Cathode Recirculation Research Articles

Analysis of the influence of cathode recirculation strategy on proton exchange membrane fuel cell from internal polarization and external characteristics perspectives

Cathode recirculation provides a new solution to the accelerated degradation of proton exchange membrane fuel cells (PEMFCs) due to low humidity and high potential at low load conditions. Analyzing the internal impact mechanisms is basis for a more appropriate use of this technology. This paper presents a systematic investigation of the mechanisms by which key operating parameters of the cathode recirculation, including current density, cathode recirculation ratio, and stoichiometry of the fresh air path, affect the internal and external properties of PEMFCs. The voltage suppression effect and humidification effect under different cathode recirculation strategies are first analyzed. Then, as a novelty, the distribution of relaxation times (DRT) method is used to deconvolve electrochemical impedance spectra and qualitatively resolve the polarization impedance under different strategies. DRT is enabled to separate up to five polarization processes with different time constants and assign them to oxygen diffusion, oxygen reduction reaction and proton transport processes affected by the cathode recirculation. Finally, DRT quantified the change in ohmic impedance due to cathode recirculation and confirmed the effect of external humidity changes. This work provides a more in-depth insight into internal mechanisms of cathode recirculation in PEMFCs, and especially provides a comprehensive theoretical basis for the development of idle controllers for automotive PEMFCs in terms of improved durability.

Optimization strategies to mitigate reactant starvation in a dead-ended hydrogen–oxygen proton exchange membrane fuel cell during cyclic loading

Investigating the dynamic characteristics in hydrogen–oxygen proton exchange membrane fuel cells (PEMFCs) can provide valuable insights into the coupling degradation mechanism present within the fuel cell, which can help avoid reactant starvation and voltage decline. In this study, a recirculation subsystem is integrated into the cathode of the PEMFC to alleviate gas starvation due to water flooding. The impact of pump speed on dynamic loading is also examined. Furthermore, obstacle devices are incorporated into the cell flow channel to enhance mass transfer. The effects of flow channel configuration under the exhaust gas recirculation mode on cell performance and dynamic characteristics are investigated as well. Our findings indicate that blockage of the gas transport channel due to accumulated liquid water is the primary issue during the loading process of PEMFCs. The cathode recirculation pumps set at 600 rpm improves cell performance by 30.6% compared to the dead-ended mode at 1000 mA·cm−2. However, excessively high pump speeds lead to an increase in voltage undershoot during the loading process. Exhaust gas recirculation is found to significantly alleviate gas starvation and water flooding during the cyclic loading process and the cathode recirculation is the primary mitigation strategy for improving dynamic characteristics.

A novel control scheme to mitigate temperature profile variation in a SOFC System

Maintaining temperature profile in a solid oxide fuel cell (SOFC) is one of the essential issues that need to be addressed during the load tracking. In this paper, a novel control scheme with three-loop structure is developed to mitigate SOFC temperature profile variation. This scheme can simply implemented by three variable-speed blowers. The first blower is an anode recirculation blower used to maintain anode inlet temperature, the second one is a cathode recirculation blower used to maintain cathode inlet temperature, and the last one is an air blower used to maintain cathode outlet temperature. Compared with other simple schemes with only one-loop or two-loop structure, the scheme with three-loop structure achieves the best performance in mitigating SOFC internal temperature profile variation and guaranteeing a safe margin on operation limits. With a 10% system load step drop, the three-loop control scheme presents the best performance in rapidity and stability, with a much smaller overshoot, a much shorter settling time, and the smallest ITAE criterion. The SOFC temperature profile variation is also smallest (deviating within 4.7 K). Time-dependent temperature gradient is much less than 3.0 K/min. Temperature gradient is significant less than 10 K/cm. Steam to carbon ratio is also kept more than 2.0. Furthermore, the three-loop control scheme can perform high system efficiency, which reaches 53.9% at the 90% load. As a result, the three-loop SOFC temperature control scheme is robust in safe and efficient operation.

