Life Cycle Assessment (LCA) is a well-established methodology that has been extensively used for the environmental evaluation of emerging technologies in the field of renewable energy conversion systems. In order to identify effective solutions for impact mitigation already at the conceptual process design stage, it is necessary to take into account the environmental aspects in the process design procedure, in consistency with economic and thermodynamic criteria. This thesis presents a systematic methodology for the integration of LCA in the conceptual design of renewable energy systems, using process design, process integration and multi-objective optimization techniques. The developed methodology is illustrated by three application case studies. The first one concerns the evaluation of different candidate technologies for the combined production of Synthetic Natural Gas and electricity from lignocellulosic biomass by thermochemical conversion. The second case study treats of the environomic optimal configurations of Enhanced Geothermal Systems for combined heat and power production, considering different potential conversion technologies. The third case study deals with the design and the synthesis of an urban energy system, accounting for the different energy services to be supplied, the different potential resources, the candidate conversion technologies, and the waste to be treated. In this last case study, the overall supply chains from resources or wastes to services are synthesized, accounting for the industrial ecology possibilities. A first major outcome of these application case studies is that accounting for the process integration in the evaluation of the environmental impacts leads to an important impact reduction. Secondly, the developed methodology allows for calculating the trade-offs between the environmental criteria and the thermo-economic ones. Therefore, the optimal promising configurations of an emerging technology or of a system considering its environmental performances can be identified. Finally, it is demonstrated that integrating the environmental impacts in the process design procedure leads to a different decision-making than if only thermo-economic criteria are considered.