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Why do we burn coal and trees to
make solar panels? Thomas A. Troszak (2019/11/14 revision)
Figure 1. Workman shovels coal and lumpy quartz (silicon ore)
into a silicon smelter in China. (photo: Getty)
1. Most commercial solar PV modules use
photovoltaic cells
(solar cells) made from
highly purified silicon (Si).
Since the early 1900s, silicon “metal” is reduced from
quartz using carbon in submerged-arc furnaces, each
powered by up to 45 megawatts* of electricity. (Fig 1,2)
Figure 2. Diagram of a silicon smelter showing the three giant carbon
electrodes that provide arc temperatures > 3,000oF for smelting quartz
into “metallurgical grade” silicon (mg-Si) using carbon as a reductant.[26]
(John Wiley and Sons, Ltd.)
2. Why do we need to burn carbon to make solar PV? -
Elemental silicon (Si) can’t be found by itself anywhere
in nature. It must be extracted from quartz (SiO2)
using carbon (C) and heat (from an electric arc) in the
“carbothermic” (carbon+heat) reduction process
called “smelting.” (Si02 + 2C = Si + 2CO) Several
carbon sources are used as reductants in the silicon
smelting plant, which requires ~20 MWh/t of
electricity, and releases CO - resulting in up to 5 - 6 t
of CO2 produced per ton of metallurgical grade
(mg-Si) silicon smelted. [1] Thus, the first step of solar
PV production is gathering, transporting, and burning
millions of tons of coal, coke and petroleum coke -
along with charcoal and wood chips made from
hardwood trees - to smelt >97% pure mg-Si from quartz
“ore” (silica rocks). [1][2][3][4][5][6][7][8][9][10]
*45 megawatts (MW) is enough for a small town (about 33,000 homes).
Figure 3. Pouring liquid metallurgical grade (~99% pure) silicon
into molds, to cool into silicon “metal”. (Getty)
3. Even more fossil fuels are burned later, to generate
electricity for the polysilicon, ingot, wafer, cell, and
module production steps shown. [21] As a result of all
these processes, the solar PV industry generates
megatons of CO and CO2. But as shown below (fig 4),
some often-cited descriptions of solar module
production omit the raw materials and smelting
process from the PV supply chain which obscures the
use of fossil fuels and the vast amount of deforestation
necessary for solar PV production. [1][3][9][27]
Figure 4. (source: National Renewable Energy Laboratory, 2018)
4. Raw materials for metallurgical-grade silicon
Raw materials for one ton (t) MG-Si (Kato, et. al) [37]
●Quartz 2.4 t
●Coal 550 kg
●Oil coke 200 kg
●Charcoal 600 kg
●Woodchip 300 kg
Raw materials for one ton (t) MG-Si (Globe) [3]
●Quartz 2.8 t
●Coal 1.4 t
●Woodchips 2.4 t
For 110,000 tpy (tons per year) MG-Si (Thorsil) [1]
●Quartz 310,000 tpy
●Coal, coke and anodes 195,000 tpy
●Wood 185,000 tpy
●Total 380,000 tpy
When calculating CO2 emissions from silicon smelting, “by
joint agreement” some authors exclude CO2 emissions from
non-fossil sources (charcoal, wood chips), power generation,
and transportation of raw material. [27]
5. Sources of carbon for solar silicon smelting
• Coal - Is a dense, rock-like fuel. The (low ash) coal
used directly for silicon smelting is mostly the ”Blue
Gem" from Cerrajón, Columbia, Kentucky, USA, or
Venezuela. [1][2][3][5][6][7][8]
The Cerrajón open-pit mine in Columbia supplies “Blue
Gem” coal for silicon smelters around the world.
A ”Slot Oven” discharging coke into a railroad car. (photo: Alamy)
• Metallurgical Coke (Metcoke) is a
tough, cinder-like solid fuel made
by "coking" coal in large “slot
ovens” - to drive out most of the
volatile tars, etc. to the atmosphere
as smoke, flame, carbon monoxide,
carbon dioxide, sulfur dioxide,
other gasses, and water vapor.
