Journal of Energy Engineering

This article reports the experimental study on a concentrated photovoltaic thermal (PV/T) solar system. A nonimaging linear Fresnel lens was used as a solar concentrator to improve the density of solar energy on a PV/T collector surface. The PV/T collector consisted of a top copper cover and a serpentine channel in which water flowed. Eight pieces of monocrystalline PV cells were connected to one another in series and stuck on the top of the copper cover to produce electrical power. Water from a water storage tank flowed through the lower serpentine channel and removed heat to cool PV cells. The experimental studies were carried out on March 2016 in Shanghai, China. Results showed that the total effective utilization of solar energy was approximately 53% for the concentrated PV/T system. Although the electrical efficiency of PV cells decreased by approximately 34.7% for the PV/T system with a nonimaging linear Fresnel lens on March 4, 2016, the output power increased by 29% at the peak point. Another benefit was that the water storage tank can obtain high water temperature because the PV/T collector received high solar energy density by the linear Fresnel lens. The temperature increment in the water tank can reach 20°C, which was remarkably good to satisfy the daily demand of hot water.

Development of a stable and efficient small-scale combustor architecture with comparable performance emission characteristics to large-scale burners is presented. Furthermore, the proposed architecture reduced susceptibility to extinction and maintained high combustion efficiency and low emission levels under ultralean operating conditions for a wide range of combustion power outputs. Prototype burner arrays were additively manufactured and demonstrated with methane/air flames. The burner sustained lean flames ($\varphi =0.65$) independent of power output, indicating good scalability. High combustion efficiencies (98%) were estimated using gas chromatography-mass spectrometry analysis of the exhaust gas. Combined unburned hydrocarbon (UHC) and carbon monoxide (CO) emission measurements were well below 0.1% by mass. Near-adiabatic flame temperatures with minimal spatial variations across the burner were observed resulting from enhanced flame interaction and reduced heat loss. Overall, this study successfully demonstrates the potential for a novel combustor architecture that can be scaled across a wide range of power outputs with minimal performance degradation.

Experiments on kerosene spray through a single-hole nozzle are carried out under various ambient pressures ranging from 1.4 to 5.6 MPa. High-speed schlieren photography is used to capture the time-dependent images of the kerosene spray. Repetitive experiments are conducted to verify the repeatability of the penetration tip of kerosene spray, and through analysis it is found that the penetration tip is repeatable to $\pm 7\%$. The density of the ambient gas is calculated with the Redlich–Kwong equation of state (EOS) instead of the ideal gas law. Because the material property of kerosene differs from that of diesel, a new modified correlation is proposed to predict the penetration tip of kerosene spray, which provides a higher accuracy than existing methods. The analysis shows that when the ambient pressure is high enough, the influence of injection pressure on the projected spray area becomes weak. The penetration tip velocities are analyzed, and the development of the spray cone angle is discussed and analyzed. All the macro characteristics and the new modified correlation discussed in this study are foundations for two-phase combustion in aeronautical engines or diesel engines fueled by kerosene as a substitution.

Methane combustion with porous honeycomb hexaaluminate catalysts is studied under high temperatures. Cobalt-substituted barium-lanthanum hexaaluminates (${\mathrm{Ba}}_{1-\mathrm{m}}{\mathrm{La}}_{\mathrm{m}}{\mathrm{CoAl}}_{11}{\mathrm{O}}_{19\pm \delta }$) are prepared by co-precipitation procedures and directly deposited onto 300-cpsi porous monolithic honeycomb supports. Catalyst characterizations are performed by scanning electron microscope, X-ray diffraction, and Brunauer-Emmett-Teller method. Effects of La substitution ratio, catalyst content and length, and air preheating temperature on catalytic combustion performances are evaluated. Increasing La substitution ratio can significantly enhance catalyst activity, thermal stability, specific surface area, and combustion performance. The monolithic honeycomb catalyst with largest specific surface area and optimal combustion performance has an optimal catalyst content of 6.0% by weight. The long honeycomb catalyst can improve flame stability limits and reduce pollutant emissions. Increasing air preheating temperature can reduce HC and CO emissions, whereas NOx formation can be improved as the air preheating temperature higher than 250°C.

