EPA
United States
Environmental Protection
Agency
Office of
Research and
Development
Industrial Environmental Research
Laboratory
iti. Ohio 45268
EPA-600/7-77-087
August 1977
ASSESSMENT OF LARGE-SCALE
PHOTOVOLTAIC MATERIALS
PRODUCTION
Interagency
Energy-Environment
Research and Development
Program Report
LIBRA:
EP 600/7
77-087
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RESEARCH REPORTING SERIES
Research reports of the Office of Research and Development, U.S. Environmental
Protection Agency, have been grouped into nine series. These nine broad cate-
gories were established to facilitate further development and application of en-
vironmental technology. Elimination of traditional grouping was consciously
planned to foster technology transfer and a maximum interface in related fields.
The nine series are:
1. Environmental Health Effects Research
2, Environmental Protection Technology
3. Ecological Research
4. Environmental Monitoring
5. Socioeconomic Environmental Studies
6. Scientific and Technical Assessment Reports (STAR)
7. Interagency Energy-Environment Research and Development
8. "Special" Reports
9. Miscellaneous Reports
This report has been assigned to the INTERAGENCY ENERGY-ENVIRONMENT
RESEARCH AND DEVELOPMENT series. Reports in this series result from the
effort funded under the 17-agency Federal Energy/Environment Research and
Development Program. These studies relate to EPA's mission to protect the public
health and welfare from adverse effects of pollutants associated with energy sys-
tems. The goal of the Program is to assure the rapid development of domestic
energy supplies in an environmentally-compatible manner by providing the nec-
essary environmental data and control technology. Investigations include analy-
ses of the transport of energy-related pollutants and their health and ecological
effects; assessments of, and development of, control technologies for energy
systems; and integrated assessments of a wide range of energy-related environ-
mental issues.
This document is available to the public through the National Technical Informa-
tion Service, Springfield, Virginia 22161.
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EPA-600/7-77-087
August 1977
-
irs
ASSESSMENT OF LARGE-SCALE PHOTOVOLTAIC
MATERIALS PRODUCTION
by
Martin G. Gandel
Paul A. Dillard
Lockheed Missiles & Space Company, Inc.
Sunnyvale, California 94088
&
D. Richard Sears
S. M. Ko
S. V. Bourgeois
Lockheed Missiles & Space Company, Inc.
Huntsville. Alabama 35807
Contract No. EPA 68-02-1331
Project Officer
Robert P. Hartley
Power Technology and Conservation Branch
Industrial Environmental Research Laboratory
Cincinnati, Ohio 45268
Industrial Environmental Research Laboratory
Office of Research and Development
U.S. Environmental Protection Agency
Cincinnati, Ohio 45268
UfiRJffiY
S. ENVIRu.i.'iiLSTAl PROTECTION AG
N. J.
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DISCLAIMER
This report has been reviewed by the Industrial Environmental Re-
search Laboratory, EPA, and approved for publication. Approval does not
signify that the contents necessarily reflect the views and policies of the
U. S. Environmental Protection Agency, nor does mention of trade names or
commercial products constitute endorsement or recommendation for use.
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FOREWORD
When energy and material resources are extracted, processed, con-
verted, and used, the related pollutional impacts on our environment and
even on our health often require that new and increasingly more efficient
pollution control methods be used. The Industrial Environmental Research
Laboratory-Cincinnati (IERL.-C1) assists in developing and demonstrating
new and improved methodologies that will meet these needs both efficiently
and economically.
This report assesses the effects of large-scale manufacture of solar
photovoltaic materials. It is intended to provide quantitative projections of
raw material and energy requirements and waste products, with an assess-
ment of how these affect the environment and natural resources. This
report is addressed to those engaged in research and development of photo-
voltaics and their raw materials and those interested in comparing the
promise of solar photo voltaic s with other energy sources.
Further information on this subject can be obtained from the Power
Technology and Conservation Branch of lERL-Ci.
David G. Stephan
Director
Industrial Environmental Research Laboratory
Cincinnati
111
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ABSTRACT
Solar cell production at rates needed to supply continuously 1% of pro-
jected U.S. power requirements in the year 2000 is examined. Silicon and
cadmium sulfide are followed from raw material extraction to finished cell;
gallium arsenide is reviewed less thoroughly. Numerical data are developed
for air, water, and solid wastes, and compared with corresponding effects of
equivalent coal-electric power. Mass and energy balance data are derived
from flow sheets developed for this report.
For Si, major problems requiring engineering solutions are material
and energy inefficiencies. A very large byproduct stream should be elimi-
nated to increase yield by as much as 59% or decrease air pollutant releases
by 37% on a process weight basis. Power consumption in cell production
creates indirect air pollutant emissions over half as large as those created
by the coal-burning plants silicon might replace.
The production of cadmium sulfide and gallium arsenide is less energy-
intensive. Their metallic raw materials are themselves byproducts of other
smelting operations. Atmospheric cadmium releases, and the potential for
cadmium or As spills are major problem areas.
Of materials known to be involved in cell production, only gallium is
resource-limited; however, use of concentrators and thin-film technology
may obviate this problem.
This report was submitted in fulfillment of Contract EPA 68-02-1331,
Task 15, my Lockheed Missiles § Space Company under the sponsorship of
the U.S. Environmental Protection Agency. The report covers the period
March 30, 1976 to July 16, 1976. Editorial revisions were incorporated in
February 1977.
IV
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CONTENTS
Foreword i-ii
Abstract iv
Figures vii
Tables viii
1. Introduction 1
Scope and Objectives 1
Basis of Assessment 1
Technologies Evaluated 5
2. Conclusions 6
Silicon 6
Cadmium Sulfide 6
Gallium Arsenide 6
3. Recommdendations 8
Silicon 8
Cadmium Sulfide 8
Gallium Arsenide 9
4. Silicon Process 10
General Process Description 10
Power Requirements 26
Environmental Considerations 28
Energy and Environmental Summary 42
Impact on Natural Resources 46
Pollution Control Technology Readiness and Projections . . 47
5. Cadmium Sulfide Process 49
General Process Description 49
Material Consumption and Production Estimates 58
Power Requirements 58
Environmental Summary 61
Impacts on Natural Resources 64
Pollution Control Technology Readiness and Projections . . 65
6. Gallium Arsenide Process 67
General Process Description 67
GaAs Process Material Requirements (Refs. 89, 97-98) . . . 74
GaAs Process Power Requirements 76
Impact on Natural Resources 76
Environmental Consequences 78
Economic Consequences 78
7. Alternate Processes 80
References 82
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Appendixes
A
Atmospheric Emission Factors for Raw Materials
Extraction and Production 90
B Glossary 95
C Conversion Factors 98
D Toxicity of Chemical Compounds Used in Solar Cell
Production 100
E Cadmium: Properties, Occurrence, and Ecological
Effects 113
vi
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FIGURES
Number Page
1 Installed electrical generating capacity in United States ... 3
2 Peak and average solar power vs time for direct
radiation on a clear day 4
3 Silicon process diagram quartz to single crystal 11
4 Current silicon yields as percent of poly silicon input
to crystal growers
5 Silicon cell fabrication flow diagram wafer preparation . . 17
6 Silicon cell fabrication flow diagram diffusion to
electrical test 18
7 Silicon wafer cross-section after diffusion 21
8 Submerged-arc furnace for silicon production (Ref. 51
modified) 37
9 CdS process flow diagram 52
10 Cross sectionback surface CdS cell 55
11 Cross section of front surface CdS cell 56
12 CdS solar cell production back surface cell-spray
process 57
13 CdS solar cell production front surface cell-vacuum
process 59
14 Process flow diagram Ga 68
15 Arsenic production from flue dusts (Ref. 91) 69
16 Process flow diagram Ga to GaAs cell 70
17 Cross-section of GaAs cell 73
vii
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TABLES
Number Page
1 Solar Cell Characteristics and Production Requirement ... 4
2 Proposed Cell Types (Ref. 7) 14
3 Materials and Products for 4500 MW Annual Silicon Cell
Production {10^ Metric Tons) 25
4 Summary of Power Requirements for Annual Production
of 4500 MW of Silicon Solar Cells 27
5 Estimated Ground Level Particulate Concentrations
Caused by 17,000 kW Open Submerged Arc Silicon
Furnace with Baghouse Collection. 38
6 National Primary and Secondary Ambient Air Quality
Standards for Particulate Matter (Ref. 53) 38
7 Primary and Secondary Pollutants Resulting from 4500
MW/yr Silicon Solar Cell Production 43
8 Primary Pollutants from 9000 MWe Power from Coal
Fired Steam Plants 44
9 Comparison of Three Criteria Pollutants as Emitted in
Silicon Cell and Feedstock Production, as Emitted in
Electric Power Generation to Supply Solar Cell Production,
and as Emitted by 9000 MWg Coal-Fired Power
Generation 45
10 Silicon Related Emissions as % of Equivalent Coal-Fired
Emissions 46
11 Emission Factors (kg/MT of Subject Material) for Raw
Materials Required in the Production of CdS Solar Cells 50
12 Quantities of Front Surface Cell Materials for 9000
MW/Year (Peak) 60
13 Quantities of Back Surface Cell Materials for 9000 MW/
Year (Peak) 60
14 Power Requirements for Annual Production of 9000 MW
(Peak) of CdS Solar Cells 6l
15 Primary and Secondary Pollutants Generated from the
Manufacture of Cadmium Sulfide Cells for 9000 MW/Year
(Peak) 62
16 Toxicity and Biological Activity of Selected Elements
(Ref. 78) 63
17 Relations Between Cadmium and Zinc Production in the
20th Century (Ref. 71) 65
18 GaAs Process Material Consumption and Production
Summary for 4500 MW 75
VI11
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Number Page
19 GaAs Yields in Cell Production 76
20 Summary of Power Requirements for Production of 100
MW/Year Peak Power GaAs Solar Cells 77
21 Environmental Consequences and Corrective Actions
GaAs 78
A-l Emission Factors for Quartzite Mining Unit Operations
(Derived from Ref. 23) 91
A-2 Elemental Analysis of Emissions from Quartzite
Processing (Derived from Ref. 23) 92
A-3 Metallurgical Coke Manufacture: Atmospheric Emissions
Related to Equivalent Quantity of Silicon Solar Cell Pro-
duced (Derived from Refs. 27, 36 and 37) 93
A-4 Coal Emission Factors for Atmospheric Particulates
(Ref. 39) 94
E-l Estimated Rates of Emission of Cadmium During Pro-
duction and Disposal of Cadmium Products for 1968
(Ref.85) 114
IX
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SECTION 1
INTRODUCTION
SCOPE AND OBJECTIVES
The objective of this task is to evaluate the possible environmental
consequences of large scale photovoltaic materials manufacture and to
assess potential hazards, relative advantages, environmental control tech-
nology requirements, and impact on energy and material resources.
The scope of this task covers silicon (Si), cadmium sulfide (CdS), and
gallium arsenide (GaAs) solar cells. The current methods and types of
materials and processes are identified in detail. Alternate materials and
methods which are being pursued have been identified. Processes, yields,
energy, raw materials and waste products are discussed in sufficient detail
to estimate quantities.
Primary emphasis is placed on Si for two reasons: (1) the technology
is more developed than for the other candidates, and (2) technology growth
continues to accelerate (Ref. 1). The principal advantages of the other cell
types low cost of CdS, and high efficiencies of GaAs have not been suffi-
ciently developed to overcome the low lifetime of CdS and high cost and
material quantity limitations for GaAs.
BASIS OF ASSESSMENT
This study addresses the area/mass/efficiency interactions that di-
rectly affect raw material quantities, energy, and pollutants generated. The
raw material quantities are influenced primarily by three factors:
1. Material in cells: The thickness of the active material in the
finished cell from which the ratio of net mass to cell area can
be calculated.
2. Production yield: A measure of the amount of material required
at the input to obtain the desired amount of material in cells.
Overall production yield =
Amount of active material in solar cells
Amount of active material in raw material
x 100%
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3. Generating efficiency: Several factors influence the efficiency
of an installed solar panel. Efficiency is defined as peak power
output of the solar cells as a percentage of the incident energy
on the solar cell area at solar noon at a location directly under
the sun's path at air mass one (AMI) conditions. Local incident
energy is affected by latitude, elevation, time of year, time of day,
array orientation, weather conditions and local air quality.
Cell output is responsive to energy in the ranges of ultraviolet
(UV), visible and near infrared (IR) wavelengths from 0.4 micron
to 1.1 micron (Ref. 2). Conversion efficiencies improve with air
mass; but, since net incident energy is lower the cell output energy
is less. The extraterrestrial solar spectral irradiance value (AMO)
of 1353 Wm~2 has been determined by extensive tests and standard-
ized {Ref s. 3 and 4). The direct solar spectral irradiance at air
mass one with attenuation due to atmospheric turbidity, water vapor
and ozone is 956 Wm~^ (Ref. 5). In addition, there is a diffuse ir-
radiance due to sky radiation which is approximately 20% of the
total radiation over all wavelengths. The ratio of diffuse, radiation
to direct radiation is very high in the UV but drops rapidly in the
visible and IR. Solar cells respond to both direct and diffuse ir-
radiance normally. When concentrators are used only the direct
component is concentrated.
Efficiency calculations in this study use the peak normal output of
cells as a percentage of 1000 Wm~2 incident on solar cell area. To
determine installed array area required for a specific site, the de-
signer 'would have to determine local solar incidence, account for
packing factor* and adjust for structural area, power conditioning,
and transmission losses.
Production quantities are based on solar cells providing 1% of the esti-
mated installed electrical generating capacity in the United States in the year
2000. The estimate derived from the Project Independence report (Ref. 6 and
others) is shown in Figure 1 to be 900,000 megawatts average. For solar
cells to be capable of producing 1% of the generation capacity or 9000 MW
average over 24 hours they must have 45,000 MW peak output capability be-
cause of the solar insolation curve (see Figure 2). Throughout this report
we calculate quantities based on peak output.
Peak Power is the power output of a solar energy system at solar noon
directly under the sun's path on a clear day due to direct peak solar radia-
tion at normal incidence. Latitiude, time of year and day (solar angle), cloud
conditions and local environment are among conditions which affect solar
radiation on the solar panel. For simplicity in calculations, peak solar
radiation is normally assumed to be 1000 W/m .
Packing factor = solar cell area as percent of total solar array area.
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10.0
0.1
1960 1965 1970 1975 1980 1985 1990 1995 2000
YEAR
General Electric Data
Estimated
Calculated from GE Data
Figure 1. Installed electrical generating capacity in United States.
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Average Power is the power integrated over time divided by 24 hours.
For this study the ratio of peak to average power is estimated to be 5 to 1
as shown in Figure 2.
, . Peak
1.04
000 0400 0800 1200 1600
Time-of-day (hrs)
2000
2400
Figure 2. Peak and average solar power vs time for direct radiation
on a clear day.
Annual production quantities are based on the best yields, lifetimes,
conversion efficiencies and thicknesses for the three cell types which, based
on review of published literature, can be achieved currently with little risk.
The values used are shown in Table 1.
TABLE 1. SOLAR CELL CHARACTERISTICS AND PRODUCTION
REQUIREMENT
Material
CdS
Si
GaAs
Lifetime,
Years
5
10
10
Energy Conversion
Efficiency, %
(Air Mass One)
5
14
16
Thickness,
m x 10'6
6
150
125
Annual
Production,
MW
9000
4500
4500
Note: Annual production is the amount required to maintain a system that
produces 9000 MW (average), based on the lifetime of the cells.
The extensive research presently under way in all energy related tech-
nologies will cause these numbers to change; therefore the derived data are
presented in forms which can be easily factored when performance improve-
ments have been verified.
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TECHNOLOGIES EVALUATED
An extensive review of published literature was conducted for each
cell type. Conversations with respective leaders in production and analysis
of the cell types were conducted to determine the confidence level and prob-
ability of fulfilling performance estimates.
Heavy emphasis is placed on silicon and the current ERDA/JPL pro-
gram (Ref. 7). ERDA programs for alternatives to silicon are being devel-
oped and funded. These include GaAs, CdS-C^S and others.
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SECTION 2
CONCLUSIONS
SILICON
A major improvement in process efficiency is needed. This should
include reduction in the very large power consumption, elimination of the
SiC^4 byproduct stream, and reduction in the amount of Si and "SiO" which
leaves the plant as solid waste. Although "SiO" is a designated component
of several streams, its true identity seems to be questionable in our view.
Its composition, and implications with respect to plant safety are unclear.
CADMIUM SULFIDE
A very promising process for the production of pure CdS, suitable for
solar cells, has been well defined in terms of energy and material balance,
pollutants and pollution control technology needs. The techniques for fabri-
cating the laminated solar cells, however, are proprietary and in the early
stages of development. Thus, quantitative environmental impact assessment
and material and energy balances could not be developed. On a gross, na-
tional scale, the atmospheric emission of cadmium or cadmium-bearing
particulates is a potentially severe primary pollution problem; discharges of
suspended solids, oil and grease effluents are expected to be the most signif-
icant secondary pollutants. Adverse local impacts may also exist because
of the toxicity of the heavy metal effluents (As, Cd, Cu, Pb, Se and Zn).
The long lead time available before widespread CdS solar cell pro-
duction occurs (about 15 years), should allow for the development and testing
of adequate pollution control technology. The only significant impact on
natural resources will be the demand for cadmium, which is a byproduct of
zinc production. Demand in the year 2000 from solar cells (36% of the total
market) would exert appreciable upward pressures on prices, but demand
could probably be met.
GALLIUM ARSENIDE
Many aspects of current and projected GaAs technology remain un-
defined because of proprietary rights.
Arsenic's environmental toxicity has been reported in research and
review literature. By contrast, almost nothing is known about gallium in
the environment. Production of GaAs at levels which we have considered in
this report will almost certainly create a potential for previously impossible,
large-scale environmental releases of gallium.
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Current GaAs technology is somewhat energy intensive, but not nearly
as much so as Si production.
Gallium may come into short supply unless current hopes for high con-
centration ratios are realized.
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SECTION 3
RECOMMENDATIONS
SILICON
Energy Consumption
Alternative processes which may be less energy intensive need in-
vestigation and development.
Byproduct and Waste Streams
Development of a hydrochlorination step to convert SiC*4 to SiHCi 3 in
a recycle loop needs development and application.
Separation of SiO and Si from the fine particle solid waste stream
needs development, so that these may be recycled to the arc furnace and
chlorination reactors, respectively.
Control Technology
Fire hazards in dust collector due to purported "SiO" needs to be
assessed.
CADMIUM SULFIDE
Process Definition
Further identification and quantification of emissions from the cell
fabrication portion of the process is required.
Control Technology
Particulate control device efficiencies for cadmium bearing atmos-
pheric particulates should be assessed.
The adequacy of trace metal removal processes for the control of As,
Cd, Cu, Pb, Se and Zn ions in effluent discharges should be determined.
Environmental Impact
After the foregoing are completed, a NEPA EIS on a hypothetical large
scale CdS plant is in order to better quantify ecological and other effects.
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As part of a comparison of solar versus conventional energy pro-
duction, a large scale CdS industry should be included in an overall environ-
mental assessment of producing 9000 MW (or more) of electricity by solar
versus coal and should include primary and secondary emissions for each.
GALLIUM ARSENIDE
Gallium in the Environment
Gallium's environmental toxicity and ecological effects must be clas-
sified. A definitive literature survey to define future research directions
should be the first task.
Proprietary Information
Feedstocks, chemical intermediates, and plant operations {which cur-
rently remains confidential) need identification and characterization.
Spills
Engineering, regulatory, and management approaches must be devel-
oped to minimize risks and consequences of accidental toxic releases.
Safety
Engineering, regulatory, and management approaches to assure work-
place safety must be developed.
Resource Limitations
The supply/demand picture for gallium needs searching examination.
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SECTION 4
SILICON PROCESS
GENERAL PROCESS DESCRIPTION
A process for large-scale manufacture of single crystal Si for solar
cells which represents application of current methods is.presented in Fig-
ure 3. It does not describe any existing facility; however, the technology
described is based on Refs.8,9 and 10 and the author's projections. Process
quantities and operating conditions are based on calculations from handbook
data since detailed process information is kept proprietary.
Quartz is reduced by coke in an electric arc furnace to metallurgical
grade Si. This process, while requiring high power is> approximately 50%
- efficient. The waste CO stream carries from 15 to 25% of the incoming Si
as SiO, which comes off as a light particulate. The SiO is separated and
filtered from the gas stream and is conveyed to storage. SiO and Si fines
collected from Si milling operations can be used for building insulation, as
a filler for lightweight concrete or bricks or they can be compacted and re-
turned to the arc furnace.
The CO stream, after being filtered of dust, could be used as a fuel,
along with waste hydrogen, in a waste heat boiler to produce steam. A small
fraction of the steam could be used to heat the silane fractionating column
reboiler; the balance could drive a steam generator.
Basic Unit Operations in Silicon Ingot Production
T richloro silane --
The metallurgical grade Si is drawn off from the arc furnace in the
liquid phase, cooled and allowed to crystallize. It is then milled and fed to
a fluid bed reactor and reacted with HCl to produce mono-, di- and tri-
chlorosilane, silicon tetrachloride and hydrogen. A small amount of Si dust
is removed from the reactor product stream and then the product gas is run
through two sequential condensers to separate a light silane phase, hydrogen
and a heavy silane phase. The light silanes, chiefly SiH2Cl2 and SiH^Cf.,
are recycled to the reactor. The heavier fraction containing trichlorosilane,
silicon tetrachloride and dichlorosilane is run through fractioning columns
to yield high purity trichlorosilane. The Sit^C*2 with SiHC^ is recycled
to the reactor.
A silicon tetrachloride product is removed to storage for sale, or it
may be used for silicon production. Another alternative is to use SiC>?4 as
10
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11
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12
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a starting material for high purity poly crystalline Si; this would give a
higher yield of polycrystalline silicon per unit of Si entering the process,
but might be more energy intensive. Two producers of polycrystalline Si
use the SiCI^ in adjacent silicone plants.
