煤的燃料(火用)分析

作为重要的一次能源,对煤的燃料(火用)进行分析以确定其最大做功能力,具有重要的理论和现实意义.通过对有关专家研究的总结和分析,认为Rant对燃料(火用)的估算方法既具有科学性,又与实际情况相符.根据电与(火用)的等效性,用发电能力来反映燃料的做功能力,使得基于第二定律的统计更加有效,并能准确揭示其潜力的大小.     

含相变过程的机械[火用]计算方法

机械(火用)作为广泛使用的稳流物系的焓(火用)的两个组成部分之一,其计算正确与否直接影响到(火用)分析结果的正确性.通过分析发现,传统的机械(火用)计算方法没有充分考虑机械(火用)计算路径下可能存在的相变及其影响.为此,通过对定温变压过程下物态变化及其做功能力的理论分析,提出了适用于含相变过程的完整的机械(火用)计算方法.应用该方法及传统方法对水的焓(火用)进行比较计算.结果表明,对存在相变过程的机械(火用)的计算,传统计算方法的误差较大,而提出的计算方法准确可靠,具有较强的实用价值.     

基于等效(火用)降品质系数的火力发电机组热经济学分析

提出等效(火用)降品质系数θe的概念,根据θe的大小判断燃料(火用)流在各热力设备处的热量(火用)变功能力,进而确定其(火用)单价,从(火用)变功的角度合理构造了热经济学补充方程;利用该系数对某1 000MW机组热力系统进行了(火用)流的(火用)单价计算,并将计算结果与Valero方法、能质系数法进行了分析比较.该系数不仅体现了(火用)流的品质和价格,而且考虑到设备处的热量(火用)变功能力对(火用)成本的影响,使(火用)成本计算更科学合理.     

纯低温余热发电系统的(火用)分析及其主蒸汽参数优化问题

采用.用分析的方法,分析计算了纯低温余热发电系统入口余热的粗(火用)流分布,指出减少出口余热(火用)损和内部换热损是提高余热锅炉(火用)回收的关键,并在此基础上分析了主蒸汽压力参数与余热锅炉最大回收炯及发电系统最大做功的关系,分析论证其进行优化的原因和必要性,为进一步研究提高纯低温余热发电系统的炯回收率提供了参考.     

能源结构调整的理论基础研究

针对能源结构调整优化的战略需要,基于单耗分析理论,从热力学第二定律的角度分析了不同品质燃料对能源利用第二定律效率的影响,并对几种典型能源品种的燃料(火用)进行了分析和评价.结果表明:燃料品质越高,其电量化的燃料(火用)越高,用于发电消耗的燃料就越少.提出了基于热力学第二定律的折算标准煤系数,为能源结构调整和优化配置提供了必要的理论基础.     

超超临界机组加热器端差对煤耗率的影响

准确而快速地评价加热器端差引起的机组热经济性变化,对热力系统的设计、运行和检修具有十分重要的意义.以热力系统热平衡方程、锅炉输入燃料(火用)方程、锅炉吸热量方程、汽轮机输入热(火用)量(火用)方程和汽轮机功率方程为基础,根据小扰动理论和微分理论,得各抽汽量、锅炉输入燃料炯、锅炉吸热量、汽轮机输入热量(火用)和汽轮机功率对端差的变化率.在锅炉(火用)效率、机组炯效率和发电煤耗率概念的基础上,由微分理论推导端差对(火用)经济性指标的影响.以N 1000 - 25/600/600机组为例,计算加热器端差对机组(火用)经济性指标的影响.     

基于流图理论的200MW机组热力系统[火用]损模型

采用流图理论对200MW机组进行热经济性分析,建立了有关加热器(火用)损与燃料(火用)值的数学模型.该模型能简单,直观的体现热力系统(火用)损与各种输入(火用)值之间的联系,利用这种联系,可以对热力系统进行微弱扰动情况下的热经济性分析.     

基于能级概念的火用经济学计价策略

正确合理地对火用流计价,确定从燃料到产品(火用)流的转换过程中费用形成及变化过程,是实现能量系统的(火用)经济学分析和优化的关键之一.将能级的概念引入热经济学计价体系,把供入能流按能级拆分,依据能级相近最大化相供的原则,解决能量取出与供入间的对应问题,提出了基于能级相近最大化相供的(火用)流计价策略,以减少求解过程中所涉及的未知变量数,说明产品(火用)流存在时的附加方程引入情况.最后以典型的热电联产系统(CGAM系统)的(火用)经济分析优化中(火用)流计价为例,介绍了该计价方法的应用.     

