Journal of Energy Engineering

A method of single vertical well combined with hydraulic fracturing is proposed to prevent short circuits and enhance heat mining. The technical and economic feasibilities of geothermal energy exploitation from a deep reservoir are analyzed based on this method. A simulation model was established to analyze the coupled heat transfer between wellbore and reservoir, and then thermodynamic performances and economic analyses of single-flash, double-flash, and flash–organic Rankine cycle geothermal power generation systems were carried out. Simulation results indicate that the heat mining rate can maintain above 3 MW after 40 years of exploitation from the reservoir with 235°C using a water circulation rate of $432{\mathrm{m}}^3/\mathrm{day}$. A combination of large horizontal permeability, high circulation flow rate, and excellent thermal-insulating tubings favors high heat mining. Thermodynamic cycle analyses show that the net power outputs from the single-flash, double-flash, and flash–organic Rankine cycle systems under the optimal condition are 513, 646, and 627.8 kW, respectively. Correspondingly, the geothermal power generation cost ranges from $\$0.086/\mathrm{kWh}$ to $\$0.095/\mathrm{kWh}$, which is a little higher than the conventional power generation cost. Double-flash or flash–organic Rankine cycle is suggested to be installed for geothermal power generation if more electricity is needed.

Due to intermittent characteristics and lack of inertia and damping properties, high penetration of renewable energy sources in power grids could bring about a series of security issues related to power system stability and control. In many countries and regions, therefore, power regulators tend to request self-frequency control ancillary services (FCAS) of renewable power generation, which could introduce further obstacles to utilize clean and inexhaustible wind power, solar energy, and the like in large scale. This paper aims to address such a challenge by presenting a tiered energy storage system (TESS) for self-provision of frequency regulation services. The TESS is composed of different types of energy storage devices aimed at rapid response speed, sufficient storage capacity, and acceptable investment/operation costs. The proposed method can be applied for the FCAS of power grids with high-penetration renewable energy integration. Based on the real wind power generation and electricity demand, simulations were carried out to demonstrate the feasibility of the self-FCAS by the developed TESS.

Skeleton-network reconfiguration is an important task and plays a most important role during power system restoration (PSR) after a blackout. By determining the skeleton network, the power system can be restored as soon as possible to minimize the burden of network reconfiguration. In this paper, a resilience-based skeleton-network reconfiguration (NR) strategy, i.e., a two-stage mixed-integer linear programming (MILP)–based NR strategy for grid-resilience enhancement, is proposed to minimize the impact of emergency power outages on power systems. The start-up sequence for non-black-start generators (NBSGs) and line energization sequences are determined in the first-stage optimization model. A serial restoration constraint and a transient frequency constraint are considered to achieve the serial restoration scheme of NBSGs. Except for the generator power, all the variables attained during the first stage are assumed fixed in the second stage. The second-stage optimization model determines the optimal skeleton network by determining the target transmission lines and critical load pickup during the NR phase. Furthermore, the sets of metrics (i.e., system flexibility, power loss ratio, and recovery time resilience) are presented to determine the grid operational resilience in the skeleton-network reconfiguration. Then, two stages of skeleton-NR are formulated as MILP, and the branch-and-bound method is presented to solve both models. Finally, simulation studies are performed on the two modified power system test cases and the results validate that the proposed strategy can efficiently determine the robust skeleton network for practical and detailed PSR planning.

The transcritical ${\mathrm{CO}}_2$ (${\mathrm{TCO}}_2$) Rankine cycle has been receiving more and more attention for the lower irreversible losses and the excellent thermophysical characters of ${\mathrm{CO}}_2$. However, it is hard to condense ${\mathrm{CO}}_2$ to liquid due to its low critical temperature (about 31°C) under ambient conditions. In this paper, a novel transcritical ${\mathrm{CO}}_2$ Rankine cycle called ${\mathrm{TCO}}_2$ is proposed, in which a vortex tube is added to condense ${\mathrm{CO}}_2$ under ambient conditions. A mathematical model is established to analyze the parametric effects on thermodynamic and economic performance based on specific equipment investment cost; the model is verified with experimental data. Nondominated sorting genetic algorithm II is used to achieve multiobjective system optimization for obtaining optimum cycle performance. Parametric analysis results show that an increase turbine inlet temperature and vortex tube outlet pressure can increase exergy efficiency. A decrease in turbine inlet pressure and turbine inlet temperature can reduce the ${\mathrm{TCO}}_2$ cycle equipment investment cost. In addition, multiobjective optimization results indicate that a conflict exists between ${\mathrm{TCO}}_2$ cycle thermodynamic and economic performance based on specific equipment investment cost.

