Why does my product system link downstream as well as upstream?

Question

I'm not sure whether I've done something silly or there's another explanation but would appreciate any suggestions.

I'm finding that when I create a product system from a unit process, the resulting product system links to both the upstream processes (providers, as you'd expect), but also to all of the downstream processes (users, not as expected). I'm not sure why!

For example, I create a product system from the process "market for electricity, high voltage | electricity, high voltage | Cutoff, U - GB" (43fe4ab4-3325-360e-a74d-af31b28d0576). I use the default settings (auto-link processes, not check multi-provider, prefer default providers, system processes, no cutoff). I'm using openLCA 1.10.3 on macOS (catalina) with ecoinvent 3.7 cutoff (unit).

The resulting product system has the various sources of electricity generation as expected (as per the inputs to the process). However, the resulting model graph also has lots of additional processes as outputs that are not expected - for example "treatment of precious metal from electronics scrap, in anode slime, precious metal extraction | palladium | Cutoff, U" (87e72e1a-8320-3222-9aba-1d1d45c9f6ae). This unexpected process has the original process as a default provider (along many others).

It's almost as if the electricity is being treated as a waste product and the additional processes as waste treatment processes. I feel like perhaps I've done something wrong but I've no idea what!

Any suggestions gratefully received!

Sam

Comments

To add - strangely, this occurs for other electricity market processes (with the processes for which those markets are default providers), but doesn't seem to happen for other processes I'm trying (e.g. foods).

When I actually analyse the product system with the downstream processes, they don't seem to show up in the impacts but this might just be that they're so small (I don't know how much of each, if any, is actually included in the inventory calculation).

Answer 1

Hi Sam,

good question - openLCA always connects "downstream" (following the supply chain), you see this e.g. in the sankey diagram; it is however that in ecoinvent and also many IO databases, processes are used also in other parts of the product system, requested by other processes. This is shown as a downstream connection in the model graph. You see that for a calculation of 1kWh (3.6 MJ) of electricity..

..overall 3.658 MJ of this process product are used, because a small amount (0.058 MJ) is input in other processes in the supply chain (screenshot from the calculation of this process product system, ecoinvent 3.7.1 cut off).

I hope this clarifies, best wishes, Andreas

Comments

Hi Andreas
Thanks so much, that does help.
To make sure I've understood - I think you're saying that those processes that the electricity is being supplied to are just those that are actually also in the supply chain for that electricity market.
I should have thought of that! I guess the default starting point for the model graph is to show supply chain for one tier either side, because potentially there are waste outputs, so that makes sense also.
Thanks again!
Best regards
Sam