The power generated by a wind turbine is directly proportional to the sweep area of the blades and the cube of wind-speed. As the generating power of wind-turbine units increases, so does the dimension of the apparatus to provide it, hence a blade span of 124 metres is required for some 5 MW units, with a unit height of 114 metres.
The use of permanent magnet generators eliminates the need for gear boxes, and can be accomplished with the application of Neodymium-Iron-Boron REE magnets, known as "Neo-magnets". These designs increase mechanical efficiency and reliability, and reduce the internal energy losses in the machines. However, some of the larger units, say 5 MW rated capacity, might require a tonne or more of neodymium. I am aware that ferrite magnets can be used instead of REEs, but my understanding is that weight for weight they are less powerful by a factor of about ten than REE magnets requiring heavier engineering of the wind-power unit to support them. The matter may not be so straightforward however, according to the following written by an experienced engineer in the magnet/wind-power field (http://www.usmagneticmaterials.com/documents/HarvestingWind.pdf):"One might think that on a pound-for-pound basis, permanent magnet generator using sintered NdFeB will provide the highest energy-conversion efficiency, but this is not necessarily true. In
order to optimize the generator efficiency over a wide speed range, it is best to have some control over the amount of magnetic flux in the circuit, so that the flux can be weakened at higher wind speeds. Sintered NdFeB, with its extremely high flux density and high coercivity, is quite “permanent.” At high speeds, this “permanence” can saturate the generator with flux, leading to greatly increased iron losses. Also, sintered NdFeB is electrically conductive, causing eddy current losses and inductive heating in the magnets.
At higher wind speeds, these parasitic losses increasingly eat into the efficiency and heat up the generator, causing a rapid degradation in performance. Non-conductive sintered ferrite magnets are better in this regard, providing a bit more efficiency headroom at higher speeds. Though the energy product of sintered NdFeB is roughly 10 times that of sintered ferrite, the cost per kilogram of sintered NdFeB is 30 times that of ferrite. The higher cost of NdFeB provides, at best, an incremental gain in overall system efficiency, and I believe that sintered ferrite can provide the best return on investment for a permanent magnet wind turbine generator. There are many interesting magnetic circuit topologies and control schemes that provide flux weakening at higher speeds, thereby increasing the overall system efficiency.
With continuing advances in power electronics and control algorithms, these designs emphasize the “electro” part of the generator with copper and steel, at the expense of the “magnet” part. Several clever generator designs employ lower-energy permanent magnets, such as injection-molded ferrite, to boost the output of what would typically be a non-permanent- magnet machine. These “hybrid magnetic circuit” designs can produce the highest efficiency over the widest speed range, providing the best long-term bang for the buck in a wind turbine system. The cost of such generators is driven by the cost of power electronics and controls, more so than the permanent magnets, wire and steel."
It seems to me that there are definite advantages to using REEs, and there is a move to having 25% of wind-power generation from units containing Neo-magnets by 2015. But since it seems clear that providing sufficient REEs to fill the hole dug by demand for this technology is unlikely, finding an alternative path to such renewable energy is critical. Most of the ongoing efforts to do so are at the "research level" and so having actual wind-devices generating significant green-electricity as fossil-fuel power declines is years away. Even were there no problem over resources or choice of technology, there is a distinct lack of manufacturing capacity for wind energy, certainly on the scale of demand to meet UK "green energy" targets set by the European Commission. It is the perennial rate of flow, conversion or recovery that limits the inauguration of all new technology.