Performance evaluation of heat exchanger using alternative refrigerant R407C

R22 (HCFC22) has been widely used as the refrigerant in air conditioners. According to the Montreal protocol for ozone layer protection, the total production of HCFCs has been capped since the beginning of 1996. Zeotropic refrigerant mixture R407C and nearly azeotropic refrigerant mixture R410A have been selected as alternatives to R22. We examined refrigerant passages in heat exchangers used in heat pump‐type room air conditioners using zeotropic refrigerant R407C through simulation, and obtained the following conclusions. In an indoor heat exchanger, a counter flow configuration when operating as a condenser has higher temperature efficiency. When an outdoor heat exchanger operates as an evaporator, a configuration that suppresses the temperature glide by partially reducing the refrigerant passage not only produces high efficiency, but also reduces the frost formation on fins. © 2002 Wiley Periodicals, Inc. Heat Trans Asian Res, 31(8): 626–638, 2002; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.10064     

Experimental investigation on the performance of ORC power system using zeotropic mixture R601a/R600a

An experimental study on the practical performance of organic Rankine cycle (ORC) system using zeotropic mixture is performed by using a small scale ORC power generation experimental setup. R601a/R600a is selected as the working fluid. The effects of mixture composition, heat source temperature, and working fluid flow rate on the performance of ORC system are investigated. The experimental results indicate that the net power output first increases and then decreases as the R600a concentration increases. The optimal mixture composition with the maximum net power output is 0.6/0.4 (mass fraction) at the heat source temperature of 115°C. The net power output of R601a/R600a (0.6/0.4) is higher than that of R601a by 25%, indicating that the performance of ORC system can be clearly improved by using the zeotropic mixture. For a fixed working fluid flow rate, both net power output and thermal efficiency first decrease slowly and then drop sharply with the decrease of the heat source temperature. The appropriate superheat degree of R601a/R600a is in the range of 15 to 20°C when the heat source temperature has a small variation. In addition, the optimal working fluid volume flow rates yielding the maximum net power output are obtained for different compositions of R601a/R600a. The experimental results in the study can be of great significance for the design and operation of ORC power system using zeotropic mixture. Copyright © 2016 John Wiley & Sons, Ltd.     

Performance analyses and improvement guidelines for organic Rankine cycles using R600a/R601a mixtures driven by heat sources of 100°C to 200°C

R600a/R601a mixtures are promising to be widely used in organic Rankine cycle (ORC) systems and also can promote the popularization of ORC technology. While, most of existing studies on ORC systems using R600a/R601a mixtures are based on certain heat source temperatures (generally below 150°C) and saturated vapor at the evaporator outlet. Variations in the optimal mixture composition and superheat degree of R600a/R601a mixtures with increasing heat source temperature remain indeterminate thus far, especially for heat sources above 150°C. Suitable approaches to further improve the system thermodynamic performance are also unclear. This study carried out a systematized analysis for subcritical ORC systems using R600a/R601a mixtures driven by heat sources of 100°C to 200°C, based on the first and second law analysis methods. Guidelines for selections of optimal mixture composition and cycle parameters were provided. Characteristics of exergy loss distribution were revealed to point out the crucial process to further improve the system thermodynamic performance. Results show that the effects of critical parameters on the selections of optimal mixture composition and evaporation pressure become remarkable for heat sources above approximately 160°C. A minimum superheat degree is optimal for heat sources below 170°C, whereas the optimal superheat degree may increase with increasing heat source temperature and R600a mass fraction for heat sources above 170°C. Moreover, reducing the exergy losses in the heat absorption process, turbine, and condenser is vital to further increase the heat‐work conversion efficiency for heat sources of approximately 100°C to 160°C, 170°C to 190°C, and 200°C, respectively.     