High-potential control for durability improvement of the vehicle fuel cell system based on oxygen partial pressure regulation under low-load conditions

In the state-of-the-art high-power self-humidifying proton exchange membrane fuel cell (PEMFC) systems for vehicles, the high potential and low water production at idle or low load conditions strongly cause corrosion and decay of key materials and thus reduce durability. Therefore, the control technology of system-level durability requires an innovative design. Cathode recirculation is beneficial in alleviating the above unfavorable factors from the perspective of regulating oxygen and vapor partial pressure. This paper presents a pioneering study on the dynamics and control of cathode recirculation in vehicle high-power self-humidifying PEMFC system under low load conditions. First, a control-oriented dynamic model of the vehicle PEMFC system with a cathode recirculation loop is developed and the steady-state and dynamic performance is verified with experimental data from a 120 kW system. Active control of the intake component is achieved by re-feeding the reacted cathode gas to the air compressor outlet through a recirculation pump. On this basis, a high-potential controller based on oxygen partial pressure regulation is designed in combination with the dynamics of cathode recirculation. Results show that the designed dynamic fuzzy logic segmented proportional integral derivative controller with feedforward compensation achieves the optimal high-potential control effect by managing the oxygen partial pressure under variable low load conditions. It not only has excellent anti-disturbance ability but also effectively reduces the dynamic response time, transient overshoot, and steady-state error to satisfy the rapid and stable voltage output. Finally, the concomitant effect of humidification brought by the implementation of the optimal high-potential controller is analyzed, and the results show that the proton membrane is completely humidified.

Концептуальная проработка энергоустановки на основе твердооксидных топливных элементов мощностью до 5000 Вт

<b>Object and purpose of research. </b>The purpose of this work performed by SET Branch of KSRC was to develop the principal technological layout and main design solutions for a breadboard SOFC-based power plant with the capacity of up to 5000 W. This work relied on existing Russian and international experience in design of SOFC-based power plants. <br><b> Subject matter and methods.</b>In-depth review and mathematical analysis of latest Russian and international experience in design of SOFC-based power plants enabled justification of a whole class of promising designs, as well as identification of main approaches to development of breadboard SOFC-based power plant (capacity up to 5000 W) suggested by KSRC SET Branch. <br><b>Main results. </b>The study analyses calorific processes and design and technological solutions implemented in Russian and foreign SOFC-based power plants. It also suggests a technological layout for a breadboard SOFC-based power plant (capacity up to 5000 W) developed by KSRC SET Branch. <br><b> Conclusion. </b>The mathematical models developed and verified in this study enabled the analysis of the most efficient SOFCbased power plant design, BlueGen by Ceramic Fuel Cells Limited (CFCL). as well as identification of further ways to the improvement of promising calorific designs for the power plants based on planar SOFCs. This study also investigated the effect of pre-reforming, anode and cathode recirculation and volt-ampere properties of the fuel cell stack upon its weight, size and thermodynamic efficiency. Another result of this work was the development of control algorithms for suggested power plant, with efficiency demonstration of the longest and the most complex among them, involving a number of limitations for startup/shutdown conditions. Finally, the paper explains design principles of 500–5000 W SOFC-based power plant family suggested by KSRC SET Branch.

Effect of cathode recirculation on high potential limitation and self-humidification of hydrogen fuel cell system

The high potential at the start and stop of the stack, as well as the flooding and film drying phenomena that occur during operation, will seriously cause the performance degradation of PEMFC. In this study, the effects of the cathode cycle on the high-voltage limiting strategy and water management characteristics of the hydrogen fuel cell system under different operating conditions was measured using the orthogonal test and the AC impedance measurement methods. The experimental results show that by reducing the fresh airflow and increasing the amount of air recirculation, the oxygen concentration can be reduced and the single piece voltage can always be kept below 0.8 V under a small load. Air recirculation improves the relative humidity at the cathode inlet close to the humidifier, thereby improving the output performance of the stack and having better dynamic characteristics. Furthermore, air recirculation can also enhance the water removal capacity of the stack outlet under high power operation with less pump power, thereby significantly improves the consistency of the distribution of oxygen along the flow channel and increasing the durability of the stack. The conclusion can provide more specific guidance for fuel cell performance improvement and control optimization.

