Life Cycle Analysis of Solar Module Recycling Process

Abstract

Since June 2003 Deutsche Solar AG is operating a recycling plant for modules with crystalline solar silicon cells. The aim of the process is to recover the silicon wafers so that they can be reprocessed and integrated in modules again. The aims of the Life Cycle Analysis of the mentioned process are (i) the verification if the process is beneficial regarding environmental aspects, (ii) the comparison to other end-of-life scenarios, (iii) the ability to include the end-of- life phase of modules in future LCA of photovoltaic modules. The results show that the recycling process makes good ecological sense, because the environmental burden during the production phase of reusable components is higher than the burden due to the recycling process. Moreover the Energy Pay Back Time of modules with recycled cells was determined.

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Life Cycle Analysis of Solar Module Recycling Process
Anja Müller 1, Karsten Wambach1 and Erik Alsema2
1Deutsche Solar AG, Solar Material, Alfred Lange Straße 18, 09599 Freiberg, Germany
2Science, Technology and Society, Copernicus Institute, Utrecht University, Utrecht,
Netherlands
ABSTRACT
Since June 2003 Deutsche Solar AG is operating a recycling plant for modules with crystalline
solar silicon cells. The aim of the process is to recover the silicon wafers so that they can be
reprocessed and integrated in modules again. The aims of the Life Cycle Analysis of the
mentioned process are (i) the verification if the process is beneficial regarding environmental
aspects, (ii) the comparison to other end-of-life scenarios, (iii) the ability to include the end-of-
life phase of modules in future LCA of photovoltaic modules. The results show that the recycling
process makes good ecological sense, because the environmental burden during the production
phase of reusable components is higher than the burden due to the recycling process. Moreover
the Energy Pay Back Time of modules with recycled cells was determined.
INTRODUCTION
In recent years a rapid growth of the production capacity and installation of photovoltaic
modules has been observed. For the next decades a growth of 15% is predicted [1]. Due to their
long lifetime the amount of end-of-life modules is still relatively small. It is estimated to be
about 14 MWp in 2006 [2], and this quantity will increase rapidly as the PV market grows. In
Europe alone, the emergence of end-of-life modules is estimated to be 290 tons in 2010 and
33,500 tons in 2040.Other concepts for recycling crystalline photovoltaic modules were
examined in scientific studies, but none of them was realized in an industrial scale until now.
Regarding the growing quantity of end-of-life modules producers of photovoltaic products have
to take the responsibility of the final treatment. Due to the curtness of silicon, it is necessary to
establish a recycling concept for all kinds of photovoltaic modules that regards environmental
aspects and statutory regulations. Competing technologies to high-value processes like the one
of Deutsche Solar AG are low-value recycling technologies like the treatment in a recycling
plant for laminated glass or disposal on a landfill after treatment in a municipal incineration
plant.
RECYCLING PROCESS OF DEUTSCHE SOLAR AG
SolarMaterial, a business unit of Deutsche Solar AG is engaged in various recycling loops
along the succession of production steps from silicon as raw material to photovoltaic modules.
The presented study deals only with the module recycling process. Regarding a sustainable use
of silicon as raw material, the other recycling activities are just as important.
Mater. Res. Soc. Symp. Proc. Vol. 895 © 2006 Materials Research Society 0895-G03-07.1

Recycling activities along the production steps from ingot growing to module assembly:
- sides of multi crystalline ingots
- bottom of multi crystalline ingots
- tops of multi crystalline ingots (partly)
- broken wafers
- faulty processed cells
- cell breakage
- production rejects of ingots
A silicon photovoltaic module is composed of silicon solar cells, metal contacts between the
cells, encapsulation layer that enclose the cells, front glass plate and a back-side foil or a second
glass plate on the back side. Often the module is framed with aluminium and contains a contact
box. The module recycling process of Deutsche Solar AG enables the recovery of wafers and the
recycling of glass and metals from crystalline solar modules [3]. By burning off the laminate in
an furnace the module compound structure is disunited, so that solar cells, glass and metals can
be separated manually. Glass and metals are and given to recycling partners, while the unbroken
cells are etched in the etching line of SolarMaterial. Broken cells are also collected for reuse as
raw material for ingot growing after etching with a different technology. In the etching line the
metallization, anti-reflection coating and pn-junction of the cell are removed subsequently. The
clean wafer, which is the final product of the recycling process, can be processed again in a
standard solar cell production line and integrated into a PV module. During the thermal treatment
energy is consumed by the furnace, afterburner and washer. In addition, the washer consumes
water and leach. Important outputs are air emissions and different waste streams. During the
chemical process different chemicals are required. Moreover, water and energy are consumed in
the line and the exhaust gas washer. The chemicals used for etching are treated chemically and
physically. The resulting sludge is disposed of. Resulting water is delivered to a sewage
treatment plant. The process and important in- and outputs are summarized in Figure 1.
In this study the transportation of the modules to the recycling plant is not considered, because it
does not depend on the recycling technology but on the collection system.
Figure 1. Important in- and outputs during module recycling.
wafers
furnace afterburner washe
r
etching line washer
end- of-life
modules
recovered
cells
energy water energy water chemicals
air used waste
emissions chemicals water
air waste glass metals
e
mi
ss
i
o
n
s
water
0895-G03-07.2

