how green is that product lecture notes week 8
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Lecture Notes for How Green is That Product? 2015 Northwestern University.
How Green is That Product?
An Introduction to Life Cycle Environmental Assessment
Coursera Lecture Notes
March 2015
Prepared by:
Eric Masanet and Yuan Chang
McCormick School of Engineering and Applied Science
Northwestern University
Evanston, IL, USA
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Lecture Notes for How Green is That Product? 2015 Northwestern University.
Table of Contents About these lecture notes ...................................................................................................................... 3
Lecture 1: The life-cycle perspective and course goals .......................................................................... 4
Lecture 1 Supplement ............................................................................................................................ 8
Lecture 2: Understanding unit processes ............................................................................................. 10
Lecture 2 Supplement .......................................................................................................................... 16
Lecture 3: Constructing unit process inventories: Part 1 ..................................................................... 18
Lecture 3 Supplement .......................................................................................................................... 23
Lecture 4: Constructing unit process inventories: Part 2 ..................................................................... 25
Lecture 4 Supplement .......................................................................................................................... 29
Lecture 5: Energy flow basics ............................................................................................................... 33
Lecture 5 Supplement .......................................................................................................................... 37
Lecture 6: Mass balances .................................................................................................................... 39
Lecture 6 Supplement .......................................................................................................................... 45
Lecture 7: Goal definition ..................................................................................................................... 48
Lecture 8: Scope definition: functional units ....................................................................................... 52
Lecture 8 Supplement .......................................................................................................................... 57
Lecture 9: Scope definition: initial system boundaries ....................................................................... 59
Lecture 10: Scope definition: requirements for data and data quality ................................................ 67
Lecture 11: Scope definition: review and reporting ............................................................................. 73
Lecture 12: Life cycle inventories: the basics ...................................................................................... 77
Lecture 13: Life cycle inventories: mass flows and cut off criteria ..................................................... 82
Lecture 14: Life cycle inventories: data estimation .............................................................................. 87
Lecture 15: Life cycle inventories: multi-functionality ........................................................................ 92
Lecture 16: Life cycle inventories: system expansion ....................................................................... 100
Lecture 17: Life cycle inventories: data quality assessment ............................................................. 109
Lecture 18: Life cycle inventories: Input-output (IO) methods ......................................................... 115
Lecture 19: Life cycle inventories: EIO-LCA ....................................................................................... 121
Lecture 20: Life cycle inventories: IO uses and limitations ................................................................ 127
Lecture 21: Life-cycle impact assessment: the basics ....................................................................... 131
Lecture 22: Life-cycle impact assessment: how it works .................................................................. 136
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Lecture 23: Life-cycle impact assessment: impact categories ........................................................... 142
Lecture 24: Interpretation: the basics ............................................................................................... 147
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About these lecture notes There are many useful resources for learning the life-cycle assessment (LCA) methodology, including
books, websites, case studies, publicly-available lecture materials, and LCA standards and
guidebooks. Rather than choose one particular resource as the assigned reading, the course staff
has prepared this compendium of lecture notes, which will serve as your primary reference for this
course. These notes make use of elements of key online LCA resources that are available to
students, and refer you to them where appropriate for additional information on different LCA
topics. Additional readings will be assigned or suggested throughout our MOOC as part of the
homework assignments, through the discussion forums, and when discussing specific LCA case
studies.
Each chapter relates to a single video lecture. The first section in each chapter contains a full
transcript of the video lecture. These transcripts will allow you to read along with the lectures as
you watch them, to write down comments at different points in a lecture, and to refer to the lecture
content when you are offline.
In many chapters, a second section has been provided, which contains additional notes that expand
upon points made within the lecture and refer you to other LCA resources as appropriate. Because
Coursera video lectures are inherently short, weve made use of the additional notes sections to
provide you with supporting information that couldnt be included in the video lectures due to time
constraints. Weve also added additional notes to further discuss topics that proved particularly
interesting or challenging in past offerings of the MOOC. Within the transcript section, youll see
blue arrows in the left hand margin that look like this:
This symbol indicates that additional notes have been provided. Each additional note has been
assigned a number, which also appears in the blue arrow symbol (in our example above, this
number is 1.1). The numbered blue arrows will allow you to easily jump back and forth between the
transcript and the additional information that is relevant to a particular topic.
Lecture notes will be released on a week-by-week basis.
We hope these lecture notes can serve as a basic, useful reference for you in your learning
experience. Suggestions for improving or expanding these lecture notes for future offerings of this
course are heartily welcomed. We hope you enjoy our journey together learning about and applying
the LCA methodology. Lets get started!
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Lecture 1: The life-cycle perspective and course goals Transcript
Hello, and welcome to How Green is that Product? An Introduction to Life Cycle Assessment. Im
Eric Masanet, and Ill be your instructor for this course. I hope youve been looking forward to this
as much as I have.
This course will provide you with a basic working knowledge of life cycle assessment, or LCA for
short. Now, you wont become a certified LCA practitioner in only nine weeks. However, you will
learn how to construct LCA studies that provide transparent results, to build basic LCA models in
spreadsheets, and to collect, analyze, and interpret environmental data in a structured manner for
better decisions.
But perhaps most importantly, youll learn that -- whatever the product -- everything has
environmental impacts and that understanding these impacts requires sound data and thorough
analysis. If you stick with me, youll be equipped with the basic skills to conduct such analyses and
begin answering environmental questions of your own.
So what exactly is LCA? LCA is a method to assess the environmental impacts of a product, process,
or service that involves four major steps:
1. Determine the goals and scope of the LCA;
2. Compile an inventory of energy and mass
inputs and outputs across all relevant life
cycle stages;
3. Evaluate relevant environmental impacts
associated with the life-cycle inputs and
releases; and
4. Interpret the results to lead to a more
informed decision.
Lets first discuss what is meant by life cycle stages using this plastic bag as an example. In this
course, well refer to five distinct stages of the product life cycle:
1. Raw materials acquisition, which includes processes related to raw materials extraction and
refining. For our plastic bag, which is made of a plastic called high-density polyethylene or
HDPE for short, raw materials acquisition would include extracting and processing natural
gas and transporting it to a chemicals plant.
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2. Manufacturing, which includes processes that convert raw materials to finished products.
In our case, plastic bags are manufactured by producing plastic pellets, melting them into a
film, and forming the bags.
3. Distribution, which includes transporting and stocking products for consumption. For
example, our plastic bag will be shipped from the manufacturer to a grocer.
4. Use/reuse, which is the stage where products perform a useful service to the consumer. In
our case, the plastic bag will carry our groceries home. Some consumers might also reuse
the bag for additional shopping trips or as a garbage can liner, which is why we often include
reuse in the use phase as well.
5. Stage 5 is the end of life stage, where products enter the waste management system.
Depending on local waste management practices, the plastic bag might be recycled,
landfilled, or incinerated to generate energy.
So what is meant by relevant impacts? As youll learn in this course, an environmental impact is a
consequence associated with inputs and outputs of energy and mass across the product life cycle.
For example, the combustion of diesel fuel in the trucks that transport plastic bags to the grocer
releases carbon dioxide, which leads to global warming. When conducting an LCA, we strive to
include all non-negligible impacts so that informed decisions can be made and any tradeoffs
between impacts are made explicit.
