life cycle assessment of photovoltaic modules · 2015-07-28 · life cycle assessment of...

UNIVERSITY OF CALIFORNIA, BERKELEY DEPARTMENT OF MECHANICAL ENGINEERING

Life Cycle Assessment of Photovoltaic Modules

Chris Gladden Euiyoung Kim

Zhou Lin Marcus Ulmefors

Prof. David Dornfeld Dr. Margot Hutchins

5/7/2012

A Life Cycle Assessment comparison on a module basis was performed for four of the most promising inorganic solar cell technologies. We found that in almost all impact categories the solar cells outperformed the grid by an order of magnitude, but that CdTe and CIGS solar panels had the lowest overall impact. The low energy use in production of thin film CIGS and CdTe more than compensates for their lower conversion efficiency.

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Contents

1. Introduction .......................................................................................................................................... 1

1.1. Background ................................................................................................................................... 1

1.2. Goal ............................................................................................................................................... 1

1.3. Methodology ................................................................................................................................. 1

2. Photovoltaic Technologies .................................................................................................................... 2

2.1. Crystalline Silicon .......................................................................................................................... 2

2.1.1. Background ........................................................................................................................... 2

2.1.2. Materials ............................................................................................................................... 2

2.1.3. Manufacturing....................................................................................................................... 3

2.1.4. Life Cycle Assessment ........................................................................................................... 4

2.2. Cadmium Telluride ........................................................................................................................ 4

2.2.1. Background ........................................................................................................................... 4

2.2.2. Materials ............................................................................................................................... 4

2.2.3. Manufacturing....................................................................................................................... 5


2.3. CIGS ............................................................................................................................................... 6

2.3.1. Background ........................................................................................................................... 6

2.3.2. Materials ............................................................................................................................... 6

2.3.3. Manufacturing....................................................................................................................... 6


2.4. Gallium Arsenide ........................................................................................................................... 7

2.4.1. Background ........................................................................................................................... 7

2.4.2. Materials ............................................................................................................................... 7

2.4.3. Manufacturing....................................................................................................................... 8


3. Methodology ......................................................................................................................................... 9

3.1. Scope ............................................................................................................................................. 9

3.2. Inventory Analysis ....................................................................................................................... 10

3.3. Impact Assessment ..................................................................................................................... 13

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4. Results ................................................................................................................................................. 13

4.1. Environmental Impact ................................................................................................................. 13

4.2. Energy Pay-Back Time ................................................................................................................. 15

4.3. Future Projection ........................................................................................................................ 16

5. Economics ........................................................................................................................................... 17

6. Social Impact ....................................................................................................................................... 19

6.1. Energy Independence ................................................................................................................. 19

6.2. Employment ................................................................................................................................ 20

6.3. Health and Safety ........................................................................................................................ 20

7. Conclusion ........................................................................................................................................... 21

8. References .......................................................................................................................................... 23

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1. Introduction

1.1. Background

Photovoltaic systems have been under active development for decades, but with improving efficiency

and declining cost, they have become an increasingly attractive method for electricity generation for

standalone or grid-connected applications. Solar technology holds the promise of clean and renewable

energy for a more sustainable world. However, the extraction of raw materials, manufacturing of solar

cells and end-of-life disposal of the modules are energy intensive processes and often involve toxic

chemicals that have a negative environmental footprint. In order to assess the impact of these

technologies it is necessary to perform a life cycle analysis (LCA) that examines all the sources of

environmental impact from material extraction to end of life disposal.

1.2. Goal

The goal of our study is to determine the relative environmental impacts of several solar cell

technologies that we have deemed most important for the future of solar energy. The four technologies

we selected for analysis are crystalline silicon (c-Si), thin film cadmium telluride (CdTe), copper indium

gallium di-selenide (CIGS), and gallium arsenide (GaAs). By providing a clear picture of where these

technologies stand, we hope to provide useful information to both manufacturers and consumers as the

solar cell technology market becomes increasingly diverse. While many other works have compared

various solar cell technologies, none has compared these four key technologies on a life cycle basis.

1.3. Methodology

LCA provides a tool that allows broad comparisons between dramatically different technologies. An

important determination in an LCA is what sustainability indicators are used to measure the

performance of certain technology. Evans et al. suggested using: cost, greenhouse gas emissions,

material availability or technological limitations, energy conversion efficiency, land use, water use, and

social impacts.1 Other LCAs are focused more specifically on greenhouse gases emissions from a wide

variety of technologies2–4 , while others examine a wide variety of indicators, such as marine water

toxicity, for a more specific set of technologies.5

In order to provide a unified framework for our analysis, we will use the CML 2000 method. The

“Handbook on life cycle assessment: operational guide to the ISO standards” (CML 2000) can provide

step-by-step guidelines for LCA studies based on ISO standards. Published in 2001 it is an updated

version of the previous work done by Heijungs et al. in 1992.6 Both publications were written at the

Centre of Environmental Science at Leiden University (CML) in the Netherlands and are recognized

globally. With the goal of facilitating comparison of results obtained by different groups, the CML 2000

handbook presents a standardized format for planning, undertaking and reporting results of an LCA.7

Since there is no existing LCA encompassing all four technologies of this study it will be necessary to

collect data from various sources. When comparing quantitative results from different studies it will be

required to either break down the numbers into their constituents, or alternatively ensure that the

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methodologies are compatible. Since LCA results typically are reported in aggregate form it is expected

that the CML 2000 standardized format will be helpful to facilitate comparison.

2. Photovoltaic Technologies The fundamental unit in photovoltaic systems is the solar cell, usually a single wafer of silicon, with a

typical size of 100 mm x 100 mm. Dozens of cells are joined together in series to create a greater voltage

drop for more efficient energy conversion. The finished solar module includes the glass encapsulation,

wiring, and metal frame required for installation of a solar power plant. As a result of the spacing

between cells, electronics and other inefficiencies solar module efficiencies are significantly lower than

the efficiency of the individual cells that make up the module. Our LCA is performed on a module basis,

in order to ensure the easiest comparison between different technologies.

