Optimal ratios of the piston speeds for a finite speed irreversible Carnot heat engine cycle

The performance of an irreversible Carnot heat engine cycle is analysed and optimized by using the theory of finite time thermodynamics based on Agrawal's [2009. A finite speed Curzon-Ahlborn engine. European Journal of Physics, 30 (3), 587–592] model of finite piston speed on the four branches and Petrescu et al.’s [2002b. Optimization of the irreversible Carnot cycle engine for maximum efficiency and maximum power through use of finite speed thermodynamic analysis. In: Proceedings of ECOS’2002, 3–5 July, Berlin, Germany, Vol. II, 1361–1368] model of a Carnot cycle engine with the finite rate of heat transfer, heat leakage from heat source to heat sink and irreversibilities caused by finite speed, friction and throttling through the valves. The finite piston speeds on the four branches are further assumed to be different, which is different from the model of constant speed of the piston on the four branches. Expressions of power output and thermal efficiency of the cycle are derived for a fixed cycle period and internal entropy generation rate. Numerical examples show that the curve of power output versus thermal efficiency is loop shaped, and there exist optimal finite piston speeds on the four branches which lead to the maximum power output and maximum thermal efficiency, respectively. The effects of the heat leakage coefficient and internal entropy generation rate on the optimal finite piston speed ratios are discussed.     

包含6种热机模型的普适内可逆热机循环(火用)经济性能优化

用有限时间热力学方法分析了工作在恒温热源TH、TL之间的普适定常流内可逆热机循环模型的炯经济性能，导出了循环利润率与工质温比、热效率与工质温比的关系式，以及利润率和效率的特性关系，并由数值计算分析了循环过程对循环性能的影响特点。所得结果包含了内可逆Carnot、Diesel、Otto、Atkinson、Brayton和Dual循环的有限时间炯经济性能。     

热漏对内可逆四热源制冷系统的影响

在恒温热源内可逆四热源吸收式制冷循环的基础上，建立不可逆吸收式制冷循环的模型，考虑环境热源到制冷空间的热漏以及工质与外部热源间的热阻损失，导出牛顿定律下循环的制冷率和制冷系数的基本优化关系、最大制冷系数及相应的制冷率和最大制冷率及相应的制冷系数；并通过数值计算分析了循环参数对循环的制冷率、制冷系数的影响。     

不可逆半导体固态热离子制冷器性能分析和优化

建立了考虑外部有限速率传热过程和热源间热漏的不可逆半导体固态热离子制冷器模型,基于非平衡热力学和有限时间热力学理论导出了热离子制冷器的制冷率和制冷系数的表达式;对比分析了不可逆热离子制冷器与可逆热离子制冷器的发射电流密度特性、电极温度特性以及制冷系数特性;研究了不可逆系统的制冷率与制冷系数最优性能,得到了制冷率和制冷系数的最优运行区间;通过数值计算,详细讨论了外部传热以及内部导热、热源间热漏损失、热源温度、外加电压、半导体材料势垒等设计参数对热离子装置性能的影响。在总传热面积一定的条件下,进一步优化了高、低温侧换热器的面积分配以获得最佳的制冷率和制冷系数特性。结果表明,由于存在内部和外部的不可逆性,热离子装置的发射电流密度及制冷系数都会明显降低;不可逆半导体固态热离子制冷器的制冷率与制冷系数特性呈扭叶型;合理地选外加电压、势垒等参数,可以使制冷器设计于最大制冷率或最大制冷系数的状态。     

The influence of heat resistance and heat leak on the performance of a four-heat-reservoir absorption refrigerator with heat transfer law of Q∝Δ(T)

On the basis of an endoreversible absorption refrigeration cycle model with Newton's heat transfer law, an irreversible four-heat-reservoir cycle model with another linear heat transfer law of Q∝Δ(T⁻¹) is built by taking account the heat leak and heat resistance losses. The fundamental optimal relation between the coefficient of performance (COP) and the cooling load, the maximum COP and the corresponding cooling load, as well as the maximum cooling load and the corresponding COP of the cycle with another linear heat transfer law coupled to constant-temperature heat reservoirs are derived by using finite-time thermodynamics. The optimal distribution relation of the heat-transfer surface areas is also obtained. Moreover, the effects of the cycle parameters on the COP and the cooling load of the cycle are studied by detailed numerical examples. The results obtained herein are of importance to the optimal design and performance improvement of a four-heat-reservoir absorption refrigeration cycle.     

Irreversible four-temperature-level absorption refrigerator

A refrigeration cycle is modeled as a demonstration of an irreversible absorption refrigeration cycle. This four-temperature-level model takes into account the heat resistance, heat leakage, and irreversibilities due to internal dissipation of the working fluid. The fundamental optimal relationships between: (1) the coefficient of performance (COP) and the cooling load; (2) the maximum COP and the corresponding cooling load; and (3) the maximum cooling load and the corresponding COP of the cycle, all coupled to constant-temperature heat reservoirs, are derived by using finite-time thermodynamics. The optimal distribution relationships of the heat-transfer surface areas are also presented. Moreover, the effects of the cycle parameters on the COP and the cooling load of the cycle are studied by detailed numerical examples. The results obtained herein are useful for optimal design and performance improvement of absorption refrigeration cycles.     

