Technical and economical evaluation of Power-to-Methane technologies, based on green H2 and biogenic CO2

The topic of this PhD thesis is focused on the techno-economic analysis of energy systems for the production of green fuels, such as hydrogen (H2) and biomethane (CH4), exploiting Renewable Energy Sources (RES) and biogenic CO2. In the frame of the Sardinia energy scenario, one of the biggest islands in Italy, the only one without a natural gas grid and, at the same time, a high availability of renewable resources, the present thesis offers a contribution to find a solution for the future decarbonization of the island. The contribution of the present study refers to the production, transport, distribution, and final economic analysis of green fuels to support the energy transition and can also be a model for other isolated energy systems. The analysis carried out allowed the evaluation of the effectiveness and economic feasibility of such innovative technologies, Power-to-Hydrogen and Power-to-Methane. With the focus on the Power-to-Methane system, different layouts have been designed to perform a comprehensive analysis of various solutions. Systems based on commercially mature or innovative technologies are analysed throughout the development of models using MATLAB software. Hydrogen is produced using RES and electric energy from the grid and converted to biomethane through biological methanation processes (BHM), employing the CO2 resulting from the biogas upgrading in anaerobic digestion plants. Two different solutions have been analysed: a BHM process with the injection of CO2 and H2, and a BHM process with the injection of Biogas and H2. Evaluation of the optimal location for the Power-to-Methane system was carried out to find the more profitable way of transporting the CH4 produced. Variations on the reference layout allow getting a comprehensive view of different approaches and integrations, with the common objective to find the solution with the lowest Levelized Cost of Biomethane (LCOBM) value. In addition, another interesting solution studied is the inclusion of a BHM process in a Hydrogen Valley, with a focus on the economic and environmental benefits. Depending on the chosen configuration, the minimum LCOBM was between 2.27 and 2.85 €/Nm3, in the case of polymeric electrolyser membrane technology (PEM) with 56% of energy from RES and alkaline electrolyser (AEL) with 75% of energy from RES, respectively. Finally, including such a system in the Sardinia energy scenario, can provide a contribution of about 44% to the forecast natural gas consumption in 2050.