Journal of Energy Engineering

Mining disasters such as coal and gas outbursts and rock bursts seriously threaten safe coal mining. Accordingly, it is important to analyze the dynamic damage characteristics of gas-bearing coal and achieve safe coal mining. In the present study, the criterion of crack arrest for gas-bearing coal was set in accordance with the theory of gas dynamics and fracture mechanics. Then, the impact failure characteristics of gas-bearing coal subjected to an impact load were investigated. In this regard, different parameters, including the impact energy, gas pressure, and gas type, were considered. Moreover, the transient effect of the free gas on the coal destruction was analyzed. The influence of free gas-induced damage on the coal structure was also simulated using ANSYS 17.0 software. The obtained results show that, during the impact crushing of gas-bearing coal, the gas moves in the coal body, which tears the coal body and increases the crushing effect of coal. The crushing effect of gas-bearing coal has a linear correlation with gas pressure, impact energy, and gas adsorption strength. It was concluded that the proposed crack arrest criterion and numerical methods are powerful measures for analyzing the failure characteristics of gas-bearing coal.

For the development and utilization of geothermal reservoirs, a thorough understanding of fluid flow and heat transfer characteristics in rock fractures is crucial. The effect of two interacting crossed-rough fractures on heat recovery capacity was studied in this research, in which a three-dimensional thermal-hydraulic-mechanical (THM) model is established to simulate heat extraction from a granite reservoir. The impact of fracture surface roughness and the angle between the two rough fractures on heat extraction performance are examined, where thermal power and reservoir recovery rate were taken as indicators to access the heat extraction performance. The geometry of the fluid flow line on the rough fracture surface changes as the fracture aperture changes, according to simulation data. From the injection well to the production well, the fluid flows in a circuitous route, skipping the smaller fracture opening and passing through the bigger one. The fracture permeability and fracture aperture increase as the distance between the location and the injection well becomes closer. When the angle between the two fracture surfaces is fixed, the high-pressure zone is concentrated, and the faster the flow rate, the faster the matrix in the rock matrix between the fractures is exploited, resulting in a reduction in production thermal power later in the mining period, which is not conducive to sustainable development. With the passage of time, the rate of reservoir recovery rises essentially linearly.

A numerical simulation method was used in this study to explore the effects of equivalence ratio, intake air temperature, intake air pressure, compression ratio, and engine speed on the combustion and performance characteristics of cyclohexane/n-heptane dual-fuel HCCI (homogeneously charged compression ignition) using a zero-dimensional single-zone combustion model. In this paper, the ignition delay time and laminar flame speed were used to verify the selected chemical reaction kinetics mechanism, and the effect of each engine parameter variation on the combustion performance of cyclohexane/n-heptane dual-fuel HCCI was numerically simulated. The results demonstrate that the advanced combustion phase occurred with an increase in equivalence ratio, intake air temperature, intake pressure, and compression ratio as well as a decrease in engine speed. The work indicated increases on average by about 27% for each 0.2 increment in equivalence ratio. The excessive rise in intake air temperature leads to higher pressure rise rate and lower indicated work. For every 10 kPa improvement in intake pressure, the starting point of combustion is advanced by about 1.3°CA, and the peak pressure in the cylinder is improved by about 11 bar. The appropriate improvement of the compression ratio increases the heat release rate and the indicated work. The effect of engine speed is not so significant. The peak in-cylinder pressure rises by only about 1.22 bar for each 100 rpm increment in engine speed, and the indicated work and indicated mean effective pressure improve only about 3%. Ultimately, it is found that the introduction of cyclohexane makes the low-temperature heat release region in HCCI combustion of cyclohexane/n-heptane weaker than that of pure heptane.