基于熵分析法的回热系统通用损分布矩阵

熵分析法的核心就是求取过程的熵产,而熵产的大小体现着过程不可逆程度.在回热系统中,节流、有限温差传热以及摩擦与扰动等过程都是不可逆过程,它们必然会导致做功能力损失即(火用)损,而这损失可以利用Gouy-Stodola关系式由这些过程所造成的熵产转化获得.针对回热系统的(火用)损分布进行讨论,在划分单一回热加热器为独立系统后,对系统采用黑箱分析,对方程进行水侧定流量法处理,通过Gouy-Stodola关系式转化最终获得通用炯损分布矩阵方程.     

中低热值燃料对燃气轮机运行特性的影响分析

中低热值燃料燃气轮机对经济发展和节能减排具有重要的意义,对其燃烧特性和运行工况的研究是本项技术的关键问题。本文根据燃气轮机压气机特性曲线、涡轮特性曲线和热力循环过程,分析计算中低热值燃料燃气的热力性质,提出了适用于不同组分的中低热值燃料的膨胀做功计算方法。通过对燃用天然气和合成气进行模拟计算,得到燃料热值对燃料系数、涡轮出口温度、燃料流量及膨胀功的影响规律,为燃气轮机的设计和工况调整提供理论依据。     

A review: Exergy analysis of PEM and PEM fuel cell based CHP systems

In recent years, growing attention has been given to new alternative energy sources and exergy analysis since fossil fuels cause emissions that have some negative impacts on earth such as global warming, greenhouse effect etc. New power generation systems have been developed in order to reduce or eliminate these impacts as possible. So that, new alternative energy systems have been taken place instead of fossil fuel based systems with nearly zero emission levels. One of them is solid polymer electrolyte or proton exchange membrane (PEM) fuel cell. Although it has significant advantages, there are some disadvantages such as cost, and hydrogen is not a fuel that can be easily obtained. For these reasons, efficiency of a PEM fuel cell has a great significance. Energy efficiency of a system is the most important parameter for utilization. But, energy analysis does not always show the capacity to do work potential of energy of a system. Exergy analysis must be investigated for a system in order to see available work of the system. Because of disadvantages of the PEM fuel cell, exergy analysis has quite importance. In this paper PEM fuel cell and exergy analysis of PEM fuel cell are combined and investigated. A detailed review of the past and recent research activities has been documented. The review focuses on exergy analysis of both PEM fuel cells and PEM based combined heat and power (CHP) systems at different operating parameters. It is concluded that there are a lot of parameters which effects the exergy efficiencies of systems.     

Fuel cell energy generation and recovery cycle analysis for residential application

Today’s concern regarding limited fossil fuel resources and their contribution to environmental pollution have changed the general trend to utilization of high efficiency power generation facilities like fuel cells. According to annual reducing capital cost of these utilities, their entrance to commercial level is completely expected. Hot exhaust gases of Solid Oxide Fuel Cells (SOFC) are potentially applicable in heat recovery systems. In the present research, a SOFC with the capacity of 215 kW has been combined with a recovery cycle for the sake of simultaneous of electric power, cooling load and domestic hot water demand of a hotel with 4600 m² area. This case study has been evaluated by energy and exergy analysis regarding exergy loss and second law efficiency in each component. The effect of fuel and air flow rate and also current density as controlling parameters of fuel cell performance have been studied and visual software for energy-exergy analysis and parametric study has been developed. At the end, an economic study of simultaneous energy generation and recovery cycle in comparison with common residential power and energy systems has been done. General results show that based on fuel lower heating value, the maximum efficiency of 83 percent for simultaneous energy generation and heat recovery cycle can be achieved. This efficiency is related to typical climate condition of July in the afternoon, while all the electrical energy, cooling load and 40 percent of hot water demand could be provided by this cycle. About 49 percent of input exergy can be efficiently recovered for energy requirements of building. Generator in absorption chiller and SOFC are the most destructive components of exergy in this system.     

