Pressurized Solid Oxide Fuel Cell Research Articles

Understanding Solid Oxide Fuel Cell Hybridization: A Critical Review

A holistic literature review of 399 articles is presented on hybrid solid oxide fuel cell (SOFC) systems and taxonomized through a unique classification methodology that considers primary energy sources, component configurations, system outputs and relevant analysis. In the reviewed research articles, primarily four different types of technologies are used to complement the energy production of the fuel cell stack. As a result, four respective categories were established in terms of the coupling components, inclusive of (1) wind turbines (WTs), (2) solar systems (SS), (3) thermal power plant turbines (TPPTs) and (4) piston engines (PEs). Additional classification was needed to further subdivide the evaluated system configurations, due to the wide spectrum of system outputs encountered in the reviewed literature. That involved (a) electrical power generation (PG), (b) four types of cogeneration (CG), (c) four types of trigeneration (TG) and (d) two types of multigeneration (MG). Depending on the type of assessment in respective research, literature was further categorized as computational, experimental, or both computational and experimental. The most prevalent hybrid system amongst the 102 designs, as adapted from the reviewed literature, is the SOFC/gas turbine (GT) application, with the overwhelming majority producing exclusively electrical power output. The particular system is physically demonstrated in an existing 220-kW experimental facility established by the National Fuel Cell Research Center (NFCRC). The identified systems were predominantly computationally examined and research focused on operating conditions around specific design load ratings. Energy, exergy, economic, and environmental assessments were typically followed by sensitivity studies in the reviewed literature. The primary decision variables in most cases revolved around SOFC operating conditions, such as temperatures over 1000 °C as well as elevated pressures. Those resulted in the deterioration of the electrochemical cell and failed to be addressed by the majority of encountered studies. Improvements in the conversion efficiency of PG applications were enabled by natural gas (NG)-fed pressurized SOFCs, directly coupled to GTs. The most efficient power configuration in terms of thermodynamic performance was achieved by coupling PEs to SOFCs and a metal hydride reactor (MHR) to unlock a maximum net system efficiency of 79.54%. CG systems integrating solar cells (SCs) and photovoltaic panels (PVs) to SOFC subsystems can result in a combined efficiency of approximately 90% and sustain this over a broad energy demand range. TG systems incorporating gasifiers to SOFC/GT subsystems achieved a combined efficiency of 93%, whereas the optimal combined efficiency in MG configurations was identified at 79.5% by coupling SCs to the SOFC/GT/organic Rankine cycle turbine (ORCT) subsystems. In terms of cost-effectiveness, configurations that relied on increasing levels of renewable energy (RE) technologies resulted in high overall levelized cost of electricity (LCOE). This cost was increased further when power storage units were deployed to address wind and solar intermittency challenges. Configurations classified under TPPTs operating on low carbon-content fuels were preferred in terms of providing lower LCOE, while applications operating on gasified biomass and coal blends should deploy additional CO2 capture and storage units to mitigate respective emissions.

Evaluation of Tubular SOFC’s Performance at Elevated Pressure for Highly-Efficient Clean Power Generation

