how green is that product lecture notes week 2

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


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 4: Constructing unit process inventories: Part 2 ..................................................................... 24


Lecture 5: Energy flow basics ............................................................................................................... 32


Lecture 6: Mass balances .................................................................................................................... 38

P a g e | 2


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, we’ve made use of the additional notes sections to

provide you with supporting information that couldn’t be included in the video lectures due to time

constraints. We’ve also added additional notes to further discuss topics that proved particularly

interesting or challenging in past offerings of the MOOC. Within the transcript section, you’ll 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. Let’s get started!

1.1

P a g e | 3


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.” I’m

Eric Masanet, and I’ll be your instructor for this course. I hope you’ve 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 won’t 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, you’ll 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, you’ll 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.

Let’s first discuss what is meant by “life cycle stages” using this plastic bag as an example. In this

course, we’ll 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


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 you’ll 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? It’s 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 we’re

making a green choice, environmental impacts may shift based on the consumption choices we

make. That’s why it’s important to consider all relevant impacts in an LCA; otherwise such shifts in

impacts might be missed when we’re 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. It’s

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


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 you’re an engineer, LCA can help you choose materials and design

features that lead to greener products and technologies. If you’re 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 you’re 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.

Let’s 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 you’ll 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 you’ll be developing a new section of the model that relates to

that week’s lecture material, so by the end of the course you’ll have built

a complete bottled soda LCA. While the product is fairly simple, by

building the model across all life cycle stages and impacts, you’ll acquire the skills and perspectives

that should allow you to move on to more complex products after you complete this course.

Lastly, we’ll 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.

I’m 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 you’ll 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


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 we’ll 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, you’ll 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 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, I’d 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.

Let’s take a look at the course schedule, which lists the topics we’ll 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, we’ll 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, let’s take a look at the “Project” section of the website,

which describes the scope and intent of the course project. You’ll 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 you’ll be developing yourself. Starting

in Week 3, you’ll 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,

you’ll 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.

I’d also like to draw your attention to the discussion forums. If you’ve taken Coursera courses in the

past, you’ll 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,” we’ve 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


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.

You’ll also notice that I’ll 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 we’d like to solve. While participation isn’t 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 I’m sure you’ll 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. We’ve 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

you’re 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 2: Understanding unit processes Transcript

Welcome back! Today we’ll begin learning about the data structure of an LCA, starting with LCA’s

most fundamental building block: the unit process model. But first let’s 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, we’ll 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


To visualize how we use unit processes, let’s look more closely at the manufacturing stage of our

plastic bag. The first step is to convert processed natural gas into ethylene, which we’ll represent by

this first unit process model.

The second step is to convert ethylene into HDPE pellets, which we’ll 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


As you’ve probably guessed, to construct a complete LCA model for the plastic bag, we’d 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. You’ll 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, don’t worry … you’ll 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


First, many unit process inventories refer to inputs and outputs as “flows” or “exchanges.” In this

course, we’ll 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, we’ll 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 don’t 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


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. We’ll learn more about impact assessment later in the

course. For now, let’s get used to organizing our unit process inventories in this way.

Lastly, we’ll 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 week’s course notes.

Additional notes

Correction: Here we’ve 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. We’ll be referring to these standards often throughout the course. We’ll 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, you’ll get a basic

understanding of these standards through our class materials and through the additional readings

we’ll 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 we’ve changed “materials and energy” to the more general and correct “energy

and mass.” See Note 1.1.

For clarity, we’ve 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


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.

You’ll 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 we’ve 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 we’ll 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.pdf

http://www.container-recycling.org/assets/pdfs/LCA-SodaContainers2009.pdf

http://physics.nist.gov/cuu/Units/

http://www.bipm.org/utils/common/pdf/si_summary_en.pdf

http://www.onlineconversion.com/

P a g e | 15



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,

let’s step through the plastic grocery bag example in a bit more detail. Furthermore, let’s try

working backwards in the life cycle so that the different types of flows are clear.

Let’s 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, we’ll include these steps in one unit process that we’ll 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, we’ll 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, we’ll 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, we’ll

ignore these flows to keep things simple.

The production of HDPE pellets occurs at a chemical factory, which converts ethylene—another

man-made product—into HDPE resin. Let’s 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 we’ll 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,

we’ll label this flow as an input from the technosphere.

