Silicon for the Chemical and Solar Industry IX
Oslo, Norway, June 23-26, 2008
Comparison of the energy consumption in different
production processes for solar grade silicon
Jan Ove Odden,1) Gunnar Halvorsen,1) Harry Rong,2) and Ronny
Gløckner1)
1) Elkem Solar AS, P.O.Box 8040 Vaagsbygd, NO-4675 KRISTIANSAND, NORWAY
2) Elkem AS, Silicon Division, Hoffsveien 65B, 0303 OSLO, NORWAY
Abstract
Today more than 95% of solar grade silicon feedstock is produced by decomposition of
silanes using Siemens, Komatsu or FBR – technology. Metallurgical refined silicon of
solar grade quality will in the coming years become increasingly available to the solar
market and will reach a market share of at least 10–15% by 2011, probably around 2030% if the probability of success for new solar grade silicon producers is taken into
consideration. In the future energy consumption and CO2- emission will become
important competitive factors. The different feedstock sources and the industry will be
faced with complete Life Cycle Assessment (LCA) studies to compare solar energy with
alternative sustainable energy sources.
The current paper deals with the energy consumption in the main gas route
processes including generation of trichlorosilane, silane and different silicon
tetrachloride recycling steps. It also evaluates the energy consumption in the Siemens
process based on literature available data. For all the processes, detailed process data
from the industry are missing, so some assumptions are included. Generally the
numbers for energy consumption for gas route processes are more than 4 times higher
than for metallurgical solar grade silicon when internal recycling and silicon yield are
included. The CO2 emissions are, dependent on the available energy source for
production, up to 16 times that of metallurgical solar grade silicon produced in Norway.
Introduction
The demand from the solar industry for silicon feedstock has escalated tremendously the
last couple of years and actually surpassing the electronic demand in 2007-2008.
The total announced volumes of polysilicon in 2007 were close to 42,000 MT, and
Fig. 1 shows the distribution of this capacity on the different production routes.
1%
14 %
6%
MS-rod deposition
5%
MS-FBR
Metallurgical route
TCS-Siemens
Other (VLD,STC red.,TCS-FBR)
74 %
Figure 1: Total polysilicon production in 2007 by production route. Source: Calculations
based on public announcements of production volumes. (MS = monosilane, TCS =
trichlorosilane, STC = silicon tetrachloride, VLD = vapor to liquid deposition, FBR =
fluidized bed reactor.)
Due to the shortage of polysilicon in the marked for PV use, generating very high
prices, a tremendous growth of polysilicon production is experienced in the solar
industry. The total announced volumes of polysilicon for 2011 amount to over 330,000
MT (not considering the probability of success for the new players and for those
planning to expand). Over one third of this apparent expansion is announced to come
from new players in China. Fig. 2 shows how the capacity of polysilicon in 2011 will be
distributed on the different production routes.
2%
2%
3%
7%
11 %
MS-rod deposition
MS-FBR
Metallurgical route
TCS-Siemens
Other (VLD,STC red.,TCS-FBR)
TCS-FBR
75 %
Figure 2: Total polysilicon production announced for 2011 by production route. (Source:
same as for Fig. 1.)
From Figs. 1 and 2 we can deduct that even if the amount of polysilicon produced
through a metallurgical route is expected to more than double from 2007 to 2011 the
traditional Siemens process decomposing TCS in Siemens type reactors will still by far
be the largest source, indicating that new players in the market also plan to use the
conventional polysilicon production method.
2
The production of polysilicon
Metallurgical grade silicon – the first step
Most of the processes for producing polysilicon starts with the generation of so called
metallurgical grade silicon (MG-Si) from silicon oxides abundantly present in the earths
crust in different qualities regarding purity. The mostly used way of generating MG-Si
from silicon oxide is through the (simplified) high-temperature (~2300 K) carbothermic
reaction:
SiO2(s) + 2C(s) → Si(l) + 2CO(g)
(1)
This is a very energy demanding step requiring about 12 kWh/kg MG-Si produced [1].
