Lessons from the use of a long-term energy model for consequential life cycle assessment: the BTL case

The main objective of this study is to develop a methodology adapted to the prospective environmental evaluation of actions in the energy sector. It describes how a bottom-up long-term energy model can be used in a life cycle assessment (LCA) framework. The proposed methodology is applied in a case study about the global warming impacts occurring as a consequence of the future production of synthetic diesel from biomass ("biomass to liquids" – BTL), a secondgeneration biofuel, in France. The results show a high sensitivity of the system-wide GHG balance to (i) the policy context and to (ii) the economic environment. Both influence the substitutions occurring within the system due to the production of BTL. Under the specific conditions of this study, the consequences of introducing BTL are not clear-cut. Therefore, we focus on the lessons from the detailed analysis of the results more than in the precise-looking projections, illustrating how this type of models can be used for strategic planning (industry and policy makers). TIMES-type models allow a detailed description of the numerous technologies affected by BTL production and how these vary underdifferent policy scenarios. Moreover, some recommendations are presented, which should contribute for a propersystematization of consequential and prospective LCA methodologies. We provide argumentation on how to define a functional unit and system boundaries that are better linked with the goal of the study. Other crucial methodological issues are also discussed: how to treat temporal aspects in such environmental evaluation and how to increase the consistency of life cycle assessments.