**Preface to "Life Cycle Assessment of Energy Systems"**

There is little doubt that the existing energy model, based on the mass consumption of fossil fuels, is utterly unsustainable. The urge for its profound transformation has intensified in recent years due to mounting evidence of global environmental degradation, potential shortages due to political instability in fossil fuel producing countries, and the economic consequences of higher prices due to a declining supply capacity. Despite unceasing warning signs, current projections from the International Energy Agency still describe a 1.3% yearly rise in energy demand until 2040, with fossil fuels remaining as the dominant source and expecting to account for 80% of the total primary energy supply in 2035. The result of a such trend will inevitably be a departure from the objectives of the 2016 UN Paris Agreement and an escalation in the strains exerted on the limits of our environment and our capacity to survive as a species.

In this context, several international initiatives are striving to redirect this situation so that a more sensible, beneficial future exists for all. For instance, the UN 2030 Agenda for Sustainable Development emphasizes, in Goal 7, the need to ensure universal access to affordable, reliable, and modern energy services. This document also states the need to increase the share of renewable energy and to improve efficiency, with actions required throughout the entire value chain of energy systems (including extraction of resources, transformation, transmission/transport, storage, and use). For all this to work, we need to develop advanced technologies and implement effective policy measures.

But, to ensure success, what will these new technologies and policies look? How can we ensure that the new technologies and plans are not flawed, that there is no transfer between impact categories and that the resulting scenario is more sustainable than the one we leave behind? How can we design the most sustainable technologies? How can they be deployed to maximize social wellbeing? How many jobs will be gained or lost in this energy transition? Will the economic cost compensate the environmental and social benefits? For this transition to be effective, all these questions and all the decisions that lay ahead cannot be taken lightly, and need to be responded to from a scientific, objective, and holistic perspective.

Life Cycle Thinking is a comprehensive and systemic framework that goes beyond the traditional focus on production sites and manufacturing processes to evaluate the sustainability of products and services. This framework has shaped a range of tools that are certainly applicable to investigating these questions and shedding light onto the sustainability assessment of energy systems. The most mature of these tools is the conventional Environmental Life Cycle Assessment (LCA), a robust procedure that is widely accepted and aimed at evaluating the attributional performance of systems, from the very simple to the highly complex. Even though it was born as a product-oriented tool focused solely on environmental issues, recent methodological extensions (such as Environmentally Extended Input–Output analysis, Hybrid IO–LCA, Consequential Analysis, Social LCA, and Environmental Life Cycle Costing) have broadened its scope and functionality.

This Special Issue on "LCA of Energy Systems" contains inspiring contributions describing the sustainability assessment of novel energy systems that are destined to shape the future energy system. These include battery-based and plug-in hybrid electric vehicles, geothermal energy, hydropower, biomass gasification, national electricity systems, and waste incineration. The identification and analysis of trends and singularities that result from these investigations will be invaluable to product designers, engineers, and policy makers. Furthermore, these exercises also contribute to refining the life cycle framework and harmonizing the methodological decisions that are specifically applicable to energy systems. We shall finish by sharing our hopes and desires that this analysis will promote the use of science and knowledge to shape a better world for everyone.

**Guillermo San Miguel, Sergio Alvarez**

*Editors*
