Journal of Energy Engineering

The objective of this work is to investigate, from both experimental and simulation points of view, the feasibility of a flywheel energy storage system (FESS) for buffering energy when implemented in off-grid (autonomous) electricity production. Toward this aim, a prototype FESS was built and measured to identify its optimal design and operational characteristics as they have been obtained theoretically. After simulating two different materials and shapes for the rotating mass, the hollow aluminum cylindrical design found to provide excellent energy storage results. By identifying the storage capacity of a prototype laboratory-scale FESS, it is also found that such a device can be promoted as a uninterruptible power supply system to cover peak loads during limited time periods. Finally, the scale of the project found to be a crucial parameter for its feasibility.

Because of the increase in wind rotor size, the Reynolds number of an airfoil profile can reach a very high value. The effect of the Reynolds number on the aerodynamic performance of airfoils is investigated, and its influence on the optimal design of a wind rotor aiming to maximize the power coefficient is discussed. Six airfoils are involved—four DU and two NACA6—as well as five Reynolds numbers varying from ${10}^6$ to ${10}^7$, which cover most commercial wind turbines. At a higher Reynolds number, all of the airfoils exhibit better performance, such as a higher lift coefficient, a lower drag coefficient, and a larger lift-to-drag ratio at a given angle of attack. The largest lift-to-drag ratio and the corresponding lift coefficient and angle of attack also change with the Reynolds number, which in turn affects both the performance and the optimal shape of a blade. The results show that a practical blade operating at a higher Reynolds number requires a duller shape with a greater twist angle, and has a better power coefficient than those operating at lower Reynolds numbers.

Integration of multiple sound generators is essential to the development of large-scale, sono-assisted processes such as combustion, evaporation, or ${\mathrm{CO}}_2$ capture with acoustic excitations. In this work, the transient and synchronized behaviors of a thermal acoustic converter (TAC) pair were investigated experimentally for various crossing angles and different separation distances between the converters. The two TACs were acoustically coupled through the air mass between their openings, and the only mode-locking operation that could be achieved was the one that was nearly 180° out of phase. The time to achieve synchronization was found to be dependent upon the initial mistuning of the frequencies and the crossing angle between the converter axes. The synchronization process could also be accelerated by turning on the converter with the lower power input first. When the separation distance between the two converters exceeded a certain value, synchronization of the TAC pair could not be achieved. As the separation distance increased, the maximum amplitude of the unsynchronized TAC pair became less and less than the sum of the maximum amplitudes of the two converters when operating individually.

The growth of photovoltaic (PV) systems on low-voltage distribution networks can create technical issues such as voltage rise, voltage unbalance, unnecessary neutral current circulation, and reverse power flow. Usually, these happen with little fluctuation. However, they tend to fluctuate seriously because of the low clear sky index in Malaysia and they become more severe when the installation and distribution of single-phase renewable energy sources are rather arbitrary and not monitored by utility companies. To ensure effective operation of distribution networks with PV systems, an improved version of a three-phase fuzzy control algorithm for distributed energy storage (ES) systems is proposed. With this algorithm, distributed energy storage systems can control individual phase voltage to mitigate the voltage unbalance factor effectively, maintaining voltage magnitude without cutting any excess renewable energy. An experimental low-voltage distribution network with two 3.6-kWp PV systems was set up. A number of case studies under various generation and demand conditions were carried out to evaluate the performance of the three single-phase energy storage systems with the novel fuzzy control algorithm. The results show that the proposed algorithm can effectively coordinate the systems to mitigate voltage unbalance and maintain the voltage level of low-voltage distribution networks with PV systems.

