Performance optimization for a two-stage thermoelectric heat-pump with internal and external irreversibilities

A model of the behaviour of a two-stage semiconductor thermoelectric heat-pump with external heat transfer is devised. Performance of the heat-pump, assuming Newton’s heat-transfer law, is analyzed using the combination of finite-time thermodynamics and non-equilibrium thermodynamics. The analytical formula describing the heating load versus working electric-current, and the coefficient of performance (COP) versus working electric-current are derived. For the fixed total number of thermoelectric elements, the ratio of number of thermoelectric elements of the top stage to the total number of thermoelectric elements is also optimized for maximizing the heating load and the COP of the thermoelectric heat-pump. The effects of design factors on the performance are analyzed.     

Thermo-economic optimization of an endoreversible four-heat-reservoir absorption-refrigerator

Based on an endoreversible four-heat-reservoir absorption-refrigeration-cycle model, the optimal thermo-economic performance of an absorption-refrigerator is analyzed and optimized assuming a linear (Newtonian) heat-transfer law applies. The optimal relation between the thermo-economic criterion and the coefficient of performance (COP), the maximum thermo-economic criterion, and the COP and specific cooling load for the maximum thermo-economic criterion of the cycle are derived using finite-time thermodynamics. Moreover, the effects of the cycle parameters on the thermo-economic performance of the cycle are studied by numerical examples.     

Performance characteristic of isothermal chemical engines

Chemical engines process mass flow to convert the differences in chemical potentials into work. An isothermal endoreversible chemical engine, in which the sole irreversibility is finite-rate mass transfer, is modeled in this paper. The power versus efficiency relationships of the chemical engine, using the analogous method of finite-time thermodynamics for a heat engine, are derived. The performance comparison between a heat engine and a chemical heat engine is presented.     

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。     

Performance analysis and optimization of a new two-stage ejector-expansion transcritical CO2 refrigeration cycle

In this paper, a new two-stage configuration of ejector-expansion transcritical CO₂ (TRCC) refrigeration cycle is presented, which uses an internal heat exchanger and intercooler to enhance the performance of the new cycle. The theoretical analysis on the performance characteristics was carried out for the new cycle based on the first and second laws of thermodynamics. Based on the simulation results, it is found that, compared with the conventional two-stage transcritical CO₂ cycle, the COP and second law efficiency of the new two-stage cycle are about 12.5–21% higher than that of conventional two-stage cycle. It is also concluded that, the performance of the new two-stage transcritical CO₂ refrigeration can be significantly improved based on the presented new two-stage cycle. Hence the new two-stage refrigeration cycle is a promising refrigeration cycle from the thermodynamically and technical point of views. A regression analysis in terms of evaporator and gas cooler exit temperatures has been used, in order to develop mathematical expressions for maximum COP, optimum discharge and inter-stage pressures and entrainment ratio.     

两级热电热泵的有限时间热力学分析

建立了考虑外部传热影响的两级半导体热电热泵模型，用有限时间热力学对牛顿传热规律下两级半导体热电热泵的性能进行分析，导出了供热率、供热系数与工作电流的一般关系式，得到了热电单元数的最优分配，并分析了多种因素对其性能的影响。     

Thermodynamic optimization of organic Rankine cycle using two-stage evaporation

Organic Rankine cycle (ORC) is a promising technology to recover low-grade heat, but it leads to a low efficiency due to the highest irreversible loss caused by the single-stage evaporation. The present work concerns the performance enhancement of a two-stage serial organic Rankine cycle (TSORC) for geothermal power generation. The heat source is divided into two separate temperature ranges. The main goal of the current simulation is to evaluate system performance of TSORC, as well as, to calculate the influence of two-stage evaporation on system performance. The ratio of the net power output to the total thermal conductance was chosen as the objective function. Results show that the system performance is coupled with geothermal water inlet temperature (GWIT), intermediate geothermal water temperature (IGWT), and evaporating temperatures. The two-stage evaporation significantly reduces the irreversible loss, thereby enhancing the net power output. The TSORC presents excellent systematic performances and deserves to be popularized in engineering applications.     

