Theory Of Finite Time Thermodynamics Research Articles

Exergetic efficiency optimization for an irreversible Brayton refrigeration cycle

Exergetic efficiency optimization that combines exergy concept and finite-time thermodynamic theory has been carried out for an irreversible Brayton refrigeration cycle. Multi-irreversibilities considered in the system include finite rate heat transfer, internal dissipation of the working fluid and heat leak between heat reservoirs. Exergetic efficiency defined as the ratio of rate of exergy output to rate of exergy input of the system is considered as the objective index. The goal of exergetic efficiency optimization is to maximize this index. The maximum value of the exergetic efficiency can be determined analytically. The results are compared with those obtained from the traditional coefficient of performance. The influences of heat leak between heat reservoirs and temperature ratio of two reservoirs on the exergetic efficiency are investigated by numerical calculations. The allocation of a fixed total thermal conductance between the two heat exchangers is also discussed. The results show that the method of exergetic efficiency optimization is an important and effective criterion for the evaluation of an irreversible Brayton refrigeration cycle.

04/02607 Effect of varying dead-state properties on energy and exergy analyses of thermal systems: Rosen, M. A. and Dincer, I. International Journal of Thermal Sciences, 2004, 43, (2), 121–133

The performance of an irreversible closed intercooled regenerator Brayton-cycle coupled to constant-temperature heat reservoirs is analyzed by using the theory of finite-time thermodynamics (FTT). Analytical formulae for dimensionless power and efficiency are derived. Especially, the intercooling pressure-ratio is optimized for the optimal power and the optimal efficiency, respectively. The effects of component (the intercooler, the regenerator, and the hot- and cold-side heat-exchangers) effectivenesses, the compressor and turbine efficiencies, the heat-reservoir temperature-ratio, and the temperature ratio of the cooling fluid in the intercooler and the cold-side heat reservoir on the optimal power and the corresponding efficiency and corresponding intercooling pressure ratio, as well as the optimal efficiency and the corresponding power and corresponding intercooling pressure-ratio are analyzed by detailed numerical examples.

Finite-time heat-transfer analysis and generalized power-optimization of an endoreversible Rankine heat-engine

This paper reports the results of a study carried out for the power optimization of a Rankine-cycle heat-engine using finite-time thermodynamic theory. This study extends the recent flurry of publications in heat-engine efficiency under the maximum power condition by incorporating the optima of heat conductance and heat capacitance ratios. While maximizing the instantaneous power output, it is shown that there is an optimum balance between the sizes of heat exchangers, between the heat capacity rates of heating and cooling fluid as well as temperature differences between the engine and thermal reservoirs. The results indicate that power output increases significantly with the increase in heat capacity rate of the heating fluid, but the thermal efficiency at maximum power remains constant. The effects of thermal conductance of the hot-side heat-exchanger on the power output and thermal efficiency are insignificant. The theoretical efficiency formulated in this study is much closer to that actually observed in well-designed power plants.

Closed intercooled regenerator Brayton-cycle with constant-temperature heat-reservoirs

The performance of an irreversible closed intercooled regenerator Brayton-cycle coupled to constant-temperature heat reservoirs is analyzed by using the theory of finite-time thermodynamics (FTT). Analytical formulae for dimensionless power and efficiency are derived. Especially, the intercooling pressure-ratio is optimized for the optimal power and the optimal efficiency, respectively. The effects of component (the intercooler, the regenerator, and the hot- and cold-side heat-exchangers) effectivenesses, the compressor and turbine efficiencies, the heat-reservoir temperature-ratio, and the temperature ratio of the cooling fluid in the intercooler and the cold-side heat reservoir on the optimal power and the corresponding efficiency and corresponding intercooling pressure ratio, as well as the optimal efficiency and the corresponding power and corresponding intercooling pressure-ratio are analyzed by detailed numerical examples.

