Page 441 - A Comprehensive Guide to Solar Energy Systems

Chapter 23 • Materials: Abundance, Purification, and the Energy Cost 453

of transportation, packaging, construction, consumer durables, electrical transmission
lines and machinery. Al is derived from bauxite ore in the Earth, which is an abundant ma-
terial. Bauxite is converted to alumina, an aluminum oxide, which is further electrolyzed
to very high purity concentrations. Al is used in the PV industry for making supports, or the
Body of System (BOS), including frames for solar panels, and electricity transmission lines.
An amount of 102 kg of Al is consumed per megawatt of overall PV installations [6,37]. The
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2016 production of worldwide Al was estimated at 57.6 × 10 t (57.6 mt) with a yearend
capacity of 72.5 mt [46]. Reserves are not calculated due to the immense global resources
of bauxite, which are sufficient to meet world Al demand well into the future. Al is also
important to PV production, as it is the primary ore for Ga.

23.3.3 Gallium
Ga is a secondary by-product from aluminum oxide production from bauxite, although
some is produced from Zn-processing residues. It is used in producing integrated circuits
and optoelectronic devices (mostly laser diodes and light-emitting diodes). For the most
part, specific methods for refining Ga to semiconductor grade are proprietary information.
Some likely processes involved include fractional distillation, electrolysis, extracting, vacu-
um distillation, fractional crystallization, zone melting, and single crystal growth. The exact
process would depend upon Ga concentrations in the alumina being refined. Ga is most
relevant to the PV industry in terms of its application in thin-film PV technology, specifically
CIGS. Amounts of between 0.12–6.17 kg of high-grade Ga are consumed per megawatt of
CIGS installations [6,33]. The 2016 worldwide production of low-grade Ga was estimated at
375 t and high-grade at 180 t. year-end low-grade capacity was estimated to be at 730 t and
high-grade at 320 t [46]. As only a very small portion of the Ga present in bauxite and zinc
ores is recoverable and factors controlling recovery are proprietary, united States Geological
Survey (uSGS) does not estimate Ga reserves, however, Grendall and hook estimate them to
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be 6.5 × 10 t (6.5 kt) [33]. The Ga contained in world resources of bauxite is estimated to ex-
ceed 1 mt and the uSGS postulates that a considerable quantity could be contained in world
Zn resources, however only a very small percentage is techno-economically recoverable [46].

23.3.4 Copper

Cu has played an important role in human development for thousands of years. It is
favored for its properties of ductility, malleability, resistance to corrosion, and thermal
and electrical conductivity. Cu is an invaluable industrial metal mostly used in build-
ing construction, building wiring, telecommunications, and electronics. It can also be
found in transportation, industrial machinery, and general consumer products. usually
associated with sulfur in natural deposits, it is currently produced by a variety of multi-
stage processes in many countries. Generally, it begins with the mining and concentrat-
ing of low-grade ores (<0.5% Cu) containing copper sulfide minerals and then smelting
and electrolytic refining to produce cathode grade Cu (>99.99% pure Cu). hydromet-
allurgical Cu processing is increasing in popularity. Given that Cu is the second-best-
known conductor of electricity, it is used in electricity transmission, distribution, and to