Cold Start Process Research Articles

Quantitative analysis on cold start process of a PEMFC stack with intake manifold

To systematically explore the low-temperature operating characteristics of polymer electrolyte membrane fuel cell (PEMFC) stack, a three-dimensional PEMFC stack model with intake manifold is developed in this study. The characteristics of different cold start modes in the stack are compared and analyzed. The distribution and transmission characteristics of water, ice, and heat in each cell of the stack are analyzed in detail. The location of water accumulation in each cell of the stack is also explored. Finally, finite difference sensitivity is calculated for the cumulated charge transfer density to quantify the effects of operating parameters on the cold start process at low temperature. And how these parameters affect the operation of the PEMFC stack at low temperature is investigated. The results show that inconsistency exists in stack operation due to the position particularity of the intermediate cell. Irreversible heat is the main heat source for the cold start of the stack, and the cathode catalyst layer is the main heat-generating component. The heat production proportion of cathode catalyst layer can reach 90%, which decreases with the increment of current density and the running time, especially for the edge cell. The initial ionomer water content is most sensitive to the cold start process of the stack, followed by the porosity of cathode catalyst layer. These parameters are sensitive to the cold start process mainly because of the change in volumetric exchange current density and oxygen concentration.

Numerical study on the influence of waveform channel and related design parameters on the cold start of proton exchange membrane fuel cell

In order to enable the proton exchange membrane fuel cell to be used under outdoor conditions, research on the cold start of the fuel cell is essential. In this paper, numerical simulation is used to study the influence of fuel cell cathode channel geometry and porosity on the fuel cell cold start process. By analyzing the internal parameters of the fuel cell at different times, including current density, ice volume fraction, temperature and water content, the optimal fuel cell design was obtained. From a comprehensive comparison, although the current density and temperature rise of the ordinary wave channel fuel cell with higher porosity increase slowly at the beginning of the cold start, the decline process and icing process of it are also relatively slow. Therefore, a fuel cell with an ordinary wave channel and a higher porosity may perform better at cold start. This research contributes to the application and commercialization of fuel cells in a wider range of practical conditions.

Control-Oriented Modeling of Purging Process and Cold Start of Proton-Exchange Membrane Fuel Cell

AbstractThe main purpose of the shutdown purge is to reduce the residual moisture inside the stack and improve the water storage capacity in the cold start process and thereby reduce icing damage. ...

The Second Discovery from the COCONUTS Program: A Cold Wide-orbit Exoplanet around a Young Field M Dwarf at 10.9 pc

Abstract We present the identification of the second discovery from the COol Companions ON Ultrawide orbiTS (COCONUTS) program, the COCONUTS-2 system, composed of the M3 dwarf L 34-26 and the T9 dwarf WISEPA J075108.79−763449.6. Given their common proper motions and parallaxes, these two field objects constitute a physically bound pair with a projected separation of 594″ (6471au). The primary star COCONUTS-2A has strong stellar activity (Hα, X-ray, and ultraviolet emission) and is rapidly rotating (Prot = 2.83 days), from which we estimate an age of 150–800 Myr. Comparing equatorial rotational velocity derived from the Transiting Exoplanet Survey Satellite (TESS) light curve to spectroscopic vsini, we find that COCONUTS-2A has a nearly edge-on inclination. The wide exoplanet COCONUTS-2b has an effective temperature of T eff = 434 ± 9 K, a surface gravity of log g = 4.11 − 0.18 + 0.11 dex, and a mass of M = 6.3 − 1.9 + 1.5 M Jup based on hot-start evolutionary models, leading to a mass ratio of 0.016 − 0.005 + 0.004 for the COCONUTS-2 system. COCONUTS-2b is the second coldest (after WD 0806−661B), and the second widest (after TYC 9486-927-1 b) exoplanet imaged to date. Comparison of COCONUTS-2b’s infrared photometry with ultracool model atmospheres suggests the presence of both condensate clouds and non-equilibrium chemistry in its photosphere. Similar to 51 Eri b, COCONUTS-2b has a sufficiently low luminosity ( log ( L bol / L ⊙ ) = − 6.384 ± 0.028 dex) to be consistent with the cold-start process that may form gas-giant (exo)planets, though its large separation means that such formation would not have occurred in situ. Finally, at a distance of 10.9 pc, COCONUTS-2b is the nearest imaged exoplanet to Earth known to date.

