Journal of Energy Engineering

Quick start at low temperature is one of the key bottleneck technologies that restrict the large-scale commercialization of proton exchange membrane (PEM) fuel cell vehicles. Ice and devices in battery systems based on thermal deicing are inevitably subjected to thermal shock. In order to study this problem, a non-Fourier heat conduction model is established to study the temperature response of a spherical ice particle subjected to thermal shock with different boundary conditions on the surface. Furthermore, distribution of thermal stress in the particle is obtained using the calculated temperature field, and the effect of thermal relaxation time and boundary conditions on the temperature response as well as the thermal stress field are also analyzed. The results, which are significantly different from that obtained using Fourier law of heat conduction, show that the mechanical stresses and the serious expansion of the ice particle may lead to the severe deformation of the devices connected to the ice during the cold start-up, imposing a great challenge in controlling structural reliability of PEM fuel cells. The numerical results are expected to provide a scientific theoretical basis for PEM fuel cell design and low-temperature start-up control.

One of the major ways of delivering renewable energy generation from large-scale energy bases in northeast, northwest, and northern China, is to bundle the power from wind-photovoltaic-thermal units and then transmit it by alternating current/direct current (AC/DC) systems. The interactions among wind power units, photovoltaic (PV) power units, and high voltage direct current (HVDC) systems make the stability of the interconnected power system more complicated. In this paper, a framework model of wind-PV-thermal-bundled power transmitted by AC/DC systems is constructed based on the mathematical models of thermal power units, wind power units, PV power units, and HVDC transmission systems. Considering the fluctuations of wind power outputs and PV power outputs, a small-signal stability model of an interconnected power system with wind-PV-thermal-bundled power transmitted by AC/DC systems is established based on an existing stability theorem. The interactions among various forms of energy and their impacts on wind-PV-thermal-bundled power transmitted by AC/DC systems are studied through theoretical analysis and dynamic simulation. A sample interconnected power system is used to demonstrate the proposed method. It is shown by simulation results that the wind-PV-thermal-bundled power transmission distance; the electrical distance among wind power units, PV power units, and thermal power units; the grid-connected capacities of wind, PV, and thermal power units; and the power loading ratios of AC lines over DC lines all have various degrees of impacts on the stability of the interconnected power system.

The low temperature difference (LTD) Stirling engine is important for solar power application. This study focuses mainly on the influence of physical and geometrical parameters on the operating characteristics of two LTD Stirling engines. It shows that a phase shift of 90°–120° implies the best performance of one of them (electrical powered engine), while the optimum phase shift is about 95°–105° for the other (solar powered engine). In addition, the friction loss increases with increasing rotation speed for both engines. It seems that the friction loss increases with the phase shift for the electrical powered engine, while it attains maximum value at 90° for the solar powered engine. Thermal efficiency first increases with the heating power and then decreases for both engines. Optimum conditions for the maximum indicated power of both Stirling engines are also indicated. Comparison between experimental results of the electrical heating engine and those obtained by simulations with several complexity levels [0-Dimension (0-D), 1-Dimension (1-D), and 2-Dimension (2-D)] has shown that the 1-D model can provide the best simulation results.

The 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. First, by establishing a fuel cell purging model integrated with a high-frequency impedance module, the relationship between the initial water content of the fuel cell stack, purging time, and gas temperature was qualitatively analyzed. Second, a constant flow shutdown purging control strategy for fuel cell stacks for vehicles was designed based on the simplified and practical purging model. Purging experiments showed that the humidity of the intake air mainly affected the degree of final purging and drying, and the initial moisture content mainly affected the total time required for purging and drying. The constant flow purging strategy adopted map-based control, which is relatively simple and can be easily controlled in real time within the specified purging time limit. After purging, the stack started successfully in the range of $-10°\mathrm{C}$ because the water still existed in the form of supercooled water. However, when the temperature reached $-20°\mathrm{C}$, the large amount of ice hindered gas transmission, which resulted in a failure of start-up. Cyclic voltammetry results show that the freezing of supercooled water will reduce the electrochemically active area by about 5% after four cycles.

