In this paper the results of a thermodynamically exact calculation method for determining SOFC (solid oxide fuel cell) system efficiencies depending on reforming method and fuel are presented. Several fuels (diesel, gasoline, liquefied petroleum gas, ethanol, methane and biogas) are studied. The aim is to identify combinations of fuels and reforming methods with the potential for high efficiencies. The influence of the S/C (steam/carbon) ratio and reformer air ratio λRef on achievable efficiency is examined, explaining the superiority of steam reforming. Only with biogas partial oxidation can achieve comparable values to steam reforming due to its specific composition containing CO2 making biogas an ideal fuel for SOFCs. Also a close look is taken at the demand of heat flux to or from the reforming step in order to reveal potential necessities of a complex heat management.
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 CO 2 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 CO 2 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 CO 2 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 CO 2 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 CO 2 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 CO 2 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.
Hybrid power plants consisting of solid oxide fuel cells (SOFC) and a gas turbine (GT) can play an essential role in the future energy scenario due to the expected high electrical efficiency, fuel flexibility and good part-load performance. A demonstration SOFC/GT hybrid power plant is being setup in Stuttgart with state of the art, commercially available electrolyte supported cell (ESC) stacks and its operation is being simulated by means of a overall system model. However, the model used in this paper, in contrast to most models in literature, accounts for heat transfer based on actual geometries and materials. In the present study, the system model is integrated with a set of sub-models that predict the heat losses of the components of the hybrid power plant with a feasible computational speed. This allows for an improved prediction of the operating range as well as for the prevention of undesired operating conditions. The results of the simulations of the stationary operation of the hybrid power plant with varying heat losses are shown and discussed. Operating limitations are analyzed as well as system performance. It is shown that it is possible to operate the hybrid power plant from design power output to 30% of it. A system electrical efficiency higher than 0.55 considering the fuel’s higher heating value is maintained throughout the entire range. Further design choices and developments could lead to an improvement of this condition. In addition, an adiabatic assumption can lead to about 4 percentage points overestimation of electrical efficiency and reduces the high power operating range by about 10%. This approach opens up a new perspective on the simulation of this type of power plant.
The low-carbon economy of the future needs low consumption of fossil and high quality renewable based fuels. This requires high efficiencies, good part-load performance and fuel flexibility. A very promising concept to achieve that is the combination of solid oxide fuel cells (SOFC) with a gas turbine (GT) in a pressurized hybrid power plant. However, experimental data for such SOFC/GT systems are rare. Thus, the DLR built a test rig to analyse such a system with 30 kW electrical output. A 30 kW SOFC module is used under pressurized conditions with components that emulate the GT. Commercially available stacks and state of the art peripheral components are installed. These include e.g. a hot anode off-gas recirculation blower, a steam reformer and recuperator. The system was put into operation and is used to experimentally analyse its operational behaviour. This publication will give insights about the current status of the experimental work. It will outline the basic SOFC/GT process, the implementation within the installed SOFC system and the degrees of freedom in comparison to a coupled system. Experimental results are shown and the impact of main parameters is analysed.
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 well-known 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.
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