Study on optimal performance and working temperatures of endoreversible forward and reverse carnot cycles

The connection between the expressions of optimization performances of Carnot heat engines, refrigerators and heat pumps, which operate subject to irreversible heat flow, is studied. We consider the endoreversible forward and reverse Carnot cycles and analyse the expressions which relate efficiency, refrigeration and heating coefficients to power, refrigeration and heating rates, respectively. It is found and proved that when one of the optimal relations is derived the others are also determined, and give the unified formulation of the related optimal working temperatures of the forward and reverse Carnot cycles by isentropic temperature ratio exponent. Finally, several new optimal performance relations are derived for forward and reverse Carnot cycles under nonlinear heat transfer, and some major results in the references are easily deduced and unified in this paper.     

Exergoeconomic performance of an endoreversible Carnot heat pump with complex heat transfer law

The finite-time exergoeconomic performance of an endoreversible Carnot heat pump with a complex heat transfer law, including generalized convective heat transfer law and generalized radiative heat transfer law q∝ (Δ T ⁿ ) ^m , is investigated in this paper. The focus of this paper is to obtain the compromised optimization between economics (profit) and the energy utilization factor (coefficient of performance, COP) for the endoreversible Carnot heat pump, by searching the optimum COP at maximum profit, which is termed as the finite-time exergoeconomic performance bound. The obtained results include those obtained in much of the literature and can provide some theoretical guidance for the design of practical heat pumps.     

考虑热漏影响的热泵装置有限时间热力学性能

本文研究热漏对热泵装置最优性能的影响,导出存在热阻和热漏损失的定常态流不可逆卡诺热泵最佳供热系数与供热率关系,所得结果与仅存在热阻损失时的内可逆卡诺热泵供热系数供热率特性关系有量和质的区别。     

Optimum allocation of heat exchanger inventory of irreversible air heat pump cycles

This study has determined the optimal ratios of heat conductance of a cold-side heat exchanger to that of a hot-side heat exchanger when the heating load and the coefficient of performance (COP) of the irreversible air heat pump cycles are taken as the optimization objectives. Both the optimum distributions of heat conductance corresponding to the maximum heating load and the maximum COP are less than 0.5 for the fixed total heat exchanger inventory. The influences of the heat reservoir temperature ratio, the total heat exchanger inventory, and the efficiencies of the compressor and expander on the optimum distribution of heat conductance and the maximum heating load and the maximum COP are analysed and shown by numerical examples.     

Performance optimization of an irreversible four-heat-reservoir absorption refrigerator

On the basis of an endoreversible absorption refrigeration cycle model with linear phenomenological heat transfer law of Q∝Δ(T⁻¹), an irreversible four-heat-reservoir cycle model is built by taking account of the heat resistance, heat leak and irreversibilities due to the internal dissipation of the working fluid. The fundamental optimal relation between the coefficient of performance (COP) and the cooling load, the maximum COP and the corresponding cooling load, as well as the maximum cooling load and the corresponding COP of the cycle coupled to constant-temperature heat reservoirs are derived by using finite-time thermodynamics. The optimal distribution relation of the heat-transfer surface areas is also obtained. Moreover, the effects of the cycle parameters on the COP and the cooling load of the cycle are studied by detailed numerical examples. The results obtained herein are of importance to the optimal design and performance improvement of real absorption refrigerators.     

高炉余能余热驱动的内可逆闭式布雷顿热电冷联产装置火用经济性能分析

用有限时间热力学理论研究恒温热源条件下由一个内可逆闭式布雷顿热机循环和一个内可逆四热源吸收式制冷循环组成的高炉余能余热驱动的热电冷联产装置的火用经济性能,导出热电冷联产装置的利润率和火用效率与压气机压比的关系。利用数值计算,分析热电比和吸收式制冷循环总放热量在吸收器和冷凝器之间的分配率对利润率与火用效率关系的影响,并研究联产装置各种参数对最大利润率及相应火用效率特性的影响。     

有限速率过程对活塞式斯特林发动机性能的影响

本文讨论在给定热源温度和压缩比的情况下,过程进行的速率有限,并受热传导不可逆影响的内可逆活塞式斯特林发动机的最优性能,导出以理想气体或范德瓦尔斯气体为工质的斯特林发动机的最大输出功率与热效率的关系,以及最大热效率与输出功率的关系,并推出了一些新的有限时间热力学的性能界限。     

不可逆吸收式热泵循环的基本优化关系及性能分析

在恒温热源内可逆四热源吸收式热泵循环的基础上，建立了线性(牛顿)传热定律下考虑泵热空间到环境热源的热漏、工质的内部耗散以及工质与外部热源间的热阻损失的不可逆吸收式热泵循环模型。导出了总换热面积一定的条件下循环的泵热率和泵热系数的基本优化关系、最大泵热率和相应的泵热系数、最大泵热系数和相应的泵热率、以及循环中最佳工质工作温度和最佳换热面积分配关系；并通过数值算例分析了循环参数对循环最优性能的影响规律。     

Performance analysis of two stage combined heat pump system based on thermoeconomic optimization criterion

A thermoeconomic performance analysis based on a new kind of optimization criterion has been performed for a two stage endoreversible combined heat pump cycle model. The optimal performances and design parameters that maximize the objective function (heating load per unit total cost) are investigated. The optimal temperatures of the working fluids, the optimum performance coefficient, the optimum specific heating load and the optimal distribution of the heat exchanger areas are determined in terms of technical and economical parameters. The effects of the economical parameter on the global and optimal performances have been discussed.     