Growing global energy needs and demand over imminent reductions of harmful greenhouse gas emissions, call for development of highly efficient, clean and reliable power generation from both renewable and fossil resources. A hybrid solid oxide fuel cell gas turbine (SOFC-GT) system is a promising candidate for fulfilling such need of a low emission, highly efficient power generation. This system can be applied to stationary, transportation, aviation, and maritime power generation applications. This study focuses on understanding operational characteristics of the pressurized SOFC as a key component for the successful design and construction of a highly efficient SOFC-GT hybrid system.Pressurization of an SOFC can cause improvement on electrode kinetics, stack lifetime and round-trip efficiency by cell integration with downstream components such as gas turbines and catalytic reactors (for electrolysis) [1-4]. However, literature data from experimental studies on the electrochemical behavior of the SOFC as a function of operating pressure is relatively scarce. This is presumably due to the complexity involved in the construction and operation of pressurized system. To address such research challenge, a pressurized SOFC test rig as shown in Fig. 1 has been constructed and commissioned at Tennessee Technological University (TTU) to allow tubular SOFCs to be characterized under a maximum operating pressure of 60 psia (~4 bar) and maximum operating temperature of 825oC. A detailed system set-up shown in Fig. 2. The power generation capacity for the test rig can be scaled up to 2.4 kW. For this study, performance of single anode supported SOFC tubes with H2 and CH4 fuels were examined by varying pressure from 1 bara to 4 bara and temperature from 675oC to 775oC, with fuel utilization held constant at 50% and an oxygen-to-carbon ratio (O/C) of 0.6 for the methane operation. The testing cells were gratuitously supplied by Special Power Sources (SPS) LLC, USA, with cathode/electrolyte outside-layers supported on the anode inner-layer as a 14”-long tube. Silver ink and wires are used for cell current collection from active surface area of ⁓100 cm2 . The connection fixture can be seen in Fig. 1. V-I curves of tubular SOFCs in Fig. 3 measured from step-wise electronic load profile indicate total performance gain of 13% as H2 pressure increases from 1 bara to 4 bara, with highest peak power density obtained at 0.61 Wcm-2. As comparison Fig.4 and 5 demonstrate pressurization suppressed gas diffusion limit for tubular SOFC at higher current density in CH4 fuel, leading to 44 % increase of peak power density to 0.51 Wcm-2 at 4 bara.Electrochemical impedance spectroscopy (EIS) studies were performed using a Solartron 1260/PAR 263 electrochemical testing system in 4-point connection measurement to understand effects of pressurization on the SOFC performance. SOFC impedance spectra as Nyquist and Bode plots in Fig.6 (b) and (d) indicate the tubular SOFC performance is mainly limited by anode mass transports, which are greatly improved by pressurization. From non-linear, least-square cell impedance fitting and previous findings on impedance of similar larger cells, an equivalent-circuit model (ECM) in Fig. 6(a) has been identified to best represent impedance responses from the most rate-limiting processes of tubular SOFCs. The Warburg component (Ws) in the ECM with resonance frequency around 10Hz typically corresponds to gas diffusion in porous electrode of tubular SOFC [3-5], while paralleled R-CPE component at <1Hz resonance frequency can be attributed to gas-conversion impedance (GCI) due to gas phases concentration gradient along fuel-flow channel resulting from reactant conversion [5,6]. Experimentally observed changes of such impedances with fuel compositions, flow rates and tubular SOFC configuration are generally in agreement with theoretical assumptions and other reports [4-6]. Overall, results of this work demonstrate pressurization can effectively boost anode-supported SOFCs’ performance by overcoming fuel-side mass-transport limit associated with practical cell configuration and testing geometry. The experimental performance data collected can then be used to fit key constants and estimate parameters for numerical modeling of the SOFC system performance at elevated pressures.References can be seen in the figure document. Figure 1

Compressor Surge Precursors for a Turbocharger Coupled to a Pressure Vessel

Abstract Dynamic compressors operating region is mainly constrained by fluid-dynamic instabilities occurring at low mass flow rate conditions, such as surge and rotating stall. This work presents a vibro-acoustic experimental investigation on a centrifugal compressor of an automotive turbocharger aimed to identify and confirm some surge precursor values in correspondence of its inception conditions. The experimental campaign was carried out on a turbocharger equipped by a vaneless diffuser compressor exploited for the pressurization of an innovative solid oxide fuel cell (SOFC) emulator. The investigated turbocharger is coupled with a pressure vessel for a former emulation activity on a pressurized SOFC. In such kind of plants, the joint effect of large volume size downstream the compressor makes more complex the dynamic behavior of the whole system during transients, thus significantly increasing surge onset risk. The main goal is to define a suitable quantitative indicator to detect in advance surge inception by relying only on system vibro-acoustic response. Several transient from a compressor stable condition to surge instability onset were performed by progressively closing specific valves in the air line. When moving close to surge , vibro-acoustic signals were acquired at a high sampling rate to investigate blade passage phenomena which might interact with rotating stall inception. Meanwhile, pressures, temperatures and mass flow rates measured at a lower sampling rate to correlate compressor vibro-acoustic and performance behavior. Cyclostationary analysis was performed on vibro-acoustic acquired signals to provide innovative diagnostic and predictive solutions (precursors) for early surge detection.

