Journal of Energy Engineering

The alteration of rock wettability is essential for improving the efficiency of low-salinity water flooding. However, the fundamental understanding of this process remains unclear. In this paper, an extended Derjguin–Landau–Verwey–Overbeek (DLVO) theoretical model was applied to study the effects of cation types, brine concentrations, and mineral types on rock wettability in an oil–brine system. The zeta potential was measured to analyze the electric double layer. The oil–rock contact angles were tested to evaluate the wettability. The disjoining pressure was also estimated to analyze the rock wettability changes. The divalent cation showed a greater ability to compress the electric double layer. The lower concentrations of cations led to a thicker water film, promoting the detachment of oil from the rock surface. Clay minerals showed a greater impact on the wettability of rocks than quartz. This study improves the fundamental understandings of alteration of wettability in brine, oil, and rock systems and elucidates the mechanisms of disjoining pressure effects on low-salinity water flooding while shedding some light on improving the performance of low-salinity water flooding.

The design of a channel has significant effects on mass transfer and water and heat management inside fuel cells. In this study, novel three-dimensional porous channels and their optimizations (coupling design of the geometric structure and surface shape) by the computational fluid dynamics (CFD) method are proposed to improve the comprehensive performance of direct ethanol fuel cells (DEFCs). The overall electrical performance of tubular DEFCs is significantly better than that of parallel channels because of the better capacity for oxygen convection and water removal of three-dimensional (3D) porous channels. Among porous channels of different shapes, the square and triangular shapes were found to perform the best. Through optimization, the oxygen mass transfer capacity was enhanced remarkably owing to better convection, while the velocity distribution was disordered in some cases. The results also indicate that square porous channels and their optimizations have a power density growth rate of 15.99% and 40.86%, respectively, at 0.2 V, compared to parallel channels. Therefore, the square tapered design further improves the pressure drop inside the electrode layers, provides a more uniform distribution of oxygen, and allows liquid water to concentrate at the outlet, and has thus been proven to be the optimal choice.

Prechamber jet ignition can significantly enhance the progress of combustion and improve fuel consumption. Based on a single-cylinder gasoline engine and a small-volume prechamber, the prechamber local air-fuel equivalence ratio, ignition type, and compression ratio were examined under lean burn conditions. The results show that spray wetting the wall of the prechamber is inevitable. As the quality of the fuel injection increases in the prechamber, the quality of the spray wetting the wall also increases, and the particle number (PN) emissions increase significantly. A rich mixture in the prechamber can improve ignition and enhance the main chamber combustion process. However, an air-fuel equivalence ratio near 1 in the prechamber has a greater fuel saving potential and lower PN emissions. When the excess air coefficient in the main chamber is below 1.4, the gross indicated thermal efficiency (GITE) of the prechamber is lower than that of the spark plug ignition. When the excess air coefficient in the main chamber is above 1.4, the prechamber significantly improves the thermal efficiency of the lean burn. The prechamber achieves a maximum GITE of 48.5% when the excess air coefficient in the main chamber is 1.8. Moreover, a prechamber combined with a high compression ratio can further improve combustion and reduce fuel consumption in the lean burn engine. Prechamber ignition extends the lean burn limit to the excess air coefficient value of 2.1 in main chamber, and nitrogen oxides emissions are as low as 58 ppm. Furthermore, the higher the compression ratio, the more the mixture gas is pushed into the prechamber, which increases the prechamber ignition energy and enhances the combustion process of the main chamber.

The battery temperature rise rate is significantly increased when a lithium battery pack is discharged at a high discharge rate or charged under high-temperature conditions. An excessively high temperature will have a great impact on battery safety. In this paper, a liquid cooling system for the battery module using a cooling plate as heat dissipation component is designed. The heat dissipation performance of the liquid cooling system was optimized by using response-surface methodology. First, the three-dimensional model of the battery module with liquid cooling system was established. Second, the influence factors of the liquid cooling effect of the battery module were analyzed. Then, the optimal conditions level and corresponding response values of the factors within the global range test were obtained by response-surface optimization design. The interaction among the different factors was analyzed, and thus the combination of the factors with optimal liquid cooling heat dissipation performance was achieved. Finally, the response value predicted by the optimal combination of influencing factors in response-surface optimization analysis was obtained. The results were compared with the results calculated by software simulation under the same conditions to verify the accuracy of optimization effect of response-surface model. The results were also compared with the maximum temperature and temperature difference results of battery pack obtained from the original model so as to evaluate the optimization effect of the response-surface method. The results showed that the feasibility and liability of response-surface optimization model can be verified. The response-surface optimization method can appropriately control the parameters that effectively reduce the heat generation of batteries, which is significant for the research of battery thermal management.

