Journal of Energy Engineering

Abstract

Limited by the small space size of electric vehicles (EVs), a more concise and lightweight battery thermal management system (BTMS) is in great demand to keep the lithium-ion battery safe. In the current study, the phase change material (PCM)-based internal and external cooling models were constructed for a cylindrical lithium-ion battery, and the effectiveness of the cooling design and the accuracy of the numerical model are verified. The effects of different cooling modes, PCM melting point, PCM mandrel size, and thermal conductivity anisotropy on the cooling performance were systematically evaluated from the perspectives of maximum temperature, maximum temperature difference, and temperature distribution. The results revealed that the internal cooling method exhibited optimal performance; the maximum temperature difference was only 1.77 K compared to 6.77 K when cooled in external mode. Moreover, the bidirectional heat transfer process using a PCM-based internal cooling mode was investigated and the results showed that the heat transfer resistance reduced, the temperature gradient lowered, and the temperature distribution more evenly distributed. When the mandrel diameter of PCM increased from 2 to 5 mm, the maximum temperature of the battery dropped from 316.65 K to 314.10 K, and the maximum temperature difference decreased from 2.23 K to 1.32 K on one accord. The internal heat transfer process of a lithium-ion battery was influenced by the radial thermal conductivity, which directly determined the uniformity of the temperature difference inside the battery. With regard to the structure design, minimizing the battery size in the stratiform direction on the premise of ensuring energy density helped to enhance temperature uniformity.

Practical Applications

The lithium-ion battery is the power source of electric vehicles. However, an excessive temperature rise during battery operation will put the battery in danger. Based on this, the research of more advanced thermal management systems for power batteries has become a hot topic in the field of new energy vehicles. A 2.6 Ah 26650-type LiFePO4 cylindrical battery was considered in the present research. The phase change material (PCM)-based internal and external cooling models were constructed for a cylindrical lithium-ion battery. The internal cooling model has been constructed by filling the PCM into the core of the battery with the help of the hollow mandrel that already exists inside the cylindrical lithium-ion battery. An external PCM cooling model was built by wrapping the 1 mm of thickness PCM around the outside surface battery. The cooling system was evaluated in terms of maximum temperature, maximum temperature difference, and temperature distribution. The results showed that the internal cooling method exhibited the best performance, lowered temperature gradients, and produced a more uniform temperature distribution. This study proposes an important reference for the design and optimization of the battery thermal management system.

Abstract

Because flow field design is an effective method for improving the performance of proton exchange membrane fuel cells, some novel flow fields have been proposed to improve mass transportation in the horizontal plane using numerical methods. Different geometrical characteristics in the horizontal plane, such as the intersection-node coverage area, intersection form, and intersection-node distribution, have been used to discuss their effects on cell performance. The simulation results were obtained from nine cases. According to the calculations, introducing a crossover node could increase the current density by up to 26%. The simulation results elucidated that expansion of the reaction area facilitates oxygen diffusion. Forced convection was induced by an inclined channel, which increased the reactant concentration in the downstream region. Therefore, the uniformity of the current density also increased. The flow rate of the gas was affected by the symmetrical mode. Among them, high gas flow velocities were more easily observed in a flow field with a centrally symmetric distribution node.

Practical Applications

Proton exchange membrane fuel cells are a promising energy conversion device for a wide range of applications. It can directly convert chemical energy into electrical energy. There is one key component in the fuel cell, named the bipolar plate. On them, there are channels with grooves formed by machining; comprehensive performances of the fuel cells are profoundly determined by the structure and distribution of these channels. Therefore, rational design of the flow field is an effective way to improve the fuel cell performance. If the channels are intersected by each other, the intersection nodes will be generated. That is, the structure of the flow field will be changed by introducing different intersection nodes in terms of coverage area, intersection form, and distribution. The purpose of these measures is to increase the residence time of the gas in the flow field. Therefore, this study will introduce the reader to how the performance of the fuel cell is affected by the flow field with intersection nodes.