As can be seen from eqn. (1) this reaction generates carbon oxides and thereby
contributing negatively to the environment by about 4.3 kg fossil CO2/kg MG-Si
produced [1]. The silicon producers operate the process with different silicon yields
(due to knowhow and capabilities) and this will also influence the environmental aspect
together with the actual energy mix the electricity is generated from.
Refining MG-Si into polysilicon
Since MG-Si doesn’t have the required quality for use in solar cells regarding the
content of impurities (MG-Si is typically ~98% pure) further refinement is necessary
before reaching the requirements of the solar industry (SOG-Si of ~99.9999% purity).
From MG-Si to solar grade silicon (SOG-Si) there are, however, different routes. The
most common way of refining MG-Si is through steps generating gaseous silicon
containing compounds followed by their separation by distillation and thermal
decomposition into purer silicon. Alternatively MG-Si can be upgraded through means
like slag treatment and leaching. In the following the most important routes are
discussed in some detail.
Producing the silicon-containing source gas
TCS production
All the major gas route processes for making SOG-Si starts with MG-Si reacting and
forming the silicon containing gas trichlorosilane (TCS) – SiHCl3.
Either through a hydrochlorination of MG-Si:
Si(s) + 3HCl(g) → SiHCl3(g) + H2(g)
(2)
Or through the reaction of MG-Si with silicon tetrachloride (STC) – SiCl4, which is also
generated as a by-product during the total process (this recycle reaction is to be
commented further below in the section “dirty recycle”):
Si(s) + 2H2(g) + SiCl4(g) → 4SiHCl3(g)
(3)
Eqn. 2 represents a strongly exothermic reaction, ∆Hreaction, 573 K = –218 kJ/mol [2],
which is kinetically hindered by the activation energy (Ea) of 116 kJ/mol [3]. The output
3
ratio TCS/byproducts (mainly STC) decreases with temperature, so cooling of the
reaction mixture is necessary to hinder the exothermic nature of the reaction in lowering
the TCS yield. The reaction is normally run at temperatures of 300–350°C to obtain a
TCS yield of between 80% [4,5] and 95% [6], the latter result obtained using numerous
filtration steps separating the TCS from i.a. high-boiling residues, silicon fines, and
aluminum chloride solids at temperatures below the sublimation temperature of 180°C,
preferably 60-80°C [7].
In order to further purify the generated crude TCS the reaction mixture is first
quenched to remove light gas waste followed by several distillation steps in different
columns. Especially low-boilers like BCl3 and certain hydrocarbons are difficult to
separate from TCS due to their similar boiling points, and thereby increasing the
necessary length of the distillation column. Next high-boilers like PCl3, POCl3, and
AsCl3 are separated from TCS. Even STC has a slightly higher boiling point than TCS,
necessitating a distillation step to separate STC for the recycling back to TCS, or
possibly for the purpose of making fumed silica. The TCS is at this stage ready to be
decomposed (at temperatures in the range of 1100°C) into solid SOG-Si. However,
there is also the opportunity to transform the TCS into monosilane (MS) – SiH4 in order
to decompose the latter into SOG-Si at somewhat lower temperatures (~850°C)
compared to TCS. The transformation process is commented on next.
Monosilane production
The most common way of producing MS is through a disproportionation chain starting
from TCS. Fig. 3 shows this chain transforming TCS by ion exchanges in a resin of
tertiary amines via dichlorosilane and monochlorosilane into MS. Other ample catalytic
functional groups for the transformation are quaternary amine or ammonium groups and
heterocyclic groups like pyridines.
CH3
R-N: +
CH3
2HSiCl3
R-N----H -----SiCl3
CH3
CH3
CH3
CH3
R-N: +
2H2SiCl2
CH3
CH3
CH3
CH3
2H3SiCl
CH3
CH3
H3SiCl + HSiCl3 + R-N:
CH3
ClH2Si------Cl
CH3
R-N----H -----H2SiCl
CH3
H2SiCl2 + SiCl4 + R-N:
Cl2HSi----Cl
R-N----H -----HSiCl2
CH3
R-N: +
CH3
H3Si--------Cl
SiH4 + H2SiCl2 +
R-N:
CH3
R=
Figure 3: Transformation of TCS into MS through ion exchanges taking place in a
polystyrene resin containing groups of tertiary amines.
