A rapid start-up strategy for polymer electrolyte fuel cells at subzero temperatures based on control of the operating current density

Gwak, G.,Ju, H. Pergamon Press ; Elsevier Science Ltd 2015 International journal of hydrogen energy Vol.40 No.35

In a previous study, we numerically described the three distinct stages of ice evolution during the cold start of a polymer electrolyte fuel cell (PEFC), namely, the freezing, undersaturated, and melting stages. Based on our numerical observations, we propose an efficient start-up strategy for achieving a rapid cell temperature rise while simultaneously mitigating the rate of ice accumulation within a cell. The key to this cold-start strategy involves raising the operating current of the PEFC in the undersaturated stage, which accelerates the cell temperature rise without any further ice accumulation. Using a three-dimensional, transient cold-start model, we numerically demonstrate that raising the cell current in the undersaturated stage is very effective and significantly improves the cold-start behavior of a PEFC. In contrast, increasing the cell current in the freezing stage has a negative impact on the PEFC cold start in that the oxygen reduction reaction causes the water production rate to increase, leading to more rapid ice growth. This ultimately results in cold-start failure and the deterioration of the porous electrode structure. This study clearly illustrates that optimization of the cold-start operation is key to improving the cold-start performance, while also pointing to the importance of the roles played by PEFC cold-start modeling and the simulations used to search for an optimum cold-start strategy.

Comparison of numerical simulation results and experimental data during cold-start of polymer electrolyte fuel cells

Ko, Johan,Ju, Hyunchul Elsevier 2012 APPLIED ENERGY Vol.94 No.-

<P><B>Highlights</B></P><P>► We newly developed a multiphase, transient PEFC cold start model. ► Lower current density startup results in a longer cold start operation time. ► Higher startup temperature shows longer cold start operation. ► Cell shutdown time is extended with lower initial water content.</P> <P><B>Abstract</B></P><P>A multiphase transient model is developed to investigate key physical and transport phenomena during the startup of a polymer electrolyte fuel cell (PEFC) from subzero temperatures. The proposed PEFC cold start model rigorously considers ice/frost formation and evolution in the cathode electrode, and their complicated interactions with heat transport, mass transport, and electrochemical reactions. The proposed cold start model is extensively validated against experimental data measured under various key cold start conditions such as startup temperature, startup current density, and initial water content in the membrane. The model predictions are in good agreement with the experimental data, demonstrating the validity and accuracy of the cold start model. In addition, detailed simulation results, including multidimensional contours, clearly elucidate the cold start behavior of a PEFC under different cell designs and operating conditions. We suggest that the proposed cold start model could aid in the development of optimal cold start strategies.</P>

Numerical investigation of cold-start behavior of polymer-electrolyte fuel-cells from subzero to normal operating temperatures - Effects of cell boundary and operating conditions

Gwak, G.,Ko, J.,Ju, H. Pergamon Press ; Elsevier Science Ltd 2014 International journal of hydrogen energy Vol.39 No.36

In this study, we performed transient cold-start simulations of polymer-electrolyte fuel-cells (PEFCs) under a wide range of PEFC operating temperatures, from subzero (-20 <SUP>o</SUP>C) to normal operating temperatures (80 <SUP>o</SUP>C). For these wide temperature range transient simulations, the cold-start model developed in a previous study is enhanced by including ice-melting phenomena and additional constitutive relationships. The model successfully predicts various stages of PEFC cold-starts, i.e., ice formation/growth, constant ice accumulation with undersaturated vapor, ice melting/membrane hydration, and membrane dehydration. In addition, the results for simulations performed under various cold-start operating conditions clearly address the effects of key cold-start factors such as cathode stoichiometry, external thermal boundary condition, inlet relative humidity, startup temperature, and co- and counter-flow configurations on PEFC cold-start behavior, especially ice freezing/melting and membrane hydration/dehydration processes. This numerical investigation attempts to develop effective cold-start strategies that can simultaneously suppress both ice formation/growth and anode dry-out, thereby ensuring rapid and stable startup of a PEFC from subzero temperatures.