Gas-bearing capacity is an important feature in the evaluation of the different properties of shale. The calculation of adsorbed gas and free gas content is the focus of the shale gas-bearing capacity evaluation, for which gas saturation is a key parameter. In the present study, the target area was the marine shales of the Wufeng–Longmaxi Formation in the Dingshan, Jiaoshiba, and Changning areas of the southern Sichuan Basin in China, while the purpose of the study was the more effective characterization of Langmuir’s volume and Langmuir’s pressure using well-logging data. The application of new well-logging technologies in the evaluation of shale gas-bearing capacity is seldom studied, and the conventional sand-mudstone saturation models calculate the shale gas-bearing capacity with low accuracy. Therefore, this study systematically analyzed the shale conductivity mechanism, which laid the foundation for a new calculation model for shale gas saturation. The analysis results of the influencing factors of shale conductivity in the study area showed that the resistivity of shale in the interlayer is mainly affected by the thin low-resistivity layers, and the resistivity of shale in laminates is affected by clay minerals, pyrite, overmature conductive organic matter, and pore fluids. Moreover, this study further clarified the main controlling factors of the conductivity mechanism by implementing a multiscale analysis. Herein, on the meter-scale, the influence of thin low-resistivity layers on the shale resistivity was characterized based on a horizontal resistivity model; on the centimeter-scale, the influence of pore fluids on shale resistivity was investigated based on the rock electrical experiments; and on the nanometer-scale, the influence of clay minerals, pyrite, and organic materials on shale resistivity was examined based on digital rock technology and numerical simulation of the electrical properties. The results showed that the factors affecting the conductivity of the shale, from the strongest to the weakest, are conductive organic matter, thin low-resistivity layer, clay mineral, pore water, and pyrite, respectively.

In this study, the influence of direct steam injection (DSI) on the performance and emissions of marine engines is investigated. Simulation models are developed and tested based on experimental data. Steam is obtained by waste heat recovery in a marine engine. The limitations of DSI parameters are investigated based on the exhaust gas temperature. The steam quantity and temperature decrease with a decrease in load. The results show that steam mass plays an important role in ${\mathrm{NO}}_{\mathrm{x}}$ reduction and improving power. For a steam/fuel ratio of 1.27 at 100% load, the brake power improved by 3.09% and ${\mathrm{NO}}_{\mathrm{x}}$ emissions decreased by 4.67%. A higher degree of improvement is obtained with an increase in steam mass. The steam temperature and injection timing only slightly influenced the brake power and ${\mathrm{NO}}_{\mathrm{x}}$ emissions. When steam-injection timing improved to 12°CA at 100% load with injection duration decreasing from 85°CA to 25°CA, brake power improved from 3,485.7 to 3,529.7 kW. All of this demonstrates that the DSI approach has excellent energy-saving and emission reduction potential for marine engines. The steam-injection strategy should be optimized in the future.

Adhering to the thermal management requirements of prismatic battery modules, an improved lightweight parallel liquid cooling structure with slender tubes and a thin heat-conducting plate is proposed. The multiobjective optimization of the structure, operating parameters of the thermal management system, and thermal characteristics of a battery module are carried out. The effects of the equivalent diameter of the square tubes and the inner diameter of the circular tubes on the maximum temperature, maximum temperature difference of the batteries, and coolant pressure drop of liquid cooling battery thermal management system (BTMS) are investigated. The more significant factors, selected as the design variables, are the equivalent diameter of the square tubes, the inner diameter of the circular tube, and the coolant inlet velocity. Combining the computational fluid dynamics (CFD) method with multiobjective optimization, the different conditions of heat management parameters obtained by Latin hypercube sampling are numerically calculated, and the optimization of these parameters is carried out by the second-generation Elitist Nondominated Sorting Genetic Algorithm (NSGA-II). Compared with the thermal characteristics of the battery module before optimization, the maximum temperature of the module after optimization is reduced by 9.3%, the maximum temperature difference is reduced by 20.7%, and the coolant pressure drop from inlet to outlet is reduced by 49.1%.

