SM5002 Chap.5 Modelling a Product System in openLCA
Modelling a Product System in openLCA
Why the course teaches the software early
The written assignment asks for a cradle-to-grave life cycle assessment of a single-use healthcare product, built in openLCA on Ecoinvent background data (cut-off, unit processes) with ReCiPe impacts under the Hierarchist perspective. Two workshops prepare you for that task.
The first introduces flows, processes, product systems and projects using a single-use surgical scissor. The second consolidates those skills by modelling a reusable scissor, adding a sterilisation process, and comparing the two per use inside a project.
This chapter follows the same route, so each skill is learned on a product whose answer is known before you try it on your own.
Setting up a database you can trust
The start-up guide recommends creating a new database named Net Zero Analytics course from scratch with complete reference data, then importing the Ecoinvent cut-off unit-process library and the openLCA impact assessment methods into it.
Keeping everything in one database gives an offline copy and flexibility later. Before modelling anything, the guide and the first workshop ask you to raise the memory limit to 10,000 MB, enable collaboration comments, set USD as the reference currency, and change the technical unit groups so that energy is in kWh, length in km, volume in litres and mass in grams.
That last change matters more than it looks, because a model built in kilograms and checked against a gram-based answer will be wrong by a factor of a thousand.
Flows, processes, product systems and projects
openLCA separates four kinds of object. A flow is a thing that moves, such as steel or a finished pair of scissors. A process is a dataset of inputs and outputs that makes one reference flow.
A product system links a process to all of its upstream providers so that the full supply chain can be calculated. A project places several product systems side by side under one impact method so they can be compared on the same chart.
In the first workshop you create a category for the case study, make a process for the single-use scissor, create a new flow for it, and set the reference flow property to number, because the product is counted rather than weighed.
The single-use scissor inventory
The functional unit is one single-use surgical scissor weighing 60 g.
Inputs are 60 g of chromium steel 18/8 from the global market process, 60 g of metal working for chromium steel products, and lorry freight entered as 0.06 x 800 kg*km using the 16 to 32 tonne EURO 5 diesel lorry for the rest of the world. The output is 60 g of waste steel. Calculated with ReCiPe Midpoint Hierarchist, the climate change result is 0.47651 kg CO2e.
Building a project that compares one pair with seven pairs then gives 3.33559 kg CO2e for seven, and dividing confirms an exact factor of seven, which is the first habit the workshops build: check numbers against an expected ratio before trusting them.
Reuse, sterilisation and the per-use functional unit
The consolidation workshop changes the functional unit to one use of a pair of surgical scissors.
The reusable pair weighs 100 g and lasts 40 uses, and its whole-lifetime process uses 100 g of steel, 100 g of metal working, 0.1 x 800 kg*km of freight and 100 g of waste steel. Its climate result of 0.79419 kg CO2e sits in a 60:100 ratio with the single-use result, exactly as the masses predict.
Reuse requires decontamination, so a second process models steam sterilisation of one instrument set: one slot of a sterilizer that uses electricity, steam, tap water, detergents and small quantities of packaging and container materials. Its municipal solid waste output must be derived by summing the solid material inputs, which gives 24.89 g.
A third process then assembles one use.
It takes the whole-lifetime process as an input with an amount of 1/40, and the sterilisation process with an amount of 100/2000, because one set holds 2 kg and a 100 g scissor occupies one twentieth of it. The resulting climate change figure is 0.11968 kg CO2e per use.
Most of that comes from sterilisation rather than manufacture, which is the central lesson of the exercise.
Reading results and avoiding common mistakes
The result editor offers several views. Top contributions lists the five processes that add the most directly. Inventory results list the summed elementary flows. Impact analysis expands each category into flows and characterisation factors.
Process results distinguish direct impacts from upstream impacts. The contribution tree and Sankey diagram show how processes connect and where the load is carried, and LCIA checks flag flows the chosen method does not cover.
