Methane production through a Solid Oxide Electrolyser with primary renewable source and carbon dioxide capture

Renewable electric energy can be transformed into storable methane via electrolysis and subsequent methanation. Power to Gas systems appear as a promising technology to exploit the surplus energy from renewable power plants; different gases can be produced (e.g. methane or hydrogen). In this Master Thesis work, two different Power to Gas systems are modelled using Matlab Simulink: a Solid Oxide Electrolyser with a CO2-methanator and a Solid Oxide Co-Electrolyser with a CO-methanator. This latter component can operate with an inlet H2/CO-ratio equal to 3 or larger. A ratio of 5 is also considered. The CO2, coming from biogas produced in the Waste Water Treatment Plant of EDAR Riu Sec situated in Sabadell (Barcelona), is captured and processed in order to obtain methane. Additionally, according to the Master Thesis’ objectives, a photovoltaic power plant and a CO2 storage are sized. An averaged power profile produced by a PV plant located in Barcelona is obtained with the PV*SOL programme. Therefore, the installation of a P2G system allows to achieve two positive effects: firstly, the CO2 is not released to the atmosphere. Secondly, the CO2 is reused to produce methane without getting it, in a more polluting way, from natural gas or coal. Nowadays not all the technologies analysed in this work are mature. Due to the complexity and partial lack of information simplified models and assumptions in operating conditions have been employed. The main goal is to evaluate what is the most efficient Power to Gas choice in terms of installed nominal power to exploit the entire amount of the carbon dioxide over one year of operation. Furthemore the photovoltaic power plant is sized assuming operativity only during the summer period; the differences with respect to yearly operation are illustrated in terms of required power and storage capacity. According to the results achieved in this work, the optimal Power to Gas system is the co-electrolyser coupled with a CO-methanator with a H2/CO-ratio equal to 3. Considering the worst case of operation, thus during winter, a PV plant of 1.2 MW has to be installed to process the entire amount of CO2 available; regarding only summer operation, a smaller plant of 0.8 MW plant is needed. The capacity of the storage, assuming only summer operation, is also reduced of almost 30% with respect to winterly operation, where 995 m3 of storage are needed. The molar fractions of the outlet gas mixture, after methanation process, are around 30% CH4, 60% H2O and 10% CO2. To obtain more precise results, improvements should be apported to the developed models, enhancing the learning of the operating conditions.