Specifying functional units and reference flows for comparable alternatives

Goal, Scope, and Background. 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. Methods. 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. Results and Discussion. 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. Conclusions. 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. Recommendations and Outlook. This work is significant because the specification of the functional unit and the definition of the reference flows dictate the materials and processes included in the LCA. Future work is needed to test the general applicability of the suggested requirements to a wide variety of product systems.