Journal of Energy Engineering

High-momentum jets, which are the key parameter to achieve moderate or intense low-oxygen dilution (MILD) combustion may lead to insufficient residence time and high CO emissions, especially for furnaces with a forward-flow configuration. A reflector for application in the furnace was proposed by two-dimensional numerical simulations to reduce the CO emissions and maintain low ${\mathrm{NO}}_{\mathrm{x}}$ emissions. The prediction showed a remarkable agreement with measurements. The effects of the reflector height $H$, the excess air factor $\lambda$ (changes in the inlet methane-mass flow rate), and the hydrogen content in the blended gas were investigated. The numerical results indicate that the reflector blocks the reactant’s flow, intensifies the combustion before it, and prolongs the residence time of flue gases both before and behind it. The added hydrogen in methane can intensify combustion, increase reaction temperature and NO emissions, and reduce CO emissions. The reaction rates of thermal, prompt, NNH, and ${\mathrm{N}}_2\mathrm{O}$-intermediate routes increase with the decrease of $\lambda$. The addition of hydrogen decreases the rate of the prompt route, but the rates of NNH and thermal routes increase. MILD combustion is achieved under the reflector conditions, considering the nearly uniform temperature distribution and the low pollutants.

Biogas has great potential as a clean alternative fuel for dual-fuel diesel engines. The present work is aimed at investigating the operating parameters such as compression ratio (CR) and injection timing (IT) for a dual-fuel diesel engine run on biogas and rice-bran biodiesel. The motivation behind this investigation is to obtain a diesel equivalent performance together with a reduction in emission. To achieve this, a single-cylinder, direct-injection, variable-compression-ratio diesel engine is modified into a dual-fuel diesel engine by incorporating a venturi gas mixer at the inlet manifold. A set of combinations composed of CRs of 17, 17.5, and 18 and ITs of 23°, 26°, 29°, and 32° before top dead center (BTDC) at different loading conditions are considered. The combination of $\mathrm{IT}=32°$ BTDC and $\mathrm{CR}=18$ is found to produce a maximum brake thermal efficiency of 25.88% and a liquid fuel replacement of 82.75% at 100% load. On average, there is a reduction of carbon monoxide as well as hydrocarbon emissions by 11.64 and 14.21%, respectively. On the other hand, for the same setting, there is an increase of oxides of nitrogen emissions by 39%.

With energy consumption increasing sharply and copper resource decreasing rapidly, the demand for a high-performance and efficient heat exchanger becomes more and more pressing. Therefore, it is very important to develop new technology, new materials, and low-cost and high-performance fin tubes, which is the main component of heat exchanger. In this paper, a new type of aluminum noncontact thermal resistance finned tube (ANCRF) is studied experimentally on the standard wind tunnel test bench. The performance curves of heat transfer and resistance and the experimental correlations were obtained in the experiment range of Reynolds number, which indicates that ANCRF has excellent heat transfer and flow resistance performance. Compared with a louvered fin tube and plate fin tube, the air-side convective heat transfer coefficient of ANCRF was approximately 63.06 and 176.33% higher on average; the friction factor was approximately $-71.28$ and 51.3% higher on average; and the heat transfer factor was approximately 135.3 and 105.9% higher on average. In addition, a simulation model was established. The positive correlation between the simulated results and the experimental data demonstrated the effectiveness of the model. With this model, the heat transfer process of ANCRF was simulated, with results indicating that the structure sizes of fin thickness, fin height, fin spacing, and fin angle all have great influence on the heat transfer and flow performance of ANCRF, and the rules of influence are different. This research has a guiding significance for the optimization design and application extending for this new type of fin tube.

