Performance of a PEMFC system integrated with a biogas chemical looping reforming processor: A theoretical analysis and comparison with other fuel processors (steam reforming,partial oxidation and auto-thermal reforming)

In this work, the performance of a PEMFC (proton exchange membrane fuel cell) system integrated with a biogas chemical looping reforming processor is analyzed. The global efficiency is investigated by means of a thermodynamic study and the application of a generalized steady-state electrochemical model. The theoretical analysis is carried out for the commercial fuel cell BCS 500W stack. From literature, chemical looping reforming (CLR) is described as an attractive process only if the system operates at high pressure. However, the present research shows that advantages of the CLR process can be obtained at atmospheric pressure if this technology is integrated with a PEMFC system. The performance of a complete fuel cell system employing a fuel processor based on CLR technology is compared with those achieved when conventional fuel processors (steam reforming (SR), partial oxidation (PO) and auto-thermal reforming (ATR)) are used. In the first part of this paper, the Gibbs energy minimization method is applied to the unit comprising the fuel- and air-reactors in CLR or to the reformer (SR, PO, ATR). The goal is to investigate the characteristics of these different types of reforming process to generate hydrogen from clean model biogas and identify the optimized operating conditions for each process. Then, in the second part of this research, material and energy balances are solved for the complete fuel cell system processing biogas, taking into account the optimized conditions found in the first part. The overall efficiency of the PEMFC stack integrated with the fuel processor is found to be dependent on the required power demand. At low loads, efficiency is around 45%, whereas, at higher power demands, efficiencies around 25% are calculated for all the fuel processors. Simulation results show that, to generate the same molar flow-rate of H₂ to operate the PEMFC stack at a given current, the global process involving SR reactor is by far much more energy demanding than the other technologies. In this case, biogas is burnt in a catalytic combustor to supply the energy required, and there is a concern with respect to CO₂ emissions. The use of fuel processors based on CLR, PO or ATR results in an auto-thermal global process. If CLR based fuel processor is employed, CO₂ can be easily recovered, since air is not mixed with the reformate. In addition, the highest values of voltage and power are achieved when the PEMFC stack is fed on the stream coming from SR and CLR fuel processors. When a H₂ mixture is produced by reforming biogas through PO and ATR technologies, the relative anode overpotential of a single cell is about 55 mV, whereas, with the use of CLR and SR processes, this value is reduced to ∼37 and 24 mV, respectively. In this way, CLR can be seen as an advantageous reforming technology, since it allows that the global process can be operated under auto-thermal conditions and, at the same time, it allows the PEMFC stack to achieve values of voltage and power closer to those obtained when SR fuel processors are used. Thus, efforts on the development of fuel processors based on CLR technology operating at atmospheric pressure can be considered by future researchers. In the case of biogas, the CO₂ captured can produce additional economical benefits in a ‘carbon market’.     

Performance evaluation of a fuel processing system based on membrane reactors technology integrated with a PEMFC stack

The development of fuel cells is promised to enable the distributed generation of electricity in the near future. However, the infrastructure for production and distribution of hydrogen, the fuel of choice for fuel cells, is currently lacking. Efficient production of hydrogen from fuels that have existing infrastructure (e.g., natural gas, gasoline or LPG) would remove a major drawback to use fuel cells for distributed power generation.The aim of this paper is to define the better operating conditions of an innovative hydrogen generation system (the fuel processing system, FP) based on LPG steam reforming, equipped with a membrane shift reactor, and integrated with a PEMFC (Proton Exchange Membrane Fuel Cell) stack of 5 kWel.With respect to the conventional hydrogen generation systems, the use of membrane reactors (MRs) technology allows to increase the hydrogen generation and to simplify the FP-PEMFC plant, because the CO removal system, needed to reduce the CO content at levels required by the PEMFC, is avoided.Therefore, in order to identify the optimal operating conditions of the FP-PEMFC system, a sensitivity analysis on the fuel processing system has been carried out by varying the main operating parameters of both the reforming reactor and the membrane water gas shift reactor. The sensitivity analysis has been performed by means of a thermochemical model properly developed.Results show that the thermal efficiency of the fuel processing system is maximize (82.4%, referred to the HHV of fuels) at a reforming temperature of 800 °C, a reforming pressure of 8 bar, and an S/C molar ratio equal to 6. In the nominal operating condition of the PEMFC stack, the FP-PEMFC system efficiency is 36.1% (39.0% respect to the LHV).     