2.1. Crystalline Silicon

2.1.1. Background

Silicon solar cells encompass a range of technologies. Monocrystalline and polycrystalline silicon cells

are based on conventional wafers while amorphous silicon is a thin film technology. In 2011 crystalline

silicon and amorphous silicon accounted for 68% and 1% of the US solar module production respectively

which motivated the choice of crystalline silicon as the reference case of this study8. Monocrystalline

modules can reach the highest efficiencies and in Q1 2012 SunPower successfully commenced

commercial manufacturing of cells capable of 24% conversion efficiency giving a module efficiency of

20.4%.9 Crystalline silicon is well documented and manufacturing cost has dropped with increasing

shipping volumes. However, since crystalline silicon modules use more material than their thin film

competitors (the semiconductor layer is typically ~100 times thicker) the manufacturing cost is

vulnerable to global silicon price volatility.

2.1.2. Materials

Silicon (Si) is the 2nd most abundant material in Earth’s crust but has to be purified before it can be used

to make solar cells. Its 1.1 eV bandgap is not too far from the optimal value of 1.4 eV as a single junction

cell. Si is also very stable and non-toxic. There’s also a great amount of expertise on silicon processing

built by the electronic industry and can be leveraged for solar cell manufacturing.

Typical silicon solar cell (Figure 1) 10 has an emitter and a base made of differently doped silicon (usually

n-type for emitter and p-type for base) and the p-n junction is formed between the two. The top surface

is usually passivated with oxide and texturized to better trap incoming light. Another layer of anti-

reflection material can also be deposited on top of the cell to reduce reflection.

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2.1.3. Manufacturing

There are two primary types of crystalline

silicon, mono-crystalline and multi-crystalline.

Mono-crystalline silicon is produced in a slow,

high temperature growth process that produces

very high quality single crystals of silicon. Multi-

crystalline silicon on the other hand is cast into

ingots which are allowed to crystallize

individually, resulting in a lower quality but less

expensive material. Silicon solar cell production

starts with the feedstock, which is then cut into

wafers using mechanical sawing and polishing.

After the wafers are created, they are doped

with ions in order to form a p-n junction that

turns the silicon into a solar cell. The next step

is the addition of metallic contacts to the p and n doped regions to allow for charge extraction. Finally

the solar cells are assembled into a module and placed in a frame. The energy consumption during

manufacturing is dominated by the silicon feedstock and wafer production, which have high heat loads

and result in large amounts of wasted materials.11

Figure 2 - Energy input for silicon module production, with the contributions of consecutive process steps. (data reproduced from Alsema et. al.

11)

Contributions from the different process steps are also shown in (Figure ) and we can see that while

poly-silicon has slightly higher energy consumption from feedstock due to its low efficiency, the mono-

crystalline silicon requires much more energy to produce wafers. The processes of cell and module

12.2

17.6

3.3 2.6 1.9

13.5

8.3

3.3 2.9 2.0

Si Feedstock Ingot+Wafer Cell Prod. Module Assem frame

Production Process

Energy Consumption During Silicon Module Production (GJ/kWp)

Mono-Crystalline

Poly-Crystalline

Figure 1 - Si solar cell schematic (figure reproduced from PVCDROM 2012)

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production, which are the same for both technologies, have less contribution to the cumulative energy

demand. For wafer production silicon carbide and other materials that are used in wafer cutting also

contribute to the energy and materials costs. 11

2.1.4. Life Cycle Assessment

Several seminal research papers from Alsema, E.A. & Wild-scholten, M.J.D and other scholars will form

the base line LCA for crystalline silicon and allow us to compare this well established technology to its

new competitors. For example, within the CrystalClear project important progress has been made to

quantify the life-cycle environmental impacts of crystalline silicon photovoltaic modules. 12 An up-to-

date set of life cycle inventory data has been established and published for the technology status of

2004 13, these data were subsequently updated to the status of end 2005 to early 2006 13. Based on

these data a detailed LCA has been made of present-day c-Si modules and PV systems 14,15.

Recent studies have shown module recycling is one promising option to decrease the negative

environmental impact of manufacturing silicon solar cells. Standard crystalline module recycling begins

with a thermal cycle for burning synthetic parts (EVA, the frame glue, the plastic backsheet of the

module etc.), which also allows raw materials- such as glass, the aluminum frame and the metals – to be

separated and then recycled. The metallic layers, anti-reflection covering and the doping layer of the

cells are removed chemically. The resulting wafers are either recycled in the manufacture of silicon

ingots (if they are damaged) or re-used to create new cells (without loss of efficiency). It is also possible

to test and re-use the cells, which are still working after delamination. Since the manufacture of wafers

from silicon is the most expensive energetic process in a crystalline module, its recycling significantly

reduces energy pay-back time (EPBT). Glass recycling is of comparatively minor importance.

2.2. Cadmium Telluride

2.2.1. Background

Cadmium Telluride (CdTe) is currently the most sold thin film technology with 23% of the US solar panel

production in 2011.8 The highest efficiency recorded for a CdTe module is 14.4% which was achieved in

December 2011 by First Solar, the company dominating CdTe production globally.16 Production costs for

CdTe modules are the lowest of all thin film technologies and in many cases already competitive with

crystalline silicon modules. However, current cost-effective manufacturing methods produce a slightly

defective CdTe layer which limits the maximum efficiency. Developing new manufacturing technologies

could likely increase the efficiency, but would result in more expensive panels.