Energy, exergetic and economic analysis and multi-objective optimization of atmospheric and pressurized SOFC based trigeneration systems

In this study, the novel trigeneration systems based on the atmospheric and pressurized SOFC are proposed and investigated. The proposed systems include the SOFC based top cycles and an innovative bottom cycle which combines a transcritical CO2 cycle, a transcritical organic Rankine cycle, and a LNG cold energy utilization system. The mathematical model is developed, and the energetic, exergetic and economic analysis is conducted accordingly. Then, the effects of key parameters, such as current density, operating temperature, steam-to-carbon ratio (STCR) on system performance are evaluated. Under design conditions, the results show that the electrical efficiencies, the overall system efficiencies, the exergy efficiencies, and the total cost rate of the integrated systems can reach 57.14%, 59.56%, 55.12%, 16.73 $/h for the atmospheric system and 68.1%, 68.8%, 65.7%, 19.84 $/h for the pressurized system, respectively. Parameter study shows that the thermodynamic performance of pressurized system is significantly better than that of the atmospheric system, but economic cost is much higher. SOFC inlet temperature has a positive effect on the pressurized system performance, while this positive effect is found when the inlet temperature is less than 510 °C for atmospheric system. Besides, the multi-objective optimization by using genetic algorithm (NSGA-II) for the novel trigeneration systems has been performed. And the technique for order preference by similarity to an ideal solution (TOPSIS) based on entropy weight is performed to choose the final optimum design point of the combined systems. The multi-objective optimization results demonstrate that the exergy efficiency and the total cost rate achieve the values of 56.1%, 16.82 $/h and 66.83%, 12.02 $/h for atmospheric and pressurized system, respectively, at the optimal balance point selected by the TOPSIS method.

Pressurized SOFC System Fuelled by Biogas: Control Approaches and Degradation Impact

Abstract This paper shows control approaches for managing a pressurized solid oxide fuel cell (SOFC) system fuelled by biogas. This is an advanced solution to integrate the high efficiency benefits of a pressurized SOFC with a renewable source. The operative conditions of these analyses are based on the matching with an emulator rig including a T100 machine for tests in cyber-physical mode. So, this paper presents a real-time model including the fuel cell, the off-gas burner (OFB), and the recirculation lines. Although the microturbine components are planned to be evaluated with the hardware devices, the model includes also the T100 expander for machine control reasons. The simulations shown in this paper regard the assessment of an innovative control tool based on the model predictive control (MPC) technology. This controller and an additional tool based on the coupling of MPC and proportional integral derivative (PID) approaches were assessed against the application of PID controllers. The control targets consider both steady-state and dynamic aspects. Moreover, different control solutions are presented to operate the system during fuel cell degradation. The results include the system response to load variations, and SOFC voltage decrease. Considering the simulations including SOFC degradation, the MPC was able to decrease the thermal stress, but it was not able to compensate the degradation. On the other hand, the tool based on the coupling of the MPC and the PID approaches produced the best results in terms of set-point matching, and SOFC thermal stress containment.

A pressurized ammonia-fed planar anode-supported solid oxide fuel cell at 1–5 atm and 750–850°C and its loaded short stability test

Ammonia is a useful energy carrier for solid oxide fuel cell (SOFC) with advantages over hydrogen. Understanding of the performance and stability of ammonia-fed SOFC operated at elevated pressure (p) is an important step towards the development of high-efficiency hybrid SOFC power system with micro gas turbine (MGT). This paper reports cell performance, electrochemical impedance spectra (EIS), and stability measurements of a pressurized ammonia-fed anode-supported SOFC at p = 1–5 atm and T = 750–850°C using a planar (50 × 50 mm2) single-full-cell (400 μm Ni-YSZ anode/3 μm YSZ electrolyte/12 μm LSC-GDC cathode). The full cell together with metallic frames and current collectors are sandwiched by a pair of rib-channel flow distributors (interconnectors) in a high-pressure testing facility. Results show that pressurization and increasing temperature enhance the ammonia-fed SOFC performance significantly having almost the same power densities as those of hydrogen/nitrogen-fed SOFC, as substantiated and explained by EIS data and an equivalence circuit model where the effects of p and T on ohmic, gas diffusion, and gas conversion impedances are shown. Moreover, the loaded short (10 h) stability tests at 700°C and 0.8 V for 1 atm/3 atm cases reveal no/little power degradation, where the microstructures without any crack within the scale of SEM observation and nearly the same element atomic percentages of Ni-YSZ anode surfaces from EDX spectra are found. These results suggest that the pressurized ammonia-fed SOFC is a promising candidate for the hybrid SOFC-MGT power generation.