The pure trichlorosilane is fed to a. hydrogen saturator where a re-
circulating hydrogen stream is being saturated with SiHC^ prior to being
fed to the Si rod deposition chambers. In these chambers a thin Si rod is
resistance heated to 1370 K where the hydrogen-trichlorosilane to Si deposi-
tion takes place. The walls of the deposition chambers are water cooled and
the gas stream of approximately 95% hydrogen is purified, resaturated with
SiHCl3 and recirculated through the chambers several times. A near-
turbulent gas flow is desired with a resultant deposition rate of approxi-
mately 10 {4/min. This slow rate (1.44 cm/day) demands three to four days
to grow a common ingot and consequently this one step is more energy in-
tensive than all else combined. A fair quantity of cooling water is required,
however, it can be recirculated through a cooling water tower thereby re-
quiring only makeup water to replace evaporative losses.
Silicon Rod Deposition--
The hydrogen-rich exit stream from the Si rod deposition chambers
carries a mixture of silanes, silicon chlorides, hydrogen-chloride and
hydrogen. Sequential condensers separate and return these products to the
process for recycling. No silanes, silicon chlorides or HCJf are vented to
atmosphere at any time. A slight excess of hydrogen gas is produced and
burned in the CO waste heat boiler.
The major potential pollution problem is dust. The following opera-
tions will require some form of dust control.
Coke and Quartz to the Arc Furnace--
Coke in briquet form and quartz in chunk form would be transported by
rail or truck to the plant site and by belt or bucket conveyor to the furnace.
Closed bulk handling equipment may be required; however, the particulates
are expected to be large at this stage and may not require special considera-
tion.
SiO and Ash from the CO Stream--
The CO stream carries fine SiO dust with it. The particle size distri-
bution of this dust was unavailable at this writing; therefore it was estimated
that a high-quality dust collection system would trap 99 percent of the solids.
The remaining fine dust would go through the waste heat boiler stack either
to additional dust precipitation equipment or, if concentrations are low
enough, to the atmosphere.
13
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Milling and Conveying of Metallurgical Silicon--
Further investigation is necessary to ascertain the fineness of the
milling required for the fluid bed reactor. Nevertheless, there will be some
fine silicon dust generated which will require enclosure of the mill and ex-
haust air filtration.
Boiler Flue Gases--
The exhaust stack from the CO waste heat boiler will contain a small
concentration of SC»2 if high grade coke is used in the arc furnace. If the
coke has a high sulfur.content it could become necessary to use a scrubber
on the stack. Some oxides of nitrogen .may also be expected in the waste heat
boiler stack gases. A small quantity of nitrogen enters the process stream
with the coke and is expected to remain in the CO gas stream to the waste
heat boiler where oxides of nitrogen can form. The operating conditions of
the boiler affect NOX generation and gas composition.
Crystal Doping--
The solar-grade polycrystalline Si proceeds to cell fabrication where
it is transformed into one of the forms of cells noted in Table 2. The poly-
crystalline material must be doped to form, either p-type material (boron,
etc.) or n-rtype material (phosphorous, etc.). The amount of dopant is deter-
mined by the desired bulk resistivity. A target resistivity for silicon of 0.25
ohm-cm requires 3 x 10^6 atoms/cm^ of n-.type dopant (As, P, Sb); or 1.2 x
1017 atoms/cm3 of p-type dopant (At, B, Ga) (Ref. 11).
TABLE 2. PROPOSED CELL, TYPES (REF. 7)
Single crystal
Wafers sliced from crystal
Sheet grown crystal by E3
Epitaxially grown crystal on substrate
Sheet grown crystal by EFG or other
Polycrystalline
Chemical vapor deposition
Zone crystallized sheet
Edge-fed film growth.
If all of the polycrystalline Si is doped with boron to form p-type ma-
terial for N on P solar cells the amount of boron required is 1.49 gm/MW
of peak array output. This equates to approximately 20 milligrams per hour
in a facility which produces 100 MW/yr. For n-type material (for P or N
14
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solar cells), 4.47 gm of phosphorus per megawatt (approximately 60 milli-
grams of phosphorus per hour for a 100 MW/yr facility) are required. The
dopant is added to the poly crystalline melt prior to single crystal growth.
For this study boron doped material is used for the bulk cell wafer. N or P
denotes n-type or p-type material.
Crystal Growth--
Single crystal silicon ingots for solar cell fabrication are grown in
sizes ranging from 5 cm to 15 cm diameter, weighing from 9 to 36 kg each.
The polycrystalline Si and dopant charge are placed in a quartz cru-
cible which, in turn, is placed into an electrically heated carbon bowl. Agi-
tation of the melt assures uniform dispersal of dopant and proper crystal
orientation during growth is accomplished in an atmosphere of high purity
argon.
A seed crystal of Si is introduced to the melt to form the crystalliza-
tion nucleus. It is slowly drawn from the melt as the crystal forms. Con-
stant monitoring of controls by a trained operator is required to ensure good
crystal yield. The operator must adjust controls in response to variations
in the crystal growth parameters. At present one operator is required for
two to four crystal growing machines.
The resultant crystal is approximately 90% of the polycrystalline Si.
The remaining 10% is crucible waste. It, along with the crucible itself, could
be recycled to the arc furnace to provide some cost savings. Figure 4a
shows the utilization of the ingot. Thirty percent is high purity Si which is
not suitable for producing cells but it is remelted for crystal growth after
etching away its oxide coating. Figure 4b shows the current net yield of
polycrystalline Si into wafers for cell production.
Cell Fabrication Sequence
Cell fabrication for this study is based on Czochralski grown single
crystal Si which is sliced into wafers. Process flow is diagrammed in
Figures 5 and 6. Currently, sliced single crystal technology has several
advantages over other methods of cell production. It is well known, proven,
and has near term growth potential (Ref. 12) to produce more kilowatts/
kilogram of Si (because of higher material utilization and higher cell effi-
ciencies).
Wafer Slicing--
The two most popular methods for wafer slicing are: (1) cutting single
slices with diamond blade, and (2) multiblade slicing with abrasive slurry.
Of these the multiblade slicing is the most promising for both high production
and minimum wafer damage. It is less sensitive to diameter than the dia-
mond blade and can accommodate multiple ingots with some modification.
The apparent limitations with diamond blade cutting are that with increasing
15
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Remelt
27%
Etch
Waste
3%
Good Crystal
Grind Waste
3%
Saw Waste
30%
Good Wafers
30%
a. Single Pull
Crucible
Waste
7%
Other
Waste
23.4%
Saw Waste
36.3%
Wafers
40.3%
b. Net Yield Assuming Remelt Recycled Twice
Figure 4. Current silicon yields as percent of polysilicon input
to crystal growers.
16
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PS
o
V
ft
N
P.
N
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ingot diameter: (1) the blade causes chatter damage to the wafer, and (2) the
blade must be thicker causing more kerf loss and reduced material utiliza-
tion.
Of the 60% of the polycrystalline entering the melt which becomes
single crystal Si and is suitable for producing solar cells, half is removed
as saw kerf. This material has large surface area which oxidizes readily.
It also is contaminated with SiC grit used in cutting and ceramic from the
blocks used to support the ingot during cutting.
Programs to reduce the saw kerf and improve surface finish as well
as to reduce wafer thickness which can be cut are being performed under
ERDA-JPL contracts (see Ref. 7).
The single crystal ingot is sliced to the maximum length that the ma-
chine can accommodate and bonded to a ceramic submount with hot melt ad-
hesive or wax. The submount is either alumina or other material which has
about the same density and cutting qualities as Si. The submount must be
thick enough to permit the blade to cut half of its depth into the submount to
assure that no tapers remain on the wafers. 0.5 kg of alumina per kilogram
of polysilicon is required. The amount of adhesive or wax is estimated to
be 0.07 kg per kilogram of polysilicon.
The cutting results from the abrasive action of steel bands rubbing
grit on the Si. The highly-tensioned and accurately-spaced steel bands are
pressed against the Si and drawn across in a reciprocating motion while the
abrasive slurry is fed to the bands. The abrasive slurry consists of: (1) the
Si carbide abrasive (usually 600 mesh), and (Z) the slurry vehicle which is an
oil base mixture with fine clay which holds the grit in suspension. The
slurry vehicle most commonly used has a flash point of 275 to 300 F and is
noted as a non-hazardous material for shipping.
The required amount of slurry is 8.3 kg per kilogram of polycrystal-
line Si. If equal parts of oil, clay and grit in the slurry are assumed then
there will be 2.77 kg of each per kilogram of polycrystalline silicon. Present
equipment recirculates 99% of the slurry, leaving 1% which may become air-
borne in the cutting area. Control of the 1% may be required to prevent in-
halation by personnel.
Other consumables during wafer slicing are the blade pack and lubri-
cants. The blade pack consists of 10 mm thick strips of steel 0.1 mm wide
spaced 0.15 mm apart. Each pack is estimated to last 10 cuttings. There-
fore the steel consumption during cutting is approximately 0.77 kg per kilo-
gram of polycrystalline Si.
Air/oil mist lubrication is used for the guide mechanism. This mist
is sprayed on the bearing cages of the four guide shafts. Most of the mist
is trapped and drains into an open collection trough in the cutting machine.
Some of the mist becomes airborne in the area around the machine. A rough
estimate for the quantity consumed is 0.3 kg lubricant/kg polycrystalline
Si with 5% becoming airborne. Cutting energy is estimated to be 7.5 kWh/kg
19
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of polycrystalline Si. For this study 40.3% of the polycrystalline Si is as-
sumed to emerge as wafers for cell processing.
Wafer Etching (Ref. 13)--
The wafers from the saw have mechanical damage resulting from the
600 grit abrasive which is approximately 15 /nm deep. To remove this
damage 15 to 25 ju of wafer is etched away in heated 30% by weight NaOH.
An additional 15 to 25 JLL of wafer is removed in a 1% NaOH solution. This
second etching produces a rough surface which has low reflectance and im-
proved adherence of metallization.
The reaction taking place in the sodium hydroxide etching is:
Si + 2 NaOH +
Each kilogram of polycrystalline Si produces approximately one square
meter of wafers and the above reaction must remove 15 /n from each side of
the wafer. On this basis it is estimated that 20 gm of NaOH are required
per kilogram of polycrystalline Si. These estimates are based on stoichio-
metric reaction the efficiencies of the etching processes are unknown to
the authors. Thirty-one grams of Na2SiO3 are produced during each etch
in addition to 4 gm of hydrogen. Following the etch the wafers are rinsed
with deionized water and dried to prepare them for the diffusion process
which follows.
Diffusion Junction Formation (Ref. 14)--
A layer of n-doped material is formed on the wafer by diffusing phos-
phorus into the crystal structure in a tube furnace. To quantify the potential
pollutants a description of the open-tube phosphorus diffusion process which
uses a phosphine (PHs) gas source is required. A gas mixture of 1% PH3
and 99% argon, with some further dilution nitrogen, is fed to the tube fur-
nace where the PH3 is dissociated.
2 PH,
713
3 Hz + 2P
Oxygen is provided to produce phosphorus pentoxide.
4 P + 50,
As the P2O5 strikes the surface of the wafer it reacts to form a glassy layer
by this reaction:
+ 5 Si
4P + 5 SiO,
20
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The portion of the phosphorus in this layer which diffuses into the Si deter-
mines the electrical property of the junction.
The Si wafer already contains 1.2 x 10 atoms of boron/cm . To
achieve the desired net density of doping of 3 x 10*" atoms of phosphorus
requires a total of 15 x lO1^ atoms/cm3 in the layer. The quantity of
dopant in the layer can be calculated. There are 6.2 x 10~6 g of phos-
phorus in the layer, per square meter of wafer area. To achieve this level
of doping requires 3 x 10-° kg of PH3» 180 cm3 of argon, 180,000 cm3 of
nitrogen and 2700 cm3 of oxygen per kg of polycrystalline Si.
At the exit from the furnace the excess phosphorus reacts with air to
form a mixture of P, PzOs and H^PO^. The disposal or recovery of these
products may include recycling PH3« Because of the toxicity of PH3 and re-
lated compounds, a negative pressure gradient must be maintained between
the inlet (where personnel load the furnace) and the exhaust from the tube
furnace. This is an occupational safety and health problem, but not an en-
vironmental problem. Diffusion process energy requirements are estimated
to be 18.6 kWh/kg of polycrystalline Si.
Expose Material--
The diffusion process leaves the wafer coated with an n-layer and an
outer surface of SiC>2 which contains phosphorus as shown in Figure 7.
must be removed from both sides of the wafer and the n-layer from
P Containing SiO2 [0.01]
Silicon Wafer
P-Type [150]
N-Type Layer [l]
-6
[ ] = Thickness (10 m)
Figure 7. Silicon wafer cross-section after diffusion.
21
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one side to expose the P material. This is accomplished with an acid etch
as follows:
4 HF
SiO2 =
4 HNO + 5 Si
= 5 SiO2 +
The resulting SiF4 gas may be scrubbed with Ca(OH)2 to form CaF2
which is relatively insoluble in water.
To remove one micron of Si requires 6.33 gm of HF and 4.2 gm of
HNO3/kg of polycrystalline Si, assuming 1 m2/kg of polycrystalline; There
are 8.5 gm of gaseous SiF4 to be scrubbed per kilogram of polycrystalline.
Reclamation of Silicon Ingot for Remelt- -
As noted above, part of the Si ingot is unsuitable for slicing into wafers
but is capable of being remelted at the crystal puller after the oxide layer is
removed. HF-HNO3 is used to remove the outer 10% of the ingot prior to
remelting. This reaction requires 0.08 kg of HF and 0.06 kg of HNOs and
produces 0.11 kg of SiF4/kg of polycrystalline Si.
Apply Contacts --
Screen printed contacts have been shown to be suitable for terrestrial
solar arrays and for high production (Ref. 13). The composition of these
contacts is proprietary but they are known to contain silver, aluminum,
silica and an organic binder. For this study we have assumed the relative
volume composition to be 10% silver, 50% aluminum, 30% silica, and 10%
binder. Assuming 75% yield of 8 jit thickness and 14% front coverage and
95% back coverage, .the amountof contact material is 11.63 cm^/kg of poly-
crystalline Si. Quantities of each constituent per kilogram of Si are alumi-
num, 16 g; silver, 13 g; SiC>2, 9g; and binder, 1 g.
Firing Contacts --
After the screen printed contacts are thoroughly dried they are fired
at approximately 923 K to sinter and drive off the binder. The products
of the binder will have to be determined and their environmental effects
assessed. Estimated energy is 7.6 kWh per kg of polycrystalline Si.
Anti-Reflective Coating --
Si covered cells made with the NaOH etch process showed a 2% drop
in output when compared with silicon covered cells with spin-on and vapor
deposited anti-reflective coatings (Ref. 15).
The spin-on process is used because it does not require a vacuum.
An 0.2 (i coating of TiO2 and SiC>2 is spun onto the front of the cell and
sintered at 523 K. This requires approximately 2.7 g each of TiO? and
22
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(assuming 10% deposited on the cells) and 10 kWh of electrical energy
per kg of polycrystalline Si.
Electrical Test--
The final step in cell production is the electrical test under a solar
simulator. Environmental and personnel safety aspects of the test include
high voltages and UV radiation. Presently tests are conducted at low levels,
but increased production would demand more detailed study to ensure per-
sonnel safety.
Cell Processing Yield
This study assumes a cell processing yield from wafer to finished cell
of 85%. The net yield based on polycrystalline Si to finished cell is 34.8%.
Material Consumption and Production Estimates
Material Utilization--
When cell wafers are cut from an ingot such as in the current
Czochralski crystal growth and slice method, the maximum material utili-
zation is limited by the saw kerf and wafer damage during sawing. During
slicing by either the internal diameter diamond saw with its 5 to 10 mil kerf
or the multiple band slurry saw with its 4 to 10 mil kerf, there is some
damage to the wafers that must be etched away or otherwise treated. With
further development of the sawing method, the potential is for 0.10 mm
wafers with 0.10 mm kerf with no wafer damage (Ref. 16) which is the utili-
zation limit for sliced wafers. There is no possibility of increasing area by
reducing wafer thickness because thinner wafers become too fragile for the
slicing operation. If thinner wafers are desired they are chemically etched
from 0.1 mm wafers. Thus the maximum area that can be produced from
sliced material is 2.1 mr/kg of net Si which corresponds to 0.10 mm (or
thinner) wafers with 0.10 mm saw kerf. This study assumes 0.15 mm thick
wafers with 0.15 mm of saw kerf, which when combined with yield factors,
produces 1 m^/kg of polycrystalline Si. Other methods of utilizing Si (e.g.,
vapor deposition in 10 \i. thickness) may permit an order of magnitude in-
crease in area to mass ratio. However, cell collection efficiency is reduced
rapidly when base diffusion lengths are less than 0.015 mm (Refs. 17 and 18).
Production Yield Czochralski--
The production of single crystal Si for solar cells involves the melting
of polycrystalline high purity Si with a suitable dopant and the growing of the
single crystal by introducing a "seed" crystal. The temperature, atmosphere,
agitation and rate of pull are carefully controlled during crystal growth to
maintain a properly oriented single crystal. The net yield of wafers from
the saw is 40.3% of the polycrystalline Si supplied to the crystal grower.
Of the wafers entering the cell production line, 60 to 90% will emerge
as completed electrical cells. Eighty-five percent is projected as 1977
capability (Ref. 13) and is used for this study.
23
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Polycrystalline Si Requirements --
The estimated 14% electrical conversion efficiency will result in 140
watts per square meter of 150 jum thick finished cells. This is equivalent
to 0.40 kW/kg of Si in finished cells or 0.14 kW/kg of polycrystalline Si
(using 34.8% yield of cells from polycrystalline silicon).
The quantity of polycrystalline Si required to produce 4500 MW (peak)
annually is, therefore, 32.14 x 106 kg. (This will produce 11.2 x 10° kg of
finished cells). The quantity used (as discussed earlier) is 8.1.54 kg/hr for
a plant which produces 100 MW/yr. Based on the polycrystalline Si require-
ment, other material requirements for the above solar cell production were
estimated and are listed in Table 3.
Silicon Byproducts --
SiO and Si fine particulates are low density, excellent insulating ma-
terials. Fine silica particles are presently being sold for high quality ther-
mal insulation. This material could also be used as lightweight filler in
concrete or bricks.
The "SiCi4" stream has the following composition:
Substance
Total
kg/h
1226.41
714.57
1.84
1942.82
63.1
36.8
0.1
or 17 x 10 metric tons annually. SiC^4 is much too valuable to be treated
as a waste stream, even were this environmentally acceptable. Consequently,
a Si tetrachloride product is removed to storage for eventual sale. It has a
market price of approximately 13 cents/kg. It can be used in Si production
or as a starting material for polycrystalline Si production by a different
process. This would give a higher yield of polycrystalline Si per unit of Si
entering the process, but might be more energy intensive. Two producers
of polycrystalline Si use the SiC^ in adjacent Si plants. SiC^ is a major
constituent of the "SiC&j" byproduct stream. We have not investigated its
effect upon the marketability of
Methods of hydrogenation of SiC*4 and SiCj?3 should be investigated.
may be economically advantageous to include within the plant an SiC44/Si
recycle loop through a hydrogenation facility which converts chlorides to
chlorosilanes, particularly SiHCj?3.
For the latter conversion, the reaction
It
HCI(g) - SiHCi3(g)
24
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TABLE 3. MATERIALS AND PRODUCTS FOR 4500 MW ANNUAL
SILICON CELL PRODUCTION (10* METRIC TONS)*
Solids
Quartz, 98% SiO2
Coke, 92% C
Boron
Si Carbide
Clay
Steel
Adhesives
Alumina
Ca(OH)2
Aluminum
Silver
SiO2-TiO2
Liquids
HCI 99.9%
Oil
Lubricant
Gases
Air
Argon
Nitrogen
Oxygen
PH3
Quantity
6.13
2.36
8 x 10~8
0.89
0.89
0.26
0.02
0.16
As req'd
0.005
0.004
0.003
7.37
0.89
0.09
108.06
0.35
0.07
0.001
in
9.6 x ID'10
Solids Quantity
Solar Cells
Si
Si Carbide
Clay
Steel
Adhesives
Alumina
CaF2
Phosphorus residues
Liquids
Sid*
n»
SiCl ?
Oil
Lubricant
Na, SiO,
M 3
Gases
C02
S02
N0**
H20**
Ar
H2
Fine Particulates ^Air-
borne and Solid Waste)
SiO
Si
Ti
Ai
Ash
Aerosol (oil and
lubricant)
0.11
0.12
0.89
0.89
0.26
0.02
0.16
As req'd
9.6 x 10-1°
4.83
7.25 x 10"3
2.82
0.89
0.09
0.02
7.92
0.04
0.04
1.34
0.35
Trace
0.87
0.17
0.02
0.05
0.04
Trace
To convert from text data (kg/h for 100 MW production) to units of this
table (105 MT/yr for 4500 MW production) multiply by 3.94 x 103.
**.
Does not include boiler feed or cooling water.
25
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has an endothermic heat of reaction
298'
= 61.8 kcal/mole (Ref. 19)
which is not prohibitive. High temperature equilibrium partial pressures
for most of the Si chlorides and chlorosilanes, H2, HCi, and monatomic Cl
have been reported in a paper by Sirtl et al., and by references quoted
therein (Ref. 19).
Were the Si chloride "byproduct" stream 100% converted to SiHC%, an
immediate 59% increase in poly crystalline Si production would result. Look-
ed at another way, this would result in a 37% reduction in mass of pollutant
released per ton of product or megawatt of power consumption at the sub-
merged arc furnace. From the vantage point of plant management, this
would make for easier compliance with current New Source Performance
Standards (NSPS) for particulate emissions. Plant performance with regard
to NSPS is discussed in a later section.
Plant Siting
The preceding discussion does not imply that the entire production
train from quartzite to finished solar cells necessarily will be located
inside one plant or that all plants will even be in the same region of the U.S.
First plants (if of new design) possibly will be located on the basis of Fed-
eral funding for demonstration plants and other forms of institutionalized
seed money. Subsequent facilities are more likely to be financed privately.
Siting may be near raw material, near cheaper hydropower, by product re-
covery plants, near product market, or added on to existing ferrosilicon
production facilities.