The present study assesses different lignite-fired plant configurations for combined heat and power (CHP) production for use in district heating (DH) networks, in terms of environmental, technical, and economic criteria. Lignite utilization guarantees reliable and cost-effective base load power generation, as well as competitive district heat supply in several European countries. The predrying technology is expected to play an important role in the next generation of brown coal power plants and, more specifically, the integration of the fluidized bed drying system with internal waste heat utilization (WTA technology) is anticipated to enhance plant efficiency. On the other hand, aged power plants will be decommissioned in the coming years, and a lack of capacity in electricity production and more importantly in district heat production will arise. In this context, a reference CHP plant configuration firing raw lignite is compared with a new plant configuration firing predried lignite produced by dedicated lignite dryer (WTA), integrated into the reference plant. Thermal cycle calculations are performed in both cases for winter and summer operating modes, and the effect of the WTA integration on the overall heat and mass balance of the steam cycle and the corresponding plant efficiency is computed, using a thermal cycle calculation tool. The electricity and thermal energy generation costs as well as the net annual profit are then calculated and compared. The techno-economic evaluation is also extended by including a comparison of these lignite utilization scenarios with a natural gas (NG) utilization scenario of a typical combined cycle gas turbine (CCGT) configuration. Finally, a sensitivity analysis of the effect of some mutable critical parameters on the economic indices is presented. The performed calculations indicate that a significant increase of both electrical and thermal efficiencies may be achieved in the predried lignite firing case. Regarding the economic study, predried lignite firing turns out to be a promising technology, which results in considerable fuel and ${\mathrm{CO}}_2$ emissions savings and hence increased competitiveness.

This work explored the use of ashes of a low-sulfur, high-alkali lignite coal for partially capturing the sulfur dioxide emissions from combustion of a high-sulfur bituminous coal. The bituminous coal was mixed with the lignite ashes and then burned in a laboratory drop-tube furnace (DTF) externally heated to 1,400 K. The gas-phase emissions in the combustion effluents of the neat bituminous coal were monitored and compared with those of the bituminous coal mixed either with the lignite ashes or with other additive compounds, such as a specially prepared sorbent from the ash of the lignite coal or with calcium oxide (CaO). All experiments were executed at a molar $\mathrm{Ca}:\mathrm{S}=0.3$ in air, under fuel-lean conditions. Coal particles were in the size range of 75–90 μm. Results showed that the addition of lignite ashes caused substantial reductions, by up to 21% in the ${\mathrm{SO}}_2$ emissions of the bituminous coal. Such reduction was akin to that caused by burning the coal mixed with the CaO sorbent. Significant reduction in ${\mathrm{NO}}_{\mathrm{x}}$ emission was also attained. This observation, in conjunction with ash analysis, showed that the alkali-rich ashes of the lignite coal acted as sulfur sorbents for the abundant ${\mathrm{SO}}_2$ emissions of the bituminous coal.

This study examines the influence of operating conditions and heat exchangers’ characteristics on the overall performance of a single-stage, dual-bed adsorption chiller. A simple lumped parameter model is utilized in order to simulate the thermal behavior of the chiller. The overall performance of the chiller is evaluated using four performance indicators, namely, (1) cooling capacity, (2) coefficient of performance, (3) chiller efficiency, and (4) waste heat recovery efficiency. A parametric study is performed in order to determine the way the overall performance of the chiller is affected by operating conditions and by overall thermal conductance of the chiller’s heat exchangers. Then a sensitivity analysis is carried out to investigate which parameter affects each performance indicator the most. It is observed that the heat sink temperature is the parameter with the biggest impact on the adsorption chiller’s performance. Other influential variables are the heat source and the chilled water inlet temperatures.

An inversion method has been established by this study for calculation of internal parameters for optimization of condensing heat recovery system of an air-source heat pump. Real-time external parameters of a retrofitted system were measured, such as temperature and mass flow rate of cold water, hot water, and air, to provide data for the inversion model. The inversion model would then be used to determine the real-time internal parameters of the refrigerant. The performance characteristics of the heat recovery process of the retrofitted system were evaluated. The internal parameters distribution and variation of heat recovery condenser was modeled and heat transfer in the heat recovery condenser was studied based on the inversion method. The result showed that approximately 70% of the area of the heat recovery condenser was in the two-phase region, and there was less than 50% heat exchange but with more than 80% pressure loss in two-phase region. This work has been demonstrably useful in identifying the shortcomings of the designed system and therefore could contribute to the improvement of the system.