Hydrogen and power generation from supercritical water reforming of glycerol and pressurized SOFC integrated system: Use of different CO2 adsorption process

The performance analysis of an integrated system of glycerol supercritical water reforming and pressurized SOFC was presented. The use of different CO2 adsorption processes that include in situ and ex situ processes was compared to determine the suitable process for hydrogen and power generations. The influence of operating condition, e.g., temperature and pressure of reformer, supercritical water to glycerol (S/G) molar ratio, and calcium oxide to glycerol (CaO/G) molar ratio was examined. Then, the electrical performance of each integrated process was considered with respect to the SOFC conditions comprising temperature, pressure, and current density. The simulation results revealed that both processes have same favourable conditions for temperature and pressure operated at 800 °C and 240 atm, respectively. The suitable S/G and CaO/G molar ratios for in situ process are 10 and 2 whereas those for ex situ process are 20 and 1. Under these conditions, maximum hydrogen can be achieved as 87% and 75% for in situ and ex situ processes, respectively. When both integrated processes are operated at the optimal SOFC conditions as 900 °C, 4 atm, and current density of 10,000 A/m2, the SOFC efficiency of 71.56% and 62.12% can provide for in situ and ex situ processes, respectively.

Theoretical and experimental study of Reversible Solid Oxide Cell (r-SOC) systems for energy storage

A theoretical system model to study different system concepts is presented in this study. An SOC reactor model was developed based on the experimental analysis in pressurized SOFC and SOEC operation mode. An experimental analysis under pressurized conditions was performed on a commercially available Electrolyte Supported Cell (ESC) type 10 layer SOC stack. A simple system model analysis was performed based on this r-SOC reactor. The effect of different system operation parameters such as pressure, temperature, current density and utilization on system performance was analysed. Endothermic operation of an r-SOC stack in SOEC operation mode can lead to higher roundtrip efficiency. Endothermic operation in SOEC mode requires thermal energy supply. A thermal energy storage system to store high temperature heat released during the SOFC operation to supply heat with high exergy content for the endothermic SOEC process was investigated. A system roundtrip efficiency around 55%–60% is achievable with current ESC type r-SOC technology whereas the theoretical limit of roundtrip efficiency for an ideal reactor (no ohmic, activation and diffusion losses) is around 98% for the same operating conditions. Endothermic operation is beneficial and a heat storage concept is an attractive approach for a prototype system.

Cell performance, impedance, and various resistances measurements of an anode-supported button cell using a new pressurized solid oxide fuel cell rig at 1–5 atm and 750–850 °C

We report a novel pressurized solid oxide fuel cell (SOFC) rig that overcomes the stagnation flow problem in classic button cell setups, allowing fuel and air to distribute uniformly over anode and cathode surfaces. Power and impedance measurements of an anode-supported full button cell (ASC; NiO-YSZ/YSZ/LSM-GDC-LSM) operated at p = 1–5 atm and T = 750–850 °C show that both pressurization and increasing temperature enhance the cell performance, which are explained by the Nyquist plots consisted of a high frequency arc with characteristic frequencies (fc) around 100–1000 Hz relating to the cathode activation overvoltage and a low frequency arc with fc around 10–30 Hz corresponding to the anode concentration overvoltage. The activation overvoltage decreases with increasing p and/or T, while the concentration overvoltage decreases with increasing p but rather insensitive to T. We find that the activation overvoltage is the major source for the polarization resistances contributing 63%–82% depending on pressure, temperature, and current density, while the concentration overvoltage is a minor one contributing 18%–37% for the present ASC. These results and the rig should be useful for our understanding and further studies of pressurized SOFCs.