Influences of hydrogen and various gas fuel addition to different liquid fuels on the performance characteristics of a spark ignition engine

This study reports the impacts of dual fuel mixtures on the theoretical performance characteristics of a spark ignition engine (SIE). The effects of addition of liquefied hydrogen, methane, butane, propane (additive fuels) into gasoline, iso-octane, benzene, toluene, hexane, ethanol and methanol fuels (primary fuels) on the variation of power, indicated mean effective pressure (IMEP), thermal efficiency, exergy efficiency, were examined by using a combustion model. The fuel additives were ranged from 10 to 50% by mass. The results exhibited that the ratios of hydrogen, methane, butane, propane noticeably affect the performance of the engine. The maximum increase ratio of power is 82.59% with 50% of toluene ratio and its maximum decrease ratio is 10.84% with 50% of methanol ratio in hydrogen mixtures. The maximum increase ratio of thermal efficiency and exergy efficiency are observed as 26.75% and 32.23% with the combustion of benzene-hydrogen mixtures. The maximum decrease ratio of thermal efficiency is 29.71% with the combustion of 50% of methanol ratio and it is 21.95% for the exergy efficieny with the combustion of 50% of ethanol ratio in hydrogen mixtures. The power, IMEP, thermal efficiency and exergy efficiency of primary fuels demonstrate different variation characteristics with respect to type and ratio of additive fuels.     

Exergy analysis of a fuel cell power system for transportation applications

An exergy analysis of a solid polymer fuel cell power system for transportation applications is reported. The analysis was completed by implementing the fundamental governing second law equations derived for the system into a fuel cell performance model developed previously. The model analyzes all components of the system including the fuel cell stack and the air compression, hydrogen supply, and cooling subsystems. From the analysis, it was determined that the largest destruction of exergy within the system occurs inside the fuel cell stack. Other important sources of exergy destruction include irreversibilities within the hydrogen ejector and the air compressor, and the exergy associated with the heat rejected from the radiator. The results may aid efforts to optimize fuel cell systems.     

Dynamic exergy analysis for capacity expansion of regional power-generation systems: Case study of far West Texas

The global electricity supply from non-hydro renewables, mainly wind and solar, is currently growing at a high rate, and it is expected that this trend persists. In the near-to-medium term, the power produced by breakthrough fossil fuel technologies might also grow intensively. These expansion patterns can be optimized in a regional context, which translates into a multidimensional problem. As part of the solution, a procedure to determine maximum allowable growth rates for alternative power-generation technologies is developed and exemplified in this paper. The model applies a dynamic exergy analysis based on the cumulative exergy-consumption concept, expanded to include emissions abatement. A Gompertz sigmoid growth is assumed and constrained by both exergetic self-sustenance and regional energy resource availability. Far West Texas is the selected study region. The deployment of alternative technologies (wind turbines, photovoltaics, hybrid solar thermal parabolic troughs, and solid oxide fuel cells) to meet the regional power demand is projected assuming backup capacity by a conventional technology (natural gas combined cycle). The results show that during the next decades the new capacity demand may largely be met by deploying alternative technologies, with a cost in primary resources that can be minimized through a proper allowance for exergy reinvestment.     

Thermodynamic model for exergetic performance of a tubular SOFC module

A tubular solid oxide fuel cell (TSOFC) module fed by methane is modelled and analyzed thermodynamically from the exergy point of view in this paper. The model of TSOFC module consists of mixer, pre-reformer, internal reforming fuel cell group, afterburner and internal pre-heater components. The model of the components forming module is given based on mass, energy and exergy balance equations. The developed thermodynamic model is simulated, and the obtained performance characteristics are compared and validated with the experimental data taken from the literature concerning TSOFC module. For exergetic performance analysis, the effects of operating variables such as current density, pressure, and fuel utilization factor on exergetic performances (module exergy efficiency, module exergetic performance coefficient, module exergy output and total exergy destruction rate, and components' exergy efficiencies, exergy destruction rates) are investigated. From the analysis, it is determined that the biggest exergy loss stems from exhaust gasses. Other important sources of exergy destruction involve fuel cell group and afterburner. Consequently, the developed thermodynamic model is expected to provide not only a convenient tool to determine the module exergetic performances and component irreversibility but also an appropriate basis to design complex hybrid power generation plants.     