Effect of catalyst layer mesoscopic pore-morphology on cold start process of PEM fuel cells

Water transport is of paramount importance to the cold start of proton exchange membrane fuel cells (PEMFCs). Analysis of water transport in cathode catalyst layer (CCL) during cold start reveals the distinct characteristics from the normal temperature operation. This work studies the effect of CCL mesoscopic poremorphology on PEMFC cold start. The CCL mesoscale morphology is characterized by two tortuosity factors of the ionomer network and pore structure, respectively. The simulation results demonstrate that the mesoscale morphology of CCL has a significant influence on the performance of PEMFC cold start. It was found that cold-starting of a cell with a CCL of less tortuous mesoscale morphology can succeed, whereas starting up a cell with a CCL of more tortuous mesoscale morphology may fail. The CCL of less tortuous pore structure reduces the water back diffusion resistance from the CCL to proton exchange membrane (PEM), thus enhancing the water storage in PEM, while reducing the tortuosity in ionomer network of CCL is found to enhance the water transport in and the water removal from CCL. For the sake of better cold start performance, novel preparation methods, which can create catalyst layers of larger size primary pores and less tortuous pore structure and ionomer network, are desirable.

Characteristics of Cold Start Behavior of PEM Fuel Cell with Metal Foam as Cathode Flow Field under Subfreezing Temperature

ABSTRACT Cold start has been realized as an important issue for PEMFC in its global commercialization. As is well-known, the conventional “channel-rib” type flow field will surely lead to several problems, such as water accumulation under the rib, resulting in slow water removal and serious ice blockage in the pores. In recent years, with the fast development of metal foam materials, metal foam has been recognized as a promising alternative replacement of the conventional “channel-rib” type flow field. In this study, a cold start model of PEMFC is established under sub-zero temperature. The coupled transport processes of heat, mass and charge in PEMFC under various subzero temperatures and startup modes are simulated. The results show that ice formation is slower in the PEMFC with metal foam as cathode flow field in cold start process, due to the superior performance of metal foam in water removal and uniform distribution of gas supply. However, the excellent thermal conductivity of metal foam could also result in faster heat loss from PEMFC. Furthermore, ice formation is highly dependent on the supercooled water transfer behavior, though small amount of supercooled water is maintained in the cell. This work aims to provide a systematic analysis for the cold start behavior of PEMFC with metal foam flow field.

Research on water and heat management in the cold start process of proton exchange membrane fuel cell with expanded graphite bipolar plate

Water storage space and heat capacity are important factors affecting the cold start ability of fuel cell, and heat capacity is affected by materials. In the present study, the lumped static model and two-dimensional dynamic prediction model of the fuel cell with expanded graphite bipolar plates were established. The relationship between water storage space, heat production, heat capacity and the maximum temperature rise is analyzed based on the theoretical calculation model, and the cold start ability is also predicted. The calculation results show that the heat capacity of the bipolar plate is 1.3 J/K·cm2, which accounts for the largest proportion of 34% of all components. With the help of the heat generated by the air compressor, the reactor can be started in 100 s at −30 ℃ and 1.2A/cm2. The temperature difference in the reactor during cold start-up is about 2℃, which proves the reasonability of the isothermal assumption in the static model. The dynamic impedance spectroscopy experiment results show that the super cooled water icing during the cold start will cause the gas transmission impedance to increase rapidly, indicating that the icing mainly occurs at the pores or interfaces inside the catalyst layer and the gas diffusion layer during the cold start.

Development of a mechanism-dynamic-selection turbulent premixed combustion model with application to gasoline engine combustion and emissions simulation

In order to overcome the drawback (tuning reaction rate coefficients) of using a single mechanism in the chemical-kinetics-based turbulent premixed combustion modelling approach, a mechanism-dynamic-selection turbulent premixed combustion model has been developed, validated, and successfully applied to the combustion and emissions simulation of spark ignition gasoline engines. In this new model, based on the local thermal and turbulent conditions of any preheat cell, a certain chemical kinetic mechanism which can achieve the target ratio of turbulent flame speed to laminar flame speed is selected. To realise this goal, based on the laminar flame speed sensitivity analysis of an improved eTRF (ethanol toluene reference fuel) base chemical kinetic mechanism, a derived mechanism library has been generated. Based on enough experimental and DNS (direct numerical simulation) data of S T/S L, a mechanism selection database (mechanism as a function of Borghi-Peters coordinates) has been constructed and implemented into a CFD (computational fluid dynamics) code, also a classification scheme for identifying unburned, preheat, flame front, and burned zones of a combustion domain has been developed and implemented into the CFD code. The developed mechanism-dynamic-selection turbulent premixed combustion model has been validated using the combustion in a spherical constant volume and applied to the prediction of combustion and emissions of a spark ignition gasoline engine under transient cold start process. Validation results show that, without tuning any model parameters, the base mechanism, the derived mechanism library, the mechanism selection function, the classification scheme, as well as a simulation setup strategy for modelling engine transient cold start process, have good prediction performances and are practical for engine combustion and emissions simulation.