Single crystal ingot production from quartzite and coke naturally falls
into three process segments, which may be remotely located (cf., Figure 3):
Crushed Si metal production terminating at the 5 kW Si mill. This
segment is most likely to be located in urban industrial areas in re-
gions near metallurgical coking coal, or abundant quality quartzite.
Pure polycrystalline Si production beginning with the fluid bed
chlorinator and ending with the 1 kW rod crusher. This segment
would find Si elastomer industry and cheap hydropower attractive.
Single crystal ingot production, namely the 5 MW pulling chamber.
This third segment is most likely to be located: near existing elec-
tronic industry.
POWER REQUIREMENTS
The estimated power requirements for the manufacture of Si solar
cells, discussed earlier, are summarized in Table 4. Power rather than
energy is used for two reasons: (1) the processes lend themselves to con-
tinuous operation, and (2) a direct comparison to solar cell output can be
26
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TABLE 4. SUMMARY OF POWER REQUIREMENTS FOR ANNUAL
PRODUCTION OF 4500 MW OF SILICON SOLAR CELLS
Equipment or Operation
Power, MW
Arc Furnace 765.00
SiO Conveyor 0.05
CO Exhaust Fan 0.90
CO Waste Heat Boiler Steam Generator -45.00
Air Blower 9.00
Si Mill and Conveyor 0.22
HCS. Condenser-Refrigeration 45.00
HCi Condensate Pump 0.05
Silane Condensate Pump 0.09
Refrigeration for Light Silanes Condenser 2.25
Silane Bottoms Pump 0.09
SiC*4 Pump 0.09
SiHC^ Pump 0.09
Condenser-Refrigeration for Silanes from Deposition Chambers 22.50
Si Deposition Recirculation Blower 0.90
Si Rod Deposit Chambers 3,240.00
Si Rod Crusher 0.05
Cooling Tower Water 45.00
Crystal Growth 253.00
Slice Wafers 27.50
Diffuse Wafers 68.30
A/R Coating and Sinter 36.70
Firing Contacts 27.90
Test 55.00
Miscellaneous Waste Handling and Environmental Controls 20.00
Total 4,574.35
made. The power estimates are for cell production only and do not include
allowances for coverglass, electrical interconnects, solar array mounting
structure, power conditioning, manufacturing facility utilities or energy re-
siduals (cf., the following section). However, these factors are expected to
be only a small fraction of the process power defined.
The 4500 MW of solar cells represents peak power, peak sun exposure,
or average power over 24 hours of 900 MW. Power payback on silicon solar
cells is then calculated as follows:
4574.35 MW to produce cells
900 MW/year from cells produced
= 5.08 years
Before conclusions are drawn a number of factors should be taken
into consideration as follows: (1) the process described and power con-
sumed are based on simply factoring up present small-scale production,
27
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and (2) new technology will replace or improve present processes. Current
efforts unders ERDA-JPL (Ref. 7) are directed toward that end. As Table 4
shows, the rod deposition of Si from SiHC/3 consumes 70% of the electrical
power requirement; any improvement in this process or its replacement by
a less energy-intensive process offers a large incentive. As discussed
earlier, a saving in power (exclusive of deposition and pulling chambers)
can be made by conversion of the Si chlorides byproduct stream to SiHC^.
Energy Residuals
Two major raw materials, quartz and coal, are open-pit mined. Coke
is used in the arc furnace reduction of silica and therefore, there is a re-
quirement for mining approximately 1/2 kg coal per kilogram of Si. Com-
parable quantities of coal as fuel for energy generation versus required
quartz and coal to make solar cells is calculated using the following assump-
tions :
1. Coal to coke conversion of 70%
2. Coal to electrical conversion of 30%
3. Ten year cell life at average power 20% of peak power.
The 100 MW of electrical power generated by the solar cells produced
annually in one hypothetical plant yields 1.732 x 109 kWh over a ten year
life. These solar cells require 13.36 x 10^ kg of quartz and 6.87 x 10& kg
of coal to be mined. To produce an equivalent amount of energy from burn-
ing coal requires the mining of 9.08 x 10^ kg of coal. Thus, the production
of solar cells requires mining only 1/45 the material consumed by coal-
fired power plants.
The direct and indirect environmental problems of silicon cell manu-
facturing process are discussed in the following section.
Energy Payback and "Ungenerated" Pollution
The amount of pollutants "saved" by the use of solar cells as com-
pared to coal-fired or other electrical power generation is proportional to
the amount of material mined and burned or reacted.
During initial production, there is an investment of the "dirty" energy
required to produce the first solar cells. Using an energy payback period
of five years and a lifetime of 10 years we obtain a 2 to 1 energy advantage.
ENVIRONMENTAL CONSIDERATIONS
For purposes of assessing its environmental effects, Si differs from
cadmium sulfide and gallium arsenide photovoltaic material in three im-
portant ways:
The principal substance is itself non-toxic.
Its production consumes large amounts of energy.
28
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Its principal raw materials must be mined and processed. For
their own value in Si production they are not byproduct minerals
arising from extraction of a more valuable resource.
(Silica) Mining (Refs. 20 and 21)
About 54.8 kg of 98% silica are required for every kilogram of solar
cell Si ultimately produced.
Quartzite mining problems are similar to those for other low-value
materials, namely, trying to obtain maximum output at the lowest possible
cost. The mining of quartzite presents a unique problem due to the abrasive
nature of this material, which causes excessive wear on drilling and process-
ing equipment. The toxic nature of Si dust is another problem which re-
quires constant attention to hygiene in both mining and processing operations
(Ref. 22).
The silica raw materials in demand for production of Si and ferro-
silicon lump quartz, quartzite, or well -cemented sandstone should con-
tain 98% or more silica. Preferred size is between 2 and 15 cm, with quartz
pebbles generally acceptable as small as 1 cm. Because of the violence in
the furnace and high volumes of waste product, large chunks of silica raw
material are preferred, although quartz sand and gravel have been used at
some plants.
Mining of silica is confined to surface operations. Conventional equip-
ment such as bulldozers, power shovels, loaders and haulage trucks are used.
After overburden is removed the exposed silica is removed by draglines and/
or power shovels and washed and crushed. Quartzite and well-cemented
sandstone require blasting with wagon drills commonly used for the primary
drilling and ammonium nitrate-fuel oil mixtures used for blasting.
High grade quartzite needs only crushing and sizing to prepare it for
the smelting furnace. The price of 99% pure quartzite produced in the Oro
Grande district of California has been estimated at approximately $6/ton plus
an additional freight cost of $2 to $3/ton to deliver it to the smelting furnace.
One of the smaller quartzite mining operations which uses blasting and size
reduction reported a productivity figure of approximately 3 tons/manhour.
This did not include processing or overhead.
The most obvious environmental impacts of open pit silica (quartzite)
mining are:
Irretrievable Land Use Committment
Fugitive Dust
Noise
Indirect Socio -Economic Effects.
Although locally significant, nationwide the amount of land disturbed in
mining silica raw materials for Si production is insignificant when compared
-------
with the mining of silica sand and gravel for construction and other uses.
Sand and gravel mined approximates 1 x 10° MT annually, while current re-
quirements for Si metal for all-uses is- estimated 'at less than 0.7 x 10^ MT.
Projected demand in the year 2000, without regard for solar applica-
tions, is about 3.5 x 10^ MT sand and gravel and 1.6 x 10^ MT metallic Si.
To supply Si solar cell fabrication needs at the rate of 4500 MW equivalence
annually would require an additional 0.6 x 10" MT Si. The total, 2.2 x 10^
MT, corrected to SiO^, represents 0.13% of sand and gravel demand for year
2000 (Refs.20 and 21). - .
'All quartzite mining' is by the open pit -method-, -and the overburden to
ore 'ratio is nominal. Thus,' the proliferating spoil- heaps-'characteristic of
coal mining are a relatively insignificant, and- tractable, -problem in quartzite
mining.
Fugitive dust emissions at quartzite mines arise mostly from loading
and movement of trucks, from drilling, and from crushing. In fact, un-
paved -roads represent 90% of all fugitive particulate emissions in the U.S.
Chalekode and Blackwood (Ref. 23) measured emission factors for various
unit operations in the quartzite mining," and reported average- trace- element
concentrations for suspended particulate1" in-open qua'rtzite mines. Their data
(translated to metric ton solar cell equivalent)' are- shown in Table s'-A-l^and
A-2. Average free silica content of dust frorrTunpaved quartzite mine roads
in the U-. S.1 wa's 1 7%-'(weight) with a 95% 'confidence-interval "of ±11%.--- The;
corresponding threshold limit-value- (TLV) for- this -mine road" dust would "be
1.5 mg/m3. The authors report that rib dust control measures were ob-
served at quartzite mining sites. Wet drilling, drill bit collars, and wet or
oil sprays on haul roads are control measures applied successfully at
granite, taprock," and limestone plants. These me"asure~s~seem applicable
also to .quartzite mining.
Even without'control measures, quartzite mining does not seem to be a
major dust and particulate source ascjribable to solar cell manufacture. "In
fact, we will see in the next section that it is trivial relative to the impact of
metallurgical coke production; ' ~ ' ' -
Effluent limitations guidelines for industrial sand and gravel mining
and processing, have been. promulgated (Ref. 24) but none exist for quarrying.
The "Best practicable control technology currently available" (BPCTCA)
degree of effluent reduction limitations" applicable "are,:
- TSS: 30
- pH: 6.0 to 9.0.
These are not translatable into pollutant release per unit mass of resource.
In any event, quarried lump quartzite is preferred to sand, as discussed
earlier. . . ., .
."Noise pollution" is dependent on plant geography, topography and .size,
for example subsurface hydrologic modifications, can result if a. quartzite.
30
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caprock is removed from its underlying limestone. Surface drainage is
effected by removal of sand and gravel deposits. These effects are however,
wholly site specific.
Coke
The discussion that follows is intended to orient the reader to some of
the literature describing current problems in coke production, and to present
a "worst case" estimate of atmospheric emissions in coke production. A
detailed discussion of water and air quality problems and environmental pol-
lution control in the manufacture of metallurgical coke is beyond the scope
of this report.
High grade metallurgical coke is used for reduction of SiO2 (quartz) in
the arc furnace. For each kilogram of Si in the solar cells ultimately pro-
duced, about 21 kg coke is required. This may correspond to as much as 30
kg coking coal, depending on the design of coke batteries used. The equiva-
lent energy consumption was discussed earlier.
Although briquetted or lump coke is used as arc furnace feedstock,
routine materials handling will generate some coke dust. Simple hooding
(Ref. 25) with exhaust air filtration seems appropriate in problem areas.
Vastly more significant environmental problems result front coke oven
operation and coke quenching.
All of these are subjects of a voluminous literature. Coke oven opera-
tion and coke quenching technology {Ref. 26) are changing rapidly. Many coke
plants are very old.' Nineteenth century "beehives" (Ref. 27) are still in
operation, for example. Nevertheless, we expect that by 1982 45%, and by
2000 most of the oldest plants will have been scrapped and replaced {Ref. 28)
with modern facilities employing sealed push and hooded techniques {Ref. 29)
or even dry quench (Ref. 30). Also, conventional byproduct coking may be
increasingly displaced by "formed coke" processes (Ref. 31) capable of uti-
lizing marginal or lower grade coking coals in a closed system and by con-
tinuous and pipeline coke, also capable of closed system operation (Ref. 32).
Both USEPA and the U.S. Labor Department' s Occupational Safety and
Health Administration (OSHA) have jurisdiction over atmospheric emissions
from coke ovens. For example, OSHA regulations taking effect in 1980 limit
employee workday ambient atmospheric exposure to 150 /Jtg (per 8-hr shift)
of benzone-soluble fraction of total atmospheric particulate (Ref. 28). This
standard must be met by engineering control (emissions control and/or, for
example, ventilation). Even 150 /Ig is not considered a "safe" exposure,
according to OSHA. Benzene solubles include established carcinogens, such
as polynuclear hydrocarbons (also called polycyclic aromatic s) (Ref. 33) and
noxious irritants such as phenols, for instance. The emissions from coke
ovens include noxious and irritant chemicals, potential carcinogens and
criteria pollutants. Trace element emissions and details of organic compo-
nents are tabulated in Ref. 34.
31
-------
Relevant analytical techniques have been evaluated by the National In-
stitute of Occupational Safety and Health (NIOSH) (Ref. 35).
Atmospheric emissions are difficult to quantify, because reported
measurements vary widely. Reasons include the diversity of coke oven and
emission control (Refs. 26 through 32), by variations during duty cycle, and
variations due to vagaries of maintenance and quality control. Rather
sketchy data in United Nations and USEPA reviews (Refs. 36 and 37) have
been combined with Smith's (Ref. 27) and translated to metric tons of Si
equivalent in Table A-3. These must be considered as "worst case11 esti-
mates, for regulatory standards and enforcement at all levels of government
are improving. Air pollution control, particularly in the quenching operation,
is more advanced in Europe and USSR than, in the U.S., but progress is accel-
erating now in the U.S. (cf., Ref. 36a). Dry quenching is not yet practical in
the U.S., although it is mandatory for new and rebuilt facilities in the USSR
and is employed elsewhere in Europe {Refs. 30b and 36a). It has several in-
herent advantages including reduction in air and water pollution and energy
saving.
Water pollution problems are reviewed in the United Nations report
(Ref. 36a) as well as in a voluminous U.S. literature. Waste water is often
used in wet quenching. Typically this water has been treated by mechanical
removal of tars and oils, and steam stripping of phenols and ammonia.
Biochemical removal of phenolics, thiocyanates, and cyanides from sour
waters is also practiced.
This "cleaned" water, containing residuals, and in excess of quenching
recycle requirements is next discharged to evaporation ponds and/or munic-
ipal sewers. Dephenolization of waste water can be economically less
onerous if phenol, cresols and xylenols can be recovered as marketable
byproducts.
Waste water, called "crude ammonia liquor" is also produced in the
coke oven off-gas, typically at a rate of about 113 liters/MT coke produced.
At the modern Weirton Steel Brown's Island plant, this liquor is stripped in
a crude liquor still with a lime leg (Ref. 38).
Additional waste water is generated at the sulfur recovery and light oil
plants at the coke batteries.
Biological waste water treatment at Brown's Island requires control of
pH, phenols, cyanide, BOD, and temperature. The biological treatment
occurs in aerated basins in an activated sludge process said to convert 99%
of phenolic wastes to COz (Ref. 38). About 98% of the sludge removed in a
clarifier is recycled to the aeration basin; the remainder is discharged to
municipal sewers, or trucked to landfill.
Coal
Actual coal-to-coke yields depend on the efficiency of each coke oven
facility. As a "worst case" figure, we accept the previously stated ratio of
-------
30 kg metallurgical coking coal to be mined in order to produce 21.4 kg coke
(needed to manufacture 1 kg of solar cell Si). This coal may be stripped or
deep mined. The potential variety of mining locations and techniques makes
it impossible to quantify the environmental impact of the coal mining needed
to supply the coke ovens.
Blackwood and Peters (Ref. 39) have measured total respirable par-
ticulate emission factors for one unidentified coal surface mine and for one
coal storage facility. Translated into kg/MT equivalent Si, their data is
presented in Table A-4. We cannot predict the source of coking coal to be
used in year 2000, nor do we know how typical these data are of present or
future coal stripping. But based on the data of Table A-4, we conclude that
coal stripping and coal storage are trivial fugitive particulate emission
sources -on any national scale, compared with current practice in coke pro-
duction. Locally, coal stripping and storage can, however, be significant
dust sources.
Although mining and reclamation technologies are advancing, there is
some evidence that regulatory standards are relaxing at some levels of
government.
Briefly, effects of strip mining include:
Hydrologic Modification (Surface and Subsurface)
Land Erosion
* Surface Water Siltation
Decreased pH and Increased 804 in Surface Water
Fugitive Dust Emissions
Removal of Land from Productivity
Destruction of Habitat
Noise Pollution
Accelerated Road Deterioration
Indirect Socio-Economic Effects.
Although deep mining appears to be more "benign" than stripping, it
can lead to hydrologic modification. Most notable is the well publicized de-
terioration in water quality due to acid mine drainage from deep drift mines.
Currently deep mining is more hazardous than stripping both accidents and
respiratory ailments contribute to disablements and fatalities. Indirect
socio-economic effects also occur. Subsidence and mountainside slumps
occur, and contribute to removal of land from productivity.
Both forms of mining create slag heaps ("gob piles"), some of which
ignite spontaneously and smoulder for years. Safe and economically attrac-
tive means of extinguishing large gob pile fires apparently do not exist. The
major noxious combustion product is SOz- Leachate from coal storage piles
as well as gob piles contribute to surface water pollution.
33
-------
Coal refuse dams are infamous hazards in the aftermath of West
Virginia's Buffalo Creek disaster. Aside from such dramatic events, coal
fines lagoons generated by coal cleaning plants irretrievably remove some
land from future use. Frequently these areas were fertile Appalachian
valleys.
Coal cleaning plants are improving. Nevertheless, they are locally
significant sources of suspended particulates, fugitive dust, and noise, as
well as energy consumers. Coal pile leachate contributes to water pollu-
tion.
New source performance standards for coal preparation plants have
been promulgated in the Federal Register (Ref. 40). Because these are pol-
lutant concentration (or effect) standards rather than process weight limits,
they are not convertible to emission factors without plant-specific informa-
tion.
Effluent limitations guidelines (EL.G) for coal preparation plants (Ref.
41) permit no pollutant discharge. For coal storage, refuse storage, and
preparation plant "ancillary areas," the ELG based on BPCTCA lists "con-
centration" limits on iron, manganese, TSS and pH.
The EL.G for acid or "ferruginous" mine drainage (Ref. 42) lists
BPCTA "concentration" limits identical to the BPCTA of coal preparation
plants. For alkaline drainage, total iron, TSS and pH are also identical.
Dissolved iron and total manganese are not listed.
duct.
These limitations cannot be translated into kilogram per ton of pro-
For definitive discussions of environmental effects and mitigating
technologies in the coal mining and preparation sectors, consult Refs. 34
and 43 through 46. A standard annual survey and statistical compilation
concerned with coal resources and industries is the Keystone Coal Industry
Manual (Ref. 47).
Atmospheric Discharge of "SiO" Dust
SiO dust produced during arc furnace operation is the largest solid
waste stream by weight. Like SiCt^, however, it has some potential for by-
product utilization, for example, in ceramics and mineral wool insulation
production.
The production facility chosen as our basic module, producing 100 MW
of Si annually, releases to the atmosphere 1.43 kg/h of SiO dust. The par-
ticle size distribution is unknown, and needs investigation. Annual release
would be 12.5 MT.
As a solid waste stream, this factory produces hourly about 286 kg of
impure SiO comprising:
34
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Substance
SiO
Si
At
Ash
Ti
Weight (%)
77.4
14.2
4.3
2.5
1.6
Undoubtedly there are trace constituents; At and Ti may be wholly or partly
oxidized; and some of the SiOrSi may be present as SK>2.
Annual production of this waste stream is 2500 MT. Not only are
ceramics and rock wool industries possible customers, but this tonnage may
be also attractive to the ferrosilicon industry.
The collected dust has to be handled in closed and exhausted conveyors
and transport equipment. Depending on specific end use, the dust could be
mixed with water and handled as a slurry.
Silicon monoxide is probably a misnomer. Unquestionably it exists in
the gas phase. Normal "SiO" in the condensed phase is known now to be 1:1
Si + SiOz, unless rapidly dry quenched (Ref. 48).
Rapidly dry quenched "SiO," whatever its composition may be, is a
brown, amorphous, pyrophoric solid which burns to SiO^ in air (Ref. 48).
Kolderup (Ref. 49) states flatly that all Si fume and SiO vapor over open sub-
merged arc ferrosilicon furnaces are oxidized to SiO2 in air.
The implications of this discussion are that: (1) the principal industrial
hygiene problem arises from respirable SiOz; (2) that any air pollution con-
trol device which permits a rapid dry quench of Si-SiO furne introduces a
fire hazard; (3) any dry collector (such as a fabric filter) should be far
enough downstream to absolutely ensure that the fume has been 100% oxi-
dized to SiO2 prior to collection; and (4) a wet scrubber would eliminate any
fire hazard due to pyrophoric dust if (3) is not a practicable solution.
It is possible that occasional baghouse and flue fires may occur (and
may have occurred) due to SiO accumulations. Kolderup and other writers
do not comment on this point. X-ray diffraction laboratories can distinguish
positively and rapidly between amorphous SiO dust and poly crystalline SiO +
Si dust. Such laboratory analyses are not suitable for continuous flue gas
monitoring, however. Development of some sort of ignition or flammability
test on samples continuously extracted from ventilation hood ducts seems to
offer some promise if control device fires become a. problem.
The Norwegian ferrosilicon alloy industry is a major source of ex-
perience in silicon alloy production and fume control. Ferrosilicon pro-
duction there is one of the most important sources among all industrial
fume releases (Ref. 49).
35
-------
In the U.S., however, in a ranking of particulate pollutant releases by
various measures of objectionability (Ref. 50), Si and ferrosilicon production
received no specific ranking at all among major sources based on:
Cummunity Complaints
Increased Ambient Urban Background Toxicity
Soiling
Materials Deterioration
Relation to Federal and State Air Quality Goals
Overall Objectionability.
Nevertheless, Si production in submerged electric arc furnaces pre-
sents important and difficult air pollution control problems.
Chief reasons are (Ref s. 49 through 51):
Large Gas Volumes
Small Particle Size (0.01 to 4.0 /jtm)
Proportional Relationship with Power Consumption
High Electrical Resistivity (> 10*° ohm-cm)
High Emission Rates (290 kg/MT)
Discontinuous Emissions Due, for Example, to
Charging Cycles.
In this industry, power consumption (rather than product tonnage) is
the conventional measure of capacity.
In 1971 only two U.S. plants were devoted exclusively to Si production.
Located in Oregon and Alabama, both were small less than 25,000 kW (Ref.