Comparison of self-humidification effect on polymer electrolyte membrane fuel cell with anodic and cathodic exhaust gas recirculation

Self-humidification with anode/cathode recirculation was found to be an alternative in the place of external humidifier according to the previous studies in the last decade, however, there is still a lack of quantitative experimental and theoretical comparison between them. In this study, the dynamic PEM fuel cell system model with anodic and cathodic exhaust gas recirculation is proposed and related model calibration and verification are carried out by measured output voltage, gaseous pressure and mole concentration during the dynamic process. The model simulation shows quite good agreement with the experimental result and source of error is discussed. Based on developed model, it is simulated that the humidification effect is much better for cathode recirculation compared with anode recirculation, resulting in performance improvement, especially under the condition of low inlet RH. However, this improved humidification effect is weakened, considering the structure limit of recirculation pump and gas channel in this case. Besides, the power consumption of cathode recirculation pump is found to be much higher than that of anode recirculation. As a result, the anode recirculation is found to be a better solution considering the energy efficiency and mechanical structure limit. In contrast, cathode recirculation conducts better humidification effect, which is more suitable for the short-time low-inlet-humidity working conditions.

Performance Evaluation of an SOFC-GT Hybrid System With Ejectors for the Anode and Cathode Recirculations

The ejectors used for the fuel cell recirculation are more reliable and low cost in maintenance than high-temperature blowers. In this paper, an anode and cathode recirculation scheme, equipped with ejectors, was designed in a solid oxide fuel cell-gas turbine (SOFC-GT) hybrid system. The ejector model, SOFC model, and other component models and the validation were conducted to investigate the performance of the hybrid system with anode and cathode ejectors. The geometric parameters of the ejectors were designed to perform the anode and cathode recirculation loops according to the design conditions of the hybrid system with a blower-based recirculation loop. The cathode ejector geometries are much larger than the anode ejector. In addition, the sensitivity analysis of the primary fluid for the standalone anode and cathode ejectors is investigated. The results show that the ejector can recirculate more secondary fluid by reducing the ejector outlet pressure. Then, the anode and cathode ejectors were integrated into the SOFC-GT hybrid system. A blower gets involved downstream, and the compressor is necessary to avoid high expensive cost of redesigning compressor. The off-design and dynamic performance were characterized after integrating the anode and cathode ejectors into the hybrid system. The dynamic and off-design performances show that the designed ejectors are effectively integrated into the anode and cathode recirculation loops to replace the blower-based recirculation loops. The safety range of relative fuel flow rate is 0.62–1.22 in the fixed rotational speed strategy, and it is 0.53–1.1 in the variable rotational speed strategy. The variable rotational speed strategy can ensure higher system efficiency, which is more than 61% at a part-load condition.

Thermodynamic analysis of a solid oxide fuel cell jet hybrid engine for long-endurance unmanned air vehicles

The turbine-less jet engine is promising due to the compressor driven by a motor and the high combustor exit temperature. It is a key problem that how to provide electric power for the motor. In this paper, a novel solid oxide fuel cell (SOFC) jet hybrid engine with anode and cathode recirculation is proposed and investigated where the SOFC power system is instead of the turbine to provide power for the compressor. For the system, compressed air entering into SOFC cathode channel is preheated by cathode recirculation exhaust. The gases reform in the reformer, which are air, fuel and steam from the anode recirculation exhaust. Reformed gases are provided for the SOFC anode channel. SOFC produces electric power to drive the compressor by a motor. Then, SOFC exhaust enters the combustor. The combustor exhaust expands and produces propulsion power in the nozzle. In order to evaluate and improve the performance of the SOFC jet hybrid engine, the thermodynamic analysis model is built. The energy and exergy analysis are accomplished. The effect of pressure ratio and flight altitude on the performance of the hybrid engine are studied. In addition, sensitivity analysis is accomplished. The preliminary results are as follows: (1) The thermal efficiency, the specific thrust and the specific impulse of the engine could reach to 57.60%, 790.00 N/(kg⋅s−1) and 4132 s under design conditions. (2) The sum of the relative exergy destruction ratio of the SOFC, the combustor, the nozzle and the exhaust is over 85%. (3) The total pressure recovery coefficient of the SOFC has a strong influence on the performance of the SOFC jet hybrid engine.

Effect of different operating strategies for a SOFC-GT hybrid system equipped with anode and cathode ejectors

Ejector technology is introduced to perform the anode and cathode recirculation loops with low maintenance costs and high reliability. Four different operating strategies were designed for the novel solid oxide fuel cell-gas turbine hybrid system with anode and cathode ejectors to keep high efficiency and safety at a part-load operating condition. The part-load characteristics under different operating strategies were compared according to the simulation results. The comparison results show that the operating strategy has great effect on the part-load performance of the hybrid system with anode and cathode ejectors. Maintaining the SOFC operating temperature with variable speed operation has a great significance on the system efficiency. Moreover, fuel utilization, turbine inlet temperature, fuel cell temperature should be controlled and monitored to guarantee safely operating. Specifically, a concept of monitoring the temperature difference between anode and cathode channel is proposed. It can effectively avoid huge fuel cell temperature differences and compressor surge. Therefore, case 4 is an effective and appropriate operating strategy, which adjusts the primary fuel flow rate of SOFC, rotational speed, assistant fuel flow rate, compressor/turbine bypass flow rate to maintain turbine inlet temperature and temperature differences between anode and cathode.

