Conditions For Maximum Efficiency Research Articles

Performance assessment and optimization of a combined heat and power system based on compressed air energy storage system and humid air turbine cycle

Renewable energy based power sources have grown rapidly in the past few years owing to the dual constraint of climate change and pollution control. Compressed air energy storage (CAES), as a large-scale energy storage system (ESS) technology, has huge potential to manage the intermittent renewable energy based power sources effectively. However, the compression heat generated during charge and waste heat carried in turbine exhaust during discharge are not fully recuperated in current stage. A combined heat and power (CHP) system consisting of a CAES system and a humid air turbine (HAT) system is proposed to utilize the both types of heat energy. The proposed system can boost the power output, enhance performance and improve efficiency through a simultaneous supply of power and heat. The thermodynamic analysis shows that the expansion train power can be improved about 26% compared with the conventional CAES system. The parametric analysis reveals that the exergy efficiency increases with the turbine inlet temperature (TIT) of high pressure turbine (HPT) and inlet pressure of low pressure turbine (LPT), but decreases with the TIT of LPT, L/G ratio and dry air inlet temperature of saturator. Meanwhile, the system optimization is carried out via particle swarm optimization (PSO) to determine the maximum power and exergy efficiency conditions.

УТИЛИЗАЦИЯ ТЕПЛОТЫ ОТРАБОТАВШИХ ГАЗОВ ГАЗОТУРБИННОЙ УСТАНОВКИ С ПОМОЩЬЮ ДВИГАТЕЛЯ СТИРЛИНГА

The article researches a feasibility of real cycles of combined power plants including gas turbine engines and Stirling engines. The variant of construction concepts of such power plants has been considered. The analysis of thermodynamic cycles has been performed, the dependences to calculate their basic parameters has been founded. Based on the research results quantify the heat utilization degree of the combustion products of a gas turbine engine for the Stirling engine has been given. Stirling engine’s efficiency, power and combined power plant’s efficiency in general has been identified. The heat exchanger’s type and heat transfer agent has been proposed, the optimal parameters by conditions of maximum efficiency of the plant has been identified. Proposals for the application and the empowerment of such power plants as part of the vehicles has been offered.

Removal of sulfide ions from water using rice husk

Sulfide is an unwanted anion present in tannery effluents. In this study, a raw material namely rice husk has been used to check its performance to remove sulfide ions from water in batch adsorption experiments. The effects of various physical parameters such as contact time, pH, shaking speed and temperature have been investigated to optimize the conditions for maximum adsorption efficiency. Rice husk was further pretreated with calcium chloride, zinc chloride and organic solvents to check any change in uptake capacity of the adsorbent. Langmuir, Freundlich and Temkin isotherms were used to explain the nature and mechanism of adsorption. The results argue that rice husk could be a useful material to treat sulfide-containing wastewaters.

Physiological Changes in the Seeds of Jatropha curcas L. at Different Stages of Fruit Maturity

The objective of present study was to determine appropriate fruit harvesting stage for Jatropha curcas L. for getting seeds with good oil content and better germination. The seeds separated from the fruits harvested at yellow capsule stage (C2) showed maximum germination efficiency, both under laboratory and nursery conditions whereas seeds separated from the fruits harvested at brownish black stage (C3) yielded highest oil content. Yellow capsule stage (C2) was physiologically mature fruit stage, which should be harvested to obtain the seeds with higher oil content besides showing maximum germination efficiency.

Photocatalytic degradation of pentachlorophenol by visible light Ν-F-TiO₂ in the presence of oxalate ions: optimization, modeling, and scavenging studies.