In consideration of the multiple split injection, heavy exhaust gas recirculation (EGR) atmosphere, high latent heat of vaporization of dimethyl carbonate (DMC), and low cetane number, the combustion variations of both diesel and D10 (a fuel blend of 10% of DMC in diesel) have been investigated with characteristic parameters of indicated mean effective pressure $p_{mi}$, peak pressure $p_{\text{max}}$, and initial 10% combustion duration ${\theta }_d$. Results show that the cyclical coefficients of variation (COVs) of three characteristic parameters are lower than 2%. This means good combustion quality thanks to the precise control of fuel injection. The COVs for diesel are much smaller than those for D10 fuel. The COVs of $p_{mi}$, $p_{\text{max}}$, and ${\theta }_d$ rise with a declining center of heat release (COHR), while the COVs show visible growth with the increase in EGR rate. The anticipation of possible combustion instability due to pilot injections did not happen. Both $p_{\text{max}}$ and ${\theta }_d$ have good corresponding relations with $p_{mi}$. The $p_{mi}$ positively rises with the increase in $p_{\text{max}}$, while the higher $p_{mi}$ adversely corresponds to the shorter ${\theta }_d$. As a whole, the correlations between $p_{mi}$ and $p_{\text{max}}$ are higher than that between $p_{mi}$ and ${\theta }_d$, which means that $p_{\text{max}}$ is more dependent on $p_{mi}$ than ${\theta }_d$ The fact that the correlations between $p_{mi}$, $p_{\text{max}}$, and ${\theta }_d$ are higher than 0.67 further points to the choice of $p_{\text{max}}$ and ${\theta }_d$ as evaluating subjects for cyclic variations in cylinder complex process.

This paper discusses a comparative investigation of the three basic nonisolated DC-DC converters used as interfaces for maximum power point tracking (MPPT) in photovoltaic (PV) generators using the direct duty ratio control tracking algorithm. Analysis of buck, boost, and buck-boost converters is undertaken to study the behavior of converter performance with respect to changing atmospheric conditions, in-turn duty ratio variation (as a result of MPPT), and tracking efficiency. As atmospheric conditions change, the operating value of resistance at the maximum power point ($R_{\text{mpp}}$) changes. To effectively drive the system at the maximum power yield point, the MPPT algorithm has to drive the system operation point close to the value of $R_{\text{mpp}}$ for the varying atmospheric profile of change in insolation and temperature. The efficiency of the MPPT algorithm lies in meeting this very requirement. The simulation study shows that, although buck, boost, and buck-boost converters are used as power converters in MPPT applications, not all converters perform with high efficiency. The effect of different resistive loads that have values other than $R_{\text{mpp}}$ on converter-side output is considered for the three topologies, and it is observed that the buck-boost converter is the only one that is able to track the maximum power point (MPP) under variations in insolation, temperature, and loading effect with the highest tracking efficiency.

In this work, a theoretical model for investigating the induced photocurrent and the corresponding power conversion efficiency is established. The quantum dot solar cells (QDSCs) model is used to determine the dependence of these figures of merits on its various parameters, such as optical generation lifetime, recombination lifetime, quantum dot size, surface density of quantum dots, effective density state into QDs, and number of QD layers. The spectral response for the power conversion efficiency is theoretically calculated. The obtained results ensure that up to 78% conversion efficiency is obtained due to the intrinsic region in the p-n junction being equipped with QD layers. Additionally, the results ensure that a wide spectrum of efficiency is obtained that is not only concentrated near 200 nm, as considered before, but also extended to longer wavelengths up to 3 μm. As a result, the QDSCs will respond to visible light, yielding a higher performance. Finally, the method overcomes the narrow-band response problem reported in the literature.

In the paper, an investigation was conducted on the combustion performance and emission characteristics of a single cylinder diesel engine burning diesel fuel containing a new oxygenate blend of propylene glycol monomethyl ether (PGMME) and ethylene glycol dimethyl ether (EGDME) in a proportion of $1.6:1$ by volume. When diesel fuel was added with 15–25% by volume of the oxygenate blend, the largest decrease in smoke opacity was observed to be as great as 90.3%, and the maximal reduction in carbon monoxide (CO) emissions amounted to 66.7%. Nitrogen oxide (NOx) emissions did not change noticeably. Hydrocarbon (HC) emissions increased more than double, but the level was still too low to take into consideration. The engine in-cylinder peak pressure and ignition delay did not alter. The in-cylinder peak pressure descended, and the peak heat release rate decreased in the premixing combustion, whereas it increased during diffusion combustion.

