negative values among the inventory inputs

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Sara
Posts: 13
Joined: 03 Jun 2013 17:04

negative values among the inventory inputs

Post by Sara » 21 Nov 2013 18:06

Hi everybody,
I'm running an LCA study and I think I'm having problems with the inventory.
The LCA in question is about onions production and the aim is to asses blue water footprint. The system is very simple (it's just a sort of "training "case study) and I'm using Ecoinvent 2.1. I have installed the latest version of openLCA. Furthermore, I created a new LCIA method in order to sum all the freshwater related values of the inventory inputs: I considered all the elementary flows in the category "resource/in water" (except for the sea/salt water ones).
When calculating the inventory, I have, as I expected to, some different water flows among the inputs. But some of them have negative values and these values affect the final result of blue water footprint that is, in fact, negative. And obviously this isn't an accettable result...

Can you help me?
Thanks a lot!

Sara

Sara
Posts: 13
Joined: 03 Jun 2013 17:04

Re: negative values among the inventory inputs

Post by Sara » 23 Nov 2013 17:57

Hi,
in order to solve the problem I mentioned in the previous post, I dowloaded and checked the matrices (both). In the matrices the values are all negative because they are all input flows. So I think that it isn't a data related issue, nor a modelling related one....but the problem is that I am probably missing something about how openLCA calculate the inventory. And, maybe, I should calculate blue water footprint in a different way.
I hope someone can help me,
thank you!

Sara

aciroth
Posts: 750
Joined: 09 May 2010 23:28

Re: negative values among the inventory inputs

Post by aciroth » 24 Nov 2013 23:20

Hi Sara,
inputs and outputs can both be negative; and then, the sign is also used to distinguish input from output therefore the matrix contains negative and positive values. From your question it seems that your process data sets - as you see them in openLCA - do not have negative values; do you model a system expansion?
Best,
Andreas

Sara
Posts: 13
Joined: 03 Jun 2013 17:04

Re: negative values among the inventory inputs

Post by Sara » 25 Nov 2013 12:49

Hi Andreas,
I did the following:
1. I chose some processes in agricultural means of production category (such as irrigating, sowing, tillage, ecc.) and I made copies of them in order to modify them entering my data (included some specific water consumption data). I didn't remove any of the existing inputs and outputs (except for electric energy consumption in the irrigation, because they used only diesel fuel) but I changed some values of them and added a few other inputs (essentially lubricating oil and its disposal).
2. I built my reference process (in plant production) with the modified processes
3. then I built my product system e and calculated the inventory.
Is it a system expansion?
Doing so, I obtained a very complex system (as i can see using the Expand All icon) and I didn't expected it.
If I remove some unit processes in the graph, then can I calculate a simplified inventory? Or is it better to simplify the system by modifying the matrices?
I hope you can give some useful suggestions,
thanks
Sara

Sara
Posts: 13
Joined: 03 Jun 2013 17:04

Re: negative values among the inventory inputs

Post by Sara » 25 Nov 2013 13:20

I forgot: my reference process has only one product that is 1 kg of onions and no emissions or waste because they are already included in the inputs processes.

aciroth
Posts: 750
Joined: 09 May 2010 23:28

Re: negative values among the inventory inputs

Post by aciroth » 25 Nov 2013 17:53

Hi Sara,
With system expansion I mean if you have avoided products in your model. You can either reduce your model by deleting branches, or, better, start again and build the model stepwise, using the complete supply chain button. I would not modify the matrices.
It does not matter that your final process is rather simple, important is that it uses some products with an obviously longer supply chain.
Best wishes,
Andreas

Sara
Posts: 13
Joined: 03 Jun 2013 17:04

Re: negative values among the inventory inputs

Post by Sara » 03 Dec 2013 19:03

Hi Andreas,
I checked the system and here are some considerations.
I didn't put any avoided products in my processes and all the water inputs in the whole system seem to be positive.
Through the analysis tool in openLCA I saw that the negative values are related to fertiliser production, that is related, at the very beginning of the supply chain, with the process "electricity, hydropower, at run-of-river power plant" that has
the elementary flow "Water, turbine use, unspecified natural origin" as an input. This flow has the highest negative value among the inventory results.
Do you think that this could be related with the use of water in this kind of hydropower plants? Because It's not really extracted from environment and maybe It's better to exclude this flow from tha LCIAM blue water footprint. In fact, if I remove it from the calculations, the result turns positive.
Actually, there are other negative values but they aren't relevant("Water, well in ground" and "Water, lake").

Let me know if, in your opinion, this is an acceptable reasoning.

I would have also another question: why we can have negative values in the input and in the output tables of the inventory?

Thanks a lot,
Sara

aciroth
Posts: 750
Joined: 09 May 2010 23:28

Re: negative values among the inventory inputs

Post by aciroth » 15 Dec 2013 18:19

Dear Sara,

sorry for my somewhat delayed reply - is it possible that "Water, turbine use, unspecified natural origin" occurs also on the output side of a process? Because openLCA tries to determine whether an elementary flow is "really" input or output by checking where a flow is used, on the input or on the output side of processes, and multiplies those that are input (typically resources) with a -1 in order to present positive values as result.

We think to make this more explicit in future, e.g. by adding a checkbox ("input?") to the elementary flows. However, if these flows are really elementary flows, then the initial "contact" with a process should clearly identify whether the flow is input- or output related.

OpenLCA treats inputs and outputs just with two different signs, inputs are negative. If, for example, you have an avoided product, you can have a net consumption of emissions, which leads to negative values in the inventory tables.

Hope this helps, best wishes,
Andreas

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