how green is that product lecture notes week 1
DESCRIPTION
How Green is That ProductTRANSCRIPT
-
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
-
P a g e | 1
Lecture Notes for How Green is That Product? 2015 Northwestern University.
Table of Contents About these lecture notes ...................................................................................................................... 2
Lecture 1: The life-cycle perspective and course goals .......................................................................... 3
Lecture 1 Supplement ............................................................................................................................ 7
Lecture 2: Understanding unit processes ............................................................................................... 9
Lecture 2 Supplement .......................................................................................................................... 15
Lecture 3: Constructing unit process inventories: Part 1 ..................................................................... 17
Lecture 3 Supplement .......................................................................................................................... 22
-
P a g e | 2
Lecture Notes for How Green is That Product? 2015 Northwestern University.
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!
1.1
-
P a g e | 3
Lecture Notes for How Green is That Product? 2015 Northwestern University.
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.
1.1
1.2
-
P a g e | 4
Lecture Notes for How Green is That Product? 2015 Northwestern University.
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
1.3
-
P a g e | 5
Lecture Notes for How Green is That Product? 2015 Northwestern University.
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
1.4
1.1
1.2
-
P a g e | 6
Lecture Notes for How Green is That Product? 2015 Northwestern University.
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.
1.3
1.4
-
P a g e | 7
Lecture Notes for How Green is That Product? 2015 Northwestern University.
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.
-
P a g e | 8
Lecture Notes for How Green is That Product? 2015 Northwestern University.
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.
-
P a g e | 9
Lecture Notes for How Green is That Product? 2015 Northwestern University.
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.
2.1
2.2
2.4
2.3
-
P a g e | 10
Lecture Notes for How Green is That Product? 2015 Northwestern University.
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.
2.5
-
P a g e | 11
Lecture Notes for How Green is That Product? 2015 Northwestern University.
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.
2.6
-
P a g e | 12
Lecture Notes for How Green is That Product? 2015 Northwestern University.
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.
2.7
-
P a g e | 13
Lecture Notes for How Green is That Product? 2015 Northwestern University.
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.
2.8
2.1
2.2
2.3
2.4
2.5
http://www.iso.org/iso/home/store/catalogue_tc/catalogue_tc_browse.htm?commid=54854
-
P a g e | 14
Lecture Notes for How Green is That Product? 2015 Northwestern University.
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/
2.6
2.8
2.7
http://www.container-recycling.org/assets/pdfs/LCA-SodaContainers2009.pdfhttp://www.container-recycling.org/assets/pdfs/LCA-SodaContainers2009.pdfhttp://physics.nist.gov/cuu/Units/http://www.bipm.org/utils/common/pdf/si_summary_en.pdfhttp://www.onlineconversion.com/
-
P a g e | 15
Lecture Notes for How Green is That Product? 2015 Northwestern University.
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.
-
P a g e | 16
Lecture Notes for How Green is That Product? 2015 Northwestern University.
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.
-
P a g e | 17
Lecture Notes for How Green is That Product? 2015 Northwestern University.
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.
3.1
3.2
-
P a g e | 18
Lecture Notes for How Green is That Product? 2015 Northwestern University.
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
3.3
3.4
-
P a g e | 19
Lecture Notes for How Green is That Product? 2015 Northwestern University.
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.
3.1
3.2
3.3
3.5
-
P a g e | 20
Lecture Notes for How Green is That Product? 2015 Northwestern University.
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
http://www.ecoinvent.org/fileadmin/documents/en/Data_Quality_Guidelines/01_DataQualityGuideline_v3_Final.pdfhttp://www.ecoinvent.org/fileadmin/documents/en/Data_Quality_Guidelines/01_DataQualityGuideline_v3_Final.pdfhttp://www.nrel.gov/lci/pdfs/users_guide.pdf
-
P a g e | 21
Lecture Notes for How Green is That Product? 2015 Northwestern University.
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.
3.4
3.5
-
P a g e | 22
Lecture Notes for How Green is That Product? 2015 Northwestern University.
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.
-
P a g e | 23
Lecture Notes for How Green is That Product? 2015 Northwestern University.
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.