HW6 Spring2015 Solutions

How Green is That Product? An Introduction to Life Cycle Environmental Assessment

Homework #6 Solutions

Correct answers in RED

Goals: In this assignment, youll do the following:

address a multi-functionality issue by using the avoided burden approach;

perform a data quality assessment of a published LCI study.

Instructions: The questions below can be answered offline. When you complete the assignment, return

to the Week 6 course page on the Coursera website. Click on the Submit Homework Assignment

Answers button, which will allow you to enter your answers into a web form for automated grading.

Grading: This assignment is worth 100 points. The point values of each answer are listed below. You

can submit a maximum of 30 attempts. The highest scoring attempt that is submitted before the

deadline will count toward your official grade. Scores for each attempt will be available immediately

after submission of your answers.

Numbers: In this assignment, 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.

Multi-functionality and the avoided burden approach

Recall from Homework #5 that Table A contains representative facility energy use and production data

for the commercial bakery example that weve discussed in the lectures. This time, assume that you

dont have process-level data; thus, you cannot divide the facility into its sub-processes as you did in

Homework #5. Instead, youll use the avoided burden approach to estimate the facility energy inputs

that are associated with just the baked bread.

Table A: Energy and production data for a commercial baking facility

Production of baked bread (kg/year) 500,000

Production of frozen dough (kg/year) 400,000

Natural gas used to generate steam (TJ/yr) 0.97

Natural gas used directly in baking ovens (TJ/yr) 2.31

Electricity use (MWh/yr) 461

Table C contains data on frozen dough production processes from another facility that manufactures

only frozen dough.

Table C: Process energy requirements for frozen dough production

Product Process Steam use (MJ/kg)

Direct natural gas use (MJ/kg)

Electricity use (kWh/kg)

Frozen dough Mix - - 0.10

Form and chill 0.37 - -

Bake 0.40 0.80 0.05

Cool - - 0.54

Finish - - 0.09

Package - - 0.15

Your job is to use the avoided burden approach based on the information in Tables A and C to assign the

commercial bakerys inputs of natural gas (for steam), natural gas (direct use), and electricity to just the

baked bread. Enter your results below. For all answers, enter only the numerical value using two

decimal places after the decimal separator (i.e., X.XX). (8.33 points for each correct answer)

Question 1: How much of the facilitys annual natural gas use for steam is attributable to just the baked bread? Use units of TJ/year in your answer.

0.66

Question 2: How much of the facilitys annual direct natural gas use is attributable to just the baked bread? Use units of TJ/year in your answer.

1.99

Question 3: How much of the facilitys annual electricity use is attributable to just the baked bread? Use units of MWh/year in your answer.

89.00

The first step in the solution is to determine the total energy intensities for steam use, direct natural gas

use, and electricity use for frozen dough production in Table C as follows:

Product Process Steam use (MJ/kg)

Direct natural gas use (MJ/kg)

Electricity use (kWh/kg)

Frozen dough

Mix - - 0.10

Form and chill 0.37 - -

Bake 0.40 0.80 0.05

Cool - - 0.54

Finish - - 0.09

Package - - 0.15

Total 0.77 0.80 0.93

Next, we multiply the total annual quantity of frozen dough that is produced by the bakery (400000 kg)

by the total energy intensities calculated above as follows:

Total annual steam use for frozen dough = 0.77 (MJ steam/kg) * 400000 (kg frozen dough/yr) =

308000 MJ/yr = 0.308 TJ/yr

Total direct natural gas use for frozen dough = 0.80 (MJ/kg) * 400000 (kg frozen dough/yr) =

320000 MJ/yr = 0.32 TJ/yr

Total electricity use for frozen dough = 0.93 (kWh/kg) * 400000 (kg frozen dough/yr) =

372000 kWh/yr = 372 MWh/yr

Finally, we use the avoided burden approach to estimate the annual natural gas use for steam, direct

natural gas use, and electricity use for baked bread production by subtracting our results for frozen

dough from the data in Table A.

Avoided burden approach:

Estimated annual natural gas use for steam in baked bread production = 0.97 TJ/yr 0.308 TJ/yr = 0.662

TJ/yr

Estimated annual direct natural gas use in baked bread production = 2.31 TJ/yr 0.32 TJ/yr = 1.99 TJ/yr

Estimated annual electricity use in baked bread production = 461 MWh/yr 372 MWh/yr = 89 MWh/yr

Now, compare your answers above to Answers 6, 7, and 8 from Homework 5. T hen, answer the

following questions (5 points for each correct answer):

Question 4: Compared to Homework 5, Answer 6, does the avoided burden approach overestimate or

underestimate the annual natural gas use for steam that is attributable to just the baked bread?

Overestimate

Underestimate

Question 5: What is the percent difference between Answer 1 (above) and Answer 6 (Homework 5)?

Chose the closest answer. Hint: Calculate the percent difference as (Answer 1 Answer 6)/(Answer

6)*100.