(photo: Getty Images)
The coking process is nearly
identical to the process used for
making charcoal from wood (see charcoal production
below). Restricting the
air supply to a large
mass of burning coal
allows about 40% of the
coal to “burn off” -
leaving behind a solid
residue (coke) with a
higher carbon content
per ton that the original
coal. It takes about 1.6 t
of coal to make a ton of
coke.
Metcoke looks like
porous, silvery grey coal.
Filling barges with petcoke outside Chicago, Ill, USA (photo)
• Petroleum Coke (Petcoke) - is a solid fuel in the form
of pellet-like granules, which are a carbon-rich
byproduct of crude oil refineries. Millions of tons of
petcoke are also made directly from raw bitumen (tar).
Due to its low price and high carbon content, petcoke
made in American refineries from "Canadian Tar
Sands” is a source of carbon exported from the U.S. to
silicon manufacturers in China. [9]
“Because it is considered a refinery byproduct, petcoke
emissions are not included in most assessments of the
climate impact of tar sands” [10]
“Beehive” charcoal ovens in Brazil (Alamy)
• Wood Charcoal - Many hardwood
trees must be burned to make wood
charcoal. In the traditional process,
wood is stacked into “beehive ovens”,
ignited, then mostly smothered to
prevent the wood from burning completely to ash. By
weight, about 75% of the wood is lost to the
atmosphere as CO, CO2, smoke, and heat.
Some silicon producers use “charcoal plantations,” but
they only supply a fraction of the current demand of
carbon for silicon production. The rest of the carbon
supply has to come from imported coal or coke, or the
cutting and burning of “virgin” rainforest. [13][14][15][16]
In Brazil, it is estimated that more than a third of the
country’s charcoal is still produced illegally from
protected species. [14] Brazil is a charcoal supplier to
silicon producers in other countries, including the
United States. Silicon smelters around the world use
charcoal from many sources, so solar silicon may be
smelted with charcoal made directly from rainforest
not grown on plantations.
This hardwood forest in the U.S. was clear cut to make wood
chips
6. Hardwood Chips (also called
Metchips) - Matchbox-sized
fragments of shredded
hardwood must be mixed into
the silicon smelter “pot” for
many reasons - to allow the
reactive gasses to circulate, so
the liquid silicon that forms can settle to the bottom
for tapping, and to allow the resulting CO (and other
gasses) to escape the smelter “charge” safely. [4]
Solar silicon quartz rocks (Wacker Chemie)
7. Silicon ore - Quartz - (silica, silicon dioxide, SiO2)
Even if sufficiently pure, silica sand won’t work in any
silicon smelter, it is too fine. Selected high-purity
quartz is mined and graded into “lumpy” (fist-sized)
gravel for smelting. Worldwide, "solar grade” deposits
of quartz are somewhat scarce, and highly valued.
A single polysilicon plant like this one in Tennessee, USA. can
draw 400 megawatts of electricity, enough power for about
300,000 homes. (Wacker Polysilicon)
8. Polysilicon production
Metallurgical grade silicon (mg-Si) from the smelter is
only about 99% pure, so it must undergo two more
energy-intensive processes before it can be made into
solar cells. First, the Siemens Process converts (mg-Si)
from the smelter into polycrystalline silicon (called
polysilicon) by a high-temperature vapor deposition
process.
This is a bit like “growing rock candy” on
hyper-pure silicon “strings” inside a pressurized-gas
filled “bell-jar” reactor. As a mixture of silicon gas
(made from mg-Si) and hydrogen gas passes through
the reactor vessel, some of the silicon gas molecules
“cling” to the electrically heated “strings” (called
filaments) causing them to grow into “rods” of
99.9999% pure (or better) polysilicon.
Left: When heated to around 1100°C the polysilicon “filaments”
standing beneath the reactor cover can “catch” about 20% of the
silicon atoms that pass through the reactor in gaseous form.