2.2.2. Materials

Cadmium Telluride (CdTe) is one of the most common materials used in thin-film solar cells. CdTe has a

direct bandgap of 1.45 eV, making it ideal for absorption of the sun’s spectrum. It can be deposited on

cheap substrates such as commercial glass with good long term stability. Despite its low cell cost, the

two compounding elements of CdTe solar cells each have significant issues. Cadmium is toxic in its

elemental form, and so is CdTe when ingested, inhaled, or handled improperly. The long term safety

issue of CdTe had been noticed by the U.S. National Institutes of Health. Requested by NIH, the U.S.

Department of Energy's Brookhaven National Laboratory did in-depth research of CdTe solar cells and

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concluded that large scale CdTe solar module deployment would not pose significant risks to human

health, and with recycling at the end of product life, resolve environment concerns associated with toxic

chemicals. Use of cadmium in PV modules appeared to be more environmentally friendly than any other

usage. Indeed, CdTe PV cells would cause Cd pollution to the environment at a rate per kWh less than

1% that of coal power plants.17 Another concern is Tellurium’s limited reserve18, estimated at 22,000

metric tons, enough to generate about 0.35 TWp of solar power if fully utilized. Te is a byproduct of

copper mining and no known Te ore exists. There’s also uncertainty on the real quantity of Te available

since mining companies often don’t release data. As the production of CdTe increases, the availability of

resources may become a constraint. With current levels of reserves, the production of CdTe solar cells

could be expanded by a factor of 1000 to 10,000 without recycling of materials.19


The structure of a typical Cadmium Telluride (CdTe)

solar cell is illustrated in Figure 3. 20 The bulk of the

cell is made of CdTe as the main absorption layer with

a thin window layer of Cadmium Sulfide (CdS) on top

of it. Tin Oxide (SnO2) is the transparent conductor

and the glass capsulation protects the cell from the

operating environment. The back conducting layer is

made of Zinc Telluride with some copper doing and

titanium.

The dominant CdTe solar cell manufacturer in the

market today is First Solar. In 2011, the company

shipped about 1.4 GW of the world’s total 1.5 GW

worth of solar panels. 21 The manufacturing method

employed by First Solar is close-spaced sublimation (CSS). The process starts with a sheet of glass,

followed by a physical vapor deposition of a transparent conducting oxide, and then a chemical

deposition of a buffer layer. Then the CdTe absorber layer is evaporated at high temperature and

deposited onto the cooled substrate. The sheets of glass used in the process are very large, and to

define individual cells, the CdTe film is mechanically scribed. The majority of the energy consumed

during production comes from the high temperature vapor deposition techniques. While researchers

are working to develop lower energy techniques for depositing these materials, none have reached the

quality of vapor deposition.


Various LCAs have been conducted to evaluate the environmental impacts of CdTe since it’s one of the

major commercialized thin film technologies 22,23,24,25,26. Overall CdTe solar cells seem to have relatively

low energy intensity during module production and low level of harmful chemical emissions on a life

cycle basis. CdTe technology has a shorter energy pay-back period, which indicates its high effectiveness

in energy measures.22 In other common impact categories such as abiotic material input, water usage,

global warming potential and acidification potential, CdTe technology scores quite high as well. We

Figure 3 - CdTe solar cell (reproduced from

Fundamentals of Photovoltiac Devices)

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hope to be able to more directly compare CdTe to its competitors by analyzing several of the seminal

CdTe LCAs.

2.3. CIGS

2.3.1. Background

Copper-Indium-Gallium-Selenide thin film solar cells are increasingly popular and accounted for 7% of

the US production in 2011.8 Laboratory CIGS have achieved cell efficiencies of 20% but as of today the

most efficient module in commercial production is made by MiaSolé at 14% conversion efficiency. 27,28

CIGS modules are of interest since the solid solution can be tuned to contain a fraction of indium and

gallium thus optimizing the band gap. The CIGS module record conversion efficiencies have increased

several percentage points in the last few years. Combined with innovations in manufacturing technology

such as the NanoSolar roll printing it is predicted that manufacturing costs are going to drop. 29

2.3.2. Materials

Copper Indium Gallium di-Selenide (CIGS) is a direct bandgap material with bandgap of 1.50 eV. Much

like CdTe, it can achieve high conversion efficiency and long term stability on cheap substrate materials 30. Unlike CdTe, the elements in CIGS are not considered to be dangerously toxic, but indium, gallium

and selenium are rare elements and are typically recovered as by-products from zinc, nickel, or copper

mining. Indium supply is of particular concern, since its price has fluctuated dramatically in the past 5

years due to shortages. Typical CIGS solar cells only use a few micrometers of material, so there is little

material restriction at current levels of production, but like CdTe, this could become an important issue

as production volume increases.


Typical CIGS cell structure resembles CdTe in that it also

has a bulk absorption layer of CIGS and a CdS window

layer with transparent electrode on the top (Figure 4).

The fact that CIGS is a four-element compound

complicates its manufacturing and there is significantly

less known about this compound than silicon. There is no

unique method for CIGS module production. A more

traditional process is used by for example the California

manufacturer MiaSolé where the CIGS semiconductor

layer is formed by co-evaporation in a controlled

atmosphere. In contrast with the layered module process

is the high volume production technique employed by

NanoSolar, also headquartered in California. Instead of

building the module in several layers using vacuum technology the cells are printed using rollers. The

key component is a nanoparticle ink which is printed onto an aluminum foil in normal atmosphere.

Individual cells are created by cutting the roll into appropriate size and are subsequently performance

matched and joined together to form panels. Eliminating the need for a controlled vacuum atmosphere

facilitates high volume production 29.