Consider again this plastic bag. Many jurisdictions have banned plastic
bags at grocery stores in an effort to reduce litter. However, several
LCA studies have shown that if consumers shift to paper bags, more
diesel trucking might be required. Why is that? Its because a paper
bag takes up more space than a plastic bag, and therefore more trucks
might be required to bring the same number bags to the grocer. So in
this case, one tradeoff of a shift from plastic to paper grocery bags
might be that plastic litter is reduced but diesel fuel use and emissions
are increased.
This case teaches us two important lessons. First, an LCA can reveal that, while we think were
making a green choice, environmental impacts may shift based on the consumption choices we
make. Thats why its important to consider all relevant impacts in an LCA; otherwise such shifts in
impacts might be missed when were evaluating our options. Second, consideration of all life cycle
stages allowed for identification of unintended consequences. That is, a reduction in plastic litter in
the end of life stage might come at the cost of increased diesel fuel use in the distribution stage. If
we just focused on non-biodegradable litter, surely paper bags would look greener than plastic. Its
only by looking at all life cycle stages did we see that paper bags might make things worse in the
distribution stage. So you see that even the simple case of plastic versus paper bags involves
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environmental tradeoffs. With proper application of the LCA method, however, these tradeoffs are
made visible so we can make the most informed decisions.
You may be wondering how LCA is used in the real world, or, more directly, how you might use LCA
after completing this course. If youre an engineer, LCA can help you choose materials and design
features that lead to greener products and technologies. If youre a policy maker, LCA can help you
design public policies and incentives that improve sustainability without simply shifting
environmental problems from one type of impact to another. If youre a consumer, LCA can arm you
with data and results that guide you to greener purchasing decisions. And no matter what you do,
LCA can give you a healthy degree of skepticism of the environmental claims that are so often made
without hard data and through analysis to back them up.
Lets wrap up with an overview of what you can expect. Each lecture will
introduce a new concept, which will be reinforced through online quizzes,
homework, and the course notes. I believe LCA is best learned by
jumping in hands on, so in this course youll build an LCA model of a
simple product that you should all be familiar with a bottled soft drink.
No special LCA software packages will be required; all that is needed is a
spreadsheet.
Each week youll be developing a new section of the model that relates to
that weeks lecture material, so by the end of the course youll have built
a complete bottled soda LCA. While the product is fairly simple, by
building the model across all life cycle stages and impacts, youll acquire the skills and perspectives
that should allow you to move on to more complex products after you complete this course.
Lastly, well also occasionally offer separate videos describing real-world LCA studies that highlight
key material, so you can easily see how the theory relates to practice in real time.
Im looking forward to this experience together. See you next time!
Additional notes
Correction: In the lecture video, I say Compile an inventory of energy and material inputs and
environmental outputs across all relevant life cycle stages when I really should have said Compile
an inventory of energy and mass inputs and outputs across all relevant life cycle stages. The goal of
LCA is to include all relevant mass flows, whether they are materials, resources (such as water or
biomass), pollutants to the environment, or products to society.
Correction: As youll see in Homework 1, natural gas must be extracted and processed before it can
be used in industrial systems. Processing is aimed at improving natural gas quality by removing
impurities. In the lecture video, I say raw materials acquisition would include extracting natural
gas and transporting it to a chemicals plant when I really should have said raw materials
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acquisition would include extracting and processing natural gas and transporting it to a chemicals
plant.
Correction: In the lecture video, I say an environmental impact is an adverse consequence
associated with inputs of resources and outputs of pollutants across the product life cycle when I
really should have said an environmental impact is a consequence associated with inputs and
outputs of energy and mass across the product life cycle. In reality, not all impacts arising from life-
cycle systems are negative. For example, a biomass system may sequester carbon dioxide from the
air and a remediation technology may remove hazardous pollutants from soil to make it safe again.
By quantifying all flows of mass and energy across a life-cycle system (and not just resource and
pollutant flows), LCA enables us to explore both adverse and positive impacts associated with these
flows. While well focus exclusively on adverse impacts in this course, it is helpful to keep in mind
that LCA can just as easily quantify positive impacts.
Starting in week 3, youll begin building your very own LCA model of a bottled soft drink packaged in
plastic. See the Course Project section of the course website for more details. (The Course
Project section can be accessed by clicking on Start Here! or Course Information in the left
hand navigation pane on the course website.) Note also that I say bottle of soda in the lecture
video, which is a term used commonly in North America to refer to bottled soft drink.
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Lecture 1 Supplement Transcript
Welcome to our first lecture video supplement. Supplements such as this one have been added to
improve the course content and to provide additional discussions and examples to help you better
understand the topics covered in our core lecture videos.
In this first supplement, Id like to give you a better idea of what to expect in this course as well as
some tips for success based on past course offerings.
First, I highly encourage you to review all of the material provided on the Start Here! section of
the website, which includes important information on policies, our course schedule, and further
details on the project.
Lets take a look at the course schedule, which lists the topics well cover in this course. The first
two weeks of this course will cover core skills that are necessary for sound LCA, such as constructing
unit process inventories, conducting energy and mass balances, and understanding data
conventions. These are the essential building blocks of an LCA. In Week 3, well begin applying
these building blocks to learn the LCA methodology and to start constructing our very own LCA
models.
For more information on the LCA models, lets take a look at the Project section of the website,
which describes the scope and intent of the course project. Youll be exposed to two different LCA
models, both of which will be developed in spreadsheets.
The first is an LCA model for a plastic grocery bag that has been developed by the course staff. The
spreadsheet consists of different tabs that contain the various elements of the LCA model, which
we'll reveal in week by week fashion as we learn each step of the LCA methodology. Think of our
plastic bag LCA model as an example of how your bottled soft drink LCA model should be
constructed and how it should function, and refer to it often for inspiration and guidance.
The second is the LCA model for a bottled soft drink, which youll be developing yourself. Starting
in Week 3, youll be given tasks to construct your model based on recent lecture topics.
Furthermore, some of the homework assignments will contain exercises that help you build specific
portions of your model. By following the development of our plastic bag LCA model, and by
completing the homework and modeling tasks to construct your own bottled soft drink LCA model,
youll gain valuable hands on experience. The course staff will also post regular solutions for the
bottled soft drink model, which you can use to check the accuracy of your spreadsheet.
Id also like to draw your attention to the discussion forums. If youve taken Coursera courses in the
past, youll know that the discussion forums can be a great way to enhance your learning
experience, but that they can also become unwieldy to navigate over time. To minimize forum
fatigue, weve established specific sub-forums for different types of posts. For example, there is an
Assignments sub-forum that you can use for posts related to specific homework assignments.
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There is also a Lectures sub-form for posts related to the lectures each week. Please review the
available sub-forums and be sure to choose the most logical sub-forum first before you make a post.
If we all do this, the discussion forums should be much more useful and manageable for everyone.
Youll also notice that Ill be suggesting discussion topics each week. These questions should be fun
to explore together, and will help us all think about how LCA relates to our own lives and the
sustainability problems wed like to solve. While participation isnt mandatory, I highly encourage
you to join in or review the posts whenever you can. The topics have been selected from some of
the most interesting and thought-provoking discussions in past offerings, so Im sure youll enjoy
them.
Finally, here are some quick tips for getting the most out of this course and earning a high grade:
First, if you need to improve your spreadsheet skills, please use the first two weeks of this course to
do so. Weve provided a specific discussion sub-forum that students can use to share spreadsheet
tips and tricks. Once we introduce the LCA models in Week 3, you may find it difficult to keep up if
youre not comfortable with spreadsheets.