Performance prediction of a RPF‐fired boiler using artificial neural networks

In order to provide adequate engineering assistance and to improve the energy efficiency in process industries, it is crucial to evaluate the operational performance of a boiler in terms of its practical requirements, viz. temperature, pressure, and mass flow rate of steam. This study was aimed at assessing and optimizing the performance of a refuse plastic fuel‐fired boiler using artificial neural networks. A feed‐forward back propagation neural network model was developed and trained using existing plant data (5 months), to predict temperature, pressure, and mass flow rate of steam, using the following input parameters: feed water pressure, feed water temperature, conveyor speed, and incinerator exit temperature. The predictive capability of the model was evaluated in terms of mean absolute percentage error between the model fitted and actual plant data, while sensitivity analysis was performed on the input parameters by determining the absolute average sensitivity values. The higher absolute average sensitivity value of the incinerator exit temperature in comparison to that of feed water pressure, feed water temperature and conveyor speed suggested that the change of incineration exit temperature has a significant influence on the selected outputs (steam properties). Overall, the good results observed from this work demonstrate the fact that artificial neural networks can efficiently predict the data on steam properties and could serve as a good tool to monitor boiler behavior under real‐time conditions. Copyright © 2013 John Wiley & Sons, Ltd.     

Long time investigation of the effect of fouling on the super‐heaters in a circulating fluidized biomass boiler

The present investigation involves measurements and theories on the mechanisms of the forming of deposit layers on super‐heater tubes in a biomass‐fired CFD boiler. The deposit layer thickness and the soot‐blowing frequency effect on the super‐heaters heat transfer are the main subject of the study that has been conducted over a 3‐year period. The measurements show a deposit growth rate on the super‐heaters of approximately 4 g m^?2 h^?1. The distribution of the deposit material varies significantly between the windward and the leeward side of the tubes, with the thickest layers on the windward side. Further down stream of the first super‐heater, the fouling problem on the super‐heater and re‐heater tubes are not so severe. A theoretical model shows that a deposit layer of 20 mm will decrease the heat transfer rate of the first super‐heater by nearly 40%. The soot‐blowing system shows a strong positive effect on the heat transfer rate of the super‐heater a few hours after a soot‐blowing sequence has been completed. However in the long run, the varied soot‐blowing frequency does not have a significant influence on the deposit layer growth rate. Copyright © 2006 John Wiley & Sons, Ltd.     

Performance evaluation of a triple‐effect evaporator with forward feed using exergy analysis

The present study evaluates the performance of a triple‐effect evaporator with forward feed (TEEFF) system by using exergy analysis based on actual operational data. The orange juice with a capacity of about 1.222 kg s^?1 is concentrated from a dry matter (DM) content of 12 to 65% in this TEEFF, which is situated in an orange juice concentrate line installed in a factory, located in Denizli, Turkey. A Visual Basic 6.0 program was also developed to show how the exergetic performance characteristics of the system vary with the feed flow rates ranging from 1.222 to 1.667 kg s^?1. The total exergy efficiency of the TEEEFF is found to be on average 0.85. The largest exergy destruction occurs in the first‐effect of the TEEFF system with 48.2% of total, followed by the third and second effects with 32.04 and 19.76% of that. Evaporator performance is also rated on the basis of steam economy, which is obtained to be in the range of 2.05–2.14 under the operation conditions. It is expected that the analysis presented here should provide a designer with a better, quantitative grasp of the inefficiencies and their relative magnitudes in the design, simulation and operation of multiple‐effect evaporators. Copyright © 2005 John Wiley & Sons, Ltd.     

