Design, modelling, evaluation and comparison of energy systems for the production and use of renewable methanol using recycled CO2

In this PhD thesis the analysis of complex energy systems to produce high-grade fuels such as methanol exploiting renewable energy sources (RES) for electrolytic hydrogen production and captured CO2 was carried out in the context of a decarbonisation of the future society. The produced fuels might be used in to generate electricity, for transportation and heat or even as a chemical feedstock. Specifically, in this work the energy storage application is considered. Hydrogen is produced using RES and/or excess electric energy from the grid and converted to methanol by a methanol synthesis process based on CO2 hydrogenation. The product is stored at ambient pressure. Subsequently, methanol is used in a fuel cell to produce electricity when required. The energy system conceived to perform such a process is analysed from the point of view of mass and energy balances of the components and sections constituting the system. Different layouts to perform a comprehensive analysis of various solutions are studied. Simulations of the single components using electrochemical and mathematical models are carried out mainly using the software Aspen Plus V8 and Matlab. The general system layout consists of water electrolysis, CO2 hydrogenation to methanol, methanol storage, methanol utilisation in a fuel cell, and a heat integration section to store the heat produced in the fuel cell. Water electrolysis via high temperature solid oxide electrolysis cells (SOEC) and alkaline electrolysis are considered, while the methanol synthesis section (MSS) is based on catalytic CO2 hydrogenation and the fuel cell section is fixed as a high temperature solid oxide fuel cell (SOFC). SOEC and SOFC can be also considered as two operational modes of the same apparatus, namely reversible solid oxide cells (RSOC). Heat integration is performed using a TES system and the opportunity of introducing an organic Rankine cycle (ORC) between the SOFC and the TES is also considered. The considered reference layout is composed of a SOEC to produce hydrogen that is sent to the MSS where CO2 hydrogenation and methanol purification take place. Subsequently, when required, methanol is sent from the storage facility to the SOFC to generate electricity and thermal energy as a by-product. Thermal energy produced during SOFC operation contained in the post-combustion exhaust gases is stored using a TES system in a latent heat packed bed of phase change materials (PCM). This energy is supplied back to the SOEC to vaporise water and optimise the energy requirements. Variations on the reference layout allow getting a comprehensive view of different approaches and integrations. The SOFC system is the same in each solution, just like the MSS that is based on catalytic CO2 hydrogenation and is not varied from one layout to the other. The SOEC is compared with commercially and industrially affirmed alkaline electrolyser. Given the difference between the temperature of the heat supplied by the SOFC and that of the heat required by the users, the application of an ORC is considered during SOFC operation. The performance indexes defined in this work allowed the comparison between the layouts. Each main subsection was characterised by an efficiency consistent with literature data for similar systems. Depending on the chosen configuration, the optimal efficiency of the overall plant is found to be between 34 and 35% in case of commercially mature technology (AEL) and innovative technology (SOEC), respectively, while the power-to-liquids efficiency is between 57 and 71%. These values are consistent with both literature data regarding similar power-to-X technologies and with other energy storage technologies. Since the two main layouts are characterised by similar efficiencies, the one based on commercially ready technology (AEL) might be considered in a short-term perspective, while the one based on innovative technology (RSOC) might be considered in a long-term perspective.