Second, while the first two weeks of this course are somewhat basic, the level of difficulty and
required effort will increase in Weeks 3 9 when we move into the LCA method and modeling.
Therefore, you should plan for a greater time commitment in the last 7 weeks of the course.
Third, please take full advantage of the discussion forums for seeking out help and providing help to
others. In past offerings, many questions related to homework assignments, project tasks, and LCA
concepts were collectively answered by students through ongoing discussion. And you may find
that assisting others deepens your own understanding of the course material.
Fourth, while I encourage students to exchange ideas, please try to complete the assignments and
project tasks on your own before seeking out answers online. Learning through trial and error is
important for any course, and especially for the LCA methodology given its many details and
nuances.
Fifth, and finally, try to review some of the additional resources that are indicated in the lecture
notes. This course only covers basic LCA concepts, but the additional resources we mention provide
a wealth of information that can bring you closer to LCA proficiency if you have the time to review
them.
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Lecture 2: Understanding unit processes Transcript
Welcome back! Today well begin learning about the data structure of an LCA, starting with LCAs
most fundamental building block: the unit process model. But first lets quickly review what we
learned yesterday.
The product life cycle can be divided into five major stages: raw materials acquisition,
manufacturing, distribution, use, and end of life. In our plastic bag example, we learned that raw
materials acquisition covers the extraction, processing, and transportation of natural gas, which is
then converted into ethylene. Ethylene is converted into HDPE and formed into a bag in the
manufacturing stage. Next, the bag is distributed to retail stores, where it is filled with groceries to
transport food home during the use stage. Lastly, at the end of life stage, the bag is either recycled,
landfilled, or incinerated to generate energy.
We also learned that a key step in all LCAs is to compile an inventory of energy and mass inputs and
outputs across all relevant life cycle stages. So how do we compile such inventories? We do so by
modeling the product life cycle as a series of unit processes. The ISO 14040 standard for LCA
defines a unit process as the smallest portion of a product system for which data are collected
when performing a life-cycle assessment.
This is a picture of a generic unit process. On the left we have inputs of energy and mass required to
generate a useful product output. On the right we have the outputs of environmental emissions
and co-products that are associated with the process, along with the product output itself. From
now on, well refer to the inputs and outputs associated with a unit process as the unit process
inventory, which is a term commonly used by LCA practitioners.
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To visualize how we use unit processes, lets look more closely at the manufacturing stage of our
plastic bag. The first step is to convert processed natural gas into ethylene, which well represent by
this first unit process model.
The second step is to convert ethylene into HDPE pellets, which well represent with this second unit
process.
The third step is to melt the HDPE pellets, extrude a film, and form the bags in the bag production
process.
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As youve probably guessed, to construct a complete LCA model for the plastic bag, wed need to
develop and apply unit process models to capture all unit processes at each life cycle stage. We can
then sum all the unit process inventories to quantify the total environmental footprint of the bag life
cycle. Youll learn how to do this later; for now, you may be asking yourself how such unit process
inventories and life-cycle models can be developed without detailed engineering knowledge.
Fortunately, we have we have databases and literature sources to help us in this regard.
For example, a unit process inventory I obtained from the literature for converting ethylene to HDPE
pellets looks like this. If this level of detail seems a bit daunting, dont worry youll learn how to
work confidently with unit process inventory data in this course.
Fortunately, the LCA community has adopted a number of conventions for organizing unit process
inventories to make our lives easier. These conventions help ensure that inventories are intuitive
and use the same data structure for easy transfer between researchers and databases. So while the
unit process inventory for HDPE pellets may look complicated, thanks to this structured organization
of data it is actually simpler than it looks.
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First, many unit process inventories refer to inputs and outputs as flows or exchanges. In this
course, well use the word flows. Unit process inventories are essentially comprised of flow
information listed in rows.
In many LCI databases, flows are further characterized as flows to or from nature or to or from the
technosphere. In this course, well adopt this convention and organize our inventories into the
following four types of flows:
1. Inputs from nature,
2. Inputs from the technosphere,
3. Outputs to nature, and
4. Outputs to the technosphere
Inputs from nature are probably pretty obvious: they include flows such as crude oil extracted from
the ground or corn harvested from a field. Conversely, outputs to nature include pollutants and
wastes that are released back into the environment. Inputs from and outputs to the technosphere
refer to any flow of energy or mass that originates from a man-made process. For example, diesel
fuel is produced from crude oil in a petroleum refinery, but we dont find diesel fuel occurring
naturally in the environment.
For our plastic bag, the extraction of natural gas describes a flow from nature. After extraction,
natural gas must be processed to remove impurities. In the next unit process, that processed
natural gas is converted into ethylene. Here, because the natural gas came from a pipe and not the
ground, it is considered an input from the technosphere. Because ethylene is an intermediate
product that is used by other unit processes, it is considered an output to the technosphere.
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Why do we need to distinguish between flows to and from nature and flows to and from the
technosphere? Besides helping us better visualize the origins and destinations of flows in our
inventory, identifying flows to and from nature allows us to quantify environmental impacts in the
life-cycle impact assessment step of an LCA. Well learn more about impact assessment later in the
course. For now, lets get used to organizing our unit process inventories in this way.
Lastly, well use SI units to describe all flows in our unit process inventories in this course. For
example, mass will be expressed in grams, energy in joules, and volume in liters. Some of you may
wish to review the SI system before proceeding with this course; further readings are provided in
this weeks course notes.
Additional notes
Correction: Here weve added in the processing step that was omitted in the lecture video. See
Note 1.2.
Correction: Here again I should have referred to energy and mass inputs and outputs instead of
energy and materials inputs and environmental releases. See Note 1.1.
The ISO 14040 series of standards are a set of best practice rules and guidelines for conducting
LCA that have been developed and revised by the international LCA expert community since the
1990s. Well be referring to these standards often throughout the course. Well use them to discuss
the step by step nature of an LCA and to reinforce best practices. Unfortunately, the actual
standards documents are not freely available to the public. However, youll get a basic
understanding of these standards through our class materials and through the additional readings
well suggest and assign. There is no need to purchase the standards to benefit from the content of
this course. For those who would like to learn more about the formal standards, please visit the
International Organization for Standardization (ISO) website at:
http://www.iso.org/iso/home/store/catalogue_tc/catalogue_tc_browse.htm?commid=54854
Correction: Here weve changed materials and energy to the more general and correct energy
and mass. See Note 1.1.
For clarity, weve specified that it is processed natural gas that is converted into ethylene.
Processed natural as is a flow from the technosphere. This change was necessary to reduce
confusion in past course offerings as to whether natural gas from nature or natural gas from the
technosphere is used in ethylene production. See the Lecture 2 supplement video for more
information.
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To give you a sense of the detail contained in a typical life-cycle inventory (LCI), and the
documentation that explains and supports such inventories, take a peek at the following report.
Youll use some of these data in this course to build you spreadsheet LCA model of a bottled soft
drink. There is no need to carefully read this report now, or to understand its contents. But looking
it over will give you an idea of the types of information sources that we rely on when constructing
LCA models.
Franklin Associates (2009). Life Cycle Inventory of Three Single-Serving Soft Drink Containers:
Revised Peer Reviewed Final Report. Prepared for the PET Resin Association. Eastern
Research Group. Prairie Village, KS. http://www.container-recycling.org/assets/pdfs/LCA-
SodaContainers2009.pdf
Similar to the reasons for Note 2.5, here weve added After extraction, natural gas must be
processed to remove impurities. In the next unit process, that processed natural gas is converted
into ethylene. See the Lecture 2 supplement video for more information.
