Internal Irreversibility Parameter Research Articles

Thermodynamic Investigation of an Irreversible Combined Stirling-Organic Rankine Cycle for Maximum Power Output Condition

Abstract A pragmatic approach is adopted to investigate irreversible thermodynamic combined cycle devices. The finite-time thermodynamic model of combined Stirling-organic Rankine cycle is formulated and evaluated for maximum output power and thermal efficiency. The influence of effectiveness of heat exchangers, heat capacitance of external fluids, and inlet temperatures of heat exchangers at heat source, heat recovery unit and heat sink on the performance of Stirling-organic Rankine cycle are investigated to get their corresponding optimum. The maximum allowable heat capacitance of external fluids of heat source and heat recovery units are about 1.1 kW/K and 1.4 kW/K, respectively, for the operating conditions considered in the present study. The maximum power output is achieved only when the effectiveness of heat exchangers is ideal. The overall performance of Stirling-organic Rankine cycle combination will be higher than either of the performances of individual cycles provided that the isothermal heat rejection from Stirling cycle takes place at temperature above 540 K. Further, a 0.2 increase in the internal irreversibility parameter from an ideal/reversible condition reduced the maximum output power and the corresponding thermal efficiency of Stirling-organic Rankine cycle by 16.1 kW and 24%, respectively.

Multi-objective optimization and analysis of performance of a four-temperature-level multi-irreversible absorption heat pump

Performance analysis and multi-objective optimization based on finite-time thermodynamics are conducted in this paper for an absorption heat pump operating between four temperature levels. The exergy-based ecological (E), the exergetic performance (EPC) and the thermo-economic (F) criteria were considered as the objective functions. The maxima of the three objective functions and the corresponding optimal conditions were obtained by applying the flowchart optimization methodology. The influences of some design factors such as the economic indicators, internal irreversibility parameters, heat-transfer coefficients of heat exchangers and the distribution ratio rate of the total heat-reject quantity between the absorber and the condenser on the maxima of the exergy-based ecological (E), the exergetic performance (EPC) and the thermo-economic (F) objective functions and the corresponding optimal parameters have been analyzed and discussed. The results show that: (i) Materials with a low heat-transfer coefficient significantly affect the performance of the system. (ii) The detrimental effects of internal irreversibilities depend on the order of magnitude of the optimal heat-transfer rates in the absorber and the condenser. Finally, (iii) the optimum performance has been obtained for certain values of the heat exchange surface area and the exergy input rate from an economic point of view.

Local stability analysis of an irreversible absorption refrigerator powered by a wood boiler

• We analyse the local stability of an irreversible absorption refrigerator powered by a waste wood boiler. • The coefficient of performance COP is taken as objective functions. • We derive the expressions of the relaxation times at the optimal steady-state of the maximum coefficient of performance . • We discuss the effects of some performance parameters on relaxation times. This paper in refrigeration analyses the effects of internal and external irreversibilities on the steady-state of a double irreversible absorption refrigeration system powered by a waste wood boiler. At the optimal steady-state of the maximum coefficient of performance, expressions of relaxation times of a double irreversible absorption refrigerator are derived. The influence of internal irreversibility parameters, heat leakage coefficient and heat transfer coefficients on relaxation times are discussed. Phase portraits for the trajectories are presented. The results of our analysis show that the eigen-values of the generator-absorber assembly are real with opposite signs while those of the evaporator-condenser are real and both negative. From these results, it follows that internal and external irreversibilities parameters have a draconian effect on the steady-state of the generator-absorber assembly which leads to the disturbance of the absorption system and consequently to its instability. The result can provide some theoretical guidelines for the designs of real absorption refrigerators.

