Page 444 - A Comprehensive Guide to Solar Energy Systems

456 A COmPREhEnSIVE GuIdE TO SOlAR EnERGy SySTEmS

23.3.9 Zinc

After Fe, Al, and Cu, Zn is the most mined metal in the world. Zinc sulfide, or sphalerite is
the primary mineral ore for Zn production and it is never found in its elemental form. It
is mostly consumed as a metal for coatings to protect iron and steel from rusting, alloys
such as bronze and brass, Zn based dye casting alloys and rolled Zn; however, it also has
applications in rubber production and medicines. It is required for proper growth and
development in plants and animals and is the second most common trace metal found in
the human body after Fe. Zn is typically refined through electrolysis. Although Zn is not
directly used in PV modules, it is necessary for PV in that it is the primary metal for Cd, Ge,
and In, which may not otherwise be produced economically. It is estimated that 29.99 kg of
Zn is required for every megawatt of overall PV installation [6]. In 2016 worldwide produc-
tion was estimated at 11.9 mt with reserves of 220 mt [46].

23.3.10 Cadmium

Cd is also a relatively new industrial metal. It is soft, malleable, and ductile. Industrial
applications include rechargeable batteries, pigments, coatings, plating, and stabilizers
for plastics. Since Cd shares certain chemical properties with Zn, it can substitute Zn in
sphalerite formations. Zn:Cd ratios in sphalerite range from about 200:1 to 400:1 [46]. Cd is
generally recovered as a by-product from the process of electrolytic refinement or smelting
of Zn. It is concentrated through a SX/EW process, although it can also be further refined
using vacuum distillation or zone refinement. One of the major uses for high-grade Cd
today is in the production of CdTe thin-film PV modules, and is also used in the produc-
tion of CdS that forms a heterojunction in CIGS manufacturing as well. Amounts as low
as 0.93 kg of Cd are consumed per megawatt of CIGS installed, compared to about 83 kg
consumed per megawatt of CdTe installed [6,33]. In 2016 worldwide production of Cd was
estimated at 23 kt [46] and reserves are not calculated by the uSGS, although Grendall and
hook estimate them to be 500 kt [33].

23.3.11 Germanium

Ge is another metal that is relatively new to industry. It is hard, has a metallic luster, the
same crystal structure as a diamond and is brittle. Ge is also a semiconductor and once
seemed very promising as a transistor material. unfortunately, among other deficiencies,
it proved difficult to concentrate to the necessary purities. It was replaced by very-high
quality Si in transistors but was then used for fiber optics, infrared systems, and polymer-
ization catalysts. Today, the energy costs associated with semiconductor grade Si are being
reevaluated and Ge is once again considered as a good option. like Cd and In, Ge is a
by-product of Zn production and occurs as germanite, which is associated with Zn ores.
Concentration of Ge is very energy intense and typically consists of precipitation, electro-
refining or leaching with solvent extraction, and then zone refinement. Today, Ge is mostly
of interest in a-Si production. It is estimated that 4.07 kg of Ge is consumed per megawatt