51). Although some larger U.S. plants, located in Washington and Ohio, ex-
ceed 75,000 kW, they produce other alloys in addition and their Si capacity
is unknown. The module used in this report is 17,000 kW. This is typical
of Si-only facilities in the U. S.
Figure 8 is a stylized view of an open submerged arc furnace. Details
vary from the air pollution control point of view. The most important likely
variations are the semi-closed furnace and the sealed furnace (cf., Refs. 50
and 51). Many aspects of air pollution control in Si production are dis-
cussed in elaborate detail in the two references cited above, and will not be
repeated here.
Table 5 presents ground-level particulate concentrations which may
occur at the stated distances from a silicon furnace. Table 6 shows for
comparison, the National Primary and Secondary Standards for particulates.
The method of arriving at the data of Table 5 clearly is crude (cf.,
footnote). Further, the figures given for 0 km relate to industrial hygiene,
rather than environmental quality.
36
-------
Electrodes
Mix Feed!
Chute I
(Typical)
i r i i i i i i » i i T
To Baghouse, Venturi
Induced Air
Charge Material
Refractory Lining
Shell
Crucible
s Tap Hole
*" " >
Ladle
Figure 8. Submerged-arc furnace for silicon production (Ref. 51 modified)
37
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TABLE 5. ESTIMATED GROUND LEVEL PARTICULATE
CONCENTRATIONS CAUSED BY 17,000 kW
OPEN SUBMERGED ARC SILICON FURNACE
WITH BAGHOUSE COLLECTION.
Emission Averaging Estimated Ground Level Particulate
Rate Times Concentration at Specified Distance
(kg/h) from Source (jig/m^)*
0 km
**
2.31 24 hr 875
1 yr 117
0.3 km
117
15
2.0 km
12
1
20 km
1
Obtained by down-scaling from data for a 7.92 kg/h source given in Ref. 51.
**.
SiO plus flyash from furnace hood baghouse.
TABLE 6. NATIONAL PRIMARY AND SECONDARY AMBIENT AIR
QUALITY STANDARDS FOR PARTICULATE MATTER
(REF. 53)
Time
Period
24 hr*
, ##
1 yr
Concentration,
Primary
Standard
260
75
/jlg/m-5
Secondary
Standard
150
60+
Maximum 24-hour concentration not to be exceeded more than once a year.
£
Annual geometric mean.
To be used as a guide in assessing implementation plans to achieve the
24-hour standard.
Nevertheless, we see at 0.3 km that the additional atmospheric par-
ticulate burden contributed by a single silicon submerged arc furnace is
comparable in magnitude to the secondary 24-hr standard and nearly half
of the primary 24-hr standard.
Ferrosilicon and Si facilities commonly will be located in urban indus
trial areas which already have many particulate sources. Unquestionably,
38
-------
additional Si submerged arc furnace capacity in the U.S. will adversely im-
pact achievement of air quality goals. The fraction of such furnaces actu-
ally ascribable to solar cell Si is irrelevant.
New Source Performance Standards for Ferroalloy Production Facil-
ities (including elemental Si) were promulgated on May 4, 1976 (Ref. 52).
Paraphrased, these standards are:
NEW SOURCE PERFORMANCE STANDARDS
SILICON PRODUCTION IN ELECTRIC SUBMERGED ARC FURNACES
(PARAPHRASED FROM REF. 52)
STANDARDS FOR PARTICULATE MATTER
Mass Loading gases shall not exit from a control device and contain
particulate matter in excess of 0.45 kg/MW-hr while silicon metal is
being produced.
Opacity gases shall not exit from a control device and exhibit 15%
opacity or greater. Nor shall there be discharged into the atmos-
phere from any dust-handling equipment any gases which exhibit 10%
opacity or greater.
Visible Emissions gases shall not exit from an electric submerged
arc furnace and esca'pe the capture system and be visible without the
aid of instruments during periods when flow rates are being established.
Nor shall gases escape the capture system at the tapping station and
be visible without the aid of instruments for more than 40% of each
tapping period. (There are no limitations on visible emissions under
this subparagraph when a blowing tap occurs.) The requirements under
this subparagraph apply only during periods when flow rates are being
established.
These mass loading restrictions would permit short term emissions
at the rate of 7.65 kg/hr from the 17,000 kW furnace in our model plant.
The average emission rate we have stipulated is 2.31 kg/hr (SiO and fly ash
combined). This leaves little allowance for transients, failure of control
equipment to meet specifications, operator error and maintenance lapses,
etc.
We conclude that this 17,000 kW reduction plant would comply with
current NSPS for SiO plus fly ash, and by itself would not cause airborne
particulate loads to exceed national primary and secondary ambient air
quality standards. At the same time, the safety margin for NSPS compli-
ance is not large. Further, it would be difficult to continue in compliance
with National Primary and Secondary Air Quality Standards in regions or
communities in which new submerged arc furnaces typically would be in-
stalled.
If a venturi scrubber or other wet collector is used in addition to or in
lieu of fabric filters, there are Effluent Limitations Guidelines (ELGs) which
39
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are applicable (Ref. 54). However, these limitations are aimed primarily
at the chromium and manganese released in ferrochrome and ferro man-
ganese production. Those portions of the ELGs relevant to Si open electric
furnaces are:
Effluent
characteristic
Maximum for
any 1 day
Average of daily values for 30
consecutive days shall
not exceed
kg/MWh
TSS
PH
TSS
pH
0.319
Within the range 6.0 to 9.0
0.024
Within the range 6.0 to 9.0
BPCTCA
0.160
BATEA
0.012
The "Best Available Technology Economically Achievable" (BATEA) limits
apply to New Sources. The TSS limits translate to 194 (BPCTCA) and 14.6
(BATEA) kg/MT solar cell silicon.
Si Dust
Depending on end use, Si fines from cutting and shaping operations in
later production steps may be combined with the SiO-Si fines waste streams
described above.
Recycle of Si and SiO
The combined Si-SiO waste stream represents 182 kg/h elemental
silicon or 389 kg/h pure SiO^. This is about 25% of the quartz feedstock.
With due attention to progressive accumulation of undesirable contaminants,
serious attention should be given to recycling this material to the sub-
merged arc furnaces.
SiO and Si fine particulates are low-density, excellent insulating ma-
terials. Fine silica particles are presently being sold for high quality
thermal insulation. This material could also be used as lightweight filler
in concrete or bricks.
40
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Carbon Monoxide
All CO is expected to be combusted in a waste heat boiler. Flares are
also usable. The relevant NSPS is given in Ref. 52 and no compliance prob-
lems are anticipated.
Oxides of Nitrogen
No provisions of the NSPS relate to NOX. The model plant will emit
10.84 kg/h from its waste heat boiler. Although this is low, it could be re-
duced further by combustion modifications. Flue gas scrubbing for NOX is
not yet common, and does not seem economically justifiable for this source.
Sulfur Dioxide
The sulfur content of the coke used in the arc furnace would exit the
process as SC>2 from the waste heat boiler. The concentration is very low,
dependent on coke sulfur content, and may be low enough to meet environ-
mental standards without further treatment. There are no standards for
SC»2 or SOX in the NSPS for electric submerged arc furnaces. The SC>2
concentration in the combustion gases from the waste heat boiler will be
0.91 g/m , assuming the nominal coke composition listed on the flow sheet.
However, actual mass of SO2 discharged hourly is less than 10 kg/h.
Methods of reducing SOX include coal cleaning prior to coking, using
the lowest-sulfur metallurgical coals available, and flue gas desulfuriza-
tion.
Fluorine Compounds
SiF4 gas is produced by the etching reaction of HF-HNC»3 on the wafer.
At present, the best method of removing the gas is reaction with Ca(OH)2 in
a wet scrubber, producing CaF2» which is relatively insoluble (~20 ppm).
The CaF2 may be placed in a land fill.
Alternatively, a recycle market may develop, since the CaF2 may be
rather pure compared with domestic fluorite deposits. Higher grade fluor-
spar (fluorite) mined in the Rosiclare and Cave-in-Rock districts, Illinois
and Kentucky, and Northgate district, Colorado, for example, are now used
in the metallurgical and glass industries. Metallurgical grade fluorspar
must contain at least 85% CaF2, certainly obtainable from this process (Ref.
55).
Domestic resources of fluorine are estimated at 5.4 x 10 MT. Based
on the present ratio of domestic production to demand, the presently known
U. S. resources of fluorine in the form of fluorspar will be depleted in 25 to
30 years (Ref. 56). This suggests that by year 2000 there will probably be
greatly increased emphasis on byproduct fluorine recovery in all segments
of industry.
41
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Environmental Effects of In Situ Damage
Silicon solar cells are non-toxic and stable. If massive damage to
solar panels occurs, the potential environmental effects would be limited to
disposal of debris.
ENERGY AND ENVIRONMENTAL SUMMARY
Table 7 summarizes emission factors (kilograms of pollutant per
metric ton of solar cell Si) and total mass of pollutant emitted annually. The
latter value is based on a solar cell production rate of 4500 MW of solar
cell Si manufactured annually. For comparison, Table 8 presents the mass
of primary pollutants from 9000 MWe coal fired steam electric generation.
In Table 1 we saw that 4500 MW of Si cells would, at steady state, maintain
9000 MWe electric generating capacity.
In Table 9 the data of Tables 7 and 8 are organized differently in
order to illuminate the air pollution impact of Si solar power generation.
We see that:
Particulates generated directly in Si and Si-cell manufacture
(960 MT/yr) represent only 68% of the total resource and
manufacturing-generated particulates (1410 MT/yr).
The very large power demands of Si and cell production add an
additional, and overwhelmingly more important contribution
(approximately 12,100 MT/yr).
When mining, manufacturing, and power generating are con-
sidered together, we see that Si photovoltaic power seems to
be "57% as dirty" as coal-fired power.
With respect to SOX and NOX, Si photovoltaic power is "57% and
53% as dirty," respectively, as coal-fired power.
The reader should realize that our numbers for coal-fired emissions do not
include'any secondaries (coal mining, steel and cement manufacturing, etc.),
while those for Si do. This is not basically why the comparison seems un-
flattering to Si, however.
Basically, the problem is that Si production is hugely energy intensive.
Any improvement in technology which significantly improves the energy
efficiency of Si production is likely to make major reductions in net atmos-
pheric pollution. This becomes obvious when we make the comparison as
in Table 10.
Other pollutant releases are not directly comparable. The very large
quantity of oils and lubricants would not be released to natural waters; con-
sequently no comparison can be made fairly with the 304 MT/yr oil and
grease discharges from coal-fired plants.
42
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TABLE 7. PRIMARY AND SECONDARY POLLUTANTS RESULTING FROM 4500 MW/YR
SILICON SOLAR CELL PRODUCTION
QUARTZ1TE MINING
Atmospheric Farticulate*
Wastewater**
COAL STRIPPING
Atmospheric Particulate*
Wastewater
COAL STORAGE
Atmospheric Particulate1
Wastewater
COAL PREPARATION
Atmospheric Emissions
Wastewater
COKING*
Atmospheric Emissions
Suspended Particulate
SOZ
CO
Total HC
Aromatic HC
NH3
H2S
Cyanides (as HCN)
Phenols
Pyridine Bases
Tars
NOX
Wastewater
SILICON AND SILICON CELL MANUFACTURE*
Atmospheric Particulate
Gaseous Emissions
CO,
NOX (as NO)
S02
Liquid Wastes
Oils and Lubricants
Aqueous Na2 5iO3
Solid Wastes
Si
SiO
Flyash
Ti (Unknown Oxidation State)
Al (Unknown Oxidation State)
SiC
Clay
Steel
Adhesive s
AI203
CaF2
Phosphorus Residues
kg/MT Solar Si
11.62
Nil
0.75
f+
0.20
++
t
t
27.9
11.8
1.0
++
5.8
0.6
6.8
1.1
0.6
0.004
++
-n-
t
86
71000
380
340
8800
180
1400
7900
260
170
2100
8000
8000
2300
180
1400
M-
Trace
MT/yr
(4500 MW/yr)
129.0
Nil
8.3
-H-
2.2
++
t
t
310
131
11
++
64
7
76
12
7
0.04
++
++
t
960
792000
4000
4000
98000
2000
28000
87000
3000
2000
5000
89000
89000
26000
2000
16000
++
Trace
See Table A-2 for elemental composition. Datum is for quartzite quarrying.
Based on BPCTCA guideline for sand and gravel mining and processing.
+ See Table A-4.
Known pollutant, but no numerical data available.
^Available data, limitations, or standards are expressed in terms of effluent concentration
and cannot be converted to mass emissions or discharges (see text).
Byproduct slot ovens with "control" and wet quench. Much domestic coke capacity is
without control.
Excludes "wastes" destined for byproduct utilization. Some pollutants have been
recombined into different categories than those in which they appeared in Table 3.
43
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TABLE 8. PRIMARY POLLUTANTS FROM 9000 MWe POWER
FROM COAL FIRED STEAM PLANTS
Pollutants
Emissions,
MT/yr
Air
Parti culates
NO
x
Cd
2.86
0.24
1.67
0.43
10'
t
10-
Water
TSS
Oil/Grease
As
Cd
Cr
Cu
Fe
P
Pb
Se
Zn
608
304
143
10.0
7.88
0.36
0.36
197
106
42.6
39.4
**.
*
1% of projected electricity generation capacity for the year 2000.
Emission factors for coal fired plants assume the application of the con-
trol technology in the relevant NSPS and Effluent Limitations Guidelines
(Ref. 57) and Development Document (Ref. 58). Trace metal emission
factors were derived from recent literature (Refs. 59, 60).
44
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TABLE 9. COMPARISON OF THREE CRITERIA POLLUTANTS
AS EMITTED IN SILICON CELL AND FEEDSTOCK
PRODUCTION, AS EMITTED IN ELECTRIC POWER
GENERATION TO SUPPLY SOLAR CELL PRO-
DUCTION, AND AS EMITTED BY 9000 MW COAL-
FIRED POWER GENERATION.
MT/yr
Pollutant and Source
Solar Cell
Silicon Mfg.
9000 MW Coal-
Fired Power*
Atmospheric Particulate
SiOz Mining and Processing 129.0
Coal Stripping 8.3
Coal Storage 2.2
Coal Preparation **
Coke Production 310.0
Silicon and Si-Cell Mfg. 960.0
Subtotal 1,409.5
Power to Produce Si and Si Cells 12,100. 23,800
Total 13.500
Sulfur Oxides
Coke Production 131.0
Silicon and Si-Cell Mfg. 4,000.
Subtotal 4,131.
Power to Produce Above 145,400.
Total 149,500. 286,000
Nitrogen Oxides
Silicon and Si-Cell Mfg. 4,000.
Power to Produce Above* 84,900.
Total 88,900. 167,000
Includes primary (direct) pollutants only, and none of the secondaries.
#*
Available data, limitations, or standards are expressed in terms of
effluent concentration and cannot be converted to mass emissions or
discharges.
See Table 8. Assumes coal-fired power.
45
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TABLE 10.
SILICON RELATED EMISSIONS AS % OF EQUIVALENT
COAL-FIRED EMISSIONS.
Silicon Related Emissions as %
of Equivalent Coal-Fired
Emissions
Including Si Power Demand
Not Including Si Power Demand
Particulates
56.7
5.9
SO
X
50.8
1.4
NO
X
53.2
2.4
Quantities of cooling water are required for the model 4500 MW solar
cell production level economy. In all, 79.5 x 10" kg/h of cooling water is
needed, discharging waste heat equivalent to a single 3500 MW power plant.
This is less than, but not trivial compared to 9000 MWe coal-fired power
plant needs.
The total power delivered to this plant, from outside generating sta-
tions, is 4574 MW. These outside generating stations reject waste heat,
also. To a crude approximation, the total waste heat rejection, inside and
outside the plant, is about that of an 8074 MW power plant. This should be
compared to the 9000 MW generating capacity the silicon is meant to replace.
Again, we see that any successful efforts toward improving energy
efficiency of Si production are likely to have substantial environmental
benefits.
In a comparison report (Ref. 61) we examine the materials require-
ments consequent to actually using Si solar cells. For a 1000 MWpk silicon
photovoltaic plant with concentration and one-axis tracking the following
major structural materials would be needed:
835,000 MT steel (for structures)
260,000 MT cement (for foundations)
164,000 m3 water (for concrete)
2,400 MT Li (for batteries)
In any thorough-going assessment of residuals, extraction and manu-
facture of such materials must be included, both for solar and for its
alternatives.
IMPACT ON NATURAL RESOURCES
Production of arrays which will put out 4500 MW (peak) using 0.15 mm
thick, 14% efficient, Si solar cells with overall production yield of 34.8% re-
quires 32.14 x 10" kg of pure polycrystalline Si. This is about 25 times the
estimated total semiconductor Si production in 1974 and 38% of the 1968
United States production of 99% pure Si (metallurgical grade). The U.S.
reserves of SiO2 which may be used to produce Si are so great that they
have not been completely cataloged.
46
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POLLUTION CONTROL TECHNOLOGY READINESS AND PROJECTIONS
A reduction in Si requirements may result if technological improve-
ments can be realized: (1) increased utilization of Si, that is, more area
from a given mass of Si, and (2) increased efficiency-more output/area. In
the ERDA-JPL Low Cost Silicon Solar Array Project (7), Task II, the prob-
lem of obtaining increased utilization of Si is approached several ways. Of
these the most promising are; (1) upgrading the single-crystal ingot cutting
used in this study, which also yields more area at higher efficiency, (2) edge-
film-growth (EFG) of ribbons, and (3) chemical vapor deposition of poly-
crystalline Si which produces the greatest area but very low efficiencies.
Of these, only the latter will make a significant reduction in Si re-
quirements. But that reduction will occur only if cells of greater than 5%
efficiency can be produced assuming 35 /j.m thickness and 50% utilization
of polycrystalline Si.
Edge-film growth of ribbons offers some labor economies in producing
area but the yields are not significantly better than ingot slicing and prob-
lems in producing cell quality ribbon need to be overcome. Sliced single-
crystal ingots offer the best near term potential for both increased material
utilization and higher efficiency.
During wafer slicing the major environmental problems are: (1) safe-
guarding employees from breathing aerosol oils containing SiC abrasive,
and (2) handling and disposal of the sludge which contains oils, clay, alumina,
adhesive, abrasive, Si and steel. Currently the employees are protected by
placing an exhaust fan over the saw to collect the vapors and pass them
through an expanded metal screen which traps them. This method appears
to be adequate for current levels of production. However, more effective
methods such as fabric filtration may be required for large scale pro-
duction. Disposal of cutting sludge should involve extraction of oils for re-
cycling before disposal into land fill.
The HF-HNO3 etching of the Si prior to remelting produces SiF4 gas.
The handling and disposal involves collection of the gas and passing it
through Ca{OH)? to produce CaF2 which is relatively insoluble in water.
Current methods of collection, neutralization, sampling and disposal appear
to be technically adequate. It remains to scale them up to handle the in-
creased volumes which will result from large scale production.
Wafer diffusion involves PH3 gas in small quantities which are signifi-
cant from an occupational health standpoint. No environmental problem
seems to exist outside plant perimeters. The chemistry of the diffusion re-
action and resulting products is not completely understood, but appears to be
adequately controlled for current production levels. Other products con-
taining phosphorus collect on the furnace tube near the exit. With increased
production these probably would be economically recycled.
47
-------
ENVIRONMENTAL CONSEQUENCES AND CORRECTIVE
ACTIONS - SILICON
High energy consumption for
Si rod deposition
HF removal of oxide for recycle
of ingot to remelt and SiF4 gen-
erated present potential personnel
hazard
Phosphine gas during wafer dif-
fusion and waste disposal
Si and abrasive dust in oil
aerosols
" byproduct stream
SiO in dusty off-gas
R&D on process improvement
Recommend safe handling methods
and safety devices. Scrub
with Ca(OH)2
Recommend handling methods and
safety devices to protect personnel
Limit personnel exposure by venting
and filtration
Develop effective technique for con-
verting Si fluorides to SiHCIj as
part of an in-plant recycle stream
Determine if SiO is true composition,
and if it constitutes fire hazard in dry
collectors. If affirmative, develop
on-line SiO monitor, and provide ap-
propriate collection system modifi-
cations.
48
-------
SECTION 5
CADMIUM SULFIDE PROCESS
GENERAL PROCESS DESCRIPTION
Raw Materials
The main source of cadmium for all cadmium alloy products is the
smelting and refining of zinc ores (Refs. 62 and 63). The cadmium by-
product concentrates are in the form of fumes from zinc calcine sintering
plants (pyrohydrometallurgical zinc refining) or precipitates from zinc
electrolyte in electrolytic zinc refining. Details of the principal techniques
used in mining and refining zinc ores and separating the cadmium-rich
streams are given elsewhere (Refs.63 through 67). The environmental im-
pact of this processing (Refs. 64, 65, 67 and 68) will not be considered, how-
ever, because these emissions would exist whether or not cadmium was
used as a byproduct for solar cells.
The other important feedstock in the production of CdS is raw sulfur.
Readily available emission factors from the manufacture of sulfur by the
Claus process indicate that sulfur dioxide is the major environmental prob-
lem. SO2 emissions from uncontrolled Claus plants are 62 to 162 kg/MT
of sulfur produced or 2 kg/MT from a controlled source (Ref. 69).
Secondary raw material requirements include process water, fuel oil
and make up quantities of sulfuric acid and argon. Hydrocarbon vapors
emitted during the storage and transfer of fuel oil are 0.21 kg/1000 liters
of fuel oil (Ref. 69). The Environmental Protection Agency performance
standard for SO2 from sulfuric acid plants is 2 kg/MT of 100% sulfuric acid,
while the standard for acid mist emissions is 0.075 kg/MT of acid (Ref. 70).
Substantial quantities of structural and protective materials are also
required to support the CdS photovoltaic material. These will include
copper, glass, zinc, lead and stannous oxide. Emission factors associated
with the mining, smelting, refining and finished production of these ma-
terials can be estimated from (Refs. 58, 64, 65, 68 and 69). This has been
done for each of these materials and the results are given in Table 11. In
all cases, the emission factors were determined by assuming that the most
advanced control technology is applied since the manufacturing will be done
in the last decade of this century.
