Analysis of pressurized operation of 10 layer solid oxide electrolysis stacks

High temperature steam electrolysis using solid oxide electrolysis cell (SOEC) technology can provide hydrogen as fuel for transport or as base chemical for chemical or pharmaceutical industry. SOECs offer a great potential for high efficiencies due to low overpotentials and the possibility for waste heat use for water evaporation. For many industrial applications hydrogen has to be pressurized before being used or stored. Pressurized operation of SOECs can provide benefits on both cell and system level, due to enhanced electrode kinetics and downstream process requirements. Experimental results of water electrolysis in a pressurized SOEC stack consisting of 10 electrolyte supported cells are presented in this paper. The pressure ranges from 1.4 to 8 bar. Steady-state and dynamically recorded U(i)-curves as well as electrochemical impedance spectroscopy (EIS) were carried out to evaluate the performance of the stack under pressurized conditions. Furthermore a long-term test over 1000 h at 1.4 bar was performed to evaluate the degradation in exothermic steam electrolysis mode. It was observed that the open circuit voltage increases with higher pressure due to wellknown thermodynamic relations. No increase of the limiting current density was observed with elevated pressure for the ESC-stacks (electrolyte supported cell) that were investigated in this study. The overall and the activation impedance were found to decrease slightly with higher pressure. Within the impedance studies, the ohmic resistance was found to be the most dominant part of the entire cell resistance of the studied electrolyte supported cells of the stack. A constant current degradation test over 1000 h at 1.4 bar with a second stack showed a voltage degradation rate of 0.56%/kh

e-XPlore: A High-Pressure Solid Oxide Cell Electrolyser in a Sea Container for Offshore Power-to-X Applications

Green hydrogen and synthesis gases are one of the main energy carriers in our attempts to combat global warming and to fulfill the transition of our fossil fuel-based society and industrial activities to carbon neutral alternatives. One of the promising technologies for the production of these gases is the high temperature electrolysis with solid oxide cells (SOCs). At the German Aerospace Center’s (DLR) Institute of Engineering Thermodynamics we experimentally investigate how SOC stacks1 and modules2 (up to 120 kW input) and use transient system simulations3 to develop operation strategies. Particularly when syngas is needed by downstream processes at elevated pressures, it can be advantageous to pressurize the electrolysis as well. Therefore, DLR is building up a transportable test environment called e-XPlore4 for an experimental analysis of a pressured electrolysis system. It comprises a system built in a 40 foot-sea container with an SOC module in a pressure vessel and includes almost all the required auxiliary components for nearly self-sufficient operation, such as cooling water, air supply, climate system, gas heaters, controls and safety system. The system only requires tap water, renewable electricity and some gases like hydrogen and nitrogen for heat-up or emergency cases. Steam electrolysis and co-electrolysis can be performed for hydrogen and syngas production with pressures up to 25 bar, an operating temperature of ca. 900 °C, and with a maximum electrical power input of 10 kW. This system supplies the synthesis gas for downstream processes, such as Fischer-Tropsch-synthesis to produce synthetic fuels within a Power-to-X context. This presentation will showcase the latest updates of planning and engineering of this system, as well as the relevant technical challenges. Operation strategies for different operating points will also be discussed. Furthermore, the off-shore application near a wind farm in the German North Sea as part of the H2Mare5 project will be presented

Operation of a Solid Oxide Fuel Cell Reactor with Multiple Stacks in a Pressured System with Fuel Gas Recirculation

Large-scale modular solid oxide fuel cell (SOFC) reactors composed of multiple stacks are regarded as an efficient form of power generation and important for the global energy transition. However, such an arrangement leads to several operational challenges, and methods are required for handling such challenges and understanding their sources. Hence, a test rig for the examination of a 30 kW SOFC reactor with multiple stacks, for operation near real system conditions, is built. The test rig, which allows operation at elevated pressure, is equipped with a high-temperature blower that recirculates the fuel gas at SOFC reactor temperature. In a measurement campaign, fuel gas, reactant conversion, and pressure are varied in stationary and transient experiments. The experimental results showed that the operating conditions of the individual stacks of large SOFC reactors vary largely due to flow distribution and heat losses. Methods for the investigation of these critical characteristic parameters are derived from the experimental results. Furthermore, the impact of pressurization and fuel gas recirculation on the SOFC reactor is analyzed. This publication shows the need to understand the behavior of large SOFC reactors with multiple stacks to increase the performance and robustness of complete process systems