Study on voltage clamping and self-humidification effects of pem fuel cell system with dual recirculation based on orthogonal test method

Durability and reliability are still major challenges of vehicular polymer electrolyte membrane fuel cell (PEMFC) systems. With exhaust gas recirculation on both the anode and cathode sides, two important functions can be achieved: the voltage clamping in low current density, and the self-humidification without any external humidifiers. The former restrains catalyst decay in small load working conditions, and the latter is beneficial for improving the cold-start ability. In this study, dynamic performances and stable characteristics of a fuel cell system with dual exhaust gas recirculation are firstly experimentally studied using an orthogonal test method. System parameters, including humidification temperature of cathode external humidifier, fresh air stoichiometric ratio (SR), current density, cathode and anode recirculation pump speeds, are regarded as key factors in the experiments based on the testing conditions of the test-bench. Two four-factor and three-level orthogonal tables are designed, and the effects of key factors on system performance indices (average cell voltage, relative humidity (RH) at cathode inlet, high frequency resistance (HFR), oxygen concentrations at cathode inlet and outlet, and the concentration difference between these two positions) are investigated. Results show that: (1) with the cathode recirculation, the cell voltage can be reduced in low current densities by coordinately adjusting the recycled gas flow and reducing fresh air SR; (2) with the dual recirculation, the fuel cell membrane can be well hydrated, and system performance only shows 3% reduction compared with a system with an external humidifier; (3) the difference between the oxygen molar concentration at the inlet and outlet of cathode gas channels becomes small using dual recirculation.

Control strategy design for a SOFC-GT hybrid system equipped with anode and cathode recirculation ejectors

The ejectors equipped for anode and cathode recirculation loops are more reliable and low-cost in maintenance than blowers in a solid oxide fuel cell-gas turbine hybrid system. However, an effective control system is one of the technical challenges associated with the development of the hybrid system. In present work, a novel control strategy was designed to restrict the hybrid system equipped with anode and cathode recirculation ejectors to a safe and feasible zone. Six control loops were carried out to control the vital parameters including power, rotation speed, fuel utilization, anode and cathode inlet temperature, and turbine inlet temperature. At the same time, the performance of another control system without anode inlet temperature control loop was also investigated. A comparison results of the two control systems reveal that both anode and cathode inlet temperature loops are necessary. If there is no anode inlet temperature control loop, the system efficiency decreases by 0.18%, the fuel cell inlet temperature differences between anode and cathode increases by 9 K and the compressor surge margin decreases by 1.8% at 95% system load. It not only causes a little reduction in efficiency but also may cause some latent dangers of the hybrid system at a part load condition. Consequently, the control system with all six control loops is effective and appropriate.

Study on control strategy for a SOFC-GT hybrid system with anode and cathode recirculation loops

The solid oxide fuel cell-gas turbine (SOFC-GT) hybrid power generation system offers a possible technology of high efficiency combined with low emissions. Due to its highly coupled system and complex dynamic characteristics, several vital parameters such as fuel cell temperature, thermal stress, steam-to-carbon, turbine inlet temperature, and compressor surge margin must be controlled and restricted to a feasible zone to guarantee the safe operation of the hybrid system. A novel control strategy was carried out for the SOFC-GT hybrid system with anode and cathode recirculation loops. In the SOFC thermal management system, two control loops were introduced to keep anode temperature and cathode temperature. The behaviors of the control system were compared with the control system without an anode temperature control loop during load following. The results reveal that the anode channel temperature and cathode channel temperature should both be controlled to avoid huge temperature differences and temperature gradient in SOFC. The designed control system 1 in this study for the hybrid system is feasible and effective.