^Depending on the information available, emissions were estimated with the
application of: NSPS, BATEA, BPCTCA, other controls, no controls; in de-
creasing order of priority.
49
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S SOLAR CELLS*
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Process Description
Cadmium extracted from zinc concentrates is a starting material for
the process assumed for this study. The recovery of cadmium and other
byproducts in the zinc refining industry is receiving increased attention be-
cause of pollution considerations. A recent comprehensive EPA report
(Ref. 71). and an industry survey by the Bureau of Mines (Ref. 72) describe
the principal processes in refining Cd and quantify pollutants and cadmium
availability. The process flow diagram and material and energy balance for
producing pure Cd, shown in Figure 9, is based on a hypothetical "Byproduct
Cadmium Subsystem" (Refs.62, 71 and 72).
In Figure 9 cadmium processing begins with receipt of densiiied cad-
mium fume from the roasting and sintering of zinc concentrates, which is
considered a necessary part of zinc processing. The chief constituents are
the oxides of zinc, cadmium, lead and copper; relative quantities of these
constituents are given for illustrative purposes. Expected contaminants are
listed, and, where possible, the chemical state, combined anions and relative
concentrations of the emissions are identified. The Cd fume is collected
and wet- milled and leached with sulfur ic acid to extract cadmium and other
acid soluble components.
Lead sulfate, silver, gold and trace metals remain as solids and are
filtered out. The sulfate liquor is then neutralized with zinc oxide which
precipitates oxides of antimony and arsenic and hydroxides of cobalt, nickel
and thallium. Sodium chlorate oxidizes ferrous ions to ferric which are
precipitated as
The antimony and arsenic oxides in the above precipitate are a poten-
tial problem; therefore the filter cake should be either processed further for
byproducts or collected in an environmentally safe landfill.
The filtrate is next treated with a limited amount of zinc dust to pre-
cipitate copper, but not cadmium. After centrifuging to remove the copper,
more zinc dust is added which precipitates the cadmium sponge, which is
then washed to remove zinc sulfate, briquetted and purified by distillation.
The oil-fired retort will emit stack gases from combustion with air; and en-
trained cadmium fines (<_8 JA) which are rapidly oxidized {Ref. 67). The waste
washwater containing very low concentrations of zinc sulfate and cadmium
can be sent to an evaporation pond. Sites for and amounts of cadmium losses
exhibited in Figure 9b were determined from pages 272, 273, and 382 of Ref.
67.
Since sulfur and sulfuric acid are already produced in large quantity
and are available in high purity, solar cell production is not expected to sig-
nificantly affect those industries. In addition the zinc smelting process pro-
duces SO2 which can be used to make sulfuric acid which is consumed in
leaching of the cadmium fume.
The pure cadmium sulfide is produced by directly reacting cadmium
vapor, in an argon gas carrier, with sulfur vapor in a reaction tube. CdS is
condensed in the cool end of the tube and excess sulfur is condensed at a
51
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lower temperature from the argon stream. In the hypothetical process
shown, argon is recirculated and used to cool the reaction tube since the
CdS formation is exothermic.
CdS/Cu2S Solar Cell Production
Two processes have been proposed for large scale production of CdS/
Cu2S solar cells. The back-surface cell process forms the cell by spraying
reagents on the back of glass sheet emerging from the "float glass" manu-
facturing process. The second process forms the cell on the front surface
of thin metal foil with vacuum deposition of the compounds to form the
heterojunction and collection grids. Cross sections of the two cell types
follow in Figures 10 and 11, respectively. Neither of these has progressed
beyond laboratory proof-of-concept experiments. However, there appear to
be no significant technical problems preventing their implementation.
The pollution problems arising from these processes have not been
examined fully because of uncertainty of process details. For this study we
have attempted to identify the hazardous chemicals and suggest further
efforts to assess environmental aspects as process details are developed.
Efficiencies as high as 9 to 10% may be achieved if several chemical
and production problems can be solved. Laboratory samples have been pro-
duced on a. batch basis with maximum efficiencies of 5%.
Lifetimes for CdS solar cells are limited by several factors not cur-
rently understood (Ref. 73). For this study we have projected a 5-year life-
time at 5% efficiency for both cell types.
Back Surface Cell-Spray Processing (Refs. 74 and 75)--
The back surface cell is being developed by D. H. Baldwin Company,
El Paso, Texas. It uses common window glass as the cover on which the
active layers of CdS and Cu£S are sprayed while'the float glass cools, see
Figures 10 and 12.
There are three spray stations which sequentially form a conductive
layer of tin oxide, the CdS layer, and the Cu2S layer. The exact compounds
used in each spray process are proprietary to the Baldwin Company. From
their report, we know that a salt of tin plus other chemicals are used to
form the SnOx layer and that CdCl£ and thiourea plus other chemicals are
used to form the desired CdS layer. Also, a combination of copper acetate
and N.N. dimethyl thiourea plus other chemicals are used to form the Cu2S
barrier. No other data are available on this process, except that Refs. 74
and 75 state it will be necessary to scale up the process before dealing with
the pollution problems arising from the spray process.
Front Surface Cell-Vacuum Processing (Ref. 76)--
The front surface cell, developed by Westinghouse, is manufactured in
sequential stages inside a large vacuum chamber. It consists of a 0.5 micron
zinc coating on 10 micron copper foil substrate, a 5 micron thick CdS layer,
54
-------
SnO Electrode [0.4]
Float Glass
Cover [3000]
Cu Electrode [10]
Inconel Contact
[0.4]*
[ ] Thickness (10~6 m)
* Cover 15% of Cell Area
CdS Layer [2]
Cu^S Layer
[1]
Pb Coating [0.5]
t
Cu Collector [lof
Figure 10. Cross section back surface CdS cell.
55
-------
Cu Collector [10]
Cu Grid Contact [ l]
Solar Influx
*
* I
V
SiO2 Cover [5]
X /,,
CdS Layer [5]
Cu Substrate [10 ]
Zn Coating [0.5]
[ ] Thickness (10~6 m)
* Cover 15% of Cell Area
Figure 11. Cross section of front surface CdS cell.
56
-------
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57
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a 0.5 micron CuzS barrier layer, a 1 micron copper grid contact and 10
micron copper collector bus, and a protective cover of 5 microns of sput-
tered SiOz as shown in Figures 11 and 13. Except for vapor degreasing of
the substrate and grid material, all processes are in vacuum. Hydrocarbon
emissions from the degreasing process will have to b« collected in hoods
and treated. The other effluents are contained within the vacuum chamber
which require periodic cleaning and removal of the wastes. However, most
of these effluents should be of sufficient purity to be recycled.
Because this cell method is only in a proposal stage, little is known
about the process details or environmental consequences which would result
from large-scale production.
MATERIAL CONSUMPTION AND PRODUCTION ESTIMATES
Assuming that U.S. electric power generating capacity for the year
ZOOO will be 900,000 MW (average) the quantities of material required for
the two cell processes are based on the following assumptions:
1. 1% (average) of generating capacity is equivalent to 45,000 MW
(peak), using 5 to 1 peak to average output as shown earlier.
2. Five-year service life means that 9000 MW of cells must be pro-
duced each year. This is twice the amount required for Si for
which a 10-year service life was assumed.
3. 5% cell electrical conversion efficiency (compared with 14% for
Si). This requires that 1.8 x 108 m2 of cells be produced each
year.
The quantities of material required for production of 9000 MW of front
surface cells and 9000 MW of back surface cells are shown in Tables 12 and
13, respectively. Quantities for front surface cells are based on processing
yields taken from Ref. 76. No process yields were available for back sur-
face cells; therefore conservative yield factors were estimated and shown
in Table 13. Quantities in both tables are based on estimated cell area
coverage and thickness.
Note that the quantities are based on end product materials in the
cells. The process chemical quantities are not shown because they were
unavailable due to developmental and proprietary status.
POWER REQUIREMENTS
The energy requirements for the production of some basic materials
used in CdS cell production are listed in Table 14. The data are limited
which reflects the level of development and the proprietary interests of the
developers.
58
-------
CO
59
-------
TABLE 12. QUANTITIES OF FRONT SURFACE CELL MATERIALS
FOR 9000 MW/YEAR (PEAK).
Material
End Product
Copper
Copper
Copper
Zinc
CdS
CUgS
Si02
Use
Substrate
Grid Contact
Collector Bus
Substrate
Coating
Semiconductor
Barrier
Cover
Thickness
(microns)
10
1
10
0.5
5
1
5
Overall Process
Yield (%)
72
64
72
Total Copper
72
60
60
60
Amount Required
(Metric tons)
23,356
252
3.348
25,956
900
7,344
1,368
3, 996
TABLE
13. QUANTITIES OF BACK SURFACE CELL
FOR 9000 MW/YEAR (PEAK).
MATERIALS
Material
End Product
Copper
Copper
Lead
Inconel
CdS
Cu2S
Stannous
Oxide
Use
Positive
Electrode
Negative
Collector
Positive
Electrode
Coaling
Negative Contact
Semiconductor
Barrier
Negative
Electrode
Thickness
(microns)
10
10
0.5
0.4
2
1
0.4
Overall Process
Yield (%)
64
64
Total Copper
64
64
20
20
40
Amount Required
(metric tons)
22,635
3,780
26,415
1,440
149
8,820
4,140
1,260
Si09
Cover
300
40
359,100
60
-------
TABLE 14.
POWER REQUIREMENTS FOR ANNUAL PRODUCTION
OF 9000 MW (PEAK) OF CdS SOLAR CELLS
Power (MW)
Copper
Glass
Zinc
CdS
Cu2S
Other Process Power
ior
**
Total
*Calculated from material production energies given in Ref. 77 and yield
percent in Table 12.
**Calculated from Figure 9.
Estimated based on relative thickness of Cu~S to CdS.
Author's estimate.
ENVIRONMENTAL SUMMARY
None of the proposed CdS cell large-scale production concepts have
progressed beyond the laboratory stage. Process formulations and quan-
tities are proprietary which limits the evaluation of potential environmental
effects. Without precise quantification and accuracy, however, discharge
estimates can be made.
Estimates of the emission and effluents resulting from the annual pro-
duction of solar cells with a generating capacity of 9000 MW/yr (peak) are
shown in Table 15. These emissions are derived from Tables 11, 12 and 13
and Figure 9 and include both primary and secondary (off-site) emissions.
Emissions from the upstream processing of cadmium-rich fumes and pri-
mary emissions from cell-spray or cell-vacuum processing, however, are
not considered.
In order to assess this level of emissions, Table 15 lists the emissions
as a percentage of those emitted from the generation of 9000 MW of electrical
power by conventional coal/fired utility boilers (see Table 8).
##
These emissions will exist regardless of the byproduct usage of the fumes.
Inadequate data are available on process emissions (species or rates).
61
-------
TABLE 15. PRIMARY AND SECONDARY POLLUTANTS GENERATED
FROM THE MANUFACTURE OF CADMIUM SULFIDE
CELLS FOR 9000 MW/YEAR (PEAK)**.
Front Surface
Cells
Pollutants
Air
S02
Particulates
CO
Hydrocarbons
NO
Cd
Water
TSS
Oil/Grease
As
Cd
Cu
P
Pb
Se
Zn
Solids
CdO
ZnSO4
kg/yr %
6.9
9.3
0.4
0.1
1.2
5.2
6.4-
5.6
5.5
1.1
1.4
2.0
5.2
5.2
1.6 .
1.6 *
1U
10
io2
IO3
io3
io1
!02
IO1
io2
IO1
IO1
io2
io2
2.4
0.4
Nil
Nil
Nil
120
0.1
1.8
0.04
1.1
3.9
0.1
0.1
0.1
Back Surface
Cells
kg/yr %+
7.6
6.0
0.5
0.1
1.5
6.3
2.5
5.0
6.1
1.3
1.5
1.8
0.6
3.1 .
3.6 -
1.9
2.0
10
10
io2
io5
io5
io2
IO1
io4
io1
io1
io2
io2
2.7
2.5
Nil
Nil
Nil
146
41.1
164.4
Nil
1.3
4.2
9.1
Nil
0.1
0.1
Excluding emissions from the raw cadmium feedstock (densified, cadmium-
rich fumes from zinc smelters) and primary emissions from cell surface
processing.
^f^f
Equivalent to the production of 8820 MT of CdS for front surface cells and
7344 MT of CdS fpr back surface cells. This represents the demand, in
the year 2000, from solar photovoltaic power plants producing 1% of the
total U.S. demand for electricity (9000 MW).
Emission as percentage of the corresponding rates emitted from coal-
fired steam generators producing 9000 MW/yr of electricity in the year
2000, assuming application of NSPS control technology (see Table 8).
62
-------
Examination of Table 15 indicates that on a gross, national scale, only
the atmospheric emission of cadmium particulates is a significant primary
pollution problem in CdS solar cells. The large suspended solids, oil and
grease effluent discharges are potentially severe secondary pollution prob-
lems if the "back surface" cells are produced. These effluents are generated
in the production of the large quantities of glass needed for the "back surface"
cells.
In addition to the gross national output of pollutants discussed above and
in Table 15, the quantities of heavy metals generated in the production of CdS
cells may create locally severe impacts. A synopsis of the toxic effects of
arsenic, cadmium, copper, .phosphorus, lead, selenium and zinc compounds
is given in Appendix D. Because of its potentially severe local impact, a
more detailed description of cadmium's properties and toxic effects is given
in Appendix E.
Micro-organisms can convert these heavy metals into biologically
active compounds by means of oxidative and reductive reactions catalyzed by
enzymes. For example, arsenic compounds are reduced and methylated by
anaerobes to give dimelthylarsine and trimethyl arsine as volatile products
of extreme toxicity (Ref. 78). A classification of the heavy metals and other
elements according to their susceptibility toward biological activity is shown
in Table 16. As indicated in that table, As, Cu, Cd, Pb, Se and Zn (all CdS
solar cell effluents) are all in the most acute category.
TABLE 16.
TOXICITY AND BIOLOGICAL ACTIVITY
OF SELECTED ELEMENTS (REF. 78).
Noncritical
Very Toxic and
Easily Accessible
Toxic but Very
Insoluble or Very
Rare
Al
Br
a
Fe
K
P
Si
S
As
Cu
Cd
Cr
Ni
Pb
Se
Sn
Zn
Ga
Ti
Zr
Relative tendency toward bioaccumulation and interaction.
The severe toxicity of these metals indicates that careful attention
should be given to their control at every significant source in the process.
More specific suggestions on control technology needs must await more quan-
tiative process designs and/or detailed environmental impact assessments.
63
-------
Currently, best available control technology for atmospheric trace
metal particulate matter is considered to be electrostatic precipators, but
fine particulates are not controlled adequately by this method (Refs. 59 and
67). Fine particulates (<8 microns) are expected to be a significant fraction
of the expected cadmium emissions to the atmosphere (Ref. 60).
The best "wastewater treatment" for heavy metals is zero discharge,
because heavy metals are toxic to micro-organisms in low concentrations
and must be removed before the wastewater can be subjected' to secondary
treatment by biological oxidation. Removal of heavy metals from wastewater
has been successfully accomplished for many years in the metal plating and
finishing industry (Refs. 79 and 80), and this technology may be applied to the
solar cell industry. Heavy ion removal from wastewater consists of pre-
cipitation with lime. Extensive studies on the toxic effects of heavy metals
on biological waste treatment systems were conducted by the Robert A. Taft
Sanitary Engineering Center (Ref. 81) and reviewed by Richard Jones (Ref. 82)
and this continues to be an active subject of research even today.
Some other potential pollutants from front and back surface processes
which are not presently quantifiable are:
Copper Acetate
N. N. Dimethyl Thiourea
Hydrochloric Acid
Hydrocyanic Acid
Chromic Acid
Nitric Acid
Sulfuric Acid
Vapor Degreasing Solvents (Halocarbons)
IMPACTS ON NATURAL RESOURCES
U.S. production of cadmium has declined from 3768 tons in 1972 to
1994 tons in 1975 (Ref. 83). This trend may be reversed as a consequence
of better controls on zinc processing, and increased awareness of environ-
mental hazards of cadmium. The following excerpt from Ref. 71 describes
the complementary roles of increased cadmium production as a consequence
of better controls on zinc processing.
"Trends in the Primary Zinc Industry--
Two trends are evident in the production of primary zinc. First
is the dramatic reduction in the domestic zinc production level over
the last 20 years, and the second is the rapid phase-out of older pyro-
metallurgical smelters in favor of electrolytic plants. Both of these
trends are of course, directly beneficial in reducing the quantity of
cadmium emitted to the air from U.S. zinc smelting operations. A
side benefit of the worldwide switching from retorting to electrolytic
64
-------
zinc is that much less cadmium is being dissipated to consumers as
an impurity in zinc. Since the trend is toward less cadmium loss to
the environment and less cadmium dissipated in the zinc products,
the result is that more recoverable cadmium is becoming available
for the primary cadmium industry. Compared to a ratio of cadmium
to zinc in ore of 0.5%, the following ratios [Table 17] of primary cad-
mium production to primary zinc production indicate that the maximum
ratio is being approached:"
The 6000 tons required for the production of solar cells will not sig-
nificantly deplete the reserves which are estimated to be 164 thousand tons.
The recovery of cadmium is dependent, however, on zinc production, with an
estimated recovery of 5 kg cadmium per ton of contained zinc in zinc re-
serves. The projected annual demand for zinc in the year 2000 ranges from
1.7 to 3.3 x 10" tons (Ref. 84). Maximum cadmium production would there-
fore approximate 16500 tons. Thus the cadmium demand from CdS solar
cells would represent 36% of the market and would exert appreciable upward
pressures on cadmium prices. Solar cell demand, however, could probably
be met.
TABLE 17.
RELATIONS BETWEEN CADMIUM AND ZINC
PRODUCTION IN THE 20TH CENTURY (REF. 71)
Years
Ratio of Cadmium Production
to Zinc Production (%)
1901-10
1910-20
1920-30
1930-40
1940-50
1950-60
1960-68
0.002
0.009
0.056
0.17
0.25
0.27
0.33
Quantities of other materials required for production of CdS are not
expected to impact resource availability (Ref. 84).
POLLUTION CONTROL TECHNOLOGY READINESS AND PROJECTIONS
Complete definition of the environmental considerations awaits devel-
opment of production processes beyond the current paper study/laboratory
sample level. However, current pollution control practices utilized in the
zinc and lead mining, smelting, and refining industries should yield 90% or
better control of cadmium losses from, the incoming feedstock during the
production of CdS. Application of control techniques recommended in Efflu-
ent Limitation Guidelines and New Source Performance Standards, for the
lead and zinc industries (Ref. 57) should adequately control emissions other
than cadmium.
65
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Although CdS is fairly stable the potential reactivity of Cu2S dictates
that cell disposal, after electrical generating lifetime, must be controlled.
The instability of Cu£S may be the cause of the short lifetime that most CdS
cells have experienced. The possibility of slow erosion and dissolution of
discarded CdS needs to be evaluated as well as the feasibility of reclamation.
66
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SECTION 6
GALLIUM ARSENIDE PROCESS
GENERAL PROCESS DESCRIPTION
GaAs Process Description
A hypothetical process has been synthesized based on Refs. 89 and 90
and informal communications with Alcoa and Eagle-Picher personnel. The
process flow? diagram, material balance and power requirements based on
100 MW/yr of solar cell production are presented in Figures 14, 15 and 16.
Gallium Production and Purification--
Gallium is produced as a byproduct of the extraction of aluminum from
bauxite.
Caustic is used to extract sodium aluminate from bauxite in the manu-
facture of aluminum. The aluminate is precipitated and the spent caustic
solution is recirculated for further bauxite extraction. Recirculation of the
caustic solution allows a buildup of gallate concentration to an A£/Ga ratio of
approximately 500/1. This solution is the starting point for gallium refining.
The bulk of the aluminate is fractionally precipitated by neutralizing
the caustic solution by bubbling carbon dioxide through it. The precipitate
is recycled to the alumina process and the filtrate is further treated with
carbon dioxide to precipitate the gallate with the balance of the aluminate.
Leaching of the precipitate with sodium hydroxide yields a gallate-rich solu-
tion from which gallium can be electrolytically deposited. 99.5% pure gal-
lium, in the liquid phase, is collected. Proprietary chemical processes are
used to further refine the gallium to up to 99.999% purity.
The chemical reactions for the processes are:
Aluminum Precipitator:
*0 -I- 2 NaOH + 2
3 H2°
Gallate Precipitator:
2 NaHGaO +
2 Ga(OH)3
67
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o
I
a
ttt
(fl
f^
d
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a
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V
U
o
u
PH
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68
-------
p^tt from focrf ing and
liMltino, topper ores
*i0«>fl.l-2%orseme
Cottrell collection of duits
from copper ore roasting,
and reverbera'ory smelting
i
Flue dusts
(10-30% arsenic}
Packaging, handling.
ond
transportation
\
Residue to
copper furnaces
Split-off point
for byproduct
Amorphous, block,
arsenic 195% AsjOj)
from higher temperotuf»
chc,-tcr: i: rt-rtf stt
Motte to
copper smelter
«astt
First chamber
nonarsenical dust
(divided ir.to "kitchen"
Subchasers ot gradually
declining operating
temperature)
White ors«nicl99.9%AszOj)
to pockosing and market
Figure 15. Arsenic production from flue dusts (Ref. 91).
69
-------
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Leach Tank:
3 H2O -I- Z NaOH
+ 4
Ga(OH)3 + NaOH
Consult Figure 14.
Arsenic Production and Purificatipn
Arsenic is produced in oxide form as a byproduct nnainly from treatment
of the arsenic bearing flue dusts collected from the smelting of such metals
as copper and lead (Figure 15).
The flue dusts of 10 to 30% arsenic content from the smelters are
roasted at 400 C followed by a condensation step to produce crude trioxide of
90 to 95% purity. The crude oxide is vaporized in reverberatory furnaces at
a roasting temperature of 550 C and the refined arsenic trioxide of 94.9%
purity is obtained as the lower temperature fraction in condensing chambers
consisting of a series of kitchens of gradually declining operating tempera-
ture. The nonarsenical dust from the first chamber is roasted and the
higher temperature fraction is re -refined. The refining furnace residue
containing about 20% of the crude oxide change to the furnace is recycled to
the smelting operation.
