Integration of solar passive heating strategies in low-cost biogas plants

This study analyzes the influence of various solar passive heating designs on the thermal performance of low-cost tubular digesters in cold climate regions through experimental and simulation tools. Five full-scale low-cost tubular digesters were monitored in the Andean region: one with both greenhouse and trench insulation, two with trench insulation only (thicknesses of 1 cm and 5 cm), one with a greenhouse only, and control (without any solar passive heating design). The most effective solar passive strategy was found to be the combination of greenhouse and insulation, achieving a slurry temperature increase of +7.4 °C above the ambient temperature. The trench insulation enhanced the slurry temperature, with temperature increases ranging from +4.3 °C to +6 °C, depending on the insulation thickness. Conversely, greenhouses alone showed minimal thermal benefit (+0.8 °C). A unique simplified dynamic thermal model was developed to simulate the thermal performance of these digesters, achieving a root mean square error below 1 °C across all configurations. These findings demonstrate that solar passive heating designs can substantially improve the efficiency of anaerobic digestion in cold climates, promoting sustainable biogas production.