Design, development and testing of SOEC-based Power-to-Gas systems for conversion and storage of RES into synthetic methane

International and national initiatives are promoting the worldwide transition of energy systems towards power production mixes increasingly based on Renewable Energy Sources (RES). The integration of large shares of RES into the actual electricity infrastructure is representing a challenge for the power grids due to the fluctuating characteristics of RES. The adoption of long-term, large-scale Electric Energy Storage (EES) is envisaged as the key-option for promoting the integration of RES in the electricity sector by overcoming the issue of temporal and spatial decoupling of electricity supply and demand. Among the several EES options, one of the most promising is the conversion of energy from the electrical into the chemical form through the synthesis of H2 and synthetic natural gas (SNG) in Power-to-Gas (P2G) systems based on the electrolysis of water (and also CO2) in Solid Oxide Cells (SOCs). The application of SOC technology in P2G solutions shows attractiveness for the high efficiency of high-temperature electrolysis and the flexibility of SOCs that can operate reversibly as electrolyzers or fuel cells (rSOC) and can directly perform the electrochemical conversion of CO2 and H2O to syngas by co-electrolysis. The capability of reversible operation also allows the application of SOC-based systems to Power-to-Power (P2P) concepts designed for deferred electricity production. This dissertation is focused on the investigation of electricity storage using Power-to-Gas/Power systems based on SOCs. The aim of this Thesis has been the investigation of the thermo-electrochemical behavior of SOCs integrated P2G/P2P systems, with the purpose to identify the system configuration and the operating conditions that ensure the most efficient electricity-to-SNG (P2G) or electricity-to-electricity (P2P) conversion within the thermal limits imposed by state-of-the art SOC materials. To this purpose, a detailed thermo-electrochemical model of an SOC has been developed at cell level, validated on experimental data, extended at stack level and coupled with models of the main P2G/P2P components for the system analysis. Model validation was performed through the characterization of planar commercial SOCs in the reversible operation as electrolyzers (SOEC) and fuel cells (SOFC) with H2/H2O and CO/CO2 fuel mixtures at different reactant fractions and temperatures. The physical consistency of electrode kinetic parameters evaluated from the model was verified with the support of literature studies. The investigation of SOC-based P2P and P2G solutions was performed using the models developed. Three different configurations were analyzed and simulated: 1) hydrogen-based P2P with rSOC, 2) SOEC-based electricity storage into hydrogen with subsequent SNG production by methanation with CO2 and 3) electricity storage by co-electrolysis of water and carbon dioxide with SOEC for syngas production and subsequent upgrading to SNG by methanation. The performance of the P2P system was thoroughly assessed by analyzing the effects of rSOC stack operating parameters (inlet gas temperature, oxidant-to-fuel ratio, oxidant recirculation rate, cell current) and system configurations (pressurized/ambient rSOC operation, air/oxygen as oxidant/sweep fluid) on stack and system efficiency. The analysis allowed to identify the most efficient configuration of the P2P system, and to select the feasible operating currents (i.e., the currents included within the limits given by the physical thermal constraints of SOC materials) for which the highest roundtrip efficiency is achieved. Pressurized rSOC operation (10 bar) with pure oxygen as oxidant/sweep gas and full recirculation of the oxidant flow ensured the highest charging and discharging effectiveness, with a system roundtrip efficiency of 72% when the stack is operating at the maximum efficiency currents (-1.3 A/cm2 in SOEC and 0.3 A/cm2 in SOFC). A dynamic analysis was performed on the rSOC to determine the characteristic times of the thermal response of an SRU coupled with variable loads. The analysis showed that the SOEC is intrinsically more suitable to work with variable loads thanks to the balance between reaction endothermicity and losses exothermicity that reduces the magnitude and the rate of temperature fluctuations originated by current variations. A case study was presented to show the application of P2P with fluctuating RES. In the case study, the sizing of an rSOC-based P2P system designed for the minimization of the imbalance (i.e., the difference between effective and forecasted electricity production) of a 1 MW grid-connected wind farm was performed. An optimal number of cells was found, for which the imbalance is reduced by 77 %. The estimated roundtrip efficiency of the optimal-size P2P system coupled with the wind farm was 54 %. The P2G systems analyzed are composed by three main sections: a hydrogen/syngas production and storage section based on an SOEC stack; a methanation section based on chemical reactors; and an SNG conditioning section for the upgrading of the produced SNG to grid-injection quality. The design and operating conditions of the SOEC section were selected following the results of the analysis performed on the P2P system, and the SNG production section was designed on the basis of a commercial methanation process based on catalytic reactors. The plant efficiency evaluated by simulations was 65.4% for the H2-based P2G and 65.5% for the co-electrolysis based P2G without considering cogeneration or thermal integration between plant sections. Even if the efficiencies were similar for the two P2G configurations, the storage capacity of the H2-based P2G plant was higher, because of the higher operating current achieved by the SOEC stack. The results suggested that even if the co-electrolysis based P2G system presents a slightly higher efficiency, the choice of a H2-based P2G option can ensure a better exploitation of the installed capacity, and also eliminates the risks of carbon-deposition in the stack related to the use of carbon containing mixtures and of stack poisoning related to contaminants potentially present in CO2 streams (e.g., hydrogen sulphide). A case study assessing the effect of H2S poisoning of the SOEC stack on the P2G system performance was also presented. The results presented in this Thesis demonstrated that hydrogen-based P2P with rSOCs is the most efficient solution for local RES storage among the different SOC-based EES options investigated. The high values of roundtrip efficiency achieved demonstrated the competitiveness of rSOC-based P2P also with other large-scale EES options (PHS, CAES). The hydrogen-based P2P is however constrained to on-site applications due to the lack of a hydrogen transport infrastructure, while P2G solutions offer the possibility of transferring the electricity stored in the SNG form through the existing natural gas infrastructure, and also allow the direct use of SNG in already existing technologies (i.e., for mobility, heating, etc.), providing the technological bridge for transferring RES power to other markets different from the electrical one.