Figure 4 - CIGS solar cell structure6

(reproduced from Fundamentals of Photovoltaic Devices)

7


In 2008 an LCA was performed by “Sustainability Evaluation of Solar Energy Systems” (SENSE) examining

CIGS module including Balance of Systems (BOS). Most of the negative impact related to the

manufacture of the CIGS module was due to the energy intensive “absorber simultaneous evaporation”

process. In addition the balance of system, or BOS (inverter, cables, aluminum frame) is responsible for a

non-negligible contribution (about 13%) of the global warming potential 31. No LCA has yet been

performed for the Nanosolar printing roller production technology. Since the energy intensive

evaporation is avoided it is possible that the impact could be greatly reduced. In addition the increased

size of the panel requires fewer rails and clamps which reduces both cost and environmental impact of

the BOS 29,32.

2.4. Gallium Arsenide

2.4.1. Background

Gallium Arsenide (GaAs) is in many ways the ideal material to use for solar cells. It has high absorption,

an ideal bandgap, and has excellent electronic properties, all of which allow GaAs to achieve record

efficiencies for single and multi-junction solar cells.33 As of early 2012 GaAs is not yet commercialized.

The industry leader AltaDevices announced in February 2012 that a module efficiency of 23.5% had

been achieved and independently verified by the National Renewable Energy Laboratory (NREL).34 GaAs

is currently only developed in laboratories and used in niche applications due to challenges related to

scaling up the manufacturing technology. However, the cells may finally have the potential to achieve

high market penetration. Since this emerging technology has long been overlooked by the solar research

community, it is important and timely to include a projected LCA for GaAs solar cells in our comparison.

2.4.2. Materials

Gallium Arsenide (GaAs) is a III-IV compound. Widely used in LED, GaAs is a direct band material,

meaning that it has a high absorptivity and only requires a few microns of material to absorb light

efficiently. Its bandgap is 1.43 eV, almost ideal for single-junction solar cell and by replacing some Ga

atoms with other group III element, the bandgap can be further tuned for even higher efficiency. Unlike

Si solar cells, GaAs cells are relatively insensitive to heat and resistant to radiation damage, making it a

good candidate for space applications.

Gallium Arsenide (GaAs) is a highly toxic material, which also has very attractive semiconductor

properties. The state of California lists GaAs as a known carcinogen, and its disposal into the

environment must be carefully controlled. During wafer production, a single crystal (boule) of GaAs is

sliced into wafers, and then these wafers are chemically and mechanically polished. This processing

accounts for the majority of the GaAs waste, and it has been estimated that 85% of the original material

is lost during processing.35 In order to reduce the impact of this waste generation, some amount of the

waste can be recycled. While gallium is considered a rare element, scarcity is not expected to be a

problem in GaAs solar cell production.

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During the energy crisis in the 1970s GaAs solar cells were explored seriously36

, but the development of

GaAs solar cells has been slow compared to silicon and other thin film technology. The high

temperature and time consuming growth processes required to produce the GaAs solar cells has

traditionally been prohibitively expensive for most applications.37

Recently several companies have

begun exploring a technique to dramatically reduce the cost of these solar cells based on the ability to

peel thin layers of GaAs off of a reusable substrate. Using this technique, originally described by

Yablonovich et al.38

, GaAs may finally have the potential to achieve high market penetration.

As a model GaAs solar cell we have chosen to analyze Alta Devices recently demonstrated high efficiency

solar cells. Alta Devices is a small Silicon Valley startup company that currently holds the world record

for single junction solar cell efficiency and is in the process of building its first small volume pilot

fabrication facility.39

High quality GaAs devices can only be built by chemically depositing layers of GaAs

and its alloys onto a single crystal GaAs wafer using high energy and high toxicity process such as metal

oxide chemical vapor deposition (MOCVD).40 The GaAs wafer has traditionally been a large part of the

cost of a GaAs solar cell, but it served no purpose other than to assist in the growth of the device layer

and provide mechanical support. Alta Devices have developed a technique to produce GaAs solar cells

by first depositing the active material onto a thick GaAs wafer and then peeling off the finished devices,

which are only a few microns thick, and placing them on a different substrate in a process called

epitaxial lift off (ELO).41,42

This technique allows the expensive GaAs substrate to be used many times

and greatly reduces cost and embedded energy in the process.


In 2003 Meijer et al. performed an LCA on GaInP solar cells, one of the first analyses of a solar cell

similar to GaAs.43

The results reported indicated that GaInP solar cells had a longer energy payback

period than multi-crystalline silicon. In 2006 a detailed LCA using the CML 2000 method by Mohr et al.

describes both GaAs thin film and GaAs/GaInP multi junction solar cells fabricated by a similar ELO

method to the one used by Alta Devices. 5 Error! Reference source not found. shows the process flow

hat was analyzed. This LCA is the best available model to guide our own analysis, since many of the

material extraction costs and process will be the same. Items such as the GaAs wafers, electronic grade

arsine and phosphine, and trimethyl compounds of gallium, indium and aluminum are commodity items,

and will have the same extraction cost regardless of the downstream processing.

An important conclusion from Mohr et al. was that the GaAs MOCVD deposition process was by far the

most energy intensive part of the fabrication of GaAs modules, consuming 66% of the total primary

energy demand, as seen in Figure 5. The deposition process (MOVPE) consumes by far the most energy,

but the wafer production is still a significant source of energy use, despite the fact it is recycled up to 20

times. Based on their analysis the study concluded that GaAs solar modules have a slightly longer energy

payback period and are similar in terms of environmental impact to multi-crystalline silicon. A follow up

study by Mohr et al. shows that while the production of cells have a relatively high environmental

impact when the electricity used comes from fossil fuels, using the solar cells themselves to power the

production dramatically drops the CO2 emissions.44

9

Figure 5 – Energy consumption during the GaAs solar cell manufacturing process. The MOVPE deposition process dominates the production, and should be the first area to be studied to reduce costs and environmental impact. (data reproduced from

Mohr et al.44

)

While the work done by Mohr and colleagues established a good initial study of these devices, the

equipment studied was off the shelf and production methods were dramatically lower volume than

what is being developed by Alta Devices. Significant cost and energy savings can be realized by

development of specialized production equipment, particularly for the MOCVD reactor. The MOCVD

reactor is the key item in the LCA of GaAs solar cells, and although thermodynamics have been well

described by several researchers, there is still significant work involved in adapting this to a new reactor

design.45,46

3. Methodology

3.1. Scope

A life cycle analysis (LCA) is a holistic analysis of a product’s impact on the environment. It can include

everything from resource procurement, production, consumption, and disposal.7 The methodology

used for an LCA is typically performed in three steps. First is the identification of goals and scope of the

analysis. Our project has the specific goal of comparing the four most promising inorganic solar cell

materials on a module production basis, with the scope of the study limited to the modules themselves.