Combined thermal characteristics analysis of steam reforming and combustion for 5 kW domestic PEMFC system

A natural gas-based steam reformer for a domestic polymer electrolyte membrane fuel cell (PEMFC) system is thermodynamically analyzed with a special focus on the heat supply mechanism, which is critical to the endothermic steam reforming process. The interdependence of the reforming and combustion processes is evaluated through a characteristic study of heat transfer from the heat source to the reforming zone. Premixed combustion patterns may be affected by the inclusion of controlling means such as a metal fiber screen or burner placement. In this study, we attempted to enhance reforming performances of a reformer embedded in a 5 kW in-house PEMFC through modification of the combustion pattern by varying the type and placement of the burner and other operating conditions. Reforming input conditions such as steam-carbon ratio (SCR) and fuel distribution ratio (FDR) are also analyzed to quantify the overall performance such as thermal efficiency and fuel conversion rate. In our experiments involving three types of combustors—cylindrical metal fiber burner, flat type metal fiber burner and nozzle-mixing burner—the operating conditions are set so that the SCR and FDR are in the range 3.0–4.0 and 0.4–0.7, respectively. It is found that the cylindrical metal fiber burner at an appropriate location could improve thermal efficiency up to 79% by 10%, compared to other devices. This maximum thermal efficiency output is obtained with 0.63 FDR, which eventually yields 99% hydrogen conversion rate when using a cylindrical metal fiber burner, while the other burners produce 95% conversion. These outputs substantiate that the overall efficiency is strongly affected by an appropriate control for uniform temperature distribution on the catalyst layer.     

Performance of an anode-supported solid oxide fuel cell with direct-internal reforming of ethanol

A theoretical study of a solid oxide fuel cell (SOFC) fed by ethanol is presented in this study. The previous studies mostly investigated the performance of ethanol-fuelled fuel cells based on a thermodynamic analysis and neglected the presence of actual losses encountered in a real SOFC operation. Therefore, the real performance of an anode-supported SOFC with direct-internal reforming operation is investigated here using a one-dimensional isothermal model coupled with a detailed electrochemical model for computing ohmic, activation, and concentration overpotentials. Effects of design and operating parameters, i.e., anode thickness, temperature, pressure, and degree of ethanol pre-reforming, on fuel cell performance are analyzed. The simulation results show that when SOFC is operated at the standard conditions (V = 0.65 V, T = 1023 K, and P = 1 atm), the average power density of 0.51 W cm⁻² is obtained and the activation overpotentials represent a major loss in the fuel cell, followed by the ohmic and concentration losses. An increase in the thickness of anode decreases fuel cell efficiency due to increased anode concentration overpotential. The performance of the anode-supported SOFC fuelled by ethanol can be improved by either increasing temperature, pressure, degree of pre-reforming of ethanol, and steam to ethanol molar ratio or decreasing the anode thickness and fuel flow rate at inlet. It is suggested that the anode thickness and operating conditions should be carefully determined to optimize fuel cell efficiency and fuel utilization.     

Efficiency improvement of a PEMFC power source by optimization of the air management

The increase of fuel cell (FC) system efficiency requires an optimal management of all its sub-systems. This paper discusses and analyses the possibilities of the improvement of the performance of a proton exchange membrane fuel cell (PEMFC) power source via the implementation of an optimal operating design of the air management sub-system. The steady-state PEMFC operation has been taken into account. This work takes into account a numerical and mixed technique for modeling of FC sub-systems, based on moving least squares approach. In has been analyzed the opportunity of using an adjustable backpressure valve. The work proposes a numerical optimization of air management, computing the optimal speed of the compressor and the optimal throttle opening, in correlation with an imposed operating point of PEMFC system. A Constrained Optimization By Linear Approximation (COBYLA) algorithm has been implemented to solve the optimization problem. The results are useful to design the control of PEMFC system and to develop an optimal configuration of it.     