To produce processed natural gas, another unit process is required that we’ll 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, let’s 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, it’s 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, you’ll 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


As you’ll 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.

You’ll get more practice with labeling flows in Homework 1.

P a g e | 17


Lecture 3: Constructing unit process inventories: Part 1 Transcript

Welcome back. In today’s lecture, we’ll 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 we’ll use in our inventories:

1. Inputs from nature,

2. Inputs from the technosphere,

3. Outputs to nature, and

4. Outputs to the technosphere

Let’s take a closer look at the complete unit process inventory for converting ethylene to HDPE

pellets. I’ve created this inventory in a spreadsheet in the same way that you’ll 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


The third column contains additional information on the origins and destinations of flows to and

from nature, which we’ll 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 we’ll 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, we’ll use a standard set of subcategories to describe inventory

flows. I’ve provided the list of subcategories we’ll 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. We’ll 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 aren’t confused by this, moving forward we’ll also use this label for our

flow types in unit process inventories.

By convention, we’ll 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 we’ll 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


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. You’ll get exposed to all of these

types of outputs and more moving forward.

Lastly, our simple example inventory focused on single unit process, but you’ll 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. You’ll 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 we’ll 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, we’ll use a simplified set of categories and

subcategories for all flows. As discussed in the lecture video, we’ll 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


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, you’ll 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. You’ll notice that many more subcategories are available

for defining flows with greater precision in practice.

U.S. LCI Database Project –User’s 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. We’ll 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. You’ll 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.pdf

http://www.ecoinvent.org/fileadmin/documents/en/Data_Quality_Guidelines/01_DataQualityGuideline_v3_Final.pdf

http://www.nrel.gov/lci/pdfs/users_guide.pdf

P a g e | 21


subcategory you’ll encounter is “unspecified.” The publicly-available data sources we’ll use in our

course projects do not contain such detailed specification of subcategories, either. This is another

reason we’ll 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. We’ll 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.

We’ve 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



In this video supplement, we’ll use the simplified system of unit processes for HDPE grocery bags

that we discussed earlier. Below the figure I’ve added in an inventory table that contains a

simplified list of flows for each unit process. In this example, we’ll only track a few flows to illustrate

how inventory aggregation works. However, you’ll practice aggregating much more complicated

inventories later in this course.

Let’s 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 let’s 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. You’ll 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, you’ll 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. You’ll 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


Another way to think of aggregation is that I’ve drawn a boundary around the entire system and I’ve

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, you’ll 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, we’ll make use of aggregated

inventories as a matter of practicality and convenience. However, we’ll 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.

P a g e | 24


Lecture 4: Constructing unit process inventories: Part 2 Transcript

In this course, we’ll 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 we’ll practice constructing our own unit process inventories to help you gain proficiency in

data compilation and analysis. We’ll also learn an important convention for ensuring we can scale

our unit process inventories for use in different LCA models.

Let’s suppose we are conducting an LCA of a residential hot water heater that is fueled by natural

gas. In today’s example, we’ll be constructing the unit process inventory for the use stage of the

water heater, which refers to its operation. I’ve 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, you’ll 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 Agency’s 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 can’t 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 process’s 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.

4.2

4.1

P a g e | 25


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 we’ve 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

can’t 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, let’s 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.

You’ve 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, you’ll 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 you’ll 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 we’ll

discuss in future lectures. For now, just be aware that you will encounter inventories with multiple

product outputs in practice, but that you’ll also learn to work with them effectively in this course.

4.4

4.6

4.3

4.5

P a g e | 26


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, we’ll 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 Agency’s 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 won’t 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 you’re seeing in the lecture

video is very simplified, as it only contains a few flows to and from nature. We’ll work with a much

more comprehensive list of flows to and from nature in the standard unit process inventory that

we’ll 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.”

4.1

4.2

4.4

4.5

4.3

http://buildingsdatabook.eren.doe.gov/

http://www.epa.gov/ttnchie1/ap42/

http://www.nrel.gov/lci/

P a g e | 27


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, you’ll

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

4.6

http://www.nrel.gov/lci/

P a g e | 28



In this video supplement, we’ll further explore the concept of unit process inventory scaling in an

LCA. Furthermore, we’ll 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 plant’s annual flows of air, water, and land

emissions and solid waste.