This study aims at modeling and simulating the transient operation of a utility supercritical steam generator fired with pulverized hard coal. The present theoretical study focuses on the development and validation of a dynamic thermodynamic model, with the aim of analyzing the steam generator’s transient state of start-up. A commercial software tool is used to this end and the steam generator model is developed based on the software package’s components and is in accordance with manufacturer’s data. In the beginning, the operation of the once-through steam generator at full load as well as at various partial-load cases is modeled at static conditions. The combustion process in the furnace is influenced by several parameters, which are taken into account in this analysis with as much detail as possible for the specifications of a one-dimensional thermodynamic computational package. For this purpose, the modeling approach adopts and utilizes a number of innovative special simulation features, which are analyzed in the present paper. Both data provided by the steam generator supplier and results derived by computational fluid dynamics (CFD) numerical runs are used for the validation of the present model. Subsequently, the transient event of the steam generator’s cold start-up is simulated dynamically and its results are displayed along with the corresponding ones from the steady-state runs. The present model’s results are in satisfactory agreement with the provided validation data, proving the usefulness of a new simulation tool to the power-plant scientific and industrial community.

The flow through a collapsible thin-walled tube established by gravity is experimentally studied by performing two-dimensional particle image velocimetry (2D PIV) velocity measurements. Two series of experiments are performed during which the flow rate and the inlet tube pressure are kept constant but the outlet pressure is varied. In the first experiment, the velocity field is examined in a plane perpendicular to the plane of tube collapse, when the tube is in a state of weak nonperiodic tube oscillations, focusing on its two-lobe-shaped diffuser formed downstream of the tube midlength. It is verified that when the tube diffuser angle exceeds 20° from a lowering of the transmural pressure, the flow separates alternatively from the opposite walls of the tube constituting a probable triggering mechanism for the initiation of tube periodic oscillations. In the second series of experiments, the velocity field is measured in the plane of the tube collapse when the tube is in a state of periodic self-oscillation. Using the tube outlet pressure as triggering signal, phase-locked PIV measurements are performed revealing the flow evolution within a period. The two-lobed shape of the tube neck cross section oscillating streamwise for a distance of 10% of the unsupported length of the tube is verified through velocity profiles, which show two maxima off axis. Between these two maxima, which are four times the inlet bulk velocity, the flow decelerates when the tube outlet pressure is reduced in each half period, while at the same time the opposite walls of the tube approach each other. In the second half of the period during which the tube expands, negative flow appears near the tube outlet, while the velocity and the width of the two jets increase. The amplitude of the central axial velocity fluctuations is enhanced streamwise, reaching a fivefold increase compared to the inlet, especially close to the tube exit, where the tube deformation is significant.

Electrostatically manipulated, laminar, non-premixed, counterflow methane flames were studied experimentally and computationally. It was established experimentally that the flame position could be controlled solely through control of the applied electric field, without any variation of the strain imposed on the flame. The computations were conducted using a commercially available finite-element software implementing a widely accepted detailed kinetic mechanism that was supplemented with a set of three reactions generating three chemi-ions: ${\mathrm{H}}_3{\mathrm{O}}^{+}$, ${\mathrm{HCO}}^{+}$, and ${\mathrm{e}}^{-}$. The electrostatic effect was coupled with the reactive flow equations through two distinct mechanisms. First, the electric field introduced a body force that affected the momentum balance. Second, for the charged species, a diffusion mechanism developed in addition to the Fickian diffusion, which involved the generation of a diffusion velocity that was determined by the charged species mobility (i.e., ambipolar diffusion). The corresponding terms were introduced appropriately into the reactive Navier-Stokes equations. The contribution of the ambipolar diffusion is observed to be relatively small by comparing results with and without the ambipolar diffusion term, which made the application of the body force the main means through which electrostatics affected the flame. Computations and experiments support the notion that the application of the electric force shifts the flame to a different location without drastically affecting its structure.