Right: Polysilicon “rods” after 5 days of growth. (Siemens AG)
Each batch of polysilicon “rods” takes several days to
grow, and a continuous, 24/7 supply of electricity to
each reactor is essential to prevent a costly “run abort.”
So polysilicon refineries depend on highly reliable
conventional power grids, and usually have two
incoming high-voltage supply feeds.
A polysilicon plant consumes ~1.6 - 6 t of
incoming mg-Si, and requires at least 175 MWh (or
more) of additional electricity per ton of polysilicon
produced - about 10 times the energy already used for
smelting each ton of mg silicon from ore. [11] After the
rods are removed from the reactor, they are sawed into
sections or broken into “chunks” for loading into
crucibles in the next step.
Polysilicon rods and sections being broken into chunks by hand
in a clean room. (Hemlock)
Polysilicon chunks being heated in a crucible. When melted, a
single crystal will be pulled out of the liquid polysilicon. (Getty)
9. Crystal growing (ingot production)
For making single-crystal solar cells (called mono PV)
the PV industry uses the Czochralski process to
further purify the polysilicon, and align the silicon
molecules into a single-crystal form.
First, polysilicon chunks are melted in a
rotating crucible in an inert atmosphere. Then a small
seed crystal of silicon is lowered into the molten
polysilicon. As the seed crystal is slowly withdrawn, a
single silicon crystal forms from the tip of the seed. As
the crucible turns, the polysilicon continues to grow
into a cylindrical ingot, leaving most of the non-silicon
impurities behind in the 5-10% of “pot scrap” remaining
after the crystal is drawn free.
Czochralski ingot being pulled from melted polysilicon.
(Image source: Siltronix)
Czochralski ingot after cooling (Image source: Getty)
This process requires several days, and uninterrupted
power. An ingot/wafer/cell plant can use more than
100 MWh additional energy per ton of incoming
polysilicon, about 6 times as much as the original
smelting of the silicon from ore. After slow cooling,
the ingot's unusable crown and tail are cut off (about
10%), the center is then ground down, the four “chords”
(long sides) are sawn off (about 25%) leaving a
rectangular “brick” so the solar wafers will be almost
square after slicing.
Czochralski process whole ingot (left), and brick and chords after
sawing (right), crown and tail (upper right) (SVM)
For multi-crystalline cells (called multi PV)
polysilicon is melted in rectangular quartz molds, then
allowed to cool slowly into a rectangular ingot of
multi-crystalline silicon. which is trimmed to remove
unusable portions, then sliced into bricks.
10. Wafer sawing
Then, like a loaf of bread, the silicon "bricks" are sliced
with wire saws into thin wafers, which will later be
processed into cells.
About half of the "brick" is lost as "sawdust" in the
wafer slicing process, and this can't be recovered. So,
after all of the energy and materials that have gone into
making each "brick", much of the incoming polysilicon
does not ever become finished wafers. Some of the
heads, tails, chords, and trimmings can be etched (to
remove contamination) and remelted using additional
energy if the purity of the scrap is sufficient to justify
the expense, otherwise they are discarded as waste.
11. Cell and module production.
Once the wafers are sliced, they are made into “cells”
by adding layers of other materials and components in
a series of additional production steps.
Diffusion Furnace in the PV-TEC at Fraunhofer ISE.
Loading of the diffusion tubes with batches of multicrystalline
silicon wafers. The wafers, sorted into quartz boats, are brought
into the (up to) 1000 °C hot quartz tubes. (Fraunhofer ISE)
Then the cells are assembled into modules. Beside
silicon wafers, most solar PV modules also require
many other energy-intensive materials - aluminum (for
the frame), silver, copper, glass, plastic, highly toxic
rare earth metals, acids, and dozens of other chemicals
for processing the polysilicon into cells and modules. A
lot of electricity is needed to power the cell production
and module assembly, a supply of natural gas is used to
provide heat in the process.