A comparison of temperature distribution in PEMFC with single-phase water cooling and two-phase HFE-7100 cooling methods by numerical study

Thermal management has been considered as one of the critical issues in proton exchange membrane fuel cell (PEMFC). Key roles of thermal management system are maintaining optimal operating temperature of PEMFC and diminishing temperature difference over a single fuel cell and stack. Severe temperature difference causes degradation of performance and deterioration of durability, so understanding temperature distribution inside a single fuel cell and stack is crucial. In this paper, two-phase HFE-7100 cooling method is suggested for PEMFC thermal management and investigated regarding temperature change inside a fuel cell. Also, the results are compared to single-phase water cooling method. Numerical study of temperature distribution inside a single PEMFC is conducted under various conditions for the two different cooling methods. Fuel cell model considering mass transfer, electrochemical reaction and heat transfer is developed.The result indicates that two-phase HFE-7100 cooling method has an advantage in temperature maintenance and temperature uniformity than single-phase water cooling method, especially in high current density region. It is also revealed that the cell temperature is less dependent on system load change with two-phase cooling method. It indicates that the fuel cell system with two-phase cooling method has high thermal stability. In addition, the effect of coolant flow rate and coolant inlet pressure in two-phase HFE-7100 cooling method are discussed. As a result, two-phase cooling method showed reliable cooling performance even with low coolant flow rate and the system temperature increased as coolant pressure rose.     

燃料重整固体氧化物燃料电池发电系统性能分析及多变量参数优化

以燃料重整的固体氧化物燃料电池发电系统为研究对象,通过数值模拟方法对固体氧化物燃料电池发电系统的性能、(火用)损、(火用)效率以及多变量运行参数优化进行了分析。研究结果表明：重整反应中燃料利用系数、电池工作温度、水碳比、电堆电流密度等参数对系统性能影响显著;电堆工作在不同电流密度下都有其对应的最佳工作温度、最佳燃料利用系数工况点;水碳比会改变重整反应产氢量,从而影响电化学反应速率,空气加热器的(火用)损所占份额最大;优化后的系统效率及(火用)效率为0.480 9和0.462 6,效率提升约4%。     

Design of an integrated fuel processor for residential PEMFCs applications

KIER has been developing a novel fuel processing system to provide hydrogen rich gas to residential PEMFCs system. For the effective design of a compact hydrogen production system, each unit process for steam reforming and water gas shift, has a steam generator and internal heat exchangers which are thermally and physically integrated into a single packaged hardware system. The newly designed fuel processor (prototype II) showed a thermal efficiency of 78% as a HHV basis with methane conversion of 89%. The preferential oxidation unit with two staged cascade reactors, reduces, the CO concentration to below 10 ppm without complicated temperature control hardware, which is the prerequisite CO limit for the PEMFC stack. After we achieve the initial performance of the fuel processor, partial load operation was carried out to test the performance and reliability of the fuel processor at various loads. The stability of the fuel processor was also demonstrated for three successive days with a stable composition of product gas and thermal efficiency. The CO concentration remained below 10 ppm during the test period and confirmed the stable performance of the two-stage PrOx reactors.     

Hydrogen energy system analysis for residential applications in the southern region of Algeria

The purpose of this paper is related to hydrogen energy system analysis for residential applications in Ghardaia (southern region of Algeria). This system is based on proton exchange membrane fuel cell (PEMFC) technology, which is supplied by fuel reforming process, for producing hydrogen fuel starting from natural gas. The exhaust heat is recovered by a Thermal Storage Tank (TST), which is used in an absorption sub-system as a generator for residential cooling system. The feasibility analysis of an absorption cooling device, using thermal energy of PEMFC sub-system, for a residence application located at the unit of applied research in renewable energy in Ghardaia has been studied and performances were analysed. Electrical and thermal powers generated by the PEMFC sub-system with variable electrical loads (Part Load Ratio) have been analysed. The feasibility study shows that using PEMFC for residential cooling in Ghardaia is a promising solution. It is shown that the temperature of the TST is sufficient to supply the absorption sub-system with a coefficient of performance equals to 0.72 and, the efficiency of the HES equals to 97%.     

