Thermodynamic and thermoeconomic analyses of an irreversible combined Carnot heat engine system

A combined cycle model which includes the irreversibilities of finite‐rate heat transfer in heat‐exchange processes and heat leak loss of the heat source is used to analyse the performance of a multi‐stage Carnot heat engine system. The efficiency, power output, ecological function and profit of operating the combined system are optimized. The optimally operating region of the combined system is determined. The optimal combined conditions between two adjacent cycles in the combined system are obtained. Moreover, the cycle model is generalized to include the internal irreversibilities of the working fluids so that the results obtained here become more general. Copyright © 2001 John Wiley & Sons, Ltd.     

复杂热源下不可逆诺维科夫热机最大功率输出

考虑实际热机工作下的旁通热漏和内部耗散等不可逆因素,建立了包括连续均匀分布、三角形分布、二次分布和帕累托分布等四种不同的统计概率分布高温热源温度下的广义不可逆诺维科夫热机模型,导出了热机最大输出功率及相应的热效率和熵产率随高温热源温度、内部不可逆性等因素变化的关系式。结果表明：热漏和内部耗散分别对热机性能有着不同的影响,热漏使统计热源温度分布下最大功率输出对应的热效率减小,同时也增大了熵产率,但对热机的最大功率输出无影响;内部耗散不可逆性使热机的最大输出功率及相应热效率均明显减小,但使熵产率先增大后减小;熵产率随高温热源温度的标准差增大而减小。研究结果对太阳能发电厂性能提升具有一定理论指导意义。     

补燃型不可逆联合循环热机的性能分析

在原有的不可逆联合动力循环模型的基础上,建立了一个存在热阻、热漏、补燃、内不可逆性的定常流联合卡诺热机循环模型。研究其在补燃作用下的功率和效率特性并对其进行优化,导出功率、效率的基本优化关系,分析了补燃对最优性能的影响。     

Efficiency of an Otto engine under alternative power optimizations

The proper optimization criterion to be chosen for the optimum design of the heat engines may differ depending on their purposes and working conditions. In this study, a comparative performance analysis is carried out for a reversible Otto cycle based on three alternative performance criteria namely maximum power (mp), maximum power density (mpd) and maximum efficient power (mep). The power density criterion is defined as the power per minimum specific volume in the cycle and the efficient power criterion is defined as multiplication of the power by the efficiency of the Otto cycle. Maximizing the efficient power gives a compromise between power and efficiency. Three different objective functions are defined and maximization of these functions is carried out under different design parameters of the Otto engine. The variations of power, power density and efficient power outputs are derived and presented with respect to the thermal efficiency of the cycle for various temperature ratios. It has been found that the design parameters at mep conditions lead to more efficient engines than that at the mp condition and the mep criterion may have a significant power advantage compared with mpd criterion. Copyright © 2009 John Wiley & Sons, Ltd.     

Ecological coefficient of performance analysis and optimization of an irreversible regenerative-Brayton heat engine

In this paper, a performance optimization based on the ecological coefficient of performance (ECOP) criterion has been carried out for an irreversible regenerative Brayton heat-engine. The results obtained were compared with those using the power-output criterion and alternative ecological performance objective-function defined in the literature. The design parameters, under the optimal conditions, have been derived analytically and their effects on the engine’s performance have been discussed. It is shown that, for the regenerative Brayton-engine, a design based on the maximum ECOP conditions is more advantageous from the point-of-view of entropy generation rate, thermal efficiency and investment cost.     

Optimization of the irreversible Stirling heat engine

The effects of inefficiencies in the compression, expansion and regeneration processes on engine performance have been evaluated theoretically for a Stirling heat engine operating in a closed regenerative thermodynamic cycle. The irreversible cycle has been optimized by using the maximum power density technique. Maximized power and maximized power density are obtained for different n_ex, τ, α_c, α_h, η_c, η_ex and η_reg values. The maximum efficiencies have been found very close to the values corresponding to the maximum power density conditions but far from the values at maximum power. It has been found that the engines designed by considering the maximum power density have high efficiencies and small sizes under the same prescribed conditions. Copyright © 1999 John Wiley & Sons, Ltd.     

线性唯象传热规律下广义不可逆卡诺热机生态学性能系数优化

以反映热机循环输出功率和火用损失率之比的生态学性能系数(ECOP)为目标,用有限时间热力学理论和方法研究广义不可逆卡诺热机的循环性能。导出了线性唯象传热规律(Q∝Δ(T-1))下ECOP的解析式,通过数值计算分析了各种目标极值条件下ECOP与循环功率、效率、熵产率、生态学函数E之间的关系,主要研究了热源温比对最优性能的影响。     

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.     

Theory and performance analysis of a new heat engine for concentrating solar power

The objectives of this paper are to introduce a new heat engine and evaluate its performance. The new heat engine uses a gas, such as air, nitrogen, or argon, as the working fluid and extracts thermal energy from a heat source as the energy input. The new heat engine may find extensive applications in renewable energy industries, such as concentrating solar power (CSP). Additionally, the heat engine may be employed to recover energy from exhaust streams of internal combustion engines, gas turbine engines, and various industrial processes. It may also work as a thermal‐to‐mechanical conversion system in a nuclear power plant and function as an external combustion engine in which the heat source is the combustion gas from an external combustion chamber. The heat engine is to mimic the performance of an air‐standard Otto cycle. This is achieved by drastically increasing the time duration of heat acquisition from the heat source in conjunction with the timing of the heat acquisition and a large heat transfer surface area. Performance simulations show that the new heat engine can potentially attain a thermal efficiency above 50% and a power output above 100 kW under open‐cycle operation. Additionally, the heat engine could significantly reduce CSP costs and operate in open cycles, effectively removing the difficulties of dry cooling requirement for CSP applications. Copyright © 2014 John Wiley & Sons, Ltd.     

