Heat transfer effect on the specific power availability of heat engines

The maximum possible specific power (specific power availability) that can be obtained from heat engines with a set of high temperature heat source and low temperature sink is analyzed. The heat engines considered in this paper include (1) externally and internally reversible, (2) externally irreversible and internally reversible, (3) externally reversible and internally irreversible and (4) externally and internally irreversible engines. The irreversibilities are assumed caused by heat transfer only. The specific power, defined as the power output per unit total heat exchanger surface area, is adopted as the objective function in determining power economics in this paper.     

不同热漏对热机功率效率特性的影响

建立了一个包含多种不可逆性的不可逆热机模型,并将热漏分为外热漏和内热漏两种方式。在此基础上求得存在热阻、热漏和内不可逆损失的定常态流不可逆卡诺热机的功率、效率关系。分析了两种热漏方式对热机最优性能的影响,发现内热漏对热机功率效率特性的影响不同于外热漏,而且与摩擦、涡流和非平衡等不可逆效应也不同;内热漏不能归结于外热漏作为整个热机的热漏或合并为除热阻和热漏外的其他不可逆性。分析表明,当有内热漏存在时,一定温比下热机的最佳功率和最佳效率工作状态分别对应不同的面积比。所得结果对热机设计具有一定指导意义。     

Effects of combined heat transfer on the thermo-economic performance of irreversible solar-driven heat engines

A thermo-economic performance analysis and optimization has been carried out for an irrversible solar-driven heat engine with losses due to heat transfer across finite temperature differences, heat leak and internal irreversibilities. In the considered heat engine model, heat transfer from the hot reservoir is assumed to be simultaneous radiation and convection mode and the heat transfer to the cold reservoir is assumed to be convection mode. The effects of the technical and economical parameters on the thermo-economic performance have been investigated in order to see the collective effects of the radiation and convection modes of heat transfer. Also the optimal performance parameters of the heat engine, such as the thermal efficiency, temperatures of the working fluid and the ratio of heat transfer areas have been discussed in detail.     

Effects of functional surface on performance of a micro heat pipe

The effect of a functional surface with the axial ladder contact angle distribution on the thermal performance of a triangular micro heat pipe has been analyzed based on a one-dimensional steady-state model. Compared with the traditional micro heat pipe (MHP) with a uniform contact angle distribution on its surface, the simulation results show that a MHP with a functional surface can remove a greater amount of heat under the same condition. The increase in thermal performance is more obvious with the increase in the ladder difference of the contact angles between the adjacent sections of the MHP. The increased thermal performance associated with the functional surface can be attributed to the increase of the liquid capillary force as well as the no obvious increase of the liquid shearing force provided by the functional surface, which also brings about the increase in condensate mass flow rate through the adiabatic section–evaporation section interface. It is also found that for the traditional MHP with uniform contact angle surface, there is an optimal contact angle leading to the maximum heat input. The deviation of the optimal value will decrease the capillary force and thermal performance of the MHP.     

Performance of a micro engine with heptane as working fluid

A micro Newcomen engine is proposed. To overcome the problems of friction and leakage of the micro mechanical parts during engine’s operation, we use a flexible ripple tube to integrate piston and cylinder into one monolithic part. Heptane is used as the working fluid, and it works in the two-phase condition for higher energy output density per thermodynamic cycle. In the experiment, the prototype engine is tested under different operational conditions. It works continuously and generates the net mechanical work of 0.833 J per cycle with an efficiency of 2.77% in the maximum. The experimental results prove its feasibility. However, the prototype engine still requires further improvement and optimization for better performance.     

有限热源斯特林热机的优化性能研究

应用有限时间热力学方法,探索有限热源、热阻和回热损失的斯特林热机的优化性能,得到一些新的性能参数,所得结论可为斯特林热机的研制和优化设计提供些新理论指导。     

Optimized thermal coupling of micro thermoelectric generators for improved output performance

