The analysis of methods to increase the temperature inside the digester has shown that the required
heating decreases as digester size increases because the retention time is sufficient to enable high
production at low temperatures. However, a net energy gain may not be guaranteed if this heat is
supplied by biogas. A sustainable solution may be the use of a combination of methods to increase the
temperature inside the digester [21]. In this context, the use of biogas to heat the digester in
combination with another heating method may be a viable solution. The present study aims to provide
farmers with a simple tool to assess the feasibility of a combined heating system (solar + biogas heater)
depending on the biomass entering the digester. This decision-support model is intended to aid in the
construction of simple digesters for cold environments that produce a constant rate of biogas throughout
the year and to define the sustainability of a heated digester in relation to available biomass.