Metallic arsenic can be obtained by thermal reduction of the refined
trioxide with charcoal in a cylindrical retort and by condensing the metallic
vapor (Ref. 92).
At present, all domestic arsenic is produced at one mill: American
Smelting and Refining Co., Tacoma, Washington. Imports account for about
75% of U.S. requirements (Ref. 29). An increase of 36% in imports would
meet projected needs.
GaAs Compounding (Fig. 16)--
High purity {six 9's) gallium and arsenic are compounded inside an
evacuated quartz bottle. The relative quantity of arsenic to gallium is
slightly below stoichiometric to minimize residual arsenic when the quartz
bottle is opened. The gallium and arsenic are heated independently within
the bottle and caused to react. Some of the arsenic reacts with the SiO2
inside the bottle. The products when the bottle is opened are:
1. GaAs
2. Some gallium which is reused
3. The quartz bottle with some arsenic contamination on the
inner wall.
Part of the quartz bottle is removed. The other part is etched with
HF-HNO3 to remove the arsenic and then it is reused. The amounts cannot
be determined at present due to the proprietary nature of this process. Safe
71
-------
handling and disposal will have to be determined after quantities are estab-
lished; further environmental study is warranted.
The high purity, polycrystalline GaAs proceeds to a single crystal
growing chamber.
Currently the bulk of n-type GaAs used for solar cells is made from
single crystals produced by a modified horizontal Bridgman process (Ref.
93), from melted (1510 K) GaAs and the dopant. This process produces
dislocation-free n-type GaAs crystals which have been doped with germanium.
During growth, which is started in vacuum, arsenic vapor pressure is care-
fully controlled to maintain stoichiometric growth.
The use of quartz as a container for gallium and molten GaAs, and as
a reaction vessel in which GaAs synthesis is carried out, may lead to Si con-
tamination of the GaAs grown in these containers. Si can act as an electron
donor or an acceptor and must be restricted to maintain electronic prop-
erties. Incorporation of Si at a level of a fraction of a part per million can
have a serious effect on the electrical behavior. To suppress such contami-
nation a small diameter tube connects the arsenic reservoir with the boat in
which the GaAs crystal is grown. A large temperature gradient is main-
tained over a portion of this tube to suppress the transport of Ga2O. This
substance, in the vapor phase at GaAs melt temperatures, is formed by the
reaction of gallium with SiO2 in the boat containing the charge. Free Si is
also produced, which can be incorporated into the growing crystal. Main-
taining a high cencentration of Ga^O vapor suppresses the reaction. This
method produces high purity n-type GaAs. Further details of this process
are proprietary.
Preparation of n GaAs Single Crystal for Cell Fabrication--
After the bulk n-type single crystal of GaAs is grown it is cleaned and
sliced into 0.125 mm wafers. The slicing is usually done by reciprocating
multiple band saws; but it is also done with water-cooled diamond saws.
GaAs is much more fragile than Si; and this limits the thinness of wafers.
The kerf material is separated from cutting grit and is recycled by the gal-
lium producer. After slicing, the wafers are cleaned with sulfuric acid and
H202.
Although the resulting wafer is of high purity and good crystal, it has
poor diffusion lengths which lower the response to long wavelength light.
This deficiency is overcome by epitaxially depositing a layer of n-type GaAs
which has good diffusion lengths onto the wafer. This layer then becomes
the base on which p-type (zinc doped) GaAJAs is grown (Ref. 94).
The liquid phase epitaxial (L.PE) growth of the good-diffusion-length
GaAs on the GaS substrate takes place in a vacuum. In a graphite boat, which
has been baked several hours in vacuum! at 1673 K n-doped GaAs is soaked
for two hours with palladium diffused H2 a-t 1120 K. The melt is brought
into contact with the heated GaAs substrate (wafer). The temperature of the
system is then reduced at a controlled rate and the GaAs crystallizes from
the melt onto the substrate (Ref. 93).
72
-------
This is followed by a similar LPE growth of 1 micron of zinc doped
AfGaAs. The zinc dopant diffuses into the n-type LPE GaAs layer producing
the p-n junction below a thin p-type layer. The resulting GaAs-AfGaAs
heterojunction confines electrons generated in the p-type GaAs layer giving
low surface recombination velocity which increases efficiency over a straight
GaAs cell (Ref. 95).
The 1 micron thick AlGaAs is a transparent conductive coating which
permits reduction of surface resistance and the corresponding series re-
sistance of the cell without excessive doping (Ref. 96). This layer thickness
could be reduced to 0.3 micron to minimize optical losses and improve cell
efficiency (Ref. 93).
Contact Application--
The front contact and grid as well as the back contact are formed by
vapor deposition.
Anti-Reflective Coating--
Anti-reflective coatings are required for high performance. The opti-
mum A/R material has not been selected. The cross section of a GaAs cell
is shown in Figure 17.
P-Type
AlGaAs
(LPE)
-Anti-Reflective Coating
'-Contact
N-Type GaAs (Wafer)
Junction
N-Type GaAs
(Epitaxially
Grown Layer)
N-Contact
Figure 17. Cross-section of GaAs cell.
73
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GaAs PROCESS MATERIAL REQUIREMENTS (REFS. 89, 97-98)
Consumption and production are summarized in Table 18 based on
annual solar cell production to generate 4500 MW of peak power and the
process described previously.
Presently, GaAs cells are produced in very low volume using labora-
tory equipment. One producer is installing equipment which will produce
larger cells but this is still of a laboratory nature.
The yields for material processing for GaAs are expected to approxi-
mate those obtained with Si. This is justified by the fact that with the ex-
ception of the liquid phase epitaxial growth steps, the processes involved in
cell justification are similar.
The yields assumed for this study are listed in Table 19.
Using 125 micron cell thickness, 5.32 specific gravity and 29% cell
yield from polycrystalline GaAs, a cell area to weight ratio of 0.43 m^/kg is
calculated. As assumed efficiency of 18% yields 0.078 kW (peak power) per
kilogram of GaAs.
From the assumed life of 10 years, we determine that arrays with 4500
MW (peak output) must be produced each year to generate 1% of year 2000
generating capacity. This is the same result we obtained for Si, but only half
the production required of CdS cells.
A "single" manufacturing facility producing only 100 MW/yr of GaAs
solar cells, which corresponds to the same capacity Si cell facility, would
have 1.271 x 10^ kg/yr of pure polycrystalline GaAs going into cell pro-
duction.
The gallium is shown to be recovered from the spent caustic liquor
from the alumina extraction process. The quantities to be processed are
large which raises the question of availability of materials. Gallium occurs
in bauxite and averages 0.005% in sphalerite from the tri-state area of
Kansas, Missouri and Oklahoma. This means that over 1 x 10^ metric tons
of bauxite would have to be processed each year to get enough gallium for
the above production. Another approach is to look at the world reserves of
gallium with respect to the above requirements. Reference 89 estimates our
domestic gallium reserves at 2.7 x 10" kg. Estimated consumption from
Figure 12 is 2.28 x 10 kg/yr. Obviously, it cannot be done as conceived.
A practical approach using GaAs would require at least a 100 to 1 con-
centration of solar energy to reduce consumption of gallium to less than 1%
of projected available domestic resources. Worldwide resources of gallium
may alleviate the demand-supply problem, but this demand would strain the
industry and impact cost.
The arsenic required sould be no problem compared to gallium. The
required quantity based on a 100:1 solar concentration factor is only about
0.25% of the U.S. annual imports of arsenic (Ref. 97).
74
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TABLE 18. GaAs PROCESS MATERIAL, CONSUMPTION
AND PRODUCTION SUMMARY FOR 4500 MW
Raw Materials
Solids:
As, High Purity
Germanium
Zn
Spent Liquor -Alumina
Extraction:
Na-Ai-jO,
224
NaOH
Na2CO3
NaH2GaO3
(as Ga, 0.297)
H20
Caustic:
NaOH
H20
Gases:
C02
Cutting and Grinding
Abrasive Grit
Clay
Slurry Vehicle
Saw Blades
Mounting Cement
Mounting Blocks
Lubricants
Quartz Reactor Tubes
Quantity,
105 MT
0.296
0.1-0.02
trace
248.
366.
122.
0.607
3059.
12.13
30.32
277.
0.89
0.89
0.89
0.26
0.02
0.16
0.09
0.01
Products and Wastes
Solids:
GaAs Solar Cells
(as Ga, 0.08)
AI203-3H20
H20
Liquors Recycled to
Alumina Process:
Na2A*2°4
NaH2GaO3
Na2C03
H20
Gases:
co2
Cutting and Grinding
Slurry for Reclamation:
Abrasive Grit
GaAs
(as Ga, 0.196)
Vehicle
Cutting and Grinding
Sludge -Discarded:
Clay
Worn Blades
Mounting Cement
Mounting Blocks
Lubricants
Quartz Waste
Quaji tity,
10b MT
0.166
209.
35.5
24.5
0
17.6
3059
12.98
0.89
0.406
0.89
0.89
0.26
0.02
0.16
0.09
0.01
75
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TABLE 19. GaAs YIELDS IN CELL PRODUCTION
,, Net Percent of
item _, , j. IT
Polyc r y s talHne
GaAs as Compounded
GaAs Crystal Yield
Wafer Yield
Cell Fabrication Yield
Electrical Cell Yield
1 00
90
40
32
29
Process
Yield (%)
90
45
80
90
Concentration of incident solar energy for use with GaAs cells has
been discussed by James and Moon (Ref. 95) and is judged feasible for very
high concentration factors.
Efforts to reduce consumption of GaAs are directed toward a process
for liquid phase epitaxial growth of a very thin (i.e., 1 to 3 micron) layer of
GaAs on a low cost substrate followed by LPE growth of p-type GaAIAs.
Similar efforts are proceeding using chemical vapor deposition (CVD) of the
layers. Successful development of either process would reduce GaAs con-
sumption by 96% and, thereby ease the necessity for high concentration
factors.
GaAs PROCESS POWER REQUIREMENTS
Table 20 is a summary of power requirements based on a plant annu-
ally manufacturing 100 MW (peak power) of GaAs solar cells. The power
quantities were collected from Figures 14 and 16 and represent estimates
of the major requirements. Based on an average peak power ratio of 5:1,
the average output of the 100 MW peak power solar cells would be 100 MW/
5 = 20 MW. Energy payback on the cell basis would be 57.98 MW/20MW/
year = 2.90 years. Since concentration of solar energy is a probability with
GaAs cells, the energy payback should be analyzed based on potential con-
centrator designs.
IMPACT ON NATURAL RESOURCES
Gallium occurs widely in nature, particularly in aluminum bearing
minerals. However, because of its very low concentration in even the
highest grade ores (0.002 to 0.008%) processing ores simply for their gal-
lium content is not economical. The two current sources for gallium are
from aluminum processing and from recycling of mine tailings. British
fly ash has been mentioned as a possible source.
The supply of gallium produced from mine tailings may be limited if
large quantities are required. The other products of this recycling include
zinc rich potassium phosphate which is used by fertilizer manufacturers,
76
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TABLE 20. SUMMARY OF POWER REQUIREMENTS FOR PRODUCTION
OF 100 MW/YEAR PEAK POWER GaAs SOLAR CELLS
Power, kW
Slurry Pump No. 1
Slurry Pump No. 2
Slurry Pump No. 3
Filtrate Pump
Filter No. 1
Filter No. 2
Filter No. 3
Gallium Electrodeposition
Gallium Refining, GaAs
Formulation and Single
Crystal Growing
Cell Production
Total
150
150
30
150
250
200
50
17,000
25,000
15,000
57,980
sulfur, and acidic sand which is used to neutralize the basic soil in the region
of the processing plant. The marketability of these products may limit the
availability of gallium from this source.
4
The probable domestic demand for gallium in the year 2000 is 3.2 x 10
kg, (Ref. 98) not including any allocation for solar cell production. To pro-
duce 4500 MW of GaAs solar cells, would require 2.98 x 10^ kg, thus in-
creasing demand by one order of magnitude. Clearly, the level of increased
production required to meet such demand is not feasible because of the de-
pendence of gallium production on other materials such as aluminum and
zinc.
As discussed earlier, large-scale usage of gallium for solar cells is
only practical if concentration factors of 100 or greater are used. With no
concentration, the estimated (Ref. 98) U.S. reserve of 2.7 x 10" kg of gallium
would be expended in meeting approximately 0.1% of the year 2000 U.S. elec-
trical energy requirements.
If gallium comes into greater demand, its cost will surely rise. The
above factors suggest that, at present, large-scale use of GaAs is limited to
applications where concentrators and/or higher cell costs can be accom-
modated.
It may become competitive if efforts to reduce material consumption
and/or to develop high concentration reflectors are successful. The large
77
-------
scale use of gallium arsenide solar cells will require both the use of con-
centrators and the reduction of cell thickness.
ENVIRONMENTAL CONSEQUENCES
The GaAs solar cell production problems which impact the environ-
ment are listed in Table 21. The pollution problems associated with arsenic
production and handling have not been addressed in this study; and, they are
being continuously reviewed by OSHA. Details on GaAs production are
limited; and there are probably hazardous chemicals to be disposed of.
TABLE 21. ENVIRONMENTAL CONSEQUENCES AND CORRECTIVE
ACTIONS-GaAs
Problem
Action
Arsenic production
and handling
HF during etching of
quartz reactor
Requirement for Ga
exceeds supply
Unknown proprietary
chemicals used during
GaAs compounding and
doping and crystal growth
Requires further definition of
safe limits by OSHA
Requires quantification and
safeguards for personnel
Consider concentrators and
alternate Ga conserving cell
production methods
Determine chemicals and
identify environmental action
required, if any
The technology readiness of environmental aspects of'GaAs cell pro-
duction related primarily to the handling of arsenic and the large amount of
process energy required. The extraction of gallium as a byproduct of alum-
inum production is a "clean" process in that it feeds all residual products
back to the aluminum production stream.
One proposed GaAs cell production method involves the use of organo-
metallic materials in a chemical vapor deposition process. Environmental
problems posed by using this and other processes must be examined further.
ECONOMIC CONSEQUENCES
Cost is the principal factor limiting wider application of solar panels.
However, usual comparisons using common utility costs do not take into
account ecological advantages and the ever increasing costs of alternate
energy sources, in part due to environmental controls.
In each solar cell system, the cost of raw materials tends to increase
with demand, since quality or concentration of mined minerals decreases
with consumption.
78
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New process development normally concentrates on making the
process work and places environmental aspects in a lower priority. Care
must be taken to allocate the development and production costs of environ-
mental control to new processes in any economic analysis.
An economic analysis of the solar cell systems was not accomplished
in this study. Costs can be estimated for the process once material re-
quirements are defined, and there is some certainty about which process
will be likely to see major industrial application.
In the case of GaAs, complications arise from the probable use of
solar concentrators which drastically reduce all requirements, but add a
large unknown. System analyses and studies should be conducted to ascer-
tain optical concentration factors and generate feasible design concepts. In
a companion report (Ref. 61) we have examined construction materials and
dollar requirements for a large concentrator Si photovoltaic utility plant.
The support structures and concentrators were found to be the principal
cost elements, rather than photovoltaic material.
79
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SECTION 7
ALTERNATE PROCESSES
The processes discussed in preceding sections are considered to have
the highest probability for large-scale production; however, other processes
are being studied.
One of the major factors affecting the environmental impact of solar
cell production is the amount of material in the finished cell. The amount
of material required to produce the cells is a function of the production
yield. The area of cells produced from supplied material is determined by
the relation:
m
Area to mass input -r =
&
production yield
cell mass to area kg/m*
The peak power-to-mass input (in kilowatts per kilogram) is the pro-
duct of area-to-mass input and cell efficiency.
Programs to reduce the amount of material, increase yields through
the process steps, and increase cell efficiency are all aimed at maximizing
material utilization, which, in turn, will reduce environmental problems. A
discussion of the potential for some of the current and proposed cell pro-
duction methods follows.
Several methods to improve utilization of Si in cell production are
being studied under ERDA/JPL, {Ref. 7) funding. Current production (point
1 in Figure 18) using sliced single crystal Si 0.6 mm thick yields 40 W/rn^
at 10% efficiency per kilogram of polycrystalline Si. This study is based
on 1977 projections (Ref. 16) for 14% efficiency cells 0.15 mm thick yield-
ing 1.0 m2 or 140 W/kg of polycrystalline Si.
The ultimate growth potential for sliced Si wafers is to produce
1.6l m^ of 14% efficiency cells or 225 W/kg of polycrystalline Si as cal-
culated from Ref. 16.
The most probable alternate Si cell manufachiring method is single
crystal ribbons growth from polycrystalline melt by edge-fed growth (EFG).
Currently, this method yields less cell area and lower efficiencies than
sliced crystals. However, if development efforts are successful, the po-
tential is for slightly higher area yields. From an environmental viewpoint,
the principal advantage of this process will be that the waste Si will be in
solid form, not in sludge as it is with wafer slicing.
80
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Projected EFG silicon utilization is for 84 W/kg of polycrystalline Si;
ultimate growth potential is for 265 W/kg as calculated from Ref. 12.
A third method of forming Si into cells is to deposit it by chemical
vapor deposition (CVD). Efforts to achieve high area utilization of the Si
have been successful; but the efficiencies have been very low, owing to the
crystalline formation during deposition. Environmental considerations of
large scale production using CVD are not understood since details of the
process methods and chemicals and energies are unavailable. The current
laboratory samples yield 6.3 m^/kg of polycrystalline Si at 1.5% efficiency.
Clearly, if efforts to improve efficiency are successful, this method will
produce the highest material utilization of the three Si methods.
Zoutendy (Ref. 9V) recently reviewed some R&D directions currently
being pursued to develop low-cost cells. Recent progress has brightened
hopes that doped amorphous Si may be developed successfully (Ref. 100).
For GaAs chemical vapor deposition (CVD) and liquid phase epitaxy
(L.PE) are being studied to improve material utilization. Efforts to produce
cells by growing very thin layers of GaAs either on thin films or on alter-
nate low cost substrates have not been successful; but the work is promising.
The method, discussed in this study, of slicing wafers from single
crystals and growing junction layers by LPE produces 0.42 m2 of cells (76
W/kg) of polycrystalline GaAs, based on 18% efficiency.
Further increases in output using this method are not expected due to
losses during slicing which are similar to those in Si cell production.
Currently, proposed CdS cells already utilize very thin (1 to 5 micron)
layers of active material. Alternate production methods are aimed at im-
proving cell life and efficiency rather than material improvement.
81
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/
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82
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14. Burger, R.M., and R. P. Donovan, Fundamentals of Silicon Integrated
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20. Cooper, J. D., "Sand and Gravel," in Mineral Facts and Problems, Cir-
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24. 41 FR 23559, June 6, 1976, and 40 CFR Part 436, Subpart D.
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26. McGannon, H.E. (ed.): The Making, Shaping, and Treating of Steel, 9th
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83
-------
b. Kemmetmueller, R., "Dry Coke Quenching Proved, Profitable
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"Dry Coke Quenching, Air Pollution and Energy: A Status Report,"
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33, Durham, N. C., 1967.
34. Cavanaugh, G. et al., "Potentially Hazardous Emissions from the Ex-
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35. Schutte, K. A., D. J. Larsen, R. W. Hornung and J. V. Crable, "Report on
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36. a. Economic Commission for Europe, Problems of Air and Water Pol-
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Cleaning Installations Used in the Iron and Steel Industry," United
Nations, New York, 1970, p. 19.
b. Compilation of Air Pollutant Emission Factors, 2nd Ed., AP-42,
USEPA, Research Triangle Park, N. C., 1973, p.7.2-1.
37. "Air Pollution Aspects of Emission Sources: Coke Ovens A Bibli-
ography with Abstracts," 450/1-74-002, USEPA, Research Triangle
Park, N. C., 1974.
38. Weirton Steel, "Brown's Island Coke Plant Pollution Control Facilities,"
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39. Blackwood, T. R., and J. A. Peters, "Relative Impacts of Open Sources of
Emissions," in Proc. of Symposium on Fugitive Emissions Measurement
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Research Triangle Park, N. C., September 1976.
40. a. 41 FR 2232FF, 15 January 1976 and 40CFR, Subpart 4, Sec. 60.250ff.
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aration Plants - Vol. 1 - Proposed Standards," EPA-450/2-74-02la, and
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Triangle Park, N. C., October 1974.
84
-------
41. 40 CFR, Part 434, Subpart A, 1976.
42. 40 CFR, Part 434, Subpart D, 1976.
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Manual," 600/2-76-138, USEPA, Research Triangle Park, N. C., 1976.
44. Grim, E.G., and R.D. Hill, "Environmental Protection in the Surface
Mining Coal," EPA-670/2-74-093, USEPA, Cincinnati, Ohio, 1974.
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Fuel Resource Extraction, On-Site Processing, and Transportation,"
EPA-600/2-76-064, USEPA, Research Triangle Park, N.C., 1976.
47. 1976 Keystone Coal Industry Manual, Mining Information Services,
McGraw-Hill, New York, 1976.
48. Sidgwick, N. V., The Chemical Elements and Their Compounds, Vol. 1,
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49. a. Kolderup, H., "Fume Emission from Ferrosilicon Submerged Arc
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b. Kolderup, H., "Particle Size Distribution of Fumes Formed by Fer-
rosilicon Production," J. Air Pollution Control Assoc., Vol. 27, 1977,
p. 127.
50. a. Vandegrift et al., Particulate Pollutant System Study Vol. 1 Mass
Emissions, APTD-0743, NTIS No. PB-203 128, USEPA, Durham, N. C.,
1971.
b. Ibid., Vol. 3 Handbook of Emission Properties, APTD-0745, NTIS
No. PB-203 522.
51. a. "Background Information for Standards of Performance: Electric
Submerged Arc Furnaces for Production of Ferro Alloys Vol. 1 Pro-
posed Standards," EPA 450/2-24-018a, USEPA, Research Triangle Park,
N. C., October 1974.
b. Ibid, Vol. 2 -"Test Data Summary," EPA 450/2-74-018b, October
1974.
c. Ibid, Vol. 3 - "Supplemental Information," EPA 450/2-74-018c,
April 1976.