This limited scope allows us to focus on the important details and subtle differences between the solar

technologies to help shed light on where improvements should be made, and which technologies are

ripe for adoption.

2.2 0.6 0.19 0.9 0.32

22

0.44 0.9 4.1 3.0

0

5

10

15

20

25

30

Production Process

Energy Consumption During Production (GJ/kWp)

10

3.2. Inventory Analysis

The second step in an LCA is inventory analysis. In this stage we identify what information is needed and

where it will be acquired from. It is important to maintain consistency when collecting data, otherwise

the analysis will be biased. In our study we have identified four key papers that will provide a significant

amount of our data. Since these previous studies were based on different conditions, the data

presented are not immediately comparable and will require significant additional analysis. Our analysis

methodology is summarized in Figure 6.

Figure 6 - Data Analysis Methodology: Orange text indicates where external data was used in the reverse engineering of the sources, blue text indicates where our own uniform data was used to perform our impact assessment.

To perform our own comparison, we have developed a framework within which several different

existing LCAs can be synthesized into a single comparison. The four source papers chosen cover life

cycle analyses of solar cell technologies we had identified as important, including crystalline silicon11,

CdTe47, GaAs5 and CIGS48. The advantage of these papers is that most contain a similar poly-silicon

reference case, and the analyses were all performed using the same CML 2001 methodology. These two

common threads makes adapting these sources for side by side comparison possible, however,

challenges still exist. There are differences among the studies with respect to system boundaries,

reference for energy/material flows, normalization factors, etc. Some of these differences can be

addressed by simply adjusting the assumptions used for the LCA, while others require extraction of data

from reverse engineering, or re-calculation of certain terms. The goal is to make a comparison

reasonable and meaningful while presenting the data in a fair and consistent context. Table 1

summarizes the key information for the source papers we will be using, with the highlighted boxes

representing areas where a conflicts needed to be resolved.

11

Table 1 - Comparison of metrics, sources and assumptions. Conflicts are highlighted in orange and assumed values are highlighted in blue.

CdTe

47 GaAs

5 Silicon

11 CIGS

48

BOS Yes No Yes Yes

Normalization Factor Europe 2000 World 1995 W-Europe 1995 None

Amortized Data No Yes Yes Both

Grid Mix UCPTE UCTE (manufacturing)

Netherlands (use) UCTE UCTE

Grid Efficiency 32% 41% 31% Not Specified

Functional Unit kWp, kWh, 1m² kWp kWp kWp, MJ

Irradiation (kWh/m²a) 1700 1000 1700 1700

Performance ratio 0.75 0.75 0.75 0.925

Life time (y) 20 30 30 20

Installation Location Southern Europe Netherlands Southern Europe Europe

Database EHT-ESU Ecoinvent 1.2 Ecoinvent 2000 GaBi 4

System Boundaries Materials, Module

Production, Use

Materials, Module

Production, Use

Materials, Module

Production, Use

Materials, Module

Production, Use, End-

of-Life

Recycling/Decomission No No No Yes

CML 2000 Yes Yes Yes Yes

LCA Software Method ISO 14040, SUMMA Eco-indicator 99 Simapro

Ecoinvent 1.01

ISO 14040

GaBi 4

Environmental Metrics GWP, AP, GER GWP, AP, EP,GER GWP, AP, EP,GER GWP, AP, EP,GER

The first issue that was resolved was to de-normalize the two papers that had used normalization

factors. This was done by dividing the data by the normalization factors provided in the literature.49,50

The next step was to remove the amortization that had been applied to extract the raw manufacturing

impact data from the papers. The equation for the lifetime energy production per kWp of a solar cell is

given as:

[

] [

] [ ]

Since a solar panel produces almost all of its impact during manufacturing it is common to assume a

lifetime and installation location and then amortize the manufacturing emissions over the entire life of

the panel. Since all the papers performed this amortization differently, we had to reverse engineer all

the data and then re-amortize the data on a uniform basis. It is important to note that the system

efficiency is different from the panel efficiency, which does not factor into this equation. Since the

analysis is per kWp of panel, two panels with different efficiency would still produce the same power

using different areas. Two papers quote a product life time of 20 years while the other two choose 30

years. For our comparison the 20 year period is used. For our analysis the functional units are 1 kWp

(kilowatt peak) for the solar cell materials and inverter, and 1 m2 for the rest of the balance of systems,

however a few important pieces from the source material are provided in the incorrect functional units

and need to be converted. The terms are interchangeable knowing the solar cell’s energy conversion

efficiency (peak power = area x solar irradiation per area x conversion efficiency). Solar irradiation is also

12

quoted differently, with values ranging from 1000 [kWh/m2 yr] to 2200 [kWh/m2 yr]. For our analysis a

value of 1784 [kWh/m2 yr] is chosen since it is representative of the average insolation in the SF Bay

Area. The final factor used is system efficiency, which is the ratio of output from the solar cells to the

actual usable energy, this is different from the solar cell efficiency. A value of 75% is used in all but one

paper, so for our analysis we also used 75%.