궤도차량 엔진의 저온 시동성 개선 사례 연구

김성빈,성기창 국방기술품질원 2021 국방품질연구논집 Vol.3 No.2

The cold start requirement of most vehicle is -32℃. So most vehicle system provides starting aid devices as water preheating system and auxiliary power unit (single cylinder engine) to prevent battery discharging problem during long starting time. Cold start-ability is performed under several severe condition, limited starting times and limited operating time of starter, 8hrs maintain time under full soaking temp condition. Limited heating condition and so on those environments are clearly stated in the system-specifications. There are various approaches to improve cold start-ability. Preferentially combustion optimization is most easy approaching method through fuel injection quantity, pressure and multiple injection control and hardware point of view, we introduced the change of the oil storage position by the application of the Oil anti-drain valve in this work which can reduce the starting load by bringing up the effect of oil heating through the preheating condition. This improvement model was applied in engine development and improved outstanding achievement in cold start-ability and this is also applicable to similar systems as extendibility.

Large-scale cold-start simulations for automotive fuel cells

Jo, Ahrae,Lee, Sungho,Kim, Whangi,Ko, Johan,Ju, Hyunchul Elsevier 2015 International journal of hydrogen energy Vol.40 No.2

<P><B>Abstract</B></P> <P>In this study, the three-dimensional transient cold-start model is applied to a real-scale polymer electrolyte fuel cell (PEFC) geometry, and transient cold-start simulations are carried out from subzero to normal temperatures. In order to reduce the computational turnaround time involved for a large numerical mesh with millions of grid points, the cold-start code is parallelized for parallel computing. The simulation results clearly show the evolution of ice, water content, temperature, and current density contours at different cold-start stages, characterizing freezing, melting, hydration, and dehydration processes. In addition, the model predictions emphasize the beneficial influence of vapor-phase diffusion from the cathode catalyst layer (CL) to the gas-diffusion layer (GDL) during cold starts, which can contribute to reducing ice accumulation in the cathode CL. As the effect of vapor-phase diffusion is substantial, more ice is accumulated in the cathode GDL than in the cathode CL. The total amount of ice accumulated inside a cell is therefore not always proportional to the amount of ice in the cathode CL, depending on the vapor-phase diffusion strength.</P> <P><B>Highlights</B></P> <P> <UL> <LI> We studied cold-start behaviors of real-scale PEFC geometry. </LI> <LI> The total cumulative ice mass was even higher in low cathode stoichiometry. </LI> <LI> As the effect of vapor phase diffusion, ice is accumulated in the cathode GDL. </LI> <LI> The total amount of ice mass in the cathode CL is always not a good indicator. </LI> </UL> </P>

Improving the cold-start capability of polymer electrolyte fuel cells (PEFCs) by using a dual-function micro-porous layer (MPL): Numerical simulations

Ko, J.,Kim, W.G.,Lim, Y.D.,Ju, H. Pergamon Press ; Elsevier Science Ltd 2013 INTERNATIONAL JOURNAL OF HYDROGEN ENERGY - Vol.38 No.1

A novel micro-porous layer (MPL) is designed to enhance the cold-start capability of a polymer electrolyte fuel cell (PEFC). The concept of designing an MPL is to expand the ice storage capacity of the electrode into the MPL region. We impose proton conduction capability and the oxygen reduction reaction (ORR) kinetic activity on the MPL via controlling the platinum (Pt) loading, ionomer fraction and weight ratio of Pt to the carbon support (wt%<SUB>Pt-C</SUB>) of the MPL. Therefore, the MPL is dual-functional, and can work as a typical MPL for normal PEFC operations and as a part of the cathode catalyst layer (CL) for cold-start operations. Three-dimensional (3-D) cold-start simulations are carried out by using a 3-D cold-start model developed in a previous study [1]. The detailed simulation results clearly suggest that the cold-start operational time can be extended significantly using a dual-function MPL, and the extended time is directly proportional to the pore volume of the MPL for ice storage. This study provides a new strategy to enhance the cold-start capability of a PEFC by properly designing and optimizing the MPL.

자동차용 고분자전해질형연료전지 스택에서의 막-전극접합체 설계인자가 저온시동에 미치는 영향성 연구

곽건희,고요한,주현철 한국수소및신에너지학회 2012 한국수소 및 신에너지학회논문집 Vol.23 No.1

This paper presents a three-dimensional, transient cold-start polymer electrolyte fuel cell (PEFC) model to numerically evaluate the effects of membrane electrode assembly (MEA) design and cell location in a PEFC stack on PEFC cold start behaviors. The cold-start simulations show that the end cell experiences significant heat loss to the sub-freezing ambient and thus finally cold-start failure due to considerable ice filling in the cathode catalyst layer. On the other hand, the middle cells in the stack successfully start from -30℃ sub-freezing temperature due to rapid cell temperature rise owing to the efficient use of waste heat generated during the cold-start. In addition, the simulation results clearly indicate that the cathode catalyst layer (CL) composition and thickness have an substantial influence on PEFC cold-start behaviors while membrane thickness has limited effect mainly due to inefficient water absorption and transport capability at subzero temperatures.