The workshops also list the mistakes that most often produce wrong answers: choosing a system process instead of a unit process, mixing grams and kilograms, confusing market and transformation processes, selecting the wrong geography, and forgetting that openLCA only calculates saved data.
If a process changes after a project exists, delete and rebuild the product system or update the process links from the model graph.
Sharing models and moving to damage results
Databases live on your own machine, so collaboration happens by export.
The first workshop recommends JSON-LD as the format for moving work between openLCA installations, and advises exporting processes rather than a product system alone, because a product system without its processes cannot be rebuilt on another computer. Imported processes are then used to recreate the product system. Results can be shared as an HTML report, as LCIA tables pasted into a document, or as a saved Sankey image.
Once midpoint results are stable, the consolidation workshop switches the calculation to the ReCiPe 2016 v1.03 Hierarchist method at damage level, where the summed total across damage categories is what is generally reported.
The same sequence of build, check, compare and report is what your own product model will follow, and the written assignment expects screenshots of inputs and outputs, the impact assessment, and the contribution analysis.
What this chapter covers
- 01
Setting up the course database and unit groups
- 02
Flows, processes, product systems and projects
- 03
Building the 60 g single-use scissor unit process
- 04
Steam sterilisation as a separate process with a derived waste output
- 05
Scaling lifetime and sterilisation processes to one use
- 06
Comparing product systems in a project and reading result tabs
Worked example · free
Climate change per use of a reusable surgical scissor
- 1Sense check the lifetime result: 0.47651 / 0.79419 = 0.60, matching the 60:100 mass ratio.
- 1Scale manufacture to one use: 0.79419 / 40 = 0.01985 kg CO2e.
- 2Find the scissor's share of a set: 100 / 2000 = 0.05, so sterilisation per use is 1.99654 x 0.05 = 0.09983 kg CO2e.
- 1Sum the two shares: 0.01985 + 0.09983 = 0.11968 kg CO2e per use.
- 1Compare: 0.47651 / 0.11968 = 3.98, and sterilisation supplies 83% of the reusable burden.
Key terms
- Unit Process
- An Ecoinvent dataset that links to its upstream providers rather than holding pre-aggregated results; the course uses the Cutoff, U versions.
- Reference Flow
- The product output a process is defined to make, set to number for counted items such as a pair of scissors.
- Product System
- A process linked to all of its upstream providers so openLCA can calculate the complete supply chain.
- Project
- An openLCA object that compares several product systems under one impact method and report.
- Market Process
- An Ecoinvent dataset named market for a product, which the workshops ask you to tell apart from transformation processes when choosing providers.
- Direct Impact
- Impact caused only by the flows inside one process dataset, excluding any supply chain upstream of it.
- Direct Calculation
- A quick calculation run from a process editor to check results before building a product system.
Modelling a Product System in openLCA FAQ
Why does the reference flow use number rather than mass?
Scissors are counted and used as whole pairs, so the functional unit is one pair or one use. Material exchanges such as steel and waste stay in grams, and openLCA scales them from the counted reference.
How do I know my openLCA result is right before I trust it?
Compare against an expected ratio. Seven pairs should give exactly seven times one pair, and 60 g against 100 g of identical materials should give 0.6. A ratio that drifts means a unit, provider or unsaved edit is wrong.
Why is the sterilisation amount 100/2000 and not 1?
One sterilizer slot treats a whole instrument set holding about 2 kg. A 100 g scissor takes one twentieth of that load, so only that fraction of the energy, steam, water and materials belongs to it.
What should I do after editing a process that is already in a project?
openLCA will not refresh the linked product system on its own. Save the process, then either delete and rebuild the product system or right click the model graph and update process links before recalculating.
Assessment move
Rebuild both scissor models yourself and check every result against the ratios given before you start your own product. Keep a running list of providers, regions and units as you go, because that list becomes the inventory table the written assignment requires.