Optimum allocation of heat transfer surface area for cooling load and COP optimization of a thermoelectric refrigerator

The theory of finite time thermodynamics is applied to analyze and optimize the performance of a thermoelectric refrigerator, which is composed of multi-elements. For the fixed total heat transfer surface area of two heat exchangers, the ratio of the heat transfer surface area of the high temperature side heat exchanger to the total heat transfer surface area of the heat exchangers is optimized for maximizing the cooling load and the coefficient of performance of the thermoelectric refrigerator. The effects of various parameters on the optimum performance are analyzed. The results may provide guides for the analysis and optimization of practical thermoelectric refrigerators.

Performance analysis for an irreversible variable temperature heat reservoir closed intercooled regenerated Brayton cycle

In this paper, the theory of finite time thermodynamics is used in the performance analysis of an irreversible closed intercooled regenerated Brayton cycle coupled to variable temperature heat reservoirs. The analytical formulae for dimensionless power and efficiency, as functions of the total pressure ratio, the intercooling pressure ratio, the component (regenerator, intercooler, hot and cold side heat exchangers) effectivenesses, the compressor and turbine efficiencies and the thermal capacity rates of the working fluid and the heat reservoirs, the pressure recovery coefficients, the heat reservoir inlet temperature ratio, and the cooling fluid in the intercooler and the cold side heat reservoir inlet temperature ratio, are derived. The intercooling pressure ratio is optimized for optimal power and optimal efficiency, respectively. The effects of component (regenerator, intercooler and hot and cold side heat exchangers) effectivenesses, the compressor and turbine efficiencies, the pressure recovery coefficients, the heat reservoir inlet temperature ratio and the cooling fluid in the intercooler and the cold side heat reservoir inlet temperature ratio on optimal power and its corresponding intercooling pressure ratio, as well as optimal efficiency and its corresponding intercooling pressure ratio are analyzed by detailed numerical examples. When the heat transfers between the working fluid and the heat reservoirs are executed ideally, the pressure drop losses are small enough to be neglected and the thermal capacity rates of the heat reservoirs are infinite, the results of this paper replicate those obtained in recent literature.

On the Curzon–Ahlborn efficiency and its lack of connection to power producing processes

Because some physicists continue to defend the nonexistent theory of finite time thermodynamics, additional incontrovertible experimental and theoretical evidence is provided about its irrationality and nonreality.

Optimum Allocation of Heat Exchanger Inventory of Irreversible Air Refrigeration Cycles

The theory of finite time thermodynamics is applied to analyze the performance of irreversible air refrigeration cycles in this paper. For the fixed total heat exchanger inventory, the ratio of heat conductance of the low-temperature side heat exchanger to that of the high-temperature side heat exchanger is optimized for maximizing the cooling load and the coefficient of performance (COP) of the cycles. The influences of various parameters on the characteristic of the cycle are analyzed. The results obtained may provide guidance for the design of practice air refrigeration plants.

On the Curzon–Ahlborn efficiency and its connection with the efficiencies of real heat engines

It is acknowledged that the Curzon–Ahlborn efficiency η CA determines the efficiency at maximum power production of heat engines only affected by the irreversibility of finite rate heat transfer (endoreversible engines), but η CA is not the upper bound of the efficiencies of heat engines. This is conceptually different from the role of the Carnot efficiency η C which is indeed the upper bound of the efficiencies of all heat engines. Some authors have erroneously criticized η CA as if it were the upper bound of the efficiencies of endoreversible heat engines. Although the efficiencies of real heat engines cannot attain the Carnot efficiency, it is possible, and often desirable, for their efficiencies to be larger than their respective maximum power efficiencies. In fact, the maximum power efficiency is the allowable lower bound of the efficiency for a given class of heat engines. These important conclusions may be expounded clearly by the theory of finite time thermodynamics.

Effects of internal irreversibility and heat leakage on the finite time thermoeconomic performance of refrigerators and heat pumps

By using finite time thermodynamic theory, a performance analysis based on the thermoeconomic criterion has been performed for an irreversible refrigerator and a heat pump. The thermoeconomic objective function is defined as the cooling load for the refrigerator and the heating load for the heat pump per unit total cost. The optimal performances and design parameters which maximize the objective functions are investigated. The design parameters under the optimal conditions have been derived analytically, and then, the effects of internal irreversibility, heat leakage and the economical parameter on the global and optimal performances have been discussed.

