Negative or Zero Impact Assessment Results During Calculation in openLCA

Question

I am currently trying to run a product system with a cradle-to-grave system boundary. I am using openLCA version 2.6.2 and the ecoinvent 3.12 database.

When I run the product system using the auto-linked connections from the upstream to downstream unit processes, the impact assessment results appear normally. However, when I try to run only the upstream unit processes, all of the impact results are negative.

I then tried to manually link the unit processes to each other, but the resulting impact assessment values were all 0. The impact assessment method I am using is IPCC.

Why could this happen? Is there possibly something wrong with the datasets I am using or the way I have linked the processes?

I am also attaching screenshots showing the negative impact results, as well as the input/output inventory data I used.

Any help or guidance would be greatly appreciated. Thank you very much.

Answer 1

Hi, thank you for your for question, it seems you combine unit and system processes, why? I do not understand 

the auto-linked connections from the upstream to downstream unit processes

vs 

when I try to run only the upstream unit processes

- what is the difference? Downstream is with end of life? Thank you. Ideally, update your question above.

Comments

Thank you for your question. Let me clarify the difference between the two situations.

My LCA model consists of several unit processes connected from upstream to downstream. When I create a product system and use the auto-linking function to connect the upstream and downstream unit processes, I can calculate the product system successfully and the impact results appear normally.

However, when I try to calculate only the upstream unit processes separately, without including the downstream processes, some of the impact results become negative or zero.

By “upstream” and “downstream,” I mean the different stages of my production system, not necessarily end-of-life. The downstream processes are simply the subsequent processes connected to the upstream processes in my production chain.

So, my question is:

Why do the impact results appear normally when the upstream and downstream unit processes are connected through auto-linking in a product system, but become negative or zero when I calculate only the upstream unit processes separately?

Could this be related to the reference flows, inputs/outputs, providers, or the way the unit processes are connected? Is it expected behavior in openLCA when an upstream unit process is calculated separately from the processes that consume its output?

Thank you for your help.

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Ideally post a screenshot of the two models - it seems you are getting only partial results when you see negative values. openLCA always calculates downstream from the reference process for product flows, and upstream from the reference process for waste flows. You can also book a modeler hour in Nexus if you do not want to share information here in public. Thank you.

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Thank you for the explanation. Here I am sending the screenshots of the two models: the upstream-to-downstream model and the upstream-only model.

Thank you for your help.