Performance comparison of an irreversible closed Brayton cycle under maximum power density and maximum power conditions

In this paper, the power density, defined as the ratio of power output to the maximum specific volume in the cycle, is taken as objective for performance analysis of an irreversible closed Brayton cycle coupled to constant-temperature heat reservoirs in the viewpoint of finite time thermodynamics (FTT) or entropy generation minimization (EGM). The analytical formulas about the relations between power density and pressure ratio are derived with the heat resistance losses in the hot- and cold-side heat exchangers and the irreversible compression and expansion losses in the compressor and turbine. The obtained results are compared with those results obtained by using the maximum power criterion. The influences of some design parameters on the maximum power density are provided by numerical examples, and the advantages and disadvantages of maximum power density design are analyzed. The power plant design with maximum power density leads to a higher efficiency and smaller size. However, the maximum power density design requires a higher pressure ratio than maximum power design. When the heat transfer is carried out ideally, the results of this paper become those obtained in recent literature.     

变温热源内可逆中冷回热布雷顿循环功率密度分析

用有限时间热力学方法分析内可逆变温热源中冷回热布雷顿循环,导出了无因次功率密度的解析式,由数值计算给出了燃气轮机功率密度特性,分析了循环中各热力参数对功率密度的影响,并对最大功率工况与最大功率密度工况下的主要参数进行了比较,得出了最大功率密度设计的优点和不足。     

恒温热源条件下内不可逆布雷顿循环的功率密度特性

用有限时间热力学方法分析恒温热源条件下不可逆布雷顿循环的功率密度特性,计入工质与高、低温侧换热器的热阻损失及压气机、透平的不可逆压缩和膨胀损失,导出了功率密度与压比间的解析式,并通过数值计算将对应于最大功率密度时的一些参数与对应于最大功率时的同样参数进行了比较,说明了功率密度设计的优点与不足.     

Performance of an Atkinson cycle with heat transfer,friction and variable specific-heats of the working fluid

The performance of an air standard Atkinson cycle with heat-transfer loss, friction-like term loss and variable specific-heats of the working fluid is analyzed using finite-time thermodynamics. The relations between the power output and the compression ratio, between the thermal efficiency and the compression ratio, as well as the optimal relation between the power output and the efficiency of the cycle are derived by detailed numerical examples. Moreover, the effects of variable specific-heats of the working fluid and the friction-like term loss on the irreversible cycle performance are analyzed. The results show that the effects of variable specific-heats of working fluid and friction-like term loss on the irreversible cycle performance should be considered in cycle analysis. The results obtained in this paper provide guidance for the design of Atkinson engines.     

燃气轮机中冷循环功率和功率密度优化比较

计入高低温侧换热器和中冷器的热阻损失、压气机和涡轮机中的不可逆压缩和膨胀损失及管路中压力损失,用有限时间热力学方法导出了变温热源条件下不可逆闭式燃气轮机中冷循环功率和功率密度（功率与循环中最大比容之比）的解析式;分别以功率和功率密度为目标,优化了中间压比、高低温侧换热器及中冷器热导率分配,并对结果进行了比较.     

变温热源条件下内可逆闭式中冷回热布雷顿循环的功率优化

计入工质与高低浊侧换热器、回热器和中冷器的热阻损失以功率为优化目标，借助数值计算，研究了变温热源条件下内可逆闭式中冷回热布雷顿循环输出功率最大时，高低温侧换热器、回热器和中冷器的热导率分配以及中间压比与总压比的关系；分析了工质与热源间的热容率匹配对双重最大功率的影响。     

Performance analysis of an Organic Rankine Cycle with superheating under different heat source temperature conditions

This paper presents an analysis of non-regenerative Organic Rankine Cycle (ORC), based on the parametric optimization, using R-12, R-123, R-134a and R-717 as working fluids superheated at constant pressure. A computer programme has been developed to parametrically optimize and compare the system and second law efficiency, irreversibility of the system, availability ratio, work output, mass flow rate with increase in turbine inlet temperature (TIT) under different heat source temperature conditions. The calculated results reveal that R-123 produces the maximum efficiencies and turbine work output with minimum irreversibility for employed constant as well as variable heat source temperature conditions. Hence, selection of a non-regenerative ORC during superheating using R-123 as working fluid appears to be a choice system for converting low-grade heat to power.     

工质线性变比热内可逆矩形循环功率密度和有效功率特性

应用有限时间热力学理论基于已有文献建立的模型,以功率密度和有效功率为优化目标,分析工质比热随温度线性变化条件下,存在传热损失的内可逆矩形循环性能特性,得到循环无因次功率密度Pˉd、无因次有效功率Wˉep与循环膨胀比γ和效率η之间的关系.结果表明,循环Pˉd-γ和Wˉep-γ关系曲线均为类抛物线型,Pˉd-η和Wˉep-...     