This paper investigates the effect of isobutane blended dimethyl ether (DME) fuels on the spray behaviors, combustion, and emission reduction characteristics in a diesel engine. To investigate the influence of DME-isobutane blends, measurements of the spray characteristics from the spray visualization, combustion characteristics, and emissions reduction characteristics were conducted under various injection timings and isobutane blending ratios. In this experiment, isobutane fuel was blended into DME to make up for the lower heating value (LHV) of DME. The isobutane blending ratio of DME-isobutane blends was changed from 10 to 30% using the isobutane mass fraction. To investigate the effect of the isobutane blending ratio on the combustion and emission characteristics of a diesel engine, five types of fuels were applied: diesel, DME, DME90LP10, DME80LP20, and DME70LP30 (where numerical values indicate the weight fraction percentage). In this paper, the results of the visualization tests show that the DME and DME-isobutane blended fuels have slightly shorter results than that of conventional diesel fuel beyond 1.7 ms of elapsed time. The spray cone angles show similar trends, although with a slight decrease as the ratio of isobutane in the DME-isobutane blends was increased. The combustion pressure of the DME-isobutane blends decreased as the isobutane blending ratio was increased in the test range, showing higher pressure than that of conventional diesel fuel. The ignition delay increased when the injection timing was retarded; it also increased with increased amounts of isobutane in the DME-isobutane mixture. The results of cumulative heat release and heat release rate show that the increase in isobutane blending ratio resulted in retarded heat release. As the isobutane blended ratio was increased, the brake-specific nitrogen oxides (NOx) of DME-isobutane blends were increased compared to DME fuel. The soot emissions of the DME and DME-isobutane blends were nearly zero compared to those of conventional diesel. The CO emissions of the DME-isobutane blending fuels increased compared to that of DME fuel with the isobutane blending ratio. The DME and DME-isobutane fuels showed lower HC emissions, with the exception of the late injection timing of the DME70LP30 fuel.

Accurate prediction of heat-transfer rates in condensers is a challenging task because of phase-change dynamics. This is further complicated if noncondensable gases are present since they tend to form an insulating layer around heat-exchange surfaces. This study examines the utilization of support vector regression in predicting the preheat temperature of seawater exiting a condenser upon its flashing in a vacuum chamber to produce fresh water. Gasses dissolved in seawater are released but not condensed. Thus, system vacuum and heat transfer slowly erode with time due to this accumulation of noncondensable gasses. The preheat temperature is modeled in this study as a function of system vacuum, seawater flow rate through the condenser, and flashed vapor temperature destined for condensation. In comparison with the least-squares polynomial method, the results indicate that support vector regression can predict the preheat temperature much more accurately, resulting in a better performance evaluation of the entire solar desalination system.

A numerical analysis for performance optimization of solar cells based on heterojunctions with copper oxide (${\mathrm{Cu}}_2\mathrm{O}$) is performed. The equations given by the single-diode model for solar cells with the Lambert $W$-function, experimental data from the literature, and an adequate fitting procedure are used. A model for the light-generated current density is proposed. The results indicate a power conversion efficiency of 19.29% without any optical confinement structure to such an ideal solar cell. The parameters of the solar cell are obtained from the analysis of the total current density on voltage ($J–V$) experimental characteristics. The influence of the series resistance, shunt resistance, ideality factor, product between the mobility and the lifetime of electron, cell temperature, and thickness of the absorber layer on the maximum output power and power conversion efficiency is analyzed, aiming to be used for the design and optimization of these solar cells.

To study the thermal effects of using nanofluid as a heat transfer fluid (HTF) in a thermocline-type packed-bed energy storage tank filled with spherical phase-changing material (PCM) capsules, an in-house Fortran code is developed and validated using existing experimental and numerical data. The current assessment study shows that use of a nanofluid as HTF is able to accelerate the charging and discharging periods, hence positively affecting the thermal storage efficiency. For instance, using a nanofluid with a 5% nanoparticle concentration as HFT would reduce the charging/discharging period by about 20%. Hence, the current findings reveal that the use of a nanofluid as HTF should be considered for future design of thermocline-type packed-bed energy storage tanks of commercial size. The fact that the nanoparticles’ concentration is directly affecting the heat transfer rate can be used as an adjustable parameter during the design and optimization stages of the thermal storage.

