Modelling parasitic energy demand

Question

I am wondering how to model parasitic energy demand in openLCA. In the anaerobic digestion system I am modelling, biogas is used to generate electricity. Some of the electricity generated is used by the system. Furthermore all of the heat generated is used also by the digester. I have tried to implement this with electricity/ heat generated as outputs in the third process in the product system used as a providers for the electricity/ heat inputs in the second process. However this does not appear to work as this results in negative values for environmental impacts for the second process when calculated.

Maybe I am making this too complicated and can exclude the parasitic demand in the model? I am using a consequential approach, so the net electricity that goes to the grid is accounted for as an avoided product.

When creating the product system, I select auto-link processes and prefer default providers.

Any thoughts on this would be appreciated.

Many thanks, Ciara

Answer 1

This is an interesting question and if I am not mistaken we also run into this issue some years ago in a German bioenergy project. You do not tell the amounts but if you model so that you have a 1-1 loop (the digester takes all the heat and gives all its product back) this loop will never converge and thus will make a calculation impossible.

As workaround, you can merge this process into your other process and only have one larger extended process included in your model. Is that your situation? If you want you can post a picture of the system.

Thank you!
Andreas

Comments

Hi Andreas,

I have posted a picture of the system in the question above. Currently it is modelled using 4 main processes. So the work around would be to model all inputs and outputs of the system in one process? Does that make analysis difficult to determine where the major impacts are coming from in the product system?

Thanks,
Ciara