52. 41 FR 18498ff, May 4, 1976, with corrections, 41 FR 20659, May 20, 1976.
53. 36 FR 18498ff, April 30, 1971.
54. 40 CFR, Part 424, Subpart A, 1976.
55. Bates, R. L,., Geology of the Industrial Rocks and Minerals, Dover, New
York, 1969, pp. 276ff.
56. MacMillan, R. T., "Fluorine," in Mineral Facts and Problems, Bulletin
650, Bureau of Mines, Washington^D. C., 1970, p. 689.
57. 40 CFR, Parts 60,415,421,423,426,436 and 440 (NSPS Part 60; Effluent
Limitations Guidelines, Parts 401-460), 1976.
85 s
-------
58. "Development Document for Effluent Limitations Guidelines and New
Source Performance Standards for the Steam Electric Power Generating
Point Source Category," EPA 4401/l-74/029a (PB 240 853) USEPA,
Washington, D. C., 1974.
59. Klein, D.H. ct al., "Pathways of Thirty-Seven Trace Elements Through
Coal-Fired Power Plant," Env. Sci. Tech., Vol.9, No. 10, 1975, pp. 973-
979.
60. Davidson, R. L., D. F.S. Natusch, J. R. Wallace and C.A. Evans, Jr.,
"Trace Elements in Fly Ash," Env. Sci. Tech., Vol.8, No. 13, 1974,
pp. 1107-1113.
61. Sears, D.R., D. V. Merrifield, M. M. Penny and W. G. Bradley, "Environ-
mental Impact Statement for a Hypothetical Photovoltaic Solar -Electric
Plant," EPA temporary No. lERL-Ci-160, July 1976. (Work performed
by Lockheed-Huntsville R8tE Center under Contract EPA 68-02-1131
with USEPA, Cincinnati, Robert P. Hartley, Project Officer.)
62. International Labour Office Encyclopaedia of Occupational Health and
Safety, Vol.1, McGraw-Hill, Tisk, Brno, Czechslovakia, 1971, p. 233.
63. Chemical and Proce_g_s_ Technology Encyclopedia, D. M. Considine, ed.,
McGraw-Hill, New York, 1974.
64, "Background Information for New Source Performance Standards: Pri-
mary Copper, Zinc, and Lead Smelters, Vol.!," EPA 450/2-74-002A,
USEPA, Research Triangle Park, N.C., 1974.
65. Nerkervis, R. J., and J. B. Hallowell, "Metals Mining and Milling Process
Profiles with Environmental Aspects," EPA-600/2-76-167, USEPA, Re-
search Triangle Park, N.C., 1974, pp. 155-165.
66. Heindl, R.A., "Zinc," In Mineral Facts and Problems, 1970, Bureau of
Mines, U.S. Department of Interior, Washington, D.C., 1970, pp. 805-
814.
67. Fleischer, M. et al., "Environmental Impact of Cadmium: A Review by
the Panel on Hazardous Trace Substances," Environmental Health Per-
spectives, Vol.7, 1974, pp. 253-323.
68. "Development Document for Effluent Guidelines and Standards of
Performance, Ore Mining and Dressing Industry," (Draft), USEPA,
Washington, D. C., 1975.
69. "Compilation of Air Pollutant Emission Factors," Second Edition, Six
Supplements, AP-42, USEPA, Research Triangle Park, N. C., 1973.
70. 40 CFR Part 60, Subpart H, 1976, and NSPS Tech. Rept. No. 5, Sulfuric
Acid Plants," USEPA, Research Triangle Park, N. C., August 1971.
71. Versar, Inc., "Technical and Microeconomic Analysis of Cadmium and
Its Compounds," EPA 560/3-75-005, USEPA, Washington, D.C., June
1975.
72. Petrick, A., Jr., H. J. Bennett, K. E. Starch and R.C. Weisner, "The
Economics of Byproduct Metals Part 2 Lead, Zinc, Uranium, Rare
Earth, Iron, Aluminum, Titanium, and Li thium Systems," 1C 8570,
Bureau of Mines, Washington, D.C., 1973.
86
-------
73. Windawi, H.M., "Performance and Stability of Cuz-S-CdS Solar Cells,"
Proc. of Conference Records of the Eleventh IEEE Photovoltaic
Specialists Conference, Scottsdale, Ariz., May 1975.
74. Jordan, J.F., "Development of Very Low Cost Solar Cells for Terres-
trial Power Generation," in Photovoltaic Power Generation (Proc. of
International Conference on Photovoltaic Power Generation, Hamburg,
Germany, September 1974). Deutsche Gesellschaft fur Luft-u.
Raumfahrt e.V., Koln, Federal German Republic, 1974, p. Z21.
75. Jordan, J.F., "Low Cost CdS/Cu^S Solar Cells by the Chemical Spray
Method," Proc. of IEEE Photovoltaic Specialists Conference, Scottsdale,
Ariz., 1975.
76. Brod, T. P., and F.D. Shirland, "Prognosis for CdS Solar Cells," Sym-
posium on the Material Aspects of Thin Film Systems for Solar Energy
Conversion, May 1974.
77. Battelle Columbus Laboratories, "Energy Use Patterns in Metallurgical
and Non-Metallic Mineral Processing (Phase 4 Energy Data and Flow
Sheets, High Priority Commodities)," OFR 8075, U.S. Bureau of Mines,
June 1975.
78. Wood, J. M., "Biological Cycles for Toxic Elements in the Environment,"
Science, Vol. 183, 1974, pp. 1049-1052.
79. Dodge, B.F., and D.C. Reems, "Disposing of Plating Room Waste,"
American Electroplaters' Society Research Report No. 9, 1947.
80. Eckenfelder, W.W., Industrial Water Pollution Control, McGraw-Hill,
New York, 1966.
81. "Interaction of Heavy Metals and Biological Sewage Treatment
Processes," Public Health Service Bulletin, 999-WP-22.
82. Jones, R.H., "Toxicity in Biological Waste Treatment Processes," Proc.
of Conference on Pollution Abatement and Control in the Wood Pre-
serving Industry, W.S. Thompson, ed., Mississippi Forest Products
Laboratory, Mississippi State University, State College, Miss., 1971,
pp. 217-231.
83. Preprint, "Cadmium" from 1974 Bureau of Mines Minerals Yearbook.
84. Brook, D. B., and P.W. Andrews, "Mineral Resources, Economic Growth
and World Population," Science, Vol. 185, No. 4145, 1974, pp. 13-19.
85. Fleischer, M. et al., "Environmental Impact of Cadmium: A Review by
the Panel on Hazardous Trace Substances," Environmental Health Per-
spectives, Vol.7, 1974, pp. 253-323.
86. Schroeder, H. A., "Cadmium, Zinc and Mercury," Air Quality Monograph
No. 70-16, American Petroleum Institute, Washington, D.C., 1970.
87. Gish, C.D., and R.E. Christensen, "Cadmium, Nickel, Lead and Zinc in
Earthworms from Roadside Soil," Env. Sci. Tech., Vol. 7, No. 11, 1973,
pp. 1060-1062.
88. 40 CFR Part 421, "Nonferrous Metals Point Source Category," 1976.
87
-------
89. Stamper, J.W., "Gallium," in Mineral Facts and Problems of Mines,
Bulletin 650, Bureau of Mines, Washington, B.C., 1970.
90. Aluminum Company of America, "Gallium and Gallium Compounds,"
March 1963.
91. Pctrick, Jr., A. et al., "The Economics of Byproduct Metals Part 1
Copper System," Circular 1C 8569> U.S. Bureau of Mines, Washington,
D.C., 1973.
92. Jones, C.H., Chem. Metall. Eng., Vol. 23 (1920), p. 957.
93. Harrison, Jr., J. W., "A Survey of Single Crystal Growth Methods for
GaAs," Solid State Technology, January 1973.
94. Havel, H. J., and J. M. Woodall, "Diffusion Length Improvements in GaAs
Associated with Zn Diffusion During Gaj_x AI^As Growth,11 Proc. of
Conference Record of the Eleventh IEEE Photovoltaic Specialists Con-
ference, Scottsdale, Arix., May 1975.
95. James, W.W., and R. JL. Moon, "GaAs Concentrator Solar Cells," ibid.
96. Ewan, J. A., S. Kamath, and R. C. Knechtli, "Large Area GaAiAs/GaAs
Solar Cell Development, ibid.
97. Paone, James, "Arsenic," in Mineral Facts and Problems, Bulletin 650,
Bureau of Mines, Washington, D. C., 1970.
98. "Gallium," in Mineral Facts and Problems, Bulletin 667, Bureau of
Mines, Washington, D.C., 1970.
99. Zoutendyk, J. A., "Development of Low-Cost Silicon Crystal Growth
Techniques for Terrestrial Photovoltaic Solar Energy Conversion," in
Sharing the Sun Solar Technologies in the Seventies, Proc. of the Joint
Conference, American Section, International Solar Energy Society, and
Solar Energy Society of Canada, Inc., Winnipeg, August 1976.
100. "Amorphous-Silicon Doping Brightens Solar-Cell Picture," Physics
Today, January 1977, p. 17.
101. Sax, N.Irving, Dangerous Properties of Industrial Materials, 2nd edition,
Reinhold, New York.
102. The Toxic Substances List, 1974 Edition, Herbert E. Christensen, ed.,
U.S. Department of Health, Education and Welfare, Public Health Service,
Center for Disease Control, NIOSH, Rockford, Md., June 1974.
103. National Institute of Occupational Safety and Health, "Suspected Carcino-
gins Subfile: An Excerpt from the NIOSH Registry of Toxic Effects of
Chemical Substances," NIOSH, Rockwell, Md., 1976.
104. U.S. Coast Guard, "Chemical Hazards Response Information System
(CHRIS)," Department of Transportation, Washington, D.C., 1974.
Volume 1: A Condensed Guide to Chemical Hazards.
Volume 2: Hazardous Chemical Data.
Volume 3: Hazard Assessment Handbook.
Volume 4: Response Methods Handbook.
88
-------
105. "Oil and Hazardous Materials Technical Assistance Data System
(OHMTADS)," Jean Wright, Project Officer, Telephone (202) 245-3057.
USEPA Division of Oil and Special Materials Control, WH-558. 401 M
Street SW, Washington, D.C. 20460.
89
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Appendix A
ATMOSPHERIC EMISSION FACTORS FOR RAW MATERIALS
EXTRACTION AND PRODUCTION
Emission factors appearing in the following tables appeared originally in
the references cited, in units conventional to their respective industries.
They have been converted into kilogram (kg) pollutant per metric ton (MT) of
final photovoltaic cell material, for example, kg (SOx)/MT(Si). To determine
the total emission of a specific pollutant, the sum of all emission factors (in-
cluding unit operations for which we have no data) would be multiplied by the
total quantity of photovoltaic solar cell material in metric tons.
90
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TABLE A-l. EMISSION FACTORS FOR QUARTZITE MINING UNIT
OPERATIONS* (DERIVED FROM REF. 23).
Unit Operations
kg/MT (of Material
Processed Through
Crusher)
kg/MT (of Solar
Cell Silicon)**
Drilling
Blasting
Loading at Quarry and Vehicular
Transport to Plant
Unloading and Primary Crushing
and Screening
Secondary Crushing and Screening
Conveying
Unloading at Stockpile
Wind Erosion of Stockpiles Un-
paved Road Traffic Between
Finished Stockpile and Nearest
Paved Highway
Total (+ 95% Confidence Interval)
Fraction Respirable, by Weight
0.030
+
0.170
0.012
0.212 (±0.100)
6%
1.64
9.32
0.66
11.62 (±5AS)
6%
jjt
No NO or SO (products of explosives combustion) were detected outside
the-plant perimeter.
Using ratio 54.8 kg 98% SiO^ required to produce 1 kg solar cell Si.
Negligible, i.e., less than 1% of total.
Only wet screening and washing procedure was used at the quartzite plant
and there was no primary crusher.
91
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TABLE A-2.
ELEMENTAL ANALYSIS OF EMISSIONS
FROM QUARTZITE PROCESSING (DE-
RIVED FROM REF.23).
Element
Mn
Fe
Cu
Zn
Pb
Na
Mg
A*
Ti
V
Cr
Si
Ca
Sn
Ni
Ag
Weight %
0.03 - 0.04
0.5 - 6.6
0.4 - 2.0
<1.06
0.16 - 0.4
0.2 - 0.53,.
0.1 - 0.2
0.4 - 1.3
, 0.0-3 -0.07
< 0.005
x 0.01 - 0.02
3.3-20
1.2 - 6.6
0.01 - 0.02
<0.02
<0.006
92
-------
z
a
§
w
o
w
yS
«3
II
QQ
2w
CTURE:
CED (DE
5 3
j
3?
alZ.
DO
JU
H in
Win
So
«
g
"
ss
a
n
a
If
S
o
6
m ivi o I
"1
Ml Mill
| * | "» - | *.
| »o | Q IM I «
1
IiD
-
.3" « S
S !
a fl
S 1
3 I
u
»
M
o-
w
g
I
93
-------
TABLE A-4. COAL EMISSION FACTORS FOR ATMOSPHERIC
PARTICULATES (REF. 39).
Source Type
Surface Coal Mining
Drilling
Coal Loading
Transport and Unloading
Blasting
Total
Auger Mining
Coal Storage
Emission Factor
kg/MT Coal
12.5 x 10-3
1.6 x 10"3
1.7 x 10~3
4.9 x 10"3
4.2 x 10"3
24.9 x 10"3
0.8 x 10~3
6.5 x 10"3
kg/MT Solar
0.375
0.048
0.051
0.147
0.126
0.747
0.024
0.195
**
Cell
Total suspended respirable particulate (< 7 |Um).
Using the ratio 30 kg coal/kg solar cell Si.
94
-------
Appendix B
GLOSSARY
Symbol
AMO
AMI
Average Power
BATE A
BOD
BPCTCA
Capacity Factor
CFR
COD
CVD
DSCF
DSCFM
DSCM
DSCMM
Efficiency
EFG
ELG
EPA
ERDA
ESP
FR
GE
HC
Description
Air mass zero (extraterrestrial solar spectral
irradiance value) = 1353 W/m2
Air mass one (direct solar spectral irradiance
at ground level) = 956 W/m2
Power output integrated over 24 hours and
divided by 24
"Best available technology economically
achievable11
Biochemical oxygen demand
"Best practicable control technology currently
available"
Ratio of actual output to maximum design output
Code of Federal Regulations
Chemical oxygen demand
Chemical vapor deposition
Dry standard cubic feet
Dry standard cubic feet per minute
Dry standard cubic meters
Dry standard cubic meters per minute
100
Peak Power Output
Incident Energy at AMI
Edge-fed film growth
Effluent Limitations Guideline(s)
Environmental Protection Agency
Energy Research and Development Admin.
Electrostatic precipitator
Federal Register
General Electric Company
Hydrocarbon
95
-------
Symbol
Insolation
IR
JPL
LDLo
LD50
MT
MW
NIOSH
NO
x
NPDES
NSFS
OSHA
Packing Factor
Peak Power
Q
Respirable Participate
R&D
Scf
Scfm
Scm
Settleable Particulate
S0x
Suspended Particulate
TCLo
Description
Actual irradiance for given geometry, altitude
location and meteorological conditions
Infrared
Jet Propulsion Laboratory
Lethal dose low
Lethal dose - 50%
Metric ton (Not megaton); 1000 kg
Megawatt
National Institute of Occupational Safety and
Health
Nitrogen oxides
National Pollutant Discharge Elimination System
New Source Performance Standard(s)
Occupational Safety and Health Administration
Solar cell area as percent of total solar array
area
Power output at solar noon under optimal
meteorological conditions with solar radiation
at normal incidence
Quad = 1015 Btu
Atmospheric particle (aerosol) small enough
to be inhaled into the lungs. Often taken to
mean < 7 Jim
Research and development
Standard cubic feet
Standard cubic feet per minute
Standard cubic meter
An atmospheric particle so large that it does
not remain suspended. Often taken to mean
> 50|Um
Sulfur oxides
Any atmospheric particle small enough to
remain airborne
Toxic concentration low
96
-------
Symbol
TDLo
TFX
THC
TLV
TSS
TWA
TXDS
UV
Description
Toxic dose low
Toxic effects
Total hydrocarbon
Threshold lethal value
Total suspended solids
Time weighted average
Qualifying toxic dose
Ultraviolet.
97
-------
Appendix C
CONVERSION FACTORS
Mass
1 tig =
1 kg =
1 MT =
1 Ib =
1 Short Ton =
Length
1 cm =
1 m =
1 in. =
1 ft =
1 mi =
Area.
1 m2 =
1 hectare =
Ift2 =
1 acre =
1 mi2
Volume
1 t =
lm3 =
1 gal =
Ift3 =
Pressure
1 N/m2 =
1 psi =
Energy
1 kWh =
1 kcal =
1 Joule =
1 Btu =
Power
1 kW =
1 Btu/hr =
Concentration*
in Air
(fig/scm)
1 ppm
(Vol., 293 K)
1 ppm
1 lb/ft3 (lb/scf)
Mg
1
10^
IO12
4.536 x IO8
9.072 x IO11
cm
1
100
2.540
30.48
1.609 x IO5
2
m
1
IO4
0.09290
4047
2.590 x IO6
1
1
1000
3.785
28.32
N/m2
1
6895
kWh
1
1.162 x IO"3
2.778 x IO"7
2.929 x IO"4
kW
1
2.929 x IO"4
»*g/-3
1
M/0. 02404
1.198 x IO3
1.602x IO10
kg
10-'
1
IO3
0.4536
90.72
m
0.01
1
0.02540
0.3048
1609
hectare
IO"4
1
9-290 x 10
0.4047
259.0
m
0.001
1
3.785 x 10
0.02832
psi
1.450 x 10
1
kcal
859.2
1
2.387 x 10
0.2520
Btu/hr
3414
1
ppm (Vol.)
0.0240/M
1
28.8/M
MT
1C"12
0.001
1
4.536 x IO"4
0.9072
in.
0.3937
39.37
1
12
63360
ft2
10.76
1.076 x IO5
"6 1
43560
2.788 x IO7
gal
0.2642
264.2
"3 1
7.481
-It
Joule
3.6 x 10°
4184
"4 1
1054
ppm (Wt>
8.347 x IO"4
M/28.8
1
3.857 x 108/M 1.337 x IO5
Ib
2.205 x IO"9
2.205
2205
1
2000
ft
0.03281
3.281
0.08333
1
5280
acre
2.471 x IO*4
2.471
2.296 x IO"5
1
640
ft3
0.0353
35.31
0.1337
1
Btu
3414
3.968
9.485 x IO"4
1
lb/ft3
6.243 x 10" ll
M/3.851 x IO8
7.48 x IO"6
1
Short Ton
1.102 x 10"U
1.102 x IO"3
1.102
5 x IO"4
1
mi
6.214x 10"°
6.214 x 10~4
1.578 x IO"5
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1.563x IO"3
1
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98
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Appendix D
TOXICITY OF CHEMICAL COMPOUNDS USED IN SOLAR
CELL PRODUCTION
This appendix summarizes the toxic hazard ratings, toxic dose and
toxic effects for chemical compounds involved in a solar cell manufacturing
facility (Refs. 101, 102). The toxic hazard rate code and acronyms are de-
fined, followed by the toxicity for each of the solar cell materials. Sev-
eral materials are suspected carcinogens (Ref. 103).
For spill hazards and emergency responses, transportation informa-
tion and production sites, etc., consult the Coast Guard "CHRIS" manuals and
the USEPA "OHMTADS" data system (Refs. 104 and 105).
Toxic Hazard Rating Code
0 NONE: (a) No harm under any conditions;
(b) Harmful only under unusual conditions or overwhelming
dosage.
1 SLIGHT: Causes readily reversible changes which disappear after
end of exposure.
2 MODERATE: May involve both irreversible and reversible changes;
not severe enough to cause death or permanent injury.
3 HIGH: May cause death or permanent injury after very short ex-
posure to small quantities.
U UNKNOWN: No information on humans considered valid by authors.
Definitions
TXDS: Qualifying toxic dose
TFX: Toxic effects
TWA: Time weighted average
TCLo: Toxic concentration low
TDLo: Toxic dose low
LDLo: Lethal dose low
LD50: Lethal dose fifty.
ALUMINUM COMPOUNDS
Toxic Hazard Rating:
Acute local: 0
Acute systemic: 0
Chronic local: Inhalation 2
Chronic systemic: 0
100
-------
Aluminum Oxide
Synonym: Alumina
Description: White Powder
Formula: A£JD~
TXDS: 3
Inhalation - mouse TCLo: 357 mg/m /60DY
Aluminum Chloride
Description: White granular crystals
Formula: Aid,
TXDS:
Oral - rat LD50: 3700 mg/kg
Oral - mouse LD50: 3800 jig/kg
ARSENIC COMPOUNDS
Toxic Hazard Rating:
Acute local: Irritant 2, Allergen 2; Ingestion 3
Acute systemic: Ingestion 3; Inhalation 3
Chronic local: Irritant 2; Allergen 2
Chronic systemic: Ingestion 3; Inhalation 3.
Arsenic Acid
Synonyms: True arsenic acid
Description: White, translucent crystals
Formula: H3AsO4 1/2 H2O.
Arsenic Trioxide
Synonyms: White arsenic; Arsenic (III) oxide; arsenic sequioxide;
arsenous oxide; arsenous oxide anhydride.