TRANSIENT OPERATING STRATEGIES FOR SOLAR HEAT SUPPORTED SOLID OXIDE ELECTROLYSIS SYSTEMS FOR HYDROGEN PRODUCTION

Considering the transformation of the energy system, there are two main challenges. First, efficient and cost competitive long-term energy storage on a large scale. Second, making renewable energy accessible for hard-to-electrify sectors like transport and heavy industry. Converting renewable electricity into green hydrogen in Solid Oxide Electrolysis Cells (SOEC) is considered a viable solution for both challenges. Besides the high efficiency, SOEC offer the possibility to supply part of the energy demand by industrial waste heat or by renewable sources, such as solar thermal energy. The SOEC technology itself is mature but the integration within large systems and coupling with up- and downstream processes still requires research to be done on the SOEC's transient behaviour as well as identifying safe and efficient operating strategies. In this contribution, a solar-SOEC coupled system concept is introduced and analysed for its capability to cope with typical fluctuations in solar irradiance. The main focus is laid on the transient behaviour of the SOEC reactor during variation of different operating parameters, namely current, feed gas temperature and reactant conversion. Results show that the current variation has the strongest effect on the stacks’ temperature, yielding relevant temperature gradients, especially in endothermic operation. Whereas by increasing the reactant conversion during endothermic operation, it was possible to reduce thermal stress in the stacks, while increasing the hydrogen output as well as the system's efficiency. It is presented how these effects can be combined and utilized for the development of control and operating strategies that aim at improving system performance. This will be exemplary illustrated for improving system performance during a period of overcast and thus reduced heat supply

Characterisation of a 10-Layer SOC Stack Under Pressurised CO2 Electrolysis Operation

One promising way of facing recent challenges to slow down the climate crisis or to reduce dependencies on fossil energy sources, e.g. natural gas, is using renewable methane and other e-fuels for storage and distribution via existing infrastructure. Solid oxide cell (SOC) reactors play an important role in the conversion of sustainable electric power into chemicals as they can be obtained from combined steam and CO2 co-electrolysis for syngas production. The pressurised electrolysis operation is a key factor for increasing the system efficiency of PtX-processes, including balance-of-plant (BoP) components, electrochemical reactors and high pressure downstream processes. In general, the yield of CO2 electrochemical reduction at atmospheric and pressurised conditions in high temperature co-electrolysis is still controversially discussed. Previously, several SOC short stacks were thoroughly analysed in pressurised steam- and co-electrolysis operation in a test-rig environment. These experimental results indicate marginal influence of pressure on the performance of electrolyte supported cells (ESC). In contrast, electrochemical impedance spectroscopy (EIS) suggests that pressurisation of pure CO2 electrolysis significantly reduces the fuel electrode impedance contribution, especially at lower temperatures around 700 °C [1,2]. This work aims to experimentally determine the kinetic behaviour of pure CO2 electrolysis by varying operating conditions like pressure, temperature, reactant conversion and feed gas composition. The investigation of kinetic parameters during these experiments could complement the formerly described research. Furthermore, the kinetic expressions can be used when studying co-electrolysis operation to identify the shares of: (i) the reverse water-gas-shift (rWGS) and (ii) the CO2 electrochemical reduction. Polarisation curves were dynamically recorded and different current densities were evaluated in steady-state operation. Additionally, EIS measurements were performed at open circuit voltage (OCV), as well as under different current densities. The kinetic parameters were estimated by curve-fitting analysis of the experimental results. The resulting expressions will be implemented in the in-house modelling framework, TEMPEST, based on [3,4] with the aim to increase the accuracy of modelling high-temperature CO2 electrolysis and co-electrolysis systems. [1] Riedel, M., Heddrich, M. P., & Friedrich, K. A. (2020). Experimental Analysis of the Co-Electrolysis Operation under Pressurized Conditions with a 10 Layer SOC Stack. Journal of The Electrochemical Society, 167(2), 024504, DOI: 10.1149/1945-7111/ab6820. [2] Riedel, M. (2020, October 20–23). Experimental analysis of SOE stacks under pressurized co- and CO2 electrolysis operation [Paper presentation]. 14th European SOFC & SOE Forum, Lucerne, Switzerland. [3] Tomberg, M., Santhanam, S., Heddrich, M. P., Ansar, A., & Friedrich, K. A. (2019). Transient Modelling of Solid Oxide Cell Modules and 50 kW Experimental Validation. ECS Transactions, 91(1), 2089, DOI: 10.1149/09101.2089ecst. [4] Srikanth, S., Heddrich, M. P., Gupta, S., & Friedrich, K. A. (2018). Transient reversiblesolid oxide cell reactor operation–Experimentally validated modeling and analysis. Applied Energy, 232, 473-488, DOI: 10.1016/j.apenergy.2018.09.186

A New Approach to Modeling Solid Oxide Cell Reactors with Multiple Stacks for Process System Simulation

Reactors with solid oxide cells (SOC) are highly efficient electrochemical energy converters, which can be used for electricity generation and production of chemical feedstocks. The technology is in an upscaling phase. Thereby demanding development of strategies for robust and efficient operation or large SOC reactors and plants. The present state of technology requires reactors with multiple stacks to achieve the appropriate power. This study aims to establish and apply a simulation framework to investigate process systems containing SOC reactors with multiple stacks. Focusing especially on the operating behavior of SOC reactors under transient conditions, by observing the performance of all cells in the reactor. For this purpose, a simulation model of the entire SOC reactor consisting of multiple stacks, pipes, manifolds, and thermal insulation was developed. After validation on stack and reactor level, the model was used to investigate the fundamental behavior of the SOC reactors and the individual stacks in various operation modes. Additionally, the influences of local degradation and reactor scaling on the performance were examined. The results show that detailed investigation of the reactors is necessary to ensure operability and to increase efficiency and robustness. Furthermore, the computing performance is sufficient to develop and validate system controls