Sources Of Irreversibility Research Articles

Optimal operation of thermal energy storage units based on stratified and fully mixed water tanks

This paper refers to thermal energy storage units based on fully mixed and stratified liquid storage tanks. The stratified unit was modeled by using three partial differential equations governing the time and space dependence of hot gas, liquid storage medium and pipes wall material, respectively. For the study of fully mixed units a standard approach based on the number of thermal units concept was slightly generalized to take into account the time dependence of the heat transfer coefficients. The performance indicator adopted here is the minimum exergy destruction. All relevant irreversibility sources were considered. Every operation regime has an optimum charging time, which increases by decreasing the hot gas inlet temperature. Also, there is an optimum gas mass flow rate that makes the exergy destruction number a minimum minimorum. The stratified units are normally more effective than the fully mixed units from the point of view of exergy conservation. The standard approach based on the number of thermal units concept tends to overestimate the performance of thermal energy storage systems. Its accuracy is better in case of fully mixed units.

Study on prediction of the effects of design and operating parameters on NO x emissions from a leanburn natural gas engine : Kesgin, U. Energy Conversion and Management, 2003, 44, (6), 907–921

In this paper we present the exergetic analysis and optimisation of a steam methane pre-reformer system for marine molten carbonate fuel cells (MCFC) fuelled by liquefied natural gas (LNG). The steam methane pre-reformer is a key component of the MCFC, reforming completely higher-chain hydrocarbons and partly methane to hydrogen. The pre-reformer system efficiency improvement is of key importance since it uses up to 10% of the fuel exergy input of the MCFC. First, we develop a dynamic mathematical model that describes the physical/chemical behaviour of the pre-reformer system using a generic process modelling framework. Then, we perform exergy analysis and we optimise the reformer with respect to its exergetic performance. The developed models and exergy balances are spatially distributed to account for the internal process characteristics, capturing the interrelation of the local exergy destruction with component design and geometry. An optimisation problem is then formulated that minimises the total irreversibility of the system subject to design, space, technical, and operational constraints. The exergy analysis and optimisation for a specific MCFC derived the sources of irreversibility and provided an optimal design of 50% less exergy destruction. The results will serve as a basis for the optimisation of the entire MCFC unit.

Dissipation and decoherence in the generalized Jaynes-Cummings model

In this paper, we present a simple approach for understanding some general aspects of the irreversible dynamics of the generalized Jaynes-Cummings model. By working in the dressed-state representation, it is possible to split the dynamics of populations and coherences. Then it can be shown that the coherence dynamics may present localization and dispersivelike decay behavior, whereas the population dynamics are characterized in terms of a classical diffusive process. These phenomena are studied by developing a perturbation theory that provides simple analytical results that could be applicable to any general source of irreversibility in the Jaynes-Cummings model. As an application, we extend and analytically characterize our previous reported results on heating and decoherence in trapped ions [Phys. Rev. A 65, 041402 (2002)].

Ejector Irreversibility Characteristics

The present study analyzes and characterizes the irreversibility of the ejector’s internal processes in an effort to improve the understanding of the making of its overall performance. The analysis presented is based on entropy production methodology. Since entropy production is equivalent to performance losses, minimizing entropy production could serve as a tool for performance optimization. The three main internal processes forming sources of ejector irreversibility are mixing, kinetic energy losses, and normal shock wave. Comparison of these with those of an ideal mixing process, an ideal turbine-compressor system and stagnation conditions (of the flow after mixing) provides the benchmarks against which the actual overall performance is measured. By identifying the sources of irreversibility, the analysis provides a diagnostic tool for performance improvements. While irreversibility due to mixing can be eliminated by appropriate choice of gas and/or inlet conditions and an appropriate adjustable throat can eliminate losses associated with normal shock wave–kinetic energy losses can only be reduced but not totally eliminated.

Experiments on a solar-assisted heat pump and an exergy analysis of the system

In this work a experimental study of a solar assisted heat pump with direct expansion of the refrigerant within the solar collector, is presented. The maximum exergy efficiency, defined as the ratio of the outlet to the inlet exergy flow in every component of the heat pump cycle, is determined taking into account the typical parameters and performance coefficients. The results of this exergy analysis point out that the main source of irreversibility can be found in the evaporator of the heat pump (that is, the solar collector) emphasizing that incoming solar radiation is not used to full advantage in this piece of equipment.

