Cold Start Process Research Articles

Anion exchange membrane electrolysis at work—Investigating impact of starting parameters and start-stop operation on cold start behavior and degradation

Water electrolysis is a key technology for the production of green hydrogen, with anion exchange membrane water electrolysis (AEMWE) showing promising properties. Future energy systems will require transient electrolysis operation to combine electrolysis with fluctuating renewably generated power. This study examines and optimizes the cold start process (ambient temperature and 0 V stack voltage) of an AEMWE stack system in terms of starting time, energy demand and degradation. The influence of relevant parameters such as voltage slope, target voltage, downtime and heating strategy on the starting process is experimentally quantified. A temporary increase in cell voltage during the starting process thereby represents a suitable compromise between acceleration of the startup and maintaining a low degradation. In addition, the start-stop degradation analysis with 150 cold starts per parameter set reveals that the degradation of the AEMWE stack during the starting process is independent of the maximum cell voltage and instead correlates with the steepness of the current slope. Using electrochemical impedance spectroscopy, the degradation is assigned to electrode processes. Under moderate starting conditions, degradation rates of 2–10 μV start−1 are observed. This shows that AEMWE is highly compatible with regular operational interruptions.

Study on the redistribution mechanism and secondary purge strategy of proton exchange membrane fuel cells

Effective control of membrane water content is essential for increasing the space for ice formation during the cold start stage and enhancing the success rate of start-up. Shutdown purge can effectively lower the membrane water content following fuel cell operation. However, during the cooling and standing process after purge, the rapid change in saturated vapor pressure can result in the redistribution of membrane dissolved water, leading to an increase in its content and a reduction in the success rate of cold start. Therefore, this study establishes a multidimensional, multiphase simulation model to comprehensively and thoroughly analyze the redistribution mechanism after purging and investigates the relationship between membrane water content and cold start. This is achieved by identifying the maximum membrane water content boundary during the cold start process and ultimately improving the success rate of cold start through a secondary purge strategy. The research results indicate that the membrane water content of the fuel cell increases from 2.31 to 8.31 after redistribution. During the cold start stage, the cold start success of the fuel cell under different environmental temperatures exhibits relatively specific boundary conditions, with the cold start process being closely related to the load current density and initial membrane water content. After implementing the secondary purging strategy, the membrane water content of the fuel cell decreases again, displaying favorable cold start characteristics in the cold start stage and successfully starting at −10 °C. This study can provide a reliable basis for the development of purging strategies during shutdown and offer a theoretical foundation for the boundary identification process of cold start.

A cold start strategy in fuel cell vehicles to provide instant hot air for stacks and passengers by vortex tubes

Green vehicles, particularly Fuel Cell Vehicles (FCVs), offer a promising solution to environmental challenges. One of the major obstacles for FCVs is starting the Polymer Electrolyte Membrane (PEM) fuel cell stacks in subfreezing temperatures, where the water produced by chemical reactions can freeze and hinder the cold-start process. Preheating the inlet air to the stack up to 80 °C is an effective approach to overcome this issue. However, conventional heating systems, such as electric heaters, are unable to heat the air quickly enough. This paper introduces a novel heating method to enhance the cold-start capability of FCVs. The proposed solution involves integrating vortex tubes, which are simple and cost-effective, with the vehicle’s existing compressor. This system not only preheats the inlet air to the stacks but also provides warm air for the passengers simultaneously. By developing a 3D-CFD model of the vortex tube, the results demonstrate that the system can preheat the inlet air to the stacks from −30 °C to 80 °C and the air entering the passenger compartment from −30 °C to nearly 37 °C in just about 5 s. In comparison, conventional heating systems require over 600 s (10 min) to achieve the same temperature rise.

