24th European Photovoltaic Solar Energy Conference and Exhibition, 21-25 September 2009, Hamburg, Germany
CONTROL OF LIFE-CYCLE EMISSIONS FROM CRYSTALLINE SILICON SOLAR CELL MANUFACTURING
Mariska de Wild-Scholten1, Martin Schottler2, Birgit Schlüter2
1
ECN Solar Energy, P.O. Box 1, 1755 ZG Petten, The Netherlands
2
M+W Zander FE GmbH, Lotterbergstr. 30, 70499 Stuttgart, Germany
E-mail: m.dewild@ecn.nl
ABSTRACT
Reduction of life-cycle emissions are needed to improve the environmental profile of silicon photovoltaics. Most
emissions are indirect via upstream processes and can be reduced for example by lowering the energy consumption for
silicon feedstock and wafer production. The direct emissions from solar cell manufacturing must be reduced by
implementing abatement strategies.
1 INTRODUCTION 1 Acid and caustic gases
Source Texturing and etching benches HF HNO3 NO2 NH3 others
Treatment Wet scrubbers / absorption towers / SCR for NOx gases
In order to decrease the environmental burden and to 2 Acid and caustic waste waters
Source Texturing and etching benches KOH NH4OH others
comply with legislation, the emission of harmful gases Treatment Neutralization
into the biosphere needs to be controlled [1]. In this 3 Fluoride containing waste waters
Source Rinsing water from benches, scrubbers HF HNO3 H2SiF6 others
study, we investigate the direct and indirect emissions of Treatment Precipitation and filtration
NOx, HF, NH3, volatile organic compounds (VOC) from 4 Concentrated acid waste
Source Texturing and etching benches HF HNO3 H2SiF6 others
silicon solar cell production. Emission control is Treatment External disposal
discussed in the frame of national and EU legislation. 5 Solvent exhaust
Source Printing, spray dopers solvents, alcohols
Treatment Condensation, oxidation, biofilter
Emissions of harmful gases are regulated in the 6 Solvent waste
Source Condensation of exhaust, residues solvents, alcohols
European directive on Integrated Pollution Prevention Treatment External disposal
and Control (IPPC) [2] and in the European Solvents 7 Silane containing gas
Source Silicon nitride deposition SiH4 NH3
Emissions Directive [3]. The IPPC Directive sets out the Treatment Oxidation, scrubbing
main principles for the permitting and control of 8 Silicon dioxide waste
Source Side reactions (in chamber), scrubbers SiO2
installations based on an integrated approach and the Treatment insitu or manual cleaning, external disposal
application of best available techniques (BAT) which are
the most effective techniques to achieve a high level of Acid and caustic gases are usually treated by scrubber
environmental protection, taking into account the costs technologies (wet absorber, mostly based on packed
and benefits. However it does not include emission limit tower systems). This neutralizes the acid or caustic gases.
values. The emission limit values are normally defined by Selective Catalytic Reduction (SCR) is used instead of
regional or national authorities, taking site-specific wet scrubbing of NOx in case of high NOx load.
conditions into account such as the technical Subsequent treatments of HF rinse water, wet scrubber
characteristics of installations and the local drain, and concentrate solutions include neutralization,
environmental situation. Emission limit values for and, in most cases, also precipitation of the liquid
Germany will be presented and abatement technologies to fluoride content.
control the emissions.
For vapors and volatile organic compounds oxidation
Accurate and up-to-date information on the direct technologies or condensation can be used. Open flame
emissions from cell manufacturing is not available in burners are used as local or centralized installations. As
open literature. A European register (EPER) publishes central treatments also catalytic oxidizers, Regenerative
detailed information on the industrial emissions into air Thermal Oxidation (RTO) or biofilters can be used.
and water from approximately 12 000 industrial facilities.
However, information about volumes produced is not Silane is usually oxidized in local installations, with
provided. Companies typically do not disclose this type open flame or flameless type. The ammonia associated
of information. Our paper is based on measured with this control step is wet scrubbed (as above).
emissions for a wide variety of gases associated with
mono- and multi-crystalline silicon cell production. Most crystalline silicon solar cell Fabs are equipped
with a central acid scrubber, because many sources of
The indirect emissions associated with upstream acid emissions are there not only from production, but
processes (i.e., silicon production, crystallization and also from support and service areas of the Fab.
wafer manufacturing) are determined using using
Simapro 7.1 software with the ecoinvent 2.0 database Installations using “acid texturing” step also need
and recent data. NOx scrubbing to comply with German legislation. These
two abatement technologies (acid and NOx scrubbers) are
2. CONTROL OF EMISSIONS therefore included for calculating the environmental
impacts.