Staged coal conversion, with pyrolysis as its foundation, is one clean and effective coal usage technology. In this study, the structural and morphological characteristics of Ximeng lignite chars obtained from low-temperature pyrolysis during preheating in staged coal conversion are studied. The pulverized lignite was pyrolyzed under ${\mathrm{N}}_2$ in a temperature range of 300–700°C. Sieve analysis was used to investigate changes in char particle size. ${\mathrm{N}}_2$ adsorption/desorption, and a scanning electron microscope (SEM) and Fourier transform infrared (FTIR) spectroscopy were used to investigate changes in pore structure, surface morphology, and chemical structure of the char particles. The mechanisms of changes in particle size are discussed. Results suggest that the char particles have the trend of being smaller with increasing pyrolysis temperature. Changes in particle size, pore structure, and chemical structure at temperatures above 500°C are more significant than those below 500°C as a result of greater extent of pyrolysis and more vigorous condensation reactions at high temperatures. Specific surface area and pore volume increase with increasing pyrolysis temperature. More micropores are developed, and the number of mesopores with an average size smaller than 10 nm decreases as the temperature ranges up to 500°C. Moreover, when the pyrolysis temperature increases above 500°C, the specific surface area and pore volume increase sharply as a result of large number of pores and wide pore-size distribution. FTIR analysis shows that the aromaticity of char increases oxygen-containing functional groups, and aliphatic structures decrease as the pyrolysis temperature increases.

In the present work, a detailed thermodynamic model of a variable-geometry turbocharged (VGT) truck diesel engine has been built in the GT-Power environment in order to study its operation in terms of turbocharger and engine performance and engine-out emissions. The studied pollutants are nitrogen oxides and smoke, the latter used here as a surrogate for regulated particulate matter. The important feature of the paper is that not only the steady-state but also the transient operation is under investigation. By doing this, the much more demanding dynamic engine operation is analyzed, and useful conclusions are derived in comparison to the fixed-geometry operation of the engine in hand. In particular, the advantages of VGT operation in terms of higher boost pressure and air supply to the engine cylinders, resulting in higher injected fuel, faster acceleration, and a smaller amount of engine-out soot compared to the base line engine, are among the most important results of the study.

Total supply capacity (TSC) is a significant index to evaluate the performance of distribution systems, which reflects the safety and economy of power system operations. With the development of the energy internet, it is necessary to analyze the TSC of electric-gas combined systems considering distributed renewable generation. Because the power output of distributed renewable generation has randomness, the system operational state becomes significantly uncertain. This paper establishes a TSC model of electric-gas combined system with stochastic chance constraints. The model takes into account the N-1 constraints of the main transformer and feeder, the flow constraints of the pressure station and pipeline, and the coupling constraints of the electric power system (EPS) and natural gas system (NGS). The method combines Latin hypercube sampling (LHS) with a simplified particle swarm optimization (SPSO) algorithm to solve the model. The simulation results demonstrate that the technology of distributed power supply can effectively improve the TSC of distribution system. Moreover, the power output of gas turbines can profile the fluctuation taken from the power output of distributed renewable generation. Finally, the feasibility and applicability of the model are verified by extensive case analyses.

The design of a future active distribution network is crucial for the deployment of a smarter grid. The increasing penetration of renewable power generation and of dynamic loads increases the demand for information and communication technologies to monitor and control the distribution system. With the use of a phasor measurement unit, the distribution system can control dynamic issues, and by utilizing an advanced metering infrastructure it can manage, control, and monitor several loads. As a way to support the bidirectional flow of data, an advanced automated machine communication infrastructure becomes essential for effective real-time monitoring and control. Therefore, the primary objective of this paper is to design and to implement a machine-to-machine communication network based on mobile technologies applied to machine-to-machine communications to access the network performance and requirements under real smart operation. By considering a smart grid scenario, besides coverage and data rate analyses, assessing the effect of growing grid smartness on power consumption is necessary. For this purpose, an accurate analysis of current operational requirements and future smart machine operation demands is carried out in this work. Two scenarios are designed, tested, and validated using the IEEE distribution systems under several communication constraints.