This paper proposes a novel approach to control the output power of a proton-exchange membrane fuel-cell (PEMFC) system by the current rate of change. First, a 70-kW PEMFC system model is established, which is validated by the experimental data with the maximum error of the output voltage being 3.68%. Then, a multi-input multi-output (MIMO) sliding mode controller (SMC) is designed based on the PEMFC model to maintain the excess oxygen ratio, the pressure difference between the cathode and anode, the stack temperature, and the output power at expectations. Finally, the simulation results show that the designed MIMO SMC performs better than the conventional proportional integral (PI) controller. When the PEMFC system responds to the dynamically changing power demand, the maximum power tracking error is 4.79%, the temperature fluctuation is about 0.06 K, and the pressure difference is maintained within 10 Pa.

The design and optimization of the hydrogen circulation system can improve the hydrogen utilization rate and overall system efficiency of the fuel cell system. The ejector and water separator are important parts of the design for the ejector-driven hydrogen circulation system. In this study, a novel ejector with water separation functionality is proposed based on a three-dimensional fluid transient simulation, which integrates the ejection and water separation functions in the traditional anode hydrogen circulation system. The separation efficiency of the novel ejector reaches 70.07% and 72.59% under low and high power, respectively, and the deviation is only about 3.47%. In addition, the entrainment ratios of 30 kW, 40 kW, and 50 kW have been improved by 12.54%, 8.04%, and 9.53%, respectively, through further optimization of the new ejector. The new ejector greatly simplifies the system structure while maintaining the original performance, which is innovative and beneficial to improving the power and energy density of the system.

In the context of global efforts to pursue carbon neutrality, the methanol-diesel reactivity controlled compression ignition (RCCI) combustion has become a promising strategy for diesel engines to reduce emissions with higher thermal efficiency. To further improve the fuel economy of the methanol–diesel RCCI engine and reduce its unregulated emissions, the effects of methanol substitution rate (MSR) and exhaust gas recirculation (EGR) on fuel economy, ${\mathrm{CO}}_2$, MeOH, and H-CHO emissions at different engine speed and load conditions were tested. The conversion efficiency of a conventional diesel oxidation catalyst (DOC) in oxidizing the unregulated emissions under the World Harmonized Steady-State Cycle (WHSC) was also evaluated. The results showed that running the engine in methanol-diesel RCCI mode can significantly improve fuel economy at medium to high loads [greater than 1.18 MPa brake mean effective pressure (BMEP)]. The equivalent brake specific fuel consumption (ESFC) in the RCCI mode was lower than that of the baseline CDC mode, and the utilization of EGR can further improved fuel economy at lower load conditions. The MeOH and H-CHO emissions increased as the MSR increased, and they decreased as the load level and EGR rate increased. The ${\mathrm{CO}}_2$ emissions decreased as the MSR increased, but they increased as the EGR rate increased. The conversion efficiencies of DOC for MeOH, H-CHO and aromatic hydrocarbons (AHC) were around 93%, 84%, and 61%, respectively, under the WHSC test. Exhaust after-treatment by DOC is an effective solution to reduce unburned MeOH and H-CHO from methanol-diesel dual-fuel engines, and to extend RCCI operating range.

To address the challenges of integrating ever-increasing renewable energy generation such as wind power and photovoltaic (PV) to the power system concerned, the wind-PV-thermal-bundled (WPTB) transmission mode could be employed to mitigate the intermittent power output from wind and PV units. In this work, based on the mathematical models of thermal generation units, wind turbines and PV units, the WPTB transmission system model is developed. Each load demand is modeled as a random variable under various operating conditions of the concerned power system. The Lyapunov linearization method is used to obtain the system matrix of each operation mode, and then all eigenvalues of the system can be attained. According to the generator participation factor, the oscillation modes of the power system are classified. The eigenvalue and damping ratio of each oscillation mode are employed to formulate the objective function of the optimal strategy for enhancing probabilistic small-signal stability, and the particle swarm optimization algorithm is used for global optimization. The secure operation of the power system can be ensured in a wider range by the designed optimization strategy in this work. The effectiveness of the method is demonstrated by simulation results of a sample power system which can operate stably under some uncertain operation conditions.