There are many useful resources online for reviewing conversions from Imperial and US Customary
units into International System (SI) units. While well use SI units in this course, you are likely to
encounter data sources in your project and in your LCA careers that are expressed in Imperial
and US Customary units. Here are some conversion resources that the course staff recommends.
International System of Units from NIST. Essentials of SI units, background, and
bibliography. http://physics.nist.gov/cuu/Units/
A concise summary of the International System of Units from BIPM.
http://www.bipm.org/utils/common/pdf/si_summary_en.pdf
OnlineConversion.com Convert just about anything to anything else.
http://www.onlineconversion.com/
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Lecture 2 Supplement Transcript
To ensure that you understand the concept of a unit process and the distinctions between inputs
from nature, inputs from the technosphere, outputs to nature, and outputs to the technosphere,
lets step through the plastic grocery bag example in a bit more detail. Furthermore, lets try
working backwards in the life cycle so that the different types of flows are clear.
Lets first consider the factory that makes plastic grocery bags. The production of plastic bags
involves melting HDPE pellets, extruding the melted plastic into a film, and cutting the film into the
shape of a bag. For simplicity, well include these steps in one unit process that well label HDPE
Bag Manufacturing. The output of this unit process is an HDPE grocery bag. Since this bag will be
shipped to a grocer for use by consumers, well label this flow as an output to the technosphere.
To manufacture the plastic bag, the bag factory requires inputs of HDPE pellets, which are a man-
made product. Therefore, well label this flow as an input from the technosphere. Of course, there
are many other flows associated with the bag factory, such as inputs of energy to power processing
equipment and outputs of mass, including emissions of air and water pollutants. For now, well
ignore these flows to keep things simple.
The production of HDPE pellets occurs at a chemical factory, which converts ethyleneanother
man-made productinto HDPE resin. Lets label this unit process as HDPE Resin Manufacturing,
and denote the flow of ethylene into the factory as an input from the technosphere.
Ethylene is manufactured from processed natural gas at an olefins plant, which well label as
Ethylene Manufacturing in our simple example. Remember that processed natural gas does not
come directly from nature; rather, it is made by removing impurities from raw natural gas. Hence,
well label this flow as an input from the technosphere.
To produce processed natural gas, another unit process is required that well call Natural Gas
Processing. This unit process requires extracted natural gas, which is yet another technosphere
product that we get as an output from natural gas drilling operations.
Finally, lets label the natural gas drilling unit process as Natural Gas Extraction. The input to this
unit process is natural gas from the ground, which is an input from nature. Observing the entire
system, its now clear that to manufacture the HDPE grocery bag, a series of different unit processes
are required. These unit processes are linked by technosphere flows that can eventually be traced
back to an original exchange with nature.
Moving forward, youll be developing more detailed inventories of energy and mass flows across
unit process systems. For example, we could further include the input of processed natural gas to
be combusted for heat in HDPE resin manufacturing as well as the smokestack emissions of carbon
dioxide and other air pollutants that arise from natural gas combustion. Here, emissions of carbon
dioxide would be labeled as a flow to nature.
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As youll come to learn in future lectures, specifying and tracking types of flows in unit process
systems is critical from an accounting perspective, because the environmental impacts of a system
are related to its flows to and from nature. In our case, you can probably imagine that the sources
of impact in our system so far are related to the resources we extract from the ground and to the
pollutants we reject into the air.
Youll get more practice with labeling flows in Homework 1.
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Lecture 3: Constructing unit process inventories: Part 1 Transcript
Welcome back. In todays lecture, well dive deeper into how unit process inventories are
structured for ease of interpretation and ease of transfer between researchers and databases. Last
time I introduced the four types of flows well use in our inventories:
1. Inputs from nature,
2. Inputs from the technosphere,
3. Outputs to nature, and
4. Outputs to the technosphere
Lets take a closer look at the complete unit process inventory for converting ethylene to HDPE
pellets. Ive created this inventory in a spreadsheet in the same way that youll be creating unit
process inventories in your spreadsheets. As we discussed last time, flow data appear in rows of the
inventory table, and they are organized into our four types of flows. In this course, the first column
in the inventory will always contain the flow type, starting with inputs from nature, followed by
outputs to nature, inputs from the technosphere, and outputs to the technosphere.
The second column will always contain the name of the flow, which, by convention, uses standard
names for products (e.g., diesel fuel), pollutants (e.g., carbon dioxide), and resources (e.g., water).
In many cases, the name of the flow will be taken directly from the LCI database from which we get
the flow data. It is critically important to use standard flow names and to use them consistently so
we can link up unit process inventories correctly when creating our LCA model.
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The third column contains additional information on the origins and destinations of flows to and
from nature, which well refer to in this course as the flow category. Inputs from nature will
always be denoted as resources in the category column, while outputs to nature will be denoted
by the medium to which they are released. There are three media well denote: air, water, and land.
The fourth column is reserved for subcategories of the third column. For example, the
subcategories for outputs to air include emissions to areas with low population density and
emissions to areas with high population density. And the subcategories for resources include
resources extracted from in the ground (like coal), from water (like drinking water), or from the
biosphere (like wood). In this course, well use a standard set of subcategories to describe inventory
flows. Ive provided the list of subcategories well use in the lecture notes because there are too
many to mention here.
Why do we need information on flow categories and subcategories? The main reason is that this
information helps us better quantify the environmental impacts caused by flows to and from nature
in the life-cycle impact assessment step of an LCA. For example, you might easily imagine that a
pollutant emitted in a high population density area will have a higher human health impact than if it
were emitted in a low population density area where there are fewer persons exposed. Well learn
more about impact assessment later in the course.
I also want to mention that in many LCI databases, flows to and from nature are referred to as
elementary flows. So you arent confused by this, moving forward well also use this label for our
flow types in unit process inventories.
By convention, well always use the category product for flows to and from the technosphere.
This makes sense when we consider that once a resource enters the technosphere, it is converted
into different forms of products for further use by industry and society.
The fifth column in our inventory table will always contain a numerical value and our sixth column
will always contain the unit in which that value is expressed. Where do these values come from?
Typically through some combination of direct measurement, engineering estimation, or literature
sourcing. Knowing where the data come from and how to determine their quality is a critical step in
any credible LCA, and one which well discuss later in this course. For now, just assume that all data
in our inventory come from reliable sources.
The numerical value expresses the amount of each flow that corresponds to the units of product
output listed in the inventory. For example, our product output is one kg of HDPE pellets, and the
emissions of CO2 to air associated with the production of one kg of HDPE pellets is 100 g CO2.
Here the product output is expressed in units of mass; however, the product output in a unit process
inventory can be expressed in many different units depending on what goods or services are
provided. The unit process of pellet production logically has product outputs expressed in units of
kg, which corresponds to physical production. However, a unit process for a diesel freight truck
might have product output expressed in units of kilogram-kilometers, which corresponds to the
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useful service provided by trucking. Or a unit process for electricity production might specify kWh of
electricity produced, which is the useful output of that process. Youll get exposed to all of these
types of outputs and more moving forward.
Lastly, our simple example inventory focused on single unit process, but youll often encounter unit
process inventories that combine several unit processes into one aggregated inventory. For
example, rather than finding every unit process step in the manufacture of the bag which would
include natural gas extraction, transportation, conversion to pellets, and bag forming you might
just find a single inventory for all of these processing steps combined. This aggregated inventory
would contain the sum of all included unit process flows to and from nature.