Modeling the effect of key cathode design parameters on the electrochemical performance of a lithium‐sulfur battery

A 1D model is developed for the Li‐S cell to predict the effect of critical cathode design parameters—carbon‐to‐sulfur (C/S) and electrolyte‐to‐sulfur (E/S) ratios in the cathode—on the electrochemical performance. Cell voltage at 60% depth of discharge corresponding to the lower voltage plateau is used as a metric for calculating the cell performance. The cathode kinetics in the lower voltage plateau is defined with a single electrochemical reaction; thus, the model has a single apparent kinetic model parameter, the cathode exchange current density (i_0,pe). The model predicts that cell voltage increases considerably with increasing carbon content until a C/S ratio of 1 is attained, whereas the enhancement in the cell voltage at higher ratios is less obvious. The model can capture the effect of the C/S ratio on the cathode kinetics by expressing the electrochemically active area in the cathode in carbon volume fraction; the C/S ratio in the cathode does not affect i_0,pe in the model. On the other hand, the electrolyte amount has a significant impact on the kinetic model parameter such that increasing electrolyte amount improves the cell voltage as a result of increasing i_0,pe. Therefore, in the model, i_0,pe needs to be defined as a function of the electrolyte volume fraction, which is known to have a crucial effect on reaction kinetics.     

Circulating fluidized bed gasification of low rank coal: Influence of O<Subscript>2</Subscript>/C molar ratio on gasification performance and sulphur transformation

To promote the utilization efficiency of coal resources, and to assist with the control of sulphur during gasification and/or downstream processes, it is essential to gain basic knowledge of sulphur transformation associated with gasification performance. In this research we investigated the influence of O₂/C molar ratio both on gasification performance and sulphur transformation of a low rank coal, and the sulphur transformation mechanism was also discussed. Experiments were performed in a circulating fluidized bed gasifier with O₂/C molar ratio ranging from 0.39 to 0.78 mol/mol. The results showed that increasing the O₂/C molar ratio from 0.39 to 0.78 mol/mol can increase carbon conversion from 57.65% to 91.92%, and increase sulphur release ratio from 29.66% to 63.11%. The increase of O₂/C molar ratio favors the formation of H₂S, and also favors the retained sulphur transforming to more stable forms. Due to the reducing conditions of coal gasification, H₂S is the main form of the released sulphur, which could be formed by decomposition of pyrite and by secondary reactions. Bottom char shows lower sulphur content than fly ash, and mainly exist as sulphates. X-ray photoelectron spectroscopy (XPS) measurements also show that the intensity of pyrite declines and the intensity of sulphates increases for fly ash and bottom char, and the change is more obvious for bottom char. During CFB gasification process, bigger char particles circulate in the system and have longer residence time for further reaction, which favors the release of sulphur species and can enhance the retained sulphur transforming to more stable forms.     

The effect of Pt/C agglomerates in electrode on PEMFC performance using 3D micro-structure lattice models

To simulate the Pt/C agglomerate phenomenon in proton exchange membrane fuel cell (PEMFC) cathode, we established 3D two-phase micro-structure lattice models, including C phase (Pt/C particles) and IP phase (mixed ionomer and pore), respectively. Based on these models, we studied the effect of Pt/C agglomerates on theoretical catalyst utilization, IP phase tortuosity and cell performance in PEMFC, and compared the electrode with linear agglomerates to the electrode with spherical agglomerates. Unexpectedly, contrary to the simulation results from macro-models, the electrode with Pt/C linear agglomerates performed better than that with Pt/C spherical agglomerates. In addition, it was found that the electrode with Pt/C agglomerates performed better than that without Pt/C agglomerates, which was different to what we believed before. By analysing the reasons of cell performance change, it was found that mass transport had a more important effect on cell performance than electrochemical reaction.     

Thermodynamic analysis of spark‐ignition engine using a gas mixture model for the working fluid