The application of heavy fuel oil (HFO) in high-speed diesel engines is a practical approach to reducing running costs. However, the physical and chemical properties of HFO may lead to a poor burning rate and increase pollutant emissions. Emulsified HFO has been studied and applied for the purpose of improving combustion and emissions of HFO. However, the stability and water content of emulsified HFO need to be improved. In this study, emulsified HFO with high stability and high water content (up to 30%) was investigated to study the combustion and emission characteristics of the HFO, and a simulation combined with experimental methods was used. The microimages obtained using an electronic microscope highlighted the high stability of emulsified HFO. According to the test results on physical characteristics, compared with low-water-content conditions, HFO has a higher viscosity along with a lower surface tension under high-water-content conditions. To investigate the engine performance with the increasing water content in the HFO, the combustion model was calibrated using engine test experiments. The simulation results showed that with the increasing water content of emulsified HFO, the maximum cylinder pressure, heat release rate, and the crank angle (CA) corresponding to 10% of total heat release (CA10) increased, while the CA corresponding to 50% of total heat release (CA50) decreased. In addition, the experimental results of nitrogen oxide (${\mathrm{NO}}_x$) and soot emissions maintained a consistent downward tendency with the increase in water content. Compared with fueling HFO, burning emulsified HFO with 30% water content can reduce ${\mathrm{NO}}_x$ and soot emissions by 40% and 32%, respectively. The simulation contour maps show the corresponding relationships among the temperature, equivalence ratio, ${\mathrm{NO}}_x$, and soot. The hydrocarbon emissions increased with the water content, and the carbon monoxide emissions similarly maintained the same increasing tendency in addition to fueling the emulsified HFO with 10% water content. Consequently, with a higher water content of the emulsified HFO used in the engine, better combustion was observed with reduced emissions.

Compressed-air energy storage has been considered as a promising technology to smooth the fluctuations of renewable energy sources and improve the peak-shaving flexibility capacity of power systems. In order to improve the energy degree of compression heat and enhance the system performance, the current paper described a novel combined heating and power system that integrates compressed-air energy storage with thermochemical technology. In the proposed system, the compression heat is coupled with methanol decomposition reaction to convert the thermal energy to chemical energy (${\mathrm{H}}_2$ and CO). A sensitivity analysis was carried out to investigate the effects of five key parameters on the system performance. The results indicated that higher values of air-to-methanol ratio, pressure ratio, and isentropic efficiency of gas turbine have positive influences on the thermodynamic performance of the proposed system. In addition, the multiobjective optimization was implemented to ascertain the optimum performance from the aspect of thermodynamics and economics. The optimal condition selected using the technique for order preference by similarity to an ideal solution (TOPSIS) method demonstrated that the exergy efficiency and levelized cost of energy of the proposed system are 39.42% and $\$96.58/\mathrm{MWh}$, respectively.

As an emerging technology, simultaneous hydraulic fracturing has shown significant potential for increasing stimulated reservoir volume (SRV) in the development of shale gas. However, due to complicated interfracture interference mechanisms, its application is quite limited. In this study, a triwell simultaneous fracturing in shale gas reservoirs was modeled to explore the interaction laws between fractures as well as illuminate the formation conditions of complex fracture networks. The coupled fluid flow-Discrete Element Method (DEM) approach was used to simulate the initiation and synchronous propagation of hydraulic fractures. Also, the effects of far-field geostress difference, well spacing, injection procedure, and injection rate were investigated. Due to interwell and interfracture interference, complex fracturing initiation and propagation behaviors occur, and three interfracture interference mechanisms, including suppression, attraction, and repulsion, were observed. The stress shadow effect and far-field geostress state compete to influence fracture initiation, while well spacing also plays a key role. For a large far-field geostress difference, a small well spacing can improve the development of hydraulic fractures by increasing interfracture interaction. However, when a fracture preferentially initiates and propagates, it significantly suppresses the propagation of later emerging fractures, leading to poorly developed fractures. Therefore, appropriate injection procedures should be used to induce more complicated fracture networks.

In this study, a primary optical element (POE) was designed to collect and distribute solar radiation uniformly on a multijunction solar cell in a concentrator photovoltaic (CPV) system. The POE combined multiple free-form sublenses to improve the uniformity of the concentrated sunlight and reduce the optical losses. Each sublens was designed based on the principles of nonimaging optics such as the edge ray theorem, Snell’s law, and the conservation of optical path length. Furthermore, the POE had multifocal points for focusing the direct sunlight, in which the number of focal points equaled the number of the sublenses. Each free-form sublens can guide and distribute the direct sunlight uniformly over the receiver; thus, the uniformity of concentrated light over the multijunction solar cell was improved significantly. The simulation results showed that the designed POE can concentrate the direct sunlight on a square area that matched the active site of the multijunction solar cell with a concentration ratio of about 484 times, uniformity of about 78%, and total optical losses smaller than 20%.