Optimization of a sensible heat cascade energy storage by lumped model

In this study, the concept of exergy is used to analyze and optimize a sensible heat cascade thermal energy storage system which is developed for storing exergy and later using it efficiently. The system is composed of a liquid–gas heat exchanger, an electrical heater, an insulated vessel, a large water bath as a storage liquid and air as the system gas. The units that should be improved are determined by evaluating the irreversibilities caused by viscous losses in the heat exchanger, heat transfer occurring in the storage, electrical heating, removal processes and mixing of the hot gases at the exit of the storage process with ambient air. First and second law efficiencies and irreversibility sources of the system are compared with the thermal energy storage system's having a unique energy resource. Removing time, optimum storage time and the number of transfer units of the heat exchanger are also determined for different operating conditions by minimizing the irreversibilities in the system.

Work and entropy production aspects of irreversible processes in closed and steady-state open systems

This paper systematically deals with the influence of different sources of irreversibilities in work transfer expressions for closed and steady-state open systems. In the various cases, the hypotheses necessary to an analytical formulation and therefore the limits of the latter are specified. The influence of the non-uniformity of temperature and pressure on the irreversibilities in the system is particularly examined. The main aim is that of deducing a criterion with the conditions sufficient to separate in distinct relations the different phenomena (mechanical, electrical, chemical, etc.) simultaneously present in the same system.

Solar hydrogen production via a two-step water-splitting thermochemical cycle based on Zn/ZnO redox reactions

The production of hydrogen from water using solar energy via a two-step thermochemical cycle is considered. The first, endothermic step is the thermal dissociation of ZnO(s) into Zn(g) and O 2 at 2300 K using concentrated solar energy as the source of process heat. The second, non-solar, exothermic step is the hydrolysis of Zn(l) at 700 K to form H 2 and ZnO(s); the latter separates naturally and is recycled to the first step. Hydrogen and oxygen are derived in different steps, thereby eliminating the need for high-temperature gas separation. A 2nd-law analysis performed on the closed cyclic process indicates a maximum exergy conversion efficiency of 29% (ratio of ΔG 298 K °| H 2+0.5 O 2→ H 2 O for the H 2 produced to the solar power input), when using a solar cavity-receiver operated at 2300 K and subjected to a solar flux concentration ratio of 5000. The major sources of irreversibility are associated with the re-radiation losses from the solar reactor and the quenching of Zn(g) and O 2 to avoid their recombination. An economic assessment for a large-scale chemical plant, having a solar thermal power input into the solar reactor of 90 MW and a hydrogen production output from the hydrolyser of 61 million-kWh/yr, indicates that the cost of solar hydrogen ranges between 0.13 and 0.15$/kWh (based on its low heating value and a heliostat field cost at 100–150$/m 2) and, thus, might be competitive vis-à-vis other renewables-based routes such as electrolysis of water using solar-generated electricity. The economic feasibility of the proposed solar process is strongly dependent on the development of an effective Zn/O 2 separation technique (either by quench or by in situ electrolytic separation) that eliminates the need for an inert gas. The chemical aspects of the reactions involved and the present status of the pertinent chemical reactor technology are summarized.

A critical review of pulse tube cryogenerator research

This paper is a synthesis of the state of the art in the design and theory of regenerative Pulse Tube cryogenic systems. A general outlook of the functional principles and of the processes occurring in these cryogenic systems is given. The main irreversibility sources are analyzed. A review of the important contributions on the subject, likely to improve the constructional and functional characteristics, is then presented, along with the evaluation of their scientific foundation.

Local entropy generation in an impinging jet: minimum entropy concept evaluating various turbulence models

Second law analysis techniques have been widely used to evaluate the sources of irreversibility in fluid flow systems. The same technique may be used to evaluate the various turbulence models. In the present study, a local entropy generation rate is computed for a fluid jet impinging on a heated wall. The standard k– ε, low-Reynolds number k– ε and two Reynolds stress models are introduced to account for the turbulence. A numerical scheme employing a control volume approach is used to solve the governing equations. The predictions are, then, compared with the experimental findings in the literature. The local volumetric entropy generation in the region close to the stagnation point is used to evaluate the turbulence models. The entropy generation gives information about the magnitude of viscous dissipation in the flow field. The minimum energy concept alone may not be used to evaluate the various turbulence models, in which case, the experimental measurements are accompanied with the results of entropy analysis.

Second Law Analysis of Diffusion Flames

The objective of this paper is to investigate the sources of volumetric irreversibilities in both laminar and turbulent diffusion flames. The theoretical background of analysis relies on the local exergy transport equation, which allows the microscopic formulation of the well-known Gouy-Stodola theorem. For laminar reacting flows, the volumetric entropy generation rate expression includes the viscous, thermal, diffusion and chemical components. Their expressions show that the corresponding irreversibilities are uncoupled if the combustion process occurs at constant pressure. The numerical simulation of a methane-air combustion process shows that the thermal, chemical and diffusive irreversibilities represent, in order of enumeration, the predominant irreversibilities in the laminar diffusion reacting flows. In the case of turbulent diffusion flames, the viscous, thermal, diffusion and chemical mean components have to be expressed in accordance with the combustion model. Two combustion models are used: the multi-species approach based on the eddy-break formulation of mean reaction rate, and the assumed probability density function for a conserved scalar that relies on the flame sheet model. For a diffusion methane-air jet flame, the distribution of mean irreversibility components is presented. Taking into account the technical importance of diffusion flames, the analysis could serve to improve the combustion geometry and the flow condition.

