摘要
The article assesses the energy R required by a system to transform a cereal or sugar plant into ethanol. From the specific consumption rj of each process j and its weight wj in the system, process consumption share Rj is deduced and hence R, sum of Rj. Depending on wj definition, Rj and R are relative to either 100 J of ethanol produced or 100 J of plant harvested. Depending on the nature of rj, Rj and R represent either only primary external energies, or all fuel and electricity consumed directly, or external and internal energies. From one definition to another R for average sugar cane based industries is the best or the worst relative to other plants. This results also from the use of cane residues as fuels while operating outdated processes. Through rj the process based analysis allows to examine for each system the impact of modern processes or different use of residues. All systems benefit except sugar beet based industry close to its best efficiency.This flexibility permits even to build a self-sufficient system where existing processes produce from system resources substitutes to external energies. R becomes an unambiguous definition of a system efficiency. It shows that all agroethanol systems are more consuming than petroleum industry. The system can be expanded to the vehicle stage to compare with alternatives to ethanol such as electricity and biogas. Wheat straw burnt to produce electricity used in an electrical vehicle will present R close to that of petroleum industry.