An experimental system for a solar ejector refrigeration system under the periodic climate condition was designed and constructed. The influences of ambient temperature and solar radiation on the performance of the refrigeration were studied experimentally. The results show that there exists critical ambient temperature in the solar ejector refrigeration system. When the ambient temperature is higher than the critical ambient temperature, the system coefficient of performance (COP), energy efficiency ratio (EER), the ejector coefficient, and the cooling capacity will undergo a sharp decline. Furthermore, this work shows that the critical ambient temperature is proportional to the generator temperature. With the solar radiation increasing, the system COP, EER, and cooling capacity increase initially and then decrease. For given temperatures at the evaporator and the condenser, there exists an optimal generator temperature, at which COP and cooling capacity can reach the maximum. The results obtained herein provide useful guidelines for the optimal design and operation of this type of refrigerant system.

Model structure selection with respect to short-term wind speed forecasting is relatively difficult due to the stochastic and intermittent nature of the wind speed distribution. In order to overcome the disadvantages in traditional approaches such as computing burden and low accuracy, a novel model structure selection technique about short-term wind speed forecasting is proposed in order to improve the computational efficiency and forecasting accuracy using the model variable selection, variable order estimation, model structure optimization techniques, and so on. The detailed and complete process flow associated to the theoretical analysis of the proposed model structure selection technique is described. Moreover, both the so-called overkill in the data filtering and so-called overfitting in the learning processing are avoided by a proper technique in the design of proposed approach. In order to verify the effectiveness of proposed strategy in a practical application, all the experimental results are evaluated based on the real data provided by a sampling device with respect to a low-wind-speed wind turbine (i.e., FD-77) of a wind power plant of north China. Finally, the developed model structure selection technique is verified by the cross-validation method.

This paper introduces and integrates effective models to describe the fundamental characteristics of high-performance lithium-ion (graphite-${\mathrm{LiFePO}}_4$) and lead-acid (VRLA) batteries with capacity fade for use in an off-grid residential photovoltaic (PV) generation system. The lithium iron phosphate (LFP) and VRLA batteries have been simulated using a 1D electrochemical model with thermal and aging components, and using the CIEMAT model, a general analytical model of the lead-acid battery, respectively. In the off-grid PV/battery/demand system, the calculated annual state of charge (SOC) distribution and time history of degradation in the LFP and VRLA batteries describe the internal battery behaviors and the blackout risk; the LFP battery is expected to accomplish a one-third lower annual blackout frequency and a 25% longer degradation time than a VRLA battery with a 24-kWh battery capacity over 15 years of operation. According to the calculated loss and surplus electricity indexes, the LFP battery has a remarkably long life and high charge/discharge efficiency, even with a variable current and in long-term operation.

Physical topology is essential for antidisaster and disaster recovery abilities of power systems. Focusing on preparedness of physical topology, differentiated planning has been proposed, which refers to supplying power continuously to important special loads under disaster conditions by upgrading the construction standard of the selected subsystem, as well as installing distributed generation, microgrid, or movable generation. In this paper, the specific procedure of differentiated planning in distribution system is proposed, which includes identification and prediction of special loads, analysis of current status, measures of power supply for special loads, and high-standard power grid planning. The construction standard in the procedure not only depends on the voltage class but also the power supply for special loads. Then the model is established to select the subsystem components with minimum cost increment over general planning considering disaster scenario and general scenario. To solve the constrained optimization problem, genetic algorithm (GA) combined with minimum spanning tree (MST) and improved radial topology maintaining method is adopted. The case study shows the effectiveness of the proposed procedure and the model.