4
The transformation from TCS to MS is enhanced by increased temperatures, but due
to the start of the decomposition of MS at 640-650°C [8-12] and the fact that the
presence of chlorosilanes combined with elevated temperatures promote the liberation
of functional groups like the above mentioned, the temperature during the
transformation is restricted to below 50-80°C giving one-pass yields of approximately
10% [13]. To obtain high total yields of MS recycling of the chlorosilanes is necessary,
including extensive distillation steps.
To obtain SOG-Si from the thus generated ultra pure silicon containing gaseous
species (TCS or MS) a thermal decomposition is necessary in one of several possible
types of reactors.
Decomposition
The Siemens process
In the 1950s the Siemens process was first used to produce silicon of high purity from
thermal decomposition of TCS. This is normally done in so-called bell-jar reactors (see
Fig. 4) at temperatures as high as ~1100°C.
Figure 4: Sketch of a typical Siemens bell-jar reactor [14].
The rods, consisting of silicon, are electrically heated to obtain the target temperature
and at the same time the walls are being water cooled in order to limit the deposition of
silicon to the rods. The rods are typically 7-8 mm in diameter before the start of the
deposition and can be grown to probably around 15-20 cm when run in solar mode. The
length of the rods is approximately 2 m [15] and the largest of the new type reactors can
contain about 100 rods (or 50 U-rods). The deposition reaction is sometimes presented
as [16,17]:
SiHCl3(g) + H2(g) → Si(s) + 3HCl(g) (+ byproducts)
(4)
However, a more realistic picture is probably the following reaction [18,19]:
5
4SiHCl3(g) ↔ Si(s) + 3SiCl4(g) + 2H2(g)
(5)
In order to prevent back reactions of the formed silicon, especially with HCl, the TCS is
strongly diluted with hydrogen (probably TCS:H2 at around 1:9) before inlet.
Decomposition of TCS in fluidized bed reactor (FBR)
The Siemens process is a typical batch process due to the fact that the reactions have to
be stopped occasionally in order to further process the rod-deposited silicon. In a TCSbased FBR, silicon is deposited from TCS onto small silicon seed particles (thereby
increasing the total surface available for deposition compared to conventional Siemens
deposition) that are floating around inside the reactor in the continuous gas flow. As the
particles grow bigger they fall to the bottom of the FBR to be collected. Due to the high
temperature necessary for TCS decomposition and the corrosive nature of the gas mix,
monosilane has been a more common source gas when using the FBR technology.
Recycling of by-products from TCS decomposition
The economy of the Siemens process can be enhanced by separation of the main
byproduct – STC (only 20–25% of the silicon in TCS is deposited as silicon). The STC
can either be used for the making of so-called fumed silica or be recycled back to TCS.
There are in fact two possibilities regarding the latter option, called “dirty”- and “clean”
recycling.
“Dirty” recycling of STC into TCS.
According to Eqn. 3 above STC reacts with hydrogen and MG-Si to TCS. The notion
“dirty” is used to indicate that the very pure silicon source gas STC (having gone
through several distillations) reacts with the most dirty silicon source in the entire
process – the MG-Si. This reaction is typically run in a fluidized bed reactor at 500700°C and highly elevated pressures. The one-pass conversion is reported to be as low
as 20 % [20], however under equilibrium conditions at 700°C and 35 atm pressure the
conversion is ~31% [2]. At 500-700°C the ∆Hreaction is 43.5 kJ/mol [21 and references
therein].
“Clean” recycling of STC into TCS.
The clean recycling of STC uses hydrogen according to the following reaction:
SiCl4(g) + H2(g) ↔ SiHCl3(g) + HCl(g)
(6)
The one-pass conversion is reported to be approximately 25% at 1100°C [22], and
under the same dilution conditions as in the decomposition of TCS (TCS/H2 = 1/9) the
conversion is 28.9% at equilibrium. The reactors at which the STC recycling
(hydrogenation) reaction takes place look very much like a bell-jar reactor similar to the
one used in the Siemens process. It is expected that two decomposition reactors can
share one conversion reactor. Heating takes place through inert electrodes and
simultaneous cooling of the reactor walls is necessary to prevent deposition thereon. In
order to prevent TCS from back-reacting a very fast quench condensation is critical,
which makes the wanted reaction somewhat difficult to run.