Experimental study on dual recirculation of polymer electrolyte membrane fuel cell

Durability and start-up ability in sub-zero environment are two technical bottlenecks of vehicular polymer electrolyte membrane (PEM) fuel cell systems. With exhaust gas recirculation on the anode and cathode side, the cell voltage at low current density can be reduced, and the membrane can be humidified without external humidifier. They may be helpful to prolong the working lifetime and to promote the start-up ability. This paper presents an experimental study on a PEM fuel cell system with anodic and cathodic recirculation. The system is built up based on a 10 kW fuel cell stack, which consists of 50 cells and has an active area of 261 cm2. A cathodic recirculation pump and a hydrogen recirculation pump are utilized on the cathode and anode side, respectively. Key parameters, e.g., stack current, stack voltage, cell voltage, air flow, relative humidity on the cathode side, oxygen concentration at the inlet and outlet of the cathode side, are measured. Results show that: 1) with a cathodic recirculation the system gets good self-humidification effect, which is similar to that with an external humidifier; 2) with a cathodic recirculation and a reduction of fresh air flux, the cell voltage can be obviously reduced; 3) with an anodic recirculation the cell voltage can also be reduced due to a reduction in the hydrogen partial pressure, the relative humidity on the cathode side is a little smaller than the case with only cathode recirculation. It indicates that, for our stack the cathodic recirculation is effective to clamp cell voltage at low current density, and a self-humidification system is possible with cathodic recirculation. Further study will focus on the dynamic model and control of the dual recirculation fuel cell system.

Simulation of an Innovative Startup Phase for SOFC Hybrid Systems Based on Recompression Technology: Emulator Test Rig

This paper presents a novel startup approach for solid oxide fuel cell (SOFC) hybrid systems (HSs) based on recompression technology. This startup approach shows a novel method of managing a complete plant to obtain better performance, which is always also a difficult task for equipment manufactures. The research activities were carried out using the HS emulator rig located in Savona (Italy) and developed by the Thermochemical Power Group (TPG) of the University of Genoa. The test rig consists of three integrated technologies: a 100 kWe recuperated microturbine modified for external connections, a high temperature modular vessel necessary to emulate the dimensions of an SOFC stack, and, for air recompression, a turbocharger necessary to increase fuel cell pressure (using part of the recuperator outlet flow) as required for efficiency increase and to manage the cathodic recirculation. It was necessary to develop a theoretical model in order to prevent abnormal plant startup conditions as well as motivated by economic considerations. This transient model of the emulator rig was developed using Matlab®-Simulink® environment to study the time-dependent (including the control system aspects) behavior during the entire system (emulator equipped with the turbocharger) startup condition. The results obtained were able to demonstrate that the HS startup phase can be safely managed with better performance developing a new control logic. In detail, the startup phase reported in this paper shows that all important parameters were always inside acceptable operating zones (surge margin kept above 1.1, turbine outlet temperature (TOT), and fuel flow maintained lower than 918.15 K and 7.7 g/s, respectively).

Effects of SOFC Exhaust Gas Recirculation on Performance of Solid Oxide Fuel Cell-Gas Turbine Hybrid System Utilizing Renewable Fuels

A solid oxide fuel cell-gas turbine (SOFC-GT) hybrid system run on renewable fuels, such as biogas and ethanol, is an interesting power generation technology due to its high efficiency. To avoid a carbon formation on the anode catalyst by the direct feed of hydrocarbon fuels to SOFC, a pre-reformer is needed to integrate with the SOFC system for hydrogen production. As the steam reformer and air preheater are units that require high external energy input, the efficient heat management of the SOFC system is necessary. Regarding the SOFC system, the anode and cathode recirculations are employed to recover high-quality waste heat for reformate gas and gas turbine cycle, affecting the electrical and thermal system efficiency. In this study, a steam reformer and SOFC-GT hybrid system with anode and cathode recirculations is investigated. The aim of this work is to analyze the effects of the anode and cathode recirculations on the performance of SOFC-GT systems. The mathematical model of SOFC, gas turbine, and auxiliary units based on mass and energy balances under steady-state operation is employed for this study. The parametric analysis of key operating parameters, such as fuel utilization, steam to carbon ratio and operating pressure, on the system performance is performed. The results indicate that the SOFC hybrid system with the anode recirculation can reduce the supplied heat for the external steam reformer; on the other hand, it degrades the gas turbine performance due to a decrease in the turbine inlet temperature. When considering the hybrid system with the cathode recirculation, it is found that the efficiency of the SOFC-GT system is higher. However, the turbine inlet temperature is extremely high, leading to the cooling problem of a micro turbine.