The efficiency of heterogeneous photocatalysis using N-F-TiO2 as photocatalyst to degrade a priority pollutant, pentachlorophenol (PCP), in the presence of oxalates (OA) was investigated in detail. Response surface methodology was used to optimize the effect of three variables (catalyst concentration, OA/PCP ratio, and pH) on the photocatalytic degradation of pentachlorophenol. A quadratic model was established as a functional relationship between three independent variables and the degradation efficiency of PCP. The results of model fitting and statistical analysis demonstrated that the pH played a key role in the degradation of PCP. Within the studied experimental ranges, the optimum conditions for maximum PCP degradation efficiency (97.5 %) were: catalyst concentration 600 mg L(-1), OA/PCP ratio 2, and pH 10. The contribution of HO(·), O2 (·-), and e(-) produced during the photocatalytic treatment was investigated with the addition of scavengers. The photocatalytic degradation was essentially proceeded through an oxidative mechanism at both acid and alkaline pH values by HO(.) and O2 (·-) radicals attack. It was found that O2 (·-) were the major reactive species involved in PCP degradation in pH 4 and HO(·) in pH 10.

Impregnation of Magnetic Particles on Oil Palm Shell Activated Carbon for Removal of Heavy Metal Ions from Aqueous Solution

Oil palm shell activated carbon magnetic particle (CAC-MP) was prepared for adsorption of metal ions (Zn2+, Pb2+, Cu2+). Fourier transform infrared spectroscopy (FTIR), thermogravimetric analysis (TGA), surface area and pore analysis (BET, BJH and t-plot method), field emission scanning electron microscopy (FESEM), vibrating sample magnetometer (VSM), and X-ray diffraction (XRD) were used to characterize CAC-MP. Its properties were compared with the parent activated carbon (CAC). The CAC-MP, with a high surface area (1007 m2/g), was used to study metal ions removal at different pH, adsorbent dosage, and contact time. The removal efficiency of metal ion increased with increasing pH, dosage, and time until equilibrium was reached. The optimum condition for maximum removal efficiency was at pH 6 and absorbent dosage of 0.5 g. Kinetic studies were also performed and the best kinetic model was fitted by pseudo-second-order model.

Thermodynamic bounds and general properties of optimal efficiency and power in linear responses.

We study the optimal exergy efficiency and power for thermodynamic systems with an Onsager-type "current-force" relationship describing the linear response to external influences. We derive, in analytic forms, the maximum efficiency and optimal efficiency for maximum power for a thermodynamic machine described by a N×N symmetric Onsager matrix with arbitrary integer N. The figure of merit is expressed in terms of the largest eigenvalue of the "coupling matrix" which is solely determined by the Onsager matrix. Some simple but general relationships between the power and efficiency at the conditions for (i) maximum efficiency and (ii) optimal efficiency for maximum power are obtained. We show how the second law of thermodynamics bounds the optimal efficiency and the Onsager matrix and relate those bounds together. The maximum power theorem (Jacobi's Law) is generalized to all thermodynamic machines with a symmetric Onsager matrix in the linear-response regime. We also discuss systems with an asymmetric Onsager matrix (such as systems under magnetic field) for a particular situation and we show that the reversible limit of efficiency can be reached at finite output power. Cooperative effects are found to improve the figure of merit significantly in systems with multiply cross-correlated responses. Application to example systems demonstrates that the theory is helpful in guiding the search for high performance materials and structures in energy researches.

Frequency mixing in boron carbide laser ablation plasmas

Nonlinear frequency mixing induced by a bichromatic field (1064nm+532nm obtained from a Q-switched Nd:YAG laser) in a boron carbide (B4C) plasma generated through laser ablation under vacuum is explored. A UV beam at the frequency of the fourth harmonic of the fundamental frequency (266nm) was generated. The dependence of the efficiency of the process as function of the intensities of the driving lasers differs from the expected behavior for four-wave mixing, and point toward a six-wave mixing process. The frequency mixing process was strongly favored for parallel polarizations of the two driving beams. Through spatiotemporal mapping, the conditions for maximum efficiency were found for a significant delay from the ablation event (200ns), when the medium is expected to be a low-ionized plasma. No late components of the harmonic signal were detected, indicating a largely atomized medium.

