Overview of the EROI, a tool to measure energy availability through the energy transition

The importance of energy consumption to allow societies to thrive is well established and prospects of energy needs is well derived through the scientific literature. Yet, lesser discussions persist on the future availability of energy for current industrial economies, a crucial indicator for development. It is defined as net-energy analysis, where one must appropriate more energy than required to get it. The most common indicators are the energy payback time and the EROI (Energy Return of Investment). These indicators are used throughout literature either for energy vectors, energy systems or for broader societal applications. Following growing concerns about climate change, and with the increasing difficulty of extraction of fossil fuels, EROIs became tools to study the global energy transition with a focus on a possible minimum EROI required to maintain a complex society. However, the indicator is used with a large variety of methods, definitions, and boundaries. This led to a lack of consensus on whether a transition to renewable-based energy systems could still provide sufficient net energy for societies to thrive. The concepts of EROI were studied by compiling its various definitions, boundaries, and limits, allowing a clear view of the indicator to understand where and how it could be used. This led to finding three main classes of indicators: the physical EROI, an indicator based on energy consumption, a price-based societal EROI, an indicator using monetary expenditures to look at energy-related expenditures, and finally a socioeconomic EROI which looks at energy expenditures within a nation's economy. A detailed review of those use cases led to understanding that the EROI is often badly calculated through wrong boundaries, goals, or with old data and that no norm exists for its calculation. These inconsistencies tend to negatively bias renewable technologies as a solution to the energy transition. Furthermore, most calculations of minimal EROIs are based on fossil fuel infrastructure, with current energy systems being highly inefficient. The previously calculated minimal EROIs through literature, penalizing renewable technologies, are challenged. The study discusses the possibility of transitioning away from fossil fuels' dependence based on updated data and literature to finally conclude that renewable can offer sufficient energy through the energy transition. This sufficiency however comes with short-term limits followed by a possible drop in net-energy due to the transitory nature of the global shift to mitigate climate change.