Thermo-Ecological Analysis of Irreversible Dual-Miller Cycle (DMC) Engine Based on the Ecological Coefficient of Performance (ECOP) Criterion

In this paper, an ecological-based numerical analysis and optimization have been carried out for an air-standard irreversible Dual-Miller cycle engine with late inlet valve closing version using the ecological coefficient of performance (ECOP) criterion which covers finite rate of heat transfer, heat leak and internal irreversibilities. A detailed computational analysis has been performed in order to examine the general and optimum performances of the cycle. The results obtained based on ECOP function are compared with a different ecological function and with the maximum power output conditions. The consequences of ECOP, ecological function and maximum power output conditions are acquired based on the compression ratio, cut-off ratio, pressure ratio, Miller cycle ratio, source temperature ratio and internal irreversibility parameter. The influences of these parameters on the optimum performances are examined in detail.

Thermo-ecological performance analyses and optimizations of irreversible gas cycle engines

This paper reports ecological performance analyses and optimization of irreversible gas cycle engines such as Joule–Brayton cycle (JB), Atkinson cycle (AC), Otto cycle (OC), Diesel cycle (DC), Miller cycle (MC), Dual-Atkinson cycle (DAC), Dual-Diesel cycle (DDC), Dual-Miller cycle (DMC) engines based on the ecological coefficient of performance (ECOP) criterion which covers internal irreversibility, heat leak and finite-rate of heat transfer. Comprehensive computational analyses have been conducted to investigate the global and optimal performances of the gas cycle engines. The results obtained based on the ECOP criterion are compared with a different ecological function which is named as the ecologic objective-function and with the maximum power output conditions. The results have been acquired introducing the compression ratio, cut-off ratio, pressure ratio, air recharging ratio, source temperature ratio and internal irreversibility parameter. The changes of cycle performances with respect to these parameters are examined and demonstrated with figures.

Thermoeconomic considerations in the allocation of heat transfer inventory for irreversible power systems

In this paper, thermoeconomic optimization of irreversible power systems with finite thermal capacitances for design situation is performed. Investigation is made with respect to the case of specified power output where exact expressions are determined without the use of an internal irreversibility parameter. The use of an internal irreversibility multiplier can omit important details even though it provides insight into real system behavior. Compared to the endoreversible case, the optimum hot-to cold-end unit cost ratio does not result in equal division of heat exchanger conductances and shows variation in the cycle thermal efficiency despite a constant fluid temperature ratio. It is also noted that optimization of the non-dimensional cost function does not translate into optimization of the thermal efficiency.

Thermoeconomic considerations in the allocation of heat transfer inventory for irreversible refrigeration and heat pump systems

In this paper, thermoeconomic considerations are given to heat exchanger inventory allocation in irreversible refrigeration cycles and heat pumps with finite thermal capacitances. Investigation is made with respect to specified rate of cooling and heating for the refrigeration cycle and heat pump, respectively. Exact expressions are obtained without the use of an internal irreversibility parameter. The optimum hot-to cold-end unit cost ratio resulted in unequal division of heat exchanger conductances for the heat pump in contrast to the endoreversible case where they were the same. At a constant evaporator to condenser fluid temperature ratio, the COP of both refrigeration and heat pump systems was found to vary in an almost asymptotic manner in contrast to the endoreversible case where it was constant. In conclusion, it is best to measure the effect of internal dissipation by deriving exact mathematical expressions as the effect on some performance parameters could be lost due to the averaging effect of the internal irreversibility parameter.

Thermodynamic optimisation of irreversible refrigerators base on NSGAII

This paper presents an investigation on irreversible refrigerator cycle based on external and internal irreversibilities considering finite heat capacities of external reservoirs. Finite temperature difference of the working fluid and external fluids causes the external irreversibility. The internal irreversibilities are caused by regenerative heat loss. Two objective functions including the coefficient of performance (COP) and has been considered simultaneously for maximisation and the normalised entropy generation rate of the refrigerator is minimised at the same time. Multi-objective evolutionary algorithms (MOEAs) based on NSGA-II algorithm has been employed while the Internal irreversibility parameter (I) and the temperatures of the fluid exchanging heat with the reservoir at the temperatures TH(Th) are considered as decision variables. Pareto optimal frontier has been performed and a final optimal solution has been chosen using various decision-making methods as well as the fuzzy Bellman-Zadeh, LINMAP and TOPSIS methods.