The optimal performance of solid oxide fuel cell-thermoelectric generator hybrid system

The model of a class of irreversible solid oxide fuel cell (SOFC) and the semiconductor thermoelectric generator (TEG) hybrid power system is established. The general expressions of several important performance parameters of the system such as the power output, efficiency, and minimum electric current density of the hybrid system are derived, based on the theory of non-equilibrium thermodynamics. The performance characteristics and optimal the performance of the hybrid system are analyzed. The optimum conditions for maximum efficiency or maximum output power of the hybrid system are given. The optimal regions of several important performance parameters of the hybrid system are determined. The influences of some of the main irreversibility on the optimal performance of the system are discussion in detail. Some significant new conclusions are obtained. The results obtained will provide theoretical foundation for the design and optimization of practical power hybrid system.

Response to “Comment on ‘Effective thermal conductivity in thermoelectric materials’” [J. Appl. Phys. 113, 204904 (2013)

It is commonly claimed that achieving maximum power from a thermoelectric generator necessitates electrical load matching conditions instead of the operating condition derived for maximum generator efficiency. Here, we explain why the electrical load matching claim for maximum power in a design optimization is flawed and show that the load condition derived for maximum efficiency always produces more power. Finally, we consider a CPM generator, and prove that the electrical condition for maximum efficiency is indeed the electrical condition for maximum power, maximum power density, maximum power/cost of thermoelectric material, and maximum power/weight of thermoelectric material, when the leg length of the thermoelectric generator is a design variable.

A new approach to optimize the energy efficiency of CO2 transcritical refrigeration plants

This paper proposes a model-free real-time optimization and control strategy for CO2 transcritical refrigeration plants that assures covering the cooling demand and continuous tracking of conditions for maximum efficiency. Our approach obtains the feedback with only three measurements, and controls the opening degree of a backpressure valve and the speed of the compressor. The strategy minimizes the power consumption of the compressor instead of maximizing the coefficient of performance, which avoids several sensors, and we demonstrate mathematically that both approaches are equivalent. We implemented the strategy with an algorithm that includes two independent auto tuned controllers, one devoted to regulate the high-pressure and another to regulate the outlet temperature of the secondary fluid of the evaporator. It also incorporates a real time perturb and observe procedure to locate online the optimum high-pressure that minimizes the compressor power consumption. The paper presents the experimental evaluation of the control strategy, verifying the stable operation of the algorithm and the energy optimization of the plant.

Hydrogen production from CO2-free thermal decomposition of methane: Design and on-sun testing of a tube-type solar thermochemical reactor

This study addresses the development of a solar thermochemical reactor for CO2-free production of hydrogen from solar-aided methane decomposition. The developed lab-scale solar reactor basically pertaining to the indirect heating concept is chiefly based on a tube-type configuration in tandem with a cavity receiver. The reactor design and performance prediction were first appraised via 3-dimensional CFD thermal simulation as a function of internal geometry. The model including coupled heat/mass transfer and chemical reaction aimed to simulate the reactor in order to determine the temperature distribution and the conditions for maximum reactor efficiency. The designed 1kW solar reactor was then constructed and installed for reaction testing at the focus of a 2m-diameter parabolic solar concentrator. Solar CH4 decomposition experiments were performed between 1300 and 1400°C to demonstrate the feasibility of hydrogen production and the reliability of the solar process using the developed reactor concept, and the results were used to validate simulations. Regarding the estimated kinetic parameters, the best fitting was obtained for an activation energy of 320kJ/mol and a pre-exponential factor of 1011s−1. The chemical conversion was improved when increasing the temperature or decreasing the inlet gas flow rate or the CH4 mole fraction. A maximum CH4 conversion (resp. H2 yield) of 90% (resp. 85%) was achieved at 1400°C and the thermochemical reactor efficiency reached 5% for the highest CH4 content in the feed.