This paper presents thermodynamic analysis of spark‐ignition engine. A theoretical model of Otto cycle, with a working fluid consisting of various gas mixtures, has been implemented. It is compared to those which use air as the working fluid with variable temperature specific heats. A wide range of engine parameters were studied, such as equivalence ratio, engine speed, maximum and outlet temperatures, brake mean effective pressure, gas pressure, and cycle thermal efficiency. For example, for the air model, the maximum temperature, brake mean effective pressure (BMEP), and efficiency were about 3000 K, 15 bar, and 32%, respectively, at 5000 rpm and 1.2 equivalence ratio. On the other hand, by using the gas mixture model under the same conditions, the maximum temperature, BMEP, and efficiency were about 2500 K, 13.7 bar, and 29%. However, for the air model, at lower engine speeds of 2000 rpm and equivalence ratio of 0.8, the maximum temperature, BMEP, and efficiency were about 2000 K, 8.7 bar, and 28%, respectively. Also, by using the gas mixture model under these conditions, the maximum temperature, BMEP, and efficiency were about 1900 K, 8.4 bar, and 27%, i.e. with insignificant differences. Therefore, it is more realistic to use gas mixture in cycle analysis instead of merely assuming air to be the working fluid, especially at high engine speeds. Copyright © 2007 John Wiley & Sons, Ltd.     

Steady‐state model of a variable speed vapor compression system using R134a as working fluid

This paper presents a steady‐state physical model for a variable speed vapor compression system. Its development and validation for a wide range of operating conditions are presented. The model requires as input parameters: compressor speed, static superheating degree and volumetric flow rates and temperatures of secondary fluids at the evaporator and condenser inlet. Using these input parameters, which can be easily obtained in this kind of facility, the model predicts the operating pressures, the temperature of secondary fluids at the evaporator and condenser outlet, the evaporator and condenser thermal capacities, the electric power consumed by the compressor and the coefficient of performance, COP. The experimental validation of the model has been carried out with 177 tests using R134a as working fluid, concluding that the model can predict the energetic performance of a variable speed vapor compression chiller with an error lower than ±10%. Copyright © 2009 John Wiley & Sons, Ltd.     

Effect of flow fields and humidification of reactant and oxidant on the performance of scaled‐up PEM‐FC using CFD code

The present study focuses on three‐dimensional two‐phase CFD investigation on scaled‐up proton exchange membrane fuel cell (PEM‐FC) for an active area of 100 cm² with different designs of serpentine and parallel flow configuration. The humidification of hydrogen and oxygen is varied from 10% to 100% to study the PEM‐FC performance. The numerical results of polarization curves predicted in this study have been numerically validated with that of the literature for both parallel and counter serpentine flow channels with active area of 24.8‐cm² PEM‐FC. Further upon validation, the numerical study is extended for scaled‐up PEM‐FC with active area of 100 cm² with different flow path designs to study its performance characteristics namely polarization curves, species concentration distribution, water content in the membrane electrolyte, and proton conductivity to evaluate the fuel cell performance. The three‐dimensional CAD models are created in SOLIDWORKS 10.0 and are discretised hexahedrally using finite volume method. The various governing equations namely conservation of mass, momentum, energy, species concentration, and electrochemical equations are solved numerically with the necessary boundary conditions using the CFD code. The novel design of straight zigzag flow path shows the better performance output over the other designs investigated which is having a higher power density of 0.3711 W/cm² for 100% relative humidity of reactant and oxidant.     

Effectiveness evaluation of passive resistive element placement on a fault ride‐through enhancement in a DFIG‐based wind energy conversion system

Over the last few decades, doubly fed induction generators (DFIG) have become one of the most successful and preferred types of wind energy generators (WEG). The DFIG has the advantages of a wide range of speed operations, a high efficiency, and partial rated converters. However, direct coupling of the stator with the grid makes the system more prone to grid disturbances. The consequences of grid disturbances, such as a rotor overcurrent, stator overcurrent, electromagnetic torque oscillations, and direct current (DC) link overvoltage, are the predominant considerations that affect the rotor circuit, stator circuit, mechanical components, and DC‐link capacitor of the DFIG, respectively. These uncertainties affect the operation of the generator and may lead to the generator to be shutdown. In this paper, a novel position for the placement of a passive resistive element (PRE) is illustrated. This position of the PRE placement is compared with all other possible locations for the PRE. The different locations for PRE placement are the stator side, rotor side, across the DC‐link capacitor, and between the rotor side converter (RSC) and grid side converter (GSC). This paper aims to determine a cost‐effective solution among all possible locations of the PRE placement. The novel position of the PRE, ie, between the RSC and GSC, is the best position among the other possible locations of the PRE, when the performance, cost, and loss are taken into consideration. The effectiveness of the PRE is further compared with the resistive‐type superconducting fault current limiter (R‐SFCL). The PRE performs better and has a lower cost than the R‐SFCL.     