Performance evaluation of irreversible Miller engine under various specific heat models

In this paper, the performance of a Miller engine is evaluated under different specific heat models (i.e., constant, linear, and fourth order polynomial). Finite-time thermodynamics is used to derive the relations between power output and thermal efficiency at different compression and expansion ratios for an ideal naturally-aspirated (air-standard) Miller cycle. The effect of the temperature-dependent specific heat of the working fluid on the irreversible cycle performance is significant. It was found that an accurate model such as fourth order polynomial is essential for accurate prediction of cycle performance. The conclusions of this investigation are of importance when considering the designs of actual Miller engines.     

IFTEM: An interval-fuzzy two-stage stochastic optimization model for regional energy systems planning under uncertainty

The development of optimization models for energy systems planning has attracted considerable interest over the past decades. However, the uncertainties that are inherent in the planning process and the complex interactions among various uncertain parameters are challenging the capabilities of these developed tools. Therefore, the objective of this study is to develop a hybrid interval-fuzzy two-stage stochastic energy systems planning model (IFTEM) to deal with various uncertainties that can be expressed as fuzzy numbers, probability distributions and discrete intervals. The developed IFTEM is then applied to a hypothetical regional energy system. The results indicate that the IFTEM has advantages in reflecting complexities of various system uncertainties as well as dealing with two-stage stochastic decision problems within energy systems.     

Exergy-based ecological optimization of linear phenomenological heat-transfer law irreversible Carnot-engines

The optimal exergy-based ecological performance of a generalized irreversible Carnot-engine with losses due to heat-resistance, heat leakage and internal irreversibility, in which the heat-transfer between the working fluid and the heat reservoirs obeys a linear phenomenological heat-transfer law, is derived by taking an exergy-based ecological optimization criterion as the objective. This consists of maximizing a function representing the best compromise between the power output and entropy-production rate of the heat engine. A numerical example is given to show the effects of heat leakage and internal irreversibility on the optimal performance of the generalized irreversible heat-engine. The results provide theoretical guidance for the design of practical engines.     

Performance investigation of transcritical carbon dioxide two-stage compression cycle with expander

The trend toward the energy efficiency improvement for transcritical carbon dioxide refrigeration cycles has led to the development of the two-stage compression process. Three different variations of transcritical carbon dioxide two-stage compression cycles with expanders are investigated by using thermodynamics analysis. They are the two-stage compression at optimal intermediate pressure (TCOP) cycle, two-stage compression with expander driving high-pressure stage (TCDH) cycle and two-stage compression with expander driving low-pressure stage (TCDL) cycle, respectively. The performance of the TCOP cycle and the single-stage compression with expander (SCE) cycle is mainly discussed and compared for a wide operating condition. It is found that the COP and exergy efficiency of the TCOP cycle are on average 9% higher than those of the SCE cycle. At given design points, the COP of the TCDH cycle outperforms the other options, showing 11.32%, 9.65% and 0.72% performance improvement over the TCDL cycle, SCE cycle and the TCOP cycle, respectively. If design and structure are also taken into account, the TCDH cycle is a feasible option since the expander and the auxiliary compressor are integrated into one unit; thus, the transfer loss and leakage loss can be decreased greatly. The key problem is to adopt some measures that control the operating conditions to avoid deviating from the design point.     

Performance optimization of a photovoltaic induction motor pumping system

The performances of a photovoltaic pumping system based on an induction motor are degraded once insolation varies far from the value called nominal, where the system was sized. To surmount this handicap, an improvement of these performances by the optimization of the motor efficiency is described in this paper. The results obtained are compared with those of similar work pieces presented in the literature where the motor effeciency and air gap flux where optimized separatly. The simulation results show that the proposed system allows at the same time to combine the performances of the system with constant efficiency and the simplicity of implementation provided by the system with constant airgap flux.     

The role of the power/efficiency misconception in the rebound effect's size debate: Does efficiency actually lead to a power enhancement?