Optimization and modeling of preparation conditions of TiO2 nanoparticles coated on hollow glass microspheres using response surface methodology

Response surface methodology was used to find and optimize the effect of the variables on the preparation, morphology and catalytic activity of nano-crystalline TiO2 thin films coated on hollow glass microspheres (HGMs) by sol–gel method. Three experimental parameters were chosen as independent variables: the calcination temperature, the amount of titanium butoxide and template F127. Morphology analysis showed that structure of TiO2–HGMs prepared was core shell, and the surface of HGMs was coated by TiO2 thin films with an average layer thickness of 728±12nm. A quadratic model was established as a functional relationship between three independent variables and the degradation efficiency of methyl orange (MO). The results of model fitting and statistical analysis demonstrated that the amount of titanium butoxide and F127 played a key role in discoloration for MO. The optimum conditions for maximum MO degradation efficiency (98.6%) were titanium butoxide of 18.64g, F127 of 3.12g and calcination temperature of 501.89°C.

Cold exergy recovery during LNG regasification through a closed Brayton cycle

The liquified natural gas (LNG) regasification process is a source of cold exergy capable of being recovered for the exploitation of energy conversion. This paper presents a novel power plant structure that makes use of the LNG regasification process to increase the energy and exergy efficiency. The power plant consists of a closed Brayton cycle (CBC) fuelled by the flue gas released by the natural gas, where the cold exergy of the LNG is used to cool the compressor intake to reduce the compression work. As alternatives, helium and nitrogen are proposed as working fluids for the Brayton cycle. The research study is centred upon energy and exergy analyses, focused on establishing the operating conditions of maximum efficiency and selecting the working fluid, which is best suited for the regasification process. The results show a relevant increase in the overall efficiency approaching 60.82, an exergy efficiency of 59.08% and a specific power of 1.746 MW/(kg/s LNG).

Comparative studies of augmentation in combined cycle power plants

SUMMARY The attractive features of a combined cycle (CC) power plant are fuel flexibility, operational flexibility, higher efficiency and low emissions. The performance of five gas turbine-steam turbine (GT-ST) combined cycle power plants (four natural gas based plants and one biomass based plant) have been studied and the degree of augmentation has been compared. They are (i) combined cycle with natural gas (CC-NG), (ii) combined cycle with water injection (CC-WI), (iii) combined cycle with steam injection (CC-SI), (iv) combined cycle with supplementary firing (CC-SF) and (v) combined cycle with biomass gasification (CC-BM). The plant performance and CO2 emissions are compared with a change in compressor pressure ratio and gas turbine inlet temperature (GTIT). The optimum pressure ratio for compressor is selected from maximum efficiency condition. The specific power, thermal efficiency and CO2 emissions of augmented power plants are compared with the CC-NG power plant at the individual optimized pressure ratios in place of a common pressure ratio. The results show that the optimum pressure ratio is increased with water injection, steam injection, supplementary firing and biomass gasification. The specific power is increased in all the plants with a loss in thermal efficiency and rise in CO2 emissions compared to CC-NG plant. Copyright © 2013 John Wiley & Sons, Ltd.

Impact of rainfall data resolution in time and space on the urban flooding evaluation

Climate change and modification of the urban environment increase the frequency and the negative effects of flooding, increasing the interest of researchers and practitioners in this topic. Usually, flood frequency analysis in urban areas is indirectly carried out by adopting advanced hydraulic models to simulate long historical rainfall series or design storms. However, their results are affected by a level of uncertainty which has been extensively investigated in recent years. A major source of uncertainty inherent to hydraulic model results is linked to the imperfect knowledge of the rainfall input data both in time and space. Several studies show that hydrological modelling in urban areas requires rainfall data with fine resolution in time and space. The present paper analyses the effect of rainfall knowledge on urban flood modelling results. A mathematical model of urban flooding propagation was applied to a real case study and the maximum efficiency conditions for the model and the uncertainty affecting the results were evaluated by means of generalised likelihood uncertainty estimation (GLUE) analysis. The added value provided by the adoption of finer temporal and spatial resolution of the rainfall was assessed.