Aggregated inventories are quite common in practice, because they can simplify a complex chain of
processes for general use. Aggregated inventories also protect private entities who may not want to
release detailed unit process data on each step in their production chain. The downside is that one
loses visibility on which of the aggregated processes might be hot spots and often the ability to
recreate the inventory using process-level knowledge.
How can you tell if you have an aggregated inventory? Good databases will tell you this in their
inventory documentation. Youll notice terms like cradle to gate, which refers to flows from
nature to a certain point in the technosphere, or gate to gate, which refers to flows between
points in the technosphere. All unit processes included in the aggregated inventory should be listed
explicitly.
Additional notes
When you gain access to the spreadsheet LCA models in Week 3, the structure and contents of this
unit process inventory will make more sense. For now, just concentrate on following the logic for
each column, and how that information will be useful when you link together many different unit
process inventories to construct a systems model.
In the models well use in the current offering of this course, the order of flows has been updated as
follows In this course, the first column in the inventory will always contain the flow type, starting
with inputs from nature, followed by outputs to nature, inputs from the technosphere, and outputs
to the technosphere. The updated order is reflected in the spreadsheet figure as well.
In our plastic bag and bottled soft drink LCA models, well use a simplified set of categories and
subcategories for all flows. As discussed in the lecture video, well adopt the convention of using
the category Product for all flows to and from the technosphere. Product flows will not be further
divided into subcategories.
Inputs from and outputs to nature that is, elementary flows will be labeled using the following
simplified set of categories and subcategories in our inventories.
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Elementary flow type Category Subcategory
Inputs from nature Resource Biotic (from biosphere)
In air
In ground
In water
Outputs to nature Air High population density
Low population density
Land Unspecified
Water Unspecified
There are several important caveats to our simplified selection of elementary flow categories and
subcategories.
First, because this is a basic introductory course, the course staff has chosen to keep our flow
conventions simple. Once you get in the habit of labeling flow categories and subcategories at a
basic level, youll be well equipped to use more detailed protocols for labeling of flow categories and
subcategories in the future. To get an idea of the level of detail that many LCA practitioners use
when conducting LCAs and working with LCA databases, take a look at the following reports:
Overview and methodology: Data quality guideline for the ecoinvent database version 3
(2013), Weidema B P, Bauer C, Hischier R, Mutel C, Nemecek T, Reinhard J, Vadenbo CO,
and Wernet G.
http://www.ecoinvent.org/fileadmin/documents/en/Data_Quality_Guidelines/01_DataQual
ityGuideline_v3_Final.pdf
The ecoinvent database is used widely by LCA practitioners and within various LCA software
packages. Take a look at Table 9.1, page 63, which lists the compartments and sub-
compartments (i.e., categories and subcategories) used for elementary exchanges (i.e.,
flows) in the ecoinvent database. Youll notice that many more subcategories are available
for defining flows with greater precision in practice.
U.S. LCI Database Project Users Guide, National Renewable Energy Laboratory (2004).
http://www.nrel.gov/lci/pdfs/users_guide.pdf.
The U.S. LCI data contains publicly-available life-cycle inventory (LCI) data that are reported
using a standardized unit process inventory structure. Well make use of some of the data
from the U.S. LCI database in this course. Take a look at the table on page 16. Youll notice
many categories and subcategories that are similar to those in the ecoinvent database, but
also some differences. Again, the subcategories listed allow for greater precision when
documenting flows.
Second, even though the categories and subcategories included in many LCA databases can be quite
detailed, in practice many LCI data sources do not include such detail in their reporting. For
example, one may find that pollutant outputs to water are reported, but that this flow is not further
specified as an output to a lake, ocean, or river. Thus, in many LCI data sources, the most common
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subcategory youll encounter is unspecified. The publicly-available data sources well use in our
course projects do not contain such detailed specification of subcategories, either. This is another
reason well keep our labeling of flow categories and subcategories simple in this course!
Third, as discussed in the lecture video, the primary benefit of identifying categories and
subcategories for elementary flows is that it can enable more sophisticated estimation of life-cycle
impacts. In your course project, the labeling of air emission flows with the subcategories high
population density and low population density can enable the estimation of human health
impacts to both types of demographic areas. Well discuss impact analysis later in this course.
In the spreadsheet models, and throughout this course, numbers will be expressed using the U.S.
numeric convention where commas separate thousands and the dot (or decimal point) is the
decimal separator. For example, the number one thousand two hundred and one-tenth is written
1,200.1 in the US numeric convention. However, when working with spreadsheets in this course,
you can change the numeric format in which data are displayed in your spreadsheet software to
match your local numeric convention.
Weve added in the term to and from nature here, because the process of aggregation eliminates
intermediate flows to and from the technosphere in the system. See the Lecture 3 supplement video
for a simple example of unit process inventory aggregation.
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Lecture 3 Supplement Transcript
In this video supplement, well use the simplified system of unit processes for HDPE grocery bags
that we discussed earlier. Below the figure Ive added in an inventory table that contains a
simplified list of flows for each unit process. In this example, well only track a few flows to illustrate
how inventory aggregation works. However, youll practice aggregating much more complicated
inventories later in this course.
Lets start with the unit process inventory for HDPE Bag Manufacturing. In this simplified inventory,
its only input is 1.02 kilograms (kg) of HDPE pellets and its only outputs are 1 kg of HDPE grocery
bags and 0.5 kg of carbon dioxide (CO2) emissions to air. By convention, the flows of HDPE pellets
and HDPE grocery bags are labeled as product flows from and to the technosphere, respectively.
Also by convention, the flow of CO2 is labeled as a flow to nature, or elementary flow, and to air.
Now lets take a closer look at the Natural Gas Extraction process. Its only input is 1.08 kg of in-
ground natural gas, which is a resource flow from nature. Its only outputs are 1.05 kg of extracted
natural gas and 0.02 kg of CO2 emissions to air. Youll notice that the next unit process, Natural Gas
Processing, requires 1.05 kg of extracted natural gas as a product input. If you look carefully at the
rest of the unit process inventories, youll also notice that the product mass output of each unit
process matches exactly the product mass input that is required by the next unit process.
This means that my unit process inventory data have all been properly scaled to produce the mass
flows necessary to ultimately manufacture 1 kg of HDPE grocery bags. Youll learn how to scale unit
process inventories later in this course. For now, you just need to understand that since the product
mass flows have been balanced across all unit processes, we can simply add up the flows of CO2 to
arrive at a total CO2 emissions footprint for the system.
In this example, to ultimately produce 1 kg of HDPE grocery bags, the unit processes in the system
will collectively emit 2.02 total kg of CO2 to the air. One can also scan the inventory data to
determine which unit processes account for the greatest share of CO2 emissions; namely, HDPE Bag
Manufacturing, HDPE Resin Manufacturing, and Ethylene Manufacturing.
In a similar fashion, I could also add up all resource inputs from nature in the system, which, in this
case, would amount to 1.08 kg of in-ground natural gas required to ultimately produce 1 kg of HDPE
grocery bags.
In fact, using these totals I could create a single inventory for the entire system, which would just
contain the inputs from nature, the outputs to nature, and the product output of the system. Such
an inventory is known as an aggregated unit process inventory, because it represents the sum totals
of flows to and from nature associated with all unit processes within its system boundaries. These
flows are expressed relative to the mass quantity of the final product output from the system, in our
case, 1 kg of HDPE grocery bags.