The physical properties of shale reservoirs directly affect the form of shale gas and its preservation conditions, and an evaluation of these properties has great significance on the storage and exploitation of shale gas. In this study, the Longmaxi Formation shale in the Sichuan Basin was considered as the research object. The parameters of porosity, permeability, mineral composition, total organic carbon content, density, in-situ stress, brittleness index, and fluid saturation of the samples were obtained through a series of tests. The influence of each parameter on the porosity and permeability was analyzed, and a weight analysis was conducted using an analytic hierarchy process (AHP). The results show that: clay mineral content in the mineral composition has a positive correlation with porosity, and the content of brittle minerals can affect the fracture development density and improve the permeability of samples; the total organic carbon content has a positive correlation with porosity and permeability; increases in density and in-situ stress will significantly reduce the porosity and permeability of samples; the brittleness index has a negative correlation with porosity and a positive correlation with permeability; water saturation has a positive correlation with porosity, gas saturation has a negative correlation with porosity, and the movable oil saturation has an obvious logarithmic relationship with porosity and a negative correlation with permeability. Furthermore, the AHP was used to analyze the specific effects of various parameters on porosity and permeability, with the weight of total organic carbon content > mineral composition > brittleness index > density > in-situ stress > fluid saturation. In this study, we evaluated the physical properties of shale reservoirs, which provides an important basis for shale reservoir evaluation and mining area selection.

In recent several decades, the study of the cooling methods for the photovoltaic/thermal (PV/T) system has stimulated the interest of many researchers due to the electrical efficiency improvement. However, the PV thermal stress and operation reliability of PV/T systems are rarely investigated. In this work, the operational performance and the PV thermal stress induced mechanism of three types of PV/T systems: tube-sheet PV/T (TS-PV/T), heat pipe PV/T (HP-PV/T), and phase change material PV/T (PCM-PV/T) are investigated. The results show that the primary reason for the development of the PV thermal stress is the different thermal expansion characteristics between PV and absorber. Thermal inhomogeneity plays a minor role in PV thermal stress. PV thermal stress can be significantly reduced by reducing pipe spacing, absorb thickness, or solar panel coverage rate. The PV thermal stress of the HP-PV/T system is 50 MPa higher than the TS-PV/T system. The PV thermal stress of the PCM-PV/T system is 20 MPa lower than the TS-PV/T collector system.

Abstract

In recent years, the increasing demand for the establishment of smart ocean systems has promoted the exploration of self-powered sensing devices. However, a traditional battery has the disadvantages of limited energy storage and frequent replacement. These disadvantages make it difficult to power electronic equipment for a long time, which has become a major obstacle to constructing smart ocean systems. To meet these challenges, a new self-powered sensing device based on a microthermoelectric generator (MTEG) and flag-type triboelectric nanogenerator (FTENG) is proposed. The prototype is a hybrid energy-harvesting system. Through this prototype, solar and wind energy can be harvested simultaneously, and it can be employed to continuously power environmental sensors. In addition, the performance of the prototype was tested. Under the condition of a hot end at 70°C and a wind speed of $10\mathrm{m}/\mathrm{s}$, the prototype can charge the supercapacitor to 5.5 V in 10 min. Additionally, the prototype can sense and transmit wind speed and direction data. This research provides a new way for the establishment of smart ocean systems.

Practical Applications

In this paper, we present a novel self-powered sensing device based on MTEG and FTENG. The prototype is a hybrid energy-harvesting system. Through this prototype, solar and wind energy can be harvested simultaneously. When the FTENG unit works alone, it can achieve 24.1 V output voltage and 12.3 $\mu \mathrm{A}$ output current at a $10\mathrm{m}/\mathrm{s}$ wind speed. The thermoelectric generator (TEG) unit can generate a 6.3 V output voltage and a 2 mA output current at 70°C. Through the rectifier circuit, the prototype can charge the supercapacitors (2.5F) to 5.5 V in 10 min. It can be used as a battery to continuously supply power to environmental sensors. In addition, environmental data such as wind speed and direction were sensed by the FTENG unit. These data can be transmitted and visually displayed with the assistance of the micro controller unit (MCU). The prototype can be used to sense information in harsh environments, reducing reliance on batteries to power electronics and the Internet of Things. This research provides a new way for the establishment of smart ocean systems.

