Ideal EROI (energy return on investment) deepens the understanding of energy systems

• The article demonstrates a new EROI metric, EROI Ideal.
• It further explains how to calculate such EROI.
• Two case studies are shown, where hydro and geothermal power plants are studied.
• A theoretical example of wind farm is shown.
• Geothermal energy production was shown to have the greatest room for improvement of the energy sources studied.

This article presents a new EROI (energy return on investment) factor named the ideal EROI, or EROIide, that provides the theoretical upper boundary of the EROI of a given system. The EROIide is the ratio between the inputs within the EROIstnd boundaries and the theoretical maximum output from a given system; i.e, all losses are omitted and the maximum theoretical energy output of the system is used. EROIide resembles the concept of the idealised Carnot heat engine. Although the EROI of a given system can never be equal to EROIide, the EROIide can be used to estimate the potential for improvement; e.g., if the difference between the EROIide and EROIstnd is small then little improvement can be expected from further research funding. Calculations using EROIide can add a valuable depth to the overall results of an EROI study, benefitting policy makers involved in energy policy planning. Three application examples of EROIide are provided, one for a hydroelectric power plant, one for a geothermal power plant and one example for a hypothetical wind farm. The calculations show that the hydro power plant could improve approximately 3 fold, whereas the geothermal plant had a potential of 27 fold improvement over its lifetime.