Fuel Composition in Pressurized SOFCs

Introduction Pressurized SOFC system combined with gas and steam turbine will achieve high power generation efficiency approaching 70% as the triple combined cycle power generation system. It has been reported that operating SOFCs under high pressure can make cell power output higher (1,2). However, there are only a limited number of studies available on the fuel composition for pressurized SOFCs. In general, fuel gas consists of carbon, hydrogen and oxygen, so that the C-H-O equilibrium composition determines anode performance. In this study, fuel gas composition on the anode side of pressurized SOCFs was derived by thermochemical equilibrium calculation for considering various operating conditions in terms of carbon deposition and each gas molar fraction affecting power generation characteristics, all described in the C-H-O diagrams. Calculation Procedure We calculated carbon deposition region and each gas molar fraction under various equilibrium conditions upon suppling fuels composed of carbon, hydrogen and oxygen. Thermochemical calculations were carried out using three approaches: (i) calculation with equilibrium constants, using (ii) HSC Chemistry (Version 9.0.5, Outotec Research Oy, Finland) and (iii) FactSage (Version 6.3.1, Thermfact Ltd., Canada). As the first approach, we defined main reactions within the fuel and solved the system of equations using relational expression of equilibrium constants and elements ratios by Mathematica (Version 10.4.1, Hulinks Inc., Japan). HSC Chemistry and FactSage are both thermochemical equilibrium calculation software based on Gibbs free energy minimization, containing various databases. It has been known that the major constituents of equilibrium products are H2(g), H2O(g), CO(g), CO2(g), CH4(g), and C(s) (graphite) (3). The line specifying the carbon deposition region means the equilibrium concentration of C(s) equal to 10-6 of the initial carbon content in the fuel. The C-H-O diagrams are described in such a way between 100 and 1000oC, and between 1 and 30 bar. Results and discussion Comparing Fig. 1 with Fig. 2, carbon deposition region boundaries vary with respect to temperature and pressure. The C-H-O diagrams clearly show that carbon deposition region expands on the oxygen-rich side and contracts on the hydrogen-rich side with decreasing temperature and/or increasing total pressure. It can be explained by the chemical equilibrium reactions (I) and (II): CH4↔C(s)+2H2 (I) 2CO↔C(s)+CO2 (II) Reaction (I) is an endothermal reaction; on the hydrogen-rich side, equilibrium reaction (I) shifts to the left side, preventing carbon deposition at lower temperatures. With increasing total pressure, the equilibrium also shifts to the left side due to the decrease in the number of molecules. In the same way, Reaction (II) is an exothermal reaction and the number of molecules is lower on the right side, so that the equilibrium reaction shifts to the right side to promote carbon deposition. The C-H-O diagrams of various gases and the corresponding theoretical open circuit voltage are also calculated. These results suggest, for example, a decrease in the fraction of H2and CO under higher total pressure.

Gas turbine advanced power systems to improve solid oxide fuel cell economic viability

Abstract Coupling a solid oxide fuel cell (SOFC) with a gas turbine provides a substantial increment in system efficiency compared to the separate technologies, which can potentially introduce economic benefits and favor an early market penetration of fuel cells. Currently, the economic viability of such systems is limited by fuel cell short lifetime due to a progressive performance degradation that leads to cell failure. Mitigating these phenomena would have a significant impact on system economic feasibility. In this study, the lifetime of a standalone, atmospheric SOFC system was compared to a pressurized SOFC gas turbine hybrid and an economic analysis was performed. In both cases, the power production was required to be constant over time, with significantly different results for the two systems in terms of fuel cell operating life, system efficiency, and economic return. In the hybrid system, an extended fuel cell lifetime is achieved while maintaining high system efficiency and improving economic performance. In this work, the optimal power density was determined for the standalone fuel cell in order to have the best economic performance. Nevertheless, the hybrid system showed better economic performance, and it was less affected by the stack cost.

The Impact of Pressurization on Anode-Supported and Electrolyte-Supported Planar Solid Oxide Fuel Cells at 750oC ~ 850oC