Thermodynamic modeling of an integrated biomass gasification and solid oxide fuel cell system

An integrated process of biomass gasification and solid oxide fuel cells (SOFC) is investigated using energy and exergy analyses. The performance of the system is assessed by calculating several parameters such as electrical efficiency, combined heat and power efficiency, power to heat ratio, exergy destruction ratio, and exergy efficiency. A performance comparison of power systems for different gasification agents is given by thermodynamic analysis. Exergy analysis is applied to investigate exergy destruction in components in the power systems. When using oxygen-enriched air as gasification agent, the gasifier reactor causes the greatest exergy destruction. About 29% of the chemical energy of the biomass is converted into net electric power, while about 17% of it is used to for producing hot water for district heating purposes. The total exergy efficiency of combined heat and power is 29%. For the case in which steam as the gasification agent, the highest exergy destruction lies in the air preheater due to the great temperature difference between the hot and cold side. The net electrical efficiency is about 40%. The exergy combined heat and power efficiency is above 36%, which is higher than that when air or oxygen-enriched air as gasification agent.     

Energy,exergy and exergo-environmental impact assessment of a solid oxide fuel cell coupled with absorption chiller & cascaded closed loop ORC for multi-generation

A novel solid oxide fuel cell (SOFC) multigeneration system fueled by biogas derived from agricultural waste (maize silage) is designed and analyzed from the view point of energy and exergy analysis. The system is proposed in order to limit the greenhouse gas emissions as it uses a renewable energy source as a fuel. Electricity, domestic hot water, hydrogen and cooling load are produced simultaneously by the system. The system includes a solid oxide fuel cell; which is the primary mover, a biogas digester subsystem, a cascaded closed loop organic Rankine cycle, a single effect LiBr-water absorption refrigeration cycle, and a proton exchange membrane electrolyzer subsystem. The proposed cascaded closed-loop ORC cycle is considered as one of the advanced heat recovery technologies that significantly improve thermal efficiency of integrated systems. The thermal performance of the proposed system is observed to be higher in comparison to the simple ORC and the recuperated ORC cycles. The integration of a splitter to govern the flue gas separation ratio is also introduced in this study to cater for particular needs/demands. The separation ratio can be used to vary the cooling load or the additional power supplied by the ORC to the system. It is deduced that net electrical power, cooling load, heating capacity of the domestic hot water and total energy and exergy efficiency are 789.7 kW, 317.3 kW, 65.75 kW, 69.86% and 47.4% respectively under integral design conditions. Using a parametric approach, the effects of main parameters on the output of the device are analyzed. Current density is an important parameter for system performance. Increasing the current density leads to increased power produced by the system, decreased exergy efficiency in the system and increased energy efficiency. After-burner, air and fuel heat exchangers are observed to have the highest exergy destruction rates. Lower current density values are desirable for better exergy-based sustainability from the exergetic environmental impact assessment. Higher current density values have negative effect on the environment.     

Thermodynamic modeling and exergy analysis of proton exchange membrane fuel cell power system

The proton exchange membrane (PEM) fuel cell (PEMFC) is equipped with a series of auxiliary components which consume considerable amount of energy. It is necessary to investigate the design and operation of the PEMFC power system for better system performance. In this study, a typical PEMFC power system is developed, and a thermodynamic model of the system is established. Simulation is carried out, and the power distribution of each auxiliary component in the system, the net power and power efficiency of the system are obtained. This power system uses cooling water for preheating inlet gases, and its energy-saving effect is also verified by the simulation. On this basis, the exergy analysis is applied on the system, and the indexes of the system exergy loss, exergy efficiency and ecological function are proposed to evaluate the system performance. The results show that fuel cell stack and heat exchanger are the two components that cause the most exergy loss. Furthermore, the system performance under various stack inlet temperatures and current densities is also analyzed. It is found that the net power, energy efficiency and exergy efficiency of the system reach the maximum when the stack inlet temperature is about 348.15 K. The ecological function is maintained at a high level when the stack inlet temperature is around 338.15 K. Lower current density increases the system ecological function and the power and exergy efficiencies, and also helps decrease the system exergy loss, but it decreases the system net power.     

从锅炉_效率分析看提高小型热电厂参数的意义

根据热量的概念推出锅炉效率公式,运用小偏差法原理导出效率与热效率及水蒸气平均吸热温度之间的相对变化率公式,分析了小型热电厂最高参数为次高温次高压的情况,得出提高电厂锅炉参数较提高热效率对增大燃料利用率更为重要的结论。