Internal irreversibilities’ influence on the new thermo-ecological criterion for real absorption refrigerators

Irreversibility is classified in two main types: internal and external irreversibilities. Internal irreversibility is due to turbulence, pressure gradients, mixing and diffusion processes within the system, which is represented by two irreversibility parameters indicating the deviation from an endoreversible case. This paper studies and analyses the effects of two internal irreversibility parameters (the internal irreversibility parameter for generator–absorber assembly and the internal irreversibility parameter for condenser–evaporator assembly) on the optimal ecological performances of an irreversible four-heat-source absorption refrigerator based on the ecological coefficient of performance (ECOP) criterion. The ECOP objective function is defined as the ratio of the cooling load to the loss rate of availability (or entropy generation rate). The results obtained show that the internal irreversibility parameter for condenser–evaporator assembly affects more the ecological performance of the system than the internal irreversibility parameter for generator–absorber assembly. This is of importance to the optimal design and performance improvement of absorption refrigeration cycles.

Performance optimization of an air-standard irreversible dual-atkinson cycle engine based on the ecological coefficient of performance criterion.

This paper presents an ecological performance analysis and optimization for an air-standard irreversible Dual-Atkinson cycle (DAC) based on the ecological coefficient of performance (ECOP) criterion which includes internal irreversibilities, heat leak, and finite-rate of heat transfer. A comprehensive numerical analysis has been realized so as to investigate the global and optimal performances of the cycle. The results obtained based on the ECOP criterion are compared with a different ecological function which is named as the ecologic objective-function and with the maximum power output conditions. The results have been attained introducing the compression ratio, cut-off ratio, pressure ratio, Atkinson cycle ratio, source temperature ratio, and internal irreversibility parameter. The change of cycle performance with respect to these parameters is investigated and graphically presented.

Influence of two internal irreversibilities on the new thermo-ecological criterion for three-heat-source refrigerators

Irreversibility is a phenomenon that affects much the performance of absorption refrigeration systems. Internal irreversibility is produced by the dissipation of the working fluid. This paper studies and analyzes the effect of two internal irreversibilities parameters (the internal irreversibility parameter for generator–absorber assembly and the internal irreversibility parameter for condenser–evaporator assembly) on the optimal ecological performances of an irreversible three-heat-source absorption refrigerator based on the ECOP criterion. The results obtained show that the internal irreversibility parameter for condenser–evaporator assembly affects more the ecological performance of the system than the internal irreversibility parameter for generator–absorber assembly. This is of importance to the optimal design and performance improvement of absorption refrigeration cycles.

基于有限时间热力学的不可逆汽机热泵性能优化研究

本文提出一种以供热抽汽驱动小汽轮机带动压缩式热泵工作的新型汽机热泵供热系统。新系统用小汽轮机代替凝气电厂的减温减压器，回收减温减压器浪费的能量，同时小汽轮机驱动热泵回收电厂循环冷却水的余热。引入汽机侧与热泵侧不可逆性能参数，对汽机侧和热泵侧进行了优化面积分配，得到了优化分配后的汽机效率和热泵性能系数关于设计参数的表达式，以供热率最大为优化目标，确定系统性能系数与制热率随设计参数与不可逆参数变化关系，并求出最大制热率下的性能系数表达式。通过某电厂改造参数对汽机热泵性能进行了数值分析，研究显示内不可逆参数对汽机热泵系统的性能比外部不可逆因素有更大的影响，研究还发现热泵侧不可逆因素对系统的影响大于汽机侧不可逆因素对系统的影响。 A new compression heat pump system driven by a small steam turbine which used the steam extraction as the power was developed to recycle the heat of cooling water in power plant. Temperature reducing was replaced by a small turbine and the energy lost in the temperature reducing was recycled. The System was considered as consist of an irreversible heat engine and an irreversible heat pump. Optimization was made on heat exchanger areas distribution. An optimal relation between performance coefficient of the system and heating load was obtained. The optimization model was tested and verified by a transformation of power plant in Dalian China. The results showed that internal irreversibility parameters have more drastic effect on performance reduction than any other external irreversibility parameters and the effects of the irreversible factors on the heat pump side was greater than the irreversible factors in the heat engine side on the system. Optimization results can provide guidance for the operation of the heat pump system driven by steam turbine.