6
Decomposition of monosilane
As mentioned above an alternative to TCS decomposition is conversion into MS
followed by thermal decomposition of MS. This takes place in reactors with an outer
appearance quite similar to the Siemens bell-jar reactors, called Komatsu reactors, but
with some differences regarding the interior such as a separate sub-chambers for all the
rods inside. The reaction takes place according to:
SiH4(g) → Si(s) + 2H2(g)
(7)
This reaction goes to completion as opposed to similar TCS reactions. Additionally the
temperatures at which the decompositions are run (>650°C, but typically 850°C) are
substantially lower than in the TCS decomposition. However, a substantial amount of
the silicon formed has a limited particle size. These particles are known as fines and
they can be very difficult to process into polysilicon of the required purity. Finesparticles are formed in homogeneous gas reactions and are considered as loss in the
process due to the difficulty of further processing into polysilicon of the required
quality. In order to keep the fines formation at a minimum the MS has to be diluted
before entering the reactor. Nevertheless, the loss due to fines formation is probably in
the range of ~10% [4].
The continuous decomposition of MS in so called Fluidized bed reactors (FBR) has
been an industry process for years [23] and, in addition to Wacker’s plans of a
production of 650 MT yearly from 2009 based on FBR technology using TCS [24],
other large plans of expansions based on FBR technology are also being promoted
[25,26]. This is mostly due to the claimed reduction in energy of 80-90% as compared
to the Siemens process for this more efficient semi-continuous process in which silicon
is being deposited on small seed particles (~50 µm in size). When the particles grow
bigger they eventually fall to the bottom of the reactor where they can be collected. The
surface available for deposition of silicon is about 200 times larger than for a
conventional Siemens process. The formation of fines is also a big disadvantage
regarding MS decomposition in FBR reactors.
Elkem Solar’s metallurgical process route to SOG-Si
Fig. 5 shows the individual steps in the metallurgical process for producing SOG-Si
developed by Elkem Solar. This process is based on well known processes operated by
the metallurgical industry today and avoids the transition of MG-Si into any form of
silicon containing gas (silanes). These facts minimize the power consumption of this
process.
Figure 5: The individual steps of Elkem Solar’s metallurgical process route to SOG-Si.
7
The process starts by producing MG-Si according to Eqn. 1 above. Impurities in the
MG-Si are removed in the following slag and leaching processes. Impurities still
remaining in the material are gathered in the top of the resulting ingot after performing
directional solidification. These impurity-rich parts of the generated solid material are
recycled back in the process.
Comparisons
In this section energy consumption for the central parts of each individual gas route
process is estimated based on the reaction details given above as well as other
characteristics stated individually.
Decomposition of TCS without recycling
Figure 6 shows a flow sheet of the central parts in the production of SOG-Si from TCS
without the recycling of any silicon containing byproducts.
Figure 6: From MG-Si, via TCS-production, to SOG-Si without recycling of STC.
About 4.4 kg MG-Si is necessary to produce 1 kg of SOG-Si using this route. The total
energy consumption of the process scheduled in Fig. 6 can be divided into two parts:
The production of TCS (including the separation steps) and the decomposition of the
separated TCS.
Production of TCS
After the activation energy is supplied to the reactor, where TCS is generated from MGSi and HCl, the reaction is exothermic, as mentioned above. This reaction therefore
hasn’t an energy demand. On the contrary, the heat produced by the reaction could
possibly be exploited in later process steps like in the distillations and preferably also
taking part in an integrated plant where there are other chemical processes being run
that can make use of the waste heat. Some of the necessary condensations of gas
mixtures, however, require energy. Heat duties of condensers and boilers taken from
[27] are used in calculations leading to estimated energy demands of ~750 kWh/MT
TCS for the condensations of the gas mixture originating from the hydrochlorination of
MG-Si, ~145 kWh/MT TCS for off-gas (H2 and HCl) treatment, and ~760 kWh/MT
TCS for the heating of the gas mixtures during the distillations. The amount of TCS
going through the system scheduled in Fig. 6 is about 95,500 MT, so the total energy
demand of the TCS production for such a process line would be ~32 kWh/kg SOG-Si
produced.
