Impact of alternating fuel feeding on a PEMFC stack durability

The integration of PEMFC for transport applications requires cost as well as system complexity reduction. A novel anode circuit architecture named “Alternating Fuel Feeding” (AFF) was developed. It combines on one hand, the benefits of the hydrogen recirculation and on the other hand, the simpleness of Dead-End Anode (DEA). A previous work demonstrated the benefits on the costs and on the anodic line weight. In the present work, investigations on stack durability, from the system scale to the cell scale, are performed during operation in two anode feeding modes: AFF and recirculation. The performances of stacks are evaluated along ageing tests, coupled with local measurements. At the end of life, postmortem analyses are performed on the aged cells. The degradation rates of stack performances are quite similar in both cases while more heterogeneous cell degradation are observed in recirculation mode along with the ageing according to a Fuel Cell Dynamic Load Cycle (FC-DLC).     

Impact of humidification by cathode exhaust gases recirculation on a PEMFC system for automotive applications

Reactant gases humidification, in most PEM fuel cell systems, is traditionally implemented to ensure both stack durability and superior performance. A cathode exhaust gases recirculation architecture allows to decrease the system volume compared to the passive humidifiers, which are classically used. Incorrect water management being responsible for irreversible degradations, a control of relative humidity at stack inlet thanks to the recirculation could be of great interest to limit their impact. In this work, investigations on performance and stability are performed during operation in recirculation mode, from the cell scale to the system scale. It was observed that high to medium recirculation ratios were able to stabilize and homogenize the cells voltages along the stack but performance was reduced due to oxygen dilution by nitrogen. Besides, large relative humidity ranges were achieved at stack inlet, which can vary from 25 to 85% and could be able to follow automotive dynamics.     

Design improvements for ammonia-fed SOFC systems through power rating,cascade design and fuel recirculation

In this paper, design strategies for improving electrical efficiency, thermal design and fuel utilization of an ammonia-fed SOFC are investigated. Three strategies are presented to improve system performances: (i) the introduction of an additional stack to distribute the power i.e. power rating, (ii) the evaluation of the anode off gasses recirculation and (iii) the use of the off gasses to operate a cascade stack (re-powering), where the anode flue gas is recuperated. A system design that integrates these new features is modelled with zero-dimension thermodynamic equations. The three strategies were evaluated for net system efficiency and the heat exchanger area as main design parameters. The power rating allows to reduce the heat exchanger surface while the recirculation and repowering are suitable to increase system efficiency. With an integration of the three solutions, it is possible to achieve an increase in net efficiency from 52.1% to 66% and a reduction in heat exchanger surface area of 67% compared to the reference design that does not consider any of the proposed design strategies.     

A literature study for DEA applied to energy and environment

This study systematically summarizes previous research efforts on Data Envelopment Analysis (DEA) applied to energy and environment in the past four decades, including concepts and methodologies on DEA environmental assessment. Industrial developments are very important for all nations in terms of their economic prosperities. A problem is that the development produces various pollutions on air, water and other types of contaminations, all of which are usually associated with our health problems and climate changes. Thus, it is necessary for us to consider how to make a balance between economic success and pollution mitigation to maintain a high level of social sustainability in the world. It is widely considered that DEA is one of the methodologies to examine the level of sustainability. This study examines a recent research trend on DEA applications from 1980s to 2010s. Nowadays, many researchers have paid serious attention on how to combat various difficulties in the areas of energy and environment. As a result, the number of articles on DEA applications on energy and environment has dramatically increased, particularly after 2000s. However, it is true that DEA has strengths and drawbacks in the applications. Therefore, it is very important for us to carefully use DEA for guiding large policy issues and business strategies such as the global warming and climate change. An underlying premise of this study is that technology innovation in engineering and natural science may solve various problems by linking it with political and managerial efforts. The use of DEA provides a methodological linkage among them, so enhancing the practicality in mitigating problems due to climate change and environmental pollutions. This literature study, along with a summary on conceptual and methodological developments, provides us with guidelines for our future research works on DEA on energy and environment issues.     

