Requirements for Specifying Functional Units and
Reference Flows
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Cooper, J.S. “Specifying Functional Units and Reference Flows for Comparable
Alternatives,” International Journal of Life Cycle Assessment, 8,
337-349 (2003) DOI:
http://dx.doi.org/10.1065/lca2003.09.134
Despite
documentation of product lifetime, performance, and system dependency issues,
requirements for specifying functional units and reference flows in LCA have not
been developed. The ISO standards simply note that selection between functions
is dependent on the goals and scope of the study, that the functional unit must
be clearly defined and measurable, and that the reference flows are the amount
of product necessary per functional unit. The goal of this work is to suggest
and demonstrate the use a set of requirements for specifying the functional unit
and reference flows for comparative LCAs.
The
suggested requirements were developed to address the lifetime, performance, and
system dependency issues described in LCA literature and to ensure adequate
information is available for the interpretation of results. Also, well
developed methods for conceptual design were used to formulate aspects of the
requirements to improve comparability of alternatives. A case study
demonstrates the use of the requirements in materials selection for aircraft
design. In the case study, functional units are specified for the component
being designed and for the aircraft. Similarly, reference flows for the
component, component interfaces, and the aircraft are quantified based on
parametric and linear estimation models. Finally, an interpretation of data
quality, uncertainty, assumptions, and limitations are presented.
The
requirements are shown to be particularly important when the product be assessed
operates as part of a larger system and when there are performance differences
among alternatives. The case study illustrates the importance of including
consideration of system and interface materials and energy flows in the
comparison of aircraft components. Specifically, because the mass of interface
materials is estimated as more than the difference in subsystem masses,
differences in the variable mass of the aircraft and the lifetime fuel
consumption are accounted for in the reference flows.
Some
practitioners have recognized difficulties in accounting for product lifetime,
performance, and system dependencies in LCA, even though a set of requirements
has not been included in literature or in the ISO standards. The suggested
requirements presented in this work were found to be useful in accounting for
differences in materials and energy flows and in providing a transparent
presentation, assessment, and interpretation of reference flows and ultimately
in the LCA results.
For
more information, contact Associate Professor Joyce Smith Cooper at
cooper@me.washington.edu