Once the data have been de-amortized, another issue to consider is the grid mix that supplies electricity

to module production. UCTE is the organization that manages the European grid, and is widely used and

all papers except the study by Mohr et. al.5 specifically use this grid mix. Mohr quotes the European grid

mix for production and the Netherlands grid mix for the use phase. Since our analysis will recalculate

the use phase the Netherlands data is unimportant, however we will have to assume that the European

average is the same as the UCTE mix since specific data cannot be found on the grid mix used for

production in the source. This assumption is well justified since the UCTE was the grid system for

Europe when the paper was published.

As for system boundary consideration, some caution needs to be taken regarding CIGS module

production since the analysis includes end-of-life decommissioning. This data is presented in

percentage contributions from recycling and manufacturing, so to account for this difference we used

the provided data remove the recycling contributions. 48

To re-amortize the data we took all the collected data and applied our own amortization factor

uniformly across all the data, which allows the output from the different LCAs to be compared directly.

In order to calculate energy payback time (EPBT) the grid energy conversion efficiency (from thermal

energy to electricity) is needed. This is commonly quoted between 31-33% for the UCTE, for our

analysis we used a value of 33%, which represents a more modern grid efficiency. The equation for

energy payback time is:

[ ] [ ]

[ ]

[

]

The primary energy use during production is found from the papers we analyzed, and the annual

primary energy offset is calculated based on the given factors (Performance Ratio = 75%, Grid Efficiency

= 33%, Insolation = 1784 kWh/kWp yr)

We also chose to include the impact of the Balance of Systems (BOS), which is all of the power

electronics, inverter, wiring, frame and other items that are required to produce useful energy from a

solar panel. To include these costs we used the detailed BOS data provided by the SENSE study{Citation}

and applied the same analysis method that was used to normalize the module data.

13

3.3. Impact Assessment

The final stage of the LCA is the impact assessment. The goal is to take the data gathered during the

inventory analysis and produce meaningful conclusions that address the goals set forth in the beginning.

Typical impact assessment will break the impact into relevant categories, each of which represents a

different environmental hazard related to the product. The impact assessment for our analysis will

include impact categories that all 4 papers have covered in common, namely global warming potential,

acidification potential, eutrophication potential and ecotoxicity. These four commonly used indicators

provide a good picture of both hazardous material emissions and energy consumption as a result of

production. Either the absolute value or a normalized value can be presented, though in many cases

normalized values are presented for simplicity. A North America normalization factor can also be

considered for more meaningful comparison, since the original studies were all normalized to European

grid standards. For our results we chose not to normalize, to allow the data to be more easily compared

to new data as it becomes available.

In addition to these environmental impact categories we also include a comparison of energy payback

time for the four technologies. The energy payback time is important because it compares the total

energy consumed during production of the solar module with its annual power production. This allows

for a diverse set of technologies to be examined to determine which can offset the most energy when

produced in large volumes.

As a final portion of our analysis, examined each technology and try to identify companies that are

increasing production and efficiency, which will reduce energy payback time and environmental impact.

To do this we gathered the latest efficiency data for production level panels from four companies,

MiaSole for CIGS, First Solar for CdTe, Alta Devices for GaAs, and SunPower for Silicon. Using these data

we projected the impact that the increased efficiency would have if the costs per m2 remained constant.

This gives a conservative estimate, since in reality the costs per m2 would be expected to reduce slightly

as well since a larger production volume is typically more efficient.

4. Results

4.1. Environmental Impact

The impact of the different solar panel technologies is compared using several different CML 2001

metrics. One of the most important metrics is the global warming potential (GWP), which is expressed

in grams of carbon dioxide equivalent per kWh of energy production.

14

Figure 7 – Global warming potential of four photovoltaic technologies compared to the European grid.

Our analysis shows that the four technologies are quite comparable, and all perform significantly better

than the grid. It is worth noting that unlike the grid electricity, the solar panels emit all of their pollution

during manufacturing, which is then amortized over the lifetime of the device. The GWP of all the

devices is most heavily influenced by the semiconductor deposition process. GaAs proves to have the

highest GWP, owing to its very energy intensive MOVPE crystal growth process. CIGS has the lowest

thanks to its lower energy input during the semiconductor deposition. In addition to module, Figure 7

shows the impact from the Balance of Systems, which is negligible for GWP, but has a more significant

impact on other metrics.

Figure 8 – Acidification Potential (AP), Eutropification Potential (EP), and Photochemical Ozone Creation Potential (POCP) for

the four different technologies. Units of AP have been adjusted to centigrams so as to fit on the same scale.

514

49 53 62 60

EuropeanGrid

CIGS CdTe GaAs Poly-Silicon

Global Warming Potential (g CO2-eq/kWh)

UCTE

BOS

Module

457

201 282

23 25 41 34 17 19 13

84

20 23 24 19

Gri

d

CIG

S

Cd

Te

GaA

s Si

Gri

d

CIG

S

Cd

Te

GaA

s Si

Gri

d

CIG

S

Cd

Te

GaA

s SiUCTE

BOS

Module

AP (cg SO2 eq/kWh) EP (mg Ph-Eq/kWh) POCP (mg Ethene-Eq/kWh)

15

The CML 2001 methodology describes a large range of environmental indicators. For our analysis we

focused on global warming potential, but we have also included the results for three other indicators. In

Figure 8 we present three of the most common impact indicators.

Acidification potential (AP) gives an indication of sulfur emissions which can cause acid rain. Again GaAs

stands out as having the highest impact due to its semiconductor deposition process. There is also a

significant contribution to AP from the Balance of Systems (BOS) which is a result of the sulfur released

during manufacturing the inverter.

Eutropification Potential (EP) measures the amount of artificial nutrients that are released into the

environment. These nutrients, such as phosphorus, can cause algae blooms and foul drinking water.