Solar module inspection on the assembly line. (Solar World)
12. Other materials and steps
Once the modules are made, the whole PV system
usually needs steel or aluminum framing, concrete, and
some empty land (or a rooftop) to position it securely
toward the sun, a lot of wiring to connect (through
DC/AC inverters and transformers) to the existing
power grid, or directly to battery banks,
PV support structure and concrete foundation (Hill & Smith)
Of course, it takes a lot of energy and resources to
make steel, aluminum, concrete, inverters, copper
wiring, and all of these other materials. In many cases,
the "balance of system" components in a PV
installation can require as much (or more) “up-front”
resources and energy to make as the modules. [21]
In addition, the amount of fossil fuels and
non-renewable resources needed to construct and
maintain new PV production infrastructure (smelters,
polysilicon refineries, etc.) is considerable, but has
been excluded from all “life cycle analysis” (LCA) of
solar PV production by definition. [38]
13. Transportation
Throughout the solar PV manufacturing process all of
the materials and products must be shipped to and
from more than a dozen countries around the world in
large barges, container ships, trains, or trucks - all
powered by non-renewable oil. [36]
14. Power
Worldwide, only a few silicon smelters, like those in
Norway, are powered primarily by hydro-electricity.
Elsewhere, the current majority of smelters, polysilicon
refineries, ingot growers, cell and module factories are
running on grids powered mostly by fossil fuels and
uranium. At present, more than 50% of all solar silicon
is made in China, where the industrial grid is powered
largely by fossil fuels, primarily low-grade coal.
Depending on the “energy mix” available, the quantity
of coal, coke, or gas that is being burned to deliver
power 24/7 to the PV factories may be far greater than
the amount needed as the carbon source for smelting
silicon. To provide a realistic assessment of the total
environmental impact of PV manufacturing, this must
be added to the “fossil fuel bill” for solar PV
production - along with the “embodied energy“ of PV
factories. [11][12][21]
15. Conclusions
Every step in the production of solar photovoltaic (PV)
power systems requires a perpetual input of fossil fuels -
as carbon reductants for smelting metals from ore, for
process heat and power, international transport, and
deployment. Silicon smelters, polysilicon refineries, and
crystal growers around the world all depend on
uninterrupted, 24/7 power that comes mostly from coal
and uranium. The only "renewable" materials consumed
in PV production are obtained by deforestation - for
wood chips, and by burning vast areas of tropical
rainforest for charcoal used as a source of carbon for
silicon smelters. So far, both media and journal claims
that solar PV can somehow "replace fossil fuels" have not
addressed the non-renewable reality of global supply
chains necessary for mining, manufacturing, and
distribution of PV power systems. Based on current
world production levels of solar PV, an attempt to
replace conventional electricity production with solar
PV would require a dramatic increase in the amount of
coal and petcoke needed for silicon smelting, along with
the increased cutting of vast areas of forest for charcoal
and wood chips.
Readers are encouraged to examine all of the references
below, to become aware of other aspects with solar pv
manufacturing and deployment that are beyond the
scope of this paper.
References
[1] Thorsil (2015) “Metallurgical Grade Silicon Plant - Helguvík, Reykjanes municipality (Reykjanesbær),
Reykjanes peninsula, Iceland Environmental Impact Assessment (EIA) Capacity: 110,000 tons”
https://www.giek.no/getfile.php/133565/web/Dokumenter/Prosjekter%20under%20vurdering/EIA-
Thorsil_Lingua-2-%20konsekvensutredning.pdf (1) “Thorsil's initial assessment report was based
on using...Coal from El Cerrajon in Columbia...for an annual production...of 110,000 tpy [of
mg-Si]…would correspond to 605,000 tpy of carbon dioxide…The Environment Agency feels
that…such exhaust would significantly increase Iceland's overall emissions“
[2] Efla (2013) “Environmental Impact Assessment of a SILICON METAL PLANT AT BAKKI IN
HÚSAVÍK”https://www.agaportal.de/_Resources/Persistent/856d55b1a3c1948e5f856f800195760741
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[3] “New York State Department of Environmental Conservation - Facility DEC ID: 9291100078 PERMIT
Under the Environmental Conservation Law (ECL) Permit Issued To: GLOBE METALLURGICAL
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petroleum coke, or other forms of coke, wood chips, and quartz are mixed and added at the top
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[4] “The Use and Market for WOOD in the ELECTROMETALLURGICAL Industry”
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large surface area for chemical reaction to take place more completely and at improved
rates…To maintain a porous charge, thereby promoting gentle and uniform - instead of violent
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[6] What Terrible Injustices Are Hiding Behind American Energy Habits?
