Page 55 - Materials Chemistry, Second Edition

2.2 Scope 39

Therefore, the CEDs of the two variants amount to
CED plastic A (3mm)= 3.6kgfU −1 × 60MJkg −1 = 216MJfU −1
−1 −1 −1
CED plastic B (2mm)= 3.6kgfU × 90MJkg = 324MJfU
This example is fictitious and, together with the above accomplished rough
estimates, only serves a better understanding of the fU and the reference flow.
From this simple example, however, it can already be perceived that, in view of
the use time as assumed, the thinner product fares substantially worse regarding
the single criterion CED. This relationship would not change if either any different
2
area or a different life time is used in the fU (e.g. 13.4 m /26.7 a).
If, however, a very short use time, below 15 a, is presumed in the fU in the
example above, another reference flow would result as a basis of the calculation.
Very short use times are, however, not reasonable assumptions related to building
products such as floor coverings, apart from rapidly changing products, for example,
in halls for trade fairs. The practical problem lies in the determination of realistic
use times of long-lived products, including the buildings themselves, which set an
upper limit for many building products. If use times as a function of thickness
cannot be determined, there can be a way out by restricting the comparison to ‘light
commodity A versus light commodity B, C, and so on’ and to ‘heavy commodity A
versus heavy commodity B, C, and so on’. It is thus assumed that life times within
the groups are similarly long.
A discussion of the correct use of fUs can be found in Technical Report ISO
TR 14049. 30) In addition, a set of product examples is provided, for example,
lamps, paints and hand-drying systems, and the limits of comparability of technical
systems are pointed out.
In view of the central importance of the fU in LCA, its correct determination has
the highest priority. It should be unambiguously feasible in all those cases, where
product systems to be compared have the same or a similar performance and fulfil
a similar function, respectively. Border cases are treated in the next two sections.

Exercise: Functional Unit and Reference Flow

Define a meaningful fU and describe the way to specify an appropriate reference
flow for the following products or services:

• Ball-point pen
• Window
• Disposal of polyethylene (PE) foil
• Spreading of a daily message.
Guideline:

• Which performance do these specified products or services have?
• Which variants can be considered to generate the performance?

30) ISO (2000a).