8
Decomposition of TCS
As mentioned above the TCS to be decomposed into SOG-Si has to be diluted with H2
in order to minimize back reactions of the formed SOG-Si. Increasing the H2 dilution
will maximize the SOG-Si/SiCl4 relation, but will also increase the energy demand
since a larger amount of gas has to be heated. In the calculations below a TCS/H2 of 1/9
is used. Using Cp’s for the gases present as well as for silicon in the rods, taken from
[2] in the temperature interval room 25 – 1100°C, an energy demand of ~17 kWh/kg
SOG-Si is calculated for the heating of gases and Si rods. This is a little less than the 19
kWh/kg calculated for the deposition details taken from [15] and using the 59 Watt/rod
length (in cm) reported necessary in [28] to heat the whole reactor to 900°C through the
Si rods.
In addition energy is constantly lost to the surroundings due to the fact that the
reactor walls have to be constantly cooled to prevent depositions thereon at the same
time as the temperature on the Si rods is held at 1100°C. The heat loss is calculated
from:
q=
k ⋅ A ⋅ ∆T
s
(8)
where,
q = heat loss (W); k = thermal conductivity of stainless steel (19 W/m·K [29]); A =
inner surface area of the reactor (m2); ∆T = difference in temperature between the inner
and outer surface of the reactor; s = thickness of the reactor wall (m).
The measures of a deposition reactor, big enough to contain 100 rods, is
estimated/calculated from pictures in [15], and in the calculations of heat loss these
input values are used: A = 31 m2; ∆T = 340 K (calculated from a measured temperature
at the inner surface of 300 °C when the deposition reactor was held at 900°C [28]; s =
0.079 m. Additionally one reactor containing 100 rods is estimated to produce close to
235 MT SOG-Si per year (expected operational run time ~6900 hours). This leads to a
total estimated heat loss of ~ 74 kWh/kg SOG-Si. The total energy demand for the
decomposition is therefore estimated to be 91 kWh/kg SOG-Si.
The energy of producing the MG-Si, of course, comes in addition to this. Using 12
kWh/kg MG-Si [1] the amount of MG-Si used accounts for approximately 53 kWh/kg
SOG-Si.
It should be noted that the economy of such a non-recycling process probably would
be improved by making use of the byproduct STC, which will be formed in an amount
of close to 100,000 MT in a mixture of other byproducts in the process scheduled in
Fig. 6. One possibility is to burn the STC and sell it as fumed silica, another is to
recycle the STC back to TCS. The latter possibility is considered in the following.
Decomposition of TCS with dirty recycling
Recycling of the byproduct STC will lower the consumption of MG-Si used in the
process, but necessitates additional purification steps as compared to the non-recycling
alternative described above. Fig. 7 shows a sketch of a process in which so called
“dirty” recycling is implemented. Approximately 1.4 kg MG-Si is necessary to produce
1 kg of SOG-Si in addition to the silicon in the SiCl4 (~ 0.3 kg Si/ kg SOG-Si) that
needs to be added in addition to the STC from the recycling. The amount of the “make
up”-STC is calculated from numbers given in [4].
9
Figure 7: From MG-Si, via TCS-production, to SOG-Si with dirty recycling of STC.
The production of TCS
The TCS formation through STC recycling (Eqn. 3 above) is endothermic as opposed to
the hydrochlorination of MG-Si described above. The energy required to heat the
reacting substances to the reaction temperature (700°C) is estimated to be ~10 kWh/ kg
SOG-Si produced.
The energy demand of temporary storing of the STC and the distillation related steps
are estimated to be 13 and 36 kWh/kg SOG-Si, respectively, using calculations based on
heat duties given in [27] like above. The total energy demand is thereby reasonably
in line with the 50 kWh/kg SOG-Si presented for the TCS production step in
[30]).
