Methane production and energy evaluation of a farm scaled biogas plant in cold climate area

•A biogas plant in a cold climate area was monitored.•Total energy demand at the plant varied between 26.9% and 88.2% of the energy produced.•Dairy cow slurry gave a total energy production of 79 kWh/m3 slurry.•Fish silage gave a total energy production of 1623 kWh/m3 substrate.•During summer most thermal energy was consumed for heating of new substrates.

The aim of this study was to investigate the specific methane production and the energy balance at a small farm scaled mesophilic biogas plant in a cold climate area. The main substrate was dairy cow slurry. Fish silage was used as co-substrate for two of the three test periods. Energy production, substrate volumes and thermal and electric energy consumption was monitored.Methane production depended mainly on type and amount of substrates, while energy consumption depended mainly on the ambient temperature. During summer the main thermal energy consumption was caused by heating of new substrates, while covering for thermal energy losses from digester and pipes required most thermal energy during winter. Fish silage gave a total energy production of 1623 kWh/m3, while the dairy cow slurry produced 79 kWh/m3 slurry. Total energy demand at the plant varied between 26.9% and 88.2% of the energy produced.