A numerical study was conducted to optimize the process of hydrogen production from dimethyl ether (DME) steam reforming. The reforming system was composed of a DME steam reforming reactor, a high-temperature water–gas shift reactor, a low-temperature water–gas shift reactor, and a preferential oxidation reactor. The mathematical model of the DME steam reforming reactor was set up and validated by a self-designed experimental equipment. Based on the model of the reforming reactor, a process optimization model accomplished by waste heat recovery was established by using commercial software. By using the model, the effects of various parameters on system thermal efficiency and the hydrogen flow rate were analyzed. The hydrogen flow rate and the system thermal efficiency increased with the increase of $\text{steam}:\mathrm{DME}$ ratio and the temperature of the steam reforming reactor. The hydrogen flow rate and the thermal efficiency changed little with the increase of temperature of the high-temperature water–gas shift reactor and low-temperature water–gas shift reactor. The system thermal efficiency decreased and the hydrogen flow rate increased with the increase of the DME inlet flow rate. The results will provide useful data to design and operate a ${\mathrm{H}}_2$ production system.

This paper presents a framework for the first and second law analysis and an exergoeconomic optimization of a solar tower power plant using supercritical ${\mathrm{CO}}_2$, integrated with a combined cycle for electricity production. The system’s energy and exergy losses are analyzed to ascertain possible thermodynamic improvement locations. The various design parameters along with direct normal irradiance (DNI) and concentration ratio are considered. Each component of the combined cycle is evaluated to test their energy and exergy performances. The receiver system recorded the highest exergy loss compared with the rest of the system. The levelized energy cost of $\$0.31/\mathrm{kWh}$ and payback of 10 years proves the economic viability of the design. The optimal design parameters for minimum cost are obtained using the thermoeconomic method. The objective function representing the total cost of the power plant ($/h) is defined as the sum of the operating cost, investment cost of purchased equipment, and maintenance costs. Subsequently, various parts of the objective function are expressed as decision variables and the optimum values of these variables are obtained by minimizing the investment cost and the cost associated with exergy destruction using the genetic algorithm.

Stable supercooling of phase change materials benefits the long-term latent heat storage. Various crystallization inhibitors were adopted to improve the stability of the supercooled sodium acetate trihydrate (SAT) solution, and the supercooling time, cycling stability, and thermal properties were investigated. The supercooling time of the SAT solution was prolonged from 6 h to more than 9 days by acetic acid, ethanol, and glycerin. The supercooled SAT with acetic acid was stable for a long period ($>14\mathrm{d}\mathrm{a}\mathrm{y}\mathrm{s}$) during the cycling test, indicating the significant effect of acetic acid in maintaining the cycling stability. Acetic acid acts through inhibiting the crystal growth, limiting the crystal size, and/or reducing the water activity of the supercooled solution. The influence of acetic acid on the heat of fusion, onset temperature, and peak temperature of the SAT system was negligible. The system thermal conductivity was slightly reduced from 0.3698 to $0.2764\mathrm{W}/(\mathrm{m}·\mathrm{K})$ by acetic acid, requiring further enhancement strategies. The SAT-water-acetic acid mixture has great potential for energy seasonal storage.

This work presents a hierarchical multiplicative framework for modeling the energy consumption of households. The constituents of the model are a lognormally distributed annual consumption, an annual consumption profile at weekly resolution, a mean weekly consumption profile, and a multiplicative lognormally distributed random variation. Further, the annual and weekly profiles of households are shown to fall naturally into a small number of rather homogeneous groups, identified by the regular decomposition method. The framework is adapted to monitor and compare populations of electricity consumers. On the other hand, it provides a convenient way to produce synthetic traces of household energy consumption with similar stochastic properties as measured traces. It is also shown how additional household information can be utilized to predict both the annual consumption and the random variation of the consumption of a household.