Study on fluidization of 0.5 µm ultrafine and 8.0 µm superfine Geldart‐C powders in a binary mixture circulating fluidized bed

The present study investigates the discharge rates of ultrafine and superfine powders from a semi‐batch circulating fluidized bed (CFB) using a binary mixture of fine Al(OH)₃ powders (mean size 0.5 and 8.0 µm, Geldart‐C) and coarse fluidized catalytic cracker (FCC) particles (mean size 6 µm, Geldart‐A). Discharge rates of fine powders were measured at the top of the cyclone. Effects of the equilibrium water content of coarse FCC particles on the performance of the CFB, in terms of discharge rates of fine powders, were investigated at a starting loading of fine powders of 5 wt%. The discharge rates of the cohesive ultrafine powders were much smaller than the discharge rates of the less cohesive superfine powders, irrespective of the water content of FCC particles. In the presence of 8.0 µm superfine powders, the discharge rates decreased with decreasing the loading of superfines in the bed. At a certain loading of superfines, specifically at high equilibrium water contents in the range of 0.061 and 0.067 kg‐water/kg‐dry FCC, discharge rates decreased with decreasing the equilibrium water content of FCC particles. For FCC particles of low equilibrium water content as 0.038 kg‐water/kg‐dry FCC, discharge rates were low at the beginning then increased to maximum, within a few minutes, then decreased again with decreasing the loading of superfines in the bed. In the presence of 0.5 µm ultrafine powders, at high water equilibrium content of FCC particles as 0.067 kg‐water/kg‐dry FCC, the discharge rates decreased with decreasing the loading of ultrafines in the bed. However, at the two relatively low equilibrium water contents of FCC particles of 0.038 and 0.061 kg‐water/kg‐dry FCC, the discharge rates were almost constant. The cohesive property of ultrafine powders prevailed. Copyright © 2013 John Wiley & Sons, Ltd.     

Computational study about the effect of electrode morphology on the performance of lithium‐ion batteries

Electrode morphology has significant influence on the performance of lithium‐ion batteries in that it controls electrical conductivity and electrode utilization by establishing electrical connectivity in the electrode. The present study investigates the effect of the electrode morphology on battery performance by combining two different mathematical models. First, a two‐dimensional, direct numerical simulation (DNS) model is introduced, which stochastically generates electrode morphology and calculates electrical conduction and electrode utilization. Various simulations are conducted to evaluate the effect of the active particle coating, conductive agent loading, particle size, and electrode compression by using the DNS model. Second, data acquired from the DNS model are applied to the blended‐electrode model to evaluate battery performance. Calculation result confirms that electrode morphologies have significant effects on both capacity and power of lithium‐ion batteries. Copyright © 2016 John Wiley & Sons, Ltd.     

Slip boundary condition effect on double‐diffusive convection in a porous medium: Brinkman Model

The model of double‐diffusive convection in a porous medium layer was analyzed using the Brinkman model and concentration based on an internal heat source. Linear instability analysis of the model was performed. Particularly, we analyzed the effect of slip boundary conditions on the instability of the system. We analyzed when the instability started and computed the critical Rayleigh number as a function of the slip coefficient.     