Interestingly silicon solar cells show very strong EP, possibly as a result of the phosphorus used during

purification and doping of the crystal.

Photochemical Ozone Creation Potential (POCP) is a measurement of the smog creation potential,

where different organic compounds can react with the atmosphere to create phenomena such as the LA

basin smog layer. While the solar technologies all had relatively low POCP scores, it is interesting to

note that the BOS accounted for a much larger portion of the POCP, largely as a result of the organic

compounds used in the electronics manufacturing.

4.2. Energy Pay-Back Time

Energy Pay-Back Time (EPBT) is a measurement of how quickly the solar panel will offset the energy

required to create it. For our analysis we used the San Francisco Bay Area as our installation location.

Figure 9 – Energy Pay-Back Time for the four technologies and the Balance of Systems. The significant advantage of the thin film technologies shows that a less energy intensive deposition process more than offsets the decreased efficiency. Cell

efficiencies are shown in orange, note the inverse correlation between efficiency and payback time.

1.5 1.6

2.5 2.1

CIGS CdTe GaAs poly-Si

Energy Pay-Back Time in Bay Area, California (yr)

BOS

Module

16

The energy payback time for all technologies is quite short (Figure 9), due to the good solar resource in

California. GaAs has the longest energy payback time, again due to the large amount of energy required

to deposit the semiconductor. What is interesting is that the high efficiency technologies (GaAs and Si)

have significantly longer energy payback times. The lower quality thin film technologies are energy

efficient enough during manufacturing to more than make up for the fact that they are only half of the

conversion efficiency. The Balance of Systems energy consumption is dominated by the inverter, which

is the same for all four technologies, so the more efficient technologies do not get a relative break from

the reduced area required for the same power. Clearly more development is needed to reduce the

energy consumption during the production of GaAs for it to be competitive with its thin film cousins.

4.3. Future Projection

As a part of our analysis, we applied the latest record efficiencies reported for the four technologies of

interest. In their respective technologies, each of these companies is the current front runner in

efficiency. Assuming the same areal costs and impact, we projected what these enhancements in

efficiency would yield in terms of EPBT and GWP.

Figure 10 – Projected Energy Pay-Back Time for the four technologies, using the current best efficiencies from the four companies shown and the areal costs derived from our analysis. The actual energy pay-back time could be even shorter,

since manufacturing may become more efficient on a per m2 basis as well.

First Solar and SunPower have made the most progress in terms of advancing beyond the previous

generation efficiencies and as a result both companies see significant drops in projected EPBT. Alta

17

Devices is still pre-production so its projected EPBT is quite uncertain, be we expect it should be around

that value or lower, owing to their significant investment in new deposition technologies.

Figure 11 – Global Warming Potential of the four technologies compared to the projected performance of the current efficiency champions.

The global warming potential results mirror those of the EBPT, again showing First Solar and SunPower

making significant gains, with SunPower edging very close to the thin film technologies. In just 5 years

between when most of the data from these LCAs was collected the market has driven significant

improvements in performance. These economically motivated developments have also had a positive

impact on the environmental performance of these different technologies.

5. Economics The profitability of solar panel production depends on price of silicon, progress in production

technology, and subsidies. In recent years production volumes have been soaring in large part due to

European subsidies and increased polysilicon production capacity. The so called feed-in tariffs are

covered by European electricity consumers and give additional financial initiatives for photovoltaic

installations. While subsidy levels have remained almost constant in recent years the solar modules

prices have dropped significantly resulting in rapidly growing volumes. For instance Italian sales

18

increased by 857% in 2010. Due to the rapid expansion of installations in Europe the feed-in tariffs have

become increasingly costly and in Germany eventually withdrawn. The expected fall in European

demand will likely limit the global growth to less than 10% annually in coming years. Many

manufacturers therefore find themselves with excess capacity and unsold modules. To avoid stocking

unsold units in a market with declining prices many suppliers have been forced to sell at very low prices,

leading to bankruptcy of numerous firms in the US and abroad, most notably perhaps Solyndra in the fall

of 2011. 51

In the United States installation rates have been lower than in for example Germany although both

production and installation volumes are now increasing as prices drop. Comparing Q4 of 2011 to Q4 of

2010 shows a drop in polysilicon module production cost from $1.92/W to $1.15/W due to an average

polysilicon price of $43/kg (down from $68/kg in 2010). US solar module installations grew from 887

MW in 2010 to 1855 MW in 2011 (+109%) accounting for 7% of global installations (up from 5%). The

average PV system price fell 20% due to decreased component costs and higher efficiencies.8

Figure 12 - US Production in 2011 by technology 8

Crystalline silicon still dominates the solar panel market although thin film technologies such as CdTe

and CIGS are increasing their market share year-on-year. In coming years it is projected that thin film

solutions will increase further. First Solar, the company dominating CdTe production, has reported that

their panel production cost in Q4 2011 was 0.75$/W which is competitive with silicon based modules.52

Today the cheapest solar panels in the US produce electricity at approximately $120-$140/MWh which

is to be compared to $70/MWh for modern American onshore wind or $70-90/MWh for gas power

plants. With directed subsidies or by the implementation of a carbon tax it will be possible for solar

power to be financially competitive with fossil fuels. In developing economies the benefits are

potentially even greater since there is often plenty of sunshine and grids are unreliable or non-existent.

In India rural populations use diesel generators to produce electricity, at a cost of $250/MWh. 51

68%

23%

7%

2%

Crystalline Silicon

CdTe

CIGS

Other

19

In following years capacity will continue to increase leading to lower prices for consumers. In addition it

is expected that CdTe and CIGS productions technologies will be further improved and thus increase

competition. GaAs modules are still in early development and not produced on industrial scale although

companies like Alta Devices are working on commercializing the highly efficient modules. In applications

such as space missions where power density is more important than cost GaAs is the preferred

technology.