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Another way to think of aggregation is that Ive drawn a boundary around the entire system and Ive
only counted the flows that cross this boundary in my aggregated inventory; namely, flows from and
to nature and flows of the final product to the technosphere. All of the intermediate product flows
in the system do not cross this boundary, and are therefore not counted. This makes sense when
you observe that all of these flows will simply cancel out; for example, the ethylene output from the
Ethylene Manufacturing unit process will subsequently be consumed as a product input by the HDPE
Resin Manufacturing unit process.
As you gain more practice with LCA, youll notice that many data sources contain aggregated unit
process inventories. Aggregation can be done as a matter of convenience, since it can be quite time
consuming to work with inventories for all intermediate unit processes in a product system, even for
simple products. Aggregation is also often done for confidentiality reasons, so that data on
individual factories or processing steps within a system are not revealed to the public. For example,
assume that you have obtained only the aggregated inventory for 1 kg of HDPE grocery bags. While
you would know the total CO2 emissions to air from the cradle to gate system, you would have no
way of identifying HDPE Bag Manufacturing, HDPE Resin Manufacturing, and Ethylene
Manufacturing as the largest contributors to this CO2 footprint.
In our spreadsheet models for our plastic bag and bottled soft drink, well make use of aggregated
inventories as a matter of practicality and convenience. However, well be sure to carefully
document the system boundaries associated with the aggregated inventories we use, so that we and
others can understand which intermediate unit processes have been included therein.
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Lecture 4: Constructing unit process inventories: Part 2 Transcript
In this course, well mostly be using data from available databases and literature sources that have
already been neatly organized into structured unit process inventories. In practice, however, LCA
analysts must often construct new unit process inventories by gathering data from various sources.
Today well practice constructing our own unit process inventories to help you gain proficiency in
data compilation and analysis. Well also learn an important convention for ensuring we can scale
our unit process inventories for use in different LCA models.
Lets suppose we are conducting an LCA of a residential hot water heater that is fueled by natural
gas. In todays example, well be constructing the unit process inventory for the use stage of the
water heater, which refers to its operation. Ive gathered some data on the average natural gas
consumption and hot water generation of US residential hot water heaters from the U.S.
Department of Energy and the U.S. Environmental Protection Agency.
The average U.S. residential hot water heater consumes 27 gigajoules (GJ) of natural gas per
year
The average U.S. residential home uses 64,000 liters of hot water per year
As you gain more experience with LCA, youll probably notice that there are typically more data
available on the energy consumption of different processes and products than there are for other
flows such as water pollutant releases and solid waste generation. The reason for this is quite
simple: energy use is easy to track because it is something we pay for and monitor closely.
Moreover, many regional governments track energy supplies and demands as part of energy policy
planning. When we have energy data, it is often fairly easy to derive air emissions data as well
based on combustion emission factors for various fuels, which are readily available.
For example, since I know our residential water heater uses natural gas, it was fairly easy to find the
following air pollutant emission factors for natural gas combustion in residential appliances. These
came from the US Environmental Protection Agencys AP-42 emission factor reports:
56,000 grams of carbon dioxide (CO2) per GJ of natural gas combusted
44 grams of nitrogen oxides (NOx) per GJ of natural gas combusted
19 grams of carbon monoxide (CO) per GJ of natural gas combusted
4 grams of particulate matter (PM) per GJ of natural gas combusted
The lesson here is that generating a unit process inventory that contains data on energy flows and
energy-related air emissions flows is often possible when we cant find existing unit process
inventories in LCI databases or literature sources. Unfortunately, data on flows of water pollutants,
solid waste generation, and other elementary flows that are not related to a unit processs energy
use are typically much harder to come by outside of LCA databases. The reason for this is also quite
simple: these flows are harder to monitor and record in practice, and many firms do not release
such data publicly.
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So now I have all the data in hand to construct a simple unit process inventory for hot water heater
operation. First, I determine the annual air emissions associated with the natural gas combustion by
simple multiplication. When I have these data, I can now create a simple inventory using the
structure weve discussed. Processed natural gas is a flow from the technosphere, and the air
emissions are flows to nature. Lastly, my product is 64,000 liters of hot water.
While this inventory is reasonable for the average U.S. home, is it highly useful in its current form?
In other words, can I easily use it in other analyses, such as to analyze hot water generation for an
LCA of a home dishwasher? For example, if a dishwasher uses less than 64,000 liters of hot water, I
cant directly apply this inventory. Luckily, one useful convention for unit processes inventories with
single product outputs is that such outputs are expressed as multipliers of 1, for example, 1 liter of
hot water or 1 kWh of electricity.
Having a multiplier of 1 in our denominator makes for much easier scaling of unit processes to
different product output quantities. In the hot water example, lets say I want to calculate the CO2
emissions associated with generating only 5,000 liters of hot water.
First I divide all inputs and outputs in my unit process inventory by the product output to get flows
per liter. Next, I recreate the inventory on this basis. Finally, I multiply by 5,000 liters to get the unit
process inventory for producing 5,000 liters of hot water.
Youve just learned the simple but powerful concept of using multiples of 1 as single product
outputs to allow for easy scaling of unit process inventories in an LCA. Trust me, youll get much
experience with scaling inventories since rarely do we analyze neat units of 1 product output in real-
world systems.
But what if you have more than one product output in the inventory, for example, a process with
multiple co-products? The fact is we encounter unit process inventories with more than one
product output quite often in LCA because many real-world plants manufacture more than one
product at a time. Take for example the unit process inventory for 1 kg of general output from
petroleum refining, a process that converts crude oil into multiple product outputs such as gasoline,
diesel fuel, kerosene, and refinery gas.
Because this inventory contains flow information for more than one product output, we need some
way of assigning a portion of the inventory to each product flow. This process is so important in LCA
that it has its own name: allocation. In this particular inventory, the author indicates that allocation
of flows to individual product outputs can be based on the percent by mass indicated for each
product output. However, as youll learn later in this course, there are other ways to allocate flows
to multiple products in a system, such as assigning portions of the inventory to each product output
based on their economic value. Each allocation method has potential drawbacks, which well
discuss in future lectures. For now, just be aware that you will encounter inventories with multiple
product outputs in practice, but that youll also learn to work with them effectively in this course.
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Additional notes
For those who may be interested in operational energy data for a wide variety of appliances and
devices, check out the U.S. Building Energy Data Book, which is where we obtained the average
natural gas use of U.S. residential hot water heaters (Table 2.1.17). Similar data are compiled by
other countries and regions in the world, and can be helpful for estimating unit process inventories
for the operation of common appliances and devices. In fact, well use U.S. Building Energy Data
Book data for residential refrigerators to build our unit process inventory for the use phase (i.e.,
beverage chilling) in our bottled soft drink LCA model. http://buildingsdatabook.eren.doe.gov/
The U.S. Environmental Protection Agencys AP-42 compendium of emission factors is an exhaustive
resource that can be used to estimate the air emissions from a wide range of combustion sources in
the absence of primary or secondary inventory data on unit processes with combustion. In our
residential hot water heater example, we used emission factors for natural gas combustion from
Chapter 1: External Combustion Sources, Section 1.4. While we wont make further use of this data
source in this course, you may find it useful in the future for estimating the air emissions associated
with burning fuels in common processes across the residential, commercial, industrial, and transport
sectors. http://www.epa.gov/ttnchie1/ap42/
Correction: As in previous lectures, here I should have said Processed natural gas is a flow from the
technosphere to be more precise. Also, note that the inventory youre seeing in the lecture
video is very simplified, as it only contains a few flows to and from nature. Well work with a much
more comprehensive list of flows to and from nature in the standard unit process inventory that
well use in our plastic bag and bottled soft drink LCA model.