This work presents the development and use of a comprehensive, quasi-dimensional, two-zone combustion model aiming at predicting the combustion characteristics, performance, nitric oxide (NO), and carbon monoxide (CO) emissions of high-speed spark-ignition (SI) engine. The model is validated against pertinent data from experimental investigation conducted at the authors’ laboratory on experimental, Ricardo E6, mono-cylinder, high-speed SI engine, having the capability to operate over a wide range of compression ratios (CR) (variable compression ratio engine, VCR) and (fuel-air) equivalence ratios (EQR). In this work, a comparison between experimental and computational results is carried out for the engine fueled with gasoline, operated under various CR and EQR values at wide open throttle (WOT) position. The developed model is a two-zone one consisting of an unburned and a burned zone. It simulates the combustion process by following closely the flame front movement and development in the combustion chamber, at each instant of time, taking into account the history of pressure, temperature, and local composition. The unburned mixture turbulent entrainment into the burning zone through the flame front area is considered with its subsequent combustion. The flame front movement determines also the wetted areas and the volumes in the two zones. To determine the concentration of the chemical species equilibrium model is employed, while pertinent chemical kinetics schemes are used for computing NO and CO concentrations. The obtained pressure, mass fraction burned (MFB), mass fraction entrained (MFE), temperatures in the two zones, and NO and CO histories assist in the understanding of the complex phenomena involved, while they facilitate the interpretation of performance indices and knocking tendency. They shed light into the underlying physical and chemical mechanisms influencing the related SI engine performance and emissions attributes. The computed results are found to be in good agreement with the respective experimental ones.

Making photovoltaic (PV) systems more energy efficient is a huge challenge toward their widespread adoption. Partial shading is one of the major contributors of power reduction in PV systems. The interconnection scheme of the modules, the pattern of partial shading, and the module setup area determine the extent of power reduction. Conventional literatures contain arrays set up in rectangular and square shapes following various topologies to neutralize the adverse effects of partial shadings. Therefore, new areas of research may include connection of solar modules in new shapes to form unevenly sized PV arrays. This paper includes PV modules configured in six types of novel triangular connections and compares their performances under shading conditions. These interconnection schemes are (1) triangular series-series (TSS), (2) triangular series-parallel (TSP), (3) triangular parallel-parallel (TPP), (4) triangular honeycomb (THC), (5) triangular bridge link (TBL), and (6) triangular total-cross-tied (TTCT). Each of the six configurations are compared to one another, taking into account their different performance parameters such as maximum power, fill factor, and power loss. The TSS configuration method is found to have a better performance edge over TSP, TBL, THC, and TTCT configurations for all the considered parameters.

Recently, light electric vehicles such as pedelecs, electric bicycles, and electric scooters have shown excellent potential for short distances. It is essential to control the power required by the electric motor in response to power demand fluctuations. Hybrid energy sources have some essential features to improve efficiency and dynamics. This paper presents a hybrid system of air-breathing proton exchange membrane (PEM) fuel cells (ABFC) and supercapacitors (SC) for operating electric bicycles. Since the supercapacitors meet some of the load variations, the hybrid system offers the opportunity to optimize the air-breathing PEM fuel cells to achieve better fuel consumption and performance. A hybrid dynamic model of the ABFC-SC system is built by using MATLAB Simulink version 9.3 and Simscape Power Systems toolbox for electric bicycles. A web-based software tool used to examine an electric bicycle is employed to create a realistic driving cycle for electric bicycles. The simulation results of the hybrid ABFC-SC system are compared with the results of the ABFC system. The power requirement of an electric bicycle fluctuates depending on the riding conditions, type of electric motor, and control system. The simulation results show that the hybrid ABFC-SC system provides efficient and sufficient energy for an electric bike.

This paper presents machine learning valley-filling (MLVF) to enhance plug-in electric vehicle (PEV) charging at the local power level while minimizing the effects of uncontrolled PEV charging. This study investigated whether a neural network algorithm could learn to identify when to begin charging a PEV by distinguishing low and high demand sections in the forecasted baseload. The results indicate that a neural network algorithm can indeed identify low and high demand sections in the forecasted baseload and learn when to begin charging electric vehicles to decrease demand loads. MLVF achieved a microaveraged $F1$ score, an indicator of a classifier’s overall accuracy, of 92.84% of selecting the correct timeslot to initiate charging.