Carnot's cycle for small systems: irreversibility and cost of operations

In the thermodynamic limit, the existence of a maximal efficiency of energy conversion attainable by a Carnot cycle consisting of quasistatic isothermal and adiabatic processes precludes the existence of a perpetual machine of the second kind, whose cycles yield positive work in an isothermal environment. We employ the recently developed framework of the energetics of stochastic processes (called "stochastic energetics") to reanalyze the Carnot cycle in detail, taking account of fluctuations, without taking the thermodynamic limit. We find that in this nonmacroscopic situation both processes of connection to and disconnection from heat baths and adiabatic processes that cause distortion of the energy distribution are sources of inevitable irreversibility within the cycle. Also, the so-called null-recurrence property of the cumulative efficiency of energy conversion over many cycles and the irreversible property of isolated, purely mechanical processes under external "macroscopic" operations are discussed in relation to the impossibility of a perpetual machine, or Maxwell's demon. This analysis may serve as the basis for the design and analysis of mesoscopic energy converters in the near future.

00/03670 EPA decision on coal-ash regulation spells temporary relief for utilities

Exergy and greenhouse gas emission analyses are performed for a novel trigeneration system driven by a solid oxide fuel cell (SOFC). The trigeneration system also consists of a generator-absorber heat exchanger (GAX) absorption refrigeration system and a heat exchanger to produce electrical energy, cooling and heating, respectively. Four operating cases are considered: electrical power generation, electrical power and cooling cogeneration, electrical power and heating cogeneration, and trigeneration. Attention is paid to numerous system and environmental performance parameters, namely, exergy efficiency, exergy destruction rate, and greenhouse gas emissions.A maximum enhancement of 46% is achieved in the exergy efficiency when the SOFC is used as the primary mover for the trigeneration system compared to the case when the SOFC is used as a standalone unit. The main sources of irreversibility are observed to be the air heat exchanger, the SOFC and the afterburner. The unit CO2 emission (in kg/MWh) is considerably higher for the case in which only electrical power is generated. This parameter is reduced by half when the system is operates in a trigeneration mode.

Recycling of Hazardous Solid Waste Material Using High-Temperature Solar Process Heat. 1. Thermodynamic Analysis

The thermochemical conversion and recycling of hazardous solid waste materials is investigated using high-temperature solar process heat. Two important sources of wastes contaminated with heavy metal oxides are considered: (1) electric are furnace dust (EAFD) and (2) automobile shredder residue (ASR). The chemical equilibrium composition of these complex materials and the energy required to process them, using carbon, methane, or pyre-coke as reducing agents, are computed for temperatures in the range 300-2000 K. Metals can be extracted from their oxides in reducing atmospheres at above 1300 K for both EAFD and ASR: Zn is obtained in the gas phase, while Fe, Pb, and Cu are obtained in the condensed phase. The thermal energy requirements for converting EAFD at 1500 K are 3008 kJ/ kg and 4143 kJ/kg using C(gr) and CH4 as reducing agents, respectively. For converting ASR at 1500 K, 2455 kJ/kg are required. The solar exergy conversion efficiency, i.e., the efficiency of converting solar energy into the chemical energy of the reaction products (given by the Gibbs free energy change of product oxidation), can be as high as 69% for the EAFD conversion and 87% for the ASR conversion. Major sources of irreversibilities are those associated with the reradiation losses of the solar reactor and the heat rejected during the quenching. The use of concentrated solar energy as the source of process heat avoids emissions of greenhouse gases and other pollutants derived from the combustion of fossil fuels and further offers the possibility of converting waste materials into valuable commodities for processes in closed and sustainable materials cycles.