There is a significant push to increase the output power of thermoelectric generators (TEGs) in order to make them more competitive energy harvesters. The thermal coupling of TEGs has a major impact on the effective temperature gradient across the generator and therefore the power output achieved. The application of micro fluidic heat transfer systems (μHTS) can significantly reduce the thermal contact resistance and thus enhance the TEG's performance. This paper reports on the characterization and optimization of a μTEG integrated with a two layer μHTS. The main advantage of the presented system is the combination of very low heat transfer resistances with small pumping powers in a compact volume. The influence of the most relevant system parameters, i.e. microchannel width, applied flow rate and the μTEG thickness on the system's net output performance are investigated. The dimensions of the μHTS/μTEG system can be optimized for specific temperature application ranges, and the maximum net power can be tracked by adjusting the heat transfer resistance during operation. A system net output power of 126 mW/cm² was achieved with a module ZT of 0.1 at a fluid flow rate of 0.07 l/min and an applied temperature difference of 95K.It was concluded that for systems with good thermal coupling, the thermoelectric material optimization should focus more on the power factor than on the figure of merit ZT itself, since the influence of the thermal resistance of the TE material is negligible.     

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.     

Optimal regenerator performance in Stirling engines

The key component of a Stirling engine is its regenerative heat exchanger. This device is subject to losses due to dissipation arising from the flow through the regenerator as well as due to imperfect heat transfer between the regenerator material and the gas. The magnitudes of these losses are characterized by the Stanton number St and the Fanning friction factor f, respectively. Using available data for the ratio St/f, results are found for the Carnot efficiency and the power output of the regenerator. They depend on the conductance and on the ratio of pressures at the two sides of the regenerator. Optimum results for efficiency and power output of the regenerator are derived in the limit of zero Mach number. The results are applied to the Stirling engine. The efficiency and the power output of the engine are found for given amplitude of the compression piston. Optimization with respect to regenerator conductance and piston phase angle leads to a maximum possible value of the power output. Under optimal conditions, the Carnot efficiency just below this maximum is close to 100%. Copyright © 2009 John Wiley & Sons, Ltd.     

Design and performance optimization of GPU-3 Stirling engines

To increase the performance of Stirling engines and analyze their operations, a second-order Stirling model, which includes thermal losses, has been developed and used to optimize the performance and design parameters of the engine. This model has been tested using the experimental data obtained from the General Motor GPU-3 Stirling engine prototype. The model has also been used to investigate the effect of the geometrical and physical parameters on Stirling engine performance and to determine the optimal parameters for acceptable operational gas pressure. When the optimal design parameters are introduced in the model, the engine efficiency increases from 39% to 51%; the engine power is enhanced by approximately 20%, whereas the engine average pressure increases slightly.     

Competitive solar heat engines

This article presents a technical innovation consisting of replacing the piston–cylinder system by metallic bellows in free-displacer Stirling engines. It deals with the advantage of heat engines of low power, integrating this innovation with plane solar collectors. A theoretical study requiring notions of finite time thermodynamics determines the optimised dimensioning of such systems and makes them appear serious competitors, on the one hand to solar cells to exploit solar energy and, on the other hand, to power stations regarding kW h unit cost.     

Thermodynamic information system for diagnosis and prognosis of power plant operation condition

A thermodynamic information system for diagnosis and prognosis of an existing power plant was developed. The system is based on an analytic approach that informs the current thermodynamic condition of all cycle components, as well as the improvement that can be obtained in the cycle performance by the elimination of the discovered anomalies. The effects induced by components anomalies and repairs in other components efficiency, which have proven to be one of the main drawbacks in the diagnosis and prognosis analyses, are taken into consideration owing to the use of performance curves and corrected performance curves together with the thermodynamic data collected from the distributed control system. The approach used to develop the system is explained, the system implementation in a real gas turbine cogeneration combined cycle is described and the results are discussed.     

A review of enhancing micro combustion to improve energy conversion performance in micro power system

With the development of microfabrication technology and micro devices, the demand for Power Micro Electro Mechanical System (Power MEMS) is ever-increasing. However, traditional chemical batteries are not suitable for Power MEMS due to their low energy density. The combustion of hydrogen and hydrocarbon fuels offers a more promising alternative to conventional batteries. However, micro combustion faces the problems of flame instability and low combustion efficiency. Therefore, improving flame stabilization and combustion efficiency in micro combustions is necessary. Studies have made considerable progress in these aspects over the last decade. This paper summarized these studies and classified the optimization schemes according to flame stabilization and combustion efficiency. Besides, research on the Field Synergy Principle was discussed. The synergy between the flow field and temperature gradient field in the micro-scale domain will become a key research area in the future. It is proposed to insert porous media in MTES and MTPVS and adopt catalytic combustion. Adding hydrogen to the mixed gas was recommended. The equivalence ratio of the mixed gas in the range of 0.9–1.1 would be best. The equivalence ratio is the ratio of the theoretical requirement of air with complete combustion to the actual supply of air.     

