Sustainability assessment of bioenergy at different scales: An emergy analysis of biogas power production

The purpose of this work is to present the application of Emergy Analysis to a biogas power plant, with the aim at assessing its integrated sustainability and studying how the emergy indicators of sustainability depend on the boundary selection. The object of the analysis is a biogas power plant fed on agriculture and zootechnical biomass. The complex interaction of the involved subsystems and the exchange of resource flows require powerful integrated analyses as well as the definition of reliable performance indicators. Emergy Analysis has been addressed as specifically useful to integrate the upstream and the downstream aspects along with the potential circularity of resource flows, providing in this way a quantitative estimation of the sustainability performance of the systems. The analysis was applied at two scales: the Reference System (biogas power plant plus the agricultural cultivations) and the Expanded System, which includes also the cattle breeding, whose main purpose is meat and milk production, but which provides the liquid cattle manure for the Reference System operation. The main emergy indicators have been calculated and reported: for the Reference System, the transformity of produced electricity results similar to fossil-based energy production systems; the Emergy Yield Ratio is about 1, showing that the system cannot be considered as an energy source, rather acting like a consumer; the Environmental Loading Ratio is very high, as the local renewable fraction of the resources exploited is very low compared to the non-renewable ones. The expansion of the system analysis allows to study the added value of linking the subsystems agriculture-breeding-energy production, which provide more products and services to the economy than the sole energy. The need of performing sustainability assessment at different scales appears to be presently the main issue in the analyses that should result suitable for the policy-making processes.