How to reduce effectively the severe impact of various environmental and energy problems is one of the most important issues in our human society. This continues to drive the development of hybrid power generation systems aiming to achieve much higher efficiencies together with much lower emissions than conventional power systems. Among only a few available hybrid energy conversion systems, a pressurized solid oxide fuel cells (PSOFC) integrating with a gas turbine (GT) or micro gas turbine (MGT), the hybrid PSOFC-GT or -MGT power system, has the highest efficiency up to 70%. Moreover, the development of such hybrid PSOFC-GT or –MGT power system is not only beneficial for the environment, but also it is important to the consolidation of the fuel cell industry and the expansion of the gas turbine industry. Although several demos of the hybrid PSOFC-MGT power system have been established, many technical challenges existing in the design of such hybrid power system and its control still need to be solved in order to assure a stable operation among different components of the hybrid system. One of the most important technical challenges is the detail experimental information on electrochemical impedance spectra (EIS) of PSOFC, which is still rare in literatures. This motivates us to measure experimentally the effect of pressurization on EIS of PSOFC. In this work, we measure the pressurization influence on power generating characteristics and EIS for both anode-supported and electrolyte-supported cells (ASC and ESC) using a recently-established high-pressure SOFC facility. It is worth of noting that the present study has an important advantage due to the fact that both ASC and ESC use the same single-cell setup, a planar full cell having 40 × 40 mm2 effective reactive area sandwiched by a pair of rib-channel flow distributors (interconnectors) on both anode and cathode, under exactly the same experimental conditions. Specifically, both ASC and ESC apply exactly the same flow rates (Q anode = 0.5 slpm H2 + 0.4 slpm N2 and Q cathode = 0.9 slpm air) at three operating temperatures (T = 750 oC, 800 oC, 850oC), each T under five different pressures (p = 1, 2, 3, 4, 5 atm), having a total of 30 data sets for comparison. Hence, a direct comparison of experimental EIS data and power generating characteristics between ASC and ESC at elevated pressures can be clearly made. To the best knowledge of the authors, the present measured EIS comparison between ASC and ESC is new, and experimental EIS data of pressurized electrolyte-supported planar SOFC are still not available in literatures. These ASC and ESC measurements under the same experimental conditions suggest the following points: (1) At T = 850oC and 0.7 V, power densities of ASC/ESC increase from 309/193 mW cm-2 to 476/250 mW cm-2, as p increases from 1 atm to 5 atm. As T increases from 750oC to 850oC at 0.7 V, power densities of ASC/ESC increase from 175/97 mW cm-2 to 309/193 mW cm-2 at p = 1 atm, while at p = 5 atm, the corresponding increases of power densities are 281/137 mW cm-2 to 476/250 mW cm-2. It is found that ASC is more sensitive to pressurization than ESC, while the latter is more temperature-sensitive than the former. (2) The ohmic resistance is not varied with the loaded current density and it is independent of pressure for both ASC and ESC cases. Furthermore, ESC has higher values of the ohmic impedance, about 16 % larger, than that of ASC. The thicker the electrolyte thickness is, the larger the ohmic impedance is. (3) The polarization impedance decreases noticeably with increasing p for both ASC and ESC cases at either unloaded or loaded conditions. This explains why power densities increase with increasing pfor both ASC and ESC. (4) These measurements reveal that the better cell performance of ASC than that of ESC is not simply due to the larger ohmic overvoltage of ESC as previously predicted by numerical simulations. The variation of the polarization overvoltage between ASC and ESC also plays an important role. We hasten to note that it is not sure whether the present conclusions can be extended to all ASCs and ESCs due to different electrolytes and anodes used in the present ASC and ESC, but these results should be of help in the further development of PSOFC integrating with micro gas turbines for future stationary power generation. Figure 1

A Simple Pressurized SOFC Test Rig for Measurements of Cell Performance, Impedance, and Various Overvoltages

Nowadays, our modern society is facing many serious environmental and energy shortage problems. Delay of action will jeopardize the ability of future generations to meet their needs. Thus, how to develop highly efficient power generation systems with very low emissions is an urgent need for our modern society. Probably, a pressurized solid oxide fuel cells (PSOFC) integrating with a gas turbine (GT) or micro gas turbine (MGT), the hybrid PSOFC-GT or -MGT power system, which has the highest efficiency up to 70%, is the best candidate. The development of such hybrid power system helps to the consolidation of the fuel cell industry, the expansion of the gas turbine industry, and the improvement of the environment. As such, the hybrid PSOFC-MGT power system has become an emerging topic in the 21th century with wide interest, as evidenced by a large volume of literature and several demonstration projects. For examples, Siemens demonstrated a 220 kW PSOFC-MGT hybrid system in 2002, the Korea Institute of Energy Research displayed a 30 kW (5 kW PSOFC integrated with 25 kW MGT) hybrid system in 2006, and Mitsubishi Heavy Industries run a 200 kW hybrid PSOFC-MGT combined-cycle power plant in 2011. Indeed, considerable progresses have been made over the past two decades in optimization of geometry and development of new materials and in understanding of mass and heat transports together with electrochemical reactions in porous microstructures. However, there are still technical challenges remaining to be solved step by step with limited but controllable parameters before a stable operation among different components of the hybrid PSOFC-MGT system can be assured and the status of the development can be then elevated from a module level. One of the challenges is the detail electrochemical impedance spectra (EIS) of PSOFC, which is still rare in literatures. This motivates us to design a simple efficient high-pressure full/half button cell testing platform for measurements of the effect of pressurization and increasing temperature on EIS and various overvoltages of PSOFC. The testing platform has several parts, from inside out including a specially thread-designed housing carrier for the full/half button cell, a serpentine heating pipe system for uniform heating of the supplied fuel and air gases, a temperature-controlled furnace, and a large high-pressure vessel together with measuring devices and their associates. Hence, current-voltage curves and AC impedance spectra of any full/half button cells can be measured. In this study, we present power-generating characteristics and EIS data of an anode-supported full button cell over wide ranges of pressure (p = 1~5 atm) and temperature (T = 700~850 oC). We apply constant gas flow rates, 200ml min-1 H2 in anode and 200ml min-1 air in cathode, for all experimental events. Results show that, for any given values of T or p, power densities increase with increasing p or T at any fixed current densities. Such enhancement on power densities is more sensitive to the increase of T than to the increase of p. These results are explained by the corresponding EIS data. The latter shows that both high and low frequency arcs of impedance spectra decrease with increasing p, resulting in a reduction of the total polarization resistance. Specifically, the high frequency arcs decrease rather weakly with increasing p as compared to that of the low frequency arcs. Further, we found that the characteristic frequencies of high and low frequency arcs occur around 100 ~ 1000 Hz and at about 10 Hz, respectively. The former may be attributed to the cathode activation polarization and the latter may be due to the diffusion processes in the anode electrode. Hence, pressurization can simultaneously decrease the cathode activation polarization and the anodic diffusion concentration polarization. In addition to the aforesaid power-generating characteristics and EIS results, this study reports analyses of activation and concentration overvoltages using the Butler-Volmer equation and the related concentration overvoltage equation based on a 1D diffusion model. We found that both activation and concentration overvoltages decrease with increasing p. Two important parameters, the exchange current density and the anodic effective diffusion coefficient, are also calculated by applying the present measured data. Results show that pressurization enhances the exchange current density leading to an increase of the electrochemical reaction rate. Though pressurization reduces slightly the anodic effective diffusion coefficient, it can increase the molar concentration and thus the overall gas-phase diffusion rate in porous electrode. These overvoltage results will be compared with previous numerical and experimental data to enhance our understanding of the effect of pressurization on the polarization mechanisms in SOFCs. Figure 1