In this example, we’re 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, we’ll focus on just these three flows for now. Let’s also display these data using

our standard unit process inventory structure.

Now let’s revisit our simplified unit process system for manufacturing HDPE grocery bags. Assume

that we’ve 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, we’ve 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 you’ll see next. To do this for

HDPE bag manufacturing, we’d 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, we’ve normalized the inventories to one unit of product output for all other plants in

the system using the same procedure.

P a g e | 29


Now it’s 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.

P a g e | 30


Let’s 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.

You’ve 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 I’ve 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,

P a g e | 31


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.

You’ll gain more practice with normalizing, relating, and scaling unit process inventories in

Homework 2.

P a g e | 32


Lecture 5: Energy flow basics Transcript

Today we’re 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.

Let’s 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, we’ll

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 we’ll 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.” We’ll 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, you’ll 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, it’s critically important to account for all energy losses that occur when converting

primary energy sources into energy carriers. Let’s 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

5.1

5.3

5.2

P a g e | 33


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? Let’s 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 let’s 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 we’ve 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, we’ll 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, we’ll 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.

5.4

P a g e | 34


We’ll do this by clearly labeling all flows of energy carriers as product inputs from the technosphere

in our unit process inventories. By doing so, we’ll 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, we’ll 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 don’t 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, you’ll 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, we’ll 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, you’ll 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 fuel’s 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.

5.1

5.5

5.2

P a g e | 35


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, we’ll 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 you’ll 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.

5.4

5.3

5.5

http://www.fao.org/docrep/u2246e/u2246e02.htm

P a g e | 36



To better understand the difference between primary energy and energy carriers, let’s 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.

We’ll use 100 megajoules (MJ) of coal input so you can track energy flows and losses easily through

the system. Furthermore, we’ll 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 plant’s boiler to

generate steam, which is an energy carrier.

Typical conversion efficiencies for U.S. power plant boilers are around 88%, which means 12 MJ of

the energy in the coal is lost as waste heat to the atmosphere. The steam is then run through a

steam turbine generator to produce electricity, which is another energy carrier.

Typical conversion efficiencies for steam turbine generators are around 45%, which means that 48

MJ of the energy in the steam is lost as waste heat to the atmosphere and only 40 MJ of electricity is

generated. Additionally, some of the generated electricity is used by the power plant itself to power

its own operations (typically 5-7%). Thus, we assume that 37 MJ of electricity leaves the power

plant and is distributed to consumers.

At this point in the system, we can calculate what is known as the net power plant efficiency, which

expresses the efficiency of converting input fuels into the net power exported from the plant.

Net power plant efficiency = [Net electricity generation (MJ)]/[Power plant fuel input (MJ)]

= 37 MJ/100 MJ = 0.37 = 37%.

Lastly, the transmission and distribution system will also incur heat losses (typically around 8% in the

United States), which means that only 34 MJ of electricity ultimately reaches the consumer for use

in the home.

Now at this point in the system, we can calculate what is known as the total system efficiency, which

expresses the efficiency of converting input fuels into the power that is ultimately provided to

consumers at the final point of use.

Total system efficiency = [Delivered electricity (MJ)]/[Power plant fuel input (MJ)]

= 34 MJ/100 MJ = 0.34 = 34%.

P a g e | 37


In the diagram, flows from nature are indicated with a green arrow and flows to and from the

technosphere (i.e., product flows) are indicated with dashed black arrows. Heat losses are indicated

with red arrows. Note that heat losses are not often listed as an output to nature in many LCA

databases, but thorough, well balanced unit process inventories can include flows of waste heat.

You can use this example to understand the distinction between primary energy and energy carriers

in unit process systems, and to visualize how we can track conversion losses that occur between

primary energy sources and energy carriers through the unit process modeling approach in an LCA.

You’ll get more practice with these concepts in your course project.

Lastly, note also that in this simple example, we’ve neglected the fuel inputs that are necessary to

power the coal mining and coal transport unit processes for the purposes of illustration. However, in

any real-world LCA (including in our plastic bag and bottled soft drink spreadsheet models), we must

account for these and other fuel inputs. If we did so here, those additional fuel inputs would add to

the primary energy used by the system by around 5%.