改变进气流量实现可调回流区位置的研究

提出了一种实现回流区可调的新方法，并对其特点进行了研究。根据流体力学的规律，实现了无移动部件和无高温部件(钝件)情况下回流区的可调性，满足不同油品正常燃烧时对回流区的要求，特别是在研究不同掺水率的“水包油”型乳化油的燃烧特性时，其特点更为突出。此研究还为工业应用提供了支持。实验和计算结果表明，此回流区可以适应不同油品的要求。     

Methodological comparison among radial,non-radial and intermediate approaches for DEA environmental assessment

This study compares three DEA approaches for environmental assessment, all of which are designed to examine the level of simultaneous achievement on economic prosperity and environmental protection, so measuring the degree of sustainability development. DEA, standing for Data Envelopment Analysis, has been widely applied for performance assessment in the past five decades. A new type of applications is referred to as “DEA environmental assessment” and it measures the performance of many organizations that utilize inputs to produce not only desirable outputs (e.g., electricity) but also undesirable outputs (e.g., CO₂). In the previous studies, DEA-based performance evaluation for environment assessment is methodologically classified into radial or non-radial category. Recently, a new “intermediate” approach, analytically locating between the radial and non-radial measures, has been proposed as the third alternative. A use of the intermediate approach has several unique features, all of which cannot be found in the radial and non-radial ones. The new approach measures the degree of unified inefficiency on each production factor and determines the level of total unified inefficiency from the average of the sum of these inefficiency scores. This study discusses the analytical features by comparing the intermediate approach with the radial and non-radial ones. The methodological comparison attempts to convey a message that DEA is indeed an important methodology, but not perfect. Rather, it is an approximation approach to examine the performance of various organizations. Many DEA applications on energy and environment often suffer from a methodological bias, implying that different approaches produce different empirical results. Thus, in guiding a large policy issue such as the global warming and climate change, it is necessary for us to compare several different approaches (e.g., models and concepts) to derive a reliable empirical suggestion. The importance of such a message is applicable to not only DEA but also the other types of empirical research in natural and social sciences. Therefore, this study discusses the methodological bias issue from the practicality of the three DEA approaches in assessing various concerns on energy and environment.     

A comparative study of single and dual ejector concepts for anodic recirculation system in high-performance vehicular proton exchange membrane fuel cells

The fuel supply system is a critical element in Proton Exchange Membrane Fuel Cell (PEMFC). In vehicular applications, an anode recirculation system (ARS) is required to return residual hydrogen to the fuel line. A 1D model of an ejector-based ARS was created and integrated with a vehicular PEMFC in this study. Therefore, two configurations, single-ejector-based and dual-ejector-based ARS, were developed to regulate the fuel supply and, thus, utilized to examine and conduct a comparative study of their advantages and disadvantages to the dynamic behavior of the stack. The use of ARS was found to improve stack performance and fuel utilization by delivering high stoichiometry, recirculation rate, and relative humidity. Contrary to a single-ejector-based ARS, whose use is restricted due to its limited working range, dual-ejectors can cover all fuel cell stack operating ranges. Sensitivity analysis revealed that primary pressure was the most significant parameter affecting ejector performance and flow characteristics.     

Real-time dynamic security assessment: fast simulation and modeling applied to emergency outage security of the electric grid

A major drawback of current dynamic security assessment (DSA) software used to analyze the transient and dynamic responses to a large number of potential power system disturbances in a transient time frame is the long simulation time required to complete the calculations, making it impractical for real-time applications. This paper describes an attempt to dramatically reduce the time for DSA calculations so that DSA analyses can be converted from offline studies to routine, online use to aid grid operators in their real-time controller analyses. Using a new distributed computational architecture among other techniques, a modified version of DSA is developed which performs calculations fast enough for real-time applications to energy management systems (EMS) that operate today's grid systems.     