Power and Efficiency Analysis of Diesel Cycle Under Alternative Criteria

Model studies of the internal combustion engine cycles are useful for illustrating some important parameters affecting engine performance. The Diesel cycle is considered as a special case of an internal combustion engine. In the diesel cycle, combustion is controlled in order to obtain constant pressure at the beginning of the expansion stroke. It is important to choose the proper optimization criterion for the optimum design of the internal combustion engines. The choice of optimization criterion can be changed depending on the purpose of engine design and working conditions of the internal combustion engine. In this study, a comparative performance analysis is carried out for a reversible air standard Diesel cycle based on three alternative performance criteria, namely, maximum power (mp), maximum power density (mpd) and maximum efficient power (mep). The effects of the design parameters such as volume ratio and extreme temperature ratio of the cycle have been investigated under mp, mpd, mep and maximum efficiency conditions. The results show that the design parameters at mep conditions lead to more efficient engines than that at the mp conditions and that the mep criterion may have a significant power advantage compared to mpd criterion.

Thermochemical conversion of fuels into hydrogen-containing gas using recuperative heat of internal combustion engines

The problem of the thermochemical recuperation of heat from the exhaust gases of internal com� bustion engines (ICEs) as a method of increasing of the efficiency of fuels has been considered. The thermo� dynamic analysis of thermochemical recuperation conditions was performed, and maximum efficiency con� ditions were determined. Catalysts for the steam conversion of oxygencontaining fuels into syngas were developed, and the Co-Mn/Al2O3 catalyst was shown to be the most promising. The model of a thermo� chemical heat recuperation system was developed and manufactured, and its bench tests in the conversion of alcohols were performed using the simulated exhaust gases from a heating device. Mathematical models for calculating units of the heat recuperation system were developed. A recuperation system was manufactured and tested in the ICEfree and ICEintegrated variants. Based on the test results, the equivalent fuel con� sumption characteristics of a recuperative ICE was revealed to decrease by 11-22% depending on its load with a decrease in the concentration of hazardous emissions by 8-12 times for CO, 2-3.5 times for CH, and 18-25 times for NO x .

Effect of Coupling Between Multiple Transmitters or Multiple Receivers on Wireless Power Transfer

The operation of wireless power transfer systems with multiple transmitters (TXs) or receivers (RXs) is investigated. With multiple TXs or RXs in a limited space, couplings occur between TXs or between RXs. The frequency conditions for maximum efficiency and power transfer under such couplings are proposed. Effective resonant frequency of the TXs or the RXs is changed due to such couplings, and driving and/or resonant frequencies should therefore be adjusted accordingly. The amount and type of the required adjustments are provided. The efficiencies in these conditions are discussed. These concepts are supported by experiments with couplings between TXs or between RXs. Using the proposed frequency adjustments, 51-65-W power is transferred with 45%-57% efficiency, even with very low coupling coefficients of 0.025-0.063 from TX to RX. This improvement is significant compared to the unadjusted cases where less than 4 W is transferred with only 5%-33% efficiency.

Optimized simultaneous saccharification and co-fermentation of rice straw for ethanol production by Saccharomyces cerevisiae and Scheffersomyces stipitis co-culture using design of experiments

Herein an ethanol production process from rice straw was optimized. Simultaneous saccharification and co-fermentation (SSCF) using Saccharomyces cerevisiae and Scheffersomyces stipitis co-culture was carried out to enhance ethanol production. The optimal saccharification solid loading was 5%. Key fermentation parameters for co-culture including cell ratio, agitation rate and temperature was rationally optimized using design of experiment (DoE). Optimized co-culture conditions for maximum ethanol production efficiency were at S. cerevisiae:S. stipitis cell ratio of 0.31, agitation rate of 116 rpm and temperature of 33.1°C. The optimized SSCF process reached ethanol titer of 15.2g/L and ethanol yield of 99% of theoretical yield, consistent with the DoE model prediction. Moreover, SSCF process under high biomass concentration resulted in high ethanol concentration of 28.6g/L. This work suggests the efficiency and scalability of the developed SSCF process which could provide an important basis for the economic feasibility of ethanol production from lignocelluloses.