Thermodynamic and Exergy Analysis of Reversible Solid Oxide Cell Systems

Reversible solid oxide cell (SOC) technology could enable efficient electrical energy storage of renewable energy, enhancing sustainability and efficiency of current energy systems, reducing at the same time equipment requirement. SOCs are devices able of converting electrical energy into chemical fuels, through electrolysis, and chemical fuels into electricity, through electrochemical oxidation. Although a SOC can theoretically work in both modes, the same can not be directly said for a system. Basically, high conversion system efficiencies largely depends on the addition of specific bottoming cycles, usage of specific components or specific operative conditions. For instance, it is expected that a SOEC system requires a large heat transfer equipment to bring liquid water from ambient (15 °C - 25 °C ) to the cell operative temperature (700 °C - 800 °C). Conversely, SOFC major requirements would concern gas and air pre-heating. Looking at pressurized systems, while SOFC systems are mainly based on gas compression, electrolysis includes compression of liquid water to a large extent. Although the same process is performed, gas and liquid compression have a completely different energy demands for different equipment (pump and compressor). In literature, reversible systems have not yet been widely investigated. Consequently, there is the need to address possible configurations and evaluate their conversion performances. The main goal of the present work is to address differences and similarities among simple SOFC and SOEC systems, based on hydrogen oxidation and steam electrolysis. In order to evaluate which are important variables in determining SOFC and SOEC conversion efficiencies, how do variables affect system efficiencies, and which are crucial auxiliary components in a SOFC or SOEC system, the present work proposes a thermodynamic analysis. Both energetic and exergetic point of views are adopted to estimate system and components performances. Systems are modelled with the flowsheet programming tool Aspen Plus. Since this software does not include a built-in model of SOFC and SOEC, these are devised ad hoc. Scope of the analysis is to identify useful operative regimes for a reversible system, comparing SOFC and SOEC system based on the same variables. Temperature, pressure, reactant composition, utilization factor and current density emerge as key variables that determine system efficiency. SOC systems are studied at atmospheric (~ 1 bar) and high pressure conditions (10 bar), as well as at different current densities. For a 90% inlet molar fraction of hydrogen and steam for SOFC and SOEC mode respectively, a fuel cell stack efficiency of ~44% at 800 °C, with a power output of 12 MW is achieved. At atmospheric conditions, overall system energy and exergy efficiencies are ~40% and ~41% in SOFC mode. However, at 10 bar, for the SOFC system, auxiliary compression work is larger than produced. Thus in absence of a bottoming cycle like a Gas Turbine (SOFC-GT systems have been shown to be highly efficient) the pressurisation does not result into a feasible system. Current densities and stack area obtained in the SOFC mode are used as inputs for SOEC simulations. Power requirements of SOEC are 18273 kW (~ 1bar) and 19890 kW (10 bar), with voltages equal to 1.142 V and 1.243 V, at atmospheric and pressurized conditions respectively. The corresponding atmospheric SOEC configuration energy and exergy efficiencies of 75.35% and 85.54% are obtained. A similar energy efficiency (76.50%) but lower exergy efficiency (81.44%) is observed for the pressurized SOEC system. Higher exergy losses in the pressurized case are because of the higher current density leading to higher exergy losses within the SOEC. Another contribution appears to be from the wasted fraction of input exergy linked to a higher system outlet temperature and pressure. Heat recovery especially of the water stream also contributes to the exergy losses. SOEC pressurization can anyway be an advantage from a system point of view. For instance in case of liquid fuel production, syngas pressurization could be replaced by pressurized electrolysis, enhancing overall efficiency. This is due to the lower energy intensity associated with liquid water pressurization, compare to gas. Looking at system operation and obtained efficiencies, it is feasible to design a reversible SOFC / SOEC system. Overall, multiplying SOFC and SOEC efficiencies, total reversible system energy efficiency is around 30%. This value could be increased with system modifications such as varying the SOFC current density, or introducing SOFC anode and cathode recirculation.