Thermodynamic Optimization Characteristics of a Type I Absorption Heat Pump within Finite Time

The absorption heat pump was studied with finite time thermodynamics. A four reservoirs model of absorption heat pump which is treated as an irreversible Carnot heat pump driven by an irreversible Carnot heat engine was established considering the heat resistance and the irreversibility of the internal cycle. A generalized optimization relationship between the main parameters and the corresponding conditions were derived. It is show that, two internal irreversibility parameters, the heat engine cycle and the heat pump cycle has different effects on system performance, and the reduction of the friction, heat loss, and internal dissipations of the equivalent heat pump cycle are more important than its reduction of heat engine cycle.

General thermodynamic performance of irreversible absorption heat pump

The absorption heat pump (AHP) was studied with thermodynamics. A four reservoirs model of absorption heat pump was established considering the heat resistance, heat leak and the internal irreversibility. The reasonable working regions, the performance effects of irreversibility, heat leak and the correlation of four components were studied. When studying the effects of internal irreversibility, two internal irreversibility parameters ( I he for generator–absorber assembly and I re for evaporator–condenser assembly) were introduced to distinguish the different effects. When studying the heat transfer relations of four components, a universal relationship between the main parameters were deduced. The results which have more realized meaning show that, the reduction of the friction, heat loss, and internal dissipations of the evaporator–condenser assembly are more important than its reduction of generator–absorber assembly, and lessening the heat leak of generator are more important than its reduction of other components to improve the AHP performance.

Optimal performance of irreversible absorption refrigerator with four heat sources

Abstract A model for an irreversible four-heat-source absorption refrigerator with continuous flow, which considers the irreversibilities resulting from thermal resistance and internal dissipations of the working fluid, is established. In this model, an absorption refrigerator is treated as a combined cycle system of an irreversible Carnot heat engine driving an irreversible Carnot refrigerator. Two parameters I he and I r are introduced to characterise the internal irreversibilities of the heat engine and refrigerator in the equivalent cycle system respectively. Under a given total heat conductance of the heat exchangers, the optimal relation between the cooling rate and coefficient of performance is derived, and the conditions for the optimal design of the system are determined. Then, this relation is used to deduce some new performance bounds. Moreover, the effects of the internal irreversibility parameters I he and I r on the optimal performance of the system are also investigated. The results...

Investigation on the optimal performance and design parameters of an irreversible solar-driven Braysson heat engine

An irreversible solar-driven Braysson thermal engine has been investigated, in which finite rate heat transfer with the radiation–convection mode from the high-temperature reservoir to the heat engine and the convection mode from the heat engine to the heat sink, and irreversible adiabatic processes are taken into account. Based on the thermodynamic analysis method, the analytic expressions of the power output and efficiency of the Braysson heat engine are derived. By using numerical value calculation, the effects of the isobaric temperature ratio, internal irreversibility parameter, temperature ratio of the thermal reservoirs as well as the allocation parameters involving the heat-transfer coefficients, and areas on the performance characteristics of the Braysson heat engine are analysed and discussed in detail. The results obtained in this paper are more general than the related conclusions published in the literature and may provide some parameter design reference for solar-driven heat engines.

