The global drive to bring
non-fossil fuel energy sources to fruition as swiftly as
possible has created several new energy industries, of which
solar power is very much at the forefront. Forecasts indicate
that photovoltaic (PV) derived electricity could be providing
up to 12% of the EU’s electricity demand by 2020.
Although they might not be
immediately obvious, for industrial minerals, there are
realisable opportunities in the PV manufacturing process
(see panel).
PV cells
There are two main types of PV
technology: crystalline silicon technology and thin film
technology. The most common technology is crystalline silicon,
which accounts for around 90% of the PV cells produced.
Crystalline silicon cells are made
from thin slices of a single crystal (monocrystalline), or
slices from a block of crystals (polycrystalline or
multicrystalline).
Thin film PV cells are produced by
deposition of very thin layers of photosensitive materials on
to a low cost backing such as glass, stainless steel, or
aluminium.
Fused silica
crucibles
In the PV manufacturing process,
high grade quartz is reduced to metallurgical grade silicon and
converted to solar grade silicon. The Siemens process makes
high purity polysilicon, which is then melted in crucibles at
1,400ºC and then crystallised out.
High quality fused silica is
essential to the manufacture of these crucibles. High grade
fused silica is processed and mixed into a slip/slurry,
slip-cast into molds, and then dried, fired, and cooled in
kilns.
There are very few fused silica raw
material producers worldwide. Leading fused silica crucible
manufacturers include Vesuvius (part of Cookson Plc), Belgium,
and Ceradyne Thermo Materials, USA. Vesuvius produces its own
fused silica raw material, while Ceradyne is supplied by
subsidiary Minco Inc., Greeneville, Tennessee, which also
produces fused silica for investment casting, filler, and
refractory markets.
Both Vesuvius and Ceradyne in
recent years have expanded plants and established Chinese
facilities to meet increased demand from the solar cell market,
although 2009 saw a major drop in the market.
As a result, Vesuvius closed its
Hautrage, Belgium plant and reduced headcount at its Feignes,
France, plant by around 30%. Although the company’s new
40,000 crucibles per annum Sunrise plant in Weiting, Jiangsu,
China was completed at the end of 2008, it remains
uncommissioned pending a pick up in demand.
In 2009, Jiangxi Sinoma New Solar
Materials Co. Ltd, China, using gel-casting, developed the
world’s largest fused silica crucible with a capacity of
800kg silicon metal for its partner LDK Solar Co. Ltd.
Simplified cross section of crystalline silicon PV cell
showing use of industrial minerals
SiC wiresawing
The molten silicon metal is
crystallised out using a silicon seed crystal, which is rotated
to produce a cylindrical ingot. The ingot is then cut into a
square, and the square ingot then is sliced into the silicon
wafers destined for solar cells.
This introduces the next major
industrial mineral application, that of a silicon carbide (SiC)
slurry used with a wiresaw. It is thought that around 90% or
more silicon wafer processors use SiC as the abrasive medium of
choice.
SiC grades of choice comprise
98-99% SiC, and in FEPA sizes F500, F600, and F800, and
occasionally, F1000. The key here is that the SiC grade must be
ultra consistent for silicon wafer processors.
Dr Oliver Anspach, manager research
and development, of PV Crystalox Solar Silicon GmbH (part of
leading PV Si wafer producer PV Crystalox Solar Plc, UK), told
IM: “There is still little understood
about what goes on in SiC wire sawing, it’s a black box
operation, so SiC grades used must be very
consistent.”
Certainly, leading SiC producers
have been quick to react to supplying this market by expanding
more into SiC microgrits, such as Washington Mills Electro
Minerals Corp. with its Carborex grades, and Saint-Gobain with
its Sika grades.
Although there appeared a shortage
of F600 SiC grades a couple of years ago, Anspach considers
that SiC supply for Si wafer cutting is satisfactory at
present.
However, Si wafer slicing is one of
the most costly steps in the Si wafer production process; it is
very slow, taking 5-6 hours, even with a multiple wiresaw, and
losses as saw dust (kerf loss) are up to 30%.
As a result, one potential cloud on
SiC’s PV market horizon might be the advent of more
widespread use of diamond wiresaws in Si wafer slicing. The
diamond option, which remains largely at R&D stage, could
offer quicker and more efficient results, perhaps only around
1.5-6.0 metres of wiresaw per wafer, compared to 400-600km
saw/wafer using SiC.