Review of evaluation methodologies and influencing factors for energy efficiency of the iron and steel industry

To promote the sustainable development of the iron and steel industry (ISI), evaluation methods and influencing factors of energy efficiency have been studied in the past decades. A systematic review of these methodologies and influencing factors is presented in this paper. Initially, this paper states the development, current situation, and energy consumption of the world's ISI. Then, the definition of energy efficiency is described. After that, the evaluation methods are categorized as single‐factor methods (including these indicators of thermodynamic, thermos‐physical, thermo‐economic, and economic) and multifactor methods (including parametric and nonparametric methods). Furthermore, typical evaluation methods are highlighted. Thermal analysis method and exergy analysis method are routinely combined to optimize the energy utilization. Data envelopment analysis (DEA) and stochastic frontier analysis (SFA) are widely concerned and well developed to gain a better economic benefit. Malmquist index method is often associated with DEA to analyze the technical process and return to scale. Some emerging methods are proposed, for example, the three‐stage DEA method together with DEA and SFA. Finally, the influencing factors of energy efficiency in the ISI are discussed, including industrial structure, technological level, energy consumption structure, price, and so on. It is suggested that optimizing energy structure and improving technology level are effective measures to improve the energy efficiency in the ISI.     

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.     

Experimental analysis of an ejector for anode recirculation in a 10 kW polymer electrolyte membrane fuel cell system

For analyzing ejector's performance in the system, an ejector for a 10 kW polymer electrolyte membrane fuel cell (PEMFC) system was first designed, manufactured, and a 10 kW PEMFC system bench was built up. A proportional valve and PI pressure feedback control method were adopted to control the hydrogen supply and anode inlet pressure. During the test, performances between dead-ended anode (DEA) mode and ejector mode were compared. Ejector's performances in the system, i.e., volume flow recirculated ratio, difference pressure, dynamic responses of primary pressure, anode inlet pressure, and recirculated gas flow rate during the purge process and current variation condition, were investigated. The results show that pressure adjustment is accurate, continuous, and fast using the proportional valve and PI pressure feedback control method. The hydrogen consumption rate in the ejector mode can reduce from 5% to 10% compared with the rate in the DEA mode except for the stack current 5 A and 10 A conditions. For better water removal out of the anode channel in ejector mode, the maximum stack power increases from 5.11 kW (DEA mode) to 9.56 kW (ejector mode). Anode pressure surge caused by the purge valve switching enhances the ejector's recirculated performance significantly.     

电厂直接空冷系统热风回流的数值模拟

阐述了热风回流的产生机理及热回流率的概念,并对汽轮机空冷系统周围不同环境风场进行了数值模拟.通过对计算结果的分析,找出了空冷系统的不利风向,给出了热回流率随风速的变化规律,并提出了减少热风回流的措施:适当增加空冷平台上部四周风墙高度,或加快空冷平台边缘风机的转速.     

Two-dimensional simulation of cold start processes for proton exchange membrane fuel cell with different hydrogen flow arrangements

Proton exchange membrane (PEM) fuel cells with an off-gas recirculation anode (ORA) or dead-ended anode (DEA) are widely adopted in engineering. However, those two hydrogen flow arrangements may cause anodic water and nitrogen accumulation in comparison with the flow-through anode (FTA) mode, which causes significant performance degradation. In this paper, a two-dimensional cold-start model is developed with detailed consideration of water phase changes and the nitrogen crossover phenomenon. A simplified electrochemical module is built to calculate the current density distribution in the model. The simulation results are consistent with the experimental data at both subzero temperatures and normal operating temperatures. The effects of hydrogen flow arrangements, flow configurations, and startup strategies are investigated during startup process from subzero to normal operating temperatures. Much less ice is generated in counter-flow cases than in co-flow cases during constant current operation. A relatively lower startup voltage can effectively shorten the cold-start process and enhance the cold-start capacity for the PEM fuel cell. The ORA mode has the best hydrogen flow arrangement due to its general abilities, including higher hydrogen utilization efficiency, higher anodic nitrogen tolerance, better output performance and better startup capability.     