Efficient multi-objective rolling strategy of photovoltaic/hydrogen system via short-term photovoltaic power forecasting

Green hydrogen serves as an effective energy carrier for achieving sustainable development goals. However, the high production cost and intermittent renewable energy output cause obstacles to hydrogen production. Various electrolyzer control methods have been proposed to promote electrolyzers' efficiency. Traditional electrolyzer control methods may lead to significant differences in electrolyzer wear among large-scale hydrogen production systems. Hence, this paper proposes a multi-objective rolling strategy based on photovoltaic power forecasting to mitigate electrolyzer switch fluctuations, enhance hydrogen production efficiency, and better manage the electrolyzers. Initially, the trend of photovoltaic output is calculated based on the predicted values of the photovoltaic output from the particle swarm algorithm (PSO)-backpropagation (BP) model, and during the power ramp-up phase, some electrolyzers are activated in advance to expedite the cold start process and enhance the hydrogen production. Simultaneously, the electrolyzers' power distribution and operational status are adjusted in conjunction with the real-time power situation. Finally, a multi-objective rolling strategy is proposed to balance the operational indicators of the electrolyzers. The results indicate that the proposed method yields 4,952,642 Nm3 hydrogens over six months, which has increased by 5.37% and 9.00% compared to the simple start-stop and rotation strategy, respectively. Moreover, switch action times are reduced by 27.68% and 43.22%, respectively. These results demonstrate the effectiveness of the proposed method in significantly enhancing hydrogen production and improving the economic feasibility of photovoltaic/hydrogen systems.

Effect analysis on the hydrocarbon adsorption performance enhancement of the different zeolite molecular sieves in the gasoline engine under the cold start process

In this work, the adsorption performance of ZSM-5, Beta-A, and NON zeolites on hydrocarbons emitted from the engine is investigated for selecting the best suited zeolite molecular sieves for hydrocarbon traps during the cold-start phase of the engine. Based on the giant canonical Monte Carlo (MC) method for adsorption simulation, the single-component, multi-component maximum adsorption, adsorption isotherms, adsorption sites of different hydrocarbon molecules on ZSM-5, Beta-A, and NON molecular sieves were investigated by using C2H2, C2H4, C3H6, and C7H8 molecules as probe molecules. The single-component adsorption results showed that for the macromolecular hydrocarbon toluene (C7H8) Beta-A zeolite had the best adsorption effect, followed by ZSM-5 zeolite, and NON had the worst adsorption effect. The adsorption of propylene (C3H6) on ZSM-5 and Beta-A is close to each other, and the adsorption of ethylene (C2H4) and acetylene (C2H2) are much different relative to C3H6 and C7H8. This is due to the fact that the adsorption properties of zeolites are determined by the molecular diameter of the hydrocarbons. The results of multicomponent adsorption showed that the adsorption amounts of multicomponent competitive adsorption were reduced relative to single-component adsorption, but were affected by other components in different ways, from small to large C7H8<C3H6<C2H4<C2H2.

Study of three factors for commercial-size PEM fuel cell stack cold start: Manifold, endplate, and assisted heating

Numerical investigation on a proton exchange membrane fuel cell stack comprising 400 single cells under subzero temperatures is essential for automotive applications. In this work, a one-dimensional multi-phase PEMFC stack cold start model is built to investigate the effect of manifold configuration and endplates on the cold start process. It is noticed that the U-shape manifold configuration is more conducive to the distribution of reaction gas than the Z-shape, while the Z-shape manifold configuration is more likely to cause damage to the stack due to the dilute oxygen concentration in certain cells. The endplate effect has an important effect on the temperature, ice fraction, and cell voltage of the 20 cells near the endplates. The number of cells affected by the endplate effect is independent of the convective heat transfer coefficient. Finally, the strategy of leveraging the PEMFC stack’s output energy to heat the stack can effectively boost the likelihood of a successful cold start, which can aid the stack achieve a successful cold start at −24℃. The arrangement of two single cells separated by a heating pad provides the best cold start capacity and consistency in the stack.

