The Relative Mass-Energy-Economic (RMEE) Method for System Boundary Selection

Abstract

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 processes 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 relationship 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.