Polycrystalline
silicon, also known as polysilicon, is a high purity form of silicon used to
manufacture photovoltaic solar panels. It is produced from metallurgical-grade
silicon using the Siemens process which involves purifying molten silicon
through distillation. Polysilicon forms the vital building block of the solar
photovoltaic industry and its production has increased tremendously over the
past few decades to meet the rising demand for solar panels and modules
worldwide.
Production Process
The production process of polysilicon involves several complex steps. It begins
with the reaction of metallurgical-grade silicon with hydrochloric acid to produce
trichlorosilane gas. This gas is then purified through distillation to obtain
high-purity trichlorosilane. In the next step, the purified trichlorosilane
reacts with hydrogen at elevated temperatures over a catalyst to deposit pure
silicon in the form of slim silicon rods. These rods are crushed and processed
to produce polysilicon granules.
Proper control of numerous process parameters like temperature, pressure,
catalyst composition is required throughout to maintain the purity and quality
of polysilicon produced. State-of-the-art purification technologies are
employed to filter out impurities down to parts-per-billion level. The entire
production cycle from feedstock to finished polysilicon takes around 2 to 3
months to complete.
Rise of Solar Energy and Polysilicon
Demand
Over the past few decades, solar energy has emerged as one of the most
promising renewable sources of energy globally. Many countries have set
ambitious targets to increase the share of solar power in their overall energy
mix to reduce greenhouse emissions. This has led to a massive capacity addition
of solar photovoltaic installations worldwide. To meet the increasing demand
for solar panels and modules, the production of polysilicon which acts as the
raw material had to correspondingly scale up.
In the early 2000s, global polysilicon capacity was just a few thousand metric
tons per annum. However, over the years several large polysilicon plants were
set up mainly in China which led to a massive increase in production. By 2010, global
polysilicon capacity crossed the 100,000 tons per year mark. This ramp up
continued and presently stands at over 500,000 tons annually to meet the
multi-gigawatt scale solar installations planned over the next few years.
Transition to Granular Polysilicon
Along with capacity expansion, the polysilicon industry also saw a transition
from chunk or chunk-based production to granular polysilicon manufacturing. In
the chunk or rod-based process, pure silicon was deposited directly from the
gas phase onto electrically heated filaments to form long cylindrical rods.
These rods were then crushed and processed to produce polysilicon chunks or
granules.
While this method was successful initially, it posed issues related to filament
breakage, impurity control and low production efficiency. To overcome these
bottlenecks, most modern polysilicon plants have shifted to the granular
process. In this direct deposition of silicon is done on seed particles
suspended in a fluidized bed reactor to yield spherically shaped granules
directly.
This granular process has several advantages like higher deposition rates,
better impurity control, simpler design and requires lesser capital investment
compared to the rod-based technology. It has now become the predominant method
adopted globally accounting for over 80% of total polysilicon
output. The transition started in the mid-2000s and was almost complete by
the end of the last decade.
Future Outlook
As dependency on renewable energy accelerates to counter climate change, solar
power will play a leading role in the future energy mix of many countries.
International bodies project that global solar installations could exceed 4,000
GW by 2050 from current levels of around 600 GW. This exponential rise in solar
capacity will correspondingly drive huge demand for polysilicon as its key
starting material.
Going forward, the polysilicon industry is anticipated to further consolidate
into larger and more efficient production facilities to achieve economies of
scale. Continued capacity ramp up along with transition to more advanced
granular processes can help meet the colossal polysilicon requirement
envisaged. Efforts are also underway to develop alternative lower-cost
production methods using coal or other feedstocks. With constant technological
improvements polysilicon is well-positioned to be the indispensable material
powering the green solar revolution worldwide.
polysilicon has become the primary building block material for the expansive
global solar PV industry. Its production has witnessed a massive multi-fold
growth fueled by soaring solar installations. Adoption of better granular
manufacturing processes has enhanced efficiencies. Looking ahead, further
capacity increases leveraging innovative technologies will be crucial to cater
to the gigantic projected polysilicon demand from the promising renewable solar
sector. Polysilicon will remain the backbone enabling the large-scale
transition to solar as a major clean energy source.
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