• Energy Research
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Abstract The reliability of proton exchange membrane fuel cell (PEMFC) tightly depends on the suitable operating conditions during dynamic operations. This study proposes an optimization framework to determine the optimal control strategy for PEMFC cold starts underpinned by a novel artificial intelligence method, to improve cold-start capacity and shorten the start-up time. The effects of constant and dynamic currents on PEMFC cold starts under various initial temperatures are studied. The numerical results from a developed PEMFC dynamic model show that the constant current slope strategy (CCSS) is more efficient than the constant current strategy (CCS) in respect of the cold-start time. In the CCSS study, a too-large current slope can lead to a voltage undershoot and then cause a failed cold start, but a too-small current slope can result in a long start-up process in the investigated range of the operating conditions. A data-driven model is developed for dynamic prediction and real-time optimization during the cold start by a semi-recurrent sliding window (SW) method coupled with artificial neural networks (NN) with the simulation data. Based on this NN-SW model, the specific safety–critical operating condition curve under the CCSS has been identified. A real-time adaptive control strategy (RACS) is further proposed to optimize the operating current during the PEMFC cold starts with various initial temperatures. Compared to the optimal CCSS, RACS proves to be more robust and efficient for PEMFC cold-start startups. Based on RACS, the start-up time for an initial temperature of −20 °C can be cut down by 26.7%. Furthermore, the ice predictions by the NN-SW model are also tested and the results are satisfying with an average R2 = 0.9773.

Abstract Biodiesel is a very promising alternative fuel in internal combustion engines. Fragmentation of the fuel injection has a fundamental influence on engine performance. The influence factors for the spray breakup process, including near nozzle fields, are still unclear. In this study, the primary of fuel sprays occurring with turbulence perturbation is devoted to simulation. The evolutionary processes of biodiesel fuels atomization affected by turbulence are investigated in present models. A validated single-phase fully developed turbulent flow is generated first to store time-varying outlet velocity database. Then, the database is mapped as the two-phase model inlet velocities boundary. A modified VOF (Volume of Fluid) coupled with DNS (direct numerical simulation) method is applied to study the evolution of fuel spray. It is found that wavy surface, ligaments, and droplets with various scales and shapes turn up gradually in jet evolution process. Meanwhile, after being sheared, distorted and stretched, different ligaments separation patterns are captured. Larger Reynolds number and higher gas densities accelerate the jet break-up process. Higher injection velocities and lower power-law indexes (n 1) fuel jet. What’s more, similar breakup patterns are detected in shear-thinning fluid (n

Abstract Anode recirculation operation is one method to both achieve self-humidification and increase hydrogen utilization for proton exchange membrane fuel cells (PEMFCs) automotive application. In this study, effect of operating conditions, including cathode inlet relative humidity, anode and cathode inlet pressure, and anode stoichiometry on performance of PEMFC with anode recirculation are analyzed by simulation work. Generally, the performance variation when the PEMFC starts anode recirculation can be divided into two stages. The performance increases first due to the self-humidification effect and then decreases due to the nitrogen crossover. The results show that performance enhancement caused by self-humidification becomes less significant with increasing cathode inlet humidity. Increasing anode inlet pressure can retard the performance decline caused by nitrogen crossover and increasing cathode inlet pressure will exacerbate it. Increasing anode stoichiometry can enhance the self-humidification by anode recirculation.

Abstract In this study, an enhanced non-isothermal, two-phase 1D analytical proton exchange membrane fuel cell (PEMFC) model is developed, which not only considers the water saturation jump, but also proposes a novel method to analytically solve the water phase changes and couple the liquid and vapor transport together. A stringent model validation procedure is used to show good agreement between the simulated results and the experimental data, taking advantage of the “three-step” and “multi-case” validation methods. It is revealed that the uncertain parameters may deteriorate model reliability and credibility, thus demonstrating the necessity to conduct sensitivity analysis. A multi-parametric screening method i.e. the elementary effect (EE) method based on Monte Carlo experiments is implemented to comprehensively analyze the total 22 uncertain parameters (including geometric, physical and electrochemical parameters), which are finally classified into very sensitive ones, rather sensitive ones and insensitive ones. The cathodic parameters are found more sensitive than the anodic ones, and the parameters of different components may have distinct sensitivity. Besides, whether the effect of each parameter is positive or negative on cell performance is also discussed. Furthermore, three cases with different groups of parameters are presented, which show almost the same polarization curve, and the two-sample Kolmogorov-Smirnov (KS) test is applied to verify the stability difference. It is concluded that those uncertain parameters not only influence the cell performance but also affect the model stability, and hence the effects of varying operating conditions should be taken into account in validation work.

Abstract The performance of high temperature proton exchange membrane fuel cell (HT-PEMFC) is significantly affected by the carbon monoxide (CO) in hydrogen fuel, and the flow channel design may influence the CO poisoning characteristics by changing the reactant flow. In this study, three-dimensional non-isothermal simulations are carried out to investigate the comprehensive flow channel design and CO poisoning effects on the performance of HT-PEMFCs. The numerical results show that when pure hydrogen is supplied, the interdigitated design produces the highest power output, the power output with serpentine design is higher than the two parallel designs, and the parallel-Z and parallel-U designs have similar power outputs. The performance degradation caused by CO poisoning is the least significant with parallel flow channel design, but the most significant with serpentine and interdigitated designs because the cross flow through the electrode is stronger. At low cell voltages (high current densities), the highest power outputs are with interdigitated and parallel flow channel designs at low and high CO fractions in the supplied hydrogen, respectively. The general distributions of absorbed hydrogen and CO coverage fractions in anode catalyst layer (CL) are similar for the different flow channel designs. The hydrogen coverage fraction is higher under the channel than under the land, and is also higher on the gas diffusion layer (GDL) side than on the membrane side; and the CO coverage distribution is opposite to the hydrogen coverage distribution.