Kuwait receives annual total radiation of roughly $5{,280\mathrm{MJ}/\mathrm{m}}^2$ with nearly 3,347 h of sunshine. The potential for reduction of greenhouse gas emissions motivates widespread of renewable energy systems. In the research reported in this paper, a numerical model is developed to study the effect of different collector parameters and operating conditions on the performance of parabolic trough solar collector (PTSC) in Kuwait climate. The proposed model considers all thermal losses which have been neglected in existing models. New equations were developed and used in the present model as well as reviewing the equations for convective heat transfer losses. The effects of heat conduction in the collector tube wall and mixed convection in the inner tube neglected in previous studies are included in the proposed model. In addition, a case study is presented to examine the feasibility of integrating renewable energy systems in existing buildings. Solar energy absorption system is considered in the research reported in this paper to satisfy cooling load of an existing building. The program conducted on an existing institutional building intending to convert it into near-net-zero energy building. The 2-story building under consideration is the main building at the College of Technological Studies, Kuwait. Computer software is adapted to simulate the performance of different solar system components. The economic calculations for the present study are based on lifecycle savings method. In addition, a numerical model is developed to assess the environmental impacts of building integrated renewable energy systems. The present results indicate that convection loss from the absorber tube to supporting structures is the largest between the other losses. The integrated parabolic trough collectors (PTC) satisfy nearly 73% of the building cooling load under all climatic conditions of Kuwait. The annual avoided ${\mathrm{CO}}_2$ emission at the optimum conditions is about 980 t/year which confirms the environmental impacts of these systems in Kuwait. The solar system net cost is almost equal to the current cost of producing electricity in Kuwait.

Biodiesel emulsions are considered one of the most promising alternatives to petrol fuels. The purpose of the present investigation is to study the performance characteristics and the exhaust emission characteristics of a diesel engine run on nanoparticle blended biodiesel emulsion fuel. The engine performance under biodiesel blended diesel fuel was similar to that of the diesel fuel but with higher fuel consumption mainly due to the reduced calorific value that ensued from mixing with biofuel. The biodiesel emulsion fuel was prepared by emulsification technique comprising 83% canola biodiesel, 15% water, and 2% surfactants (Span80 and Tween80) with the help of a mechanical agitator. The prepared biodiesel emulsion fuel was mixed with the cerium nanoparticles in a mass fraction of 50 parts per million (ppm) with the help of an ultrasonicator. The investigation was carried out in a constant-speed diesel engine. The cerium oxide acts as an oxygen-donating medium and also provides oxygen for the oxidation of carbon monoxide (CO) or absorbs oxygen for the decrease of nitrogen oxides (${\mathrm{NO}}_x$). The establishment force of cerium oxide burns off carbon deposits within the engine cylinder and prevents the deposition of nonpolar compounds on the cylinder wall, resulting in the reduction of hydrocarbon (HC) emissions. The experimental results showed considerable improvements in the performance and a reduction in harmful emissions with the biodiesel emulsion fuels compared with that of neat biodiesel.

Ambient wind has adverse impacts on the thermal-flow performances of air-cooled steam condensers (ACSCs). It is useful to understand the mechanisms of such influences so as to improve and optimize the performance of condensers. The flow fields around an ACSC at different wind speeds and directions are numerically investigated in this paper. It is found that the negative pressure region under the ACSC platform results in volumetric effectiveness reduction of the windward fans with increasing wind speed, and hot air recirculation or reversed flow in condenser cells causes a fan inlet air temperature increase. Under windy conditions, if a main building blockage effect exists, either hot air recirculation will occur, or else reversed flow may arise. The negative pressure region under the platform plays a leading role in the heat transfer effectiveness reduction. From the scale of condenser cell instead of the whole ACSC, stable backpressures corresponding to the windy conditions are forecasted by the combination of the effectiveness-NTU method and the numerical method, which makes the model closed to the true status. The increasing amplitudes of the backpressures reach, respectively, 14.3, 15.4, and 6.4 kPa at $+X$, $+Y$ and west-northwest wind direction as the wind speed increases from 0 to $9\mathrm{m}/\mathrm{s}$.

The aim of this work is to study the possibility of improving the thermal transmittance of a single-leaf wall made with lightened clay bricks. A first aspect is to study the possibility of lightening the clay by using additives, reducing the clay’s conductivity without decreasing its resistive ability. The clay’s conductivity has been decreased by up to 40% through the use of a suitable proportion of additives, with no reduction in the brick’s compressive strength. This drop in conductivity (40%) represents a reduction of 20% in the wall’s thermal transmittance. The internal and external geometry of the bricks has been studied by the finite element method (FEM) for two types of clay, decreasing brick conductivity by 35%. Different wall assemblies with these bricks have been studied, obtaining a 24% improvement with a thin horizontal joint assembly. To quantify the savings made, a standard building has been studied, which has shown that decreasing the wall’s thermal transmittance by 43% would save 7.9% on the annual heating demand, according to calculations made with specialized energy efficiency rating software.