Heating load,heating-load density and COP optimizations of an endoreversible air heat-pump

The finite-time thermodynamic performance has been studied of an endoreversible air heat-pump with constant-temperature heat-reservoirs. The heating load, the coefficient of performance (COP), and the heating-load density, i.e. the ratio of heating load to the maximum specific volume in the cycle, are the optimization objectives. The analytical formulae relating the heating load and pressure-ratio, between the COP and pressure-ratio, as well as between the heating-load density and pressure-ratio are derived assuming heat resistance losses occur in the hot- and cold-side heat-exchangers. The influences of the effectiveness of the heat-exchangers and the heat-reservoir temperature-ratio on the heating load, the COP and the heating-load density are analyzed. The cycle performance optimizations are performed by searching the optimal distribution of heat conductance of the hot- and cold-side heat-exchangers for the fixed total heat-exchanger inventory. The influences of some design parameters, including heat-capacity rate of the working fluid, heat-reservoir temperature-ratio and heat-exchanger inventory on the optimal distribution of heat conductance, the maximum heating load and the maximum heating-load density are indicated by numerical examples. The different results obtained from the heating-load optimization and the heating-load density optimization are shown. The air heat-pump design, with heat-loading density optimization, leads to smaller size equipment.     

普适传热定律下广义不可逆卡诺制冷循环火用经济最佳性能

应用有限时间热力学理论,以广义不可逆卡诺制冷循环为对象,以[火用]经济性能为优化目标,研究了普适传热定律下循环的最佳性能,导出了循环有限时间[火用]经济性能界限以及优化目标之间的最佳关系,并分析了不同传热定律、各种损失和价格比因素对最佳[火用]经济性能的影响。所得结果具有普适性并可以为实际制冷机进行改良提供一种参考。     

Effect of heat transfer law on the finite-time exergoeconomic performance of a Carnot refrigerator

The operation of a Carnot refrigerator is viewed as a production process with exergy as its output. The economic optimization of the endoreversible refrigerator is carried out in this paper. The Coefficient of Performance (COP) of the refrigerator is a secondary consideration of the practical engineering effort of maximizing cooling rate and exergy whose goodness is constrained by economical considerations. Therefore, the profit of the refrigerator is taken as the optimization objective. Using the method of finite-time exergoeconomic analysis, which emphasizes the compromise optimization between economics (profit) and the appropriate energy utilization factor (Coefficient of Performance, COP) for finite-time (endoreversible) thermodynamic cycles, this paper derives the relation between optimal profit and COP of an endoreversible Carnot refrigerator based on a relatively general heat transfer law q∝Δ(Tⁿ). The COP at the maximum profit is also obtained. The results obtained involve those for three common heat transfer laws: Newton's law (n=1), the linear phenomenological law in irreversible thermodynamics (n=−1), and the radiative heat transfer law (n=4).     

内可逆四热源吸收式热变换器生态学最优性能

基于能量分析的观点，建立了反映四热源吸收式热变换器泵热率与熵产率之间最佳折衷的生态学准则。导出了线性(牛顿)传热定律下生态学目标与泵热系数的优化关系、最大生态学目标值及其相对应的泵热系数、泵热率和熵产率以及最大泵热率时的生态学目标和熵产率。通过数值算例分析得到了吸收式热变换器的生态学优化准则。计算发现，与最大泵热率目标相比，最大生态学目标牺牲了27．3％的泵热率。使循环熵产率降低了77．0％。泵热系数增加了55．4％，表明生态学准则对吸收式热变换器优化设计是一种具有长期效应的可选优化目标。     

Optimization between heating load and entropy-production rate for endoreversible absorption heat-transformers

For an endoreversible four-heat-reservoir absorption heat-transformer cycle, for which a linear (Newtonian) heat-transfer law applies, an ecological optimization criterion is proposed for the best mode of operation of the cycle. This involves maximizing a function representing the compromise between the heating load and the entropy-production rate. The optimal relation between the ecological criterion and the COP (coefficient of performance), the maximum ecological criterion and the corresponding COP, heating load and entropy production rate, as well as the ecological criterion and entropy-production rate at the maximum heating load are derived using finite-time thermodynamics. Moreover, compared with the heating-load criterion, the effects of the cycle parameters on the ecological performance are studied by numerical examples. These show that achieving the maximum ecological criterion makes the entropy-production rate decrease by 77.0% and the COP increase by 55.4% with only 27.3% heating-load losses compared with the maximum heating-load objective. The results reflect that the ecological criterion has long-term significance for optimal design of absorption heat-transformers.