A Simple Pressurized SOFC Test Rig for Measurements of Cell Performance, Impedance, and Various Overvoltages

This paper reports a newly designed high-pressure full button cell test platform for measurements of various effects on pressurized solid oxide fuel cells (PSOFC). The testing platform has several parts, from inside out including a specially thread-designed housing carrier for the full button cell, a serpentine heating pipe system for uniform heating of the supplied fuel and air gases, a temperature-controlled furnace, and a large high-pressure vessel together with measuring devices and their associates. Hence, current-voltage curves and AC impedance spectra of any full/half button cells can be conveniently measured. In this study, we present power-generating characteristics and electrochemical impedance spectra (EIS) data of an anode-supported full button cell over a range of pressure (p = 1~5 atm) at a temperature T = 800 oC. Results show that power densities increase with increasing p at any given current densities. These results are explained by the corresponding EIS data. In addition to the aforesaid power-generating characteristics and EIS results, this study also presents analyses of activation and concentration overvoltages using the Butler-Volmer equation and the related concentration overvoltage equation based on a 1D diffusion model. These overvoltage results will be compared with previous numerical and experimental data to enhance our understanding of the effect of pressurization on the polarization mechanisms in SOFCs.

Operational Aspects for Direct Coupling of Gas Turbine and Solid Oxide Fuel Cells

This contribution gives an overview of the theoretical and experimental investigations that have been carried out at DLR concerning the pressurized operation of SOFC. The influence of pressure on SOFC performance is analyzed and the influence of internal reforming reactions is evaluated. Furthermore, the influence of pressure on degradation mechanisms is theoretically analyzed. Results show that pressure can strongly influence the power density of SOFC. A decrease in SOFC performance caused by reformate gases is mainly due to temperature effects. Nickel oxidation and carbon deposition propensity are both pressure dependent and should be addressed when designing pressurized SOFC systems.