This paper addresses the question of whether the rebound effect's size is bigger or smaller than one. After a brief review of the related economic literature, a thermodynamic perspective tackles this topic by demonstrating that the dispute over the size of the rebound effect relies on a misconception of the thermodynamic nature of energy efficiency. The dichotomy, in fact, concerns the relationship between efficiency and power output rather than the scale of the economic side effects generated by energy efficiency mutations. Early intuitions of the dichotomy efficiency/power belong to the pioneering works of Stanley Jevons, in the field of economics, and Alfred Lotka in that of biology. Their findings are here approached using the basis of finite-time thermodynamics with a simple amendment, the addition of the time variable to the Carnot machinery. The model shows how a process of power maximization always leads to a sub-optimal efficiency level and additionally, that any efficiency improvement, in the context of low energy costs, will shift the power output of the machine instead of reducing energy consumption. A case study taken from the transport system is presented to elucidate this argument.     

Frequency-dependent performance of an endoreversible Carnot engine with a linear phenomenological heat-transfer law

On the basis of an endoreversible Carnot heat-engine model, the frequency-dependent performance of the engine is analyzed when the heat transfers between the working fluid and the heat reservoirs obey a linear phenomenological heat-transfer law, i.e., Q ∝ (ΔT⁻¹). The relations among average power-output, efficiency, available temperature-drop, cycle frequency and ratio of the heat-transfer times are derived. They are different from those obtained with Newton’s law. The results can provide guidance for selecting the appropriate working points of heat engines.     

Efficiency of a Miller engine

Using finite-time thermodynamics, the relations between thermal efficiency, compression and expansion ratios for an ideal naturally-aspirated (air-standard) Miller cycle have been derived. The effect of the temperature-dependent specific heat of the working fluid on the irreversible cycle performance is significant. The conclusions of this investigation are of importance when considering the designs of actual Miller-engines.     

Performance of an Atkinson cycle with heat transfer,friction and variable specific-heats of the working fluid

The performance of an air standard Atkinson cycle with heat-transfer loss, friction-like term loss and variable specific-heats of the working fluid is analyzed using finite-time thermodynamics. The relations between the power output and the compression ratio, between the thermal efficiency and the compression ratio, as well as the optimal relation between the power output and the efficiency of the cycle are derived by detailed numerical examples. Moreover, the effects of variable specific-heats of the working fluid and the friction-like term loss on the irreversible cycle performance are analyzed. The results show that the effects of variable specific-heats of working fluid and friction-like term loss on the irreversible cycle performance should be considered in cycle analysis. The results obtained in this paper provide guidance for the design of Atkinson engines.     

CFD modeling of two-stage ignition in a rapid compression machine: Assessment of zero-dimensional approach

In modeling rapid compression machine (RCM) experiments, zero-dimensional approach is commonly used along with an associated heat loss model. The adequacy of such approach has not been validated for hydrocarbon fuels. The existence of multi-dimensional effects inside an RCM due to the boundary layer, roll-up vortex, non-uniform heat release, and piston crevice could result in deviation from the zero-dimensional assumption, particularly for hydrocarbons exhibiting two-stage ignition and strong thermokinetic interactions. The objective of this investigation is to assess the adequacy of zero-dimensional approach in modeling RCM experiments under conditions of two-stage ignition and negative temperature coefficient (NTC) response. Computational fluid dynamics simulations are conducted for n-heptane ignition in an RCM and the validity of zero-dimensional approach is assessed through comparisons over the entire NTC region. Results show that the zero-dimensional model based on the approach of ‘adiabatic volume expansion’ performs very well in adequately predicting the first-stage ignition delays, although quantitative discrepancy for the prediction of the total ignition delays and pressure rise in the first-stage ignition is noted even when the roll-up vortex is suppressed and a well-defined homogeneous core is retained within an RCM. Furthermore, the discrepancy is pressure dependent and decreases as compressed pressure is increased. Also, as ignition response becomes single-stage at higher compressed temperatures, discrepancy from the zero-dimensional simulations reduces. Despite of some quantitative discrepancy, the zero-dimensional modeling approach is deemed satisfactory from the viewpoint of the ignition delay simulation.     