ENERGY CONSUMPTION
An assessment of the total energy demand during the recycling process gives a first insight on
the environmental effects of the process. The total energy consumption is composed of the
demand of natural gas and electrical energy of the exhaust gas cleaning during the thermal
treatment as well as the consumption of electrical energy of the etching line. The amount of
primary energy was converted to electrical energy with an assumed efficiency of 35 percent. For
the calculation of the energy generation per year a Middle-European location with 1000
kWh/m²/year and a performance ratio of 0.75 is assumed. The results of a comparison of a
module with new wafers and a module with recovered wafers of the recycling process of
Deutsche Solar is shown in Table I. For wafer production a high energy input is necessary. The
recycling of 72 wafers for a new module takes 92 kWhel. Which is 30 % of the energy input for
the production of 72 new wafers (306 kWhel).The calculation shows that the Energy Pay-Back
Time (EPBT) of a module with recycled wafer is 1.7 years shorter than of a standard module
(EPBT of a new module with above mentioned assumptions: 3.3 years).
Table I: Energy consumption and generation during the production and use phase of a module of
160 Wp, 72 multicrystalline cells 12.5 x 12.5 cm (energy consumption during production are
based on data of 2004 [4]).
module with
new wafers
module with
recycled wafers
Unit
wafer production (multi) 306 kWhel
recycling process 92 kWhel
cell processing 49 49 kWhel
module assembly 45 45 kWhel
Total 400 186 kWhel
energy generation 120 120 kWhel/year
EPBT 3.3 1.6 years
Figure 2. Field photovoltaic installation on the German island Pellworm before disassembling
for recycling
0895-G03-07.3

The calculation is based on average data of the treatment at the pilot plant in Freiberg. In the next
months modules of the oldest field photovoltaic installation in Germany will be treated [5]. This
installation was installed 1983 with an efficiency of 8%. After reprocessing and module
assembly the recycled installation will have an efficiency of about 14% thanks to improved cell
processing.
LCA
In the presented LCA a standard module was investigated with 72 cells (12.5 x 12.5 cm),
Tedlar as backside foil and an aluminum frame. For the evaluation of the environmental impacts,
the CML Baseline-2000 method of the institute of Environmental Science in Leiden (CML) was
used. The analysis was performed with the software Simapro 6.0. Calculations are based on
Deutsche Solar data as well as data from the ecoinvent 2000 database [6]. The dataset for
production of silicon wafers is based on analysis of the years 1995 to 2000. The presented LCA
was done before more recent data were available. The described in- and outflows including the
treatment of wastewater and used chemicals are considered. The environmental impact of
producing the collected amount of glass and metals as well as of producing the amount of
recovered wafers is credited to the impacts of the recycling process itself. In the following the
results of the characterization is presented. The process is evaluated regarding seven impact
categories, for example “climate change”. For each category a specific indicator (for example kg
CO2-equivalent) is calculated as the weighted sum of individual emissions. In Figure 3 the
environmental burden (positive contribution) are opposed to the environmental disburden
(negative contribution) of the recycling process. The sum of negative and positive contribution is
scaled to 100%, because each category is evaluated by a different indicator with its own unit.
Figure 3. Disburden and burden of the recycling process of Deutsche Solar AG.
Due to the avoidance of new wafers and recycling of glass and metals the absolute impact values
are negative for each category. This shows the superiority of high value recycling processes
compared to disposal solutions with low environmental impacts but without material recycling or
the possibility to reuse individual components. The results also show that the burden of the
environment is mainly related to the energy consumption during the thermal treatment and the
use of chemicals in the etching line. As a result it is important to decrease the energy
-100%
-50%
0%
50%
abiotic
depletion
global
warming
ozone layer
depletion
human
toxicity
photochemical
oxidation
acidification
eutrophication
disburden by material recycling and reuse of wafers
burden by the thermal and chemical treatment
0895-G03-07.4

