Modeling and Performance Analysis of Solid Oxide Fuel Cell Power Generation System for Hypersonic Vehicles

Layout design and performance evaluation of hydrogen-fueled solid oxide fuel cell power generation system

In conventional gas turbine systems combustion results in high exergy losses (∼30%) of fuel exergy input. Replacing the combustor with a high temperature fuel cell, like the Solid Oxide Fuel Cell (SOFC), will significantly reduce these exergy losses. As the SOFC electrochemically converts the natural gas, exergy losses are far lower (∼10%) compared to combustion. Natural gas entering a SOFC system has to be reformed first to hydrogen and carbon monoxide by steam reforming. Here it is chosen to use the heat generated by the fuel cell to drive the endothermic reforming reactions: internal reforming. The SOFC-GT system has the advantage that both fuel cell and gas turbine technology contribute to power production. In earlier work [1] several fuel cell system configurations with PEMFC, MCFC or SOFC, were analyzed studying the exergy flows. Here is focused on the SOFC-GT configuration, to get a detailed understanding of the exergy flows and losses through all individual components. Several configurations, combining the SOFC with the GT are possible. The selected operating conditions should prevent carbon deposition. Systems studies are performed to get more insight in the exergy losses in these combined systems. Exergy analysis facilitates the search for the high efficient SOFC-GT hybrid systems. Using exergy analysis, several useful configurations are found. Exergy losses are minimized by varying pressure ratio and turbine inlet temperature. Sensitivity studies, of equivalent cell resistance and fuel cell temperature, show that total system exergy efficiencies of more than 80% are conceivable, without using a bottoming cycle.

Nowadays, biogas is a globally significant alternative source of green energy. In Thailand, agricultural industries generate a vast volume of sewage, which can be utilized to produce biogas from anaerobic fermentation. Due to advanced research and technologies, biogas can produce a more valued option as hydrogen through a tri-reforming process. This hydrogen is introduced to generate power by integrating the tri-reforming process with a fuel cell. Therefore, power generation by a tri-reforming process combined with two types of fuel cells: molten carbonate fuel cell (MCFC) and solid oxide fuel cell (SOFC), was studied in this work by the Aspen Plus simulation program. Biogas obtained from a palm oil mill plant was employed as feedstock. The effect of several parameters, including tri-reforming temperature, S/C molar ratio, O2/C molar ratio, MCFC temperature, and SOFC temperature on power generation at atmospheric pressure, was investigated to find the optimal operating conditions maximizing hydrogen and power. The simulation results showed that the optimal operating conditions of tri-reforming were 700 °C of tri-reforming temperature, 3.0 S/C molar ratio, and 0.1 O2/C molar ratio. In addition, the optimal temperatures of MCFC and SOFC were 750 °C and 1000 °C, respectively. Moreover, the system efficiency was evaluated, and the power generation system with molten carbonate fuel cell produced 2809 W/m2 of power density with 63.27% of system efficiency. The power generation system with a solid oxide fuel cell made 5526 W/m2 of power density with 76.09% of system efficiency.

Recent developments in control and integration of solid oxide fuel cells: From stack to system

In this paper the operation and performance of a high temperature solid oxide fuel cell (SOFC) stack on biomass syn-gas from a demonstration biomass gasification combined heat and power (CHP) plant is investigated. The objective of this work is to develop a computer simulation model of a biomass-SOFC CHP system, flexible enough for use in industry, capable of predicting system performance under various operating conditions and using diverse fuels. The biomass gasifier is of the dual fluidised bed (DFB) type with steam as the gasifying agent and is operated at atmospheric pressure. The tubular SOFC configuration, developed by Siemens Power Generation Inc (SPGI), is selected. It is considered to be the most advanced design and is approaching commercialisation. The SOFC stack model, developed using the chemical process flowsheet simulator Aspen Plus, is of equilibrium type and is based on Gibbs free energy minimisation. The SOFC model performs heat and mass balances and considers the ohmic, activation and concentration losses for the voltage calculation. Data available in the literature on the SPGI SOFC operating on natural gas is used to validate the model. The system model predicts thermodynamic condition and composition of all internal flow streams, the heat generated by the SOFC stack, voltage (V), current (I) and efficiency. Operating parameters are varied over a wide range, parameters such as fuel utilisation factor (Uf), current density (j) and steam to carbon ratio (STCR) have significant influence. The results indicate that there must be a trade-off between voltage, efficiency and power with respect to j and the SOFC stack should be operated at low STCR and high Uf, within certain limits. SOFC stack operation on biomass syn-gas is compared to operation on natural gas and as expected there is a drop in performance, which is attributed to increased input fuel and air flow due to the lower quality of the fuel gas. The optimum realistic operating conditions with regard to SOFC stack performance are identified. High electrical efficiencies are predicted making these systems very attractive for CHP applications.