The Zhu-III sag is a key area of resource exploration with great potential of resources in the Pearl River Mouth Basin. The reservoir in the Zhuhai Formation is a typical tight sandstone reservoir. Tight sandstone has the characteristics of low porosity, low permeability, complex pore structure, and diagenesis. However, past studies in the Zhu-III sag have mainly focused on sedimentology, and there has been little research on diagenesis and diagenetic facies. Diagenetic facies are a significant tool for determining the key parameters of tight sandstone reservoir properties. In this work, diagenesis of the tight sandstone reservoir in the Zhuhai Formation in the Zhu-III sag was investigated by multiple experiments using borehole core, thin sections observation, scanning electron microscope, energy dispersive spectrometry, X-ray diffraction, conventional rock analysis, and pore quantitative analysis to determine the influence of the main diagenesis and minerals. Based on these experiments, the diagenetic sequence was clarified. Four diagenetic facies were identified: strong-compaction facies, medium-compaction facies, carbonate-cemented facies, and clay mineral filling facies. The strong compaction and carbonate cements were the primary reasons for the development of low-quality reservoirs. The medium-compaction facies and clay mineral filling facies commonly corresponded to the high-quality reservoirs. The facies prediction system was established, and the favorable reservoir via well logs could be predicted. The findings showed that the reservoir quality or oil-bearing properties are dominated by diagenetic facies. This study provides guidance to distinguish the productive zones and evaluate the pore-throat systems and reservoir quality in the Zhu-III sag and the Pearl River Mouth Basin.

In this work, the characteristic data of the peaks of the incremental capacity (IC) curves, the constant-current (CC) charging time, and their neighborhoods were determined during the CC charging phases of a battery. These data were transformed into an aging characteristic series and input into a long short-term memory (LSTM) recurrent neural network to achieve an accurate short-term capacity estimate. A method was then developed to predict the long-term remaining useful life (RUL). Specifically, a double exponential empirical model (DEEM) was employed to describe the fade trend of the battery capacity. The DEEM model parameters were initialized based on the offline nonlinear least squares (NLS) method. The particle filter (PF) algorithm was then used to update the DEEM model parameters and predict the RUL based on the short-term capacity estimated by the LSTM network. The experimental results revealed that the proposed method could effectively overcome the phenomenon of lithium-ion battery capacity regeneration and inconsistency. In addition, this method could realize the RUL prediction of the continuous prediction start point (SP). Under the same prediction SP setting, the proposed method outperformed other prediction methods.

The objective of the present work is to investigate the effect of ethanol fraction on combustion, performance, and emission characteristics of a reactivity controlled compression ignition (RCCI) engine fueled with diesel, cottonseed oil methyl ester (CSME), and hemp oil methyl ester (HOME). The low-reactivity fuel ethanol was injected into the intake manifold using a timed manifold injection system controlled by an electronic control unit, whereas the high-reactivity fuel (diesel, CSME, HOME) was directly injected into the combustion chamber at different injection timings to initiate ignition. The injection timing of 23° crank angle (CA) before top dead center (bTDC) promises a flexible combustion phasing and better results in terms of maximum pressure rise, heat release rate, brake thermal efficiency, and emission characteristics using ethanol as a primary fuel. The RCCI mode of operation with a 20% ethanol premixed ratio resulted in increases in brake thermal efficiency by 28.07%, 10.70%, and 12.34% for diesel, CSME, and HOME at part load (50%), respectively, and 13.22%, 3.09%, 5.92% for the same fuels at full load (100%) conditions owing to uniform mixing and better atomization of fuel particles. The specific fuel consumption decreased by 13.79%, 10.34%, and 8.62% for 20E+80Diesel, 20E+80HOME, 20E+80CSME energy ratios, respectively, at part load conditions and 14.29%, 12.50%, and 10.71% at full load conditions versus the conventional combustion mode. The exhaust gas temperature slightly decreased for 50% and 100% load conditions compared with conventional diesel combustion. A reduction of ${\mathrm{CO}}_2$ emissions was observed for 20E+80Diesel, 20E+80HOME, and 20E+80CSME fuel fractions by 2.37%, 2.06%, and 2.85% respectively, at full load condition compared with neat diesel fuel. While increasing the load in all fuel shares, smoke emissions increased slightly. The low-temperature RCCI combustion strategy achieved a reduction of NOx emissions by up to 18.90% at a full load condition for 20E+80HOME fuel fractions.

