Irreversible Refrigeration Cycle Research Articles

Finite Time Thermodynamic Analysis of a Real Vapour Absorption Refrigeration System with and without Heat Leak

This article presents finite time thermodynamic analysis of a real vapour absorption refrigeration system taking into account both external as well as internal irreversibility. The complete VAR model is assumed to be consisting of two sub-systems. One sub-system (generator-absorber assembly) is assumed as an irreversible heat engine cycle whereas another sub-system (evaporator-condenser assembly) is assumed as an irreversible refrigeration cycle. This study analyzes the effect of presence of heat leak on performance of irreversible (real) vapour absorption refrigeration (VAR) system. This is achieved by adding a heat leakage term into an irreversible model of VAR system. Overall coefficient of performance (COP) of the VAR system is optimized with respect to source/sink side heat exchanger areas using Langrangian multiplier method. Furthermore, the effects of the variations in the cycle parameters on performance deviation of the VAR system are compared with and without heat leak. Results show that effect of rate of heat leak is significant along with the other irreversibilities variations on performance deviation of the VAR system.

Exergetic sustainability evaluation of irreversible Carnot refrigerator

Purpose of this paper is to assess irreversible refrigeration cycle by using exergetic sustainability index. In literature, there is no application of exergetic sustainability index for the refrigerators and, indeed, this index has not been derived for refrigerators. In this study, exergetic sustainability indicator is presented for the refrigeration cycle and its relationships with other thermodynamics parameters including COP, exergy efficiency, cooling load, exergy destruction, ecological function and work input are investigated. Calculations are conducted for endoreversible and reversible cycles and then results obtained from the ecological function are compared. It is found that exergy efficiency, exergetic sustainable index reduce 47.595% and 59.689% and rising at the COP is 99.888% is obtained for endoreversible cycle. Similarly, exergy efficiency and exergetic sustainability index reduce 90.163% and 93.711% and rising of the COP is equal to 99.362%.

Thermoeconomic considerations in the allocation of heat transfer inventory for irreversible refrigeration and heat pump systems

In this paper, thermoeconomic considerations are given to heat exchanger inventory allocation in irreversible refrigeration cycles and heat pumps with finite thermal capacitances. Investigation is made with respect to specified rate of cooling and heating for the refrigeration cycle and heat pump, respectively. Exact expressions are obtained without the use of an internal irreversibility parameter. The optimum hot-to cold-end unit cost ratio resulted in unequal division of heat exchanger conductances for the heat pump in contrast to the endoreversible case where they were the same. At a constant evaporator to condenser fluid temperature ratio, the COP of both refrigeration and heat pump systems was found to vary in an almost asymptotic manner in contrast to the endoreversible case where it was constant. In conclusion, it is best to measure the effect of internal dissipation by deriving exact mathematical expressions as the effect on some performance parameters could be lost due to the averaging effect of the internal irreversibility parameter.

Entransy analysis of irreversible Carnot-like heat engine and refrigeration cycles and the relationships among various thermodynamic parameters

Because of the energy needs of the world and the issues involved with global warming, analyzing and optimizing power cycles have increased in importance. In this paper, the concepts of entransy dissipation, entropy generation, power output, exergy output, energy, exergy efficiencies for irreversible heat engine cycles and entransy dissipation, entropy generation, power inputs, exergy input, entropy generation, COP and the exergy of efficiency for the irreversible refrigeration cycle are applied as a means of analyzing them. The results are obtained numerically, and the optimum or critical values are determined for a dimensionless temperature ratio and a dimensionless heat conductance ratio. Finally, recommendations on the design and range of operating conditions for the cycles are presented. Values of TC/TE (x), can be selected between 0.5 and 0.55 and values of kE/kC (y) in the range of 0.3–0.5 can be selected for high performance and low losses in a heat engine. Choosing values of TC/TE (x) and kC/kE (y) as low as possible for high performance, besides low thermodynamic losses (entropy generation and entransy dissipation) for the refrigeration cycle.