Effect analysis on hydrocarbon adsorption performance of transition metal modified zeolites in the gasoline engine during cold start process using Monte Carlo method

Zeolite molecular sieves are commonly used for adsorption of hydrocarbons(HCs) during cold start phase of gasoline engines. In this work, the adsorption properties of three zeolites modified with three transition metal ions(Fe2+, Cu2+ and Zn2+) were investigated for the HCs adsorption using the Monte Carlo method. Adsorption simulations were carried out with ethylene and propylene as probe molecules or with a mixture of five hydrocarbon and water molecules. The introduction of transition metal cations enhances the local positive electric field within the molecular sieve, enabling it to engage in stronger adsorption interactions with adsorbate molecules, which significantly improves the adsorption performance of the molecular sieve. For single-component adsorption, Zn2+-MFI with a Si/Al ratio of 7 showed the best adsorption of ethylene, with a high adsorption capacity of 149.19 molecules per cell at 298 K, which was 1.47 times higher than that of pure silicon. The enhanced hydrophilicity of the modified zeolite resulted in a substantial increase in the adsorption of water molecules during multicomponent adsorption. The limited adsorption sites and intermolecular competition for adsorption resulted in the occupation of adsorption sites by large hydrocarbons and a decrease in the adsorption of the remaining small molecules.

Mesoscopic process of multiphase reactive flow and heat transfer during direct cold start process in Pt/C particle coated in ionomer of PEMFC

To explore multiphase reactive flow and heat transfer process during direct cold start in catalyst layer of proton exchange membrane fuel cell, a pore scale model was established. Effects of released gaseous water particle number, platinum volume fraction, ionomer thickness, and carbon support diameter were studied. The results reveal that due to obstruction of ice at bottom, oxygen cannot fully enter the bottom. After motion of solid ice is completely melted, the obstruction of the bottom on oxygen transport weakens, and oxygen gradually enters the bottom of the catalyst layer. As the number of gaseous water particles increases, the frost layer becomes thicker, and the resistance of the frost layer to oxygen diffusion becomes more significant. The increase in platinum particles elevates temperature within the catalyst layer, accelerating melting of solid ice within the catalyst layer. The carbon support diameter only has an effect on the intermediate concentration region. When the carbon support diameter is 26 nm, more oxygen enters the ionomer due to the weakened obstruction of the frost layer. Increasing the carbon support diameter lowers the temperature within the catalyst layer.

Investigation on the hydrocarbon adsorption performance enhancement of the ZSM-5 zeolite with different Si/Al ratio in the cold start process of the gasoline engine

In response to the issue of hydrocarbon emissions during the cold start process of gasoline engines, a hydrocarbon catcher coated with ZSM-5 zeolite was manufactured. The post-treatment system of the prototype vehicle was modified to install the hydrocarbon catcher, and cold start experiments on the vehicle under World Light-vehicle Test Cycle (WLTC) conditions were conducted. The experimental results showed that the hydrocarbon emissions in the low-speed stage of WLTC were much higher than those in other stages. In the early stages of cold start, the hydrocarbon catcher can reduce hydrocarbon emissions by up to 61 %. The analysis results of engine emissions showed that the content of various hydrocarbon components during cold start was: 60.1 % butene, 13.6 % acetylene, 11.3 % propylene, 7.3 % ethylene, 7.1 % acetaldehyde, and 0.6 % other components. Further molecular simulation studies were conducted on zeolite of different types and Si/Al ratios. The results showed that the adsorption of hydrocarbons on pure Si-MFI and -FER zeolites were less affected by H2O and CO2, making them ideal hydrocarbon capture materials. Al substitution introduced compensating cations into zeolite, which increased the adsorption capacity of H2O, CO2, and C2H4O with polar functional groups. Reducing the degree of Si/Al substitution is beneficial for hydrocarbon adsorption.

Numerical investigation on the cold-start/restart process of a linear range extender for faster response

The free-piston linear range extender (LRE) system is an innovative energy conversion system characterized by a simple structure, high reliability, superior conversion efficiency, minimized energy loss, and excellent adaptability to various fuels. This paper establishes a numerical model for the cold start process of the LRE system, and experimental data are utilized to validate the model's accuracy. Subsequently, the influence of cylinder wall temperature on the cold start of the LRE system is investigated. Results reveal that increasing the cylinder wall temperature from 300K to 600K enhances system performance, evidenced by increased peak cylinder pressure (from 8.18 bar to 12.46 bar), peak piston speed (from 1.54 m/s to 1.91 m/s), and operating frequency (from 9.76 Hz to 11.66 Hz), alongside a reduction in ignition time (from 0.462 s to 0.229 s). Additionally, the required motor force for ignition decreases from 219 N to 194 N, showing a linear relationship with cylinder wall temperature. The study indicates that employing the cylinder wall heating method can decrease the motor force needed for ignition, leading to a notable enhancement in the system's cold start capability. This presents a promising strategy for quickly restarting the system following a misfire.