To view an example of expressing product outputs in multiples of 1 in a unit process inventory for
ease of scaling, take a look at the unit process inventory for corrugated product in the U.S. LCI
database. Follow the steps below. Can you identify other unit process inventories that follow this
convention?
1. Go to http://www.nrel.gov/lci/
2. Click on the Database link in the left side navigation box
3. Select the checkbox for Paper manufacturing within the Category list
4. Select the checkbox for Converted Paper Product Manufacturing
5. Click on Corrugated Product, which appears in the list at right
6. Click on the Exchanges tab, and look for the Corrugated Product output
Correction: In the lecture video, I should have said for example, a process with multiple co-
products instead of for example, a product with multiple co-products.
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Check out these inventory data for yourself in the U.S. LCI database, and note how many flow data
are provided. Petroleum refining is a complicated process, with many co-products and emissions to
account for in an inventory. As you gain proficiency working with unit process inventory data, youll
be well equipped to understand and apply even the most complicated inventory data.
1. Go to http://www.nrel.gov/lci/
2. Click on the Database link in the left side navigation box
3. Select the checkbox for Petroleum and Coal Products Manufacturing within the
Category list
4. Click on Diesel, at refinery (Petroleum refining, at refinery), which appears in the list at
right
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Lecture 4 Supplement Transcript
In this video supplement, well further explore the concept of unit process inventory scaling in an
LCA. Furthermore, well use the simplified unit process system for manufacturing of HDPE grocery
bags from previous supplemental videos to illustrate this concept.
Recall that each unit process in the system has a unit process inventory, which documents its flows
to and from nature and to and from the technosphere. You may be wondering how we obtain such
flow data to construct a unit process inventory in practice. Typically, such data are compiled from
real-world facilities and operations, and can be based on direct process measurements, engineering
estimation, or annual facility reporting.
Take for example the HDPE bag manufacturing plant. It would typically be straightforward to gather
data on the total tons of HDPE grocery bags manufactured at this plant in a year, since any business
should know this quantity. It can also be straightforward to gather data on some other annual flow
quantities, such as the amounts of natural gas, electricity, HDPE pellets, water, and other production
inputs that are purchased by the plant. Through process-level measurements and/or engineering
estimation, it can also be possible to determine the plants annual flows of air, water, and land
emissions and solid waste.
In this example, were showing data gathered for the annual raw material inputs, CO2 emissions
outputs, and manufactured product outputs for an example HDPE bag manufacturing plant. Of
course, in a real LCA we would account for many other flows in our unit process inventories, but to
keep things simple, well focus on just these three flows for now. Lets also display these data using
our standard unit process inventory structure.
Now lets revisit our simplified unit process system for manufacturing HDPE grocery bags. Assume
that weve gathered similar flow data on the annual raw material inputs, CO2 emissions outputs, and
manufactured product outputs for each plant in our system. As you see here, weve listed annual
flow data for each plant in our system using our standard unit process inventory structure.
Recall from the hot water heater example in Lecture 4 that it is most convenient to express unit
process inventories on the basis of one unit of product output whenever possible. We do this
because it makes unit process scaling in a system much easier, as youll see next. To do this for
HDPE bag manufacturing, wed divide all flows by the total manufactured product output as shown
in this table. This calculation produces an inventory in which all flows are expressed on the basis of
one unit of product output; in our case, 1 kg of HDPE grocery bags.
In this table, weve normalized the inventories to one unit of product output for all other plants in
the system using the same procedure.
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Now its time to connect our unit processes into a simple system model in which mass and energy
requirements are balanced. A straightforward way to do this is to start with a given quantity of final
product output, and to work our way backward to calculate the quantities of inputs required from
each proceeding unit process.
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Lets assume we want to produce 1 kg of HDPE grocery bags. Based on the unit process inventory
for HDPE bag manufacturing, we see that manufacturing 1 kg of HDPE grocery bags requires 1.02 kg
of HDPE pellets. Therefore, the HDPE resin manufacturing plant must produce 1.02 kg of HDPE
pellets to meet the mass input requirements of the HDPE bag manufacturing plant. So we must
scale up all flows in our unit process inventory for HDPE resin manufacturing by a factor of 1.02 to
meet this level of production output.
This procedure reveals to us that to produce 1.02 kg of HDPE pellets, 1.02 kg of ethylene is required
at the HDPE resin manufacturing plant. Now we must scale up all flows in our unit process inventory
for ethylene manufacturing by multiplying by a factor of 1.02. Doing so shows us that to produce
1.02 kg of ethylene, 1.04 kg of processed natural gas is needed at the ethylene manufacturing plant.
Next, we need to scale up all flows in our unit process inventory for natural gas processing by a
factor of 1.04, which reveals that, to produce 1.04 kg of processed natural gas, 1.05 kg of kg of
extracted natural gas are required by the natural gas processing plant.
Lastly, this means we must scale up all flows in our unit process inventory for natural gas extraction
by a factor of 1.05. Doing so indicates that 1.08 kg of in-ground natural gas is required as an input
from nature into the natural gas extraction process.
Youve just witnessed a simple example of normalizing plant-level flow data into unit process
inventories expressed on the basis of one unit of product output, and then how those unit processes
can be related and scaled into a simple unit process system model.
Note that the final inventory table Ive generated is the same one that allowed us to construct an
aggregated inventory of all of these unit processes in the Lecture 3 Supplement video. In that video,
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I mentioned that each unit process had been properly scaled to represent the mass flows required
by the system to ultimately produce 1 kg of HDPE grocery bags. I hope that statement is clearer to
you now, as is the need to properly scale unit process inventory data before we can aggregate them.
Youll gain more practice with normalizing, relating, and scaling unit process inventories in
Homework 2.
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Lecture 5: Energy flow basics Transcript
Today were going to discuss nomenclature and conventions for flows of energy in unit processes
inventories. Energy flows are common to nearly every type of unit process, and for many products,
the emissions related to energy flows account for a significant fraction of total life-cycle impacts.
Therefore, careful consideration of energy flows is critical for credible LCAs.
Lets first distinguish between two different types of energy flows: energy as a fuel and energy in
materials. Just as it sounds, energy as a fuel refers to the energy that performs useful work in a
process. Typical fuels include diesel fuel, gasoline, electricity, and natural gas. In this course, well
typically document flows of energy used as a fuel in physical units, such as the liters of gasoline or
the cubic meters of natural gas consumed in a unit process. One major exception is electricity,
which well always document using kilowatt-hours.
Energy in materials refers to the inherent energy value of materials used to create products. For
example, in the United States, our plastic bag contains HDPE that was derived from natural gas. As
such, the bag itself could be used as a fuel after it is discarded, and it often is in waste to energy
incinerators. By convention, unit process inventories account for the energy content of such
materials and denote this as feedstock energy. Well follow that convention in this course as well,
by making a note in our unit process inventories for any energy flow that should be treated as a
feedstock. In fact, youll do this yourself when you build your LCA model of bottled soda.