Sub-23 nm Solid Particle Number Emission Characteristics for a Heavy-duty Engine Fuelled with Compression Natural Gas

Cold and hot WHTC (World harmonized Transient-State Cycle) were separately run on the engine test bed for a heavy-duty natural gas engine fuelled with high calorific value natural gas and low calorific value natural gas. The particle emissions including PN10 (number of particles above 10nm) and PN23 (number of particles above 23nm) were measured. The results show that the transient emission of PN10 and PN23 have basically the same trends. The weighted specific emission of PN10 is 21.6 times of PN23. Cold start PN emissions account for a relatively large proportion. Fuel property has a great influence on the PN emissions of natural gas engines. The increase of carbon-containing fuels such as methane and ethane in the fuel gas will lead to a rapid increase in PN emissions especially in the cold start process. It is extremely important to strengthen the control of sub-23nm particulate emissions for heavy-duty natural gas engines.

Numerical study of subfreezing temperature cold start of proton exchange membrane fuel cells with zigzag-channeled flow field

An automotive proton exchange membrane fuel cell (PEMFC) engine needs to be capable of surviving from sub-freezing temperatures. Cold starting up a PEMFC engine presents great technical challenge due mainly to the intra-electrode ice formation that is closely related to the involved multi-physical transport. The flow field configuration to some extent dictates the reactants and product transport and is thus of important influence on PEMFC cold start performance. The present work carefully analyzes the cold start process of PEMFCs with zigzag-channeled flow field (ZZFF). A transient three-dimensional numerical model is established and experimentally validated first, and then we make comparisons of cold-start performance between zigzag and straight channel flow-field PEMFCs to reveal the effects of ZZFF. It is found that the ZZFF better distributes the reactants/product and current density in the PEMFC as it enhances the transport along the flow direction in-between the under-land and under-channel regions inside the cell. The process of cold starting the ZZFF PEMFC is found to generate slightly less heat, which is not good to cold start. Therefore, the ZZFF enhances the survivability of PEMFC engine cold starting from subfreezing temperatures, whereas it slightly degrades the quick self-startup ability of PEMFC engine. Case study with PEMFCs of more zigzagged flow fields further corroborates the effects of ZZFF configuration on PEMFC cold start performance.

Parametric sensitivity analysis on the cold start process of a polymer electrolyte membrane fuel cell

SummarySubzero start‐up of the polymer electrolyte membrane fuel cell (PEMFC) is one of the most challenging tasks to be solved before commercialization. During the subzero start‐up process, water generated in the oxygen reduction reaction at the cathode side of PEMFC is susceptible to freeze，which makes active sites covered by the ice and gases failed to reach the surface of the catalyst layer (CL), leading to a substantial decay and even ending up with a failure of the start‐up. Given that many factors affect the cold start process, the relative contribution of the essential factors on the cold start process is independently analyzed using first‐order finite‐difference sensitivity analysis from −20°C to −30°C. The effect of essential parameters on the cold start process is quantified. The investigated parameters include the ratio of bipolar plate (BP) thickness to that of the CL (RBP/CL), the starting voltage (Vini), stoichiometry ratios, inlet gas temperature (Tin), the porosity of the CL (εCL), initial membrane water content (λini), membrane thickness (θmem), the volume fraction of ionomer (ωCL) in CLs, and the heat capacity of the BP (ρcpBP). Results show that the cold start process is most sensitive to RBP/CL and λini. Significant improvement of cold start performance can be achieved by appropriately adjusting RBP/CL, λini, Tin, and εCL. Appropriately increasing the Vini also can be a method to improve cold start performance, especially for the cold start from −30°C. Besides, optimized ωCL in CLs, proper θmem, and lower ρcpBP can contribute to better performance, especially for the cold start from −20°C.