Effect of gas crossover on the cold start process of proton exchange membrane fuel cells

This study develops a transient three-dimensional model to investigate the effect of reactant gas crossover through the membrane on the cold start process of PEM fuel cells. Key operating and structure parameters such as current density, anode/cathode inlet pressure, and membrane thickness are explored. The results reveal that gas crossover enhances ice formation rate and cell temperature rise due to additional hydrogen–oxygen side reactions, increasing both heat and water generations. The dominance of the ice formation rate over the temperature rise rate by gas crossover adds complexity to successful cold starts. Notably, the influence of gas crossover is more pronounced at a lower current density. As the operating current density increases from 0.04 A cm−2 to 0.12 A cm−2, the contribution of gas crossover to the temperature rise rate and ice formation rate decrease from 31.6 % to 11.3 % and from 54.5 % to 16.8 %, respectively. The increase of anode inlet pressure accelerates ice formation and temperature rise, whereas cathode inlet pressure has minimal effect. A thicker membrane mitigates gas crossover and is beneficial to cold start success, especially under a wet initial condition. This study emphasizes the significant role of gas crossover in the PEM fuel cell cold start process, highlighting the imperative consideration of gas crossover in both cold start models and strategy developments.

Unique κ-Ce2Zr2O8 Superstructure Promoting the NOx Adsorption-Selective Catalytic Reduction (AdSCR) Performance of the WO3/CeZrOx Catalyst.

Selective catalytic reduction of NOx by NH3 (NH3-SCR) for diesel emission control at low temperatures is still a great challenge due to the limit of the urea injection threshold and inferior SCR activity of state-of-the-art catalyst systems below 200 °C. Fabricating bifunctional catalysts with both low temperature NOx adsorption-storage capacity and medium-high temperature NOx reduction activity is an effective strategy to solve the issues mentioned above but is rarely investigated. Herein, the WO3/Ce0.68Zr0.32Ox (W/CZ) catalyst containing the κ-Ce2Zr2O8 pyrochlore structure was successfully developed by a simple H2 reduction method, not only showing superior NOx adsorption-storage ability below 180 °C but also exhibiting excellent NH3-SCR activity above 180 °C. The presence of the pyrochlore structure effectively increased the oxygen vacancies on the κ-Ce2Zr2O8-containing W/CZ catalyst with enhanced redox property, which significantly promoted the NOx adsorption-storage as active nitrate species below 180 °C. Upon NH3 introduction above 180 °C, the κ-Ce2Zr2O8-containing W/CZ catalyst showed greatly improved NOx reduction performance, suggesting that the pyrochlore structure played a vital role in improving the NOx adsorption-selective catalytic reduction (AdSCR) performance. This work provides a new perspective for designing bifunctional CeZrOx-based catalysts to efficiently control the NOx emissions from diesel engines during the cold-start process.

Control-oriented cold start modelling and experimental validation of PEM fuel cell stack system

The cold-start of proton exchange membrane fuel cell (PEMFC) has been one of the leading technical challenges for fuel cell vehicle commercialization. During the cold start process, a new control-oriented modelling methodology of a water-cooled PEMFC stack system was present. The voltage and temperature change throughout the stack may be forecast using the model technique. Then the model was validated in a 5 Kw, 20 cells, water-cooled stack at room and subzero temperatures. The result shows that the model accuracy can capture the stationary and transient states of the fuel cell stack well. For the cold start of PEMFCs in automotive applications, the suggested model may be utilized to investigate real-time state estimates and model-based control approaches.