When it comes to energy as fuels, you also need to understand the difference between primary and
converted forms of energy. In most energy statistics, primary energy refers to the calorific value of
fuels found in nature, which includes coal, natural gas, uranium, crude oil, wind, sunlight, and
biomass. Converted forms of energy are not found in nature, but rather are created by converting
primary energy sources into more convenient or useful forms. For example, to generate electricity
we might convert the thermal energy in coal into electricity in a power plant. Or to generate steam,
we might convert the thermal energy in natural gas into steam in a boiler. Converted forms of
energy are also commonly called energy carriers. For ease of reference, a list of primary energy
sources and common energy carriers has been provided in the lecture notes.
In an LCA, its critically important to account for all energy losses that occur when converting
primary energy sources into energy carriers. Lets use the example of electricity generation to
illustrate.
First, the thermal energy in the input fuel is converted into mechanical work in a turbine, which is
then converted into electricity in a generator. During the conversion processes, a significant fraction
of the thermal energy in the input fuel is lost as waste heat to the environment. Some of the
electricity generated is used in the power plant itself, resulting in additional energy losses. Lastly,
there are also energy losses in the systems that transmit and distribute electricity from the power
plant to the consumer. As a result of all these losses, only a fraction of the thermal energy that was
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contained in the input fuel remains in the electricity that is delivered to the customer. For example,
in the United States, on average only around 1/3 of the energy that goes into a fossil-fuel-fired
power plant is contained in the electricity that obtained at the wall plug.
Why is it important to account for such conversion losses? Lets use a simple example to illustrate.
Assume we can bake a loaf of bread in a natural gas oven or electric oven. Further assume that the
energy required to bake the bread is the same in both ovens, say, 5 MJ per loaf. Note that 5 MJ is
equivalent to 1.4 kWh of electricity. It might seem that both ovens use the same amount of energy,
and are therefore comparable from an energy use perspective. But lets not forget about the energy
losses associated with generating and transmitting the electricity used by the electric oven. If we
assume that the electricity comes from a natural gas-fired power plant, and that the power grid is
33% efficient, it means that 15 MJ of natural gas are required to provide 5 MJ of electricity to the
electric oven. In other words, in this particular example the electric oven requires 3 times the
natural gas to bake a loaf of bread as the natural gas oven.
What weve just done is to convert an energy carrier (i.e., electricity) back into its original primary
energy form (i.e., natural gas) in order to facilitate a fair comparison between the two oven options.
In LCA, well always compare the life-cycle energy use of different products on a primary energy
basis. In this course, such calculations will be enabled by including all unit processes associated with
converting primary energy sources into the energy carriers that are ultimately consumed in the life
cycle system. Or, in other words, well apply life-cycle thinking by considering not just the direct
energy use of a unit process, but also the cradle-to-gate systems that supply the energy forms used
the unit process.
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Well do this by clearly labeling all flows of energy carriers as product inputs from the technosphere
in our unit process inventories. By doing so, well be forced to trace all energy carriers in the system
back to the original elementary flows of energy from nature. See the lecture notes for some
examples of this approach.
By following this approach, well minimize our risk of mistakenly adding primary energy values and
energy carrier values to each other when summing up energy flows across unit processes, which
would invalidate our results. Good data sources will always make the distinction between primary
energy data and energy carrier data in their unit process inventories explicit, but dont be surprised
if you come across data sources where this distinction is not made. Unfortunately, this is a common
omission than can render a data source useless.
Lastly, note that conversion losses vary greatly by input fuel type, energy carrier type, and
conversion technology type, and all of these can vary greatly by location. For example, a coal-fired
power grid in China will have different conversion losses than a natural gas power grid in the United
States. So if our electric oven were in China, a different amount of primary energy would be
required to bake the bread than if that electric oven were in the United States. As you gain more
experience with LCA, youll become accustomed to choosing the right unit processes inventories to
accurately capture conversion losses in different geographical regions.
Additional notes
The concept of feedstock energy is most commonly applied in LCA to materials that are derived
from fossil fuels, including plastics, chemicals, paints, synthetic rubber, and bitumen, to name a few.
However, feedstock energy is technically relevant to any material that has energetic value, including
biogenic materials such as wood. In this course, well only denote feedstock energy for plastics and
paper products, because these two products are the only relevant materials used in our simplified
grocery bag and bottled soft drink life cycles. In practice, however, youll encounter other product
life-cycle systems and LCA data sources that track feedstock energy for a much broader range of
materials.
In LCA, we also need to be aware that the calorific energy value of fuels can be reported on either a
higher heating value (HHV) or a lower heating value (LHV) basis in energy statistics. The HHV of a
fuel, which is also known as its gross calorific value, includes the latent heat of vaporization of water
in the combustion process. The LHV of a fuel, which is also known as its net calorific value, does not
include the latent heat of vaporization of water. Therefore, a fuels HHV is higher than its LHV. The
difference between HHV and LHV depends on the fuel. Ideally, in an LCA one should establish
whether HHV or LHV bases are used in life cycle inventory data and consistently use only one basis
throughout the analysis.
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For a helpful primer on basic energy units and concepts, see the following reference:
Food and Agriculture Organization of the United Nations, 1991, Energy for sustainable rural
development projects - Vol.1: A reader: Chapter 1 - Basic energy concepts. Rome.
http://www.fao.org/docrep/u2246e/u2246e02.htm
In practice, you may encounter slight differences in the definition of primary energy across the
various agencies and institutions that compile energy statistics or create regional energy balances.
In this course, well define primary energy as the energy content or calorific value of fuels found in
nature prior to any significant conversion or transformation. Energy carriers are defined as more
convenient forms of energy that are created through conversion or transformation processes from
primary energy sources. The following table contains the major primary energy sources and energy
carriers in use in many societies. In the data one uses to compile unit process inventories, one may
sometimes encounter energy inputs expressed in units of energy carriers, such as kWh or electricity
or MJ of steam. The important point to remember is that we must consider the primary energy that
was used to generate each energy carrier, otherwise the true energy cost of a system might be
undercounted!
Primary energy sources Common energy carriers
Biomass Compressed air Coal Conditioned air Crude oil Conditioned water Geothermal heat Electricity Natural gas Mechanical work Running or falling water Refined fuels (gasoline, diesel, kerosene, etc.) Solar energy Steam Tidal energy Uranium
Wind
In fact, the average system efficiency of electricity generation, transmission, and distribution in the
United States has been getting higher in recent years due to technological improvements and a shift
away from coal and toward natural gas in the electricity grid. As youll learn in Homework 2, the
efficiency of electricity generation in the United States is closely tied to the type of fossil fuels used
in its power plants.
See the Lecture 5 Supplementary video for an example of primary energy versus energy carriers for
a coal-fired electricity production system.
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Lecture 5 Supplement Transcript
To better understand the difference between primary energy and energy carriers, lets consider
again the example of electricity generation in the United States. This simplified figure depicts the
major unit processes within a coal-fired electricity system, starting with the coal mine and ending
with a residential home that consumes the electricity.
Well use 100 megajoules (MJ) of coal input so you can track energy flows and losses easily through
the system. Furthermore, well just consider energy flows related to coal and its conversion to
electricity in this system to keep things simple. In reality, there are many other flows of mass and
energy associated with these unit processes, which we would normally track in a full life cycle
assessment.
First, in-ground coal is extracted from nature and transported by rail to a power plant. Given that
coal is a raw fuel from nature with minimal processing before it is combusted in the power plant, it
is considered a form of primary energy. The coal is then combusted in the power plants boiler to
generate steam, which is an energy carrier.
Typical co
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