Avoiding “system expansion”?

When producing a product footprint – or life cycle assessment (LCA) result – we want it as far as possible to be useful for decision support. Therefore, the way we model the product systems should maintain mass, energy and monetary balances intact, fully reflecting physical and economic causalities, based on empirical relations rather than normative assumptions and arbitrary cut-offs. This also implies avoiding allocation (partitioning) of systems with joint production of co-products, since this is an inherently normative procedure that cuts off part of a product system and therefore can provide misleading results.

To avoid allocation of systems with joint production, we use a procedure that the ISO 14044 standard on LCA calls “system expansion”. This makes it sound like something was missing in the original system and that a lot of additional work may be involved. But this is not the case.

When the text of the ISO 14044 standard was originally drafted, before the turn of the century, procedures for LCA were not as developed as they are now. Product systems were manually created using foreground data to painstakingly describe flows to and from nature for each unit process in the product system. Cut-offs were needed due to lack of data or time. In this context, the term “system expansion” appeared natural for describing the inclusion of modelling of the further fate of the by-products and wastes and the resulting changes (substitutions) in the product system, as opposed to cutting them off by allocation.

Today we have environmentally extended input-output (I/O) databases based on national statistics that cover the entire global economy. Our product systems are therefore inherently complete from the outset. When we perform “system expansion”, we therefore do not actually expand the product system, but simply follow the fate of the by-products and wastes within the already complete system, accounting for the increase in treatment activities and decrease of the upstream activities that supply the same markets as the useful by-products. The only manual work left today is to ensure the correct distinction between the products that determine the volume of the joint production and the by-products for which we must ensure that they are correctly linked to the markets where the substitution occurs. More details and examples of this can be found at www.consequential-lca.org.

The term “system expansion” is therefore no longer appropriate for what we do in practice, and we therefore tend to rather use the more general and neutral term “substitution” for this procedure. This is in accordance with the way “system expansion” was described in the original ISO text, as explained by Bo Weidema in a previous blog post.