The mission of the fuel cell systems program of the Department of Energy, Office of Fossil Energy, in partnership with its customers and stakeholders, is to foster the creation of a new domestic fuel cell industry. This industry should be capable of commercialization of new, improved fuel cell power generation systems and thereby provide significant economic and environmental benefits. This program is aligned with the Department of Energy`s core mission (business line) of energy resources. The Department of Energy (DOE), Office of Fossil Energy, is participating with the private sector in sponsoring the development of molten carbonate fuel cell (MCFC) and solid oxide fuel cell (SOFC) technologies for application in the utility, commercial and industrial sectors. Phosphoric acid fuel cell (PAFC) development was sponsored by the Office of Fossil Energy in previous years and is now being commercialized by the private sector. This document describes the fuel cell activities of the DOE Office of Fossil Energy.

An H2 Polymer Electrolyte Membrane (PEM) Fuel Cell power generation system composed of three subsystems: fuel reforming, fuel cell stack, and post combustion is examined. The system is simulated and optimized with a fuel cell model integrated within the process flow sheet. A case study optimization of the entire power generation system is presented and the influence of heat integration on the optimization is demonstrated. Our objectives are to maximize the energy and system efficiencies as well as the system profit. Here, optimization formulations with heat integration are used to realize our objectives, which explore the synergy between optimization and heat integration. Our results showed that we can achieve an energy efficiency as high as 58.30% and a system efficiency as high as 28.35%. For comparison, we provide results obtained from sequential process optimization followed by heat integration. The gain from the synergy is mostly exemplified in the profit maximization study, where the profit increases by almost 27.15% in the best case. Finally, we obtain optimization results that include feed vaporization of water to steam as a comparison for the heat integration. These results demonstrate the importance of simultaneous optimal heat integration for fuel cell‐based processes. © 2006 American Institute of Chemical Engineers AIChE J, 2006

Thermodynamic analysis of biogas fed solid oxide fuel cell power plants

Cycle analysis of planar SOFC power generation with serial connection of low and high temperature SOFCs

This study investigates one of the most fundamental issues of integrating fuel cell (FC) generation into power systems – its effect on power system small-signal stability when it operates jointly with conventional power generation. The study first presents a comprehensive mathematical model of the solid oxide fuel cell (SOFC) power plant integrated with the single-machine infinite-bus power system. Based on the model, conventional damping torque analysis is carried out to study the effect of SOFC power generation on power system small-signal stability. The analysis concludes that system small-signal stability can be affected either positively or negatively by the SOFC power plant when system operating conditions change. Two examples of power systems with gird-connected SOFC power plants are presented. Small-signal stability of the first example of single-machine power system was examined when the power system operated at different load conditions and levels of mixtures of conventional and FC power generation. The second example is a four-machine two-area power system where the power supplied by the gird-connected SOFC power plant is variable. Results of simulation using full non-linear model of the power systems and the SOFC power plants are given. All the results from the example power systems confirm and further demonstrate the analysis presented and conclusions obtained.

Numerical analysis of the effects of interconnector design and operating parameters on solid oxide fuel cell performance