Performance analysis of a two-stage irreversible heat pump under maximum heating load per unit total cost conditions

A performance optimization of a two-stage irreversible combined heat-pump system has been carried out. The irreversibility of heat transfer across finite temperature differences, the heat-leak loss between the external heat reservoirs and the internal dissipation of the working fluids are considered. The heating load per unit total cost is taken as an objective function for the optimization. The maximum of the objective function and the corresponding optimal performance and design parameters have been derived analytically. The global and the optimal performance characteristics curves are presented in terms of technical and economical parameters. The irreversibility effects and economical aspects on the general and optimal performances have been investigated.     

Influence of heat loss on the performance of an air-standard Atkinson cycle

This study is aimed at investigating the effects of heat loss, as characterized by a percentage of fuel’s energy, friction and variable specific heats of the working fluid, on the performance of an air-standard Atkinson cycle under the restriction of the maximum cycle-temperature. A more realistic and precise relationship between the fuel’s chemical-energy and the heat leakage is derived through the resulting temperature. The variations in power output and thermal efficiency with compression ratio, and the relations between the power output and the thermal efficiency of the cycle are presented. The results show that the power output as well as the efficiency, for which the maximum power-output occurs, will rise with the increase of maximum cycle-temperature. The temperature-dependent specific heats of the working fluid have a significant influence on the performance. The power output and the working range of the cycle increase while the efficiency decreases with the rise of specific heats of working fluid. The friction loss has a negative effect on the performance. Therefore, the power output and efficiency of the Atkinson cycle decrease with increasing friction loss. It is noteworthy that the results obtained in the present study are of significance for providing guidance with respect to the performance evaluation and improvement of practical Atkinson-cycle engines.     

Performance analysis of an irreversible cogeneration heat pump cycle

An irreversible heat engine-driven vapour compression and absorption heat pump system is considered as a cogeneration cycle. The effects of thermal resistances and internal irreversibilities on the coefficient of performance (COP) of this cogeneration cycle were investigated using finite-time thermodynamic approach. An improved equation for the COP of the system under consideration was obtained. The results obtained here may serve as a good guide for the evaluation of existing real cogeneration heat pumps or provide some theoretical bases for the optimal design of future cogeneration heat pumps. © 1998 John Wiley & Sons, Ltd.     

Analysis of the stirling heat engine at maximum power conditions

The Stirling heat engine operating in a closed regenerative thermodynamic cycle is analyzed. Polytropic processes are used for the power and displacement pistons. Following regeneration, the maximum power density and efficiency are found and the compression ratio at maximum power density is determined.     

Performance analysis and parametric optimum criteria of an irreversible Atkinson heat-engine

Multi-irreversibilities, mainly resulting from the adiabatic processes, finite-time processes and heat loss through the cylinder wall, are considered in the cycle model of an Atkinson heat engine. The power output and efficiency of the cycle are derived by introducing the pressure ratio and the compression and expansion efficiencies. The performance characteristic curves of the cycle are presented. The bounds of the power output and efficiency are determined. The optimum criteria of some important parameters, such as the power output, efficiency and pressure ratio are given. The influences of the various design parameters on the performance of the cycle are analyzed in detail. The results obtained may provide a theoretical basis for both the optimal design and operation of real Atkinson heat engines.     

Low temperature heat source for power generation: Exhaustive analysis of a carbon dioxide transcritical power cycle

The main results of a theoretical work on the use of a low temperature heat source for power generation through a carbon dioxide transcritical power cycle are reported in this paper. The procedure for analyzing the behaviour of the proposed cycle consisted in modifying the input pressure to the turbine from 66 bar, maintained constant each evaluated temperature (60 °C, 90 °C, 120 °C and 150 °C) until the net work was approximately zero. As a result, the maximum exergy efficiency was 50%, while the energy efficiencies obtained were 9.8%, 7.3%, 4.9% and 2.4% and the net specific work was 18.2 kJ/kg, 12.8 kJ/kg, 7.8 kJ/kg and 3.5 kJ/kg, respectively. Furthermore, the effect of the addition of an internal heat exchanger, which obviously supposed an increase in the efficiency, was analyzed. The analysis of the proposed system shows the viability of implementing this type of process as an energy alternative and/or strengthener of non-conventional energy sources in non-provided zones, or for increasing the energy efficiency in the industry.     

