Output power analyses for the thermodynamic cycles of thermal power plants

Thermal power plant is one of the important thermodynamic devices, which is very common in all kinds of power generation systems. In this paper, we use a new concept, entransy loss, as well as exergy destruction, to analyze the single reheating Rankine cycle unit and the single stage steam extraction regenerative Rankine cycle unit in power plants. This is the first time that the concept of entransy loss is applied to the analysis of the power plant Rankine cycles with reheating and steam extraction regeneration. In order to obtain the maximum output power, the operating conditions under variant vapor mass flow rates are optimized numerically, as well as the combustion temperatures and the off-design flow rates of the flue gas. The relationship between the output power and the exergy destruction rate and that between the output power and the entransy loss rate are discussed. It is found that both the minimum exergy destruction rate and the maximum entransy loss rate lead to the maximum output power when the combustion temperature and heat capacity flow rate of the flue gas are prescribed. Unlike the minimum exergy destruction rate, the maximum entransy loss rate is related to the maximum output power when the highest temperature and heat capacity flow rate of the flue gas are not prescribed.

Output power analyses of an endoreversible Carnot heat engine with irreversible heat transfer processes based on generalized heat transfer law

In this paper, an endoreversible Carnot heat engine with irreversible heat transfer processes is analyzed based on generalized heat transfer law. The applicability of the entropy generation minimization, exergy analyses method, and entransy theory to the analyses is discussed. Three numerical cases are presented. It is shown that the results obtained from the entransy theory are different from those from the entropy generation minimization, which is equivalent to the exergy analyses method. For the first case in which the application preconditions of the entropy generation minimization and entransy loss maximization are satisfied, both smaller entropy generation rate and larger entransy loss rate lead to larger output power. For the second and third cases in which the preconditions are not satisfied, the entropy generation minimization does not lead to the maximum output power, while larger entransy loss rate still leads to larger output power in the third case. For the discussed cases, the concept of entransy dissipation is not applicable for the analyses of output power.The problems in the negative comments on the entransy theory are pointed out and discussed. The related researchers are advised to focus on some new specific application cases to show if the entransy theory is the same as some other theories.

Exergy and exergoeconomic analyses for integration of aromatics separation with aromatics upgrading

Methanol to aromatics produces multiple products,resulting in a limited selectivity of xylene.Aromatics upgrading is an effective way to produce more valuable xylene product,and different feed ratios generate discrepant product distributions.This work integrates the aromatics separation with toluene disproportionation,transalkylation of toluene and trimethylbenzene,and isomerization of xylene and trimethylbenzene.Exergy and exergoeconomic analyses are conducted to give insights in the splitting ratios of benzene,toluene and heavy aromatics for aromatics upgrading.First,a detailed simulation model is developed in Aspen HYSYS.Then,300 splitting ratio sets of benzene and toluene for conversion are studied to investigate the process performances.The results indicate that there are different preferences for the splitting ratios of benzene and toluene in terms of exergy and exergoeconomic performances.The process generates lower total exergy destruction when the splitting ratio of toluene varies between 0.07 and 0.18,and that of benzene fluctuates between 0.55 and 0.6.Nevertheless,the process presents lower total product unit cost with the splitting ratio of toluene less than 0.18 and that of benzene fluctuating between 0.44 and 0.89.Besides,it is found that distillation is the biggest contributor to the total exergy destruction,accounting for 94.97%.

Exergy analysis of R1234ze（Z） as high temperature heat pump working fluid with multi-stage compression

In this paper, the simulation approach and exergy analysis of multi-stage compression high tempera- ture heat pump （HTHP） systems with R1234ze（Z） working fluid are conducted. Both the single-stage and multi-stage compression cycles are analyzed to compare the system performance with 120℃ pressurized hot water supply based upon waste heat recovery. The exergy destruction ratios of each component for different stage compression systems are compared. The results show that the exergy loss ratios of the compressor are bigger than that of the evaporator and the condenser for the single-stage compres- sion system. The multi-stage compression system has better energy and exergy etticiencies with the increase of compression stage number. Compared with the single- stage compression system, the coefficient of performance （COP） improvements of the two-stage and three-stage compression system are 9.1% and 14.6%, respectively. When the waste heat source temperature is 60℃, the exergy efficiencies increase about 6.9% and 11.8% for the two-stage and three-stage compression system respec- tively.

Quantification on the Effects of Liquid-Vapor Separation in Air-Conditioning System by using Advanced Exergy Analysis

Air-conditioning system consumes a large amount of electricity in residential sections,and its efficiency has drawn extensive concerns in energy-conscious era.Liquid-vapor separation is a heat transfer enhancement technology that can effectively improve the performance of the heat exchanger as well as the system.In this paper,a regular air-conditioning system as the baseline(system-A)and other two air-conditioning systems with liquid-vapor separation heat exchanger(system-B and system-C)are comparatively studied.The component behaviors and system performances are deeply explored by using advanced exergy analysis with a focus on quantifying how much consequences come from the variants,i.e.liquid-vapor separation.The results indicate that the system-B has large reduced exergy destruction from the compressor and condenser at cooling mode relative to the system-A.The system-C has mainly diminished exergy destruction in the compressor caused by other components relative to the system-B.At heating mode,the system-C has an enhanced system exergy efficiency of 9.6%over the system-A,and it also has the decreased avoidable exergy destruction which is dominantly contributed by the compressor and evaporator.Furthermore,it is found that liquid-vapor separation mainly benefits the compressor and outdoor heat exchanger where it locates,leading to the system performance improvements.