MCFC-based CO2 capture system for small scale CHP plants

Abstract Carbon dioxide emissions into the atmosphere are considered among the main reasons of the greenhouse effect. The largest share of CO 2 is emitted by power plants using fossil fuels. Nowadays there are several technologies to capture CO 2 from power plants' exhaust gas but each of them consumes a significant part of the electric power generated by the plant. The Molten Carbonate Fuel Cell (MCFC) can be used as concentrator of CO 2 , due to the chemical reactions that occurs in the cell stack: carbon dioxide entering into the cathode side is transported to the anode side via CO 3 = ions and is finally concentrated in the anodic exhaust. MCFC systems can be integrated in existing power plants (retro fitting) to separate CO 2 in the exhaust gas and, at the same time, produce additional energy. The aim of this study is to find a feasible system design for medium scale cogeneration plants which are not considered economically and technically interesting for existing technologies for carbon capture, but are increasing in numbers with respect to large size power plants. This trend, if confirmed, will increase number of medium cogeneration plants with consequent benefit for both MCFC market for this application and effect on global CO 2 emissions. System concept has been developed in a numerical model, using AspenTech engineering software. The model simulates a plant, which separates CO 2 from a cogeneration plant exhaust gases and produces electric power. Data showing the effect of CO 2 on cell voltage and cogenerator exhaust gas composition were taken from experimental activities in the fuel cell laboratory of the University of Perugia, FCLab, and from existing CHP plants. The innovative aspect of this model is the introduction of recirculation to optimize the performance of the MCFC. Cathode recirculation allows to decrease the carbon dioxide utilization factor of the cell keeping at the same time system CO 2 removal efficiency at high level. At anode side, recirculation is used to reduce the fuel consumption (due to the unreacted hydrogen) and to increase the CO 2 purity in the stored gas. The system design was completely introduced in the model and several analyses were performed. CO 2 removal efficiency of 63% was reached with correspondent total efficiency of about 35%. System outlet is also thermal power, due to the high temperature of cathode exhaust off gases, and it is possible to consider integration of this outlet with the cogeneration system. This system, compared to other post-combustion CO 2 removal technologies, does not consume energy, but produces additional electrical and thermal power with a global efficiency of about 70%.

Ionic composition and transport mechanisms in microbial desalination cells

Microbial desalination cell (MDC) offers a new and sustainable approach to desalinate saltwater by directly utilizing the electrical power generated by bacteria during organic matter oxidation. The successful MDC development relies on the fundamental understanding of the interactions and removal mechanisms of different ion species present in saline water or wastewater, but there is limited understanding of ion transport mechanisms in MDCs and potential membrane fouling/scaling during treatment of wastewater and saline water. In this study, we investigated the transport behavior of multiple ions in MDCs and the effects of ionic composition on system performance and membrane scaling and fouling. The results showed that the presence of sparingly soluble cations in saltwater negatively affected MDC power generation and desalination. Membrane characterization revealed that the majority of such ions precipitated on the ion exchange membrane surface and caused membrane scaling. Anions such as Br− and SO42− with Na+ as counter-ion did not show significant effects on system performance. Sharp pH changes were observed during MDC operation, which resulted in the inhibited MDC anode microbial activity and the accelerated formation of alkaline precipitations on both sides of the cation exchange membrane. An anode–cathode recirculation approach was proved to be effective to solve such problems and improved the desalination rate by 152% and the electron harvest rate by 98%.

외부 개질형 평판형 고체 산화물 연료전지 시스템 구성법에 따른 효율특성

본 연구에서는 외부 개질기에 열원을 공급하기 위한 시스템 내에 가용한 열에너지의 활용 및 확보에 대한 해석을 위해서 외부 개질기를 연계한 평판형 SOFC 시스템의 해석 모델을 구축하고자 한다. 이러한 해석을 위한 모델 구축을 위해 Matlab simulink® 기반의 ThermoLib module을 사용하였으며, 구축된 해석 모델을 통하여 시스템의 성능 향상을 위한 구성 기법에 대해서 연구를 하였다. 시스템 구성 방법은 기존 시스템의 layout을 바꾸기 위해 공기극 출구가스 재순환 및 외부개질기와 촉매연소기를 통합한 개질반응시스템 적용, 개질기에 공급되는 혼합연료의 예열, 연료극 출구가스의 응축을 통한 연료 농도 향상 등을 고려하였다. 시뮬레이션의 해석 결과에서는 SOFC 시스템에 있어서 일반 연소기를 적용한 기준 시스템에 비하여 촉매 연소기를 사용한 시스템의 전기 효율이 12.13% 향상되었으며, 연료극 출구 가스를 응축시켜 버너로 연소시킨 시스템에서는 열효율이 76.12%로 가장 높았다.