study on characteristics and control strategies of cold start operation for improvement of harmful exhaust emissions in SI engines

Duk-Sang Kim,Young-Joon Park,Seang-Wock Lee,Yong-Seok Cho 대한기계학회 2008 JOURNAL OF MECHANICAL SCIENCE AND TECHNOLOGY Vol.22 No.1

Emission regulations for automobiles have become more stringent and the improvement of emission during cold start has been a major key issue to meet these regulations. Among many kinds of factors that affect cold start operation, ignition timing is crucial to improve emission characteristics due to the influence on exhaust gas temperature. Recent progress in variable valve timing allows optimized valve event strategies under various ranges of engine operating conditions including cold start. This study investigates effects of ignition and exhaust valve timing on exhaust gas temperature, combustion stability and emission characteristics through cold start bench tests of an SI engine. Experimental results show that exhaust valve timings and ignition timings significantly affect exhaust gas temperature and stability of engine operation under cold start condition. Exhaust valve timing also affects CO and NOx emission due to changes in residual gas fraction of the combustion chamber. Ignition timing mainly affects exhaust gas temperature and HC emission. A control strategy, advanced exhaust valve timing and retarded ignition, is plausible in order to achieve reduction of exhaust emission while maintaining stability under cold start operation of SI engines.

SI엔진의 냉간초기 HC 저감을 위한 포집백 및 컨트롤러 개발

김종일,차정연,손정배 조선대학교 환경연구소 2001 環境硏究 Vol.17 No.1

It is well known that unburned hydrocarbons are abundantly emitted into the atmosphere ate cold start in SI engine. Many nations tend to enforce regulations of emission much more strictly. This study was conducted to develop a system which reduces HC emissions at cold start using the device of temporary storage and recombustion after analyzing the emission characteristics of hydrocarbons during the cold start. The exhaust gas measurements are focused upon the first 30 seconds of operation after starting, with the engine and coolant initially at ambient temperature because this period has a major effect on HC emissions, over the FTP cycle. The engine examined is a 4-cylinder, 16-valve SI engine. K type thermocouples attached on the O_2 sensor, the catalytic converter and the coolant, and each sensor for the engine sere connected to the A/D board to find the parameters for storage time and reconbustion mode. The emissions were initially enriched in light fuel alkanes and depleted in heavy aromatic species. It was decided that the maximum storage time was 30 seconds at cold start. The time for storage was decreased with warmed start because the O_2 sensor's conversion at warmed start is much faster than that at cold start. The recombustion mode was selected by using the rate of throttle valve, engine speed, and operation conditions of purge solenoid valve to avoid engine torque fluctuation.

Effect of the Fuel Injection Strategy on First-cycle Firing and Combustion Characteristics during Cold Start in a TSDI Gasoline Engine

Q. FAN,J. BIAN,H. LU,L. LI,J. DENG 한국자동차공학회 2012 International journal of automotive technology Vol.13 No.4

The first firing cycle is very important during cold-start for all types of spark ignition engines. In addition, the combustion characteristics of the first firing cycle affect combustion and emissions in the following cycles. However, the firstcycle fuel-air mixing, combustion and emissions generation within the cylinder of a two-stage direct-injection (TSDI) engine during cold start is not completely understood. Based on the total stoichiometric air-fuel ratio and local richer mixture startup strategy, the first-cycle firing and combustion characteristic at cold start were investigated in a two-stage direct injection (TSDI) gasoline engine. In addition, the effects of the first injection timing, second injection timing, 1st and 2nd fuel injection proportion and total excess air ratio on the in-cylinder pressure, heat release rate and accumulated heat release were analyzed on the basis of a cycle-by-cycle analysis. It is shown that a larger 2nd fuel injection amount and later 2nd injection timing are more beneficial to the firing of the first cycle in the case of a total excess air ratio of 1.0. The optimum 1st and 2nd injection timing fuel injection proportions are 120oCA ATDC during the intake stroke, 60oCA BTDC during the compression stroke and 1:1. In addition, the firing boundary is a 2nd injection timing later than 90oCA BTDC during the compression stroke in the case of the 1st injection timing from 60oCA to 180oCA ATDC during an intake stroke and involves a 1st and 2nd fuel injection proportion of 1:1 and an excess air ratio of 1.0. The study provides a detailed understanding of cold-start combustion characteristics and a guide for optimizing the reliable first-cycle firing at cold start.