Application of exergy method to an irreversible inter-cooled refrigeration cycle

The exergy method, based on the maximum exergetic efficiency criterion, has been applied to an irreversible inter-cooled refrigeration cycle. The exergetic efficiency defined as the ratio of the rate of exergy output to the rate of exergy input is taken as the objective function to be maximized. Multi-irreversibilities include finite-rate heat transfer, internal dissipation of the working fluid, and heat leaks between heat reservoirs. The maximum exergetic efficiency can be determined analytically by introducing the internal irreversibility parameter, which represents the degree of internal irreversibility. The corresponding performances of the irreversible refrigeration system are obtained simultaneously. The results show that the exergy method can be used as an effective criterion in designing an irreversible inter-cooled refrigeration system.

General performance characteristics of an irreversible vapour absorption refrigeration system using finite time thermodynamic approach

This communication presents finite time thermodynamic analysis of vapour absorption refrigeration system considering both external as well as internal irreversibility. System is considered as consisting of an irreversible heat engine between the generator and absorber and an irreversible refrigerator between the evaporator and condenser. Optimization is made w.r.t. source/sink side heat exchanger areas, source/sink side heat transfer time and heat engine cycle and refrigeration cycle time. An optimal relation between overall coefficient of performance and maximum cooling load is obtained. Two internal irreversibility parameters (say, R Δ S is for generator–absorber assembly and R Δ S ′ is for evaporator–condenser assembly) have been introduced in the analysis, having fractional value for irreversible (real) system while R Δ S = R Δ S ′ = 1.0 corresponds to endoreversible system. A detailed parametric study shows that internal irreversibility parameters have more drastic effect on performance reduction than any other external irreversibility parameter. It is also found that internal irreversibility parameters are being dominant factor for performance reduction but out of these two parameters, internal irreversibility parameter of generator–absorber assembly is more sensitive for performance reduction than the internal irreversibility parameter of evaporator–condenser assembly.

Performance analysis and optimum criteria of an irreversible Braysson heat engine

An irreversible cycle model of a Braysson heat engine operating between two heat reservoirs is used to investigate the performance of the cycle affected by the finite-rate heat transfer between the working fluid and the heat reservoirs, heat leak loss between the heat reservoirs and irreversibility inside the cycle. The specific power output is maximized with respect to the cycle temperatures along with the isobaric temperature ratio. The specific power output is found to be a decreasing function of the internal irreversibility parameter and isobaric temperature ratio while there exist the optimal values of the state point temperatures at which the specific power output attains its maximum value for a typical set of operating parameters. Moreover, the maximum specific power output and other cycle parameters are calculated for different sets of operating conditions. The optimally operating regions of the important parameters in the cycle are determined. The results obtained here may provide some useful criteria for the optimal design and performance improvement of a realistic Braysson heat engine.

Optimum Criteria on the Performance of an Irreversible Braysson Heat Engine Based on the new Thermoeconomic Approach

An irreversible cycle model of a Braysson heat engine operating between two heat reservoirs is used to investigate the thermoeconomic performance of the cycle affected by the finite-rate heat transfer between the working fluid and the heat reservoirs, heat leak loss from the heat source to the ambient and the irreversibility within the cycle. The thermoeconomic objective function, defined as the total cost per unit power output, is minimized with respect to the cycle temperatures along with the isobaric temperature ratio for a given set of operating parameters. The objective function is found to be an increasing function of the internal irreversibility parameter, economic parameters and the isobaric temperature ratio. On the other hand, there exist the optimal values of the state point temperatures, power output and thermal efficiency at which the objective function attains its minimum for a typical set of operating parameters. Moreover, the objective function and the corresponding power output are also plotted against the state point temperature and thermal efficiency for a different set of operating parameters. The optimally operating regions of these important parameters in the cycle are also determined. The results obtained here may provide some useful criteria for the optimal design and performance improvements, from the point of view of economics as well as from the point of view of thermodynamics of an irreversible Braysson heat engine cycle and other similar cycles as well.