Article published in IEEE Xplore: Link
The abundance of relatively cheap fossil fuels has been a key driver for the development of our modern societies. The world needs to quickly decarbonise and go from a system largely based on the combustion of those fossil fuels to a system based on low-carbon technologies. Many studies focus on the development and deployment of those technologies.
Much less discussions focus on the future availability of energy during the energy transition, a crucial indicator for development. The most common indicators to study this are the energy payback time and the EROI (Energy Return of Investment). The basic idea is that energy is necessary to build the energy infrastructure and that, similar to the case of financial investments, the produced energy needs to be higher than the invested energy.
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 a system largely based on renewables could still provide sufficient net energy for societies to thrive.
Making sense of the different indicators
In this article, the concepts of EROI were studied by compiling the various definitions, boundaries, and limits, allowing a clear view of the indicator to understand where and how it could be used. Three main classes of indicators were reviewed: the physical EROI, an indicator based on energy consumption, either for energy systems, or resources; 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 large discrepancy exists in the reported EROI values which can be interpreted by differences in the definition of the scope of analysis, or in the considered energy vector.
Furthermore, most calculations of minimal EROIs are based on the existing fossil fuel infrastructure, which is highly inefficient. There is thus a need to understand the current transitory state of energy systems. Such example would be the electrification of supply chains to reduce the required energy investments for energy systems. Nevertheless, renewable technologies are constrained by material usage and not energy scarcity.
Enough energy?
Finally, given the updated results for fossil fuels leading to lower-than-expected EROI values as well as slower decreases, the idea of a required high EROI is challenged: transitioning away from fossil fuels could thus lower the “minimal” required EROI for societies to sustain. Utilizing previous minimal EROI values for future energy systems seems unfounded as global infrastructure transformations are underway.
The study finally concludes that renewable can offer sufficient energy through the energy transition. The previously calculated minimal EROIs through literature, penalizing renewable technologies, are challenged. 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.