Net energy and EROI (Energy-Return-On-Investment). A biophysical approach to energy transition

In order to highlight the importance of the concept of net energy in the evolution of biological and human systems, the author first comments the relationships between net energy and biological evolution, and between net energy and societal complexity. In the next part, the author presents the E-ROI (energy return on investment). He explains how this concept is derived from net energy, and indicates its calculation formulas in different cases (primary energy, final energy). He outlines the various information obtained through the EROI analysis. In the third part, EROI values are discussed for different energy systems and sources (coal, oil, gas, nuclear, hydroelectricity, geothermal, wave, solar, biomass) as they have been published in different studies. Time series of EROIs are also discussed. The author then discusses possible consequences of energy transition in terms of net energy: increasing difficulties to extract resources, possibility of a 'net energy cliff', higher energy prices, and interconnection of energy and metal extraction sectors. Perspectives for research are discussed in conclusion.