consumption during the thermal treatment and to cut down the consumption of chemicals in the
etching line. The actual operational mode of the furnace and the etching line posses further room
for improvement regarding both aspects [7].
COMPARISION WITH ALTERNATIVE PROCESSES
At the present time there are two alternative disposal scenarios for photovoltaic modules. The
simplest possibility is a treatment in a municipal waste incineration and subsequent disposal at a
landfill for inert waste. At least in Europe a thermal treatment before the deposit on a landfill is
necessary to fulfill the criteria for the acceptance of waste at landfill sites [8]. The advantage of
this solution is that it is not necessary to acquire end-of-life modules separately to commercial or
industrial waste. The main disadvantage is the loss of raw material like silicon. In the examined
scenario it was assumed that the aluminum frame is removed before the thermal pre-treatment
because of its high economic value. A municipal incineration plant is a large-scale plant unlike
the furnace of DS. Hence the energy consumption of a municipal incineration plant per kilogram
is substantially lower.
A comparison of the process of DS and the described scenario is shown in Figure 4. In this figure
the recycling of the aluminum frame is not considered, because it is carried out in both scenarios
and only a part of photovoltaic modules is framed with aluminum.
Figure 4. Comparison of an incineration scenario with the recycling process of DS.
Another scenario is a shredder process with subsequent sorting and thermal treatment of one
fraction, that is deposited on a landfill. It is expected that the aluminium frame is removed before
the shredder process. The recovered glass fraction can be put back into glass production. The
second fraction consists of organic material, metals and crushed solar cells. Due to its high
organic content the thermal treatment before a deposit on a landfill is necessary in Germany and
other countries. The energy consumption of a shredder process is two orders of magnitudes
lower than the recycling process. But in the shredder scenario only glass and metals can be
recycled. The silicon of wafers is lost in this scenario. The expenditure of energy for the
-100%
-80%
-60%
-40%
-20%
0%
20%
40%
abiotic
depletion
global
warming
(GWP100)
ozone layer
depletion
(ODP)
human
toxicity
photochemical
oxidation
acidification
eutrophication
incineration scenario recycling Deutsche Solar
0895-G03-07.5

production of silicon wafers and material for ingot growing is relatively high (see table I). This
fact justifies the operation of the DS process even with higher energy consumption. In general
high grade recycling solutions are preferable to low grade solutions. In consideration of the
scarcity of silicon for the PV industry the reuse of wafers is an additional advantage.
SUMMARY
The energy consumption during the recycling process is essential. Nevertheless the use of
recycled wafers for wafer production instead of new ones can halve the EPBT of a module.
Due to the reuse of recovered wafers and the recycling of glass and metals the recycling process
of Deutsche Solar AG leads to a decrease of environmental burden by avoidance of the
production of new wafers and material like glass. Other examined disposal scenarios do not
represent a high value recycling, because the material with the highest value is lost by deposition
on al landfill.
REFERENCES
1. K. Sander, S. Zangl, M. Reichmuth, G. Schröder, Stoffbezogene Anforderungen an
Photovoltaikprodukte und deren Entsorgung, Institut für Ökologie und Umwelt Hamburg,
Institut für Energetik und Umwelt GmbH Leipzig, 2003.
2. K.Wambach, S. Schlenker, Stoffkreisläufe in der Photovoltaik, Freiberger Solartage 2005,
(http://www.saxonia-freiberg.de/start/index?action=download&id=140)
3. E. Bombach, K. Wambach, A. Müller, I. Röver, Recycling of Solar Cells and Modules –
Recent Improvements, 20th European Photovoltaic Solar Energy Conference, Barcelona
2005
4. E.A. Alsema, M.J. de Wild-Scholten, Environmental Impacts of Crystalline Silicon
Photovoltaic Module Production, MRS Fall Meeting, Boston, USA, Nov.29-Dec.2 2005
5. J. Siemer, Neues Leben für ein altes Kraftwerk 6 - 88, Photon November 2005
6. Jungbluth, N., Life Cycle Assessment of Crystalline Photovoltaics in the Swiss ecoinvent
Database. Progress in Photovoltaics: Research and Applications, 2005 (in press).
7. I. Röver, K. Wambach, W. Weinreich, G. Roewer, Processs controlling of the etching
system HF/HNO3/HNO2, 20th European Photovoltaic Solar Energy Conference, Barcelona
2005
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acceptance of waste at landfills pursuant to Article 16 of and Annex II to Directive
1999/31/EC; Official Jounal of the European Communities L 11/27
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