P a g e | 38


Lecture 6: Mass balances Transcript

Today is the last lecture in which we’ll cover the structural basics of an LCA. Moving forward, we’ll

begin discussing the four major steps in the LCA process. In future lectures, you’ll also begin

“learning by doing” by applying your knowledge to build an LCA model of bottled soft drink.

However, before we move on I’d like you to gain some proficiency in mass balancing across life-cycle

systems. If you’ve had a basic physics class, you’ll probably recall the law of mass conservation. We

strive to apply the same principle in LCA; that is, all mass that goes into a life cycle system must be

accounted for as either a product flow within or out of the system or an elementary flow out of the

system.

Let’s again consider the life cycle of a plastic bag, and let’s focus on the end of life stage.

Suppose we want to analyze the CO2 emissions of different end of life scenarios for plastic grocery

bags in an urban region in an effort to inform policy makers. In this example, we’ll consider two end

of life processes for waste plastic bags collected in our urban region: landfill and recycling. Let’s

designate the variable m as the total mass of plastic bags collected from consumers in our region

each year. To help us visualize, assume that this stack of blocks represents our total mass m.

6.1

6.2

P a g e | 39


Now let’s designate the variables ac, al, as, and ar to represent emissions to air of carbon dioxide

(CO2) per unit mass processed in collection, landfill, sorting, and recycling, respectively (kg CO2/kg).

Finally, let’s assume that currently all collected mass is being sent to landfill. In this case, the CO2

emissions of the end of life stage would be calculated as:

mac + mal = m(ac + al)

Now suppose we wanted to evaluate changing this system by recovering some portion of the

collected mass for recycling. Let’s represent the fraction of collected mass sent to sorting for

recycling by the variable x. Therefore the mass quantity to sorting is represented by mx and the

mass quantity to landfill is represented as m(1-x).

The sorting process will also generate waste due to contaminants and inefficiencies. We’ll represent

the fraction of sorted mass that gets sent to the plastic recycling plant as y. Therefore, the mass

quantity from sorting to recycling can be represented as mxy and the mass quantity generated from

the sorting process as waste to landfill can be represented as mx(1-y).

P a g e | 40


Lastly, we need to consider that the recycling process will also generate waste. Let’s represent the

mass fraction that is recycled into pellets as z. Therefore, the mass quantity of recycled pellets is

mxyz and the mass quantity of waste generated by the recycling process is mxy(1-z), which we’ll

send to the landfill.

Now let’s verify that we’ve conserved mass across all flows in our system by summing up the mass

flows that terminate in the landfill process or as recycled pellets. That is:

mxyz + mxy(1-z) + mx(1-y) + m(1-x) = m

mxyz + mxy – mxyz + mx – mxy + m - mx = m

m = m

which we verify by collecting our blocks.

P a g e | 41


Great, now that mass is conserved, let’s calculate the CO2 emissions associated with this new

scenario for the end of life stage. You’ll note in this expression that I’ve included a new

CO2emissions factor c, which allocates possible CO2 emissions savings associated with plastic bag

recycling to the pellets themselves (kg CO2/kg recycled). We’ll cover allocation in more detail in

future lectures.

For now, we’ll use this factor to solve for the value of c that will ensure that the alternative recycling

scenario reduces the CO2 emissions of the end of life stage compared to existing landfill scenario.

Here’s the math:

[mac + mal] – [mac + [m(1-x) + mx(1-y) + mxy(1-z)]al + mxas + mxyar + mxyzc]> 0

mac + mal - mac -[m(1-x) + mx(1-y) + mxy(1-z)]al - mxas - mxyar - mxyzc > 0

mac + mal - mac - mal + mxal - mxal + mxyal - mxyal + mxyzal - mxas - mxyar - mxyzc > 0

(mac - mac) + (mal - mal) + (mxal - mxal) + (mxyal - mxyal) + mxyzal - mxas - mxyar - mxyzc > 0

mxyzal - mxas - mxyar > mxyzc

c < al - as/yz - ar/z

which shows us that for our recycling scenario to reduce CO2 emissions at the end of life stage, the

value of c must be less than the value of the expression on the right-hand side of this relation.

6.3

6.4

P a g e | 42


In fact, you’ll analyze a similar expression yourself as part of the next homework assignment, using

numerical values for the variables.

