There are also some innovations in how wind farms are laid out that can help make the monster-sized machines a reality. For example, lowering the power output of the first row of turbines allows for higher overall wind farm efficiency. And putting sensors on one wind turbine would allow operators to estimate the fatigue loading on the other turbines if the relationship of fatigue loading between the wind turbines is known. Wind farms operators might also take preventative measures to lower fatigue loads by evaluating upcoming wind gusts before they arrive at the turbines. This would be possible thanks to nacelle-mounted LIDAR units which have become sufficiently accurate to handle wind energy applications.
Researchers also took a look at promising direct-drive permanent magnet (PM) generators which include the axial flux (AF), radial flux (RF) and transverse flux (TF) machines. In case of RFPM machines, they concluded that the machines are almost optimised electromagnetically, so that it is hard to reduce the active material weight and cost of the machines significantly. TFPM machines have disadvantages such as complicated construction and manufacturing, and low power factor, although the machines have advantages such as high force density and simple winding with low copper losses.
Researchers also looked at optimizing the generator design and found difficulties in that a generator design that minimizes active mass leads to a design that maximizes inactive or structural mass. Traditionally in the design of the active mass, the air gap is kept as small as possible to optimize the electromagnetic performance. However, optimizing for both electromagnetic and structural qualities indicated that machines with a larger air gap will result in lower mass. The aspect ratio of the generator (ratio of length to air gap radius), depends upon the optimization criteria.
For minimum mass, large aspect ratios with a larger air gap are desirable, leading to a sausage shaped machine. If cost is key, researchers say small aspect ratios or pancake machines are more desirable, because active mass drops with radius, and this is the most expensive part of the generator.
Researchers also tried to identify the most suitable generator type for large direct-drive wind turbines based on electromagnetic
analysis models for various configurations of PM machines. A single-sided single-winding flux-concentrating TFPM generator turned out to be most promising. To further reduce the active mass of TFPM machines, researchers propose a configuration with multiple-slots to shorten the length of the flux path, which yields more potential to reduce the active mass than RFPM generators. The active mass of a claw pole design with limited pole area and up to four slots per phase seems to have potential, researchers say.
You can get a free copy of the report here: http://www.ewea.org/fileadmin/ewea_documents/documents/upwind/21895_UpWind_Report_low_web.pdf
And further review of oversized-insect physics, look here: http://tvtropes.org/pmwiki/pmwiki.php/Main/SquareCubeLaw