Geology Reference
In-Depth Information
coupled with lower maintenance costs with respect to conventional electromagnet
generators.
Such turbines are constructed using designs with permanents magnets since
the generator requires no excitation power for the generation of an electric field.
Permanent magnets, PM, are the strongest known magnets and are composed of
neodymium-iron-boron with dysprosium, or other rare earths (REE) like praseo-
dymium (RE
2
Fe
14
B as basic formula). The approximate content by weight in
Nd is 27.5-31% and 2-4% in Dy. Dysprosium is added to avoid de-magnetisation
stress in generators at temperatures higher than 80
o
C (DOE, 2011). It is estimated
that direct-drive turbines require up to 600 kg of PM/MW (Constantinides, 2011).
A 5 MW wind turbine contains about two tonnes of permanent magnet
18
. Such
quantities dramatically increase the demand for rare earths.
Rare earths present a particularly challenging set of problems. The USGS re-
ports that the world's rare earth oxides reserves could be as large as 110 million
tonnes but many of them are in deep ocean sediments with doubtful exploitation
feasibility (USGS, 2011b). Found in monazite and bastnaesite minerals, the world's
largest continental reserves are predominately located in China (50%), followed by
the former states of the Soviet Republic (17%) and the United States (12%).
Particularly, neodymium although not as rare in Nature as it might seem, is
widely dispersed, making its extraction very expensive and polluting. Natural con-
centrations are maintained in a range from ten to a few hundred parts per million
by weight. Dysprosium is even rarer since it constitutes less than 1% of all rare
earths and its occurrence is commonly accompanied by radioactive thorium or ura-
nium thus complicating its production. China (Longnan, Jiangxi province) is the
only country that has dysprosium compounds containing no radioactive minerals
(lateritic ion-adsorption clays) but with an expected 15-25 year commercial lifespan,
this possibility is not one for the long term (Constantinides, 2011). It is certainly
much shorter than the static index of depletion of REE generally (700 years), as
will be seen later in Chap. 13.
According to Alonso et al. (2012), just under 80% of all the electric and hybrid
vehicles plus all the wind energy needed to meet the IEA's 450 ppm target by 2035,
would lead to a more than 700% and 2600% respective increase in demand for Nd
and Dy, compared to 2010 baseline levels. According to Constantinides (2011), the
production of rare earth permanent magnets will increase from 56,640 tonnes in 2010
to 90,258 in 2015, while the dysprosium available in the market will not change at
1,277 tonnes. This means a shortfall of 40,788 tonnes that could affect the worldwide
wind energy expansion unless the situation alters. To oppose this trend, research is
being undertaken to obtain other designs including new hybrid machines that could
reduce the need for rare earths by up to a third, or superconducting generator
direct-drive turbines that would not use rare earths at all (see for example AMSC
(2011)). In addition the use of samarium-cobalt magnets instead of dysprosium
18
Alonso et al. (2012) assume 171 kg of REE per MW turbine.
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