This paper proposes a multiagent (MA) smart generation control (SGC) scheme for the coordination of automatic generation control (AGC) in power grids with system uncertainties. Under the control performance standards, SGC will undergo a non-Markov random process, of which the optimal solution can be resolved online by the reinforcement learning. Therefore, an MA decentralized correlated equilibrium Q($\lambda$)-learning algorithm, and an MA stochastic dynamic game-based SGC simulation platform (SGC-SP) have been proposed for its implementation, which can achieve AGC coordination in a highly uncertain environment resulting from the increasing penetration of renewable energy. Single-agent Q-learning, Q($\lambda$)-learning, R($\lambda$)-learning, and proportional integral control are implemented and embedded in SGC-SP for the control performance analysis. Two case studies on both a two-area power system and the China Southern Power Grid model have been done, which verify its effectiveness and scalability.

In this study, a high-density polyethylene helical coil heat exchanger (HCHE) is firstly adopted by a seawater-source heat pump system (SWHP) and experiments are conducted to determine the thermal performance of the heat exchanger in the Bohai Sea, Tianjin, North China. To study the effects of seawater flow on the convective heat transfer, experiments are performed at the bottom and surface of the seawater. The heat transfer coefficients of the HCHE in seawater are studied in this article in consideration of the flow of seawater. The effects of parameters such as the water-inlet temperature and the velocity in the HCHE on heat transfer are discussed with respect to cooling and heating conditions. In addition, the heat transfer analysis of the HCHE in icy conditions in winter is performed. The results show that 90% of the transferred heat energy is transported on the first 50 m of the HCHE. The external convective heat transfer coefficients under various conditions are calculated iteratively.

In this paper, two miniature horizontal-axis wind-turbine systems (MHAWTs) with and without gear transmission consisting of commercially available off-the-shelf components were designed for harvesting energy from ambient airflow to power wireless sensors. The basic performance of the MHAWTs was tested at various wind speeds ($2–8\mathrm{m}/\mathrm{s}$) and resistive loads (10–500 Ω). The maximum output powers of the MHAWT with and without gear were 8.6 and 11 mW at the wind speed of $4\mathrm{m}/\mathrm{s}$, which suggested that both MHAWTs enabled powering low-power wireless sensors in natural airflow environment. To analyze the system efficiency and predict the optimal resistive load of the MHAWTs, an equivalent-circuit model was used. According to this model, the maximum system efficiency of the MHAWT system was predicted to be 14.8%. The actual maximum system efficiency of the MHAWTs with and without gears was tested as 8.4% and 7.5% at the wind speed of 8 and $6.5\mathrm{m}/\mathrm{s}$. In addition, the predicted optimal resistive loads of the MHAWTs with and without gear were 128 and 34 Ω, which correlated well with the tested results. The equivalent-circuit model is well suited to estimating and optimizing the performance of the MHAWTs.

An experimental study is conducted to investigate the heat transfer and the pressure drop properties of a single-phase flow in a bayonet tube heat exchanger. A bayonet tube heat exchanger has only one circulating fluid, i.e., tap water. The circulating fluid is supplied at an ambient temperature (cold medium) and, as a result, it will be heated by an electrical heater (heat source). The bayonet tube heat exchanger is a two-pass heat exchanger. Different experiments were performed to show the relationships and the effects of different parameters such as the Reynolds number, friction factor, Nusselt number, overall heat transfer coefficient, and effectiveness on the purposed heat exchanger. To enhance the heat transfer rates, a helical tape is installed in the annulus between the inner and outer tube. The flow is considered as turbulent in both the plain tube and the helical tape. The recorded Nusselt number for heat exchanger with the helical tape is about 35.6% higher than that without helical tape. In addition, the helical tape increased the effectiveness by approximately 30.2%.

A battery pack system composed of 32 lithium iron phosphate (${\mathrm{LiFePO}}_4$) batteries and a battery management system (BMS) were assembled according to the actual load demand of a standard 110 kV power substation. Float-charging characteristics of the system were investigated and the results showed that 97% of its initial capacity was retained after a 1-year float-charging test. Even though the voltages of two batteries showed abnormal changes, which indicated that internal changes had occurred after long-term float charging, 94% of the batteries remained stable. All of the results suggested that the use of a lithium iron phosphate battery holds promise as a substitute for the conventional valve-regulated lead-acid (VRLA) battery in a direct-current power supply system.