Investigation into the performance of a micro gravitational heat pipe and a micro gravitational heat pipe with artery

The performance of a normal micro gravitational heat pipe was investigated using the analytical and numerical models previously developed. An innovative structure of the heat pipe, i.e. the micro gravitational heat pipe with artery, was then proposed in an attempt to overcome some of the drawbacks of the normal pipe. The thermal behaviour of the new type of heat pipe was simulated, and this was compared with that of a normal micro heat pipe. A performance estimation of both pipes was carried out based on the simulation results. Copyright © 2002 John Wiley & Sons, Ltd.     

Occult parasitic energy loss in heat engines

When a reversible process, such as the compression of a working gas, is treated as a feedback mechanism coupling adjacent heat engine cycles, analysis reveals a parasitic energy loss unaccounted for by traditional theory. Including this feedback‐induced energy loss in the derivation of an efficiency relation yields predictions that reproduce experimental observations with sufficient accuracy as to propose this effect as being the primary source of disparity between practice and existing theory. Further, it is shown that both the Carnot and Curzon–Ahlborn efficiency relations are derivable special cases of this more general result. Finally, it is postulated that substantial gains in thermodynamic efficiency may be affected in practical heat engines, including increases on the order of 30% in internal combustion engines, by frustrating these feedback mechanisms through simple mechanical modifications. Copyright © 2007 John Wiley & Sons, Ltd.     

热源自调节有机朗肯循环发电系统热力性能分析

为提高基本ORC(有机朗肯循环)系统换热器内冷热流体换热温差匹配程度,提升系统热力性能,提出一种ORC-R(热源自调节有机朗肯循环发电)系统,基于热力学第一定律和第二定律,建立了系统的数学模型并编制计算机程序进行分析,研究表明:当热源与有机工质换热温差不匹配时,采用热源自调节方式可有效提升基本ORC系统热力性能;热源自调节系数不同,ORC-R系统热力性能提升程度不同,存在随热源温度不同而有所变化的极限调节值;同时,ORC-R系统较基本ORC系统达到性能最优值时的蒸发温度降低,ORC-R系统净输出功、火用效率随热源自调节系数增加呈现先增加后减小的变化规律,可找到热源自调节系数的最佳值使ORC-R系统热力性能达到最优;热源温度Tg=373、383、393和403 K时,ORC-R系统净输出功Wnet较基本ORC系统分别增加35.52%、42.75%、51.15%和57.63%;ORC-R系统火用效率ηex分别为基本ORC系统的0.879 9倍、1.174 9倍、1.485 8倍和1.807 8倍。     

普适内可逆热机循环模型的(火用)经济性能优化

用有限时间热力学方法分析工作在恒温热源TH、TL之间内可逆普适热机循环模型的经济性能,导出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。所得结果包含了内可逆D iese、lO tto、A tk inson和B rayton循环的有限时间经济性能。     

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.     

以Dieterici气体为工质的两类热机循环性能分析

对Dieterici实际气体作了简要分析,并以Dieterici实际气体为工质,分别导出卡诺热机和斯特林热机的输出功和效率的一般表达式.最后通过数值计算,讨论了卡诺热机及斯特林热机的输出功和效率分别与体积和温度之间的关系.所得结论可为热机的运行条件和优化设计提供理论参考.     

Finite-time power limit for solar-radiant Ericsson engines in space applications

The power output and thermal efficiency of a finite-time optimized solar-radiant Ericsson heat engine is studied. The thermodynamic model adopted is a regenerative gas Ericsson cycle coupled to a heat source and heat sink by radiant heat transfer. Both the heat source and heat sink have infinite heat capacity rates. Mathematical expressions for optimum power and the efficiency at optimum power are obtained for the cycle based on higher and lower temperature bounds. The results of this theoretical work provide a base line criteria for use in the performance evaluation and design of such engines as well as for use in performance comparisons with existing extra-terrestrial solar power plants.