What is an inventory analysis? 

Now that you have decided what the scope of your project will be, you have to start collecting data about the products and processes you said you would include in your project scope. This list of products and processes needed to make up the parts that make up the product or process you are doing your LCA on is called your inventory. Think of it as the list of ingredients of what your LCA is about. The analysis part happens once you have gotten your inventory together and you want to start learning more about the impact of each item in it. This would be like checking the receipt after you bought all of the ingredients to see how much each ingredient cost. 

 

What data to use for an inventory

There are two specific data formats for the data that is used as “the ingredients” in an inventory. These two formats are called ‘unit processes’ and ‘system processes’. A unit process (U process) is the simplest data form and gives all of the inputs and outputs for making a single product or doing a single process. A system process (S process) is a more complex data form made up of several unit processes to give all of the inputs and outputs of a product or process. A visualization of the difference between a unit process and system process can be seen in the image below.

 

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An image showing that unit processes are single items that make up an inventory together while a system process is a single items that is made up of unit processes sourced from:  https://ask.openlca.org/53/difference-between-system-process-and-unit-process

 

Of the two data forms, unit processes are used most often as they show all aspects of the product or process they represent and allow for the most customization of the LCA as they can be altered. System process are mainly used when an LCA needs to be done without giving away industry secrets. System processes give inputs and outputs of the product or process they represent but the process to go from input to output is not given and cannot be altered. The dataset that we will be using in this module and introducing you to in the next section, the United States Life-Cycle Inventory National Renewable Energy Laboratory Processes dataset, has 718 unit processes available and only 2 system processes. 

 

 

Where to get data for an inventory

If you were doing an inventory from scratch, you would need to think of all of the unit and system processes needed to make the thing your inventory is on and all of the inputs and outputs of them. This is not how most inventories are made though. Like we mentioned in the last section, there are datasets of unit and system processes and even software that can select data from the dataset for you. 

 

These software tools and datasets contain almost anything you could ever need to include in an inventory of a common product or process.

The most common software tools  are:

• SimaPro

• GABI

• BEES

• openLCA

​You can follow the links to read more about each of these tools but, in general, each tool will be connected to datasets that have information about common products and the materials and processes used to make them. Some datasets used by these tools include:

• EcoInvent 3

• AGRBYLASE v 1.3

• European Life Cycle Database v 3.2

The tools are able to use these datasets to calculate the impacts of nearly ay common product or process.  Since these tools are very advanced and useful they are not usually free. Many companies and universities purchase access to them so that employees and students can use them to do LCAs. Datasets similar to the ones used by these tools are available free to the public to use for LCAs without access to these tools though. 

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The best dataset available independent of any software tool is the United States Life-Cycle Inventory (USLCI) Database, National Renewable Energy Laboratory Group/Repository, Processes Model Type  that can be found in the Federal LCA Commons. This dataset is maintained by federal agencies and is available for free to anyone who would like to use it. Take some time to explore the Federal LCA Commons and see what the data, both unit and system processes, used for inventories looks like.

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How to make an inventory

Now that you know what types of data make up an inventory and where to get the data from you can start to create the inventory itself. Inventories are assembled as a matrix with two main parts and two sections that contain those two main parts. The two main parts of the inventory matrix are called the ‘linear space’ and the ‘values’. The two sections are called the  ‘technology matrix’ and ‘intervention matrix’ even though they are both part of the same inventory matrix . An example of a blank inventory matrix can be seen in the image below.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

An image showing a blank inventory matrix

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As you can see in the blank example matrix above, the linear space of an inventory matrix, colored red, contains a list of materials used to make the things in the inventory and the units used for each material. An example entry from the linear space of a toothbrush could be “plastic, kg” since plastic is used to make the handle of most toothbrushes and plastic is most commonly measured by the kilogram (kg). What the linear space says determines what the whole row represents throughout the matrix.

 

Each column that is colored blue in the values part of the matrix represents a product or process needed to make the thing that the LCA is on. For example, a column in an inventory matrix for a toothbrush could represent the handle of a toothbrush. For each row that represents a material needed by a column the amount of that material should be substituted for the “#”. For rows that represent a material that is not needed by the product or process a column represents a zero should be substituted for the ‘#’. The amount of material used to make the product or do the process represented by the column should be negative since it is consumed. The amount of material output as a result making the product or doing the process represented by the column should be positive since it is being created. 

