The Inventory Phase2
The various steps of the inventory phase are shown in Fig. 15.2. The aim of the inventory phase is to comprehensively identify and quantify flows between the technical product system and the environment, i. e., emissions and resources. An example of an inventory of a product system is shown in Fig. 15.3 and Table 15.1.
FIGURE 15.2 Steps of the inventory Phase.
FIGURE 15.3 Example of a product system, production and use of steel sheet metal, for life cycle inventory analysis.
TABLE 15.1 Calculation of Total Elementary Flows from the Steel-Sheet Product System of Fig. 15.3
Note: Figures are fictitious, but not unrealistic
The boxes in Fig. 15.3 represent so-called unit processes. Each one can consist of several actual processes, which for practical purposes are treated together as unit processes.
A unit process is defined as the smallest unit for which flows are specified. For each one of the unit processes, there may be mass and energy flows to and from
• other unit processes (intermediate product flows),
• the environment (elementary flows), and
• product systems outside the system boundaries (product flows).
For each unit process, a reference flow may be defined, and the inputs and outputs to the unit process calculated in relation to the reference flow. For instance, the reference flow for mining of iron ore is the mass of iron ore mined per year, and the emissions to the air may be expressed as kg dust per metric ton of ore.
Reference flow and the functional unit are defined for the entire product system, and the elementary flows are calculated in relation to these. The flow figures are normally aggregated, and the total flow of each substance recorded and used for impact assessment.
Inventory calculations can be made in several ways. An example oi one method of calculating is shown in Table 15.1. The calculation process is simi lar for other emissions and resources.
In practical product design, a full LCA is seldom performed. The unit operations represent whole systems, which are used as building blocks. Typical examples include elementary flows determined for the “manufacture of 1 m2 1 — mm sheet metal of galvanized steel,” “incineration of 1 kg of polyethylene.” or “production and distribution of 1 MJ electricity.”
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