Ceramic high-temperature fuel cells and electrolysers are efficient energy conversion systems for electrical power generation and hydrogen production. Their core components are constituted by a stack of electroactive Solid Oxide Cells (SOCs) in which the electrochemical reactions take place. Thanks to their flexibility, the same stack can be alternatively operated in both fuel cell and electrolysis modes. However, the insufficient durability of SOCs still constitutes a major limitation for the technology. The present study addresses this issue and aims to bring some new insights on the effect of the Solid Oxide Fuel Cell (SOFC) versus Solid Oxide Electrolysis Cell (SOEC) operating modes on degradation of a typical Ni-YSZ//YSZ//CGO//LSCF-CGO cell. The electrochemical degradations are generally attributed to several underlying phenomena such as electrode microstructural evolution, material chemical decomposition or electroactive sites poisoning by contaminants. Among them, it is generally considered that Ni agglomeration in the Ni-YSZ cermet and Lanthanum Strontium Cobalt Ferrite (LSCF) material destabilization are two prevalent mechanisms involved in the cell performance deterioration. Therefore, these two mechanisms have been specifically investigated by a coupled approach of long-term testing in both SOFC and SOEC modes (1000 ≤ t (h) ≤9000) and post-test characterizations. The experimental results have then been analyzed in the frame of an in-house multi-scale model with the purpose to interpret them and to quantify the effect of material ageing on cell performances. The extent of Ni agglomeration has been characterized by three-dimensional electrode reconstructions obtained by X-ray nano-holotomography at European Synchrotron Radiation Facility (ESRF) (on the new Nano-Imaging beamline ID16A-NI). The new set-up and protocol enable the reconstructions of valuable 3D volumes (Fig. 1) with a large field of view (~50 µm) along with a high spatial resolution (~50 nm). The electrode morphological properties, which have been measured on the 3D volumes, have revealed a substantial Ni coarsening over time at 850°C and 750°C, whereas no Ni depletion was detected at the electrolyte interface. The increase of the Ni particle size is found to induce a decrease in both (i) the density of Triple Phase Boundary (TPBs) lines and (ii) the interfacial surface area between Ni and gas. Moreover, it was found that the Ni/YSZ interfacial surface area does not evolve during the experiments. This statement indicates that the ceramic backbone in the cermet prevents a massive Ni agglomeration at the SOFC/SOEC operating temperature. The compilation of all experimental data have allowed fitting the parameters of a physically-based law for Ni coarsening that was introduced in the modelling framework. The simulations have revealed that Ni agglomeration explains around 20-25% of the electrochemical degradation at 850°C after 1000 hrs of operation. However, the electrode microstructural evolution is found not to be affected by the cell polarizations. Therefore, the mechanism cannot explain the higher degradation rates recorded in electrolysis mode compared to the fuel cell ones. To explain the impact of the operating modes (SOFC or SOEC) on the degradation rates, several post-test analyses (i.e. Scanning Electron Microscopy, Transmission Electron Microscopy, X-ray µfluorescence and µdiffraction techniques) have been employed to investigate the phase reactivity in the region of the CGO barrier layer. The characterizations have revealed that Sr diffusion across the barrier layer and formation of SrZrO3 secondary phase occur mainly during electrolysis operation, whereas the process is very limited in fuel cell mode (Fig. 2). As a consequence, LSCF destabilization is found not to be involved in the degradation of cell performances during fuel cell operation while it could explain the highest degradation rates recorded in electrolysis mode. The post-test analyses have also revealed a diffusion and an accumulation of Co in the region of the barrier layer which is concomitant with the formation of SrZrO3. The formation of these Co-rich segregates in contact with SrZrO3 grains have been identified as cobalt-ferrite type compound. The in-house multi-scale model has been used to interpret the role of the cell operating mode on the LSCF destabilization mechanism. The cell polarization curves and the local quantities within the O2 electrode have been computed in both fuel cell and electrolysis modes. The simulations have shown that the electrolysis operation leads to a strong depletion of oxygen vacancies in the LSCF material. It has been proposed that the depletion in oxygen vacancies under electrolysis polarization could drive the Sr release from the structure, and in turn, could explain the experimental results. Based on this proposition, a possible mechanism for the LSCF destabilization and SrZrO3 formation has been detailed. Figure 1

Determination of optimum ejector operating pressures for anodic recirculation in SOFC systems

A hybrid system that combines a solid oxide fuel cell (SOFC) with a proton exchange membrane fuel cell (PEMFC) is presented in this paper. The SOFC stack acts as both an electricity producer and the fuel reformer for the PEMFC stack to generate additional power. A thermoeconomic model for the design optimization of a 220 kW SOFC-PEMFC hybrid system is developed in this work. Optimization of two objectives, i.e., the life cycle cost and the net electrical efficiency, are considered individually to find the optimum system configuration and component designs. Then, a multiparameter sensitivity analysis is performed to estimate the relative importance of the decision variables on the objectives. The optimization results indicate that the life cycle cost of the hybrid system is 3800–5,600 $/kW, and the maximum net electrical efficiency can reach around 63%, which is higher than an SOFC-only system, a reformer-PEMFC system, and an SOFC-gas turbine (GT) system with a similar output power. The sensitivity analysis shows that minimizing the size of the SOFC is most crucial to the system cost optimization. The hydrogen utilization factor in the SOFC is found to be sensitive to the net electrical efficiency.

Thermodynamic performance study of the MR SOFC-HAT-CCHP system