A numerical investigation of injection pressure effects on wall-impinging ignition at low-temperatures for heavy-duty diesel engine

The effect of injection pressure on impinging ignition at low temperatures is not systematic or even confusing. This work is to reveal the impinging ignition mechanism by the numerical method to further improve the success rate and thermal efficiency at cold-start conditions in a diesel engine. In which, n-dodecane was used as the diesel surrogate, and the intermediate reaction radicals, equivalence ratio, and temperature were studied to analyze chemical reaction processes and local burn conditions. The simulation well represents the experiments. The results show that increasing injection pressure with the same mass leads to a faster fuel spreading and makes the mixture region wider and leaner for the impinging spray simulations, and the spray-wall impingement can instantly change the direction of the fuel movement and make fuel more dispersed. Under low-temperature cold-wall conditions, the physical ignition conditions are gradually destroyed due to the diffusion during longer ignition delay and finally result in misfire at higher injection pressure. In the misfire case, there is a generation of CH2O, but no obvious generation of OH. It proves that the low-temperature reactions occurred but transition into high-temperature reaction was stopped even the local temperature has reached the decomposition threshold (1000 K) of H2O2. However, lengthening injection duration can guarantee the success of ignition at higher injection pressures. It indicates that concentration is a key parameter for triggering the high-temperature reaction, and which can be made up by extending the injection duration. This research proves and explains why the lower injection pressure is helpful to the combustion in the cold start process from the mechanism level, and also has important implications for the design of fuel injection strategy in diesel engine cold start.

FEATURES OF REPRESENTING THE TORQUE OF A DIESEL ENGINE AS AN INDEPENDENT FACTOR OF INFLUENCE ON ITS STARTING QUALITIES

In this study, we presented an original method for a high-speed small displacement diesel engine (HSSDDE) start with changing the minimum cold start temperature without using a climate chamber. This work deals with the diesel engine torque reduced to the crankshaft while its electric starter motoring in the cold start process. The characteristic, numerical values and influence of the average torque factor and its diesel engine start process efficiency are given. Scheme of the laboratory bench intended for study of the HSSDDE starting qualities in the conditions of a higher education institution is presented. The structure of the information-measuring system for determining the parameters, used in current study, is given. Empirically shown, that the equivalent cold start temperature can be presented as a dependent factor of the adjustable diesel engine torque during cold start. 4DTNA series HSSDDE empirical dependencies coefficients are determined. The torque of crankshaft motoring was determined with the classical method of a calibrated starter using the correlation of electric starter shaft torque while motoring the crankshaft with the diesel engine torque. Some standard motor oils viscosity-temperature characteristics regression dependences in the range of starting temperatures and the empirical torque dependence on the oil kinematic viscosity are determined. As a result, a table of equivalent cold start temperatures was created using variable loads during HSSDDE startup. On the basis of the results of the experiment the theoretically calculated values of equivalent cold start temperatures for other oils used in the operation of HSSDDE are given. Specified range of variation of the cold start equivalent temperature factor for further polyfactorial investigation of the HSSDDE start process, which will take values of 0 °C, 5 °C and 10 °C.

Numerical Study of the Cold-Start Process of PEM Fuel Cells with Different Current Density Operating Modes

A 1D transient model was proposed to investigate the effects of the current density operating modes on the cold-start process of proton exchange membrane (PEM) fuel cells. The temperature evolution of the cell is solved by considering the reversible heat, reaction heat, Joule heat, and phase change heat. Four current density operating modes were studied based on this model: constant mode, gradually increasing mode, stepwise increasing mode, and zigzag mode. The results show that there is a threshold for the initial current density that influences the relationship between the successful cold start and current density. The cell can successfully start at a lower operating temperature by increasing the initial current density when the initial current density is below this threshold value. However, the lowest temperature for successful cold start increases if the initial current density exceeds the threshold value. Finally, a chart was plotted to indicate the regions of successful and failed self-start processes at different operating temperatures for the studied current density operating modes, respectively. The chart suggests that the cells with gradually and stepwise increasing current density modes have nearly equivalent cold-start performance, and are both better than the constant and zigzag modes.

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.

Effects of reactants/coolant non-uniform inflow on the cold start performance of PEMFC stack

The failure at equally distributing reactants among different channels within the stack leads to uneven reaction and gas concentration distribution in the catalyst layers, which consequently impacts the performance and durability of proton exchange membrane fuel cell stacks (PEMFCs). A three-dimensional, transient, non-isothermal cold start model for PEMFCs with parallel flow-field configuration and coolant circulation is developed in this work to investigate the effects of non-uniform distribution of reactants/coolant inflow rates on the cold start process. The results show that the effect of non-uniform inflow on ice formation amount is obvious and that on the distribution uniformity of current density is apparent over the cold start survival time. Additionally, the simulation predictions show that the non-uniform initial membrane water content distribution due to the purge procedure can significantly increase the rate of ice growth and deteriorate the uniformity of current density distribution in the membrane. It is found that high stoichiometry operating condition is favorable to cold startup, but may result in drying in the membrane at regions close to the channel inlet side. As non-uniform inflow rates issue is inevitable in actual PEMFC stack operation conditions, our results demonstrate that the initial membrane water content and cathode stoichiometry ratio need to be identified to moderate the effects of reactants/coolant inflow maldistribution and to maintain a stable cold start performance for the PEMFC stack.

