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I doing LCA using OpenLCA, 1.6.3 I'm still at the beginning. And I would like to know about the "avoided product", when should I check this option on outputs?

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Hi, thank you for your question. I will just add something to the precise explanation Jonas provided related to "avoided products" in waste modelling, which is a new feature of openLCA 1.7.
In general, "avoided products" is used (also in openLCA 1.6) for system expansion, which is an alternative to allocation. System expansion and allocation are two strategies used when you have two or more products as outputs of one process and you need to know how the flows (and then impacts) contributions are distributed between the different product outputs. For instance, from cogeneration you can produce heat and electricity. If you want to see only the impacts related to electricity production in LCIA results, you can apply system expansion: you assume then that the heat produced elsewhere (outside the product system) will be substituted via the heat produced by your modelled product system.
The “co-generation” product system is thus “credited” with the avoided impacts of the alternative heat-producing process “Heat, gas heating” , i.e.  the environmental impacts of the process “Heat, gas heating” will be subtracted from the overall environmental impact of co-generation. Therefore, you can tick the box "avoided products" for the byproduct represented by heat. A process providing the avoided product flow should exist in the database, of course.
by (260 points)
Thank you for your answer, it helped me a lot.
by (240 points)
This explanation is clearer than what's in the user manual; I suggest adding it.
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by (23.4k points)

Hi, this is explained in our openLCA 1.7 manual http://www.openlca.org/learning/

One of the most important new features in OpenLCA 1.7.0 is the possibility to create a waste treatment process from a waste flow which results then as an input process.

Create a waste treatment process:

Add a waste flow as an inpit in a waste treatment process:

When applying system expansion, it is possible to mark an input waste flow as an avoided waste. This means an expense in the cost column because the flow is considered as an output, as shown in grey in the model graph of the product system.

Still, it is also possible to model waste flows in the say way as it was done in the previous versions of openLCA. This means considering waste treatment as a service for the process to eliminate the product. In this case, the waste flow is a output in the waste treatment process and the waste treatment process is an input in the considered process. Attention must be paid to the positive or negative values that are insered in the amount column.

More information on this improvement in openLCA 1.7 were also announced in our blog http://www.openlca.org/we-are-happy-to-announce-the-release-of-openlca-1-7/

openLCA ≤ 1.7 – Required modelling according to the value flow direction: Previously, some cases of waste flow modelling in openLCA required a workaround. Usually, production processes are considered as of value for the receiving process. Therefore, wastes from disposal processes had to be considered as an output and returned to the production process as an input to counter the direction of material flow.

openLCA ≥ 1.7 – Allows modelling according to the actual flow direction: The introduction of a new flow type ‘waste’ allows to model waste flows directly as outputs of a production process which can be assigned as input to waste treatment processes.

Moreover, quantitative references in openLCA ≤ 1.7 had to be to the output side. This is no longer the case.

by (23.4k points)
To my knowledge, the feature is new to openLCA 1.7 and does not exist in openLCA 1.6.