Performance improvement of PEM fuel cell power system using fuzzy logic controller-based MPPT technique to extract the maximum power under various conditions

Power generation of a fuel cell (FC) is mostly dependent upon operational variables such as cell temperature and membrane water content. There is an individual maximum power point (MPP) on the P-I curve of the FC. The location of the MPP varies with respect to the MPP position. Thus, an MPP tracking (MPPT) system should exist to guarantee that the FC works at the MPP in order to maximize the functionality. Due to their straightforward structure, prevalent MPPT methods had strong functionality. However, their primary limitations include fluctuations around the MPP and inefficiency under abrupt variations of operating conditions. The primary objective of this paper is to maintain the PEMFCs operation at an efficient power point. To this purpose, the efficiency of PEM-FC is tested and enhanced using a variety of MPPT-based smart controller techniques. To determine the appropriate MPPT controller parameters, the modified fluid search optimization (MFSO) approach and fuzzy logic controller (FLC) are employed. Furthermore, the MFSO method is deployed to adjust the membership functions (MFs) of the FLC. The MFSO is an excellent approach for coping with the stochastic behavior of the PEM-FC system when the temperature and water content of the membrane change. In terms of improved dynamic behavior, better convergence rate, reduced oscillations, and better tracking of the MPP, the results obtained by employing the suggested strategy demonstrate the superior functionality of the system compared to case using other methods. Moreover, the power generated by the PEMFC system is less than the nominal capacity for the temperature's rated capacity. Therefore, the deficit in power would be covered by transacting power with the grid.     

Performance analysis of combined cycle power plant

Combined cycle power plants (CCPPs) are in operation with diverse thermodynamic cycle configurations. Assortment of thermodynamic cycle for scrupulous locality is dependent on the type of fuel available and different utilities obtained from the plant. In the present paper, seven of the practically applicable configurations of CCPP are taken into consideration. Exergetic and energetic analysis of each component of the seven configurations is conducted with the help of computer programming tool, i.e., engineering equation solver (EES) at different pressure ratios. For Case 7, the effects of pressure ratio, turbine inlet temperature and ambient relative humidity on the first and second law is studied. The thermodynamics analysis indicates that the exergy destruction in various components of the combined cycle is significantly affected by the overall pressure ratio, turbine inlet temperature and pressure loss in air filter and less affected by the ambient relative humidity.     

水源CO2热泵的全工况性能与匹配性分析

通过对60kW水源CO_2热泵的实验测试,在获得的911组实验数据的基础上,建立了用于反映热泵系统性能和循环参数随运行工况变化趋势的BP神经网络拟合模型,综合分析了系统在供水温度为55~100℃、回水温度为10~50℃、热源温度为5~50℃和电子膨胀阀开度为50~400步的全工况范围内的性能,从而为该类型热泵系统性能的预测和系统设计提供了数据参考。在对四种工况下,即15/55/15℃、15/90/15℃、15/90/30℃和30/90/30℃时系统运行参数的比较分析的基础上,定性评价了系统在全工况范围内的匹配性。     

Performance effects of combined cycle power plant with variable condenser pressure and loading

Utility power producers are being driven by the inherently high efficiency and attractive installed cost of Combined Cycle Power Plants (CCPP) to consider them as the dominant choice for least-cost power. However, in a CCPP the power capability is significantly affected by the ambient temperature [Sue DC, Chuang CC, Lin PH. Performance improvement for gas turbine combined cycle power plants (GTCCPP) in Taiwan. In: Johnson D, editor. Electric power. Fourth annual conference and exhibition, vol. 4A; 2004. St Louis, MI, USA: American Center. p. 1–15. [1]], condenser pressure and power demand. In the Power Purchase Agreements (PPA) written in Taiwan, the capacity payment is based on the guaranteed power output during the operating period. Therefore, operating the CCPP at the highest performance and maintaining the guaranteed or contractual power output are important factors to get the full capacity and energy payments from the power purchaser and reduce the fuel and operating costs for the power producer.     

Finite time thermodynamic evaluation of endoreversible Stirling heat engine at maximum power conditions

Maximum power and efficiency at the maximum power point of an endoreversible Stirling heat engine with finite heat capacitance rate of external fluids in the heat source/sink reservoirs with regenerative losses are treated. It was found that the thermal efficiency depends on the regenerator effectiveness and the internal irreversibility resulting from the working fluid for a given value of reservoir temperature. It was also concluded that it is desirable to have larger heat capacity of the heat sink in comparison to the heat source reservoir for higher maximum power output and lower heat input.     

恒温热源内可逆Lenoir循环功率效率特性分析与优化

基于有限时间热力学理论,对恒温热源内可逆Lenoir循环进行功率效率特性分析与优化,得到换热器总热导给定的条件下循环的最大功率和最大效率。结果表明:在给定高、低温侧换热器热导的条件下,循环的功率、效率特性呈现"点"的特征;在高、低温侧换热器热导可优化的条件下,存在最佳的热导分配,使得循环的功率或效率取得最大值。高、低温热源温比增大或换热器总热导增大时,循环的功率、效率都将增大。