Parametric optimization of a new combined refrigeration cycle – active thermal potentiostatting system

The active thermal potentiostatting system proposed by Martinovskii and Tsirlin is directly generalized to a more practical case, in which one intermediate chamber, besides a thermal potentiostatting chamber, and two irreversible refrigeration cycles are included and the influence of the thermal resistance between the working fluid and the reservoirs, the heat leakages from the environment to the intermediate chamber and from the intermediate chamber to the potentiostatting chamber are taken into account. Expressions for the main parameters of the system are derived. By using the optimal control theory, the minimum total power input of the system with non-zero cooling rates is calculated and the temperatures of the working fluid in the isothermal processes of the refrigeration cycles are optimized. The optimal allocation of the heat-transfer areas of the heat-exchangers in the refrigeration cycles is discussed in detail. The results obtained here are more general and useful than the relevant results in literature and can provide some valuable guidance for the optimal design and operation of real active thermal potentiostatting systems.

General performance of an irreversible three heat source refrigerator

Abstract This work deals with the study of an irreversible refrigeration cycle controlled by three heat sources. Following the basic idea to treat the entire system as a combined cycle of an internally irreversible two heat source engine driving an internally irreversible two heat source refrigerator, we select a classical configuration to derive a fundamental optimum formula. Each one of these irreversible engine are characterized by the irreversibility factors I w and I r , respectively. Therefore, the predicted optimal plant performance coefficient and the relationship of that with the cooling effect reveal a unified description of various refrigerators of three heat sources previously reported in finite time thermodynamics. The numerical results show clearly the physical influence of these irreversibility factors together with the thermal resistances on the global performance of the refrigeration plant.

Ecological optimization of a multi-stage irreversible combined refrigeration system

A universal irreversible refrigeration cycle model which includes the irreversibility of heat transfer across finite temperature differences, the heat leak loss between the external heat reservoirs and the internal irreversibility of the working fluid is used to analyze the performance of a multi-stage combined refrigeration system. The ecological function is taken as an objective function for optimization. The maximum ecological function, its corresponding coefficient of performance, rate of refrigeration and rate of entropy production are determined. The optimal problems relative to the ecological function are discussed. Some characteristic curves of the refrigeration system are presented. The results obtained here are suitable for an arbitrary stage irreversible combined refrigeration system.

The maximum coefficient of performance of internally irreversible refrigerators and heat pumps

A class of irreversible refrigeration cycles is investigated to determine the maximum coefficient of performance in the heat pump mode and the refrigerator mode. For the purpose of generality and simplicity of the results, finite-time heat transfer in the condenser and evaporator is expressed in terms of arithmetic mean temperature differences. The generic source of internal irreversibility is measured by a single irreversibility factor which transforms the Clausius inequality into an equality to simplify the cycle model. These optimum cycle performances are obtained as closed form analytical expressions in which the irreversibility factor has been shown to be simply related to the ratio of the actual and endoreversible cycle coefficients of performance.

A class of internally irreversible refrigeration cycles

A Carnot-like irreversible refrigeration cycle is modelled with two isothermal and two non-adiabatic, irreversible processes. The generic source of internal irreversibility, measured by the Clausius inequality, is a general irreversibility term which could include any heat leaks into the Joule - Thompson expansion valve, the evaporator and compressor cold boxes. This cycle is optimized first for maximum refrigeration power and maximum refrigeration load, then for maximum coefficient of performance. Its performances are compared with those of the endoreversible refrigeration cycle, based on a propane stage of a classical cascade liquefaction cycle example. Both cycle models achieve optimum power and maximum refrigeration load at nearly the same refrigeration temperature, but only the coefficient of performance of the irreversible refrigeration cycle reaches a maximum. Moreover, its prediction of heat conductance allocation between evaporator and condenser appears to be not only more conservative, but also more realistic for actual design considerations of refrigeration cycles.