Impact of ambient temperature on the effectiveness of inlet air cooling in a co-digestion biogas plant equipped with a mGT

Biogas plants are an interesting solution for production of clean energy. Biogas produced in an anaerobic digester can be used locally to produce electric energy in an internal combustion engine or in a micro-gas turbine. A portion of the waste heat is used to keep the digester at a constant temperature which is a necessary condition for a correct operation of the digester. Generally, waste heat overcomes that necessary to keep the digester at the desired temperature and a big portion of this thermal energy is dissipated. Many solutions exist to increase the amount of heat recovered. In this study, the use of an absorption chiller to decrease the inlet temperature of a micro-gas turbine operating with biogas was considered. The advantage of this solution relies in a more stable operation of the micro gas turbine and an increase in the power output. The integration of the absorber chiller in the biogas plant and the effect of the ambient conditions were investigated in detail. The study was based on an existing plant operating near Pisa (Italy). A model of the system has been developed in AMESim and a numerical simulation has been performed. The effects of different temperature profiles, corresponding to different climate conditions, have been investigated both from the energy and economic points of view. The results showed that local climate conditions strongly influence the effectiveness and the profitability of the inlet turbine air cooling technique. In those climates where the temperatures are constantly high over the year, this technique may lead to interesting benefits and profitability.