Solutions for improving the energy efficiency in wastewater treatment plants based on solid oxide fuel cell technology

Polygeneration configurations for small power generation systems offer significant potential for energy saving and reducing carbon emissions in wastewater treatment facilities. In this work, a biogas-fed solid oxide fuel cell system operating in a wastewater treatment plant (located in Turin, Italy) is analyzed in terms of its potential improvements through novel polygeneration systems. In its present combined heat and power configuration, along with electrical power, thermal energy from the exhaust gas is recovered to provide required heat to the plant's anaerobic digester. The analysis is focusing on different energy efficiency solutions for this type of plant by using solar thermal collectors, microturbines, a trilateral Rankine cycle, and an absorption chiller. Results reveal that, despite of higher efficiency for the trigeneration case using both trilateral Rankine cycle and absorption chiller (up to 88.4%), the solar integrated system results in the lowest natural gas consumption, which is 38.5% lower than the baseline scenario. This same scenario is also the worst in economic terms due to the high capital costs of solar collectors. In a short-term cost trajectory of the solid oxide fuel cell technology, the most economically favorable scenario is the microturbine integrated case in which the calculated levelized cost of electricity is 0.11 €/kWh, lower than grid electricity price, and with payback time of 6.5 years. Long-term cost trajectory is indeed generating effective investments for all of the four scenarios with payback time between 3 and 5 years in all cases. The analysis has been developed to the entire European Union area: the most suitable market conditions are found in Germany, Denmark, Slovakia, and Italy.