It is understood that two
processors in Japan have each switched 50% of their sawing
capacity to using diamond saws. However, Dr Anspach warned:
“But there are problems with the look of the wafer, and it
will be up to what the solar cell companies desire. The diamond
wiresawing creates scratches on the wafer surface, and overall
it looks ugly.”
Anspach envisages that diamond
wiresawing might emerge in the second half of 2010 for
monocrystalline Si wafer cutting, but much later for
polycrystalline cutting.
Talking to IM at
the Ceramitec fair, Munich, in October 2009, Anne Marie Moe,
manager technology, Washington Mills AS, Norway, considered
that diamond wiresawing was “not a big threat, no-one
knows how many years it will take to develop. The sawing of Si
wafers is safe as a new growth market for SiC.”
The main reasons for this view stem
not only from diamond wiresawing’s perceived infancy, but
also that most Si wafer processors have wiresaw plants not
suitably set up for diamond wiresaws.
PV manufacturing process from quartz to solar cell
system
Module mineral fillers
After cleaning and treatment, the
Si wafers are then screen printed with electrical contacts, to
form the solar cell. The solar cells are embedded between two
glass panes and a special resin is filled between the panes,
securely wrapping the solar cells on all sides to form a
module. Modules are then linked together to form a system which
generates a direct electrical current for a variety of
applications.
Industrial minerals may find low
volume, but high value niche applications in the composition of
some of the above mentioned components of the solar cell
modules.
Polymeric materials are used as
encapsulators and backsheets (or backskins) in solar cell
modules. One of the main encapsulation polymers of choice is
ethylene vinyl acetate (EVA), which maybe mineral filled, and
one example has used silver coated wollastonite.
For crystalline silicon solar
cells, the cells can be encapsulated such that a transparent
encapsulant is used between a transparent superstrate (usually
glass Ð also using minerals for its manufacture) and the
solar cell. In this case, a second layer of encapsulant, which
may be pigmented, can be used between the solar cells and the
backsheet material.
For thin film solar cell modules a
single layer of encapsulant is employed.
A recent patent for encapsulation
of PV cells using a silicone based material includes the use of
fillers which substantially match the refractive index of the
sheet material and maybe selected from one or more of
wollastonite, silica, titanium dioxide, glass fibre, hollow
glass spheres, and clays.
The backsheet material may
additionally comprise one or more fillers to reduce weight and
lower cost and to change colour or reflectivity. These may
comprise one or more finely divided, reinforcing fillers such
as high surface area fumed and precipitated silicas and to a
certain degree calcium carbonate, or additional extending
fillers such as crushed quartz, diatomaceous earths, barium
sulphate, iron oxide, titanium dioxide, and carbon black, talc,
and wollastonite.
Flame retardant mineral fillers are
also considered for use in solar cell encapsulation materials,
and would encompass alumina trihydrate, magnesium hydroxide,
and wollastonite.
Building for a solar future: DuPont anticipates
the PV market to grow by 30% in 2010. Courtesy DuPont.
Fluoropolymer backsheets
Fluoropolymer films (derived from
fluorochemicals, whose starting raw material is fluorspar) have
played and continue to play multiple roles in the PV module
package.
The backsheet provides physical
protection, electrical insulation, moisture protection and in
some circumstances unique colour identification all while
adhering to the encapsulant over the life of the module.
Solar cell module backsheets can
contain polyviny fluoride, but also in combination with a range
of additives including mineral fillers and pigments, with
loading levels of up to 30%.
In January 2010, DuPont, a major
consumer of fluorspar, completed a $295m. investment in PV film
manufacturing, with a $175m. expansion to its DuPont
Circleville, Ohio facility producing Tedlar®
film.
Manufacturing
Tedlar® film includes producing vinyl fluoride
(VF) monomer, which is converted into polyvinyl fluoride (PVF)
polymer resins, and extruded into the Tedlar®
film.
Tedlar® films are a
main component of PV cell backsheets, and this expansion
capacity is claimed to support global demand of over 10GW of PV
module production.
DuPont plans to increase monomer
and polymer resin capacity by more than 50%. Construction is
underway for these new monomer and resin facilities at
Louisville, Kentucky, and Fayetteville, North Carolina,
respectively, and are scheduled to start up in mid-2010.
DuPont is clearly bullish about the long term future of PV,
since it expects that overall sales of its PV products will
exceed $1,000m. by 2012 (sales exceeded $500m. in 2009). DuPont
anticipates that the PV market will grow by about 30% in 2010,
and continue to grow rapidly over the next three years.