Besides giving us practice with mass balancing, tracking mass flows in this fashion has another

benefit. Doing so allows us to easily assess different end of life scenarios simply by changing the

values of the mass fractions in an LCA model.

In fact, I’ve found that it’s good practice to balance mass in this way in all of my spreadsheet LCA

models. For example, in my LCA model of the plastic bag life cycle, I’ve specified mass flows with

mass fractions indicated right on my life cycle system diagram to make all mass flows explicit and

intuitive.

Now, if you’re working with a commercial LCA software package you’ll find that it tracks and displays

mass flows automatically, which is a big help, especially for complicated life cycle systems!

However, you’ll be balancing mass flows manually in your spreadsheet model of the bottled soda

life-cycle. Even if mass balancing is fairly trivial in your class project, getting in the habit of carefully

tracking mass flows is important for visualizing the system you are modeling, ensuring mass

conservation, and analyzing “what if” scenarios analytically as you’ll do in the homework

assignment.

In practice, we might be able to neglect small mass flows in the system if doing so doesn’t

substantially affect our results. Neglecting small mass flows is something we’ll cover when

discussing “cut off rules” for life-cycle inventories a bit later in this course. For now, I want you to

assume that all mass flows are important to track until you later determine otherwise.

Additional notes

This lecture is designed to provide a basic understanding of mass balancing across unit process

systems for those without experience conducting mass and energy balances. For those of you who

already have such experience, the most important information to take away from this chapter is

that manual mass and energy balances are often necessary in an LCA, especially when compiling

one’s own unit process inventory data. It will also be necessary to understand the notion of “cut

off” rules, which refer to criteria for excluding certain mass and energy flows from an LCA on the

basis of insignificance or acceptable uncertainties. We’ll cover cut off rules in more detail later in

this course.

Correction: Mass entering a system can leave the system as an elementary flow or as a product flow.

Additionally, mass can be converted to energy within a system, such as when materials or their by-

products are combusted for their energy value. For example, a biorefinery may combust biomass

feedstock byproducts to generate heat and/or electricity for use onsite. We’ll only focus on simple

mass and energy balances in this course, with the goal of ensuring that we account for inputs and

outputs of all flows in our systems. However, in practice, the balancing of mass and energy across

systems can become complicated, especially when mass-to-energy conversions occur within a

6.1

6.2

2

6.5

P a g e | 43


system. Fortunately, most commercial LCA software packages will ensure balancing of mass and

energy flows as part of the model building process.

As you’ll learn later in this course when we discuss allocation and partitioning, emissions savings

assigned to recycled materials are known as “avoided burdens” in an LCA. When we recycle

materials, environmental impacts can be avoided in two ways. First, when we don’t send those

materials to the landfill, we avoid the emissions that would have otherwise been associated with

hauling that waste to the landfill and compacting it there. Second, by recycling materials, we avoid

demand for virgin materials in the marketplace and the emissions that would have otherwise been

associated with manufacturing those virgin materials. The amount of avoided emissions can be

estimated analytically, as you’ll see in the Lecture 6 Supplement video. Such avoided burdens have

a negative value, which indicates environmental impact savings at the societal level. For a classic

explanation of impacts avoided by recycling, read the following white paper by Ian Boustead, one of

the LCA field’s longstanding experts. Note that there are other views on allocation and partitioning

within the LCA community (Boustead’s discussion is in the context of plastics recycling). You’ll also

notice that our mass balancing approach is similar to that discussed by Boustead.

Boustead (2001). Who gets the credit?

http://www.plasticseurope.org/Documents/Document/20100312112214-

WHOGETSTHECREDITS-20050701-005-EN-v1.pdf

To help you understand this example, we’ve added the Lecture 6 Supplement video. Please be sure

to watch it, especially if the equations presented and the concept of “avoided emissions” due to

recycling are not clear.

Now that we’ve added the Lecture 6 Supplement video, you will not solve this expression in the next

homework assignment. Therefore, please just ignore this statement. However, you will get further

experience with mass and energy balances and “avoided impacts” later in this course.

6.3

6.4

6.5

http://www.plasticseurope.org/Documents/Document/20100312112214-WHOGETSTHECREDITS-20050701-005-EN-v1.pdf

http://www.plasticseurope.org/Documents/Document/20100312112214-WHOGETSTHECREDITS-20050701-005-EN-v1.pdf