The Cold Bound on Household Biogas: Temperature, Heating Cost, and the Climates a Small Digester Cannot Serve Year-Round

Methane production slows as a digester cools, and heating a small digester with its own gas consumes much of what it makes. This simulated study, not a measurement, takes a household unit producing 2.0 m3 of biogas per day at 35 degrees C, a cooking minimum of 1.6 m3 per day, and a temperature factor with Q10 equal to 2.0. Unheated, the unit meets cooking demand only when digester temperature stays at or above 31.78 degrees C; at 20 degrees C it produces 0.707 m3 per day and at 10 degrees C only 0.354 m3 per day. Heating the unit to 35 degrees C from an ambient temperature of 20 degrees C costs 1.53 m3 of gas per day uninsulated, leaving a net 0.47 m3 per day, and 0.682 m3 per day insulated, leaving 1.318 m3 per day, still below cooking demand. Even with insulation at the stated level, heating holds cooking output only down to an ambient temperature of 26.2 degrees C. Climates with a cold season below these thresholds cannot be served year-round for cooking by this household unit without a different heat source, substantially better insulation, or a non-biogas cooking supplement.