A consequential LCA always studies a specific decision and the resulting changes. A decision may involve choosing different ways to achieve the same result (e.g., going from home to work by train or by car), or whether to carry out a specific activity or not (e.g., whether to subsidize organic agriculture or not).
Decisions always involve a choice between alternatives: alternative activities or alternative products. In reality, these are two sides of the same coin: activities provide products; products are the result of activities, as in the case of dairy farming (activity) >< milk (product).
Therefore, alternatives can be modeled as product systems: interconnected activities (unitary processes) that together approximate what happens when a product, such as a car or a bicycle, is produced, used, and disposed of. To define a product system, it is necessary to start with a quantified description of the performance requirements that the product system meets. This quantified description is called the "functional unit" of the product system. The functional unit provides the reference against which all other data in product systems are normalized.
When modeling the consequences of a decision, what interests us is the difference between alternative product systems. However, to be considered alternative, product systems must have comparable results, meet the same performance requirements, and have the same "functional unit".
The following figure illustrates a step-by-step procedure for defining the functional unit and identifying the reference flows for each alternative product system.
FIGURE 1
Caption: The image represents the flow of information between different phases to identify the functional unit. In practice, product properties (phase 1) can be determined simultaneously or even starting from market segmentation information (phase 2). In other cases, product alternatives are provided in advance and therefore contribute to defining their respective product properties (Weidema et al. 2004).
For further information on the topic, refer to the following video: (go to the Youtube video).
Next lesson - In-depth "Product properties"