Exergy and economic analysis of a CaO-looping gasifier for IGFC–CCS and IGCC–CCS

We present exergy and economic analyses for two potential advanced coal-based power plants with CO2 capture and sequestration. Each system generates three products: electricity, carbon dioxide compressed to 15MPa, and pre-calcined feedstock for cement kilns. First, we analyzed a system that integrates a CaO-looping gasifier with a solid oxide fuel cell (SOFC), labeled here as IGFC–CCS. The SOFC is modeled based on a commercial pressurized SOFC system. Second, we analyzed a system that integrates a CaO-looping gasifier with a Brayton–Rankine combined cycle, labeled here as IGCC–CCS. The exergy analyses evaluated both the power generation/consumption and the exergy destruction in each of the major sub-systems within the power plant. The economic analyses evaluated the internal rate of return on investment (IRR), including the upfront construction costs and the yearly net revenue. Using recent capital cost estimates, we performed a parametric study of this IGFC–CCS system to determine the effect on the IRR on the four key SOFC parameters: current density, air pressure, fuel utilization, and air stoichiometric ratio. For this IGFC–CCS configuration, the calculated exergetic efficiency was 60% at the parameters values that maximized the IRR. For the IGCC–CCS system with same gasifier specifications, the efficiency was 46%; although, this configuration often achieved a higher value of IRR than the IGFC–CCS configuration, depending on the assumptions made on gas turbine and fuel cell equipment costs. We conducted a sensitivity analysis to determine how the IRR was affected by assumptions, such as capital costs and the sale price of CO2. Most importantly, we analyzed the effect of the coal gasification kinetic rate and CaO capture degradation rate on the IRR so that experimental researchers have goal posts as far as required rates for this CaO-looping process. From our sensitivity analysis, we conclude that the addition of alkali catalysts to CaO-looping gasification process can increase the IRR compared with a CaO-looping process without the addition of alkali catalysts, such as KOH.

Temperature Effect due to Internal Reforming in Pressurized SOFC

For the integration in a hybrid power plant the behavior of pressurized SOFC stacks was examined. A detailed electrochemical model was validated allowing for a thorough interpretation of experimental data. This paper focuses on the use of reformate gases and the influence of internal reforming. Experimental and simulated current-voltage characteristics and electrochemical impedance spectra were used to examine the temperature differences due to internal reforming within the stack which were found to have a major impact on performance. It is shown that similar performance can be obtained for different fuel gas compositions if temperature within the stack is controlled.

Temperature Effect Due to Internal Reforming in Pressurized SOFC

For the integration in a hybrid power plant the behavior of pressurized SOFC stacks was examined. A detailed electrochemical model was validated allowing for a thorough interpretation of experimental data. This contribution focuses on the use of reformate gases and the influence of internal reforming. Experimental and simulated current-voltage characteristics and electrochemical impedance spectra were used to examine the temperature differences due to internal reforming which were found to have a major impact on performance. It is shown that similar performance can be obtained for different fuel gas compositions if temperature within the stack is controlled.

Analysis of a pressurized solid oxide fuel cell–gas turbine hybrid power system with cathode gas recirculation

A pressurized solid oxide fuel cell–gas turbine hybrid system (SOFC–GT system) has been received much attention for a distributed power generation due to its high efficiency. When considering an energy management of the system, it is found that a heat input is highly required to preheat air before being fed to the SOFC stack. The recirculation of a high-temperature cathode exhaust gas is probably an interesting option to reduce the requirement of an external heat for the SOFC–GT system. This study aims to analyze the pressurized SOFC–GT hybrid system fed by ethanol with the recycle of a cathode exhaust gas via a simulation study. Effect of important operating parameters on the electrical efficiency and heat management of the system is investigated. The results indicate that an increase in the operating pressure dramatically improves the system electrical efficiency. The suitable pressure is in a range of 4–6 bar, achieving the highest system electrical efficiency and the lowest recuperation energy from the waste heat of the GT exhaust gas. In addition, it is found that the waste heat obtained from the GT is higher than the heat required for the system, leading to a possibility of the SOFC–GT system to be operated at a self-sustainable condition. Under a high pressure operation, the SOFC–GT system requires a high recirculation of the cathode exhaust gas to maintain the system without supplying the external heat; however, the increased recirculation ratio of the cathode exhaust gas reduces the system electrical efficiency.

Cathode–anode side interaction in SOFC hybrid systems

Cathode–anode interaction, mainly based on cathode versus anode volume influence, recirculation performance, and turbomachinery integration, is an important issue for pressurised SOFC hybrid systems, and this aspect must be carefully considered to prevent fuel cell ceramic material failures through a reliable control system. Over the last 10years, several theoretical analyses of this issue have been carried out at the University of Genoa. These interaction studies have been analysed and an experimental approach (for model validation, system development and prototype design activities) has been applied using emulator facilities or real plants. In particular, general hybrid system layouts based on the coupling of pressurized SOFC stacks of different geometries (planar, tubular, etc.) with a gas turbine bottoming cycle have been investigated using the hybrid system emulator facility of the University of Genoa. The experimental results are focused on the interaction between gas turbine and anodic circuit and on cathode–anode differential pressure behaviour for design, off-design and transient hybrid system operative conditions. The information obtained in these tests is essential to understand the main features of the variables that drive the phenomena and to design a suitable control system that can mitigate the differential pressure values during all plant operating conditions.