Water transport in coal is a multiscale, multiphysical process that is affected by the coupling effect of multiple systems, such as pore and microfracture systems. In this study, numerical simulation of the microlevel of a coal matrix structure during simulated uniaxial compression mechanical experiments is performed. Based on the results, a realistic deformation pore structure model is developed to simulate coal seam water injection seepage. Under the effect of different loading directions, different results show that the distribution of pores and the loading direction of stress have a significant impact on the failure mode of coal matrix materials. As time proceeds, the porosity decreases from 64.2% to 61.7%. The porosity begins to increase again after 0.8 s. By setting the pressure gradient in the $y$-direction of the pore model to simulate seepage, the permeability of the coal matrix material is found to increase with the passage of time. A positive correlation exists between permeability and porosity. Avizo version 9.0, LS-DYNA version 14.0, and Fluent version 18.0 are used to study the mechanical characteristics of the pore structure of coal, as well as the seepage characteristics from the microperspective, which provides a new idea.

An investigation of the characteristics of turbulent prevaporized n-heptane-air flames, stabilized in the downstream wake region of a stratifier/bluff-body combustor configuration under the effect of inlet fuel-air mixture stratification and preheat, is presented. Experimental and computational results for different fuel-air mixing wake topologies have been obtained by regulating the fuel injection level and the incoming fuel-air ratio gradient. Fourier transform infrared analysis and particle image velocimetry measurements were taken to assess the mixing field characteristics in the near-wake region under nonreacting operation. Chemiluminescence measurements were also obtained to assess the flame topology under reacting conditions. In the computations, an implicit, finite-volume-based large eddy simulation method, coupled with a 25-species reduced $\mathrm{n}\text{-}{\mathrm{C}}_7{\mathrm{H}}_{16}$ kinetic scheme, was utilized for reproducing the mixing and the reacting field characteristics.

Coal reservoirs are highly heterogeneous, and different macrolithotypes of coal have differences in material composition and pore structure, resulting in different coalbed methane (CBM) adsorption/desorption performance and gas-water migration characteristics. In order to explore the pore development characteristics of different macrolithotypes in low-rank coal and their influence on adsorption and desorption, the low-rank coal of the Yan’an Formation in the Huanglong coalfield is taken as the sample of the research object. Vitrain and durain are collected and then separated. The results show that the porosity of coal is 2.92%–11.81%, and the total porosity is relatively developed. The porosity of durain is slightly higher than that of vitrain, the pore throat of durain is thicker, and the macropores are more developed. The Brunauer-Emmett-Teller (BET) specific surface area (SSA) and Barrett-Joyner-Halenda (BJH) total pore volume (TPV) of vitrain is slightly larger than durain, the micropores are more developed, and most of them are semiclosed and ink-bottle-shaped pores. The pore-fracture connectivity of the durain is better than vitrain via nuclear magnetic resonance (NMR). Vitrain has more developed micropores than durain, with larger specific surface area and stronger adsorption capacity. In the process of methane adsorption/desorption, the phenomenon of desorption hysteresis generally exists. Moreover, the pore connectivity of durain is relatively good, the degree of desorption hysteresis is weaker than vitrain, and the theoretical desorption efficiency is higher than vitrain. Temperature has a significant influence on adsorption and desorption of methane. The increase in temperature is beneficial to desorption and weakens the hysteresis effect of desorption. At the same temperature, the isosteric adsorption heat in the desorption process is greater than the adsorption process, which further explains the incomplete reversibility of the adsorption and desorption process. Under the same conditions, the isosteric adsorption heat of vitrain in the adsorption process is greater than durain, and adsorption capacity of vitrain is stronger than durain. Similarly, the isosteric adsorption heat of vitrain in the desorption process is greater than durain, and desorption needs to absorb more heat. The desorption capacity is weaker than durain.

Composite walls are widely used in the external walls of buildings for their favorable thermal performance in building energy efficiency. However, the thermal performance difference under various occupant behaviors’ impacts is often ignored. This study aims to investigate the influence mechanisms of occupant behaviors on typical composite wall thermal performance in the heating rooms with combined numerical and experimental methods. Firstly, the methodology of inherent heat transfer matrix analysis is introduced to predict the original thermal performance by solving the characteristic value. Secondly, a four-in-one platform of grid-wall including four typical composite walls, single-layer insulation, exterior insulation, interior insulation, and intermediate insulation is built for the specific analysis. Thirdly, based on the specified conditions of typical walls and occupant behaviors, field test and numerical simulation are used to validate the theoretical analysis. The results reveal that the original thermal performance of composite walls may be greatly influenced under behavior intervention. It also indicates that the occupant behaviors are significant to building energy efficiency, especially in the actual operations of energy efficient buildings with composite walls.