LCA Methodology

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DOI: http://dx.doi.org/10.1065/lca199911.003 - Life-Cycle Assessment (LCA) is a decision analysis tool used to compare alternatives to providing a given product or service. To ensure a fair comparison, LCA must select system boundaries in a consistent manner. The Relative Mass-Energy-Economic (RMEE) (pronounced ‘army’) of system boundary selection is a practical and quantitative method of defining system boundaries. RMEE compares each input to a system with the system’s functional unit on a mass, energy and economic basis. If this ratio of input to functional unit is less than a selected "cut-off" (defined as ‘Z RMEE ’) then the input is excluded from the analysis and all unit pro-cesses upstream of that input are outside the system boundary. Ignoring unit processes outside the system boundary limits the size of the LCA analysis but adds a source of uncertainty for the overall results. The lower the value of the Z cut-off ratio the larger the system boundary is, resulting in a greater number of unit processes. The relationship between the Z RMEE cut-off and the uncertainty introduced to the results of an LCA are explored. The relation-ship between Z RMEE and uncertainty has been derived by analyzing 800 random systems of four different types, with the RMEE method of system boundary selection applied at different Z RMEE values. The uncertainty introduced to the overall results increases as the selected Z RMEE becomes larger.