Decomposition of TCS
The heating of the gases and silicon rods to the decomposition temperature is estimated
to be ~17 kWh/kg SOG-Si in the same way as above. The heat loss of approximately 74
kWh/kg SOG-Si is the same as above. The total energy demand for the step is therefore
91 kWh/kg SOG-Si.
The amount of MG-Si necessary is lower than the non-recycling case, of course, the
energy required for producing the MG-Si being some 16.5 kWh/kg SOG-Si in the
present case.
Dirty recycling of STC
The ~100,000 MT of STC in the off-gas coming from the decomposition reactor has to
be separated before reacting with MG-Si for conversion to TCS. Similar calculations to
the separation of TCS above give an estimated energy demand for the STC distillations
of ~11 kWh/kg SOG-Si.
10
Decomposition of TCS with clean recycle
Figure 8 below shows a flow sheet of the process of producing SOG-Si from TCS
decomposition when a so called clean recycle is implemented.
Figure 8: From MG-Si, via TCS-production, to SOG-Si with clean recycling of STC.
The production of TCS
As for the non-recycling case the only energy demand related to the production of TCS
here is related to the distillations. The demand is estimated to be 11 kWh/kg SOG-Si
using similar calculations based on heat duties as above.
Decomposition of TCS
Like the two cases above the energy for heating the gases and the silicon rods and
covering the heat loss through the walls is approximately 91 kWh/kg SOG-Si.
The energy needed for the production of the MG-Si necessary is estimated to be
close to 17 kWh/kg SOG-Si in the present case and the consumption of MG-Si amounts
to approximately 1.4 kg MG-Si per kg SOG-Si.
Clean recycling of STC
The ~100000 MT of STC in the off-gas coming from the decomposition reactor needs
to be converted back to TCS to deposit more SOG-Si. Heating the STC and the H2 for
dilution to 1000°C in the conversion reactor requires ~13 kWh/kg.
The heat loss from the reactor walls due to the continuous cooling during the
conversion is estimated using Eqn. 8 above, but with an anticipated lower inner wall
temperature (330 °C used) than expected for the decomposition reactor. The calculated
heat loss in the conversion reactor is thus estimated to be ~33 kWh/kg SOG-Si.
Additionally quench condensation is necessary as well as distillations to separate
both the reactants going into and the product coming out of the conversion reactor. The
total energy required for these steps is estimated to be ~21 kWh/kg SOG-Si.
To sum up the energy demand of the total clean recycling step is thereby 67 kWh/kg
SOG-Si.
11
Decomposition of monosilane (MS)
Figure 9 illustrates the process of depositing SOG-Si from MS converted from initially
formed TCS
Figure 9: From MG-Si, via TCS-production and conversion into MS before deposition of
SOG-Si.
The production of TCS
The energy demand for the TCS formation is in line with the case described for the
process involving dirty recycling and is in the range of 59 kWh/kg SOG-Si.
The redistribution of TCS into MS
The running of the redistribution reactor itself contributes negligibly to the total energy
consumption for the process (~0.2 kWh/kg SOG-Si) due to the low temperatures used in
this step and the ease of separating the very low-boiling MS. However, the STC and
other chlorosilanes, generated during the redistribution, needs to be separated through
distillations before being sent back again in the cycle. These steps require
approximately 14 kWh/kg SOG-Si, again calculated using heat duties from [27].
12
Decomposition of MS
The heating of gas and silicon rods to the decomposition temperature is estimated to be
only around 1 kWh/kg SOG-Si when decomposing MS. The deposition rate of silicon is
lower when decomposing MS as compared to TCS. If we estimate that a 24-rod reactor
can produce 37.5 MT of SOG-Si per year and stipulate the size of the reactor based on
the number of rods the heat loss obtained is ~83 kWh/kg SOG-Si. The amount of MGSi necessary is some 16.5 kWh/kg SOG-Si in the present case.
