Page 116 - Materials Chemistry, Second Edition

8 Scope Definition 101

8.6.1 Ideal System Boundaries

Ideally, within the system boundaries should be all the unit processes required to
deliver the reference flow(s) defined by the functional unit. In cases where multi-
functionality is handled by system expansion, this also includes processes from other
systems that interact with the studied system. System boundaries should ideally be
set so that all flows crossing them are elementary flows (resources and emissions). In
other words, no material, energy, product or waste to treatment flows should cross
the system boundaries. Ideal system boundaries thereby contain all the unit pro-
cesses used to deliver the reference flow(s) by (1) generating energy and products
(materials for other unit processes) from extracted resources and (2) treating waste
flows to the point where the only outputs are emissions. Figure 8.11 illustrates an
ideal system boundary for a simple hypothetical product system containing just
fifteen unit processes. In this case, the inventory model is fully complete, because all
unit processes needed to deliver the reference flows are inside the system boundaries.
Outside the system boundaries lies the rest of the technosphere (not shown in
Fig. 8.9), i.e. the total body of other product systems in the global economy, and the
ecosphere, i.e. which is affected by resource uses and emissions from the
technosphere.

Ecosphere
Technosphere
System boundaries

Reference flow

Legend

Process Product or Elementary
waste flow flow

Fig. 8.11 Setting of system boundaries for a simple hypothetical product system. The boundary
contains all the unit processes required to deliver the reference flow (bold), and the only flows
crossing the system boundaries are elementary flows (blue). Note that the rest of the technosphere
is not shown