Control of emissions is needed to reach the emission
limit values. The available abatement technologies are Depending on process tools, additional abatement
specific for the type of effluent to be treated. An techniques might be necessary, e.g. to reduce fire risks,
overview is given in table I. and emissions of ammonia. Some variations for scenario
comparison include doping with an oven vs. spray doper,
Table I. Overview of different abatement technologies vacuum SiN deposition vs. oven techniques. Product
performance is normally the driving force for selecting
24th European Photovoltaic Solar Energy Conference and Exhibition, 21-25 September 2009, Hamburg, Germany
industrial processes. Hence, environmental more the energy consumption of upstream processes for
favorable processes may be disregarded. example by reducing the wafer thickness.
Printing produces a VOC exhaust gas that is difficult to
handle because of condensation and polymerization 4 HYDROGEN FLUORIDE (HF)
processes. Three options are available: treatment with
local burners, local condensors plus central thermal The SiO2 layer is removed by reaction with HF:
oxidation or biofilters.
Perfluorocarbons with high global warming potential are SiO2 + 4 HF SiF4 + 2 H2O
in use only to a minor extent in crystalline Si production
and will be phased out due to replacement of the dry edge Moreover, acid texturing of raw silicon with mixtures of
isolation by laser or wet processing and because good HF and HNO3 generates significant emissions of HF by
alternatives exist for PECVD chamber cleaning. vaporization of the latter.
The next paragraphs give information on NOx, HF, HF is toxic and impacts plants even at low levels in
NH3 and volatile organic compounds (VOC): air. In the EU, immission is limited to 1 µg/m3.
- emission generation, Accordingly, HF emission limits are usually strict: 3
- environmental effects of the emission, mg/Nm3 in Germany (TA Luft).
- German legal emission limit values,
- control of the emission and HF process emissions are led to a central acid
- direct and indirect emissions. scrubber and then to precipitation of CaF2. This is for
landfill.
3 NITROGEN OXIDES (NOx)
HF emissions are modeled according to source
NOx is produced during etching (texturing) of silicon strength and usual removal rate of the scrubbing systems
wafers with nitric acid. The oxidized silicon is solubilized installed. Only 10% of the life-cycle HF emission is
in the form of H2SiF6. direct for monocrystalline silicon solar cell production
whereas for multicrystalline silicon solar cell production
3 Si + 4 HNO3 + 18 HF 3 H2SiF6 + 4 NO + 8 H2O the direct emissions are 40% of the total life-cycle
emissions. Again this means that, especially for
Assuming that 10 micron of silicon is etched, 1.7 monocrystalline silicon, the major opportunity for
gram HNO3 is consumed per 156 mm x 156 mm wafer improving the impact is to reduce the contribution from
and 0.81 g NO is produced. However, in practice it is upstream processes.
found that NOx emissions are less than the stoichiometric
calculation indicates. For the modeling of the effluents, 5 AMMONIA (NH3)
values based on measurement were used instead.
Crystalline silicon solar cell production uses ammonia
NOx emissions have acidification and eutrophication in PECVD of silicon nitride to provide nitrogen sources.
effects. Silane is used as silicon source.
The NOx emission limit values in Germany (TA Luft) The NH3 emission limit value is 30 mg/Nm3 in
are 350 mg/Nm3. Germany (TA Luft).
