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.     

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.     

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.     

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.     

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。     

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.     

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.     

不同优化准则下斯特林发动机的性能分析

应用有限时间热力学原理.建立了一个考虑热阻、热漏和回热损失等不可逆因素的斯特林发动机模型;推导了最大输出功率、最大效率和生态学优化准则下,斯特林发动机性能的表达式;比较了三种优化准则下,热漏系数和回热器有效性对斯特林发动机性能的影响.研究表明:对热漏损失和回热损失较大的斯特林发动机,宜选用生态学优化准则.为斯特林发动机...     

Thermodynamic optimization for work-assisted heating and drying operations

We construct thermodynamic criteria for optimization of sequential work-assisted heating and drying operations which run jointly with ‘endoreversible’ thermal machines. The total power input is minimized with constraints which take into account the dynamics of heat and mass transport and rate of work consumption. Finite-rate, endoreversible models include the irreducible losses of classical exergy potential caused by the thermal resistances. Extremum performance functions for optimal work, which incorporate residual entropy production, are formulated in terms of end states, duration and (in discrete processes) number of stages. Formal analogies between entropy production expressions for work-assisted and conventional drying operations help formulate optimization models of the former. An optimization procedure for a two-stage drying operation with an endoreversible heat pump at each stage is outlined.     

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.     

Performance optimization of a solar driven heat engine with finite-rate heat transfer

An optimal performance analysis for an equivalent Carnot-like cycle heat engine of a parabolic-trough direct-steam-generation solar driven Rankine cycle power plant at maximum power and maximum power density conditions is performed. Simultaneous radiation-convection and only radiation heat transfer mechanisms from solar concentrating collector, which is the high temperature thermal reservoir, are considered separately. Heat rejection to the low temperature thermal reservoir is assumed to be convection dominated. Irreversibilities are taken into account through the finite-rate heat transfer between the fixed temperature thermal reservoirs and the internally reversible heat engine. Comparisons proved that the performance of a solar driven Carnot-like heat engine at maximum power density conditions, which receives thermal energy by either radiation-convection or only radiation heat transfer mechanism and rejects its unavailable portion to surroundings by convective heat transfer through heat exchangers, has the characteristics of (1) a solar driven Carnot heat engine at maximum power conditions, having radiation heat transfer at high and convective heat transfer at low temperature heat exchangers respectively, as the allocation parameter takes small values, and of (2) a Carnot heat engine at maximum power density conditions, having convective heat transfer at both heat exchangers, as the allocation parameter takes large values. Comprehensive discussions on the effect of heat transfer mechanisms are provided.     

Optimal performance characteristics of a solar-driven heat pump at maximum COP

The effect of the irreversibility of finite-rate heat transfer on the performance of a solar-driven heat pump is investigated by using the theory of finite time thermodynamics. Maximizing the COP of the system leads to some novel rules for the optimum choices of primary performance parameters, such as the operating temperatures of the solar collector and the working fluid in the heat exchangers and the heat transfer areas of the heat exchangers. These rules can guide the evaluation of existing real solar-driven heat pumps or influence the design of future solar-driven heat pumps.     

Non-equilibrium thermodynamics of small-scale systems

Small thermodynamic systems exhibit peculiar behavior different from that observed in long-scale systems. Non-equilibrium processes taking place in those systems are strongly influenced by the presence of fluctuations which can be large. Contributions to the free energy which vanish at the infinite number of particles limit cannot be neglected and may exert an important influence on the dynamics. We show that in spite of these important differences, the method of non-equilibrium thermodynamics still applies when reducing the size of the system. By using this method, assumption of local equilibrium at the mesoscale thereby leads to the formulation of a mesoscopic non-equilibrium thermodynamics from which expressions for the non-equilibrium currents and kinetic equations for the probability density can be obtained.