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.     

Effect of variable heat capacities on performance of an irreversible Miller heat engine

Based on the variable heat capacities of the working fluid, the irreversibility coming from the compression and expansion processes, and the heat leak losses through the cylinder wall, an irreversible cycle model of the Miller heat engine was established, from which expressions for the efficiency and work output of the cycle were derived. The performance characteristic curves of the Miller heat engine were generated through numerical calculation, from which the optimal regions of some main parameters such as the work output, efficiency and pressure ratio were determined. Moreover, the influence of the compression and expansion efficiencies, the variable heat capacities and the heat leak losses on the performance of the cycle was discussed in detail, and consequently, some significant results were obtained.     

An analytical formula for the estimation of a rankine-cycle heat engine efficiency at maximum power

The authors develop an analytical formula for estimating the Rankine power cycle efficiency at maximum power, which can be extracted from the given mass flow rates of heating and cooling fluids. This formula does not need any detailed thermodynamic data. The accuracy of the procedure is shown by comparisons between analytical values and those calculated using detailed thermodynamic data. The results indicate that the thermal efficiency at maximum power depends primarily on the initial temperatures of the heating and cooling fluids and pinch-temperature differences between the working fluid and the heating and cooling fluids. The efficiency at maximum power provides a measure of the power available in a Rankine heat engine.     

Performance characteristics of an irreversible solar-driven Braysson heat engine at maximum efficiency

A novel model of the solar-driven thermodynamic cycle system which consists of a solar collector and a Braysson heat engine is established. The performance characteristics of the system are optimized on the basis of the linear heat-loss model of a solar collector and the irreversible cycle model of a Braysson heat engine. The maximum efficiency of the system and the optimally operating temperature of the solar collector are determined and other relevant performance characteristics of the system are discussed. The results obtained here may provide some theoretical guidance for the optimal design and operation of solar-driven Braysson and Carnot heat engines.     

Thermoeconomic analysis of a solar driven heat engine

Thermoeconomic optimization has been carried out for an endoreversible solar driven heat engine using finite-time/finite-size thermodynamic theory. In the considered heat engine model, heat transfer from the hot reservoir is assumed to be radiation mode and the heat transfer to the cold reservoir is assumed to be convection mode. The power output per unit total cost is taken as objective function and the optimum performance and design parameters have been investigated. The effects of the technical and economical parameters on the thermoeconomic performances have been also discussed.     

一类广义不可逆普适热机循环的生态学性能

用有限时间热力学的方法分析了热漏、热阻和其它不可逆效应对一类定常流普适热机循环模型性能的影响,导出了由两个绝热过程、两个等热容加热过程以及两个等热容放热过程组成的循环的功率、效率和生态学性能,并由数值计算分析了循环过程对循环性能的影响特点。所得结果包含了存在热阻、热漏和内不可逆损失的Diesel、Otto、Brayton、Atkinson、Dual和Miller循环的特性。     

多孔介质发动机再热循环有限时间热力学分析

建立了多孔介质（PM）发动机循环的有限时间热力学模型，对PM循环进行了分析，导出了存在摩擦及传热损失时循环功率与压缩比、效率与压缩比以及功率效率的特性关系，同时由数值计算分析了压缩比、预胀比、传热损失和摩擦损失对循环性能的影响特点。将PM循环与Otto循环进行了比较，结果表明：PM循环的性能要优于Otto循环的性能。     

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.     

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。     

Optimum performance characteristics of an irreversible solar-driven Brayton heat engine at the maximum overall efficiency

An irreversible cycle model of a solar-driven Brayton heat engine is established, in which the heat losses of the solar collector and the external and internal irreversibilities of the heat engine are taken into account, and used to investigate the optimal performance of the cycle system. The maximum overall efficiency of the system is determined. The operating temperature of the solar collector and the temperature ratio in the isobaric process are optimized. The influence of the heat losses of the solar collector and the external and internal irreversibilities of the heat engine on the cyclic performance is discussed in detail. Some important curves which can reveal the optimum performance characteristics of the system are given. The results obtained here are general, and consequently, may be directly used to discuss the optimal performance of other solar-driven heat engines.     

Optimal performance of a spin quantum Carnot heat engine with multi-irreversibilities

By using quantum master equation, semi-group approach and finite time thermodynamics (FTT), this paper derives the expressions of cycle period, power and efficiency of an irreversible quantum Carnot heat engine with irreversibilities of heat resistance, internal friction and bypass heat leakage, and provides detailed numerical examples. The irreversible quantum Carnot heat engine uses working medium consisting of many non-interacting spin-1/2 systems and its cycle is composed of two isothermal processes and two irreversible adiabatic processes. The optimal performance of the quantum heat engine at high temperature limit is deduced and analyzed by numerical examples. Effects of internal friction and bypass heat leakage on the optimal performance are discussed. The endoreversible case, frictionless case and the case without bypass heat leakage are also briefly discussed.