The integration of environmental concerns into the product design process has highlighted a new problem that arises when confronted with complex systems. Indeed environmental assessment methodologies like Life Cycle Assessment (LCA) become in this case particularly heavy to implement. Considering aluminium electrolysis substations as a complex industrial system, we propose a new eco-design methodology based on a Lean Six Sigma approach. Including the environmental parameter as the fourth dimension of the Quality, Costs, Time triangle this methodology has the advantage to cover and systematize the entire eco-design process. It answers to most of the limits raised in our study and allows managing a part of the complexity that appears in particular during the goal and scope definition and the inventory phases of LCA. An application of aluminium electrolysis substations is mentioned.