Energy-and exergy-based performance evaluation of solar powered combined cycle(recompression supercritical carbon dioxide cycle/organic Rankine cycle)

Nowadays,the recompression supercritical carbon dioxide(R-SCO_(2))cycle has emerged as a promising option for power conversion systems because of its boundless potential to tackle energy and environmental issues.In this study,we examined the performance of the solar parabolic trough collector(SPTC)integrated combined cogeneration system for the purpose of power generation as well as recovery of waste exhaust heat from the R-SCO_(2) cycle with the help of the organic Rankine cycle(ORC).An exergy and energy analysis was performed for a combined recompression cycle(R-SCO_(2)-ORC)by varying the input variables such as intensity of solar irradiation(Gb),pressure at the inlet of SCO_(2) turbine(P_(5)),mass flow rate of SCO_(2)()&mSCO_(2) inlet temperature of SCO_(2) turbine(T5),inlet temperature of main compressor(T_(9))and effectiveness of the high-and low-temperature recuperator( HTR and LTR).Eight organic working fluids were considered for the ORC:R123,R290,isobutane,R1234yf,R1234ze,toluene,isopentane and cyclohexane.The study revealed that R123-based R-SCO_(2)-ORC demonstrates the highest thermal and exergy efficiency:~73.4 and 40.89%at G_(b)=0.5 kW/m^(2);78.8 and 43.9%at P_(5)=14 MPa;63.86 and 35.57%at T5=650 K;74.84 and 41.69%at&mSCO 7kg s;2=/85.83 and 47.82%at T_(9)=300 K;84.57 and 47.11%at HTR 65;=0.85.06 and 47.38%at LTR 65,=0.respectively.Alternatively,R290 showed the minimum value of exergy and thermal efficiency.As can be seen,the maximum amount of exergy destruction or exergy loss occurs in a solar collector field,~58.25%of the total exergy destruction rate(i.e.6703 kW)and 18.99%of the solar inlet exergy(i.e.20562 kJ).Moreover,R123 has the highest net work output,~4594 kJ at T5=650 K and 6176 kJ at T_(9)=300 K.

A comprehensive approach to understanding irreversibility in a turbojet

Regarding interest in and concerns about high efficiency in recent times,an irreversibility assessment of energy conversion systems is significant.Turbojets,a type of energy conversion system,are widely used to provide thrust for aerial vehicles,such as military aircraft missiles,commercial aircraft and so on.From this point of view,the current study aims to introduce a comprehensive irreversibility assessment methodology exemplification for a turbojet.First of all,a basic irreversibility assessment methodology is explained with an application.Following this,a comprehensive assessment is performed.Within this framework,a number of a novel measures are defined by derivations in addition to previously well-known indicators.These measures are beneficial for the decomposition of the irreversibility in a turbojet and its components.At the end of the study,the highest endogenous irreversibility is determined to be in the turbine component of the turbojet engine whereas the highest avoidable irreversibility is found to be in the compressor component of the turbojet engine.The current paper is considered to be of use for researchers and scientists interested in aero-engine performance,thermal engineering and aerospace engineering.

Performance Analysis and Multi-Objective Optimization of Two Organic Rankine Cycles with Different Fluids for Low Grade Waste Heat Recovery

The basic organic Rankine cycle(BORC)and ORC with an internal heat exchanger(IHORC)are studied with different working fluids under a given heat source condition to analyse the thermodynamic performances and net power output.The results demonstrate that the external exergy efficiency of IHORC is lower than that of BORC while the internal exergy efficiency is on the opposite with the same overall exergy efficiencies.A multi-objective optimization model with inlet pressure and temperature of expander as independent parameters and exergy and heat recovery efficiencies as objective functions is solved by NSGA-II(the second non-dominated sorting genetic algorithm).The Pareto optimal solutions are obtained by the optimization models.By calculation with the optimum conditions,it is determined that R236ea has the best comprehensive performance with exergy efficiencies being 40.69%and 41.38%,and heat recovery efficiencies being 83.2%and 75.6%in IHORC and BORC,respectively.The evaporators occupy the maximum exergy destruction,which can be reduced by decreasing pinch point temperatures and increasing evaporation pressures.

Performance analysis of combined cycle power plant

Combined cycle power plants （CCPPs） are in operation with diverse thermodynamic cycle configura- tions. 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.

Defining ecologic thermo-environmental index for aero-engines as a novel performance criterion

Aviation fleet globally grows in consequence of rise in ridership,number,and hour of flight.That makes the aviation sector one of the major contributors to the global warming threat.For this reason,environmental and ecologic aspects should be considered at least as much as performance at design step of an aircraft engine,namely aero-engine.To evaluate thermodynamics,environmental and ecologic aspects of an aero-engine a holistic approach is lack of the existing literature.The current paper presents the ecologic thermo-environmental index,as a novel measure,for evaluation of an aero-engine performance from a joint perspective of thermodynamics,ecological and environmental aspects.For better understanding of the dependence of ecologic thermo-environmental index on design parameters of the aero-engine a parametric study is also included.The rise in pressure ratio has an increasing impact on the ecologic thermo-environmental index whereas increase in the turbine inlet temperature also leads an increasing impact on the ecologic thermo-environmental index.On the other hand,the ecologic thermo-environmental index is found to be inversely proportional to the exergy efficiency.At the end of the study,the exergy efficiency of the turbojet engine is calculated to be varying from 44.46%to 57.12%.Additionally,the value of the ecologic thermo-environmental index of the turbojet engine ranges between 0.02 and 0.15.The author considers the present study to be beneficial to those interested in aerospace,mechanical,and environmental engineering.