Hydraulic fracture (HF) propagation behavior is significant when building an enhanced geothermal system for hot dry rock (HDR) and evaluating the simulated reservoir volume (SRV) for shale gas reservoirs. The HF propagation behaviors are closely related to geologically structural discontinuities (SDs), which differ significantly between HDR and shale. Granite, one of the most common HDRs, mainly possesses natural fractures (NFs), quartz veins (QVs), and lithological interfaces (LIs). The SDs in the shale are mainly bedding planes (BPs) and NFs. According to the physical and mechanical property differences regarding the rock matrix, SDs can be divided into discontinuous planes and discontinuous rocks. The physical simulation experiment of hydraulic fracturing is an effective way to assess the geometry and propagation behaviors of HFs. However, the HF propagation behaviors are not generally well understood, especially the influence of multiple SDs on the HF geometry. To clarify this further, a comparative study of hydraulic fracturing on granite and shale was conducted to investigate the intersection mechanism between HFs and different SDs. The results show that HF propagation behaviors are characterized by six basic patterns: along the SD, crossing without dilation, crossing and dilation, captured by the SD, branching, and deflection. The intersection behaviors are closely associated with the cementing strength of the SD and differences in the fracture toughness of discontinuous rocks. Moreover, the fluctuation degree of the pressure-time curve and complexity of the HFs seem positively correlated. HF propagation in a rock matrix or cross interference with multiple SDs would induce a higher injection pressure and frequent fluctuations. The acoustic emission (AE) energy in granite was higher than that observed in shale. In addition, the generation of new HFs in the rock matrix can induce more AE events than those created simply by propagating along SDs. Experimental investigations can provide a theoretical basis to optimize the engineering parameters of field fracturing.

Nuclear magnetic resonance (NMR) has been widely used in monitoring the imbibition behavior of oil/gas reservoirs, especially the $T_2$ spectrum. Most $T_2$ spectra are obtained from one dimensional (1D) NMR, but less research is related to $T_2$ spectra from two dimensional (2D) NMR. Understanding the difference between these two methods helps take advantage of NMR in monitoring imbibition behavior. The main experiment is the imbibition behavior monitored by 1D and 2D NMR. Samples are tight sand from the Upper Paleozoic Taiyuan and Shihezi formations in the eastern Ordos Basin, which is full of natural gas, and the testing liquid is distilled water. The $T_2$ spectra from 1D and 2D NMR increase with imbibition time, which shows that liquid increases among pore spaces. More peaks are in 2D $T_2$ spectra than in 1D $T_2$ spectra, which shows that 2D $T_2$ spectra can differentiate more types of ${}^1\mathrm{H}$ proton signal. In dry state, a higher signal is 0.01–0.1 ms in 2D $T_2$ spectra than in 1D $T_2$ spectra, whereas the signal 0.1–1 ms is the opposite. This indicates that more signals are interpreted as short $T_2$ values in 2D $T_2$ spectra, and these signals mainly correspond to crystal water. In spontaneous imbibition with 30 min and 1 h, the signal of 0.01–0.1 ms in 2D $T_2$ spectra is much higher than in 1D $T_2$ spectra, because of the crystal water and water film effect. The signals in 1–10 ms are close between 1D and 2D $T_2$ spectra, which indicates that imbibed liquid in this interval is interpreted the same. Overall, compared with 1D $T_2$ spectra, 2D $T_2$ spectra can differentiate more types of ${}^1\mathrm{H}$ proton and are suggested to monitor the imbibition behavior of tight reservoirs.