Phasic icing phenomena of part single cells during the successful cold start processes of PEMFC stacks

Herein, phasic icing of part single cells during successful cold start process of PEMFC stacks is experimentally studied considering temperature, gas velocity and cell number; meanwhile, a heat transfer model is developed for stacks to investigate thermal evolution characteristics. The results reveal that temperature difference between end cells and middle cells can be 10 °C, and that for a cell inside may reach 7 °C. For self-starts, no apparent icing phenomenon is observed when PEM stack is successfully started at ≥ −20 °C. Phasic icing occurs commonly at ≤ −30 °C and performance degradation due to icing is less than 0.5%. In a 20-cell stack, when cathode gas channel velocity is 6.2 m/s, phasic icing in middle cells is observed due to uneven flow distribution, which can be avoided by increasing the velocity to 12.4 m/s if velocity is 3.1 m/s or less, cold start fails. Additionally, coolant circuit cannot eliminate endplate effect and phasic icing phenomenon.

Numerical Simulation of the Cold-Start Process of Polymer Electrolyte Fuel Cell

In this study, the cold-start process of a polymer electrolyte fuel cell has been numerically investigated under various ambient temperatures and operating currents, ranging from subzero to 283 K. The water desorbed from the electrolyte, when the cell temperature is below the freezing point, is assumed to exist in a state of either supercooled water or ice. The evolution of cell voltage, temperature, membrane water content, and the averaged volume fraction of supercooled water or ice in the catalyst layer and gas diffusion layer are presented. The results indicate that the cold-start process may fail due to ice blocking of the cathode catalyst layer when the desorbed water is in the form of ice and the ambient temperature is sufficiently low. However, when the desorbed water is in a supercooled state, it can diffuse from the cathode catalyst layer to the cathode gas diffusion layer, avoiding water clogging and enabling a successful cold-start process. During the cold-start process, as the ice undergoes a melting process, the membrane water content inside the membrane would increase rapidly, and a larger operation current with anode gas humidification is helpful to the cold-start process.

A detailed multidimensional simulation for the cold start process of a gasoline direct injection engine using a fractal engine simulation model

The cold start process for a gasoline direct injection (GDI) engine was studied through multidimensional simulations using a Fractal Engine Simulation (FES) model integrated into CONVERGE CFD. The simulations were focused on the very first firing cycle in the cold start process, as most of the hydrocarbon emissions derive from this cycle. The dramatically changing engine speed and low wall temperatures in the cylinder present challenges for simulation. It turned out that the FES model was able to predict the in-cylinder pressure traces for all four cylinders for the first firing cycle, and gave good agreement with the experimental measurements under these extremely transient conditions. More comprehensive analysis demonstrated the causes for the different behavior of the combustion in different cylinders: more injected fuel and a higher induced tumble ratio in cylinder 3, the first to fire, led to a higher average equivalence ratio and better fuel distribution, which resulted in the highest peak pressure; the worse fuel distribution from the lower tumble ratio, together with the lower normalized turbulence, caused weaker combustion in cylinder 4; the peak pressures were similar and on the low side for cylinders 2 and 1, mainly determined by the low average equivalence ratio. An improved strategy with multiple injections was proposed, with the first two in the intake stroke and two later injections in the compression stroke rather than one injection during each stroke. Although the total injected fuel was the same as the baseline strategy, more fuel evaporated due to the enhanced fuel evaporation from both droplets and wall films, resulting in a higher average equivalence ratio in the cylinder. The peak cylinder pressure and IMEP rose by 33.3% and 8.4%, respectively, using the 4-injection strategy compared to the baseline 2-injection strategy, and the 4-injection strategy was verified by the experiments, as well.

Pore scale study of ice melting and multiphase flow in gas diffusion layer with porosity gradient in PEMFC

An enthalpy-based multi-component multiphase flow model is established by using the lattice Boltzmann method (LBM), and the ice melting and liquid water flow behavior during cold start process in gas diffusion layers (GDLs) with porosity gradients of proton exchange membrane fuel cell (PEMFC) are captured. GDLs with five porosity gradients are considered to explore the ice melting efficiency, temperature distribution characteristics and liquid water transport. The results show that the ice melting rate is significantly affected by the porosity distribution. The model of porosity decreasing gradually from near catalyst layer to near channel side (M4) greatly improves ice melt efficiency by up to 61.44%, while the 'V' porosity gradient (M2) and the model of porosity increasing gradually from near catalyst layer to near channel side (M3) prolong the heating and melting time of 15.6% and 25.56% respectively. The arrangement of small porosity structure near the channel side enhance the convective and conduction heat transfer. The hydrophilicity of the surface of fibers is beneficial to ice melting but hinders the discharge of liquid water. Considering the ice melting efficiency and drainage performance, the GDL with porosity gradient can be used when selecting the auxiliary measure of inlet preheating.