In this paper, a detailed survey has been carried out in order to evaluate the performance of a micro combined heat and power (CHP) system, based on an internal combustion engine (ICE) fed with natural gas as the prime mover. In particular, several operating modes of the micro combined heat and power system are proposed to satisfy the electric load demand deriving from civil users. These operating modes consider a variable number of users and prime movers, as well as a variable strategy of load sharing among them. Moreover, the analysis takes into account the utilization of an electric energy storage system and a converter allowing the operation of the engine at variable speed as enabling technologies. The comparison has been done using as input a statistical profile of domestic electric load. The results, compared with the performance of the conventional systems, have highlighted a maximum natural gas savings up to 22% with consequent reduction of carbon dioxide emissions. Moreover, the results of simulations show that the number of engines and the engine operation at variable speed determines the greatest benefits on fuel consumption, followed by the utilization of an electric energy storage system. The load-sharing strategy, among the operating engines, has, on the contrary, a secondary effect.

Passive solar design strategies for buildings are necessary for annual heating energy demand reduction. This paper presents a mathematical model for interior room temperature calculation of a building that integrates a combined passive solar system consisting of an unvented Trombe wall and a direct passive solar system for direct insolation. The model was applied to a residential building located in Niš, Serbia, which is not additionally heated by conventional fuels. Two variations of a combined passive system were taken into consideration. The first combined passive system consisted of a Trombe wall made of 0.45-m-thick concrete and the second combined passive system consisted of a 0.20-m-thick concrete Trombe wall. To calculate the complex mathematical model, depending on the size of the window opening, window slope (slanting or vertical position), and orientation of the room, two software packages were developed, RMSun and InSunTr. In addition, the indoor air temperatures of a thermally insulated residential building with an unvented Trombe wall and a direct passive solar system for different building orientations and different Trombe wall thicknesses were analyzed during the winter period between January and March.

Hydrokinetic turbines have gained importance due to rapid developments in the field of renewable energy. There are several applications for large-scale hydrokinetic turbines in marine environments; however, the number of small-scale designs for rivers and channels is limited. In the present study, a fixed-speed, stall-regulated riverine hydrokinetic turbine rotor has been designed through an optimization process and simulated using computational fluid dynamics (CFD) analysis methods. The blades and rotor were optimized by genetic algorithm methods. Simulations were conducted using commercially available software with a shear-stress transport (SST) $k\text{-}\omega$ turbulence closure model. In the simulations, the performance in terms of power, torque, and thrust were found to be in good agreement with the optimization parameters. The optimized hydrokinetic turbine is named TIGRIS-27 H, and is a three-bladed horizontal-axis energy converter rotating at 45 revolutions per minute (rpm) and generating up to 27 kW power at the rated velocity of $2.7{\mathrm{ms}}^{-1}$ with an average power coefficient of 0.43.

Commercial and industrial customers are subject to monthly maximum demand charges, which can be as high as 30% of the total electricity bill. A battery-based energy storage system (BESS) can be used to reduce the monthly maximum demand charges. A number of control strategies have been developed for the BESS to reduce the daily peak demands. However, the existing control strategies may fail to reduce the peaks on some occasions because the energy of the BESS runs out during the process of peak reduction. Therefore, a new active control strategy for the BESS is developed and presented in this paper. This strategy is able to reduce the peaks even though the peaks are different from the forecasted ones. This strategy is able to reduce the monthly maximum demands by 9% when the BESS capacity is reduced to 37% of the full capacity due to financial constraints. Hence, the customers can achieve the payback period of 11 years over the project lifespan of 21 years with the reduced capacity of the BESS.

A thermoelectric generator’s battery charging performance using an application of step-up direct current (DC)–DC converter to harvest energy was researched. In the first-stage study, temperature gradients from different potential heat or cold sources around individuals’ daily lives were harnessed as power supplies for personal electronic gadgets. A primary prototype comprised four components including four bulk thermoelectric modules (with 127 thermoelectric-element couples per module). Its charging characteristics was evaluated under five conditions of use. It produced the maximum power of 4.82 W, and utilized the lowest starting temperature difference of 26°C. Thermoelectric-conversion charging using a constant-heat electric stove yielded charging characteristics comparisons among three prototypes and two commercial products in the second-stage study. Despite approximately 40% less Seebeck coefficient under the same experimental conditions, the prototype indicated competitive characteristics in battery charging comparison with higher output power and more charging efficiency in the first 45 min of charging of the lower temperature-difference condition. All prototypes, with competitive maximum unit cost of US$110, yielded maximum thermal-to-electric charging efficiency and boosted the converter’s efficiency in the range of 3.8–4.8% and 60–78%, respectively.