A conditional exchange is an Exchange that is only considered for a specified System model. This concept, developed by the ecoinvent database, addresses the fundamental challenge that different system modelling approaches require different representations of the same activity within LCA datasets.
The need for conditional exchanges arises from the distinct ways that attributional and consequential system models handle certain flows within a product system. Whilst ISO 14040 defines exchanges as causal, directional relationships between activities or between activities and the environment, it does not specify how these exchanges should be adapted when applying different system models to the same underlying activity data.
In practical terms, conditional exchanges allow database developers to maintain a single activity dataset that can be appropriately configured for different modelling contexts. For example, an exchange representing the use of a by-product might be included in an attributional model but replaced with a substitution credit in a consequential model. Rather than maintaining separate datasets for each system model, conditional exchanges enable the same dataset to behave differently depending on the specified system model context.
This flexibility is particularly valuable when working with large-scale LCA databases, as it reduces data duplication whilst ensuring that the modelling logic remains consistent with the chosen system model's underlying assumptions. When a user selects a system model for their analysis, only the exchanges relevant to that model are activated, whilst conditional exchanges tied to other system models are excluded from the calculation.
The conditional exchange mechanism supports the practical implementation of different system models without compromising the theoretical rigour of either attributional or consequential LCA approaches.