Real-Time Control of Gas Supply System for a PEMFC Cold-Start Based on the MADDPG Algorithm

During the cold-start process of a PEMFC, the supply of air and hydrogen in the gas supply system has a great influence on the cold-start performance. The cold-start of a PEMFC is a complex nonlinear coupling process, and the traditional control strategy is not sensitive to the real-time characteristics of the system. Inspired by the strong perception and decision-making abilities of deep reinforcement learning, this paper proposes a cold-start control strategy for a gas supply system based on the MADDPG algorithm, and designs an air supply controller and a hydrogen supply controller based on this algorithm. The proposed strategy can optimize the control parameters of the gas supply system in real time according to the temperature rise rate of the stack during the cold-start process, the fluctuation of the OER, and the voltage output characteristics. After the strategy is trained offline according to the designed reward function, the detailed in-loop simulation experiment results are given and compared with the traditional control strategy for the gas supply system. From the results, it can be seen that the proposed MADDPG control strategy has a more effective coordination control effect.

Experimental and modeling study on energy flow of 250 kW alkaline water electrolysis system under steady state conditions and cold start process

Improving the energy utilization efficiency of alkaline water electrolysis system (AWE) is one of the urgent problems at this stage. This study explores the heat flow and energy distribution under steady state and cold start processes by establishing a multi-physical field coupled AWE system model including heat transfer, mass transfer and electrochemistry, based on this, the methods to improve energy utilization efficiency are proposed. The system power consumption at full load is 272.7 kW, the electrolyzer accounts for up to 88.4%, of which, the useless heat production and parasitic current are the main factors that cause the electrolyzer energy loss, accounting for 21.2% and 3.1% of the total power consumption, respectively. The electrolyzer useless heat production also significantly contributes to the chiller energy consumption, which accounts for 5% of the system power consumption. Improving the electrolyzer performance and minimizing its heat production is essential to optimize the system efficiency. During the AWE cold start process, the constant voltage control has a shorter cold start time than the constant current control, but their power consumption differences in terms of heat and hydrogen production are small. Reducing the system thermal capacity is extremely effective in speeding up the cold start process compared to optimizing the power load type, which can cut the cold start time to less than 1 h.

Development of a fractal engine simulation model in a multidimensional simulation for the cold start process of a gasoline direct injection engine

A fractal engine simulation (FES) sub-model was integrated into three-dimensional simulations for modeling turbulent combustion for a gasoline direct injection (GDI) engine. The FES model assumes that the effects of turbulence on flame propagation are to wrinkle and stretch the flame, and fractal geometry is used to predict the surface area increase and thus the turbulent burning velocity. Different formulas for the four sequential stages of combustion in SI engines are proposed to account for the changing effects of turbulence throughout the combustion process. However, most prior studies related to the FES model were quasi-dimensional simulations, with few found in multi-dimensional studies, and none under cold start conditions or stratified charges. This paper describes how the model was implemented into multidimensional simulations in CONVERGE CFD, and what the formulas are in the four sequential stages of combustion in SI engines. The capabilities of the FES model for simulating the cold start cases, under the conditions of the dramatically changing engine speed and mixture stratification in a complex engine geometry, are presented in this study. The FES model was able to not only simulate the steady-state cases with constant engine speed, but also predict the in-cylinder pressure traces in all four cylinders for the very first firing cycle with transient engine speed, and gave good agreement with the experimental measurements under these extremely transient conditions. The uncertain maximum fractal dimension was chosen as 2.37 in this research, and a simple linear correlation with engine speed was used to obtain the coefficient used in calculating the kernel formation time which controls the so-called combustion or ignition delay.