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After long and successful development history of Solid Oxide Cells (SOC) continuous improvement in performance, longevity, manufacturing, and system integration it is necessary to bring this highly efficient technology to the market. The activities on material development and optimization at IKTS are focused mainly on enhancement of durability for SOFC, SOEC, and rSOC operation and on boosting the power density. During recent years considerable efforts on simplification and automation of cell and stack manufacturing processes have been addressed. The processes for electrode manufacturing have been adjusted for high yield automated printing on thin electrolytes with integrated quality control measures. Efficient ways for reduction of time and energy consumption for sealing process of SOC stacks have been found and demonstrated in pilot production as well as automated assembling of components to stacks was shown. The increasing interest in “green hydrogen” created multiple opportunities for SOEC technology to be considered as inherent part of industrial and chemical processes. IKTS pioneering work on coupled operation of SOEC module with Fischer-Tropsch reactor provided first demonstration of feasibility of this approach for wax production. However, high power electrolysis applications (>10 MW) will need new approaches for stack design and put higher requirements on durability.

Renewable energy sources based on solar and wind energy provide clean and efficient energy. The intermittent behaviour of these sources is challenging. At the same time, the needs for efficient, continuous and clean energy sources are increased for serving both electricity and thermal demands for residential buildings. Consequently, complimentary systems are essential in order to ensure a continuous power generation. One of the promising energy sources that helps in reducing CO2 emissions, in addition to providing electrical and thermal energy efficiently, is a Solid Oxide Fuel Cell (SOFC) system operated in a combined heat and power (CHP) mode, due to high electrical efficiencies (in full and part load) and the fuel flexibility. Currently, most studies tend to focus on fuel cell model details with basic information about the building’s energy requirements. Nevertheless, a deep understanding of integrating fuel cell micro-CHP systems with renewable energy systems for the residential sector is required. Moreover, it is important to define an operating strategy for the system with a specific controlling method. This helps in evaluating the performance and the efficiency of the building energy system. In this study, an investigation of different configurations of a hybrid power system (HPS) was carried out. The intended aim of this investigation was to optimize a HPS with minimal CO2 emissions, serving the energy demands for a single-family house efficiently and continuously. As a result of this study, a photovoltaic (PV)/SOFC micro-CHP system has satisfied the intended goal, where the CO2 emissions are significantly reduced by 88.6% compared to conventional systems. The SOFC micro-CHP plant operated as a complimentary back-up generator that serves the energy demands during the absence of the solar energy. Integrating the Power to Gas (PtG) technology leads to a similar emission reduction, while the PtG plant provided a seasonal energy storage. The excess energy produced during summer by the PV system is stored in the fuel storage for a later use (during winter). This SOFC micro-CHP configuration is recommended from an energy and environmental perspective. In terms of feasibility, the costs of SOFC based micro-CHP systems are significantly higher than traditional technologies. However, further technology developments and the effect of economy of scale may cause a substantial drop in costs and the micro-CHP shall become economically competitive and available for residential users; thus, enabling a self-sufficient and efficient energy production on site.

A pulsed discharge regime (GITT) was used to investigate the ohmic internal resistance of Na/NiCl2 cells, also known as ZEBRA Cells. Two methods were chosen to determine the internal resistance. On the one side the voltage response of the corresponding current enables a calculation. On the other side the potentiostatic electrochemical impedance spectroscopy was used to measure cell spectra including the ohmic internal resistance. The cells were investigated within five different operating temperatures (300°C – 180°C with 30 K steps). Results show that the ohmic internal resistance of a Na/NiCl2 is increasing when the operating temperature is decreasing, mainly due to the decrease of ionic conductivity of β^'' separator and the secondary electrolyte NaAlCl4. As a direct consequence, there is a significant capacity loss. Therefore, the operating temperature can be identified as capacity limiting factor.

Solid Oxide Fuel Cell (SOFC) technology is of high interest for stationary decentralized generation of electricity and heat in combined heat and power systems (CHP) for the residential sector. Application scenarios for SOFC systems in an electricity-regulated mode play an important role, especially in places where an electrical grid connection is not available or rather unstable. The advantages of SOFC systems are the high fuel flexibility and the high efficiencies also under partial load operation compared to other decentralized power generation technologies. Due to the long, energy-consuming system heat-up and the limited partial load capability, SOFC systems do not reach the performance of conventional power generation technologies. Furthermore, stack thermal cycling is associated with power degradation and should be minimized. In this paper, the improvement of these drawbacks are investigated for hotbox-based SOFC systems in the 1 kWel-class for residential applications. Since experimental investigations of the high-temperature systems are limited, modeling tools are established, enabling the visualization of internal system characteristics and providing the opportunity to simulate system operation in critical regions. To achieve this, a methodology for dynamic SOFC system modeling in a process engineering manner is developed based on the modeling language Modelica. A suitable approach is particularly important for modeling and simulation of the strong thermal interaction between the hot system components within the hotbox. The parametrized and validated models are used for the investigation of different dynamic effects, such as the system heat-up and the operation in low partial load points. A second reduced thermal system model aims for annual simulations of the SOFC system together with a battery to investigate the number of thermal cycles and the advantage of a hot standby operation. As a result, it is found that an adequate control of the power input at the start-up device and the cathode air flow has a high improvement potential to increase the stack heating rate and accelerate the heat-up in an energy-saving way. The hotbox-internal thermal management is identified as a crucial issue to reach low partial load points. To avoid the risk of stack cooling, lower heat losses and/or additional heat sources are of importance. Furthermore, the robustness of the tail gas oxidizer is found to be crucial for a higher load flexibility during partial load and the end of life stack operation. The annual simulation results indicate that operating the battery hybrid system with a hot standby mode requires much lower battery capacity for a high grid independence and a complete avoidance of system shutdown and associated power degradation.