Description: White, odorless, tasteless amorphous powder
Formula: As_,O,
TXDS:
23"
Oral - human;
Inhalation - human;
TFX: skin
Inhalation - human;
TFX: carcinogen
Oral - rat
Oral -
Oral - rabbit
LDLo: 1 mg/kg ,
TCLo: 110 fig (As)/m
TCLo: 200 jig (As)/m3
LD50: 45 mg/kg
LD50: 43 mg (As)/kg
LD50: 4 mg/kg
U. S. Occupational Standard USOS - air: ,
TWA: 0.5 mg (As)/m
NIOSH Received Standard - air:
TWA: 50 jig (As)/m
101
-------
Arsenic Pentoxide
Synonyms: Arsenic oxide, arsenic acid anhydride
Description: White, amorphous solid
Formula:
TXDS:
Oral - rat
Intravenous - rabbit
LD50: 8 mg/kg
LDLo: 6 mg/kg
Sodium Arsenate
Synonyms: Sodium metaarsenate, sodium ortho-arsenate
Description: Clear, colorless crystals: mild alkaline taste
Formula: NaAsO,
TXDS:
Inhalation - human
TFX - carcinogen
Intraperitoneal - mouse
Intraperitoneal - mouse
TDLo: 4 mg/m
LD50: 9 mg/kg
LDLo: 45 mg/kg
BORON COMPOUNDS
T oxi c H a z a r d R a ting;
Acute local: Ingestion 2; Inhalation 2
Acute systemic: 0
Chronic local: 0
Chronic systemic: Ingestion 2; Inhalation 2; Skin Absorption 2
TXDS:
Oral - mouse
LD50: 2000 mg/kg
CADMIUM COMPOUNDS (See also Appendix E)
Toxic Hazard Rating;
Acute local: Irritant 3; Ingestion 3; Inhalation 3
Acute systemic: Ingestion 3; Inhalation 3
Chronic local: Variable
Chronic systemic: Ingestion 3; Inhalation 3
Cadmium
Description: Hexagonal crystals; silver-white malleable metal
TXDS:
LDLo: 15 mg/kg
TDLo: 70 mg/kg
LDLo: 25 mg/kg
U.S. Occupational Standard USOS - Air: TWA 200 /Ltg/rrT
102
Intramuscular - rat
Intramuscular - rat
Intramuscular - human
-------
Cadmium Oxide
Description: Amorphous, brown crystals; cubic, brown crystals
Formula: CdO
TXDS:
Inhalation - man
Oral - rat
Inhalation - rat
Subcutaneous - rat
Inhalation - mouse
Inhalation - dog
Inhalation - rabbit
Inhalation - guinea pig
TCLo: 9 mg/m /5H
LD50: 72 mg/kg -
LC50: 500 mg/m
TDLo: 90 mg/kg,
LC50: 700 mg/m /10M
LD50: 4000 mg/m3/10M
LC50: 2500 mg/m3/10M
LC50: 3500 mg/m3/10M
U. S. Occupational Standard USOS - Air: TWA 100
Cadmium Chloride
Description: Hexagonal, colorless crystals
Formula: CdCl -,
TXDS:
Oral - rat
Intraperitoneal - rat
Subcutaneous - rat
Subcutaneous - rat
Intramuscular - rat
Parenteral - rat
Inhalation - dog
Subcutaneous - mouse
Subcutaneous - mouse
Inhalation - dog
Subcutaneous - rabbit
Cadmium Sulfide
Synonym: Greenockite
Description: Yellow-orange crystals
Formula: CdS
LD50: 88 mg/kg
TDLo: 3 mg/kg/(9D preg)
TDLo: 4 mg/kg/(14- 17D preg)
TDLo: 2.2 mg/kg
TDLo: 2 mg/kg
TDLo: 2 mg/kg 3
LCLo: 320 mg7m
TDLo: 16 mg/kg (14-17D preg)
TDLo: 6 mg/kg
LC90: 8 mg/m3/30MC
TDLo: 9 mg/kg
TXDS:
Intramuscular - rat
Subcutaneous - rat
Intramuscular - rat
TDLo: 150 mg/kg
TDLo: 90 mg/kg
TDLo: 250 mg/kg
CHROMIUM COMPOUNDS
Toxic Hazard Rating;
Acute local: Irritant 3; Ingestion 3, Inhalation 3
Acute systemic: Unknown
103
-------
Chronic local: Irritant 3, Ingestion 3; Inhalation 3
Chronic systemic: Ingestion 3, Inhalation 3.
Chromium Trioxide
Synonyms: Chromium (60) oxide, chromic acid, chromic anhydride
Description: Dark, purple-red crystals
Formula: CrO,.
TXDS:
Subcutaneous - dog:
Implant - rat:
TFX - carcinogen:
LDLo: 330 mg/kg
TDLo: 125 mg/kg
U.S. Occupational Standard USDA - air; TWA: 100 jUg/m (AsCrOJ
Sodium Chromate
Description: Yellow crystals
Formu
TXDS:
Formula: Na,CrO4.
Subcutaneous - rabbit:
Sodium Dichromate
LDLo: 243 mg/kg
Synonyms: Sodium acid chromate, sodium bichromate
Description: Red crystals
2H,O
Formula:
TXDS:
Intermuscular - rat:
TDLo: 140 mg/kg
U.S. Occupational Standards USOS - air: TWA: 100 jLtg/m (AsCrO,)
Potassium Chromate
Synonyms: Neutral potassium chromate tarapacaite
Description: Yellow crystals
Formula: K^CrO.
TXDS:
Oral - human:
Potassium Dichromate
LDLo: 430 mg/kg
Synonyms: Potassium dichromate, red potassium chromate
Description: Bright, yellowish-red, transparent crystals, bitter
metallic taste
Formula: K2CrO?
Toxicity: See Chromium Compounds.
104
-------
Chromic Acid
Synonyms: Chromic anhydride, chromic trioxide
Description: Dark, purple-red crystals
Formula: CrO,
TXDS:
Inhalation - human TCLo: 110 fig/M
U.S. Occupational Standard USOS - air: CL 100 JIg/m (as CrO,)
Chromic Acid, Lead Salt
TXDS
Subcutaneous - rat TDLo: 1200 mg/kg
Intraperitoneal - guinea pig LD50: 400 mg/kg
U.S. Occupational Standard USOS - air: CL 100 |lg/m (as CrO,)
Chromic Acid, Calcium Salt
TXDS:
Subcutaneous - rat
Intramuscular - rat
Implant - rat
Subcutaneous - mouse
Implant - mouse
TDLo: 76 mg/kg/20W
TDLo: 95 mg/kg/20W1
TDLo: 63 mg/kg
TDLo: 50 mg/kg
TDLo: 50 mg/kg
Implant - mouse * - ,
*%
U.S. Occupational Standard USOS - air: CL 100 fig/mj (as CrOj)
COPPER COMPOUNDS
Toxic Hazard Rating;
Acute local: Irritant 1; Allergen 1; Ingestion 1; Inhalation 1 '
Acute systemic: Ingestion 2; Inhalation 2
Chronic local: Allergen 1
Chronic systemic: Ingestion 1; Inhalation 1
Cupric Oxide
Synonyms: Tenorite
Description: Fine, black powder
Formula: CuO
TXDS:
None available.
Copper Sulfate
Synonyms: Blue vitriol; blue stone; roman vitriol, cupric sulfate
Description: Blue crystals or blue crystalline granules of powder
Formula: CuSO, 5H~O
105
-------
TXDS:
Oral - rat LD50: 300 mg/kg
Intraperitoneal - mouse LD50: 7 mg/kg
U. S. Occupational Standard: USOS - air: TWA 1 mg/m (as Cu)
Copper Carbonate
Synonyms: Copper bicarbonate, cupric carbonate
Description: Green powder
Formula: CuCo3 Cu(OH)2
TXDS:
Oral - rat LD50: 159 mg/kg
U.S. Occupational Standard: USOS - air: TWA: 1 mg/m3 (as Cu)
Copper Acetate
Synonyms: Cupric acetate; neutral verdigris
Description: Greenish-blue, fine powder or small crystals
Formula: Cu(C,H3O2) H2O
TXDS:
Oral - rat
GALLIUM COMPOUNDS
Toxic Hazard Rating;
Acute local: U
Acute systemic: Ingestion 1
Chromic local: U
Chromic Systemic: Ingestion 1
TXDS:
Subcutaneous - rat
Gallium Arsenide
LD50: 710 mg/kg
LDLo: 200 mg/kg
Description: Cubic crystals with dark gray metallic sheen
Formula: GaAs
TXDS:
Oral - rat
A recognized carcinogen
LD50: 4700 mg/kg
106
-------
Gallium Chloride
TXDS:
Inhalation - rat
Intraperitoneal - mouse
LDLo: 191 mg/m /3H
LJD50: 37 mg/kg
HYDROCHLORIC ACID
Toxic Hazard Rating;
Acute local: Irritant 3
Acute systemic: Ingestion 3; Inhalation 3
Chronic local: Irritant 2
Chronic systemic: U
TXDS:
Inhalation - rat
Inhalation - mouse
Intraperitoneal - mouse
Inhalation - rabbit
Inhalation - mammal
LC50: 4701 ppm/30M
LC50: 2142 ppm/30M
40 mg/kg
LCLo: 4416 ppm/30M
LCLo: 1000 mg/m3/2H
U.S. Occupational Standard: USOS - air: CL 5 ppm
LEAD COMPOUNDS
Toxic Hazard Rating:
Acute local: 0
Acute systemic: Ingestion 3; Inhalation 3
Chronic local: 0
Chronic systemic: Ingestion 3; Inhalation 3; Skin Absorption 3
U.S. Occupational Standard: USOS - Air: TWA 200 Jig/m3
Lead Arsenate
Synonyms: Lead o-arsenate; lead di-o-arsenate; lead mono-o-
arsenate
Description: White crystals
TXDS:
Oral - rat
Oral - rabbit
LD50: 100 mg/kg
LD50: 121 mg/kg
U.S. Occupational Standard: USOS - air: TWA 150 flg/m"
Lead Chloride
Synonym: Cotunnite
Description: White crystals
107
-------
TXDS:
Oral - guinia pig
LDLo: 200 mg/kg
U. S. Occupational Standard: USOS - air: TWA 269 /Ltg/m
NITRIC ACID
Toxic Hazard Rating;
Acute local: Irritant 3; Ingestion 3; Inhalation 3
Acute systemic: Inhalation 3
Chronic local: Irritant 2
Chronic systemic: Inhalation 3
TXDS:
Inhalation - rat LC50: 49 ppm/4H
U.S. Occupational Standard: USOS - air: TWA 2 ppm
Nitric Acid, Cadmium Salt
TXDS:
Oral - rat LD50: 300 mg/kg
Nitric Acid, Copper Salt
TXDS:
Oral - rat LD50: 940 mg/kg
3
U. S. Occupational Standard: USOS - air: TWA 1 mg/m (as Cu)
Nitric Acid, Zinc Salt
TXDS:
Oral - rat LD50: 1190 mg/kg
PHOSPHORUS COMPOUNDS, INORGANIC
Phosphorus
TXDS:
Oral - human
Subcutaneous - dog
Oral - rabbit
Subcutaneous - rabbit
LDLo: 1.4 mg/kg
LDLo: 2 mg/kg
LDLo: 10 mg/kg
LDLo: 13 mg/kg
Phosphoric Acid
Description: Colorless liquid or rhombic crystals
108
-------
Toxic Hazard Rating:
Acute local: Irritant 2
Acute systemic: Ingestion 2; Inhalation 2
Chronic local: Irritant 2; Inhalation 2
Chronic systemic: U
Phosphine
Synonyms: Hydrogen phosphide; phosphoretted hydrogen
Description: Colorless gas
Toxic Hazard Rating:
Acute local: Irritant 2; Inhalation 2
Acute systemic: Inhalation 3
Chronic local: U
Chronic systemic: Inhalation 3
TXDS:
Inhalation - human LCLo: 8 ppm
Inhalation - rat LC50: 11 ppm/4H
. a
U.S. Occupational Standard: USOS - air: TWA 400 fig/m"
SELENIUM COMPOUNDS
Toxic Hazard Rating;
Acute local: Irritant 2
Acute systemic: Ingestion 2; Inhalation 2-3
TXDS:
LCLo: 33 mg/kg/8H
LD50: 6 mg/kg
U.S. Occupational Standard: USOS - air: TWA 0.2 mg/m
Inhalation - rat
Intravenous - rat
Selenium Dioxide
Description: White to slightly reddish, lustrous, crystalline powder
or needles
Formula: SeO->
TXDS:
Subcutaneous - rat
LD50: '4 mg/kg
U.S. Occupational Standard: USOS - air: TWA 0.2 mg/nV
109
-------
LD50: a830 mg/kg
SILICON COMPOUNDS
Silicon
TXDS:
Oral - rat
Silicon Chloride (SiCljJ
Description: Colorless, fuming liquid, suffocating odor
TXDS:
Inhalation - rat LC50: 8000 ppm/4H
Silica (Silicon Dioxide)
Synonyms: Silicic anhydride; crystabalite
Description: Colorless crystals
TXDS:
Oral - rat
Intravenous - rat
Intrapleural - rat
Silane (SiH4)
Synonyms: Siliconhydride, disilane
Description: Gas or liquid
TXDS:
Inhalation - rat
Trichloro Silane (SiHC/3)
LD50: 3160 mg/kg
LJD50: 15 mg/kg
TDLoj 200 mg/kg
LCLo: 9600 ppm/4H
TXDS:
Oral - rat
Inhalation - rat
LD50: 1030 mg/kg
LCLo: 1000 ppm/4H
SULFURIC ACID
Toxic Hazard Rating;
Acute local: Irritant 3; Ingestion 3; Inhalation 3
Acute systemic: U
Chronic local: Irritant 2; Inhalation 2
Chronic systemic: U
TXDS:
Inhalation - human
Inhalation - human
Oral - rat
TCLo: 800 fJis/m
TCLo: 5 mg/m3/15M
LD50: 2140 mg/kg
110
-------
Inhalation - rat
Inhalation - mouse
Inhalation - guinea pig
LCLo: 178 ppm/7H
LCLo: 140 ppm/2lOM
LCLo: 48 ppm/H
U.S. Occupational Standard: USDS - air: TWA 1 mg/m
TIN COMPOUNDS
Elemental tin is not generally considered toxic. Some inorganic
salts are irritants or can liberate toxic fumes on decomposition.
Organo tin compounds are used as pesticides.
TITANIUM COMPOUNDS
Toxic Hazard Rating;
Acute local: Ingestion 1; Inhalation 1
Acute systemic: U
Chronic local: U
Chronic systemic: U
Titanium Chloride (TiC_Jt4)
TXDS:
Intramuscular - rat
Inhalation - mouse
Intramuscular - human
LDLo: 50 mg/kg
LCLo: 10 mg/m3/2H
LDLo: 83 mg/kg
Titanium Oxide
Synonym: Rutile
Description: Blue crystals
U.S. Occupational Standard: USOS - air: TWA 15 mg/m"
ZINC COMPOUNDS
Toxicity: Variable, generally of low toxicity.
Zinc Chloride
Synonyms: Butter of zinc, zinc dichloride
Description: White deliquescent crystals
Formula: ZnCj£,
TXDS:
Zinc Oxide
Inhalation - human
Intravenous - rat
TCLo: 4800 mg/m"
LDLo: 30 mg/kg
Synonyms: Zincite, Chinese white, zinc white
Description: White or yellowish powder
111
-------
Formula: ZnO
TXDS:
A disinfectant
Inhalation - rat
Inhalation - guinea pig
U. S. Occulational Standard: USOS - air: TWA 5 mg m3
LDLo: 400 mg/m ,
LCLo: 2500 mg/m
Zinc Sulfate
Synonyms: Zinkosite, white vitriol
Description: Colorless crystals
Formula: ZnSO4
TXDS:
Intraperitoneal - rat
Intraperitoneal - mouse
Subcutaneous - rabbit
LD50: 40 mg/kg
LD50: 29 mg/kg
LJDLo: 2.5 mg/kg
112
-------
Appendix E
CADMIUM: PROPERTIES, OCCURRENCE, AND ECOLOGICAL EFFECTS
Chemical and Physical Properties (Refs.48, 85)
Cadmium metal is a bluish-white to silver-white metal that melts at
321 C and boils at 765 C. Its vapor pressure is 1.4 torr at 400 C and 16 torr
at 500 C. Thus, significant losses by vaporization can be expected during
metallurgical processing or during accidents such as fires or explosions. In
the vapor phase, it is very reactive, quickly forming finely divided CdO in the
air.
Cadmium exhibits only valence +2 in its compounds with a tendency to
form covalent bonds, especially with sulfur. Alkyl cadmium compounds are
very unstable, rapidly reacting with water or moist air under ambient envi-
ronmental conditions. In water, CdS and CdCO3 are slightly soluble, but it
forms a wide variety of soluble complexes, notably with cyanides and
amines.
Occurrence
Major sources of atmospheric emissions of cadmium are the smelting
of ores, manufacture of metallic alloys, reprocessing of cadmium-plated
materials and cadmium-containing alloys, combustion of coal and oil, the
burning of cadmium-weighted plastics and of sewage sludge. Estimated
rates of emission to air, water and land are given in Table E-l.
Most fresh waters contain less than 1 ag/l (Ref.85), while a typical
atmospheric level is reported as 0.025 pg/rrH (Ref. 86). High concentrations
of cadmium have been reported in the vicinity of major emission sources
(Refs.85 and 87).
Ecological Effects
Cadmium is toxic to most living organisms, including mammals (Ref.
86).
Damage to plants from, excess cadmium has been reported, but the re-
quired levels of cadmium were greater than even those in soils of contami-
nated areas.
No evidence exists that cadmium concentrates in marine food chains,
but mollusks and plankton-eating birds possess the highest concentrations
recorded. Adverse effects on reproduction of fish have been reported at
113
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In humans, cadmium is concentrated in
Exposure to fumes or dusts of cadmium metal or cadmium oxide is
known to cause acute pulmonary edema. Chronic exposure through the
respiratory tract produces chronic emphysema. Repeated suggestions have
been stated that build-up of cadmium in the body is related to the occurrence
of hypertension in man. Rats and rabbits have developed hypertension after
ingestion or injection of cadmium (Refs. 85 and 86). Carcinogenic effects
have not yet been recognized in humans.
Large doses of cadmium have resulted in itai-itai {"ouch-ouch") di-
sease in Japan. The disease leads to progressive decalcification of bone,
with intense joint pain and increased vulnerability to fractures. About 200
cases have been described, of which 100 died (Ref. 86).
Current Regulations
Effluent guidelines for the zinc and lead smelting and refining indus-
tries have been promulgated by the Environmental Protection Agency at
levels of 0.0027 kg/MT zinc and 0.0004 kg/MT of lead or 0.5 mg/l of dis-
charge water (Ref. 88).
The only air standards in effect for cadmium are those of the Occu-
pational Safety and Health Administration (OHSA). The current OSHA limit
is: for fumes, 100 jlg/m^ (over 8 hours), with a 300 fig/m? short term
ceiling; for dust, 200 and 600 Ug/m3, respectively. Newly recommended
OHSA standards are 40 jlg/m* over a 15-minute period.
115
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TECHNICAL REPORT DATA
(t'leoie read limniftivni OH tli>- invrth' bcfare
I.HtPORTNO. 3.
EPA-600/7-77-087 |
4. TITLE AND SUBTITLE
Assessment of Large-Scale Photovoltaic Materials
Production
Martin G. Gandel, Paul A. Dillard, D. Richard Sears,
S. M. Ko, and S. V. Bourgeois
3. RECIPIENT'S ACCCSSION-NO.
!>. Pf PORT .DATE
_ August 1977 issuing_date_
C. Pt RFORMING ORGANIZATION CODE
8. PERFORMING ORGANIZATION REPORT NO.
LMSC-HREC TR D497252
»; PERFORMING ORGANIZATION NAME ANO ADDRESS
Lockheed Missiles & Space Company, Inc.
Huntsville Research & Engineering Center
Huntsville, Alabama 35807
10. PROGRAM ELEMENT NO.
EHE 624B
11. CONTRACT/GRANT NO.
Contract No. 68-02-1331,
Task 15
12. SPONSORING AGENCY NAME ANO ADDRESS
Industrial Environmental Kesearch Lab-Gin., OH
Office of Research and Development
U.S. Environmental Protection Agency
Cincinnati, Ohio 45268
13. TYPE OF REPORT AND PERIOD COVERED
Final Report .
14. SPONSORING AGENCY CODE
EPA/600/12
IS. SUPPLEMENTARY NOTES
16. ABSTRACT
Solar cell production at rates needed to supply continuously 1% of projected U. S.
power requirements in the year 2000 is examined. Si and CdS. are followed from raw
material extraction to finished cell; GaAs is reviewed less thoroughly. .Numerical
data are developed for air, water, and solid wastes, and compared with corresponding
effects of equivalent coal-electric power. Mass and energy balance data are derived
from flow sheets developed for this report.
For Si, major problems requiring engineering solutions are material and energy
inefficiencies. Very large byproduct streams should be eliminated to increase yield
by as much as 59% or decrease air pollutant releases by 37% on a process weight
basis. Power consumption in cell production creates indirect air pollutant emissions
over half as large as those created by the coal-burning plants silicon might replace.
CdS and GaAs are not as energy inefficient. Their metallic raw materials are
themselves byproducts of other smelting operations. Atmospheric cadmium re-
leases, and the potential for Cd or As spills are major problem areas.
Of materials known to be involved in cell production, only gallium is resource-
limited; however, use of concentrators and thin-film technology may obviate this
problem.
17.
KEY WORDS AND DOCUMENT ANALYSIS
DESCRIPTORS
Solar power generation
Solar cells
Silicon
Cadmium sulfide
Environmental engineering
Gallium arsenide
Resources
Pollution
13. DISTRIBUTION STATEMENT
Release unlimited
b.lDENTIFIERS/OPEN ENDED TERMS
environmental assess
ment
19. SECURITY CLASS (ThisReport)
unclassified
20. SECURITY CLASS (Thispage)
unclassified
COSATI Field/Group
10A
10B
13B
13H
NO. OF PAGES
126
22. PRICE
EPA Form 2220-1 (9-73)
U.S. GOYEBHM£NT PRINTING OFFICE 1977-7S7-056/&5I3 Region No. 5-1
116
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JEP 600/7 EPA
I 7 7 "087 I a d . Env. Re s. Lab.
ITHOR
Assessment of large-scale
TITLE photovoltaic materials pro-
duction
DATE DUE
CAVLOftO 4§
SORROWER'S NAME
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DATE DUE
BORROWER'S NAME
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(T
DATE DUE
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