Because the complexity of sulfur capture and release during solid-fuel combustion in a circulating fluidized bed combustors (CFBC), especially in the oxygen-enriched combustion, has not been sufficiently recognized, the development of a simple model, which can correctly predict the ${\mathrm{SO}}_2$ emissions from such units over a wide range of operating conditions is of practical significance. The artificial neural network (ANN) approach is proposed in this paper, which may overcome the shortcomings of the experimental procedures and the programmed computing approach. The Ca:S molar ratio, oxygen concentration in inlet gas, excess oxygen, average riser temperature, mean diameter of the coal particles, average gas velocity in the riser, flue gas recycle ratio, and inlet gas pressure are taken into account by the model as the input parameters. The [8-3-7-1] ANN model with hyperbolic tangent sigmoid activation function was successfully applied to calculate the ${\mathrm{SO}}_2$ emissions from coal combustion in several CFB boilers operating under both air-fired and oxygen-enriched conditions.

A novel swirl burner with an adjustable inner secondary air (ISA)-flaring angle $\beta$ is proposed. The authors conducted a combustion experiment in a 0.6 MW wall-fired pilot-scale furnace with the novel swirl burner and numerically simulated the combustion characteristic correspondingly. The effects of the inner secondary air-flaring angle of the burner on coal combustion, including gas temperature, wall temperature distributions, unburned carbon in the fly ash, and ${\mathrm{NO}}_{\mathrm{x}}$ emissions, are reported in detail. Both experimental and numerical results indicate that increasing $\beta$ can improve coal particle ignition and move the flame center close to the burner. To avoid hot corrosion, larger $\beta$ is recommended, because it helps to lower the temperature level of the furnace wall. The ISA-flaring angle of burner can be optimized to improve burnout and limit ${\mathrm{NO}}_{\mathrm{x}}$ emissions. The condition of the highest burnout corresponds to that of the highest ${\mathrm{NO}}_{\mathrm{x}}$ emissions with the same ISA-flaring angle of the burner. Combustion efficiency and ${\mathrm{NO}}_{\mathrm{x}}$ emissions can be balanced through the variation of $\beta$. With this new ISA-flaring angle adjustable burner, coal combustion will become more controllable and cleaner, while being more reliable.

The output of wind power is variable and random, which makes it different from conventional energy resources in practical utilization. Up to now, large-scale wind power integration has not been achieved because of some technical and political reasons. The bundled transmission model is highly recommended by many scholars. Fiscal policies on wind power industry are defective in China. Reasonable and scientific stimulation mechanisms are needed for the parties involved. On the basis of the data envelopment analysis (DEA) game theory, this paper formulates a model on excess profit allocation, and an example is set up to analyze the feasibility. This model establishes the theoretical basis for solving the excess profit allocation problem, which has significant implication for China’s energy policy initiatives.

A castor oil fatty acid methyl ester (COFAME) epoxide was prepared by structural modification of high free fatty acid castor oil. The process involved epoxidation of COFAMEs using hydrogen peroxide as an oxygen donor and IR-120 as a heterogeneous acid catalyst. Response surface methodology was implemented to optimize the epoxidation process variables and to maximize COFAME conversion to oxirane oxygen content (OOC). The effect of the process parameters on epoxidation was examined. The best optimum conditions inferred were as follows: ${\mathrm{H}}_2{\mathrm{O}}_2$ molar ratio, 2.34; temperature, 50°C; catalyst loading, 19.43% by weight; and reaction time 5.26 h. The experimental OOC value at optimum conditions was 4.38%, which agreed closely with the model-predicted value (4.44%) with a percentage error of 1.35%. The completion of epoxidation was confirmed by ${}^{13}\mathrm{C}\text{-}\mathrm{NMR}$. Molecular weight, oxidative stability, and rheological properties were determined by standard methods. The blending study results revealed that epoxide blended with COFAME showed an improved pour point compared with pure epoxide.