Optimization of fuel cell system operating conditions for fuel cell vehicles

Proton exchange membrane fuel cell (PEMFC) technology for use in fuel cell vehicles and other applications has been intensively developed in recent decades. Besides the fuel cell stack, air and fuel control and thermal and water management are major challenges in the development of the fuel cell for vehicle applications. The air supply system can have a major impact on overall system efficiency. In this paper a fuel cell system model for optimizing system operating conditions was developed which includes the transient dynamics of the air system with varying back pressure. Compared to the conventional fixed back pressure operation, the optimal operation discussed in this paper can achieve higher system efficiency over the full load range. Finally, the model is applied as part of a dynamic forward-looking vehicle model of a load-following direct hydrogen fuel cell vehicle to explore the energy economy optimization potential of fuel cell vehicles.     

Energy efficiency analysis of an integrated glycerin processor for PEM fuel cells: Comparison with an ethanol-based system

The aim of this work is to analyze energetically the use of glycerin as the primary hydrogen source to operate a proton exchange membrane fuel cell. A glycerin processor system based on its steam reforming is described departing from a previous process model developed for ethanol processing. Since about 10% w/w of glycerin is produced as a byproduct when vegetable oils are converted into biodiesel, and due to the later is increasing its production abruptly, a large glycerin excess is expected to oversaturate the market. The reformed stream contains mainly H₂ but also CO, CO₂, H₂O and CH₄. As CO is a poison for PEM fuel cell type, a stream purification step is previously required. The purification subsystem consists of two water gas shift reactors and a CO preferential oxidation reactor to reduce the CO levels below 10 ppm. The reforming process is governed by endothermic reactions, requiring thus energy to proceed. Depending on the system operation point, the energy requirements can be fulfilled by burning an extra glycerin amount (to be determined), which is the minimal that meets the energy requirements. In addition a self-sufficient operation region can be distinguished. In this context, the water/glycerin molar ratio, the glycerin steam reformer temperature, the system pressure, and the extra glycerin amount to be burned (if necessary) are the main decision variables subject to analysis. Process variables are calculated simultaneously, updating the composite curves at each iteration to obtain the best possible energy integration of the process. The highest net system efficiency value computed is 38.56% based on the lower heating value, and 34.71% based on the higher heating value. These efficiency values correspond to a pressure of 2 atm, a water/glycerin molar ratio of 5, a glycerin steam reformer temperature of 953 K, and an extra glycerin amount burned of 0.27 mol h⁻¹. Based on the main process variables, suitable system operation zones are identified. As in practice, most PEM fuel cells operate at 3 atm, optimal variable values obtained at this condition are also reported. Finally, some results and aspects on the system performance of both glycerin and ethanol processors operated at 3 atm are compared and discussed.     

Hydrogen from ethanol: Theoretical optimization of a PEMFC system integrated with a steam reforming processor

In this paper the energetic optimization of a proton exchange membrane fuel cell integrated with a steam reforming system using ethanol as fuel is analysed. In order to obtain high hydrogen production, a thermodynamic analysis of the steam reforming process has been carried out and the optimal operating conditions has been defined. Moreover, the overall efficiency of the PEMFC-SR system has been investigated as a function of the fuel utilization factor and the effects of the anodic off-gas recirculation have been evaluated.     

A parametric comparison of three fuel recirculation system in the closed loop fuel supply system of PEM fuel cell

Anodic fuel recirculation system has a significant role on the parasitic power of proton exchange membrane fuel cell (PEMFC). In this paper, different fuel supply systems for a PEMFC including a mechanical compressor, an ejector and an electrochemical pump are evaluated. Furthermore, the performances of ejector and electrochemical pump are studied at different operating conditions including operating temperature of 333 K–353 K, operating pressure of 2 bar–4 bar, relative humidity of 20%–100%, stack cells number from 150 to 400 and PEMFC active area of 0.03 m²–0.1 m². The results reveal that higher temperature of PEMFC leads to lower power consumption of the electrochemical pump, because activation over-potential of electrochemical pump decreases at higher temperatures. Moreover, higher operating temperature and pressure of PEMFC leads to higher stoichiometric ratio and hydrogen recirculation ratio because the motive flow energy in ejector enhances. In addition, the recirculation ratio and hydrogen stoichiometric ratio increase, almost linearly, with increase of anodic relative humidity. Utilization of mechanical compressor leads to lower system efficiency than other fuel recirculating devices due to more power consumption. Utilization of electrochemical pump in anodic recirculation system is a promising alternative to ejector due to lower noise level, better controllability and wide range of operating conditions.     