Thermal stability analysis of cold start processes in PEM fuel cells

Cold-start issue of proton exchange membrane (PEM) fuel cells is one of the major factors hindering its commercialization. The key element involved is the competition between ice formation and melting, behind which is the thermal behavior accompanied by nonlinear effects. In this work, the steady-state multiplicity feature during cold-start has been discovered based on the thermal stability analysis approach. The nonlinear heat generation term Qg and linear removal term Qr are separated from conservation equations and constructed against ice fraction. The plot thus obtained could identify the existence of multiplicity which is the intersection of the above two functions. Results indicate that one or two steady states arise for different operating conditions. The startup with only one steady state, namely the “extinguished” state is the worst condition and bound to fail. The startup with two steady states also incorporates an unstable one which denotes not only the critical transition from ice formation to melting but also the limit of the operational domain. Based on the theoretical analysis, simple and explicit criteria are quantitatively developed for the prediction of startup feasibility. Moreover, in order to ensure a reasonable and efficient operation, the impacts of key parameters are further summarized and discussed on the cold-start operability. The proposed approach that develops analytical expressions for the steady-state points provides a systematic yet simple way to reveal the essential physics of cold start.

Analysis of the Failure Modes in the Polymer Electrolyte Fuel Cell Cold-Start Process—Anode Dehydration or Cathode Pore Blockage

In this study, the cold-start failure processes of a polymer electrolyte fuel cell have been investigated numerically for different initial membrane water content λ 0 and the startup current densities I 0 . The result shows that the failure of the cell cold-start process is mostly attributed to the anode dehydration when the cell operates with relatively large current density. However, the failure is dominated by the cathode pore blockage when the cell starts with relatively high initial membrane water content. Corresponding maps for the classification of startup failure modes are plotted on the λ 0 − I 0 plane with different startup temperatures. Three zones, including the anode dehydration, the cathode pore blockage, and the ambiguous region, can be observed. They can be distinguished with different startup failure mechanisms. The anode dehydration zone is expanded as the cell startup temperature drops due to the weakening of the membrane water back-diffusion ability. In the ambiguous region, the startup failure phenomena may be either anode dehydration or cathode pore blockage, which depends on the stochastic freezing process of the supercooled water.

Numerical study of cold start performance of proton exchange membrane fuel cell with coolant circulation

It has been well recognized that cold start is one of the key issues of proton exchange membrane fuel cell (PEMFC) used as the engine of vehicles. Coolant circulation is usually launched synchronously with the fuel cell during cold start to avoid sudden large temperature variation, which greatly increases the cell thermal mass, lowers the heating rate, and worsens the cell performance. Considering the flow and heat transfer of coolant circulation, a three-dimensional, transient, multi-disciplinary model for cold start is built up. The numerical results agree reasonably well with experimental data, indicating that the model can be used for the investigation of PEMFC cold start processes. The analysis of circulation parameter effects shows that increasing the coolant flow rate or coolant tank capacity has little influence on the cell voltage, but will increase the non-uniformity of temperature distribution along flow direction. At lower start-up temperature, this non-uniformity is more obvious. With higher coolant flow rate, although the distribution of current density becomes more evenly, the ice formation amount increases and its distribution and location are greatly affected.

Effects of Clamping Pressure on Cold Start Behavior of Polymer Electrolyte Fuel Cells

AbstractUnderstanding the cold start process of polymer electrolyte fuel cell (PEFC) is crucial to the development of an advanced PEFC of good cold start performance or to the design of advanced cold start strategies. In this study, a three‐dimensional cold start model has been adapted and further developed to numerically investigate the cold start behavior under the applied clamping pressure, which was not considered in previous cold start modeling and simulation work. The PEFC cold start performance is studied under various assembly pressures in terms of polarization curves, ice formation, water content profile, and current density distribution, etc. The results indicate that, using a large clamping pressure leads to a significant decline on the cold start performance; therefore, using an optimum clamping pressure is important to obtain a better cold start performance. It is found that increasing the clamping pressure not only increases the ice accumulation in cathode catalyst layer, but also causes the dehydration of membrane and decreases the cold start performance. The proposed model can be used as a powerful tool to study the realistic cold start performance of PEFC and to assist the development of more advanced PEFC cold start strategies.