 

The technology matrix and intervention matrix both have linear space and values and use the same columns. The difference between the two is where the materials in each come from. All of the materials included in the rows of the technology matrix should come from the technosphere. The technosphere just means anything that is purposefully made by humans. For example, the plastic in a toothbrush is purposefully man made and should be included in the technology matrix of the inventory matrix for a toothbrush. All of the materials included in the rows of the intervention matrix should come directly from nature. For example, the petroleum fresh out of the ground used to make the plastic in a toothbrush should be included in the intervention matrix of the inventory matrix for a toothbrush. 

 

While the amount of material taken from nature to make a product or do a process is important information, the amount of materials emitted to nature is the most important information in the intervention matrix. For example, making the plastic used to make a toothbrush emits CO2 to the atmosphere that should be included in the intervention matrix of the inventory matrix of a toothbrush. The amount of these emissions to nature is the key information needed for the next step in doing and LCA, impact assessment. 

 

A simplified example of a filled inventory matrix using the toothbrush examples used in this section can be seen in the image below. 

 

 

 

 

 

 

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An image showing a simplified inventory matrix for a toothbrush

 

How to do an analysis of the inventory

While the full inventory matrix is interesting to look at it is usually too much to analyze. Our examples are very simplified so they are very small. A full inventory matrix for a simple product is typically around 50 columns by 50 rows while the full inventory matrix for a complex product can be hundreds of rows by hundreds of columns. 

 

There is complex math that starts with inverting the inventory matrix that allows us to get summary numbers for an intervention matrix when the intervention matrix is square (e.g. 2x2, 40x40, 400x400) and a principle called allocation that allows us to make an intervention matrix that is not square into a square matrix, but this is a bit too much to cover in this module. These calculations are also often done by software which is typically more accurate than doing them by hand anyway. You will be given the numbers you need to use from each inventory matrix for the rest of this learning module but we thought that you should know why they are different from the ones in the intervention matrix. 

 

The numbers you will be given, the summary numbers, are called ‘final demand’ and ‘total emissions’. Final demand is a column matrix, a matrix with only one column but many rows, that represents the total of each material used to make the product or do the process the LCA is on. Total emissions is another column matrix that represents the total emissions of each type of emission made by making the product or doing the process the LCA is on. As mentioned earlier, these emissions amounts are key to the next step in an LCA, impact assessment, and the total emissions values are the values actually used in this step. 

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Summary of Inventory Analysis

Inventory = This list of products and processes needed to make up the parts that make up the product or process you are doing your LCA on is called your inventory. The list of ingredients for the thing your LCA is about.

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Unit Process = The simplest data form that gives all of the inputs and outputs for making a single product or doing a single process. Shows all aspects of the product or process they represent and allows for the most customization of the LCA since it can be altered. Used most often.

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System Process = The more complex data form made up of several unit processes that gives all of the inputs and outputs for making a single product or doing a single process. Gives inputs and outputs of the product or process they represent but the process to go from input to output is not given and cannot be altered. Used to protect or industry secrets or when no alteration is needed. 

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Inventory Matrix = A matrix showing the list of products and processes needed to make up the parts of the product or process you are doing your LCA on, the amount of each needed, the amount of each output, and the amount of emissions created as a result.

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Linear Space = A column in the inventory matrix that gives the material and unit represented by each row in an inventory matrix. 

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Values = Columns that represent the parts used to make up the product or process the LCA is on that contain the amount of material needed by or made from making each part. 

Technology Matrix = A matrix  within the inventory matrix that has linear space and values for products and processes that are intentionally man-made. 

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Intervention Matrix = A matrix within the inventory matrix that has linear space and values for products and processes that come directly from or go directly to nature. 

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Final Demand = A column vector of summary numbers that give how much of each product or process is needed to make the product or process the LCA is on.

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Total Emissions = A column vector of summary numbers that give the amount of each type of emissions created when making the product or doing the process the LCA is on. 

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​Use the information you have learned in this section to complete the activity below, check your answers using the answer key, and then move on to the next section, Step 3- Impact Assessment.

 

Activity

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Click this red pdf button to download the activity for step 2. 

 

Answer Key

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Click this red pdf button to download the answer key for the activity for step 2 and check your answers.

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