Coal to liquid (CTL) is one of the potential methods to obtain the alternative fuel used in diesel engines. There are two methods to obtain alternative fuel from coal, namely, indirect and direct liquefaction processes. This paper evaluates the effects of indirect coal-to-liquid (ICL) fuel and direct coal-to-liquid (DCL) fuel on the combustion, performance, and emissions on a heavy-duty diesel engine. The engine was run at 1,200 and 2,000 rpm without exhaust gas recirculation (EGR) and three engine loads (25, 50, and 75%) were operated at each speed. Three injection pressures (80, 100, and 160 MPa) were applied. Results show that the cetane number is the dominating factor in heat-release patterns at the light load; however, the difference of cetane number between CTL fuels and diesel only has a minor effect on ignition delay at high loads. The brake specific fuel consumption (BSFC) of ICL is lower than that of diesel fuel, while the BSFC of DCL is higher than that of diesel fuel. The brake thermal efficiency (BTE) values of two CTL fuels are lower than that of diesel baseline due to their poor lubricity and lower mechanical efficiency. The soot emissions of ICL are always lower than those of diesel fuel at all tested operating conditions. However, the soot emissions of DCL are lower than those of diesel fuel at the low load but a little higher than those of diesel fuel at the high load. The NOx emissions of ICL fuel are lower than those of the other two fuels under low injection pressure, while the NOx emissions of ICL are a little higher than those of diesel fuel at high injection pressure but lower than those of DCL. Unburned hydrocarbons and carbon monoxide emissions of both CTL fuels remain low under both light- and high-load conditions. In general, ICL fuel is more preferable than DCL and diesel fuel from the perspectives of engine performance and emissions.

Energy and exergy analyses were conducted for air conditioning (AC) systems with and without an enthalpy exchanger, based on four common refrigerants: R404A, R407C, R410A, and R510A. The investigated AC system had better performance, with cleaner operation, when integrated with an enthalpy exchanger, which lowered its energy demand. Key performance indicators assessed were COP, evaporator cooling load, compressor input power, exergy efficiency, exergy destruction rate, irreversibility ratio, fuel depletion ratio, and productivity lack. Although the improvement in COP when an enthalpy exchanger was used was shown to be small, cooling load and input power decreased significantly. Moreover, the exergy destruction rate was significantly reduced (by more than 55%) and consequently exergy efficiency was improved. The AC system operating on R407C had the highest exergy efficiency (33%). However, a system operating on R410A is recommended because it requires the lowest compressor input power to maintain a given temperature and thus provides the best performance ($\mathrm{COP}=5.7$). On average, the compressor and the evaporator were found to have the highest irreversibility ratios: 0.26 and 0.37, respectively.

Bad data must be detected in the microgrid because they mislead the decision making of energy management systems (EMSs). The authors propose a robust detection approach that combines an improved robust extreme learning machine (R-ELM) and density-based spatial clustering algorithm with noise (DBSCAN). To resist the impact of outliers in training data, R-ELM applies robust estimation and orthogonal transformation to the ELM training process. After training, R-ELM is used to construct an error-filtering map to extract the characteristics of microgrid measurements. These characteristics are analyzed by DBSCAN to identify bad data. The detection performance of this proposed approach is verified by historical data from a four-terminal ring-shaped DC microgrid prototype. Compared with the back-propagation neural network and ELM, R-ELM is validated to have good robustness. DBSCAN is also verified to outperform traditional K-means clustering. Overall, the approach described here maintains its robustness against outliers and achieves fast and effective detection of bad data in the microgrid.

This study presents a novel and interesting investigation regarding the use of exhaust gas recirculation (EGR) in free-piston engines by exploring EGR impact on both engine performance and emissions. A single-cylinder, two-stroke free-piston engine with an electrical generator model was investigated under the influence of EGR. A new dimethyl ether (DME) reaction mechanism was modified and used to simulate the DME combustion process under homogenous charge compression ignition conditions. The combustion process was described using the one-dimensional approach in which the in-cylinder gas mixture was treated as a single-zone reactor. Chemical kinetics approach was applied to calculate the combustion heat releasing rate and to include EGR effects. Many engine parameters were investigated by applying EGR in different ratios and in different temperatures. The results showed that EGR influences the combustion process by shifting the top dead center location toward the cylinder head, which yields higher compression ratio but shorter ignition delay. The engine ran with less generator power output when EGR was applied but with higher overall efficiency, meaning that the engine could convert more energy out of the combustion process at constant or low generator power demands. The study includes EGR impact on engine emissions in which both oxides of nitrogen (${\mathrm{NO}}_x$) and carbon monoxide (CO) emissions were investigated. Exhaust gas recirculation reduced the maximum bulk mean temperature of the gas mixture, which decreased the reaction rate of ${\mathrm{NO}}_x$ emissions. When running under stoichiometric conditions, EGR seemed to decrease CO emission by reducing the dissociation rate of carbon dioxide. Increasing the intake temperature by applying hot EGR affected both the engine power and ${\mathrm{NO}}_x$ emissions. A reduction in the engine efficiency and generator power output was observed as hot EGR was applied; additionally, high intake temperatures decreased EGR’s ability to reduce ${\mathrm{NO}}_x$.