Low Reynolds number flow inside straight micro channels with irregular cross sections

This paper presents an analysis of the compound effect of finite temperature differences and fluid friction on the existence of an optimum laminar flow regime in singly connected micro channels with complex free flow area cross sections. A widespread conviction has been established that the two competing irreversibility sources in a channel flow with heat transfer lead to the existence of an optimum flow regime. The results presented in this paper clearly shows the opposite. When an objective function is represented by the entropy generation rate per unit heat capacity rate of the fluid stream, the thermodynamic optimum flow regime represents a rather rare occurrence in the laminar region of irregularly shaped ducts. The presence of an extremum is more probable for very small diameters, the ones of an order of magnitude of O(≤10−3 m). The analysis is performed for selected ranges of relevant geometric, flow, and thermal parameters of a set of straight micro channels with irregular free flow area cross-sections. The following geometries of the free flow area cross section were investigated: (i) sine duct, (ii) circular duct, (iii) elliptical duct, (iv) moon-shaped ducts, and (v) four-cuspped duct. The range of Reynolds numbers has been established between O(102) and O(104). The existence of the objective function minimum is confirmed for ducts with an irregular cross section only for very small hydraulic diameters. These minima are relatively weak, and as a general rule, the sets of optimum parameters are close to the onset of turbulence or possibly even in the transitional or turbulent regions.

Study on refrigerant circuitry of condenser coils with exergy destruction analysis

A numerical model of air-cooled condenser coils is proposed to investigate the coil performance. In this model, the properties of refrigerant and air, as well as six exergy destruction components associated with different sources of irreversibility are computed locally along the coil based on numerous control volumes. The governing equations for a control volume are presented together with a computer simulation procedure to link all the control volumes together for a coil. Using this model, the coil characteristics in heat transfer, fluid flow and exergy destruction are analyzed with an emphasis on the refrigerant circuitry investigation. The study shows that the thermal resistance of refrigerant side is comparable to that of air side and the coil performance can be improved by varying the refrigerant mass velocity along the flow path. Compared with a common coil, using a complex refrigerant circuitry that the refrigerant circuits are properly branched or joined may reduce the heat transfer area by approximately 5% in coil design. This study also shows that the quantitative analysis on the exergy destruction components gives a clear picture in the coil performance evaluation. The results show that the criterion of exergy destruction ratio conforms well to that of coil heat flux in coil performance evaluation under the specified conditions.

On entropy generation in thermoelectric devices

In this paper, a comparison between the Entropy Generation Minimization method and the Power Maximization technique is presented. The assessment is performed by analyzing, as a typical example of direct conversion heat engines, the thermoelectric generator. The effects of heat-leak and finite-rate heat transfer on the performance of the generator, which is modeled as a Carnot-like engine with internal irreversibilities, are studied. Even though both methods lead to the same conclusions, the entropy generation minimization method, when applied for such an inherently irreversible device, is shown to be less straight forward than the power maximization technique requiring careful accounting of the different sources of irreversibility. Moreover, the entropy generation rate vs. efficiency behavior of the generator reveals that the efficiency at minimum entropy generation rate and the maximum efficiency are distinct.

Ecological Interaction as a Source of Economic Irreversibility

Irreversibility can be either physical or economic in origin. For example, the extinction of a species is physically irreversible. On the other hand, contamination of lake-bottom sediments by mercury is not physically irreversible (the mercury and/or sediments can be physically removed), but the cost is so high that it can be said to be economically irreversible. This paper argues that economic irreversibility associated with environmental change is much more common than typically discussed in the economics literature. The source of the problem is the inherent complexity of ecological relationships. The paper discusses the origin and policy importance of these indirect irreversibilities.

Ecological Interaction as a Source of Economic Irreversibility

Irreversibility can be either physical or economic in origin. For example, the extinction of a species is physically irreversible. On the other hand, contamination of lake‐bottom sediments by mercury is not physically irreversible (the mercury and/or sediments can be physically removed), but the cost is so high that it can be said to be economically irreversible. This paper argues that economic irreversibility associated with environmental change is much more common than typically discussed in the economics literature. The source of the problem is the inherent complexity of ecological relationships. The paper discusses the origin and policy importance of these indirect irreversibilities.

Steady-state Mantle–Melt Interactions in One Dimension: I. Equilibrium Transport and Melt Focusing

Abstract Mantle–melt interaction during melt transport is explored in one dimension and steady state. We reconsider the equivalence between one-dimensional steady equilibrium transport and batch melting. In the absence of diffusion and radioactivity, conservation of mass flux requires that the major and trace element compositions of melt and solid at each point are the same as is generated by batch melting the source composition at the same pressure and temperature. Energy conservation requires that temperature and extent of melting are independent of melt migration except for irreversible source terms related to viscous compaction and gravitational energy release. The equivalence of phase compositions at each pressure between steady-state equilibrium transport and batch melting simplifies melt transport calculations. We examine the effects of increasing the melt flux to simulate melt focusing by channeling or by two-dimensional flow with converging melt streamlines. Melt focusing modifies the mineralogy of both residual matrix and erupted melt. We use MELTS calculations to model the formation of dunite by this mechanism and quantify the melt flux required to exhaust orthopyroxene from the residue as a function of pressure. The model dunites are found to be similar to natural dunites observed in the mantle section of ophiolite sequences.