The synergistic effect of mixing ethylene with propane on the nanostructure and morphology evolution of soot in laminar coflow diffusion flames was investigated via combining thermophoresis sampling and transmission electron microscopy (TEM). A strong synergistic effect was observed regarding the soot yields when 0.05 and 0.1 propane ratios were applied. However, it was inhibited at 0.2 propane ratio. At the height above burner (HAB) of 20 mm, the soot size generated from pure ethylene flame was larger than other mixed fuel flames. At the lower and higher HABs, the addition of propane could increase the mean primary soot diameter in different degrees. The high-resolution transmission electron microscopy (HRTEM) analysis showed that the propane addition resulted in larger fringe length, smaller fringe tortuosity, and spacing at 0.1 and 0.2 propane ratios—especially at 0.2 ratio—indicating the soot have a more compact structure and higher degree of graphitization.

To improve energy efficiency and reduce water consumption simultaneously in lignite-fired power plants, an improved lignite predrying system integrated with heat and water recovery from wet flue gas is proposed in this study. In the proposed system, the heat source for the lignite predrying system was the latent heat from the flue gas instead of the extracted steam, so the extracted steam could return to the turbine to generate more power. Moreover, both the water from the raw lignite and wet flue gas desulfurization (WFGD) processes were recovered, which minimized the water consumption of lignite-fired power plants. This study evaluated the energy- and water-saving potential of the proposed system at both air-cooling and wet-cooling power plants firing three types of lignite separately. The results showed that the higher the moisture content of lignite, the larger the net efficiency improvement and the make-up water rate reduction. The net efficiency improvement and make-up water rate reduction in power plants firing Yimin lignite reached 2.33% points and $0.370\mathrm{t}/\mathrm{MWh}$, respectively, at wet-cooling power plants, and 2.18% points and $0.443\mathrm{t}/\mathrm{MWh}$ at air-cooling power plants. This means that zero water consumption can be achieved, and water and electricity can be output at the same time at air-cooling power plants. The technical and economic analysis showed that the net present value in 10-years of power plants firing different lignite types could reach 49.31, 36.19, and 16.44 million USD depending on the predry degree. The discounted payback period was 2.34–3.67 years, which showed good economic viability in all cases.

To improve the thermal efficiency of a shale-gas engine and reduce the cycle variations in the engine, this study adopts the experimental method and simulation to examine the engine’s combustion process and cycle variations under different ignition modes. Nonlinear dynamics are employed to explore the dynamic characteristics of the combustion process. The influence of the number of spark plugs on flame propagation in the cylinder is evaluated. The results indicate that when dual spark plugs are used for ignition, the turbulence intensity increases, and the flame propagation speed increases. This approach is conducive to improving the power of the shale-gas engine. Synchronous dual ignition positively affects engine power and combustion stability. The cycle variation coefficient, correlation dimension, and maximum Lyapunov exponent of synchronous dual ignition reach their lowest values, which are 1.56%, 1.73%, and 0.073%, respectively. The phase space trajectory distribution area of the synchronous dual ignition is small. The dense distribution of Poincaré map scatter points and return map points indicate that the combustion process tends to be stable. This study pays particular attention to the significance of reducing the cycle variations of a shale-gas engine and improving its stability.