Modelling of tubular-designed solid oxide fuel cell with indirect internal reforming operation fed by different primary fuels

Mathematical models of an indirect internal reforming solid oxide fuel cell (IIR-SOFC) fed by four different primary fuels, i.e., methane, biogas, methanol and ethanol, are developed based on steady-state, heterogeneous, two-dimensional and tubular-design SOFC models. The effect of fuel type on the thermal coupling between internal endothermic reforming with exothermic electrochemical reactions and system performance are determined. The simulation reveals that an IIR-SOFC fuelled by methanol provides the smoothest temperature gradient with high electrochemical efficiency. Furthermore, the content of CO₂ in biogas plays an important role on system performance since electrical efficiency is improved by the removal of some CO₂ from biogas but a larger temperature gradient is expected.Sensitivity analysis of three parameters, namely, a operating pressure, inlet steam to carbon (S:C) ratio and flow direction is then performed. By increasing the operating pressure up to 10 bar, the system efficiency increases and the temperature gradient can be minimized. The use of a high inlet S:C ratio reduces the cooling spot at the entrance of reformer channel but the electrical efficiency is considerably decreased. An IIR-SOFC with a counter-flow pattern (as based case) is compared with that with co-flow pattern (co-flow of air and fuel streams through fuel cell). The IIR-SOFC with co-flow pattern provides higher voltage and a smoother temperature gradient along the system due to superior matching between heat supplied from electrochemical reaction and heat required for steam reforming reaction; thus it is expected to be a better option for practical applications.     

A study on the integration of micro‐reformer and high‐ temperature PEM fuel cell

This study presented an integration platform for a methanol reformer and high‐temperature proton exchange membrane fuel cell (PEMFC). The methanol micro‐reformer was combined with the catalytic reaction section and reforming section, whereas the catalytic reaction section with Pt catalysis maintained the constant temperature environment for a reforming process. SRM reforming results showed that 74 to 74.9% hydrogen and 23.5 to 25.7% of carbon dioxide in the mixture product, and less than 2% of carbon monoxide, was produced. Using the reforming product of low carbon monoxide concentration and the highest methanol conversion rate, a micro reformer link with a fuel cell integration experiment was performed. Results showed a high temperature PEMFC with 3 to 4 W power output under methanol flow rates of 15 ml/hr. Due to the lower hydrogen pressure supplied from the micro reformer, the fuel cell power output may become unstable. © 2010 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley Online Library ( wileyonlinelibrary.com ). DOI 10.1002/htj.20322     

Thermodynamic study of integrated proton exchange membrane fuel cell with vapour adsorption refrigeration system

With the rising usage of fossil fuels, there is an urgent need to develop new technologies specifically based on renewable energy sources to power the vehicles running on fuel. A fuel cell is an electrochemical cell that is used to convert the chemical energy of a fuel directly to electric power. Fuels cells possess advantages such as smaller size, high efficiency, silent operation, etc. However, there can be significant variations in the size and power output of the fuel cells depending upon the application. The focus of this paper is to estimate the performance of an integrated system comprising of Polymer Exchange Membrane Fuel cell (PEMFC) and vapour adsorption refrigeration system to produce electric output and cooling effect simultaneously. The adsorption system in this study is based on activated carbon and methanol combination. The effect of operating parameters such as the operating temperature, current density and evaporator temperature on the energy and exergy efficiency of the system is presented. The study shows a remarkable improvement in the performance of the integrated system compared to PEMFC alone. The results show that the system energy and exergy efficiency decrease as the current density value increases. Maximum system energy and exergy efficiency of 63.01% and 29.88% are achieved. In addition, a maximum energy efficiency of 65.39% was reported at an evaporator temperature of 5 °C and a current density of 0.8 A/cm².     