Numerical study of ferrofluid flow and heat transfer in the presence of a non‐uniform magnetic field in rectangular microchannels

This paper presents a numerical investigation of the hydro‐thermal behavior of a ferrofluid in rectangular minichannels in the presence of a non‐uniform magnetic field using a two‐phase mixture model and control volume technique. Effects of increasing the diameter of nanoparticles, and channel aspect ratio have also been studied. It is concluded that the magnetic field with a negative gradient increases the Nusselt number and the rate of this increment depends on the channel aspect ratio. © 2012 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley Online Library ( wileyonlinelibrary.com/journal/htj ). DOI 10.1002/htj.21004     

Study on electrochemical and thermal characteristics of lithium‐ion battery using the electrochemical‐thermal coupled model

The higher specific energy leads to more heat generation of a battery, which affects the performance and cycle life of a battery and even results in some security problems. In this paper, the capacity calibration, Hybrid Pulse Power Characteristic (HPPC), constant current (dis)charging, and entropy heat coefficient tests of chosen 11‐Ah lithium‐ion batteries are carried out. The entropy heat coefficient increases firstly and then decreases with the increase of the depth of discharge (DOD) and reaches the maximum value near 50% DOD. An electrochemical‐thermal coupled model of the chosen battery is established and then verified by the tests. The simulation voltage and temperature trends are in agreement with the test results. The maximum voltage and temperature error is within 2.06% and 0.4°C, respectively. Based on the established model, the effects of adjustable parameters on electrochemical characteristic are systematically studied. Results show that the average current density, the thickness of the positive electrode, the initial and maximum lithium concentration of the positive electrode, and the radius of the positive electrode particle have great influence on battery capacity and voltage. In addition, the influence degree of the internal resistance of the solid electrolyte interface (SEI) layer, the thickness of negative electrode, and the initial and maximum lithium concentration of the negative electrode on the capacity and voltage is associated with certain constraints. Meanwhile, the influences of adjustable parameters related to thermal characteristic are also systematically analyzed. Results show that the average current density, the convective heat transfer coefficient, the thickness, and the maximum lithium concentration of the positive electrode have great influence on the temperature rise. Besides, the uniformity of the temperature distribution deteriorates with the increase of the convective heat transfer coefficient.     

Effects of geometrical parameters of a dish concentrator on the optical performance of a cavity receiver in a solar dish‐Stirling system

Dish‐Stirling concentrated solar power (DS‐CSP) system is a complex system for solar energy‐thermal‐electric conversion. The dish concentrator and cavity receiver are optical devices for collecting the solar energy in DS‐CSP system; to determine the geometric parameters of dish concentrator is one of the important steps for design and development of DS‐CSP system, because it directly affects the optical performance of the cavity receiver. In this paper, the effects of the geometric parameters of a dish concentrator including aperture radius, focal length, unfilled radius, and fan‐shaped unfilled angle on optical performance (ie, optical efficiency and flux distribution) of a cavity receiver were studied. Furthermore, the influence of the receiver‐window radius of the cavity receiver and solar direct normal irradiance is also investigated. The cavity receiver is a novel structure that is equipped with a reflecting cone at bottom of the cavity to increases the optical efficiency of the cavity receiver. Moreover, a 2‐dimensional ray‐tracking program is developed to simulate the sunlight transmission path in DS‐CSP system, for helping understanding the effects mechanism of above parameters on optical performance of the cavity receiver. The analysis indicates that the optical efficiency of the cavity receiver with and without the reflecting cone is 89.88% and 85.70%, respectively, and former significantly increased 4.18% for 38 kW XEM‐Dish system. The uniformity factor of the flux distribution on the absorber surface decreases with the decreases of the rim angle of the dish concentrator, but the optical efficiency of the cavity receiver increases with the decreases of the rim angle and the increase amplitude becomes smaller and smaller when the rim angle range from 30° to 75°, So the optical efficiency and uniformity factor are conflicting performance index. Moreover, the unfilled radius has small effect on the optical efficiency, while the fan‐shaped unfilled angle and direct normal irradiance both not affect the optical efficiency. In addition, reducing the receiver‐window radius can improve the optical efficiency, but the effect is limited. This work could provide reference for design and optimization of the dish concentrator and establishing the foundation for further research on optical‐to‐thermal energy conversion.