Design and development of modular fuel cell stacks for various applications

Polymer electrolyte membrane fuel cell (PEMFC) stacks are conventionally made by assembling a large numbers of cells together connected electrically in series. The number of cells and the area of the electrodes determine the capacity of such a stack. One of the reasons for non-proliferation of the fuel cell systems in many applications is due to the high cost involved in making these units. Cost reduction of the fuel cell components, fuel cell stacks and fuel cell system are being extensively pursued. One of the options for cost reduction is thro’ fabricating the fuel cells in smaller scale industrial units as their overheads are low. To attract small scale entrepreneurs (SSE) the capital investment has to be minimum. If compact fuel cell stacks of size ranging from 200 to 300 W can be produced comfortably, the SSE's can be encouraged to set up manufacturing units and they can become original equipment manufacturer (OEM) supplier to system integrators who could develop fuel cell systems of various capacities by simply connecting these compact stacks electrically in series or parallel depending on the end use. Such modular architecture would reduce the cost and improve the reliability of manufacture and increase the range of applications. This modular construction not only allows the materials and manufacturing technologies for components and stacks for uniformity but also suitable for homogenous large volume production. The geometry of the stacks allows easy installation even in crowded or compact areas. Centre for Fuel Cell Technology (CFCT) has demonstrated PEMFC stacks of 1–3 kW capacities. These stacks are single units with fixed voltage and current capabilities. CFCT is now embarked on a program to explore the possibility of introducing modular architecture for various applications of fuel cell systems. In this context, CFCT has recently developed a 250 W module which can be cooled either by air or water. The present paper discusses the concept and design of a modular architecture.     

Controller design for polymer electrolyte membrane fuel cell systems for automotive applications

Continuous developments in Proton Exchange Membrane Fuel Cells (PEMFC) make them a promising technology to achieve zero emissions in multiple applications including mobility. Incremental advancements in fuel cells materials and manufacture processes make them now suitable for commercialization. However, the complex operation of fuel cell systems in automotive applications has some open issues yet. This work develops and compares three different controllers for PEMFC systems in automotive applications. All the controllers have a cascade control structure, where a generator of setpoints sends references to the subsystems controllers with the objective to maximize operational efficiency. To develop the setpoints generators, two techniques are evaluated: off-line optimization and Model Predictive Control (MPC). With the first technique, the optimal setpoints are given by a map, obtained off-line, of the optimal steady state conditions and corresponding setpoints. With the second technique, the setpoints time profiles that maximize the efficiency in an incoming time horizon are continuously computed. The proposed MPC architecture divides the fast and slow dynamics in order to reduce the computational cost. Two different MPC solutions have been implemented to deal with this fast/slow dynamics separation. After the integration of the setpoints generators with the subsystems controllers, the different control systems are tested and compared using a dynamic detailed model of the automotive system in the INN-BALANCE project running under the New European Driving Cycle.     

A combustion concept for oxyfuel processes with low recirculation rate – Experimental validation

Oxyfuel combustion is a technology for Carbon Capture & Storage from coal fired power plants. One drawback is the large necessary amount of recirculation of cold flue gases into the combustion chamber to avoid inadmissible high flame temperatures. The new concept of Controlled Staging with Non-stoichiometric Burners (CSNB) makes a reduction of the recirculation rate possible without inadmissible high flame temperatures. This reduction promises more compact boiler designs. We present in this paper experiments with the new combustion concept in a 3 × 70 kW natural gas combustion test rig with dry flue gas recirculation of 50% of the cold flue gases. The new concept was compared to a reference air combustion case and a reference oxyfuel combustion case with recirculation of 70% of the cold flue gases. FTIR emission spectroscopy measurements allowed the estimation of spectral radiative heat fluxes in the 2–5.5 μm range. The mixing of the gases in the furnace was good as the burnout and the emissions were comparable to the reference cases. The flame temperatures of the CSNB case could be controlled by the burner operation stoichiometry and were also similar to the reference cases. The heat flux in the furnace through radiation to the wall was higher compared to the oxyfuel reference case. This is an effect of the lowered recirculation rate as the mass flow out of the furnace and therefore the sensible heat leaving the furnace decreases. The higher oxygen consumption with lower recirculation rate could be compensated by a lower furnace stoichiometry. This was possible due to better burnout with increased oxygen concentrations in the burner. The results prove that a reduction of the flue gas recirculation rate in oxyfuel natural gas combustion from 70% down to 50% is possible while avoiding inadmissible high flame temperatures with the concept of Controlled Staging with Non-stoichiometric Burners.     