By Itai Vardi • Friday, November
16, 2018 (link) (6) “There is a clear ‘consumer blindness’ and citizens and residents are often
unaware of where the fuel they consume is coming from and what injustices were inflicted on
communities within those sites of fossil fuel extraction,” said Healy. “Exposing these injustices
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could be critical for future energy policy decision-making.”
[7] 2017/06/18/why-this-part-of-coal-country-loves-solar-power-215272 (7)“the seam in Whitley County
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need the blue gem to make the solar panels, and that’s what people don’t know,” Moses told
me, articulating a simple truth: “Without Coal Valley, there’s no Silicon Valley”
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[10]Petroleum Coke: The Coal Hiding in the Tar Sands (10) “Because it is considered a refinery
byproduct, petcoke emissions are not included in most assessments of the climate impact of
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[11] https://www.sightline.org/2018/06/25/small-town-silicon-smelter-plan-tees-up-big-questions/
(11) “these furnaces would have a voracious appetite for electricity: around 105 megawatts on a
continuous basis, roughly the equivalent of 68,000 homes...the facility would demand more
power than the dam could provide....Producing one ton of silicon metal requires about six tons
of raw materials...Nearby sawmills would send seven or eight trucks per day to deliver wood
chips, which are integral to the smelting process….“The smelting process requires a rare type
of metallurgic coal known as “blue gem,” … Operations at the smelter would demand
approximately 48,000 metric tons of coal per year—roughly 40 rail cars each month.”
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hydro."
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methods of charcoal making, which are still widely used in these continents, are inefficient
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[16] 2017/10/burning-down-the-house-myanmars-destructive-charcoal-trade/
(16) “Dehong's silicon
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share of non-fossil fuels was actually lower, as growth in renewables has failed to compensate
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this statistic (1000 homes per MW) to intermittent renewable power sources...Intermittent
renewables generally produce far fewer kilowatt-hours per MW than conventional power
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they are already operating at a deficit...These technologies require large, ‘up-front’ energetic
investments…A fractional [energy] re-investment of greater than 100% … means that the
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deficit”
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[24]Lessons Learned Report - Electrical Energy Storage DOCUMENT NUMBER CLNR-L163
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[26] Luque, A., & Hegedus, S. (Eds.). (2011). Handbook of photovoltaic science and engineering
.
John Wiley & Sons. (link) (26) “Photovoltaics is polluting just like all high-technology or
high-energy industries only with different toxic emissions … Manufacturing of PV modules on
a large scale requires the handling of large quantities of hazardous or potentially hazardous
materials (e.g. heavy metals, reactive chemical solutions, toxic gases”
[27] https://www.researchgate.net/publication/311440469_CO2_Emissions_from_the_Production_of_Ferrosilic
on_and_Silicon_metal_in_Norway (27) “These emission factors only include CO2 emitted from fossil raw
materials in the reduction process. CO2 from biological, renewable sources is not included (according to
joint agreement). Neither is CO2 emitted from electric power production or during transportation of raw
materials.”
[28] Cleaning Up Clean Energy - https://web.stanford.edu/group/sjir/pdf/Solar_11.2.pdf (28) “the
(PV) industry has largely overlooked investigative reports revealing current problems with
production waste, particularly pertaining to Chinese manufacturing. Until these concerns
receive more attention, promises of panel recycling will quell any public anxiety, preventing
the creation of necessary safeguards to stop rogue firms from unsafe manufacturing practices”
[29]https://www.forbes.com/sites/michaelshellenberger/2018/05/23/if-solar-panels-are-so-clean-
why-do-they-produce-so-much-toxic-waste/#256668c121cc (29) “We estimate there are 100,000
pounds of cadmium contained in the 1.8 million panels,” Sean Fogarty of the group told me.