Analysis and optimization of a fuel cell integrated with series two-stage organic Rankine cycle with zeotropic mixtures

In this article, thermodynamic modeling of a cogeneration system consisting of a series two-stage organic Rankine cycle (STORC) and a proton exchange membrane (PEM) fuel cell is conducted. The fuel cell dissipated heat is utilized as STORC plant input. In order to gain a higher efficiency for the proposed cogeneration system, the condenser of the organic Rankin cycle is replaced by a thermoelectric generator (TEG) to minimize heat loss. Moreover, zeotropic mixtures have been employed due to their lower irreversibility compared to single working fluid. Simulation code is developed in MATLAB software linked with the REFPROP software to extract the thermodynamic properties. This simulation code calculates the exergy efficiency and system's total cost rate. Since the performance of the system is affected by the working fluid, three zeotropic mixtures are compared with R123. The parametric study shows that high pressure (HP) and low pressure (LP) evaporator temperature, current density, and PEM operating pressure significantly affect the total cost rate and the second law efficiency. The results indicate that Ipentane-cis Butane has better efficiency among the selected zeotropic mixtures. Furthermore, the genetic algorithm multi-objective optimization is applied to determine the optimal design parameters of the system in a scatter distribution schematic. Finally, the normalized Pareto frontier of Ipentane-cis Butane is given and the related best point of working as a higher exergy efficiency and lower cost rate are specified. Eventually, it is concluded that the integration of STORC with primary PEM fuel cell improves overall exergy efficiency by 1.9%. The total cost rate for optimum point can be in a range of 1.36–14.94 ($/h), depending on the hydrogen production process.     

Novel hybrid two-stage alkali metal thermoelectric converter and thermally regenerative electrochemical cycle to exploit waste heat of solid oxide fuel cell: Performance assessment and optimization

An innovative combination of a two-stage alkali metal thermoelectric converter (TAMTEC), and thermally regenerative electrical cycle (TREC) is employed to utilize the high-quality heat dissipated from solid oxide fuel cell (SOFC) for further electricity production. The superiority and effectiveness of the SOFC-TAMTEC-TREC system are verified compared to existing SOFC-based hybrid systems and sole SOFC. The performance of the system based on energy, exergy, and economic indicators is evaluated by varying the main design parameters. Parametric assessment demonstrates that the SOFC-TAMTEC-TREC system can reach the maximum power density of 12126 W m^?2 with energy and exergy efficiencies of 47.13% and 50.46% as TAMTEC proportional constant increases to 10^7.5 m² and rising SOFC pore and gain diameters to 3.77 × 10^?6 m and 2.5 × 10^?6 m, respectively reduce the cost rate density of system by 3.55 $ h^?1 m^?2. Furthermore, to achieve the maximum power density and exergy efficiency, and minimum cost rate density, NSGA-III multi-criteria optimization, and decision-making techniques are conducted. Outcomes indicate that Shannon entropy leads to the maximum power density of 8597.2 W m^?2 with a 35.94% enhancement relative to a single SOFC and 1 $ h^?1 m^?2 increment in cost rate density of the hybrid system, while LINMAP and TOPSIS ascertain the minimum increase in the cost rate density by 0.6 $ h^?1 m^?2 with 31.04% improvement in power density relative to single SOFC.     

CO deep removal with a method of two-stage methanation

CO methanation experiments showed that it was difficult to reach both goals of CO removal depth of below 10 ppm and CO₂ conversion rate of below 5% by using a single catalyst in this paper. A two-stage methanation method by applying two kinds of catalysts is proposed, that is, one catalyst with relatively low activity and high selectivity for the first stage at higher temperatures, and another one with relatively high activity for the second stage at lower temperatures. CO can be removed from 1% to below 0.1% at the first stage and to below 10 ppm at the second stage with CO₂ conversion rate below 1% and below 4% at each stage respectively. In addition, results also showed that the reverse water-gas shift (RWGS) reaction at the second stage was the dominant factor of CO removal depth. Temperature programmed reduction (TPR) and H₂ chemisorption were applied to characterize the catalysts.