30% reduction in concentration is possible when Since silane (SiH4) is the more critical gas in exhaust
using a non-specific central scrubber. 60% reduction in control, NH3 treatment is designed on a case-by-case
concentration is possible when using local scrubber basis. Using either ambient or electrically heated
without H2O2. This usually meets German requirements. flameless oxidation of the silane, ammonia is not affected
90% reduction in concentration is possible when using by the oxidation step and is removed by subsequent water
local scrubber with H2O2. scrubbing. Using an open flame burner, ammonia is burnt
All wet scrubbers bring the NOx load to the waste water to N2 with some risk of secondary formation of NOx. The
in form of nitrites and nitrates. Although nitrates are latter are normally not treated.
predominant, a significant fraction remains in the form of
nitrite. Usually tight emission limits for nitrites exist, and Ammonia emissions have been calculated using the
a subsequent nitrite oxidation has to be included to emission of the process, the removal rate of a local
convert nitrite to nitrate. electrically heated oxidation with subsequent water
For high throughput nitrogen output of the fab also via scrubbing and no central installation to remove ammonia.
the wastewater will be over the emission limits. Then a 1% of the life-cycle HF emission is direct for
real conversion of all oxidized nitrogen to N2 is required. monocrystalline silicon solar cell production whereas for
This is provided by the Selective Catalytic Reduction multicrystalline silicon solar cell production the direct
(SCR) technique. This SCR local scrubber is a based on emissions are 10% of the total life-cycle emissions.
reduction of NOx to nitrogen N2 and water using for
example NH3 as reducing agent:
4 NO + 4 NH3 + O2 4 N2 + 6 H2O 6 VOLATILE ORGANIC COMPOUNDS (VOCs)
10% of the life-cycle emissions are direct from the Crystalline silicon solar cell production uses ethanol
texturing of multicrystalline wafers. This means that the and/or isopropanol to dilute the spray of dopant, and for
largest opportunity to reduce the impact is by lowering cleaning purposes. Some technologies use volatile
24th European Photovoltaic Solar Energy Conference and Exhibition, 21-25 September 2009, Hamburg, Germany
alcohols in heated baths in benches for cleaning or [3] Council directive 1999/13/EC of 11 March 1999 on
etching, thus inducing VOC emissions. the limitation of emissions of volatile organic compounds
Metallization pastes contain VOCs such as aromatics, due to the use of organic solvents in certain activities and
terpineol and softeners and others. installations
http://ec.europa.eu/environment/air/pollutants/stationary/s
VOC’s contribute to photochemical oxidant olvents.htm
formation. [4] TA Luft, Germany (24 July 2002)
http://www.bmu.de/files/pdfs/allgemein/application/pdf/t
The VOC emission limit value in Germany (TA Luft) aluft_engl.pdf
is 50 mg/Nm3 as total carbon.
There is a variety of concepts for VOC control. Since
composition and concentration of VOC show large
variation, more than one VOC collection and treatment
system may be required. Low emissions, mostly from the
printing, are treated by fixed bed carbon absorbers with
external regeneration, while the polymerizing parts of the
printer emissions have to be removed mechanically or by
condensation. Local burners and central biofilters can be
applied. High emissions can be treated by so-called
"RTO" systems (flame-based oxidation with internal heat
recovery), but also by direct flame based oxidation
(including external heat recovery) or catalytic oxidation
systems have been in use.
7 SUMMARY OF EMISSIONS
Table II shows the direct and life-cycle (from cradle
to the solar cell factory gate) emission values for the
production of crystalline silicon solar cells.
Table II. Direct/life-cycle emissions for crystalline
silicon solar cell production (size 156 mm x 156 mm)
Mono Multi
0.0/4.8 0.3/4.0 g NOx/cell
3/36 16/42 mg HF/cell
1/110 12/112 mg NH3/cell
8 CONCLUSIONS
Reduction of life-cycle emissions are needed to
improve the environmental profile of silicon
photovoltaics. Emissions of NOx, HF (mono Si) and NH3
are mostly (>90%) indirect via upstream processes and
can be reduced by lowering the energy consumption for
silicon feedstock and wafer production. The direct
emissions from solar cell manufacturing must be reduced
to comply with legislation by implementing abatement
strategies.
9 ACKNOWLEDGEMENTS
M+W Zander received funding in the frame of the
“O2 reduction” project FKZ 01LS05090.
10 REFERENCES
[1] Fthenakis V.M., H.C. Kim, E. Alsema (2008)
Emissions from Photovoltaic Life Cycles,
Environ.Sci.Technol. 42(6) 2168-2174
[2] Directive 2008/1/EC of the European Parliament and
of the Council of 15 January 2008 concerning integrated
pollution prevention and control (Official Journal of the
European Union 29.1.2008)
http://ec.europa.eu/environment/air/pollutants/stationary/i
ndex.htm