Moderate and intense low-oxygen dilution (MILD) combustion, characterized by low reaction rate, uniform heat flux, and significantly reduced pollutants, is a promising technology which is widely used in various industries. This paper presents experimental research on the successful application of the MILD combustion mode to a U-shaped recuperative radiant tube (RRT) with a nominal power capacity of 70 kW in order to optimize the performances of the RRT. The combustion characteristics and the effects of air-staged proportion, one of the key operating parameters, on the performance of RRT are investigated. Tube wall temperature uniformity and ${\mathrm{NO}}_{\mathrm{x}}$ emission are improved obviously when the air-staged proportion decreases from 3:7 to 1:9 in the conventional combustion mode. It is shown that there are some limitations to further performance improvements. Accordingly, the MILD combustion mode is successfully introduced and applied by varying burner configuration and operational processes, and the key parameters (e.g., recirculation ratio $K_A$ and flue gas temperature $T_f$) to establish the MILD combustion mode are analyzed quantitatively. It is concluded that to establish the MILD combustion mode, $K_A$ as a fundamental key parameter must first meet a critical value and the values of $K_A$ and $T_f$ then are acquired jointly. With the increase of $K_A$, the qualified flue gas temperature $T_f$ required to implement the MILD combustion decreases accordingly, and the fifth-order polynomial relationship between $T_f$ and $K_A$ with adjusted $R^2$ of 0.97 is determined based on the experimental data to accurately judge the establishment of the MILD combustion mode. Thus the application range of the MILD combustion mode, the superiority of which is reflected in significant enhancement of performance, can be successfully expanded to U-shaped RRTs, providing the theoretical basis for the industrial production of such high-quality heating items.

The analysis result of liquid fuel combustion performance shows that the factors affecting chamber combustion are the vaporization and atomization of liquid fuel. At the same time, the combustion chamber conditions and the property of liquid fuel, in turn, affect the heat transfer process and atomization quality. The present work aims to study the effect of in-cylinder conditions on spray characteristics of diesel, biodiesel, and biodiesel-ethanol blended fuels. Besides, research of the addition of ethanol in biodiesel blends compared with diesel is also discussed in this paper. Meanwhile, for the sake of contrastive analysis, two prediction models are adopted, and the corresponding research is conducted. With the experimental results and theoretical analysis approach, the extent and main influencing factors of droplet vaporization and breakup in the spray process are reached. The liquid-fuel penetration and spray tip penetration all present a decreasing tendency with the increase of in-cylinder pressure and percentage of ethanol in blended fuel, and these parameters are affected slightly by the change of in-cylinder temperature in certain range.

In recent years, researchers around the world have focused on biofuels such as 2,5-dimethylfuran (DMF) and n-pentanol because of their potential for reducing harmful emissions. Studies show that oxygenated biofuels can decrease soot emissions from the exhaust gas recirculation (EGR) engine. This study compared the combustion and emission characteristics of diesel, D20, and P20 fuels on HD diesel engines without EGR. Results show that D20 and P20 fuels show no obvious advantage in emissions compared with diesel fuel. For some load conditions, D20 and P20 have relatively better fuel economy, but overall fuel economy has little difference for the three fuels. Although D20 and P20 can be used without any engine modifications, the performance and emission characteristics of P20 fuel are closer to those of diesel fuel than to D20 fuel. These results can be used in the development of alternative bio-fuels for use in the auto industry.