Looking a few years into the future it is predicted that solar panel prices will continue to drop since

manufacturing capacity is currently growing more quickly than sales, in part due to withdrawn subsidies

in large European markets. In the short term there is consequently a risk of decreasing margins putting

pressure on manufacturers to innovate and adopt an optimum business strategy. As innovation in

manufacturing technology continues it is expected that thin film technologies will continue to increase

their share of the market. CdTe panels are likely to lead since the production cost is already competitive.

The promise of superior efficiency will possibly make CIGS panels more competitive as manufacturing

technology matures. GaAs will continue to lag in the short term but is interesting in the medium to long

term since it delivers the highest efficiency. Overall it is believed that thin film modules will play an

increasingly important role in the global market. The speed of growth will among other parameters

depend on the price of silicon since it determines the comparative cost advantage.

6. Social Impact

6.1. Energy Independence

We can explain social impact of solar technology by taking a look at current Energy Independence

movement in the United States that includes the crucial role of solar energy among other energy

sources. Briefly, Energy Independence is a powerful verbal icon defined in the context of the 1973 Arab

oil embargo, and now it is regarded as the Vision of America’s energy future and the title of America’s

new energy policy. According to the American Energy Independence official website, over last few

decades, the Unite States’ economy has become dependent on imported energy resources; 57% of all oil

consumed in nationwide is imported.53

To increase energy independence, solar energy is considered as a great substitute of imported energy

sources by being produced in domestic for local demands. Scientists believe that, theoretically, Energy

Independence can be achieved with solar energy alone, if we increase the efficiency of solar panels in a

way that absorbs the entire amount of sun radiation into the earth, which is greater than the demand.

Actually, the amount of sun radiations per hour is more than the demand of the entire population uses

during one year. So it is convincing where the opportunity area of the solar energy will be in future

energy production market. 54,55

Moreover, the cost of solar panel has been significantly decreasing approximately 10% per year, and this

fact expedites the proliferation of solar energy usage over other energy sources like fossil fuels.

According to the Figure 13, the installed cost of electricity expected to be under $0.05 in 2020.56

20

Figure 13 - Cumulative production GWp (figure reproduced from Global Warming Policy Foundation)56

6.2. Employment

Solar energy industry creates new job opportunities by hiring new employees in the entire stages;

production, manufacturing, usage, and recycling. According to SEIA (Solar Energy Industries Association),

Solar energy can create 1.5M new jobs by 2020 in this energy field if the government has strong

commitment to deploy solar energy, remove latent market barriers, and educate the public about the

benefit of solar energy.57

In this sense, the importance of the government’s commitment cannot be overemphasized. One

example can be found from the Treasury Grant 1603 program below. Treasury Grant 1603 is the funding

program provided by The US government to provide grants and support the development of Solar

energy (as well as renewable energies). This program has been creating more than 59K new jobs and the

number is still growing. Moreover, the overall number of job creation from solar industry including

Treasury Grant 1603 program so far is much larger. There were approximately 93K people who were

employed by the end of Aug. 2010.58

6.3. Health and Safety

Since solar energy doesn’t burn fossil fuels and doesn’t cause air pollution, it is considered relatively

safer and healthier than other traditional energy sources. One of criteria we could compare safety rate is

to see the difference of death rate among these energy sources. It is used to measure safety of energy

resources. According to the comparison shown in Figure 14, solar energy has a 0.44 death rate (per

TWh), which is higher than 0.04 of nuclear energy, but still significantly lower than the ones of

traditional fossil fuels such as coal and oil as they cause severe health related illnesses and deaths

mainly from air pollution.59 The air pollution data was collected from the WHO and the Externe Study

conducted to evaluate the externalities of energy production.

21

Figure 14 – Death rate per TWh based on data collected from all energy industries. (data reproduced from NextBigFuture.com)

59

7. Conclusion All four solar PV technologies we studied have smaller environmental impacts than electricity

generations in the European grid in the four impact categories. In almost all cases the grid has roughly

ten times the environmental footprint than the four technologies under investigation. The only

measurement where PV technology is of significant percentage in comparison to the grid is crystalline

silicon solar cell’s eutrophication potential, due to the large quantity of phosphate used in silicon wafer

purification process. The technologies are comparable among themselves in regard to environmental

impacts and the levels of equivalent emissions are quite close.

With both the module and balance of system (BOS) production taken into consideration, the four

technologies have relatively short energy payback periods (EPBT), from 1.5 years in the case of CIGS to

2.5 years for GaAs. That means in about 2 years, those solar PV systems can generate enough electricity

to compensate for the amount of energy put into manufacturing them. Considering the designed

lifetime for those systems are well over 20 years, solar PV systems can achieve energy neutral status

fairly early in the life cycle and have a net positive energy output for most of its operation time.

Additionally, by adopting state of the art cells from industry leaders, the energy payback period can be

shortened even further. For example First Solar’s 14.4% efficient CdTe module can be energy neutral in

just one year. One note on EPBT is that the calculation is region specific and operating the same PV

system in a different region from the Bay Area (used in our calculation) can have different EPBT value.

LCA is comprehensive and systematic (reproducible and easy to check) when conducted properly. By

summarizing and extracting information from multiple studies, we were able to compare the

environmental impacts of different solar PV technologies with reference to the European grid baseline.

Taking into account the limit and constrains posed by incomplete or outdated data, we’re confident to

conclude that solar PV systems put less burden on the environment than the current grid mix. To further

161

36 4 12 12 0.44 0.15 1.4 0.04

Death Rate (deaths per TWh)

22

validate the conclusion, more accurate and up-to-date data should be used when possible, and the use

and disposal phases should also be incorporated to produce an even more convincing result.

23

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