Energy balance and economic analysis of rural distributed generation systems using biogas

Energy recovery from agricultural residues through anaerobic digestion represents a viable pathway for renewable electricity generation and rural energy self-sufficiency. This study evaluated the energy and economic performance of five full-scale distributed generation biogas plants in southern Brazil using swine and poultry manure as feedstock. Energy performance was assessed through output-input ratios (EOR), volatile solids energy factors, electrical efficiency, and renewable energy efficiency. Economic feasibility was evaluated across electricity tariff scenarios (USD 0.07-0.15 kWh^-1) and discount rates (3.5%, 8.5%, and 13%), using net present value, internal rate of return (IRR), discounted payback period, and levelized cost of energy (LCOE). The plant equipped with a continuously stirred tank reactor achieved the highest energy performance, reaching EOR₂ of 16% and net electrical efficiency of 20%. However, despite its superior technical performance, its high capital cost resulted in the lowest economic attractiveness, with LCOE values reaching USD 0.19 kWh^-1 and payback periods exceeding 19 years under high-interest scenarios. By contrast, the covered lagoon system (Plant B) demonstrated the strongest economic performance, achieving the lowest LCOE (USD 0.05 kWh^-1), an IRR of 42%, and a payback period under 3 years, despite its comparatively lower energy efficiency. The results demonstrate that greater technological sophistication does not necessarily translate into superior economic viability in rural biogas systems. Economic feasibility was primarily governed by capital investment, electricity tariff levels, and financing conditions. These findings offer evidence-based guidance for policymakers and rural energy planners seeking to scale up distributed biogas generation from agricultural residues in Brazil.