A universal expression for optimum specific work of reciprocating heat engines having endoreversible carnot efficiencies

Irreversible heat transfer (finite time) analysis is used to obtain the optimum thermodynamic specific work potential at maximum power for various practical reciprocating cycles having endoreversible Carnot efficiencies. The theory of finite-time thermodynamics for reciprocating endoreversible cycles with heat transfer irreversibilities gives rise to an optimum efficiency at maximum power output, of η=1−(TL/TH)0·5 for Carnot-like cycles in contrast to the upper limit for Carnot-like cycles of η=1−(TL/TH) obtained from infinite-time thermodynamics. It is shown here that, additionally, for this same general family of regenerative reciprocating cycles which includes the Stirling, the Ericsson and the reciprocating Carnot cycle, the finite-time optimum specific work output at maximum power, (wopt), is exactly half of that obtained for infinite-time reversible cycles (Carnot work, wrev) operating between the same temperature limits (i.e., wopt=½wrev). To accomplish this, the analysis makes use of time symmetry to minimize overall cycle time and to thus optimize net cycle power. Based on linear heat transfer laws, the expression for optimum specific work is shown to be independent of heat conductances. Moreover, this optimum specific work output is the same expression for all of the members of this family of cycles. This analysis makes use of the ideal gas model with constant specific heats, though the results are shown to be universal for the Carnot cycle for vapours and real gases. A sample calculation is given which shows that while operating under the same optimized conditions, the endoreversible Stirling engine will have the same thermal efficiency as the endoreversible Ericsson, but will have a higher optimum power output. The optimum power of the reciprocating endoreversible Carnot engine will be superior to both. Copyright © 1999 John Wiley & Sons, Ltd.

The optimal operating temperature of the collector of an irreversible solar-driven refrigerator

A universal irreversible solar-driven refrigerator model is presented, in which not only the irreversibility of heat conduction but also the irreversibilities resulting from the friction, eddies and other irreversible effects inside the working fluid are considered. On the basis of this model and the linear heat-loss model of a solar collector, one of the important parameters, called the optimal operating temperature of the collector of a solar-driven refrigerator, is derived by using the finite-time thermodynamic theory. From the result, the maximum overall coefficient of performance of the refrigerator is determined and some significant problems are discussed. The results obtained here are quite realistic and universal, insofar as all the corresponding results derived by using the reversible and endoreversible models and the model considering only the internal irreversibility cycle can be deduced from them. Thus, they may provide some new theoretical bases for further exploitation of solar-driven refrigerators. Furthermore, some shortcoming in the related literature are pointed out.

Finite Time Thermodynamic Optimization or Entropy Generation Minimization of Energy Systems

The historical background, research development, and the state-of-the-art of finite time thermodynamic theory and applications are reviewed from the point of view of both physics and engineering. The emphasis is on the performance optimization of thermodynamic processes and devices with finite-time and/or finite-size constraints, including heat engines, refrigerators, heat pumps, chemical reactions and some other processes, with respect to the following aspects: the study of Newton's law systems, an analysis of the effect of heat resistance and other irreversible loss models on the performance, an analysis of the effect of heat reservoir models on the performance, as well as the application for real thermodynamic processes and devices. It is pointed out that the generalized thermodynamic optimization theory is the development direction of finite thermodynamics in the future.

Optimization of the rate of exergy output for an endoreversible Carnot refrigerator

By using finite-time thermodynamic theory, the optimal performances related to exergy of a Carnot refrigerator under the influence of thermal resistance are studied. The optimal relations between the rate of exergy output and the coefficient of performance and between the rate of exergy output and the rate of refrigeration are derived. Moreover, the main characteristics of the two optimal relations are discussed and some uses of the concept of exergy in finite-time thermodynamic theory are expounded. Some novel and useful results are obtained. Finally, it is pointed out that to extend the traditional exergy analysis using the theory of finite-time thermodynamics is quite important.