Abstract Thermoelectric generator (TEG) has attracted considerable attention for the waste heat recovery of internal combustion engine. In this study, a 3-D numerical model for engine exhaust-based thermoelectric generator (ETEG) system is developed. By considering the detailed geometry of thermoelectric generator (TEG) and exhaust channel, the various transport phenomena are investigated, and design optimization suggestions are given. It is found that the exhaust channel size needs to be moderate to balance the heat transfer to TEG modules and pressure drop along channel. Increasing the number of exhaust channels may improve the performance, however, since more space and TEG modules are needed, the system size and cost need to be considered as well. Although only placing bafflers at the channel inlet could increase the heat transfer coefficient for the whole channel, the near wall temperature downstream might decrease significantly, leading to performance degradation of the TEG modules downstream. To ensure effective utilization of hot exhaust gas, the baffler angle needs to be sufficiently large, especially for the downstream locations. Since larger baffler angles increase the pressure drop significantly, it is suggested that variable baffler angles, with the angle increasing along the flow direction, might be a middle course for balancing the heat transfer and pressure drop. A single ETEG design may not be suitable to all the engine operating conditions, and making the number of exhaust channels and baffler angle adjustable according to different engine operating conditions might improve the performance.

Abstract By developing a thermoelectric generator (TEG) model coupled with exhaust and cooling channels for an exhaust-based TEG (ETEG) system, the influence of the cooling type, coolant flow rate, length, number and location of bafflers, and flow arrangement are investigated. It is found that the net output power is generally higher with liquid cooling than air cooling. Since a very low velocity of liquid coolant is sufficient for cooling the TEG modules, the flow resistance is negligible, and inserting a baffler, increasing the baffler length or the flow velocity generally improves the performance. However, both the baffler length and flow velocity of air cooling need to be moderate. Placing one baffler in front of a TEG module is sufficient to guide the cooling flow. The performance is generally unaffected by the change of baffler location. By maintaining sufficient temperature difference for all the TEG modules, the counter-flow arrangement leads to higher output power than the co-flow arrangement. Although liquid cooling is more complicated, and extra cooling power may be needed to cool down the circulating coolant, the temperature increment of liquid coolant through cooling channel is insignificant for cooling 20 TEG modules producing about 250 W of power.

Thermoelectric generators (TEGs) have become a promising technology for vehicle exhaust heat recovery. Although the complex vehicle driving conditions may lead to significant variation of TEG performance, such influence was rarely paid attention to. In this study, a numerical model of thermoelectric generator (TEG) based on vehicle waste heat recovery is developed. When the acceleration duration is short, the hot side temperature increases quickly at first with an overshoot phenomenon. When the acceleration durations increase or the acceleration range becomes smaller, the overshoot phenomenon becomes weaker. The change of the voltage and power generally follows the same trend. The performance variation of TEGs becomes more significant with faster acceleration or deceleration. The transient response of the hot and cold side temperatures, voltage and power in deceleration is less significant than acceleration, because in deceleration, the cold side temperature increases first due to the weakened heat convection. For the step change of vehicle speed, when the speed is low, the voltage and power curves and the speed curve are more consistent, and a longer step duration leads to better consistency. A higher road grade can increase the power output of TEG significantly, and lead to a faster transient response. The Japanese 10–15 cycle, New European Driving Cycle (NEDC) and Urban Driving Dynamometer Schedule (UDDS) are selected to evaluate the impact of different driving cycles. The results suggest that a highly frequent change of driving condition may have a negative effect on the TEG performance.

Abstract Multidimensional numerical models are useful tools for understanding the heat transfer mechanisms and performance optimization of thermoelectric generators (TEGs). In this study, two three-dimensional numerical models are developed for TEGs based on different formulations, but with similar abilities for heat and electricity transfer analysis and performance prediction. Model 1 solves the conservation equations of the Seebeck potential and the Ohmic potential separately, and the total built-in potential can be obtained based on the solved Seebeck and Ohmic potentials. Model 2 solves the conservation equation of the total built-in potential directly, and the conservation equation for the Ohmic potential is also solved. The comparison between Model 1 and Model 2 shows that Model 2 is slightly more precise for power output prediction. The detailed formulations of these two models are described, and the difference among the present and previous models is also discussed. Some important modeling aspects are elucidated for the TEG models, such as the conservation equations and boundary conditions. Parametric studies are carried out based on various thermal boundary conditions. The influence of the TEG semiconductor shape on performance is investigated in details. It is found that for the nearly same volume of semiconductor materials, changing the shape from normal cuboid (constant cross-sectional area) to hexahedrons (variable cross-sectional area) could increase the power output significantly. The reason is that the temperature gradient could be enhanced when proper variable cross-sectional areas are used.