Power consumption for producing SOG-Si by Elkem Solar’s
metallurgical process
The process consumes 1.5–2 kg of MG-Si for every kg of SOG-Si produced. In the
calculations below 2 kg MG-Si/kg SOG-Si is used, so the energy required for this part
is thereby 24 kWh/kg SOG-Si. In addition the slag treatment, leaching and solidification
steps consume ~16 kWh/kg SOG-Si all together, which means the total power
consumption is ~40 kWh/kg SOG-Si. The Elkem Solar SOG-Si have shown to function
well in solar cells even in a pure, unblended form (see below).
Conclusions
Power consumption
Fig. 10 shows a comparison of the power consumption, based on the above calculations,
of the different gas-route processes divided into the main parts as well as for the Elkem
Solar metallurgical upgrading process to SOG-Si.
200
180
160
kWh/kg SOG-Si
140
Metallurgical upgrade
120
Metallurgical Si
100
Recycling
Decomposition
80
TCS prod.
60
40
20
So
la
r
El
ke
m
po
sit
io
n
S
M
TC
S
wi
th
de
co
m
re
cy
cli
ng
in
g
cl
ea
n
re
cy
cl
di
rty
wi
th
TC
S
TC
S
wi
th
ou
tr
ec
yc
lin
g
0
Figure 10: Comparison of the energy consumption of the main process steps for
different gas-route processes and for the Elkem Solar process.
13
Fig. 10 points out the low power consumption of the Elkem Solar metallurgical process
route. This is because energy intensive steps like the conversion from MG-Si to gaseous
silicon containing species with the necessary separation steps and the decomposition
step, in which a lot of heat is lost through the cooling of the reactor walls, are avoided.
Environmental aspects related to the power consumption of
producing SOG-Si
In Fig. 11 the emission of CO2 equivalents related to the generation of the power
necessary to run the above processes in addition to the process emissions from
generating the MG-Si is compared. The German energy mix resulting in an emission of
~560 g CO2 equivalents/kWh [31] is used in the comparison.
600000
500000
MT CO2
400000
300000
200000
100000
ix
)
en
er
gy
m
ar
So
l
ar
(N
or
we
gi
an
M
S
cl
El
ke
m
So
l
TC
S
wi
th
El
ke
m
po
sit
io
n
ea
n
de
co
m
re
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cli
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in
g
re
cy
cl
rty
di
wi
th
TC
S
TC
S
wi
th
ou
tr
ec
yc
li
ng
0
Figure 11: The emissions of CO2 (in MT CO2 equivalents) from the generation of the
power needed in the production of 5000 MT SOG-Si by different processes (the energy
mix for Germany is used in this example, which is 560 g CO2 equivalents/kWh [31],
except for the last column where a Norwegian energy mix (~8 g CO2 equivalents/kWh
[31]) is used. The CO2 emissions from the process of making MG-Si are included.
The CO2-savings related to the Elkem Solar metallurgical process is obvious when
looking at Fig. 11. The picture gets even more interesting by comparing the Elkem
Solar process using the Norwegian energy mix (mostly based on hydro power) [31] with
a gas route process ran in China using the Chinese energy mix [31], which is a relevant
comparison considering most of the new announced volumes for the future comes from
Chinese producers. In this case the total CO2 emissions from the generation of the
power needed to produce 5000 MT in China is close to 750,000 MT and for Elkem
Solar in Norway ~45,000 MT CO2.
14
Quality aspects
Fig. 12 below shows a comparison of efficiencies achieved in solar cells made of SOGSi from Elkem Solar on one side and ordinary electronic grade silicon (EG-Si) on the
other (reference).
Efficiency (%)
18
16
14
12
10
8
6
4
2
0
Reference
65% ESS
75% ESS
100% ESS
Figure 12: Comparison of solar cell efficiencies reached with Elkem Solar Silicon (ESS)
and polysilicon reference (UKON lab) from [32]. The cells are tested and qualified for
commercial release through customers and partners.
As can be seen from Fig. 12 pure Elkem Solar SOG-Si can not be distinguished
from high-quality material on the market when it comes to measured efficiency on solar
cells made from it. Even an efficiency of above 18% is reported for solar cells made of
100% SOG-Si from Elkem Solars metallurgical process route [33].
SOG-Si from Elkem Solar is thereby very competitive not only when it comes to
production cost and environmental aspects, but also indeed regarding quality.
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