The cocombustion of biomass and coal has the potential to reduce ${\mathrm{NO}}_x$ emissions at high temperatures. In this study, an experimental investigation was conducted on a drop-tube furnace to investigate ${\mathrm{NO}}_x$ formation characteristics for cocombustion. The effects of temperature, blending ratio, and stoichiometric ratio on ${\mathrm{NO}}_x$ formation characteristics were explored. The results indicated that the addition of rice husks had a positive effect on ${\mathrm{NO}}_x$ reduction with stoichiometric ratios higher than 1 at temperatures of 1,400°C–1,600°C. Shallow air-staged combustion (stoichiometric ratio slightly lower than 1) effectively reduced ${\mathrm{NO}}_x$ formation when the blending ratio was 40%. Regardless of whether rice husk was blended or not, high-temperature air-staged combustion can maintain ${\mathrm{NO}}_x$ formation at a low level. When coal was blended with 40% rice husks in the thermal fraction, temperature had a slight effect on ${\mathrm{NO}}_x$ formation for the temperature range 1,400°C–1,600°C. The blending of coal and rice husks mainly reduced thermal ${\mathrm{NO}}_x$ formation and slightly reduced fuel ${\mathrm{NO}}_x$ formation at high temperatures with stoichiometric ratios higher than 1. This study proposed a possible mechanism for ${\mathrm{NO}}_x$ reduction by bending coal with rice husks. The reduction of ${\mathrm{NO}}_x$ was due to an increase in volatile matter, a reduction in group and biomass char, and a decrease in N content by blending in rice husks.

Coal is a complex porous medium. When gas exists in the coal body, the fissure structure of coal and the occurrence of gas adsorption make the stress state of gas-bearing coal different from that of ordinary coal. Therefore, the stress state of gas-bearing coal cannot be accurately described by the Terzaghi effective stress, which ignores the effects of crack deformation and methane. To improve the accuracy of gas-bearing coal analysis, this study considered the corrosive effect of free gas and adsorbed gas and the crack closure effect to establish a novel statistical damage constitutive model with consideration to the combined effects of free gas, adsorbed gas, and coal cracking. The established model was used to elucidate the effects of gas-bearing coal on the coal’s mechanical deformation and damage characteristics under different stress conditions. The model was validated through full stress–strain experiments on gas-bearing coal under different axial pressure and gas pressure. The results obtained by the proposed statistical damage constitutive model can better describe the stress state of the gas-bearing coal, particularly in the initial stage of void compaction, and are in good agreement with the experimental data. The proposed model provides the theoretical foundation for future studies on the characteristic mechanism of gas-bearing coal.

Fracture development in ultralow-permeability reservoirs results in gas channeling and lower oil recovery during ${\mathrm{CO}}_2$ flooding. In this study, the physical properties of a light oil-${\mathrm{CO}}_2$ system were experimented, the physical properties subsequently simulated, and the effects of fracture distribution on gas channeling analyzed using the fracture mechanism model. Then, a geological reservoir model with added hydraulic fractures was used to predict ${\mathrm{CO}}_2$ injection. It is shown that gas channeling time was affected by the fracture angle and the distance between the production well fracture and injection well fracture. First, set $Y$ (north) direction as 0°, keep the angle of the production well fracture unchanged, and only change the angle of the injection well fracture. In this case, a smaller angle of the gas injection well fracture resulted in more gas sweep area and gas channeling more delayed. Second, keep the injection well fracture angle constant and only change the angle of the production well fracture; in this regard, two cases are considered. The one is when the injection well fracture angle is smaller than 45°; in this case, the effect of distance between the production and injection fractures is more than production pressure difference, the distance between the production and injection fractures is greater, the gas sweep area is larger, and the gas channeling is later. The other is when the injection well fracture angle is greater than 45°, which results in a closer distance between the production well and injection well fractures and affects the gas channeling less than the production pressure difference. Third, the angle between the gas flooding direction (along $X$) and the production well fracture is greater, the production pressure difference is more homogeneous, and the gas channeling is later. In the geological model without connected fractures, ${\mathrm{CO}}_2$ flooding achieved gas channeling all over the study area after 7.5 years under the effect of distance between the production and injection fractures and the production pressure difference. The cumulative oil increase is 2.32 millon standard barrel (MSTB) after 20 years.