Optimization and efficiency analysis of methanol SOFC-PEMFC hybrid system

In order to improve the power generation efficiency of fuel cell systems employing liquid fuels, a hybrid system consisting of solid oxide fuel cell (SOFC) and proton exchange membrane fuel cell (PEMFC) is proposed. Utilize the high temperature heat generated by SOFC to reform as much methanol as possible to improve the overall energy efficiency of the system. When SOFC has a stable output of 100 kW, the amount of hydrogen after reforming is changed by changing the methanol flow rate. Three hybrid systems are proposed to compare and select the best system process suitable for different situations. The results show that the combined combustion system has the highest power generation, which can reach 350 kW and the total electrical efficiency is 57%. When the power of the tail gas preheating system is 160 kW, the electrical efficiency can reach 75%. The PEM water preheating system has the most balanced performance, with the electric power of 300 kW and the efficiency of 66%.     

Exergy analysis on non-catalyzed partial oxidation reforming using homogeneous charge compression ignition engine in a solid oxide fuel cell system

Based on the recent improvements in high-temperature fuel cells, distributed power generation fuel cell system of small scale (～hundreds kilowatts) has been widely investigated. To improve the system efficiency, most developments focused on the fuel cell stack, but little was paid attention to the intrinsic exergy destructions at the other parts of a typical configuration. The main objective of this study is to investigate a feasibility of reducing the exergy destruction in the reforming process of fuel cell system, by using a homogeneous charge compression ignition (HCCI) engine as a replacement of existing reforming subsystems, i.e. steam methane reforming (SMR), partial oxidation (POX), or autothermal reforming (ATR), in a solid oxide fuel cell (SOFC) system. To do this, parametric studies with exergy analysis were conducted by using in-house 1-D SOFC and 0-D HCCI simulation models. In results, due to the work production from HCCI reforming engine in addition to the work of the fuel stack, it is demonstrated that HCCI-SOFC system has higher system efficiency than partial oxidation (POX) and autothermal reforming (ATR) systems, which use similar partial oxidation reaction for reformer operation. Furthermore, because of no requirement for catalyst, the HCCI system demonstrates wider operating range than that of POX and ATR systems. When compared to the steam methane reforming (SMR)-SOFC system, the HCCI-SOFC system has the lower total work but slightly higher exergetic system efficiency, mainly caused by large amount of heat exergy needed to operate endothermic reforming process in the SMR process. Based on our simulation data, the exergetic efficiency of the HCCI-SOFC system shows 6.0%, 2.1% and 0.4% higher than POX, ATR and SMR systems at the highest efficiency points of each strategy, while 5.5%, 5.8% and 3.8% higher than POX, ATR and SMR systems at 99% methane conversion points in each reformer, respectively.     

质子交换膜燃料电池堆的热力学分析

建立了质子交换膜燃料电池(PEMFC)堆的热力学分析模型,研究了运行温度、气体分压和阳极流量等工作参数对燃料电池堆能量效率和火用效率的影响。结果表明:对气体加压,能提高热力学能效率和火用效率;温度升高时,系统性能无明显变化;阳极流量增加时,系统的热力学能效率和火用效率有所降低。     

Performance analysis of a 5 kW PEMFC with a natural gas reformer

Power systems based on fuel cells have been considered for residential and commercial applications in energy Distributed Generation (DG) markets. In this work we present an experimental analysis of a power generation system formed by a 5 kW proton exchange membrane fuel cell (PEMFC) unit and a natural gas reformer (fuel processor) for hydrogen production. The performance analysis developed simultaneously the energy and economic viewpoints and enabled the determination of the best technical and economic conditions of this energy generation power plant, and the best operating strategies, enabling the optimization of the overall performance of the stationary cogeneration fuel cell unit. It was determined the electrical performance of the cogeneration system in function of the design and operational power plant parameters. Additionally, it was verified the influence of the activation conditions of the fuel cell electrocatalytic system on the system performance. It also appeared that the use of hydrogen produced from the natural gas catalytic reforming provided the system operation in excellent electrothermal stability conditions resulting in increase of the energy conversion efficiency and of the economicity of the cogeneration power plant.