Modeling and Energy Analysis of CO2 Reuse in a Power-to-Gas Polygeneration system Based on SOEC

Nowadays the effects of CO2 emissions are well known to human being. The world is in front of one of the biggest challenges ever faced. In 2020 CO2 concentration in atmosphere overcome 410 ppm causing an increase of 1°C of earth mean temperature. Despite the effort of many developing countries in reducing emissions, CO2 emissions reached in 2019 33.1 Gt, the historical highest value, due to the large reliance to fossil fuels of China, India and other South America countries. In these last decades any developing countries are trying to do better, especially Europe, with its ambitious plan to reach CO2 neutrality in 2050. This target will require a great effort in energy efficiency, renewable energy and grid infrastructures. In fact, the continuous increase of power from non-programmable renewable energy sources installed, will bring problems related to grid management and flexibility. A possible solution to grid flexibility problems and overgeneration caused by renewable energy sources is electric energy storage (EES). Today almost all the energy storage is based on hydro-pumped storage, with the drawbacks of site impact and low energy densities. This thesis focus on a Power-to-Gas system based on Solid Oxide Electrolytic Cells (SOECs) for storage of Renewable energy and CO2 recovery. Solid Oxide Electrolyzers (SOE), still on a demonstration scale, are able to perform, unlike Proton membrane and Alkaline Electrolyzers both water and CO2 electrolysis, producing syngas. The syngas is then used in a methanation section in order to produce synthetic methane to inject to the grid. In this way an interconnection between electricity grid and natural gas grid is created, mitigating overgeneration, improving demand response and increasing electric grid flexibility. In this work Power-to-Gas system was integrated to DEMOSOFC project, a wastewater treatment plant combined with a Solid Oxide Fuel Cell (SOFC). The CO2 source for SOEC is obtained through upgrade of biogas produced from anaerobic digestion of DEMOSOFC’s sludge, avoiding its combustion and flaring. After SOEC sizing and system modeling, a general steady state analysis was performed followed by a thermal integration analysis. Then, using as input biogas production of DEMOSFC during 2015, a year time dependent analysis has been done for two cases: PV-driven and CO2-driven. PV driven-case foresees that SOEC absorbs mostly power from PV (and a part form the grid if PV power available goes below 20% SOEC nominal power) while for CO2-driven case when PV available power went below SOEC nominal operation, remaining energy is taken from the grid shifting the priority to a higher CO2 reutilization instead of renewable energy storage. Results showed good system efficiency and integration with DEMOSOFC thanks also to the use of storage of CO2 and syngas (in order to match the operation of the different sections of the system). Almost the whole CO2 utilization for CO2-driven case and not negligible reduction in CO2 emissions with the PV-driven case were obtained.