12%

-42%

2%

-22%

-88%

(0.66 0.85)/0.85 *100 = -22.3%


underestimate the annual direct natural gas use that is attributable to just the baked bread?

Overestimate

Underestimate


Chose the closest answer.

25%

-5%

-23%

42%

11%

(1.99-1.79)/1.79 * 100 = 11.2%


underestimate the annual electricity use that is attributable to just the baked bread?

Overestimate

Underestimate


Chose the closest answer.

-46%

75%

-50%

50%

-42%

(89-165)/165 * 100 = -46.1%

This exercise demonstrates to us how the avoided burden approachwhile useful for partitioning

inventories between co-products in multi-functional processes when such processes cannot be

subdividedcan often introduce errors and uncertainties into an analysis. Our answers to Questions 6,

7, and 8 of Homework 5 were based on subdividing into separate product systems based on plant data,

which is the preferred approach in the ISO hierarchy of strategies for solving multi-functionality issues.

The use of partitioning via the avoided burden approach should only be pursued in cases when

subdividing isnt possible.

As we can see through the above results, here the avoided burden approach led to significant

differences in the resulting inventory for baked bread production compared to the real inventory we

generated in Homework 5. Of course, the errors will vary in practice based on how closely the

alternative production system resembles the real production system that is being subtracted out of

a co-product inventory. However, the challenge for the LCA analyst is that he/she does not typically

know how large or small such errors are, because if he/she could subdivide the processes to determine

the error (as we did), he/she would not need to use the avoided burden approach in the first place!

Data quality assessment (5 points for each correct answer)

To answer these questions, youll need to first download and read the following article:

Aldo Roberto Ometto, Michael Zwicky Hauschild, Woodrow Nelson Lopes Roma (2009). Lifecycle

assessment of fuel ethanol from sugarcane in Brazil. The International Journal of Life Cycle

Assessment, May 2009, Volume 14, Issue 3, pp 236-247.

http://users.ugent.be/~jdewulf/Rodrigo%20Alvarenga/Ometto%20et%20al%202009.pdf

Refer to the Week 17 lecture notes and review the Pedigree matrix with 5 data quality indicators table

from Weidema and Wesnaes (1996). Your job is to read the above article and assess the overall

temporal correlation of all the data used for each unit process by giving the unit process inventory a

single score from 1 to 5 based on the guidance in the pedigree matrix. Note that the year of the

authors analysis as stated in the paper is 2008.

Important: further note that they have obtained primary data for the years 2001-2008 for some of

their foreground data; for the purposes of this assignment, treat those primary data as having less

than 6 years difference from 2008 for the purpose of scoring their temporal correlation, since these

data are an average of a 7 year period that extends up to the data of their analysis. Note that they

mix primary data and secondary data in each unit process inventory; the primary data are reported

without a source, and the secondary data are reported with a source.

Also, use the following simplifying assumptions:

When a publication is cited for particular data, assume that the year of publication is the same as the

year of the data within that publication (this is certainly not always the case, but a necessary shortcut

for us in this assignment since we dont have access to all cited sources)

If 2008 the year of the data = 6 or less, choose less than 6 years difference



If 2008 the year of the data = greater than 15, choose more than 15 years difference

(These data year simplifications are needed because we dont have the months and the years of

publication for cited data, we just have the years. However, a study published in December 1998 is

less than ten years prior to January 2008, so even though 2008-1998 = 10, the study could be less than

10 years old.)
http://users.ugent.be/~jdewulf/Rodrigo%20Alvarenga/Ometto%20et%20al%202009.pdf

Enter your answers in the boxes below. (Hint: take a conservative approach! If there are multiple years

of data for the non-primary data that have been used in a given unit process inventory, use the oldest

year to assign your single score for that unit process inventory.)

Question Unit process inventory Your score for temporal correlation to 2008

10 Unit Process 1: Soil preparation 2

11 Unit Process 2: Sugar cane plantation 2

12 Unit Process 3: Chemicals application 3

13 Unit Process 4: Harvesting 5

14 Unit process 5: Fuel hydrated ethanol industrial process 2

15 Unit process 6: electrical energy cogeneration 3

16 Unit process 7: irrigation 2

17 Unit Process 8: Ethanol distribution 2

18 Unit Process 9: Use of fuel ethanol 5

The scores above were assigned based on the rationale we stipulated for primary data in the problem

description, the listed dates for the sources in each unit process inventory in the Ometto et al. (2009)

paper, and the scoring guidance provided by Weidema and Wesnaes (1996). In practice, one might

assign slightly different scores based on ones interpretation of the data years and the scoring guidance,

which is why we always provide a rationale for why a given score was chosen in a final LCA report. You

werent asked to provide this rationale in this homework problem to keep things simple, but you will do

so in your bottled soft drink LCA spreadsheet. As with all decisions in an LCA, it is critical to document

your rationale so that the audience and readers of your study can fully understand your logic and

determine for themselves whether or not they agree with that logic. Without such documentation, a

study is much less transparentand, hence, must less usefulto the reader.

Documents

HW6 Spring2015 Solutions