“Leaching from broken panels damaged during natural events — hail storms, tornadoes,
hurricanes, earthquakes, etc. — and at decommissioning is a big concern.”
[30]https://www.scmp.com/news/china/society/article/2104162/chinas-ageing-solar-panels-are-going-be-b
ig-environmental-problem (30) Lu Fang, secretary general of the photovoltaics decision in the China
Renewable Energy Society, wrote...By 2050 these waste panels would add up to 20 million
tonnes, or 2,000 times the weight of the Eiffel Tower...Tian Min, general manager of Nanjing
Fangrun Materials, a recycling company in Jiangsu province that collects retired solar panels, said
the solar power industry was a ticking time bomb.“It will explode with full force in two or three
decades and wreck the environment, if the estimate is correct,”
[31] https://www.solarpowerworldonline.com/2018/04/its-time-to-plan-for-solar-panel-recycling-in-the-unite
d-states/ (31) “We’ve conducted some toxicity testing on modules, and we have seen results showing
that the presence of lead is higher than the threshold allowed by the TCLP (toxicity
characteristic leaching procedure)...There is a potential for leaching of toxic materials such as lead
in landfill environments. If modules are intact, it’s a low risk, but as soon as they’re broken or
crushed, then the potential for leaching is increased.”
[32] https://www.welt.de/wirtschaft/article176294243/Studie-Umweltrisiken-durch-Schadstoffe-in-Solarmod
ulen.html (32) "Based on installed power and performance weight, we can estimate that by the year
2016, photovoltaics has spread about 11,000 tonnes of lead and about 800 tonnes of Cd
(cadmium)," the study said”
[33] https://www.solarpowerinternational.com/wp-content/uploads/2016/09/N253_9-14-1530.pdf (33)
“disposal in “regular landfills [is] not recommended in case modules break and toxic materials
leach into the soil” and so “disposal is potentially a major issue.”
[34] Tao, Coby S., Jiechao Jiang, and Meng Tao. "Natural resource limitations to terawatt-scale solar
cells." Solar Energy Materials and Solar Cells
95, no. 12 (2011): 3176-3180.
https://doi.org/10.1016/j.solmat.2011.06.013 “Material scarcity prevents most current solar cell
technologies from reaching terawatt scales. (…) Scarce materials in solar cells include indium,
gallium, tellurium, ruthenium, and silver. - Natural resource limitations to terawatt-scale solar
cells.”
[35] Metal-demand-for-renewable-electricity-generation-in-the-netherlands “The current global
supply of several critical metals is insufficient to transition to a renewable energy system.
…production of wind turbines and photovoltaic (PV) solar panels already requires a
significant share of the annual global production of some critical metals… Furthermore,
mining is often associated with significant environmental and social costs”
[36] INCREASES IN EFFICIENCY HAVE NOT REDUCED ABSOLUTE CO2 EMISSIONS FROM SHIPS
“Although the CO2 intensity of many major ship classes decreased (i.e., they became more
efficient) from 2013 to 2015, total CO2 emissions from ships increased. For example, although
the CO2 intensity of general cargo ships (measured as emissions per unit of transport supply)
decreased by 5%, CO2 emissions increased by 9% Thus, increases in distance traveled due to a
greater demand for shipping more than offset gains in operational efficiency during the
period studied”
[37] Kato, K., Murata, A., & Sakuta, K. (1998). Energy pay-back time and life-cycle CO2 emission of
residential PV power system with silicon PV module. Progress in Photovoltaics: Research and
Applications
, 6
(2), 105-115.
[38] Fthenakis, V., Kim, H., Frischknecht, R., Raugei, M., Sinha, P., & Stucki, M. (2011). Life cycle
inventories and life cycle assessment of photovoltaic systems. International Energy Agency (IEA)
PVPS Task
, 12
. http://www.clca.columbia.edu/Task12_LCI_LCA_10_21_Final_Report.pdf
