Page 106 - Materials Chemistry, Second Edition

8 Scope Definition 91

Emissions to air, Emissions to air,
water and soil water and soil

Materials Materials

Product 1 Product 1
Unit process
Product 2 Product 2

Fig. 8.6 Solving the multifunctionality problem by increasing the modelling resolution and
sub-dividing the process into minor units which can unambiguously be assigned to either of the
functional outputs

number of processes and that the processes needed for the production of the first
product are physically separated from the processes needed for the production of
the second product. This approach to solving multifunctionality does not always
work. Even if you zoom to the molecular level of a cow, it is not possible to
physically separate the metabolic processes in the cow that lead to the production of
milk from the ones that lead to the production of meat or hide.
System Expansion
If subdivision fails to solve the multifunctionality problem, the ISO standard rec-
ommends trying to solve the problem by system expansion. In a comparison of two
processes, this means expanding the second process with the most likely alternative
way of providing the secondary function of the first process. In the comparison of
power plant 1, which has district heating with co-generated heat as a secondary
function, with a power plant 2, which only produces electricity, this means
expanding the system of plant 2 with the most likely alternative way or combination
of ways of providing district heat in that region (see Fig. 8.7).
Expansion of system 2 with the alternative way to produce the secondary
function of system 1 is equivalent to subtracting the alternative way from system 1
(which provides the function). This is also called to credit system 1 with the inputs
and outputs which are avoided when its secondary service replaces this alternative
production. In the case of district heating being the secondary function, system
expansion would thus be the same as crediting the power plant, which produces the
district heat, through subtracting the impacts from the most likely alternative way of
producing this heat as illustrated in Fig. 8.6.
In Fig. 8.6 equation B follows from equation A by subtraction of the alternative
way of district heating from both sides of the equal sign. The approach of system
expansion is thus mathematically equivalent to crediting for avoided production.
Crediting for avoided production is typically used to account for secondary func-
tions in a hotspot analysis where there is not a comparison of two alternative
systems. For example, a product system that includes incineration can be credited
for the avoided impacts from the production of heat and electricity by subtracting
the avoided elementary flows in the inventory of the process (see Chap. 9 for
technical details). In the milk example, system expansion can be performed by