In the pursuit of minimizing a considerable amount of imported fossil diesel and greenhouse gas (GHG) emissions, green fuels like biodiesel, ethanol, and biogas can be used efficiently under dual fuel mode (DFM) of a diesel engine. The present study aims to investigate the performance, combustion, and emissions (designated as overall performance) of a DFM engine run on various ternary blends of diesel-biodiesel-ethanol (D-B-E) as the pilot (but primary) fuel and biogas as the secondary fuel. The blends chosen were D70-B20-E10, D75-B20-E5, D72-B20-E8, D77-B15-E8, D67-B25-E8, and D100-B0-E0, for which the numerical values represent the volume percentage of the individual component in the blends. The blends are designated as TB-1, TB-2, TB-3, TB-4, TB-5, and TB-0, respectively. All experiments were conducted with these blends at the intake charge preheating temperature of 55°C ($\pm 2°\mathrm{C}$) and at the optimized total fuel-air equivalence ratio (${\mathrm{\varphi }}_{total}$). In order to have a direct comparison, experiments were also conducted without preheating the intake charge (WPI), and this is designated as TB-0-WPI. On the basis of the engine’s overall performance, the TB-3 blend is found to be optimum among the tested blends with preheating. The maximum brake thermal efficiency (BTE) of 26.73% is achieved with the TB-3 blend, which is 5.04% higher in contrast to TB-1. The highest drop of carbon monoxide (CO) and hydrocarbon (HC) were 86.23 and 83.97%, respectively, in contrast to TB-0-WPI, whereas the maximum reduction as compared to pure diesel of 63.04% of oxides of nitrogen (${\mathrm{NO}}_{\mathrm{x}}$) is achieved with TB-0-WPI.

The integration of large-scale wind power in a power grid brings tremendous challenges to the power system operation because of the variability of wind power. Adequate spinning reserve is always required for effective accommodation of wind power variability by the automatic generation control system. The generation schedule and required spinning reserve are usually determined by hourly unit commitment, but the system frequency regulating process under wind power intrahour fluctuation is ignored. This increases the system operation risk. A novel wind-thermal generating unit commitment method considering short-term ($<1\mathrm{h}$) fluctuation of wind power (SFWP) is presented. A simulation model based on the $k$-means clustering technique is developed to describe the SFWP information, and the system frequency regulation is incorporated as the spinning reserve constraint of the unit commitment model. The proposed method enables more accurate assessment of the required spinning reserve capacity than the existing methods. It also improves the security of power system operation. Case studies are presented to demonstrate the effectiveness of the proposed method.

This study proposes a novel control strategy based on deep Q learning (DQL) for smart generation control of large-scale interconnected power systems. In this novel DQL algorithm, a deep neural network with higher intelligence is adopted by the action selector of Q learning. Based on the forecasting and online adjustment, DQL could speed up the convergence rate of Q learning, and mitigate the curse of dimensionality. The feasibility and effectiveness of the designed DQL controller are demonstrated in the simulations considering an IEEE two-area load frequency control model and a four-area model based on China Southern Power Grid with 23,328 different parameters. Simulation results verify that the DQL-based controllers can optimize the active power in multiareas and minimize the overall system frequency deviations.

The microscale organic Rankine cycle (ORC) system is considered a promising technology to recover thermal energy from widely distributed low-temperature heat sources. By using HFC-R225fa as working fluid, a 5-kW low-temperature ORC system with a hermetic axial-flow turbo generator was designed, and performance of the ORC system was analyzed based on the experimental data. During the design period, the actual electric power output of the ORC system was 2.5 kW, and the internal efficiency of the axial turbine was 23.6%. However, it was found that at the turbine rotational speed of 16,000 rotation per minute (rpm), such aforementioned performances are reduced to approximately half of the design values. Hence, the disagreement between the design value and experimental data implies that the conventional axial turbine design model is inaccurate at low turbine rotational speeds, and the thermal energy grade is decreased in the generator coil cooling process of the microscale integrated turbo generator structure. The experimental results also showed that the axial turbine performance is improved at high turbine rotational speed for the microscale ORC system. Finally, a suitable turbine rotational speed is recommended in this research.

This paper investigates a combined cooling and power (CCP) system driven by a low-grade heat source. This system is a combination of the Kalina cycle and an ammonia-water absorption refrigeration cycle. Based on a thermoeconomic mathematical model, a numerical simulation and an exergy destruction analysis are conducted. A parametric analysis is also performed to examine the effects of several key parameters on the thermoeconomic performances of the CCP system. The results show that condensers contribute most exergy destruction to the system. The parametric analysis verifies that the key parameters have a significant impact on thermoeconomic performance. A multi-objective optimization is conducted to obtain the Pareto frontier solution for the CCP system, which provides diversified choices for different design requirements.