A new energy efficiency evaluation solution for ethylene cracking furnace based on integrated TOPSIS-DEA model

Ethylene-cracking furnace has the highest energy consumption of all the devices used in the ethylene industry. The accurate energy-efficiency evaluation of an ethylene-cracking furnace is therefore of great significance for energy saving and productivity gain in ethylene production. Traditionally, a data envelopment analysis (DEA) model has been used to evaluate the energy efficiency. However, ethylene-cracking furnace also involves a multi-index and multiworking-condition energy-efficiency evaluation problem that results from there being multiple material flows, multiple energy flows, and multiple production loads. Therefore, this paper proposes a new energy efficiency evaluation solution for an ethylene-cracking furnace that is based on the integrated technique for order preference by similarity to ideal solution (TOPSIS)-DEA model. First, from the perspective of material and energy flows, an energy-efficiency vector (EEV) is designed and integrated into the DEA model to meet the demand for multi-index evaluation. Second, to eliminate interference of changes in working condition on energy efficiency, double virtual energy-efficiency benchmarks (DVEEBs) are built using the relationship between EEV, operating conditions, and nonoperational factors, based on the established cracking model. Finally, the rearranged values of the EEV is solved by the TOPSIS method and then evaluated by the DEA model. The actual numerical results show that the proposed solution can support energy-efficiency evaluation of an ethylene-cracking furnace subject to the multiple indexes and multiple working conditions. Specifically, the interference of changes in the working condition on the evaluation results can be reduced greatly which ensures the accuracy and objectivity of the energy-efficiency evaluation results.     

DEA window analysis for environmental assessment in a dynamic time shift: Performance assessment of U.S. coal-fired power plants

This study discusses a new use of window analysis for DEA environmental assessment in a time horizon where DEA stands for Data Envelopment Analysis. The data sets on environmental protection are often structured by time series. In applying DEA to environmental assessment, it is necessary for us to examine a frontier shift between different periods because it indicates a technology progress on desirable and undesirable outputs. An important feature of the proposed approach is that it incorporates the concept of natural and managerial disposability into the computational framework of DEA and extends the two disposability concepts in a time horizon. To capture the frontier shift, this study proposes a new type of DEA window analysis for environmental assessment. This study applies the proposed DEA window analysis to a data set on U.S. coal-fired power plants during 1995–2007. The application finds that the coal-fired power plants have gradually paid attention to environmental protections under Clean Air Act (CAA). Consequently, their performance under managerial disposability has increased from 1996 to 2007. This indicates the importance of CAA and regulation on industrial pollutions. Thus, it is necessary for the United States to extend the scope of CAA for controlling the amount of CO₂ emission because current regulation has a limited policy influence on the source of global warming and climate change in our modern society.     

简单而有效的废气再循环装置

采用废气再循环能够降低柴油机NOx排放。介绍了一个简单而有效的废气再循环装置,简要列举了该装置装在发动机台架上的部分试验结果,为更好地利用废气再循环技术减少柴油机NOx排放有借鉴作用。     

New theoretical model for convergent nozzle ejector in the proton exchange membrane fuel cell system

A new theoretical model for the convergent nozzle ejector in the anode recirculation line of the polymer electrolyte membrane (PEM) fuel cell system is established in this paper. A velocity function for analyzing the flow characteristics of the PEM ejector is proposed by employing a two-dimensional (2D) concave exponential curve. This treatment of velocity is an improvement compared to the conventional 1D “constant area mixing” or “constant pressure mixing” ejector theories. The computational fluid dynamics (CFD) technique together with the data regression and parameter identification methods are applied in the determination of the velocity function's exponent. Based on the model, the anode recirculation performances of a hybrid PEM system are studied under various stack currents. Results show that the model is capable of evaluating the performance of ejector in both the critical mode and subcritical mode.