A universal optimum work rate potential for continuous endoreversible Carnot heat engine cycles

The theory of finite-time thermodynamics for continuous endoreversible cycles gives rise to an optimum efficiency at maximum power output of η=1−(TL/TH)0.5 in contrast to the upper limit of η=1−(TL/TH) obtained from infinite-time thermodynamics. It is shown here that, additionally, for continuous Carnot cycles, the finite-time optimum rate of work output (Ẇopt), is exactly half of that obtained for infinite-time reversible cycles (Carnot work rate, Ẇrev) operating between the same temperature limits (i.e. Ẇopt=1/2Ẇrev). This expression is true for both unoptimized as well as optimized mass flow rate conditions. The formulation used in the analysis is universal for all working substances for cycles using linear heat transfer laws.

The theoretical efficiency limits for a combined cycle under the condition of maximum power output

Using the theory of finite-time thermodynamics, an optimal performance analysis of an internally and externally irreversible Carnot-like single heat engine was studied to determine the maximum power and the efficiency at the maximum power output. Based on this study, the maximum power and the efficiency at the maximum power output of an irreversible combined heat engine (two single irreversible heat engines in a cascade) were obtained for steady state operation. The optimal performance of the combined cycle was expressed in terms of two design parameters. The effects of these parameters on the performance of the combined cycle were examined. The upper and lower bounds to the maximum power and the efficiency at the maximum power were determined. These results were compared with those obtained for a single cycle. It was shown that it is possible to design a combined engine which generates more power and is more efficient than a single cycle.

Optimal choice of the performance parameters of an absorption heat transformer

The optimal performance of an absorption heat transformer, i.e. a type II absorption heat pump, is investigated by using the cyclic model with continuous flow. The effect of thermal resistances between the heat transformer and the heat reservoirs is considered in the model. A general expression related to the rate of heat-pumping, the coefficient of performance and the overall heat transfer area of the heat transformer is derived. The expression is used to optimise the main performance parameters of the heat transformer. The maximum rate of heat-pumping and the corresponding coefficient of performance are calculated. For a given overall heat transfer area of the heat transformer, the optimal relation of the heat transfer areas of the heat exchangers is obtained. The problems concerning the optimal choices of other performance parameters are discussed. The results obtained here can not only enrich the theory of finite time thermodynamics, but also provide some new theoretical bases for the optimal design and operation of real absorption heat transformers.

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.

Equivalent combined systems of three-heat-source heat pumps

In this paper, the equivalent combined systems of three-heat-source heat pumps are studied. It is pointed out that there may be three combined ways for us to conceive a reversible three-heat-source heat pump as a combined system having a reversible Carnot heat pump driven by a reversible Carnot engine. Whereas for an endoreversible three-heat-source heat pump affected by the heat resistances, there is only one combined way for us to conceive it as a combined system having an endoreversible Carnot heat pump driven by an endoreversible Carnot engine. Then, the fundamental optimum formula of a three-heat-source heat pump is derived simply and directly by using those of two-heat-source cycles. The optimal peformance of an endoreversible three-heat-source heat pump is thus discussed. The inherent relation between the two- and the three-heat-source cycles as well as the fundamental distinction between the reversible and the endoreversible three-heat-source cycles is also revealed. These results may enrich the theory of finite time thermodynamics, and provide some new theoretical bases for the exploitation and application of three-heat-source apparatus such as chemical heat pumps, absorption heat pumps, and so on.

An optimal endoreversible three-heat-source refrigerator

An endoreversible three-heat-source refrigerator only affected by thermal resistance, like a reversible three-heat-source refrigerator, may be treated as a combined cycle of a two-heat-source engine driving a two-heat-source refrigerator. The theory of finite time thermodynamics in two-heat-source cycles is then used to analyze it and derive its basic optimum relation. Thus, the fundamental effect of thermal resistance on the optimal performance of a three-heat-source refrigerator is expounded. The conclusions obtained here are more realistic than those of classical thermodynamics. They provide some new theoretical bases for further exploitation of the three-heat-source refrigeration apparatus that applies to ‘‘cheap’’ heat sources, such as solar energy, geothermal energy, waste heat, and so on.