Page 117 - Materials Chemistry, Second Edition

102 A. Bjørn et al.

8.6.2 Reasons to Divert from Ideal System Boundaries

There are three reasons to divert from working with ideal system boundaries:
First, if a study does not take a full life cycle perspective the rule of only
allowing elementary flows to cross the system boundary does not apply. A study
taking a full life cycle perspective aims to cover all the processes that are needed to
deliver the function(s) of interest upstream (extraction and production of raw
materials and manufacturing) and downstream (disposal) to the use stage. By
contrast, a so-called “cradle-to-gate” study is an example of a study not taking a full
life cycle perspective because the system boundary ends at the gate of the factory
where the studied product is produced. In this case, the product flow thus crosses
the system boundary, as shown in Fig. 8.12 (based on the simple hypothetical
product system shown in Fig. 8.11). The goal definition’s intended applications of
results decides whether a full life cycle perspective should be taken (see Chap. 7).
This decision is usually also reflected by the functional unit (see Sect. 8.4.2).
Second, in comparative studies it is justified to exclude identical processes if
they deliver identical quantities of services (energy, materials or treatment of waste)
in the systems studied. For example, in the illustrative case study on window frames
(Chap. 39) comparing four windows, the processes involved in cleaning the

Ecosphere
Technosphere
System boundaries

Reference flow

Legend

Process Product or Elementary
waste flow flow

Fig. 8.12 Setting of system boundaries for a simple hypothetical product system in a
cradle-to-gate assessment. In this case, the reference flow (bold) is crossing the system
boundaries, to the rest of the technosphere; in addition to the elementary flows (blue) entering or
leaving the ecosphere