Extended exergy-based sustainability accounting of a household biogas project in rural China

Biogas has been earmarked as one of the leading renewable energy sources capable of mitigating environmental emissions in rural areas. Thus, developing an accounting technique is of particular importance in coping with increasing problems related to renewable agriculture and rural energy supply. In this study, extended exergy was generalised for the sustainability evaluation of biogas projects. Furthermore, a series of extended exergy-based indicators was presented as benchmarking from the perspectives of resources, economics and greenhouse gas (GHG) emissions. The sustainability of a “Three-in-One” biogas production system in southern China was thereby evaluated based on the proposed framework. The results show that economic costs concentrate in the construction phase. GHG emissions are mainly derived from bricks and cement, with proportions of 36.23% and 34.91%, respectively. The largest resource depletion occurs during the consumption of feedstock (87.06%) in the operation phase. Compared with other renewable energy conversion systems, the biogas project has a higher renewability (0.925) and economic return on investment ratio (6.82) and a lower GHG emission intensity (0.012). With the merit of bridging thermodynamics and externality, the extended exergy-based approach presented in this study may effectively appraise the energy and environmental performance of biogas projects.