Investigation on the heat loss characteristic of underground household biogas digester using dynamic simulations and experiments

Underground household biogas digesters (UHBD) may achieve higher gas production in cold environments by adopting heating and insulation technologies. Fully understanding the heat loss characteristic of UHBD is the prerequisite to put these technologies into application. Here, a 3-D unsteady heat transfer model coupled with the digester and surrounding soil was developed to more accurately calculate the dynamic heat loss of UHBD. The solar-air temperature (SAT) was taken as the ground surface boundary condition parameter to describe the comprehensive influences of air convection, solar radiation, and long-wave radiation. Field experiments provided the initial and boundary conditions of the model. Furthermore, the changes in the digester heat loss process and the heat response process of the soil temperature under experimental condition were measured to create a more accurate model. The comparison between the numerical and experimental results showed that the errors were minimal when adopting SAT as the boundary condition parameter, in which the maximum error of average heat flux was 5.9%. Therefore, using the SAT boundary model, heat dissipation was calculated under different fermentation temperatures, burial depths, materials, and tectonic forms. The optimal fermentation temperature was 35 °C for maximum net energy production. Heat loss exponentially decreased with increase in burial depth. Insulation could significantly reduce the heat loss of a digester. The heat loss intensity could be decreased by 67.1% when the insulation was increased to 30 mm.